CA1261003A - Method and device for seeking a free channel in a mobile radio system - Google Patents
Method and device for seeking a free channel in a mobile radio systemInfo
- Publication number
- CA1261003A CA1261003A CA000513002A CA513002A CA1261003A CA 1261003 A CA1261003 A CA 1261003A CA 000513002 A CA000513002 A CA 000513002A CA 513002 A CA513002 A CA 513002A CA 1261003 A CA1261003 A CA 1261003A
- Authority
- CA
- Canada
- Prior art keywords
- channels
- duplex
- fixed
- mobile station
- path
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W48/00—Access restriction; Network selection; Access point selection
- H04W48/20—Selecting an access point
Abstract
A method and device for seeking a free channel in a mobile radio system.
In this method the search for a free channel for communication between a fixed (F) and a mobile (M) station is conducted in the mobile station. The fixed station (F) detects the engaged duplex channels in the rising path and transmits the list of them via the semaphore path. According to the invention, all the fixed stations (Fj), (F) also transmit via the semaphore channels allocated to them the list of channels which they use. In the mobile station the received level of the fixed stations (Fj) in the semaphore path channels is measured and compared with an interference threshold to provide the interfering fixed stations. The mobile station (M) determines the engaged channels in the descending path as being those used by interfering fixed stations. A free channel may then be selected in the mobile station (M) from among those which are engaged neither in the rising nor in the descending path.
Application : mobile radio Reference : Figure 4
In this method the search for a free channel for communication between a fixed (F) and a mobile (M) station is conducted in the mobile station. The fixed station (F) detects the engaged duplex channels in the rising path and transmits the list of them via the semaphore path. According to the invention, all the fixed stations (Fj), (F) also transmit via the semaphore channels allocated to them the list of channels which they use. In the mobile station the received level of the fixed stations (Fj) in the semaphore path channels is measured and compared with an interference threshold to provide the interfering fixed stations. The mobile station (M) determines the engaged channels in the descending path as being those used by interfering fixed stations. A free channel may then be selected in the mobile station (M) from among those which are engaged neither in the rising nor in the descending path.
Application : mobile radio Reference : Figure 4
Description
2010~-~080 A method and device for seeking a free channel in a mobile radio system.
The invention relates to a method of seeking a free channel in a mobile radio system with a cellular network in which each channel of the rising communications path is associated with a given channel in the descending communications path to form a duplex channel providing communication between any one of the fixed stations and any one o~ the mobile stations, in accordance with which, in order to seek a free duplex channel between a fi~ed station F and a mobile sta-tion M:
- in fixed station F, the engaged duplex channels in the rising path for which the received power is higher than a first given threshold are detected and the addresses of said engaged channels are transmitted by one of the channels of a semaphore path formed oE equidistant time intervals successively allocated periodically to all the fixed stations of a set of cells constituting the cellular system;
- in mobile station M, the addresses must be determined of the engaged duplex channels in the descending path for which the received power is higher than a second given threshold, while a free duplex channel is selected Erom among those with a different address from those of the engaged channels to be determined in mobile station M.
The invention also relates to the device for implementing this method in the fixed and mobile stations.
Such a free channel seeking method for a cellular mobile : radio system was described in Canadian Patent 1,235,187 in the applicant's name. This method had the advantage of making it : 1 , .. ~ , ..
possible to allocate all the communication channels to each cell in the system, whereas, in conventlonal systems, each set of channels is distributed in a group of "motif" cells, with the "motif" repeated throughout the communications system.
However, the implementation of this known method may give rise to difficulties in certain types of mobile radio systems with respect to the determination in a mobile station oE the engaged channels in the descending communications path.
Such implementation is convenient only in systems of the la /~
:'. : . ,.
, -. . :.
`
PHF 85555 2 17.0~.1986 time-share multiple-access type (abbreviated to TSMA) in which the communication channels forming the rising and descending paths are time-multiplexed time channels. In a TSMA system, a mobile station has a wide-band receiver sequentially receiving all the channels of the descending path so that, in it, it is easy to detect those channels which are engaged simply by measuring the level of the signal received in the ti~e intervals corresponding to the various channels an~ by comparing these levels with a predetermined threshold.
In mobile radio systems of the frequency-distribution multiple-access type (abbreviated to FDMA), the communication channels are requency-multiplexed frequency channels and the receiver of a mobile station can receive only one channel at a ti~e so that, in order to measure the level in all the channels of the descending path, the receiver s frequency must vary in steps in order successively to explore all the channels. In a 255-channel system, for instance, to be explored in 125 ms (the frame period of a comparable TSMA system), each frequency step must last about 0.5 ms. To take account of the fading phenomenon caused by multiple tracks, an average of the measurement of the levels in the channels over two or three exploration processes must be struck.
The relatively long time (2 to 3 times 125 ms) needed to seek a free channel by this method raises to major drawbacks during the stand-by period, but is highly annoying if the channel has to be changed during a communication, if, for example, the channel in use becomes cluttered.
Communication must, in fact, be suspended while a free channel is being sought. To avoid this drawback, it is possible to use an auxiliary receiver continuously exploring the channels to seek a second free one in anticipation of the ti~e when the main receiver has to change channels, but this is a complex solution.
Finally, it should be noted that there is a growing ~endency in practice to use mixed systems combining frequency and time-distributed multiple-access systems making it easier than with purely TSMA of FDMA to fit the ~ixed stations with a variable number of channels depending on the area (urban or rural) or on the time to take account of the gradual increase in the number of users.
It is thus possible to produce a mixed TSMA/FDMA variable capacity system by frequency-multiplexing a variable number of TSMA
frames. A distinction may be made between TSMA wide-band/FDMA systems ,: :
, ., .
~6~
2010~-8080 in which each TSMA frame occupies a greater band-width than the coherence band of a channel, oE the order of 200 KHz, and narrow-band TSMA/FDMA systems in which each TSMA frame occupies a coherence band of a channel. In both cases the fixed stations may be fitted with a variable number of channels depending on the location and the time, with an increment equal to the number of communication channels contained in a TSMA frame. Finally, it is also possible to set up a variable-capacity system with a variant of a mixed narrow-band TSMA/FDMA system in which frequency jumps are made at the rate o the TSMA frames and co-ordinated among the users so as to avoid mutual interference. In such a system, a communications channel uses a TSMA frame time lnterval with a carrier frequency according to a code determined by the law of the frequency jumps used. In what follows, this mixed system is referred to as the TSMA/CDMA/FDMA system, where CDMA signifies -"code-distribution multiple-access".
Clearly, if the free channel seeking method described in Canadian Patent 1,235,187 is to be applied to the mixed systems referred to above, all of which comprise frequency-distribution multiple-access, the same difficulties arise as with systems of the pure FDMA systems.
It is the intention of this invention to provide a method which avoids these diEficulties by making it possible to select a free channel and to change channels in any mobile radio system, especially the three mixed systems described, without any interruption during communications~
In accordance with the invention, there is provided a method of enabling a mobile station (M) to select a free duplex , :: :
L~
20104-~0~0 channel for communicating with a ~ixed station (F) in a cellular mobile radio system having a plurality of duplex channels each of which has a rising communication path and an associated descending communication path, respective duplex channels having respective addresses and providing communication between respective ones of a plurality of fixed stations and a plurality of mobile stations, the fixed stations being located in a pattern of cells constituting the cellular system; such method comprising:
- each :Eixed station identifying the addresses of duplex channels having rising paths which are engaged by detecting whether the signal power level in each received rising path exceeds a first given threshold, and transmitting the addresses of such channels to the mobile stations over a signalling path comprising equidistant time interval channels which are successively and periodically allotted to the respective fixed stations in the cellular system;
- determination by each mobile station of -the addresses of duplex channels having descending paths which are engaged by detecting whether the signal power level in each received descending path exceeds a second given threshold; and - selection by the mobile station (M) of a free duplex channel from among duplex channels having addresses differing from those of channels which are engaged in the rising paths and those which are engaged in the descending paths;
such method being characterized by:
- continuous transmission by each of -the fixed stations, in the signalling path time interval channels respectively allotted to them, of the addresses of duplex channels which they are : 3a -~ . ~ ., 3~
2010~-8080 using and of wh:ich of such channels have enyaged descending paths;
- detection by the mobile station (M) of the signal power level ln the s:ignalling path tlme interval channels received from the respective flxed stations, and identification as interfering stations those for which such received signal power level exceeds a predetermined interference threshold but is below the maximum received signal power level; and - determination by the mobile station (M) of the addresses of duplex channels of fixed station (F) having engaged descending paths by eliminating from the addresses of all duplex channels having engaged descending paths the addresses of duplex channels of interfering fixed stations.
- 3b f ~
,: :` ", , P~IF 85555 4 17.~.19~6 For some fixed stations the information from the duplex channels used which is transmitted vic~ the semaphore pa-th may be received in the mobile stations at a level on the threshold of interference, and this may give rise to errors in restoring the information.
To avoid thi.s, it is best for the interference threshold to be set at the threshold of good reception and for the fixed stations to transmit alon~ the semaphore path channels at a level which is higher than those in the co~munication channels by an amount substantially equal to the difference between the threshold of good reception and said second threshold.
The description below, given with reference to the attached drawings, all given by way of example, will show how the invention may be implemented.
