CA2053313C - Rapid received signal strength indication - Google Patents
Rapid received signal strength indicationInfo
- Publication number
- CA2053313C CA2053313C CA002053313A CA2053313A CA2053313C CA 2053313 C CA2053313 C CA 2053313C CA 002053313 A CA002053313 A CA 002053313A CA 2053313 A CA2053313 A CA 2053313A CA 2053313 C CA2053313 C CA 2053313C
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- CA
- Canada
- Prior art keywords
- time
- signal
- dispersed
- rssi
- receiver
- 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.)
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04M—TELEPHONIC COMMUNICATION
- H04M11/00—Telephonic communication systems specially adapted for combination with other electrical systems
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B17/00—Monitoring; Testing
- H04B17/30—Monitoring; Testing of propagation channels
- H04B17/309—Measuring or estimating channel quality parameters
- H04B17/318—Received signal strength
Abstract
There is provided a method of rapid Received Signal Strength Indication (RSSI) of a time-dispersed signal having echoes (where the time dispersal represents a significant fraction or more of the transmitted symbol interval). It comprises a sliding correlation (304) of a received signal against a known sequence or via channel sounding to obtain the time-dispersal function of the communications channel upon which the signal is transmitted and a summation of the squares of the quadrature components of the energies at relative maxima of the correlations (306) (correlation peaks determined with reference to a threshold level) (305) to integrate the energy defined by the correlation function to determine the energy present among the time-dispersed echoes utiliz-ing the time-dispersal function. Thus, the energy present in the time-dispersed echoes is related to the received signal strength (307), the RSSI comprises a scalar of the energy present and the scalar comprises a function of the receiver's gain, RSSI determi-nations during any given interval are less dependent upon the ambulatory velocity of the receiver, and RSSI determinations of equivalent accuracy are obtained more rapidly or more accurate RSSI determinations are obtained per unit time.
Description
2i333313 RAP~D RFCF,TVFD SIGNAI, STRFNGTH INDICATrON
Field of Invention The present invention relates to rapid received signal strength indication.
More particularly, the present invention relates to a method of rapid Received Signal Strength Indication (RSSI) of a time-dispersed signal having echoes. That is, the signal arriving at the receiver can be considered to be made up of the energy of the tr:~n.cmitte~ signal having been broken into many parts, each part of the energylS having taken a different path to the receiver and hence arriving at dirrelell~ times.
The range of these times is one way of describing the delay spread. In the present invention, the strength of such a signal is determined when the delay spread is at least a significant fraction of the transmitted bit time.
Background of the Invention One of the problems in acsescing the signal strength in a mobile, or non-stationary, radio receiver is that the phases of the time-dispersed reflected waves (or echoes) will combine to produce a composite signal. This composite signal, when processed conventionally for any particular measurement interval or sequence of intervals, may yield inaccurate ~cces~ cnlC of signal power, due to res~11t:~nt wild fluctuations in signal strength from moment to moment. This imposes long filtering time CollSlallb or long averaging intervals in order to establish a reasonably accurate signal strength ~cces.cm~.nt.
Other measu~ ellt methods have a weakness in that their accuracy is sensitive to the rate of change of the signal envelope, the square root of the signal's inct~n~neous power, so that slowly-changing instantaneous signal powers impose 21J53 ~ ! 3 relatively long time constants or averaging intervals for reasonable measurementaccuracy.
In cellular telephony, a system must be able to determine, rapidly and accurately, a representative signal strength on the channel to which it is currently 5 listening in order to properly decide whether the connection should be changed to a channel on another cell site, or to another channel on the current cell site. The process of changing the connection is called a handoff. "Signal strength" for these purposes may be in terrns of a certain received voltage level, which may be related, if necessary, to a power.
In the normal course of a cellular telephone call, subscribers may be nearly motionless, as in the case of a handheld portable telephone, or may be moving rapidly, as in the case of a mobile telephone on a freeway. The result is a broad spectrum of rates of change in the received signal's envelope. Under these conditions, a receiver capable of acsessing the signal strength more rapidly to the same accuracy, or more accurately in the same time, and in addition be relatively insensitive to the subscriber's motion, would be extremely advantageous.
This invention, then, takes as its object to address these challenges and to realize these and certain other advantages presented below.
Summary of the Invention This invention is to be used in receivers employing channel sounding to obtain the channel impulse response. It recognizes that the impulse response contains information which, properly processed, provides a more representative value for the received signal's strength more rapidly and accurately than by simply periodically sampling the signal's envelope.
In accordance with one aspect of the present invention, there is provided a method of controlling signal reception and processing in a receiver. The method employs received signal strength indications (RSSI's) of time-dispersed signals. The method comprises the steps of obtaining from a time-dispersed signal a characteristic time-dispersal function of the collllllunications channel upon which the signal is transmitted; determininp; the energy present in the time-dispersed signal from the time-dispersal function obtained; and employing that determination in controlling one of signal reception and processing in saidreceiver.
~ .
Field of Invention The present invention relates to rapid received signal strength indication.
More particularly, the present invention relates to a method of rapid Received Signal Strength Indication (RSSI) of a time-dispersed signal having echoes. That is, the signal arriving at the receiver can be considered to be made up of the energy of the tr:~n.cmitte~ signal having been broken into many parts, each part of the energylS having taken a different path to the receiver and hence arriving at dirrelell~ times.
The range of these times is one way of describing the delay spread. In the present invention, the strength of such a signal is determined when the delay spread is at least a significant fraction of the transmitted bit time.