1S Figure 1 shows the relative positions of the fixed and mobile stations in the "interference" areas.
~ igures 2 and 3 are time-frequency graphs illustrating the structure of the semaphore path used in the method of the invention and that of the descending communication path. Figure 2 relates to a mixed time and frequency-distribution multiple-access system. Figure 3 relates to a mixed time, code and frequency-distribution system.
Figure 4 is a diagram of the device for implementing the method of the invention in a mobile station.
Figure 5 is a diagram of a devic~ for implementing the method of the invention in a fixed station.
In order to explain the method of the invention, Figure 1 shows diagrammatically the relative positions of various fixed and mobile stations in a cellular mobile radio system. In these known systems, the area to be covered is divided into generally hexagonal cells, like cell H, each having a fixed station, like F, at its centre.
Each fixed station can transmit towards the mobile stations along communication channels forming a descending path. Let it be assumed that bi-directional communications are to be established between fixed station F, essentially covering an area defined by cell H, and a mobile station M. Mobile station M can transmit towards fixed stations in communication channels forming a rising path. The fixed stations may be fitted out to transmit and receive along all the systems s communication .
~, ,,, ~
channels. The aim of the method of the invention is to seek a pair oE free rising and descending channels in order to establish bi-directional comrnunications between a Eixed station F and a mobile station M. As for the method described in the Canadian Patent 1~235l187, each channel in the rising path is associated wi-th a given channel in -the descending path to form a duplex communication channel which must be regarded as engaged if the rising or descending path channel is engaged. The concept of interference areas, to be specified with reference to Figure 1, will also be used.
It is possible to define around a fixed station like F
an interference area CF within which any mobile station like MR
could interfere with communications between F and mobile station M
if the reception power in a channel in the rising path is greater than a first predetermined threshold.
It is also possib]e to define around a mobile station like M an interference area CM within which any fixed station like Fj could interfere with communications between M and fixed station F if the reception power in a channel in the descending path is greater than a second predetermined threshold.
In the free channel seeking method described in Canadian Patent l,235,187 for the establishment of communications between stations F and M, fixed station F detect the channels in the rising path engaged by communications with mobile station Mk located in interference area CF. To do so, fixed station F
measures the power levels received in all the channels of the rising communication path and compares the measured levels with a ~: , ."
- ;. ...
::
'~26~
2010~-8080 those for which the measured level is higher than the threshold.
rrhe addresses of the engaged duplex channels in the rising path are then transmitted to the mobile stations ia a semaphore path formed of equidistant time intervals successively and periodically allocated to all the -Eixed stations in a pattern of cells from which the cellular network is constituted. Alsol according to the known method, mobile station M determines the descending path channels which are engaged by communications with fixed sta-tions Fj located in its interference area CM by a direct method similar to that used in the Eixed stations, i.e. by measuring the power level received in all the channels of the descending communications path and comparing the 5a ,, .
,. , :
:
' '~ : ~:
- ' , ~
~;~6~ D3 PHF 85555 6 17.06.1986 measured levels with a second predetermined threshold, where the engaged channels are those for which the level measures is higher than this threshold. From the addresses of the engaged duplex channels in the rising path transmitted via the semaphore path and those of the engaged S descending channels (or engaged duplex channels in the descending path) detected in mobile sta-tion M, the latter may easily select a free duplex communications channel with a different address from those of all the engaged d~plex channels.
As has already been pointed out, however, the detection in mobile station ~ of the engaged duplex channels in the descending path by the direct method described above makes the known method hard to use in all systems with frequency-distribution multiple-access systems, systems of the purely FDMA type or mixed systems.
According to the present invention, the engaged duplex channels in the descending path are determined in a different manner, totally avoiding the difficulties of the known method. The determination is based on the observation that, in mobile station M, the level of the signal received is the same for all the descending channels used by a fixed station, communications and semaphore channels.
Therefore, to identify the duplex communications channels in the descending path engaged by communications with fixed stations F j located in interference area CM, it is possible, in this invention, to use the following two items of information in mobile station M:
- an initial item of information consists of a list of the interfering neighbouring stations Fj. This list may easily be made in mobile station M by measuring the levels and comparing them with a threshold in the various channels of the semaphore path which carries signals sequentially arising from all the fixed stations in a cell pattern.
- a second item of information is a list of duplex commun.ication channels used by all stations Fj, including station F with which communication are to be established. This obviously known list by each fixed station may be continuously issued towards the mobile stations via the semaphore path.
It is then easy to determine the engaged duplex channels in the descending path by taking from among the duplex communication channels used as supplied by the second item of information those arising from the interfering fixed stations provided by the first item ~,, . . . . .
'' : ' , ""', "' :
.;
.
~2~
oE informationO
It may be noted that the first item of information requires virtually no additional equipment in a mobile station.
The level in the semaphore path channels may already be measured to determine, as is necessary, the allocated fixed station which is the one received a-t the highest level and likely to change with the movement of the vehicle conveying the mobile station~
Moreover, to avoid any error in the reception of the second item oE information (the list of channels used) in a mobile station, which can receive the semaphore path channels at a level on the interference threshold, it is best to increase, by a fixed and known number of decibels, the level transmitted in the semaphore path channels so as to take the interference threshold to the threshold of good reception. The difference between these two levels is of the order of 15 to 20 dB. Thus the fixed stations transmit at two power levels, one used for the corlimunication channels and the other, higher, one for the semaphore path channels.
Finally, with the method of this invention, each fixed station transmi-t on the semaphore pa-th:
- for all the mobile stations, a list of the duplex communication channels which it uses (-to determine the engaged channels in the descending path, as has just been explained);
- for the mobile stations allocated to it~ a list of the engaged duplex channels in the rising path (as in the method of the cited Patent Application).
Clearly, these two items of information transmitted by :~ "'` `= . .
., ~ .
.
- ~ : - : ' :
', : ~` ':
each fixed stat;.on may be combined in order to use a minimum `
number of binary elements~
A mobile station thus selects a free duplex channel except Eor:
- the engaged duplex channels in the rising path, which interEere wlth its allocated stati.on and are in the 11st provided to it by the latter via the semaphore path;
- the engaged duplex channels ln the descending path which are determined by the me-thod of this invention.
~his method may be used in the same circumstances as those described in Canadian Patent 1,235,187:
: 7a ' ;' : . .
.
: -:, , :
.
.
:
`: :
.
.
PHF 85555 3 17.06.1986 - when mobile station M wishes to call a subscriber linked to the system or another mobile station, by measuring the levels in the semaphore path, it selects the fixed station giving the best reception, which becomes its allocated station. When, mobile station M selects a free duplex channel for its link with M.
- when a fixed station F wishes to call one of the mobile stations ~ in its radius of action, it broadcasts the number of the mobile station called ~-a the semaphore path and the latter station selects a free duplex channel using the method of the invention.
- when, during communications between fixed station F and mobile station M, the latter is afflicted by interference fro~ another fixed station Fj, station M seeks a fresh free duplex channel by the methoa of the invention and changes channels.
- when, during a communication between mobile station W and fixed station F, the latter is afflicted by interference from another mobile station Mk, fixed station F informs mobile station M thereof by once again broadcasting its number vla the semaphore path and mobile station M then seeks a free channel and changes channels.
- finally, when, during a communication between fixed station F and mobile station M, the latter detects a fixed station F which is better received than station F, mobile station M performs the same free-channel-seeking and channel-changing operations with, in addition, the transmission to the fixed stations F and F concerned the service information relating to the change of station.
The invention relates to a method of seeking a free channel in a mobile radio system with a cellular network in which each channel of the rising communications path is associated with a given channel in the descending communications path to form a duplex channel providing communication between any one of the fixed stations and any one o~ the mobile stations, in accordance with which, in order to seek a free duplex channel between a fi~ed station F and a mobile sta-tion M:
- in fixed station F, the engaged duplex channels in the rising path for which the received power is higher than a first given threshold are detected and the addresses of said engaged channels are transmitted by one of the channels of a semaphore path formed oE equidistant time intervals successively allocated periodically to all the fixed stations of a set of cells constituting the cellular system;
- in mobile station M, the addresses must be determined of the engaged duplex channels in the descending path for which the received power is higher than a second given threshold, while a free duplex channel is selected Erom among those with a different address from those of the engaged channels to be determined in mobile station M.
The invention also relates to the device for implementing this method in the fixed and mobile stations.
Such a free channel seeking method for a cellular mobile : radio system was described in Canadian Patent 1,235,187 in the applicant's name. This method had the advantage of making it : 1 , .. ~ , ..
possible to allocate all the communication channels to each cell in the system, whereas, in conventlonal systems, each set of channels is distributed in a group of "motif" cells, with the "motif" repeated throughout the communications system.
However, the implementation of this known method may give rise to difficulties in certain types of mobile radio systems with respect to the determination in a mobile station oE the engaged channels in the descending communications path.
Such implementation is convenient only in systems of the la /~
:'. : . ,.
, -. . :.
`
PHF 85555 2 17.0~.1986 time-share multiple-access type (abbreviated to TSMA) in which the communication channels forming the rising and descending paths are time-multiplexed time channels. In a TSMA system, a mobile station has a wide-band receiver sequentially receiving all the channels of the descending path so that, in it, it is easy to detect those channels which are engaged simply by measuring the level of the signal received in the ti~e intervals corresponding to the various channels an~ by comparing these levels with a predetermined threshold.