Background of the Invention One of the problems in acsescing the signal strength in a mobile, or non-stationary, radio receiver is that the phases of the time-dispersed reflected waves (or echoes) will combine to produce a composite signal. This composite signal, when processed conventionally for any particular measurement interval or sequence of intervals, may yield inaccurate ~cces~ cnlC of signal power, due to res~11t:~nt wild fluctuations in signal strength from moment to moment. This imposes long filtering time CollSlallb or long averaging intervals in order to establish a reasonably accurate signal strength ~cces.cm~.nt.
Other measu~ ellt methods have a weakness in that their accuracy is sensitive to the rate of change of the signal envelope, the square root of the signal's inct~n~neous power, so that slowly-changing instantaneous signal powers impose 21J53 ~ ! 3 relatively long time constants or averaging intervals for reasonable measurementaccuracy.
In cellular telephony, a system must be able to determine, rapidly and accurately, a representative signal strength on the channel to which it is currently 5 listening in order to properly decide whether the connection should be changed to a channel on another cell site, or to another channel on the current cell site. The process of changing the connection is called a handoff. "Signal strength" for these purposes may be in terrns of a certain received voltage level, which may be related, if necessary, to a power.
In the normal course of a cellular telephone call, subscribers may be nearly motionless, as in the case of a handheld portable telephone, or may be moving rapidly, as in the case of a mobile telephone on a freeway. The result is a broad spectrum of rates of change in the received signal's envelope. Under these conditions, a receiver capable of acsessing the signal strength more rapidly to the same accuracy, or more accurately in the same time, and in addition be relatively insensitive to the subscriber's motion, would be extremely advantageous.
This invention, then, takes as its object to address these challenges and to realize these and certain other advantages presented below.
Summary of the Invention This invention is to be used in receivers employing channel sounding to obtain the channel impulse response. It recognizes that the impulse response contains information which, properly processed, provides a more representative value for the received signal's strength more rapidly and accurately than by simply periodically sampling the signal's envelope.
In accordance with one aspect of the present invention, there is provided a method of controlling signal reception and processing in a receiver. The method employs received signal strength indications (RSSI's) of time-dispersed signals. The method comprises the steps of obtaining from a time-dispersed signal a characteristic time-dispersal function of the collllllunications channel upon which the signal is transmitted; determininp; the energy present in the time-dispersed signal from the time-dispersal function obtained; and employing that determination in controlling one of signal reception and processing in saidreceiver.
~ .
3 20~33 1 3 In accordance with another aspect of the present invention, there is provided a method of controlling signal reception and processing in a receiver, - the method employing received signal strength indications (RSSI's) of time-dispersed signals, the method comprising the steps of a sliding complex quadrature correlation of a received, time-dispersed signal against a known sequence to obtain a characteristic time-dispersal function of the commllnications channel upon which the signal is tr~n~mitt~cl; a sllmmzttion of the squares of the quadrature components of the complex correlation at relative m,txim,t of the correlation to determine the energy present at the relative maxima of such correlation; and employing that determination in the control of one of signal reception and processing in said receiver.
In accordance with another aspect of the present invention, there is provided a method of controlling a device utilized in a cellular radiotelephone system, the method employing received signal strength indications (RSSI's) of time-dispersed signals, the method comprising the steps of obtaining from a time-dispersed signal a characteristic time-dispersal function of the communications channel upon which the signal is tr~n.~mittecl; del~l ,llinil~g the energy present in the time-dispersed signal from the time-dispersal function obtained; and controlling, based on the determined energy present, said device utilized in said cellular radiotelephone system. Thus, energy present in the time-dispersed sigral is related to the RSSI of the time-dispersed signal. The RSSI comprises a scalar of the energy present and the scalar comprises a function of the receiver's gain, RSSI deterrninations during any given interval are less dependent upon the ~mbltl~tory velocity of the receiver, and RSSI determinations of equivalent accuracy are obtained more rapidly or more accurate RSSI
determinations are obtained per unit time. This depends upon a simplification tothe actual physical process of transrnitting and receiving a radio signal. In the actual physical case, the energy seen at the receiver is distributed more or less continuously over time. For the present invention we use a simplified model, valid for the cases of interest, which treats the received signal as if it were made up of individual reflections, spaced apart in time. These reflections may also be called rays or echoes.
'~S
- 3a-The method comprises using a known part of the signal, such as a synchronizing sequence, to obtain the time-dispersal function, namely, the impulse response, of the communication channel upon which the signal is transmitted and then determining the energy present among the time-dispersed echoes in order to S arrive at the RSSI. Thus, First a correlation function is obtained, which represents the channel impulse response, the correlation being taken between the part of the received signal containing a known training pattern and a copy of the pattern stored in the receiver, , 10 using a threshold, peaks of the correlation function are obtained, each peak being proportionate to the power in one of the rays making up the simplified model of the received signal;
the signal power is taken to be a scaled sum of these ray powers, where the scaling accounts for the receiver's electrical characteristics such as amplifier gain and voltage output, and the time period over which the measurement was taken. This measure of the signal power is called the fast RSSI
mea~ultlllent.
In accordance with another aspect of the present invention, there is provided a method of controlling a device utilized in a cellular radiotelephone system, the method employing received signal strength indications (RSSI's) of time-dispersed signals, the method comprising the steps of obtaining from a time-dispersed signal a characteristic time-dispersal function of the communications channel upon which the signal is tr~n.~mittecl; del~l ,llinil~g the energy present in the time-dispersed signal from the time-dispersal function obtained; and controlling, based on the determined energy present, said device utilized in said cellular radiotelephone system. Thus, energy present in the time-dispersed sigral is related to the RSSI of the time-dispersed signal. The RSSI comprises a scalar of the energy present and the scalar comprises a function of the receiver's gain, RSSI deterrninations during any given interval are less dependent upon the ~mbltl~tory velocity of the receiver, and RSSI determinations of equivalent accuracy are obtained more rapidly or more accurate RSSI
determinations are obtained per unit time. This depends upon a simplification tothe actual physical process of transrnitting and receiving a radio signal. In the actual physical case, the energy seen at the receiver is distributed more or less continuously over time. For the present invention we use a simplified model, valid for the cases of interest, which treats the received signal as if it were made up of individual reflections, spaced apart in time. These reflections may also be called rays or echoes.
'~S
- 3a-The method comprises using a known part of the signal, such as a synchronizing sequence, to obtain the time-dispersal function, namely, the impulse response, of the communication channel upon which the signal is transmitted and then determining the energy present among the time-dispersed echoes in order to S arrive at the RSSI. Thus, First a correlation function is obtained, which represents the channel impulse response, the correlation being taken between the part of the received signal containing a known training pattern and a copy of the pattern stored in the receiver, , 10 using a threshold, peaks of the correlation function are obtained, each peak being proportionate to the power in one of the rays making up the simplified model of the received signal;
the signal power is taken to be a scaled sum of these ray powers, where the scaling accounts for the receiver's electrical characteristics such as amplifier gain and voltage output, and the time period over which the measurement was taken. This measure of the signal power is called the fast RSSI
mea~ultlllent.
4 ~053~1 3 Description of the Drawings Ad-lition~l objects, features, and advantages of the invention will be more clearly understood and the best mode c f)~ . ,pl~ted for practicing it in its preferred S embodiment will be appreciated (by way of unrestricted example) from the following detailed description, taken together with the acculllpanying drawings in which:
Figure 1 illustrates an exemplary TDMA signal that is received and assessed according to the preferred embodiment of the present invention.
Figure 2 is a block diagram of the receiver of the preferred embodiment of the present invention.
Figure 3 is an illustration of the process for determining the signal strength indication according to the present invention.
Detailed Description The present invention is employed in a cellular radiotelephone system using Time Division Multiple Access (TDMA).
Figure 1 illustrates an exemplary TDMA signal that is received and assessed according to the plcfcllcd embodiment of the present invention.
As illustrated in Figure la, frames of 4.616 millisecond duration are sequentially tr~n~mitted throughout the system. Each frame (Frame) consists of eight time slots (Time Slot 0-7) for ongoing speech or data traffic. An exception is that, on only one carrier frequency duplex pair per cell, one time slot (Time Slot 0) is reserved for system control data, such as access requests and paging.
As illustrated in Figure lb, each time slot of 0.577 ms. duration contains 148 bits of information in a 156.25-bit interval, including a sequence upon which receivers may synchronize themselves to the tr~nsmiccion (Synchronizing Sequence), allowing for delayed, time-dispersed arrival of the signal (D) and its echoes at the receiver, as further illustrated in Figure lc.
Figure 2 is a block diagram of the receiver of the preferred embodiment of the present invention.
The receiver employs conventional radio techniques from the antenna through the mixer and band-pass filter. The mixed and filtered signal is amplified WO 9(~/147~U ~ 1~ 5 3 ~ 1 3 in an intermediate frequency amplifier 205, the gain of which is fixed for the reception of each time-slot at a predetermined value established by a latching digital-to-analog converter 239. The amplified signal is followed by fixed-gain, conventional quadrature detection. The quadrature detector consisting of a local5 oscillator 207, a 90-degree phase shifter 209 and a pair of demodulators 211 and 213; the local oscillator 207 is phase-locked to the received carrier. The I and Q
analog signal components are filtered (217 and 215) and digitized in a pair of fixed gain high speed analog-to-digital converters (A/Ds: 219 and 221) at eight times per symbol interval.
The receiver synchronizes to the system by conventional techniques, and once synchronized, is able to calculate the proper time to begin processing each time slot of the signal. The receiver receives and stores samples of the entire time-slot and later determines the time-dispersal function, namely the channel impulse response, and calculates the fast RSSI as follows.
Throughout the time slot, the DSP 223, through its DMA 229, enables tristate gates (225 and 227) to gate digitized number pairs into M sequential pairs of Random Access Memory locations (RAM: 235 and 237), thus preserving the entire real part of the signal s(t) = si(t) + sq(t), and preserving its inherenttime-dispersal characteristics.
Read-Only Memory (ROM: 243, 245 and 247) contains a local replica of the expected synchronizing sequence r(t) = ri(t) + rq(t), composed of N sequential pairs of values, as well as containing other memory in which constants and results are stored. We then write this sampled version of the local replica as r(n) = ri(n) + rq(n), n = l, 2, . . ., N.
Likewise, the sampled version of the received signal is s(n) = si(n) + sq(n), n = 1, 2, . . ., M, where M is likely to be greater than or equal to N, inasmuch as the exact location of the synchronizing sequence is unknown to the receiver.
It is instructive at this point to describe what the sampled points r(n) are describing. These points are the sampled version of a time-continuous function 35 which describes a signal which, once launched from the transrnitter, is reflected by 20~313 W O 90/14730 P ~ /US9O/02472 many objects between tr~ncmitter and receiver. Reflecting causes the tr~ncmitredsignal's energy to arrive over a period of time, determined by how far each echofrom each reflecting object had to go to get to the receiver. In the received signal, energy which was tr~ncmitted during a signaling symbol time is spread out, or 5 time-dispersed by the reflection process over a time longer than the transmitted symbol time. A model for tr~ncmiccion, now well-known in the literature (for example, "Activities of the COST 207 Working Group on Propagation", R. W.
Lorenz, In~ernational Conference on Digital Land Mobile Radio Comrnunications June 30 - July 3, 1987, Venice, Italy), treats the received signal as being composed 10 of a finite number of rays, or echoes, of different arrival times and signal strengths.