In mobile radio systems of the frequency-distribution multiple-access type (abbreviated to FDMA), the communication channels are requency-multiplexed frequency channels and the receiver of a mobile station can receive only one channel at a ti~e so that, in order to measure the level in all the channels of the descending path, the receiver s frequency must vary in steps in order successively to explore all the channels. In a 255-channel system, for instance, to be explored in 125 ms (the frame period of a comparable TSMA system), each frequency step must last about 0.5 ms. To take account of the fading phenomenon caused by multiple tracks, an average of the measurement of the levels in the channels over two or three exploration processes must be struck.
The relatively long time (2 to 3 times 125 ms) needed to seek a free channel by this method raises to major drawbacks during the stand-by period, but is highly annoying if the channel has to be changed during a communication, if, for example, the channel in use becomes cluttered.
Communication must, in fact, be suspended while a free channel is being sought. To avoid this drawback, it is possible to use an auxiliary receiver continuously exploring the channels to seek a second free one in anticipation of the ti~e when the main receiver has to change channels, but this is a complex solution.
Finally, it should be noted that there is a growing ~endency in practice to use mixed systems combining frequency and time-distributed multiple-access systems making it easier than with purely TSMA of FDMA to fit the ~ixed stations with a variable number of channels depending on the area (urban or rural) or on the time to take account of the gradual increase in the number of users.
It is thus possible to produce a mixed TSMA/FDMA variable capacity system by frequency-multiplexing a variable number of TSMA
frames. A distinction may be made between TSMA wide-band/FDMA systems ,: :
, ., .
~6~
2010~-8080 in which each TSMA frame occupies a greater band-width than the coherence band of a channel, oE the order of 200 KHz, and narrow-band TSMA/FDMA systems in which each TSMA frame occupies a coherence band of a channel. In both cases the fixed stations may be fitted with a variable number of channels depending on the location and the time, with an increment equal to the number of communication channels contained in a TSMA frame. Finally, it is also possible to set up a variable-capacity system with a variant of a mixed narrow-band TSMA/FDMA system in which frequency jumps are made at the rate o the TSMA frames and co-ordinated among the users so as to avoid mutual interference. In such a system, a communications channel uses a TSMA frame time lnterval with a carrier frequency according to a code determined by the law of the frequency jumps used. In what follows, this mixed system is referred to as the TSMA/CDMA/FDMA system, where CDMA signifies -"code-distribution multiple-access".
Clearly, if the free channel seeking method described in Canadian Patent 1,235,187 is to be applied to the mixed systems referred to above, all of which comprise frequency-distribution multiple-access, the same difficulties arise as with systems of the pure FDMA systems.
It is the intention of this invention to provide a method which avoids these diEficulties by making it possible to select a free channel and to change channels in any mobile radio system, especially the three mixed systems described, without any interruption during communications~
In accordance with the invention, there is provided a method of enabling a mobile station (M) to select a free duplex , :: :
L~
20104-~0~0 channel for communicating with a ~ixed station (F) in a cellular mobile radio system having a plurality of duplex channels each of which has a rising communication path and an associated descending communication path, respective duplex channels having respective addresses and providing communication between respective ones of a plurality of fixed stations and a plurality of mobile stations, the fixed stations being located in a pattern of cells constituting the cellular system; such method comprising:
- each :Eixed station identifying the addresses of duplex channels having rising paths which are engaged by detecting whether the signal power level in each received rising path exceeds a first given threshold, and transmitting the addresses of such channels to the mobile stations over a signalling path comprising equidistant time interval channels which are successively and periodically allotted to the respective fixed stations in the cellular system;
- determination by each mobile station of -the addresses of duplex channels having descending paths which are engaged by detecting whether the signal power level in each received descending path exceeds a second given threshold; and - selection by the mobile station (M) of a free duplex channel from among duplex channels having addresses differing from those of channels which are engaged in the rising paths and those which are engaged in the descending paths;
such method being characterized by:
- continuous transmission by each of -the fixed stations, in the signalling path time interval channels respectively allotted to them, of the addresses of duplex channels which they are : 3a -~ . ~ ., 3~
2010~-8080 using and of wh:ich of such channels have enyaged descending paths;
- detection by the mobile station (M) of the signal power level ln the s:ignalling path tlme interval channels received from the respective flxed stations, and identification as interfering stations those for which such received signal power level exceeds a predetermined interference threshold but is below the maximum received signal power level; and - determination by the mobile station (M) of the addresses of duplex channels of fixed station (F) having engaged descending paths by eliminating from the addresses of all duplex channels having engaged descending paths the addresses of duplex channels of interfering fixed stations.
- 3b f ~
,: :` ", , P~IF 85555 4 17.~.19~6 For some fixed stations the information from the duplex channels used which is transmitted vic~ the semaphore pa-th may be received in the mobile stations at a level on the threshold of interference, and this may give rise to errors in restoring the information.
To avoid thi.s, it is best for the interference threshold to be set at the threshold of good reception and for the fixed stations to transmit alon~ the semaphore path channels at a level which is higher than those in the co~munication channels by an amount substantially equal to the difference between the threshold of good reception and said second threshold.
The description below, given with reference to the attached drawings, all given by way of example, will show how the invention may be implemented.
1S Figure 1 shows the relative positions of the fixed and mobile stations in the "interference" areas.
~ igures 2 and 3 are time-frequency graphs illustrating the structure of the semaphore path used in the method of the invention and that of the descending communication path. Figure 2 relates to a mixed time and frequency-distribution multiple-access system. Figure 3 relates to a mixed time, code and frequency-distribution system.
Figure 4 is a diagram of the device for implementing the method of the invention in a mobile station.
Figure 5 is a diagram of a devic~ for implementing the method of the invention in a fixed station.
In order to explain the method of the invention, Figure 1 shows diagrammatically the relative positions of various fixed and mobile stations in a cellular mobile radio system. In these known systems, the area to be covered is divided into generally hexagonal cells, like cell H, each having a fixed station, like F, at its centre.
Each fixed station can transmit towards the mobile stations along communication channels forming a descending path. Let it be assumed that bi-directional communications are to be established between fixed station F, essentially covering an area defined by cell H, and a mobile station M. Mobile station M can transmit towards fixed stations in communication channels forming a rising path. The fixed stations may be fitted out to transmit and receive along all the systems s communication .
~, ,,, ~
channels. The aim of the method of the invention is to seek a pair oE free rising and descending channels in order to establish bi-directional comrnunications between a Eixed station F and a mobile station M. As for the method described in the Canadian Patent 1~235l187, each channel in the rising path is associated wi-th a given channel in -the descending path to form a duplex communication channel which must be regarded as engaged if the rising or descending path channel is engaged. The concept of interference areas, to be specified with reference to Figure 1, will also be used.
It is possible to define around a fixed station like F
an interference area CF within which any mobile station like MR
could interfere with communications between F and mobile station M
if the reception power in a channel in the rising path is greater than a first predetermined threshold.
It is also possib]e to define around a mobile station like M an interference area CM within which any fixed station like Fj could interfere with communications between M and fixed station F if the reception power in a channel in the descending path is greater than a second predetermined threshold.
In the free channel seeking method described in Canadian Patent l,235,187 for the establishment of communications between stations F and M, fixed station F detect the channels in the rising path engaged by communications with mobile station Mk located in interference area CF. To do so, fixed station F
measures the power levels received in all the channels of the rising communication path and compares the measured levels with a ~: , ."
- ;. ...
::
'~26~
2010~-8080 those for which the measured level is higher than the threshold.
rrhe addresses of the engaged duplex channels in the rising path are then transmitted to the mobile stations ia a semaphore path formed of equidistant time intervals successively and periodically allocated to all the -Eixed stations in a pattern of cells from which the cellular network is constituted. Alsol according to the known method, mobile station M determines the descending path channels which are engaged by communications with fixed sta-tions Fj located in its interference area CM by a direct method similar to that used in the Eixed stations, i.e. by measuring the power level received in all the channels of the descending communications path and comparing the 5a ,, .
,. , :
:
' '~ : ~:
- ' , ~
~;~6~ D3 PHF 85555 6 17.06.1986 measured levels with a second predetermined threshold, where the engaged channels are those for which the level measures is higher than this threshold. From the addresses of the engaged duplex channels in the rising path transmitted via the semaphore path and those of the engaged S descending channels (or engaged duplex channels in the descending path) detected in mobile sta-tion M, the latter may easily select a free duplex communications channel with a different address from those of all the engaged d~plex channels.
As has already been pointed out, however, the detection in mobile station ~ of the engaged duplex channels in the descending path by the direct method described above makes the known method hard to use in all systems with frequency-distribution multiple-access systems, systems of the purely FDMA type or mixed systems.
According to the present invention, the engaged duplex channels in the descending path are determined in a different manner, totally avoiding the difficulties of the known method. The determination is based on the observation that, in mobile station M, the level of the signal received is the same for all the descending channels used by a fixed station, communications and semaphore channels.
Therefore, to identify the duplex communications channels in the descending path engaged by communications with fixed stations F j located in interference area CM, it is possible, in this invention, to use the following two items of information in mobile station M:
- an initial item of information consists of a list of the interfering neighbouring stations Fj. This list may easily be made in mobile station M by measuring the levels and comparing them with a threshold in the various channels of the semaphore path which carries signals sequentially arising from all the fixed stations in a cell pattern.