Using time-continuous notation now, the signal at the receiver's antenna is s(t)= ~, Ai(t)ai(t-~i) 15 when L rays are considered. Now we develop what the signal looks like at the receiver just before sampling by the A/D converters. In the case of L = 1, a single ray arrives at the receiver and the received signal can be written in the so-called quadrature form s(t)=l(t) cos(cl~ct)-Q(t) sin(~)ct) where Cl)c is the radian frequency of the RF carrier.
In the "I", or inphase branch, the local oscillator is cos(cl~ct), 213, and in the "Q" or quadrature branch, the local oscillator is shifted by -90, 211. Each25 picks out its tr~n~mitted branch from the received signal according to si(t) = ( s(t) * cos(c)ct) } LPF ;
Sq(t) = { s(t) * sin((dct) } UF
30 where { } LPF indicates the bracketed quantity is taken through a low pass filter.
Working out the trigonometry, the response to a single ray in the I branch is l(t) * 1 W ~J ~U/ 1'1 / JU r~ l / u~u/
205~13 and similarly in the Q branch is -Q(t) * 2-When two rays are involved, it can be shown that on the I branch si(t) = 2( ) + 2 ) cos ( ~ ~ ) ~ Q2~ ~ sin ( - ~ ) 10 and similarly on the Q branch sq(t)=Q2() + 2) sin(~)- Q() cos(Q) where l'(t) and Q'(t) are the time varying parts of a second ray, and ~ is the phase 15 offset of the second ray's carrier relative to the first ray's carrier. The instantaneous power of the signal s(t) is given by ps(t) = si2(t) + Sq2(t) 20 which works out to be = 12(t) + Q2(t) + 1'2(t) +Q'2(t) + 2(1(t) l'(t) + Q(t) Q'(t) ) cos( ~ ) + 2(1(t) Q'(t) - Q(t) l'(t) ) sin( ~ ).
The last four terms represent rapid variations in the in~t;~nt~neous power. We would like to get only the first four terms, but there is no way to do this looking at the envelope of the sampled version of s(t) directly. The present invention shows how to get 12(t) + Q2(t) for each received ray, which is the power received due to 30 each ray. The sum of these powers, or of something proportional to them, gives a more stable and accurate estimate of signal strength than merely measuring the received signal's envelope.
WO 90/14730 2 0 5~ PCr/US90/02472 Figure 3 is an illustration of the process for deterrnining the signal strength indication according to the present invention. The process illustrated at 301 and 303 has just been described above.
Now, the channel impulse response is obtained by c~lcul~ting the complex S correlation function Rsr(z) between s(n) and r(n) 304. This is handled math~m~ti~-~lly by treating both s(n) and r(n) as complex qu~ntities The complexcorrelation is a function of the parameter z, representing the relative shift between s(n) and r(n). The complex correlation function is well known and is given by N
Rsl(z)= ~, s(i-z)r't(i) i=l where the r*(i) denotes the complex conjugate and the sum is taken over all non-zero samples of the locally stored reference, for each shift z. For those shifts z =
Zn where the correlation is a local maximum, one of the linearly combined rays has l S aligned with the local reference and the correlation at that point for that ray is given by Rsr(Zn) = a ~, s2(i) i = 1 = ~ S j2(i) + Sq2(i) i = 1 where s2(t) is defined, as is customary for complex numbers, as s(t) s*(t). This is proportional to the energy in one of the rays or echoes. The factor in the above equations takes into account the expected receiver output voltage level for a given input signal strength and scaling for the use of multiple discrete sarnples in the c~lsul~rion. Herein lies the essence of the technique. The process of calculating the complex correlation function ess~nti~lly reveals the location in time by knowing Zn for each ray or echo and, most importantly for the present invention, reveals its strength by the m:~gnitude of Rsr(Zn).
There may be several local peaks in the correlation function, and only those above a certain threshold are to be considered 305. When all of the local maxima, above a predeterrnined threshold, are found 306, the final step is to sum the ,,~ 1 , ~, .
r-_ 1/ U~YU/U~4 /,!
-V~ V 2~533l3 energies, or voltages squared, or absolute powers, whichever is computationally convenient 307. It is only important that the decision process which is to use the measurement knows which of the measures is being used, which can be included in the scaling factor . This then represents the fast RSSI measurement. Finally, the 5 samples from the next time slot are awaited 308.
When the rate of change of the impulse response is sufficiently slow, co,~ d to the repetition rate of the training sequence used for determining the channel impulse response, the calculated fast RSSI may be taken alone, or even as a predictor value of the strength of the next received data field. When the rate of 10 change of the impulse response is faster, it is appropriate to somehow average or filter the sequence of RSSI values. The process may employ infinite impulse response filtering, running or sliding averages to yield useable results.
It is to be further noted that this method is not limited to burst tr~ncmicsions, but is equally applicable to the reception of any type of transmission 15 or transmissions in which certain known sequences occur at predictable intervals.
It is also true that the technique in the present invention need not be limited to discrete-time, or sampled, processing. Essentially the same calculations would apply where the correlation is performed by a continuous time device, such as a Surface Acoustic Wave device.
In summary, there has been provided a method of rapid Received Signal Strength Indication (RSSI) of a time-dispersed signal having echoes, where the time dispersal represents a signifir~nt fraction or more of the transmitted symbol interval. It comprises producing a complex correlation function between the received signal and a known sequence stored in the receiver. The resulting correlation function is examined in regions above a threshold generated at each new measurement trial for local maxima, each of which is proportional to the energy,and, since the time scale of the signal is known, an individual echo's power. These energies, or powers, or even peak voltages are summed to give an RSSI
deL~l-llination during any given interval less dependent upon the velocity of the receiver than earlier methods. Furthermore, RSSI determinations of equivalent accuracy are obtained more rapidly, or more accurate RSSI determinations are obtained in a given time than simple envelope sampling methods.