- a second item of information is a list of duplex commun.ication channels used by all stations Fj, including station F with which communication are to be established. This obviously known list by each fixed station may be continuously issued towards the mobile stations via the semaphore path.
It is then easy to determine the engaged duplex channels in the descending path by taking from among the duplex communication channels used as supplied by the second item of information those arising from the interfering fixed stations provided by the first item ~,, . . . . .
'' : ' , ""', "' :
.;
.
~2~
oE informationO
It may be noted that the first item of information requires virtually no additional equipment in a mobile station.
The level in the semaphore path channels may already be measured to determine, as is necessary, the allocated fixed station which is the one received a-t the highest level and likely to change with the movement of the vehicle conveying the mobile station~
Moreover, to avoid any error in the reception of the second item oE information (the list of channels used) in a mobile station, which can receive the semaphore path channels at a level on the interference threshold, it is best to increase, by a fixed and known number of decibels, the level transmitted in the semaphore path channels so as to take the interference threshold to the threshold of good reception. The difference between these two levels is of the order of 15 to 20 dB. Thus the fixed stations transmit at two power levels, one used for the corlimunication channels and the other, higher, one for the semaphore path channels.
Finally, with the method of this invention, each fixed station transmi-t on the semaphore pa-th:
- for all the mobile stations, a list of the duplex communication channels which it uses (-to determine the engaged channels in the descending path, as has just been explained);
- for the mobile stations allocated to it~ a list of the engaged duplex channels in the rising path (as in the method of the cited Patent Application).
Clearly, these two items of information transmitted by :~ "'` `= . .
., ~ .
.
- ~ : - : ' :
', : ~` ':
each fixed stat;.on may be combined in order to use a minimum `
number of binary elements~
A mobile station thus selects a free duplex channel except Eor:
- the engaged duplex channels in the rising path, which interEere wlth its allocated stati.on and are in the 11st provided to it by the latter via the semaphore path;
- the engaged duplex channels ln the descending path which are determined by the me-thod of this invention.
~his method may be used in the same circumstances as those described in Canadian Patent 1,235,187:
: 7a ' ;' : . .
.
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.
.
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.
.
PHF 85555 3 17.06.1986 - when mobile station M wishes to call a subscriber linked to the system or another mobile station, by measuring the levels in the semaphore path, it selects the fixed station giving the best reception, which becomes its allocated station. When, mobile station M selects a free duplex channel for its link with M.
- when a fixed station F wishes to call one of the mobile stations ~ in its radius of action, it broadcasts the number of the mobile station called ~-a the semaphore path and the latter station selects a free duplex channel using the method of the invention.
- when, during communications between fixed station F and mobile station M, the latter is afflicted by interference fro~ another fixed station Fj, station M seeks a fresh free duplex channel by the methoa of the invention and changes channels.
- when, during a communication between mobile station W and fixed station F, the latter is afflicted by interference from another mobile station Mk, fixed station F informs mobile station M thereof by once again broadcasting its number vla the semaphore path and mobile station M then seeks a free channel and changes channels.
- finally, when, during a communication between fixed station F and mobile station M, the latter detects a fixed station F which is better received than station F, mobile station M performs the same free-channel-seeking and channel-changing operations with, in addition, the transmission to the fixed stations F and F concerned the service information relating to the change of station.
2~ The following description is of a structure for a semaphore path making it possible to seek a free channel by the method of the invention without interrupting communications between a fixed and a mobile station, whatever the mobile radio system.
Here, all that is needed is for mobile station W to receive information from the semaphore path regardless of the descending path channel used in communication with fixed station F. If this condition is provided, stand-by in a mobile station can, of course, concern any channel in the descending path, which has the additional advantage that the method is insensitive to interference in a particular channel during a call.
The semaphore path consists of equidistant time intervals allocated successively and periodically to all the fixed stations in a :, : . . :
:~ .
,., : , ., .
' . ~
o~;~
PHF 85555 9 17.06.1986 pattern of cells, said time intervals occurring simultaneously for all carrier frequencies usable by a fixed station. Of course all the time intervals of the semaphore path must be synchronised for all the fixed stations in a mobile radio system and this syn~hronisation must be recovered in the mobile stations.
In a purely TSMA system corresponding to the special case of a single carrier frequency, the semaphore pat~ described in Patent Application 2 556 532 is founa~ In a purely TSMA system, there will be as many simultaneous ti~e intervals for the se~aphore path as there are communication channels. As has already been pointed out, however, this system does not in practice permit different and pro~ressive fitting out of the fixed stations.
It will now be shown with reference to the diagrams of Figures 2 and 3 how it i.s possible to organise the semaphore path in mixed systems making it possible to fit fixed stations out more easily in accordance with requirements.
On a time t-frequency f graph, Fi.gure 2 shows the structure of the descending communication and semaphore paths for a wide-band TSMA/FDMA system. A fixed station may be fitted with a numbex Nf of carrier frequencies upto a maximum of Nfm, each used to transmit TSMA
frames of period T1. A TSMA frame comprises x time channels and occupies a band of width QF. The bands ~F are contiguous or all the carrier frequencies.
In the example shown, the maximum number of carrier frequences is Nfm = 12 and the band width ~F is 2 MHz, so that the maximum band occupied by the descending communication path is 24 MHz. A
T5MA frame comprises n = 60 time channels 0.5 ms wide, so that a TSMA
frame has a period T1 = 0.5 ms x 60 = 30 ms.
The semaphore path is made up with the aid of time intervals marked S with an index showing the number of time intervals in a semaphore frame period T2. All the time intervals S occur at the same moment for all the carrier frequencies F1 to F12 and are separated by time D which is produced by a whole number N of the TSWA frame period T1. In the example chosen, N = 4. In a semaphore frame period T2 there is a number of time intervals S equal to the size Ch of the selected pattern (number of cells in the pattern) so that all the fixed stations in the pattern can transmit successively during a period T2. In the -, ,, ~ .
' , . .
, -, vo~ ~
PHF 85555 10 17.06.19a6 example selected for Figure 2, Ch = 25 and the time intervals S shown are S1, 52, S3, ..., 525. If the time interval S of the semaphore channel is of the same duration 0.5 ms as that of a communication channel, the period of the semaphore frame is T2 = 0.5 ms x Ch (N.n + 1) = 0.5 ms x 25 (240 ~ 1) = 3.0125 s.
The fixed stations transmit by circular permutation in both time and frequency. Thus, as shown in the example of Figure 2 by numbers in the boxe3 corresponding to the time intervals S and the carrier frequencies, for carrier frequency F1, the successive time intervals S1, S2, S3, ..., S25 serve respectively for the traDsmission from fixed stations 1, 2, 3, ..., 25; for carrier frequency F2, timP
intervals S1, S2, S3, ..., S25 serve for the transmission from fixed stations 2, 3, 4, ..., 1; and so on until carrier frequency Fl2, for which the time intervals S1, S2, S3, ..., SZ5 serve for the transmission from fixed stations 12, 31, 14, ..., 11. At any time interval S1 to S25 in the semaphore path, all the fixed stations in a pattern transmit in this semaphore path. It is of course possible to use another example of circular permutation, provided that all the fixed stations transmit in the Ch timP intervals S of a semaphore frame period. The condition for this is that the difference between the numbers of the fixed stations transmitting in succession is a first number with the size Ch of the pattern. In the example of Ch = 25, the number 4 may be chosen for this difference. For carrier frequency F1, there would then be in succession for the intervals Sl to 525 the transmissions from fixed stations 1, 5, 25 9, 13, 17, 21, 25, 4, a, 12, 16, 20, 24, 3, 7, 11, 15, 19, 23, 2, 6, 10, 14, 18, 22. For carrier frequency F2 there would be successive . .
transmissions from fixed stations 5 to 1 in the above order. Finally, for carrier frequency F12, there would be in succession transmissions from fixed stations 20 to 16 in the above order.
With the semaphQre path structure just described, it is clear that on any communication channel (with any carrier frequency) where there is a mobile station on stand-by or in communication with a fixed station, this mobile station receives vla the semaphore path all the information from the fixed stations in pattern enabling it to seek a free channel. Moreover, the fixed stations may be fitted with a variable number Nf of carrier frequencies upto a maximum of Nfm depending on the time and space requirements. In the example of Figure :
.: :
' ..
.
': , . . ' PHF 85555 11 17.06.198S
2, Nfm = 12. Whatever the carrier frequencies used, a mobile station will always receive,via the semaphore path, the information needed to seek a free channel..
A TSMA narrow-band/TSMA system needs no special description. In relation to the system of Figure 2, there is simply a narrower band ~F,~g~ 200 kHz, for the TSMA channel frame, said frame comprising a smaller number of time channels,e.q. 6. The maximum number of carrier frequencies may be greater to give the same total number of communication channels. The structure of the semaphore channel may be the same as in Figure 2 with the same distance D between the intervals S of the semaphore path and the same period T2 of the semaphore path.
The time-frequency graph of Figure 3 will rather be used -to describe a TSMA narrow-band/CDMA/FDMA system. Here, each communication channel using a time interval of a TSMA frame is not supported by a fixed carrier frequency but by one which varies in jumps according to a given law.