W O 90/14730 2 0 5 ~3 1 3 P ~ /US90/02472 There is provided a method of rapid Received Signal Strength Tn~lic~tiQn (RSSI) of a time-dispersed signal having echoes (where the time dispersal represents a signifi~nt fraction or more of the tlAnc, . 1;1 led symbol interval). It comprises a sliding correlation of a received signal against a known sequence or via 5 channel sounding to obtain the time-dispersal function of the cv"~ ;c~tions channel upon which the signal is tr~ncmitted and a s~lmm~tion of the squares of the quadrature COI1J~Onent~ of the energies at relative maxima of the correlations (correlation peaks det~ ed with l~;Ç~lc.lce to a threshold level) to illle~,ldl~ the energy defined by the correlation function to de~el~ e the energy present among 10 the time-dispersed echoes utilizing the time-dispersal function. Thus, the energy present in the time-dispersed echoes is related to the received signal strength, the RSSI comprises a scalar of the energy present and the scalar comprises a function of the receiver's gain, RSSI dek".li.~tionS during any given interval are less dependent upon the ~mbul~t~ ry velocity of the receiver, and RSSI determinations of 15 equivalent accuracy are obtained more rapidly or more accurate RSSI
det~nin~rions are obtained per unit time.
While the preferred embodiment of the invention has been described and shown, it will be appreciated by those skilled in the art that yet other variations and 20 modifications of this invention may be implemented. These and all other variations and adaptations are expected to fall within the ambit of the appended claims.
Figure 1 illustrates an exemplary TDMA signal that is received and assessed according to the preferred embodiment of the present invention.
Figure 2 is a block diagram of the receiver of the preferred embodiment of the present invention.
Figure 3 is an illustration of the process for determining the signal strength indication according to the present invention.
Detailed Description The present invention is employed in a cellular radiotelephone system using Time Division Multiple Access (TDMA).
Figure 1 illustrates an exemplary TDMA signal that is received and assessed according to the plcfcllcd embodiment of the present invention.
As illustrated in Figure la, frames of 4.616 millisecond duration are sequentially tr~n~mitted throughout the system. Each frame (Frame) consists of eight time slots (Time Slot 0-7) for ongoing speech or data traffic. An exception is that, on only one carrier frequency duplex pair per cell, one time slot (Time Slot 0) is reserved for system control data, such as access requests and paging.
As illustrated in Figure lb, each time slot of 0.577 ms. duration contains 148 bits of information in a 156.25-bit interval, including a sequence upon which receivers may synchronize themselves to the tr~nsmiccion (Synchronizing Sequence), allowing for delayed, time-dispersed arrival of the signal (D) and its echoes at the receiver, as further illustrated in Figure lc.
Figure 2 is a block diagram of the receiver of the preferred embodiment of the present invention.
The receiver employs conventional radio techniques from the antenna through the mixer and band-pass filter. The mixed and filtered signal is amplified WO 9(~/147~U ~ 1~ 5 3 ~ 1 3 in an intermediate frequency amplifier 205, the gain of which is fixed for the reception of each time-slot at a predetermined value established by a latching digital-to-analog converter 239. The amplified signal is followed by fixed-gain, conventional quadrature detection. The quadrature detector consisting of a local5 oscillator 207, a 90-degree phase shifter 209 and a pair of demodulators 211 and 213; the local oscillator 207 is phase-locked to the received carrier. The I and Q
analog signal components are filtered (217 and 215) and digitized in a pair of fixed gain high speed analog-to-digital converters (A/Ds: 219 and 221) at eight times per symbol interval.
The receiver synchronizes to the system by conventional techniques, and once synchronized, is able to calculate the proper time to begin processing each time slot of the signal. The receiver receives and stores samples of the entire time-slot and later determines the time-dispersal function, namely the channel impulse response, and calculates the fast RSSI as follows.
Throughout the time slot, the DSP 223, through its DMA 229, enables tristate gates (225 and 227) to gate digitized number pairs into M sequential pairs of Random Access Memory locations (RAM: 235 and 237), thus preserving the entire real part of the signal s(t) = si(t) + sq(t), and preserving its inherenttime-dispersal characteristics.
Read-Only Memory (ROM: 243, 245 and 247) contains a local replica of the expected synchronizing sequence r(t) = ri(t) + rq(t), composed of N sequential pairs of values, as well as containing other memory in which constants and results are stored. We then write this sampled version of the local replica as r(n) = ri(n) + rq(n), n = l, 2, . . ., N.
Likewise, the sampled version of the received signal is s(n) = si(n) + sq(n), n = 1, 2, . . ., M, where M is likely to be greater than or equal to N, inasmuch as the exact location of the synchronizing sequence is unknown to the receiver.
It is instructive at this point to describe what the sampled points r(n) are describing. These points are the sampled version of a time-continuous function 35 which describes a signal which, once launched from the transrnitter, is reflected by 20~313 W O 90/14730 P ~ /US9O/02472 many objects between tr~ncmitter and receiver. Reflecting causes the tr~ncmitredsignal's energy to arrive over a period of time, determined by how far each echofrom each reflecting object had to go to get to the receiver. In the received signal, energy which was tr~ncmitted during a signaling symbol time is spread out, or 5 time-dispersed by the reflection process over a time longer than the transmitted symbol time. A model for tr~ncmiccion, now well-known in the literature (for example, "Activities of the COST 207 Working Group on Propagation", R. W.
Lorenz, In~ernational Conference on Digital Land Mobile Radio Comrnunications June 30 - July 3, 1987, Venice, Italy), treats the received signal as being composed 10 of a finite number of rays, or echoes, of different arrival times and signal strengths.
Using time-continuous notation now, the signal at the receiver's antenna is s(t)= ~, Ai(t)ai(t-~i) 15 when L rays are considered. Now we develop what the signal looks like at the receiver just before sampling by the A/D converters. In the case of L = 1, a single ray arrives at the receiver and the received signal can be written in the so-called quadrature form s(t)=l(t) cos(cl~ct)-Q(t) sin(~)ct) where Cl)c is the radian frequency of the RF carrier.
In the "I", or inphase branch, the local oscillator is cos(cl~ct), 213, and in the "Q" or quadrature branch, the local oscillator is shifted by -90, 211. Each25 picks out its tr~n~mitted branch from the received signal according to si(t) = ( s(t) * cos(c)ct) } LPF ;
Sq(t) = { s(t) * sin((dct) } UF
30 where { } LPF indicates the bracketed quantity is taken through a low pass filter.
Working out the trigonometry, the response to a single ray in the I branch is l(t) * 1 W ~J ~U/ 1'1 / JU r~ l / u~u/
205~13 and similarly in the Q branch is -Q(t) * 2-When two rays are involved, it can be shown that on the I branch si(t) = 2( ) + 2 ) cos ( ~ ~ ) ~ Q2~ ~ sin ( - ~ ) 10 and similarly on the Q branch sq(t)=Q2() + 2) sin(~)- Q() cos(Q) where l'(t) and Q'(t) are the time varying parts of a second ray, and ~ is the phase 15 offset of the second ray's carrier relative to the first ray's carrier. The instantaneous power of the signal s(t) is given by ps(t) = si2(t) + Sq2(t) 20 which works out to be = 12(t) + Q2(t) + 1'2(t) +Q'2(t) + 2(1(t) l'(t) + Q(t) Q'(t) ) cos( ~ ) + 2(1(t) Q'(t) - Q(t) l'(t) ) sin( ~ ).
The last four terms represent rapid variations in the in~t;~nt~neous power. We would like to get only the first four terms, but there is no way to do this looking at the envelope of the sampled version of s(t) directly. The present invention shows how to get 12(t) + Q2(t) for each received ray, which is the power received due to 30 each ray. The sum of these powers, or of something proportional to them, gives a more stable and accurate estimate of signal strength than merely measuring the received signal's envelope.
WO 90/14730 2 0 5~ PCr/US90/02472 Figure 3 is an illustration of the process for deterrnining the signal strength indication according to the present invention. The process illustrated at 301 and 303 has just been described above.
Now, the channel impulse response is obtained by c~lcul~ting the complex S correlation function Rsr(z) between s(n) and r(n) 304. This is handled math~m~ti~-~lly by treating both s(n) and r(n) as complex qu~ntities The complexcorrelation is a function of the parameter z, representing the relative shift between s(n) and r(n). The complex correlation function is well known and is given by N
Rsl(z)= ~, s(i-z)r't(i) i=l where the r*(i) denotes the complex conjugate and the sum is taken over all non-zero samples of the locally stored reference, for each shift z. For those shifts z =
Zn where the correlation is a local maximum, one of the linearly combined rays has l S aligned with the local reference and the correlation at that point for that ray is given by Rsr(Zn) = a ~, s2(i) i = 1 = ~ S j2(i) + Sq2(i) i = 1 where s2(t) is defined, as is customary for complex numbers, as s(t) s*(t). This is proportional to the energy in one of the rays or echoes. The factor in the above equations takes into account the expected receiver output voltage level for a given input signal strength and scaling for the use of multiple discrete sarnples in the c~lsul~rion. Herein lies the essence of the technique. The process of calculating the complex correlation function ess~nti~lly reveals the location in time by knowing Zn for each ray or echo and, most importantly for the present invention, reveals its strength by the m:~gnitude of Rsr(Zn).
There may be several local peaks in the correlation function, and only those above a certain threshold are to be considered 305. When all of the local maxima, above a predeterrnined threshold, are found 306, the final step is to sum the ,,~ 1 , ~, .
r-_ 1/ U~YU/U~4 /,!
-V~ V 2~533l3 energies, or voltages squared, or absolute powers, whichever is computationally convenient 307. It is only important that the decision process which is to use the measurement knows which of the measures is being used, which can be included in the scaling factor . This then represents the fast RSSI measurement. Finally, the 5 samples from the next time slot are awaited 308.
When the rate of change of the impulse response is sufficiently slow, co,~ d to the repetition rate of the training sequence used for determining the channel impulse response, the calculated fast RSSI may be taken alone, or even as a predictor value of the strength of the next received data field. When the rate of 10 change of the impulse response is faster, it is appropriate to somehow average or filter the sequence of RSSI values. The process may employ infinite impulse response filtering, running or sliding averages to yield useable results.
It is to be further noted that this method is not limited to burst tr~ncmicsions, but is equally applicable to the reception of any type of transmission 15 or transmissions in which certain known sequences occur at predictable intervals.
It is also true that the technique in the present invention need not be limited to discrete-time, or sampled, processing. Essentially the same calculations would apply where the correlation is performed by a continuous time device, such as a Surface Acoustic Wave device.
In summary, there has been provided a method of rapid Received Signal Strength Indication (RSSI) of a time-dispersed signal having echoes, where the time dispersal represents a signifir~nt fraction or more of the transmitted symbol interval. It comprises producing a complex correlation function between the received signal and a known sequence stored in the receiver. The resulting correlation function is examined in regions above a threshold generated at each new measurement trial for local maxima, each of which is proportional to the energy,and, since the time scale of the signal is known, an individual echo's power. These energies, or powers, or even peak voltages are summed to give an RSSI
deL~l-llination during any given interval less dependent upon the velocity of the receiver than earlier methods. Furthermore, RSSI determinations of equivalent accuracy are obtained more rapidly, or more accurate RSSI determinations are obtained in a given time than simple envelope sampling methods.