In a system like that of Figure 3, each fixed station must be fitted with a number Nf of multiple carrier frequencies of the size Ch of the selected pattern : ~f = KCh, where K is a whole number determining the number of groups in which the carrier frequencies are distributed. In the example of Figure 3, these are distributed over 5 groups G1 to G5 each comprising 25 carrier frequencies, thus making the maximum number Nfm of carrier frequencies 125. The band width used around each carrier frequency to transmit the TSMA frame is Qf = 0.2 MHz. These bands ~f are contiguous for all the carrier frequencies so that the maximum band occupied by the descending communication channel is 0.2 MHz x 25 x 5 = 25 MHz.
The system operates in a similar way for each of the groups of carrier frequencies G1 to ~5 and the description will relate more particularly to the structure of the communication channels and the semaphore path for group G1.
A TSMA frame comprises 6 time intervals of duration 5 ms so that a TSMA frame has a period T1 = 5 x 6 = 30 ms. Each communication channel is made up of a time interval of the TSMA frame supported by a carrier frequency which varies in jumps according to a given law, with the time interval between two successive jumps being the - .
PHF 85555 12 17.06.1986 period T1 of the TSMA frame. On the diagram of Figure 3 the hatched boxes represent the time-frequency space occupied by a communication channel using a time interval and a frequency-jump law. In the example shown, the carrier frequncies used by the channel during a period of the frequency-jump law increase in the order from f1 to f25. Another order may of course be used, provided that the 25 carrier frequencies are used in this period. ~or group G1 of 25 carrier frequencies, 25 frequency-jump laws are possible, these laws being obtained by multiple ~ime shifts of period T1 of the TSMA frame. Thus, the use in a fixed station of a group of carrier frequencies like G provides G x 25 = 150 communication chamlels. By juxtaposing 5 groups of carrier frequencies acting like G1 it i5 possible to o~tain 5 x 150 = 750 communication channels.
The semaphore path is made up in a similar way to that shown in Fi~ure 2 with the aid of equidistant time intervals S, numbering Ch = 25 in a semaphore frame period T2. These time intervals marked S1 to 525 in a period T2 are produced at the same moments for all the carriex frequencies of the 5 frequency groups G1 to G5. These time intervals S of the semaphore path are separated by a duration D which is the product of a whole number N of the TSMA frame period T1. The Ch fixed stations in a cell pattern may successively transmit in the Ch time intervals S of the semaphore frame if N is a first number with Ch, which is produced in the example of Figure 3, with Ch = 25 and N = 9.
If the time interval S of the semaphore channel is of the same duration 5 ms as that of a communication channel, the duration of the semaphore frame period is T2 = 5 ms x Ch (Nn + 1) = 5 ms x 25 ~24 + 1) = 3.125 s.
The fixed stations transmit on the semaphore path in circular permutation in both time and frequency. In the example of Figure 3, where the carrier ~requencies in group G1 are used in the frequency-jump laws in the order from f1 to f25, it will be seen that, for carrier frequency f1, the time intervals S1 to 525 serYe for transmission from fixed stations 1 to 25, for carrier frequency f2, the time intervals S1 to S25 serve for transmission from fixed stations 2 to 1 and so on upto carrier frequency f25, for which time intervals S1 to 525 serve for txansmission from fixed stations 25 to 24.
With this semaphore path structure, if the fixed stations ~' ; .
.
o~
PHF 85555 13 17.06.1986 are fitted with carLier frequency group 51, a mobile station on stand~by or in communic~tion with a fixed station on any communication channel using one of the 25 possible frequency-jump laws must receive via the semaphore path the information from all the fixed stations in a pattern, enabling it to seek a free channel to establish communication or change channel.
If the fixed stat.ions are fitted with several carrier frequency groups G1 to G5, each group of Ch carrier frequencies tCh = 25) may use independent frequency-jump laws similar to those described for group G1. Likewisè, the structure of the semaphore path may be s.imilar for each yroup to that described for group G1. Thus, any mobile station on stand-by or in communication on a given channel of one of groups G1 to G5 must receive,vla the semaphore channel, all the information it needs to seek a free channel.
The semaphore messages transmitted by the fixed stations in all the carrier frequencies which they use ~Nfm carrier frequencies at the most) have an identical content, viz :
- the address of the fixed station transmitting the message;
- the service information concerning the calls,l.e. the number of the mobile subscriber~s) sought and the nature of the call ~call, return of call, acknowledgment of receipt, end of communication, order to change channels, etc.);
- for free channel seeXing according to the invention :
* list L1 of the duplex channels used by the fixed 2~ station;
* list L2 of the duplex channels recognised as engaged by the fixed station, using level measurements in the rising path channels;
- in addition, a semaphore message states :
* in a TSMA wide-band/FDMA system, the caxrier frequency~ies) Nf fitted to the fixed station, and preferably a single carrier frequency selected by the fixed station and for use by the mobile station in selecting a communication channel;
* in a TSMA narrow-band/C3MAIFDMA system, the carrier frequency group~s) fitted to the fixed station ~i.e.
the value~s) of K in Nf = RCh), and preferably a single ~ ':
PHF 85555 14 17.06.1986 g~oup selected by the fixed station for use by the mobile station in selecting a communication channel.
By transmitting these last two items of information, a fixed station can thus at any time guide the choice of the free channel by the mobile stations to the carrier freguency with the lightest engaged channel load or to the least-loaded group of carrier frequencies. When, for instance, a carrier frequency (or group thereof) is fitted to a fixed station, but not its neighbours, this fixed station will guide the choice of the mobile stations to this frequency or group of frequencies. Moreover, if several requests for calls are transmitted in a message by the fixed station, it shows in the corresponding semaphore messages a carrier frequency or ~roup thereof which differs for each mobile station. It is thus possible to avoid the risk that several mobile stations called will select the same channel at the same time.
In addition, by transmitting the two latter items of information, the fixed station can limit the channels in lists L1 and L2 to those forming part of the carrier frequency or group thereof selected, thus reducing the number of bits in the semaphore message.
The method of the invention may be implemented in a mobile station as shown in the diagram of Figure 4. This diagram shows only the main components involved in the method.
According to Figure 4, a mobile station comprises a radio receiver 1 connected to a receiving aerial 2. Output 3 of the receiver supplies the demodulated output signal which is applied to a decoder 4.
The latter, at its output 5, provides the decoded data or voice signal transmitted to the receiver in the selected communication channel.
Output 6 of the decoder provides the information received in the semaphore path concerning the search for the free channel. Finally, assuming that synchro-bit and synchro-frame devices are incorporated into the decoder, output 7 of the decoder supplies the synchro-frame signal of the semaphore path (frame with Ch successive time intervals determining the Ch channels of the semaphore path). Radio receiver 1 also has an auxiliary low-frequency output 8 in front of the limiter and thus making it possible to measure the reception level. The signal at this output 8 is applied to a level-measuring device 9 which also receives the synchro-frame signal of the semaphore path and thus ,~:
~ ~- . '; ' PI~F 85555 15 17.06.1986 provides the level measured in the Ch channels thereof. The measured levels leaving device 9 are guided in device 10 to Ch low-pass filters, each providing a mean of the level measured over several semaphore frame periods in order to avoid any irregularities in the level measurements.
The level in the Ch channels of the semaphore path ~rom the Ch fixed station of a cell pattern is thus obtained at the outputs of device 10.
The levels supplied by device 10 are compared together in comparator circuit 11 at the c~aractexistic moments S of the synchro-frame signal.
Comparator circuit 11 determines the highest level and the position of the corresponding channel in the semaphore frame, which gives the address of the allocated fixed station, which is stored in memory 12.
This address is supplied to radio transmitter 13 for broadcasting via transmission aerial 14. Transmitter 14, of course, receives other signals to be broadcast tprocedure messages, voice or data si~nals) but these have not been shown for the sake of simplicity.
In order to implement the method of the invention, the Ch measured levels supplied by device 10 are applied to Ch threshold circuits like 15. The threshold shared by these circuits is the "interference" threshold. If the level received in a channel exceeds this interference threshold, the fixed station which has transmitted on this channel is regarded as interfering. List L of the addresses of the fixed interfering stations which have shown a Figure beyond the interference thre.shold are stored in Ch-bit memory 16.
Moreover, output 6 of decoder 4 provides the information received in the various channels of the semaphore path,l.e. for each semaphore path, list L1 of the communication channels used by the fixed station transmitting into the semaphore channel, list L2 of the channels recognised as engaged by this fixed station, finally information I
showing the carrier fxequency(ies) (or groups thereof) to be used to see~ a free channel. This information received in the semaphore path are stored in memory 17.
List L of the interfering fixed stations stored in memory 16 and lists L1, L2 and information I stored in memory 17 are read out of these memories on a call of circumstances requiring a change of channel. From L, L1, L2 and I, a free duplex channel may easily be selected using a method of the invention described above. In short, a selected free duplex channel :
.. . . . . . .
` '', "' .
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. .
~'HF 85555 16 17.06.1986 - must be one of the channels determined by information I;
- must not ~e found in list L2 of the engaged channels in the rising path;
- must not be found in list L1 of the channels used and originating from interfering fixed channels in list L.