W O 90/14730 2 0 5 ~3 1 3 P ~ /US90/02472 There is provided a method of rapid Received Signal Strength Tn~lic~tiQn (RSSI) of a time-dispersed signal having echoes (where the time dispersal represents a signifi~nt fraction or more of the tlAnc, . 1;1 led symbol interval). It comprises a sliding correlation of a received signal against a known sequence or via 5 channel sounding to obtain the time-dispersal function of the cv"~ ;c~tions channel upon which the signal is tr~ncmitted and a s~lmm~tion of the squares of the quadrature COI1J~Onent~ of the energies at relative maxima of the correlations (correlation peaks det~ ed with l~;Ç~lc.lce to a threshold level) to illle~,ldl~ the energy defined by the correlation function to de~el~ e the energy present among 10 the time-dispersed echoes utilizing the time-dispersal function. Thus, the energy present in the time-dispersed echoes is related to the received signal strength, the RSSI comprises a scalar of the energy present and the scalar comprises a function of the receiver's gain, RSSI dek".li.~tionS during any given interval are less dependent upon the ~mbul~t~ ry velocity of the receiver, and RSSI determinations of 15 equivalent accuracy are obtained more rapidly or more accurate RSSI
det~nin~rions are obtained per unit time.
While the preferred embodiment of the invention has been described and shown, it will be appreciated by those skilled in the art that yet other variations and 20 modifications of this invention may be implemented. These and all other variations and adaptations are expected to fall within the ambit of the appended claims.
Claims (10)
PROPERTY OF PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of controlling signal reception and processing in a receiver, the method employing received signal strength indications (RSSI's) of time-dispersed signals, the method comprising the steps of:
obtaining from a time-dispersed signal a characteristic time-dispersal function of the communications channel upon which the signal is transmitted;
determining the energy present in the time-dispersed signal from the time-dispersal function obtained; and employing that determination in controlling one of signal reception and processing in said receiver.
obtaining from a time-dispersed signal a characteristic time-dispersal function of the communications channel upon which the signal is transmitted;
determining the energy present in the time-dispersed signal from the time-dispersal function obtained; and employing that determination in controlling one of signal reception and processing in said receiver.
2. A method as claimed in claim 1 above, wherein obtaining the time-dispersal function comprises correlating the time-dispersed signal against a known sequence to produce a correlation function having local peaks.
3. A method as claimed in claim 2 above, wherein determining the energy comprises summing the energies of said local peaks having magnitudes greater than a threshold level.
4. A method as claimed in claim 2 above, wherein correlating comprises complexly correlating a quadrature time-dispersed signal against a quadrature reference signal to produce a complex correlation function having local peaks.
5. A method as claimed in claim 4 above, wherein determining the energy comprises summing the squares of said local peaks having a magnitude greater than a threshold level.
6. A method as claimed in claim 1 above, wherein the RSSI is employed to facilitate handoff decisions in a cellular radiotelephone system.
7. A method of controlling signal reception and processing in a receiver, the method employing received signal strength indications (RSSI's) of time-dispersed signals, the method comprising the steps of:
a sliding complex quadrature correlation of a received, time-dispersed signal against a known sequence to obtain a characteristic time-dispersal function of the communications channel upon which the signal is transmitted;
a summation of the squares of the quadrature components of the complex correlation at relative maxima of the correlation to determine the energy present at the relative maxima of such correlation; and employing that determination in the control of one of signal reception and processing in said receiver.
a sliding complex quadrature correlation of a received, time-dispersed signal against a known sequence to obtain a characteristic time-dispersal function of the communications channel upon which the signal is transmitted;
a summation of the squares of the quadrature components of the complex correlation at relative maxima of the correlation to determine the energy present at the relative maxima of such correlation; and employing that determination in the control of one of signal reception and processing in said receiver.
8. A method of controlling a device utilized in a cellular radiotelephone system, the method employing received signal strength indications (RSSI's) of time-dispersed signals, the method comprising the steps of:
obtaining from a time-dispersed signal a characteristic time-dispersal function of the communications channel upon which the signal is transmitted;
determining the energy present in the time-dispersed signal from the time-dispersal function obtained; and controlling, based on the determined energy present, said device utilized in said cellular radiotelephone system, whereby the determined energy present in the time-dispersed signal is related to a RSSI of the time-dispersed signal.
obtaining from a time-dispersed signal a characteristic time-dispersal function of the communications channel upon which the signal is transmitted;
determining the energy present in the time-dispersed signal from the time-dispersal function obtained; and controlling, based on the determined energy present, said device utilized in said cellular radiotelephone system, whereby the determined energy present in the time-dispersed signal is related to a RSSI of the time-dispersed signal.
9. The method as claimed in claim 8 above, wherein said step of controlling said device utilized in said cellular radiotelephone system further comprises the step of controlling a receiver utilized in a cellular radiotelephone system.