The means used to imple~ent the method of the invention in a fixed station are shown in Figure 5. If the fixed station is fitted out for Nf carrier frequencies, it comprises Nf fixed-frequency radio receivers like 20 and 21, fitted with receiver aerials and having, like receiver 1 of the mobile station of Figure 4, an auxiliary low-frequency output in front of the receiver limiter. The signals at these low-frequency outputs 22, 23 are applied to level measuring devices 24, 25 which also receive, from a generator 26 the synchro-frame signal of the TSMA communication channels (frame with n successive time intervals). Each measuring device 24, 25 thus supplies the measured level of the n channels of the rising communication path. The measured levels supplied by 24, 25 are guided in each of devices 27, 28 to n low-pass filters. This, at the output of all the devices like 27, 28 are obtained the levels measured in the nNf channels of the rising communication path. These measured levels are applied to nNf regulated threshold circuits like 29, 30 and 31, 32. All these circuits are set to the same threshold ~referred to above as the first) and the channels where this threshold is exceeded are recognised as engaged. The information on these excesses of the thresholds is applied to nNf-bit memory 33 and, when this is read out, it provides list L2 of the channels recognised as engaged by the fixed station. Moreover, list L
of the channels used by the fixed station is supplied to memory 34.
Finally, information I of the carrier frequency~ies) (or groups thereof) to be used in seeking a ~ree channel is supplied to memory 35.
The message to be transmitted in the semaphore path and comprising lists L1 and L2 and information ~ is formed in 36 for application to ~ran~mitter 37, in turn for broadcasting via aerial 38.
: :~
~: :
., :. ... .
-.
. .
. . ' .
. ~ . -:
Here, all that is needed is for mobile station W to receive information from the semaphore path regardless of the descending path channel used in communication with fixed station F. If this condition is provided, stand-by in a mobile station can, of course, concern any channel in the descending path, which has the additional advantage that the method is insensitive to interference in a particular channel during a call.
The semaphore path consists of equidistant time intervals allocated successively and periodically to all the fixed stations in a :, : . . :
:~ .
,., : , ., .
' . ~
o~;~
PHF 85555 9 17.06.1986 pattern of cells, said time intervals occurring simultaneously for all carrier frequencies usable by a fixed station. Of course all the time intervals of the semaphore path must be synchronised for all the fixed stations in a mobile radio system and this syn~hronisation must be recovered in the mobile stations.
In a purely TSMA system corresponding to the special case of a single carrier frequency, the semaphore pat~ described in Patent Application 2 556 532 is founa~ In a purely TSMA system, there will be as many simultaneous ti~e intervals for the se~aphore path as there are communication channels. As has already been pointed out, however, this system does not in practice permit different and pro~ressive fitting out of the fixed stations.
It will now be shown with reference to the diagrams of Figures 2 and 3 how it i.s possible to organise the semaphore path in mixed systems making it possible to fit fixed stations out more easily in accordance with requirements.
On a time t-frequency f graph, Fi.gure 2 shows the structure of the descending communication and semaphore paths for a wide-band TSMA/FDMA system. A fixed station may be fitted with a numbex Nf of carrier frequencies upto a maximum of Nfm, each used to transmit TSMA
frames of period T1. A TSMA frame comprises x time channels and occupies a band of width QF. The bands ~F are contiguous or all the carrier frequencies.
In the example shown, the maximum number of carrier frequences is Nfm = 12 and the band width ~F is 2 MHz, so that the maximum band occupied by the descending communication path is 24 MHz. A
T5MA frame comprises n = 60 time channels 0.5 ms wide, so that a TSMA
frame has a period T1 = 0.5 ms x 60 = 30 ms.
The semaphore path is made up with the aid of time intervals marked S with an index showing the number of time intervals in a semaphore frame period T2. All the time intervals S occur at the same moment for all the carrier frequencies F1 to F12 and are separated by time D which is produced by a whole number N of the TSWA frame period T1. In the example chosen, N = 4. In a semaphore frame period T2 there is a number of time intervals S equal to the size Ch of the selected pattern (number of cells in the pattern) so that all the fixed stations in the pattern can transmit successively during a period T2. In the -, ,, ~ .
' , . .
, -, vo~ ~
PHF 85555 10 17.06.19a6 example selected for Figure 2, Ch = 25 and the time intervals S shown are S1, 52, S3, ..., 525. If the time interval S of the semaphore channel is of the same duration 0.5 ms as that of a communication channel, the period of the semaphore frame is T2 = 0.5 ms x Ch (N.n + 1) = 0.5 ms x 25 (240 ~ 1) = 3.0125 s.
The fixed stations transmit by circular permutation in both time and frequency. Thus, as shown in the example of Figure 2 by numbers in the boxe3 corresponding to the time intervals S and the carrier frequencies, for carrier frequency F1, the successive time intervals S1, S2, S3, ..., S25 serve respectively for the traDsmission from fixed stations 1, 2, 3, ..., 25; for carrier frequency F2, timP
intervals S1, S2, S3, ..., S25 serve for the transmission from fixed stations 2, 3, 4, ..., 1; and so on until carrier frequency Fl2, for which the time intervals S1, S2, S3, ..., SZ5 serve for the transmission from fixed stations 12, 31, 14, ..., 11. At any time interval S1 to S25 in the semaphore path, all the fixed stations in a pattern transmit in this semaphore path. It is of course possible to use another example of circular permutation, provided that all the fixed stations transmit in the Ch timP intervals S of a semaphore frame period. The condition for this is that the difference between the numbers of the fixed stations transmitting in succession is a first number with the size Ch of the pattern. In the example of Ch = 25, the number 4 may be chosen for this difference. For carrier frequency F1, there would then be in succession for the intervals Sl to 525 the transmissions from fixed stations 1, 5, 25 9, 13, 17, 21, 25, 4, a, 12, 16, 20, 24, 3, 7, 11, 15, 19, 23, 2, 6, 10, 14, 18, 22. For carrier frequency F2 there would be successive . .
transmissions from fixed stations 5 to 1 in the above order. Finally, for carrier frequency F12, there would be in succession transmissions from fixed stations 20 to 16 in the above order.
With the semaphQre path structure just described, it is clear that on any communication channel (with any carrier frequency) where there is a mobile station on stand-by or in communication with a fixed station, this mobile station receives vla the semaphore path all the information from the fixed stations in pattern enabling it to seek a free channel. Moreover, the fixed stations may be fitted with a variable number Nf of carrier frequencies upto a maximum of Nfm depending on the time and space requirements. In the example of Figure :
.: :
' ..
.
': , . . ' PHF 85555 11 17.06.198S
2, Nfm = 12. Whatever the carrier frequencies used, a mobile station will always receive,via the semaphore path, the information needed to seek a free channel..
A TSMA narrow-band/TSMA system needs no special description. In relation to the system of Figure 2, there is simply a narrower band ~F,~g~ 200 kHz, for the TSMA channel frame, said frame comprising a smaller number of time channels,e.q. 6. The maximum number of carrier frequencies may be greater to give the same total number of communication channels. The structure of the semaphore channel may be the same as in Figure 2 with the same distance D between the intervals S of the semaphore path and the same period T2 of the semaphore path.
The time-frequency graph of Figure 3 will rather be used -to describe a TSMA narrow-band/CDMA/FDMA system. Here, each communication channel using a time interval of a TSMA frame is not supported by a fixed carrier frequency but by one which varies in jumps according to a given law.
In a system like that of Figure 3, each fixed station must be fitted with a number Nf of multiple carrier frequencies of the size Ch of the selected pattern : ~f = KCh, where K is a whole number determining the number of groups in which the carrier frequencies are distributed. In the example of Figure 3, these are distributed over 5 groups G1 to G5 each comprising 25 carrier frequencies, thus making the maximum number Nfm of carrier frequencies 125. The band width used around each carrier frequency to transmit the TSMA frame is Qf = 0.2 MHz. These bands ~f are contiguous for all the carrier frequencies so that the maximum band occupied by the descending communication channel is 0.2 MHz x 25 x 5 = 25 MHz.
The system operates in a similar way for each of the groups of carrier frequencies G1 to ~5 and the description will relate more particularly to the structure of the communication channels and the semaphore path for group G1.
A TSMA frame comprises 6 time intervals of duration 5 ms so that a TSMA frame has a period T1 = 5 x 6 = 30 ms. Each communication channel is made up of a time interval of the TSMA frame supported by a carrier frequency which varies in jumps according to a given law, with the time interval between two successive jumps being the - .
PHF 85555 12 17.06.1986 period T1 of the TSMA frame. On the diagram of Figure 3 the hatched boxes represent the time-frequency space occupied by a communication channel using a time interval and a frequency-jump law. In the example shown, the carrier frequncies used by the channel during a period of the frequency-jump law increase in the order from f1 to f25. Another order may of course be used, provided that the 25 carrier frequencies are used in this period. ~or group G1 of 25 carrier frequencies, 25 frequency-jump laws are possible, these laws being obtained by multiple ~ime shifts of period T1 of the TSMA frame. Thus, the use in a fixed station of a group of carrier frequencies like G provides G x 25 = 150 communication chamlels. By juxtaposing 5 groups of carrier frequencies acting like G1 it i5 possible to o~tain 5 x 150 = 750 communication channels.
The semaphore path is made up in a similar way to that shown in Fi~ure 2 with the aid of equidistant time intervals S, numbering Ch = 25 in a semaphore frame period T2. These time intervals marked S1 to 525 in a period T2 are produced at the same moments for all the carriex frequencies of the 5 frequency groups G1 to G5. These time intervals S of the semaphore path are separated by a duration D which is the product of a whole number N of the TSMA frame period T1. The Ch fixed stations in a cell pattern may successively transmit in the Ch time intervals S of the semaphore frame if N is a first number with Ch, which is produced in the example of Figure 3, with Ch = 25 and N = 9.
If the time interval S of the semaphore channel is of the same duration 5 ms as that of a communication channel, the duration of the semaphore frame period is T2 = 5 ms x Ch (Nn + 1) = 5 ms x 25 ~24 + 1) = 3.125 s.
The fixed stations transmit on the semaphore path in circular permutation in both time and frequency. In the example of Figure 3, where the carrier ~requencies in group G1 are used in the frequency-jump laws in the order from f1 to f25, it will be seen that, for carrier frequency f1, the time intervals S1 to 525 serYe for transmission from fixed stations 1 to 25, for carrier frequency f2, the time intervals S1 to S25 serve for transmission from fixed stations 2 to 1 and so on upto carrier frequency f25, for which time intervals S1 to 525 serve for txansmission from fixed stations 25 to 24.
With this semaphore path structure, if the fixed stations ~' ; .
.
o~
PHF 85555 13 17.06.1986 are fitted with carLier frequency group 51, a mobile station on stand~by or in communic~tion with a fixed station on any communication channel using one of the 25 possible frequency-jump laws must receive via the semaphore path the information from all the fixed stations in a pattern, enabling it to seek a free channel to establish communication or change channel.
If the fixed stat.ions are fitted with several carrier frequency groups G1 to G5, each group of Ch carrier frequencies tCh = 25) may use independent frequency-jump laws similar to those described for group G1. Likewisè, the structure of the semaphore path may be s.imilar for each yroup to that described for group G1. Thus, any mobile station on stand-by or in communication on a given channel of one of groups G1 to G5 must receive,vla the semaphore channel, all the information it needs to seek a free channel.
The semaphore messages transmitted by the fixed stations in all the carrier frequencies which they use ~Nfm carrier frequencies at the most) have an identical content, viz :
- the address of the fixed station transmitting the message;
- the service information concerning the calls,l.e. the number of the mobile subscriber~s) sought and the nature of the call ~call, return of call, acknowledgment of receipt, end of communication, order to change channels, etc.);
- for free channel seeXing according to the invention :
* list L1 of the duplex channels used by the fixed 2~ station;
* list L2 of the duplex channels recognised as engaged by the fixed station, using level measurements in the rising path channels;
- in addition, a semaphore message states :
* in a TSMA wide-band/FDMA system, the caxrier frequency~ies) Nf fitted to the fixed station, and preferably a single carrier frequency selected by the fixed station and for use by the mobile station in selecting a communication channel;
* in a TSMA narrow-band/C3MAIFDMA system, the carrier frequency group~s) fitted to the fixed station ~i.e.
the value~s) of K in Nf = RCh), and preferably a single ~ ':
PHF 85555 14 17.06.1986 g~oup selected by the fixed station for use by the mobile station in selecting a communication channel.
By transmitting these last two items of information, a fixed station can thus at any time guide the choice of the free channel by the mobile stations to the carrier freguency with the lightest engaged channel load or to the least-loaded group of carrier frequencies. When, for instance, a carrier frequency (or group thereof) is fitted to a fixed station, but not its neighbours, this fixed station will guide the choice of the mobile stations to this frequency or group of frequencies. Moreover, if several requests for calls are transmitted in a message by the fixed station, it shows in the corresponding semaphore messages a carrier frequency or ~roup thereof which differs for each mobile station. It is thus possible to avoid the risk that several mobile stations called will select the same channel at the same time.
In addition, by transmitting the two latter items of information, the fixed station can limit the channels in lists L1 and L2 to those forming part of the carrier frequency or group thereof selected, thus reducing the number of bits in the semaphore message.
The method of the invention may be implemented in a mobile station as shown in the diagram of Figure 4. This diagram shows only the main components involved in the method.
According to Figure 4, a mobile station comprises a radio receiver 1 connected to a receiving aerial 2. Output 3 of the receiver supplies the demodulated output signal which is applied to a decoder 4.
The latter, at its output 5, provides the decoded data or voice signal transmitted to the receiver in the selected communication channel.
Output 6 of the decoder provides the information received in the semaphore path concerning the search for the free channel. Finally, assuming that synchro-bit and synchro-frame devices are incorporated into the decoder, output 7 of the decoder supplies the synchro-frame signal of the semaphore path (frame with Ch successive time intervals determining the Ch channels of the semaphore path). Radio receiver 1 also has an auxiliary low-frequency output 8 in front of the limiter and thus making it possible to measure the reception level. The signal at this output 8 is applied to a level-measuring device 9 which also receives the synchro-frame signal of the semaphore path and thus ,~:
~ ~- . '; ' PI~F 85555 15 17.06.1986 provides the level measured in the Ch channels thereof. The measured levels leaving device 9 are guided in device 10 to Ch low-pass filters, each providing a mean of the level measured over several semaphore frame periods in order to avoid any irregularities in the level measurements.
The level in the Ch channels of the semaphore path ~rom the Ch fixed station of a cell pattern is thus obtained at the outputs of device 10.
The levels supplied by device 10 are compared together in comparator circuit 11 at the c~aractexistic moments S of the synchro-frame signal.
Comparator circuit 11 determines the highest level and the position of the corresponding channel in the semaphore frame, which gives the address of the allocated fixed station, which is stored in memory 12.
This address is supplied to radio transmitter 13 for broadcasting via transmission aerial 14. Transmitter 14, of course, receives other signals to be broadcast tprocedure messages, voice or data si~nals) but these have not been shown for the sake of simplicity.
In order to implement the method of the invention, the Ch measured levels supplied by device 10 are applied to Ch threshold circuits like 15. The threshold shared by these circuits is the "interference" threshold. If the level received in a channel exceeds this interference threshold, the fixed station which has transmitted on this channel is regarded as interfering. List L of the addresses of the fixed interfering stations which have shown a Figure beyond the interference thre.shold are stored in Ch-bit memory 16.
Moreover, output 6 of decoder 4 provides the information received in the various channels of the semaphore path,l.e. for each semaphore path, list L1 of the communication channels used by the fixed station transmitting into the semaphore channel, list L2 of the channels recognised as engaged by this fixed station, finally information I
showing the carrier fxequency(ies) (or groups thereof) to be used to see~ a free channel. This information received in the semaphore path are stored in memory 17.
List L of the interfering fixed stations stored in memory 16 and lists L1, L2 and information I stored in memory 17 are read out of these memories on a call of circumstances requiring a change of channel. From L, L1, L2 and I, a free duplex channel may easily be selected using a method of the invention described above. In short, a selected free duplex channel :
.. . . . . . .
` '', "' .
' '''':. ; : " :
. .
~'HF 85555 16 17.06.1986 - must be one of the channels determined by information I;
- must not ~e found in list L2 of the engaged channels in the rising path;
- must not be found in list L1 of the channels used and originating from interfering fixed channels in list L.
The means used to imple~ent the method of the invention in a fixed station are shown in Figure 5. If the fixed station is fitted out for Nf carrier frequencies, it comprises Nf fixed-frequency radio receivers like 20 and 21, fitted with receiver aerials and having, like receiver 1 of the mobile station of Figure 4, an auxiliary low-frequency output in front of the receiver limiter. The signals at these low-frequency outputs 22, 23 are applied to level measuring devices 24, 25 which also receive, from a generator 26 the synchro-frame signal of the TSMA communication channels (frame with n successive time intervals). Each measuring device 24, 25 thus supplies the measured level of the n channels of the rising communication path. The measured levels supplied by 24, 25 are guided in each of devices 27, 28 to n low-pass filters. This, at the output of all the devices like 27, 28 are obtained the levels measured in the nNf channels of the rising communication path. These measured levels are applied to nNf regulated threshold circuits like 29, 30 and 31, 32. All these circuits are set to the same threshold ~referred to above as the first) and the channels where this threshold is exceeded are recognised as engaged. The information on these excesses of the thresholds is applied to nNf-bit memory 33 and, when this is read out, it provides list L2 of the channels recognised as engaged by the fixed station. Moreover, list L
of the channels used by the fixed station is supplied to memory 34.
Finally, information I of the carrier frequency~ies) (or groups thereof) to be used in seeking a ~ree channel is supplied to memory 35.
The message to be transmitted in the semaphore path and comprising lists L1 and L2 and information ~ is formed in 36 for application to ~ran~mitter 37, in turn for broadcasting via aerial 38.
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Claims (9)
- THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
l. A method of enabling a mobile station (M) to select a free duplex channel for communicating with a fixed station (F) in a cellular mobile radio system having a plurality of duplex channels each of which has a rising communication path and an associated descending communication path, respective duplex channels having respective addresses and providing communication between respective ones of a plurality of fixed stations and a plurality of mobile stations, the fixed stations being located in a pattern of cells constituting the cellular system; such method comprising:
- each fixed station identifying the addresses of duplex channels having rising paths which are engaged by detecting whether the signal power level in each received rising path exceeds a first given threshold, and transmitting the addresses of such channels to the mobile stations over a signalling path comprising equidistant time interval channels which are successively and periodically allotted to the respective fixed stations in the cellular system;
- determination by each mobile station of the addresses of duplex channels having descending paths which are engaged by detecting whether the signal power level in each received descending path exceeds a second given threshold; and - selection by the mobile station (M) of a free duplex channel from among duplex channels having addresses differing from those of channels which are engaged in the rising paths and those which are engaged in the descending paths;
such method being characterized by:
- continuous transmission by each of the fixed stations, in the signalling path time interval channels respectively allotted to them, of the addresses of duplex channels which they are using and of which of such channels have engaged descending paths;
- detection by the mobile station (M) of the signal power level in the signalling path time interval channels received from the respective fixed stations, and identification as interfering stations those for which such received signal power level exceeds a predetermined interference threshold but is below the maximum received signal power level; and - determination by the mobile station (M) of the addresses of duplex channels of fixed station (F) having engaged descending paths by eliminating from the addresses of all duplex channels having engaged descending paths the addresses of duplex channels of interfering fixed stations. - 2. A method as claimed in claim 1, wherein said predetermined interference threshold is set at the threshold of good reception, and the fixed stations transmit in their respective signalling path channels at a power level which exceeds the transmission power level in the duplex channels by an amount substantially equal to the difference between said threshold of good reception and said second given threshold of the received power level in descending paths of engaged duplex channels.
- 3. A method as claimed in claim 1, wherein the time interval channels in the signalling path for the respective fixed stations are simultaneously produced at each of a plurality of carrier frequencies assigned to each of the fixed stations for communication over the duplex channels.
- 4. A method as claimed in claim 3, wherein the mobile radio system employs a mixture of time-sharing multiple access (TSMA) and frequency-division multiple access (FDMA), each fixed station having a variable number of FDMA carrier frequencies assigned to it and transmitting at each such frequency successive TSMA frames respectively comprising a number N of time-division multiplexed communication channels; such method being further characterized in that the time interval channels in the signalling path are separated by a duration which is a multiple of the TSMA frame period, the number of such time interval channels in the signalling path being equal to the number Ch of cells in the pattern constituting said cellular radio system; and the fixed stations in such pattern transmit by circular permutation of the time interval channels in the signalling path and the carrier frequencies respectively assigned to such stations.
- 5. A method as claimed in claim 4, wherein to enable a mobile station (M) to select a free duplex channel for communication with a fixed station (F) the fixed station (F) also transmits over its time interval channel in the signalling path information which identifies the carrier frequencies to be used by the mobile station (M) for making such a selection.
- 6. A method as claimed in claim 4, wherein the mobile radio system additionally employs code-division multiple access (CDMA), each fixed station having a variable number of FDMA carrier frequencies assigned to it and each TSMA frame comprising N
multiplexed time-division channels which are transmitted at a carrier frequency which is varied in jumps according to a predetermined code; such method being further characterized in that the number of carrier frequencies is divided into an integral number of groups (K), and the time interval channels in the signalling path are separated by a duration (D) which is an integral multiple of the TSMA frame period; and the fixed stations in the cell pattern transmit by circular permutation of the time interval channels in the signalling path and the carrier frequencies in each of said K groups of frequencies. - 7. A method as claimed in claim 6, wherein to enable a mobile station (M) to select a free duplex channel for communication with a fixed station (F) the fixed station (F) also transmits over its time interval channel in the signalling path information which identifies the group of carrier frequencies to be used by the mobile station (M) for making such a selection.
- 8. A receiver for use in a mobile station of a cellular mobile radio system wherein a free duplex communication channel is selected by such mobile station by a method as claimed in claim 1, comprising:
a signal power level measurement circuit for measuring the power level of signals received by the mobile station in the respective time interval channels in the signalling path;
a plurality of low-pass filters for receiving the power levels measured in the signal power level measurement circuit and determining the mean values thereof over several frame periods of the channels in the signalling path;
a plurality of threshold circuits connected to outputs of said low-pass filters for measuring whether the mean values determined thereby exceed the interference threshold level of interfering fixed stations and supplying, when such threshold is exceeded, the addresses of the interfering fixed stations;
first memory means for receiving and storing the interfering station addresses supplied by said threshold circuits;
second memory means for storing the information transmitted in the signalling channel to the mobile station by the fixed stations identifying duplex channels having descending paths which are engaged; and means for reading out the information stored in said first and second memories to enable the mobile station to select a free duplex channel. - 9. Apparatus for use in a fixed station (F) of a cellular mobile radio system wherein a free duplex communication channel is selected by a mobile station (M) by a method as claimed in claim 5, such system having a plurality of fixed stations which receive signals from a plurality of mobile stations in rising paths of a plurality of duplex communication channels, such apparatus comprising:
a level measuring circuit for measuring the power levels of signals received by the fixed station (F) from the rising paths of the duplex communication channels;
a plurality of low-pass filters which receive the measured power levels from said level measuring circuit;
a plurality of threshold circuits connected to the low-pass filters and respectively producing, when a first threshold power level of signals in said rising paths of the duplex communication channels is exceeded, the addresses of the duplex channels having such rising paths, such addresses identifying duplex channels which are engaged;
first memory means connected to said threshold circuits for storing a list of the engaged duplex channels;
second memory means for storing the addresses of duplex channels used by the fixed station (F) and also storing carrier frequencies to be used by the mobile station (M) for identifying a free duplex communication channel; and means for combining the information stored in said first and second memory means and supplying the combined information for transmission to mobile station (M) over a signalling path channel allocated to said fixed station (F).
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FR8510490A FR2584884B1 (en) | 1985-07-09 | 1985-07-09 | FREE CHANNEL SEARCHING METHOD AND DEVICE FOR A MOBILE RADIO SYSTEM |
| FR8510490 | 1985-07-09 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1261003A true CA1261003A (en) | 1989-09-26 |
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ID=9321117
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000513002A Expired CA1261003A (en) | 1985-07-09 | 1986-07-03 | Method and device for seeking a free channel in a mobile radio system |
Country Status (7)
| Country | Link |
|---|---|
| US (1) | US4783780A (en) |
| EP (1) | EP0209185B1 (en) |
| JP (1) | JP2572377B2 (en) |
| AU (1) | AU588895B2 (en) |
| CA (1) | CA1261003A (en) |
| DE (1) | DE3670938D1 (en) |
| FR (1) | FR2584884B1 (en) |
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Family Cites Families (9)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE2537683C2 (en) * | 1975-08-23 | 1986-06-26 | Licentia Patent-Verwaltungs-Gmbh, 6000 Frankfurt | Arrangement for channel allocation in a radio transmission system with fixed radio stations and vehicle stations |
| DE2937018A1 (en) * | 1979-09-13 | 1981-04-02 | TE KA DE Felten & Guilleaume Fernmeldeanlagen GmbH, 8500 Nürnberg | Programmed HF channel selection for vehicle radio stations - minimises same channel interference and includes relay stations with memory bank with inhibiting circuits |
| DE3014859C2 (en) * | 1980-04-17 | 1982-06-24 | Siemens AG, 1000 Berlin und 8000 München | Mobile radio network |
| US4545071A (en) * | 1982-11-12 | 1985-10-01 | Motorola, Inc. | Portable radio for a zoned data communications system communicating message signals between portable radios and a host computer |
| FR2556532B1 (en) * | 1983-12-09 | 1986-10-24 | Trt Telecom Radio Electr | METHOD FOR TWO-WAY RADIOCOMMUNICATION BETWEEN FIXED STATIONS AND MOBILE STATIONS |
| SE458734B (en) * | 1984-10-31 | 1989-04-24 | Ericsson Telefon Ab L M | PROCEDURE TO PROVIDE THE NUMBER OF CALL OPPORTUNITIES IN A MOBILE PHONE SYSTEM |
| CA1246681A (en) * | 1985-01-30 | 1988-12-13 | Northern Telecom Limited | Terminal address assignment in a broadcast transmission system |
| JPS61177040A (en) * | 1985-01-31 | 1986-08-08 | Nec Corp | Radio circuit control system |
| US4670906A (en) * | 1986-04-02 | 1987-06-02 | Motorola, Inc. | Data communications system transmitter selection method and apparatus |
-
1985
- 1985-07-09 FR FR8510490A patent/FR2584884B1/en not_active Expired
-
1986
- 1986-06-23 US US06/877,557 patent/US4783780A/en not_active Expired - Lifetime
- 1986-07-03 CA CA000513002A patent/CA1261003A/en not_active Expired
- 1986-07-07 DE DE8686201188T patent/DE3670938D1/en not_active Expired - Lifetime
- 1986-07-07 EP EP86201188A patent/EP0209185B1/en not_active Expired - Lifetime
- 1986-07-07 AU AU59795/86A patent/AU588895B2/en not_active Ceased
- 1986-07-09 JP JP61159875A patent/JP2572377B2/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| FR2584884A1 (en) | 1987-01-16 |
| AU588895B2 (en) | 1989-09-28 |
| EP0209185B1 (en) | 1990-05-02 |
| EP0209185A1 (en) | 1987-01-21 |
| AU5979586A (en) | 1987-01-15 |
| FR2584884B1 (en) | 1987-10-09 |
| JPS6215941A (en) | 1987-01-24 |
| DE3670938D1 (en) | 1990-06-07 |
| US4783780A (en) | 1988-11-08 |
| JP2572377B2 (en) | 1997-01-16 |
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