10. The method as claimed in claim 9 above, wherein said step of controlling a receiver utilized in said cellular radiotelephone system further comprises the step of controlling automatic gain control (AGC) in a receiver utilized in a cellular radiotelephone system.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US35832589A | 1989-05-26 | 1989-05-26 | |
| US358,325 | 1989-05-26 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2053313C true CA2053313C (en) | 1994-12-06 |
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ID=23409223
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002053313A Expired - Fee Related CA2053313C (en) | 1989-05-26 | 1990-05-02 | Rapid received signal strength indication |
Country Status (15)
| Country | Link |
|---|---|
| US (1) | US5740530A (en) |
| EP (1) | EP0399845B1 (en) |
| JP (1) | JP2789388B2 (en) |
| KR (1) | KR960009925B1 (en) |
| CN (1) | CN1018690B (en) |
| AR (1) | AR247636A1 (en) |
| AT (1) | ATE132676T1 (en) |
| AU (1) | AU635742B2 (en) |
| CA (1) | CA2053313C (en) |
| DE (1) | DE69024533T2 (en) |
| HK (1) | HK1000980A1 (en) |
| MX (1) | MX174520B (en) |
| PT (1) | PT94149B (en) |
| TR (1) | TR25382A (en) |
| WO (1) | WO1990014730A1 (en) |
Families Citing this family (19)
| Publication number | Priority date | Publication date | Assignee | Title |
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| KR890700492A (en) * | 1987-04-29 | 1989-04-25 | 윌리엄 큐밍 | Tipping and Lifting Device |
| JPH01191462A (en) * | 1988-01-27 | 1989-08-01 | Toshiba Corp | Semiconductor device |
| US5257401A (en) * | 1989-04-17 | 1993-10-26 | Telefonaktiebolaget L M Ericsson | Method of maintaining an established connection in a mobile radio system comprising both analog and digital radio channels |
| JP2988742B2 (en) * | 1991-05-20 | 1999-12-13 | 株式会社東芝 | Electric field strength detection circuit of wireless communication device |
| US5261119A (en) * | 1991-06-21 | 1993-11-09 | Motorola, Inc. | Signal level measuring system for a radio receiver |
| US5918184A (en) * | 1992-09-21 | 1999-06-29 | Lucent Technologies Inc. | Method and apparatus for detecting a supervisory audio tone |
| US6097957A (en) * | 1997-11-14 | 2000-08-01 | Motorola, Inc. | Radiotelephone service planning system and method for determining a best server for a communication connection |
| US6157820A (en) * | 1998-06-12 | 2000-12-05 | Ericsson Inc. | Pilot strength measurement and multipath delay searcher for CDMA receiver |
| JP4478340B2 (en) * | 2000-01-24 | 2010-06-09 | 株式会社日立国際電気 | Receiving machine |
| US20040047284A1 (en) * | 2002-03-13 | 2004-03-11 | Eidson Donald Brian | Transmit diversity framing structure for multipath channels |
| KR100459715B1 (en) * | 2002-08-09 | 2004-12-03 | 삼성전자주식회사 | High speed digital RSSI circuit |
| US7113760B1 (en) | 2003-04-29 | 2006-09-26 | Ami Semiconductor, Inc. | Direct conversion receiver for amplitude modulated signals using linear/log filtering |
| US7139546B1 (en) | 2003-04-29 | 2006-11-21 | Ami Semiconductor, Inc. | Up-conversion of a down-converted baseband signal in a direct conversion architecture without the baseband signal passing through active elements |
| US7006809B2 (en) * | 2003-05-06 | 2006-02-28 | Ami Semiconductor, Inc. | Adaptive diversity receiver architecture |
| US7197091B1 (en) | 2003-05-06 | 2007-03-27 | Ami Semiconductor, Inc. | Direct conversion receiver with direct current offset correction circuitry |
| GB2423449B (en) * | 2005-02-21 | 2009-10-07 | British Broadcasting Corp | Signal meter for digital systems |
| KR100896205B1 (en) * | 2006-02-20 | 2009-05-12 | 삼성전자주식회사 | Apparatus and method for reporting cahnnel state on providing transparent relay service to the mobile station in a multi-hop relay broadband wireless access communication system |
| EP2259084A1 (en) | 2009-06-03 | 2010-12-08 | Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. | Method and wireless device for detecting a movement |
| KR101746008B1 (en) * | 2009-09-07 | 2017-06-27 | 프라운호퍼 게젤샤프트 쭈르 푀르데룽 데어 안겐반텐 포르슝 에. 베. | Method and radio device for detecting a movement |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3766477A (en) * | 1971-12-20 | 1973-10-16 | Sperry Rand Corp | Spread spectrum, linear fm communications system |
| US3815028A (en) * | 1972-08-09 | 1974-06-04 | Itt | Maximum-likelihood detection system |
| JPS56109023A (en) * | 1980-01-31 | 1981-08-29 | Nec Corp | Channel selection system |
| WO1987006072A1 (en) * | 1986-04-03 | 1987-10-08 | Motorola, Inc. | Fm receiver with noise suppression during rayleigh faded received signals |
| US4718081A (en) * | 1986-11-13 | 1988-01-05 | General Electric Company | Method and apparatus for reducing handoff errors in a cellular radio telephone communications system |
| SE457399B (en) * | 1987-04-23 | 1988-12-19 | Ericsson Telefon Ab L M | DEVICE IN A COHERENT MOBILE PHONE RECEIVER FOR REDUCING BIT ERRORS |
| US5115514A (en) * | 1987-08-03 | 1992-05-19 | Orion Industries, Inc. | Measuring and controlling signal feedback between the transmit and receive antennas of a communications booster |
| US4829543A (en) * | 1987-12-04 | 1989-05-09 | Motorola, Inc. | Phase-coherent TDMA quadrature receiver for multipath fading channels |
| US4881028A (en) * | 1988-06-13 | 1989-11-14 | Bright James A | Fault detector |
| US4918437A (en) * | 1988-10-13 | 1990-04-17 | Motorola, Inc. | High data rate simulcast communication system |
-
1990
- 1990-05-02 AU AU56430/90A patent/AU635742B2/en not_active Ceased
- 1990-05-02 CA CA002053313A patent/CA2053313C/en not_active Expired - Fee Related
- 1990-05-02 KR KR1019910701641A patent/KR960009925B1/en not_active IP Right Cessation
- 1990-05-02 WO PCT/US1990/002472 patent/WO1990014730A1/en active Application Filing
- 1990-05-02 JP JP2507573A patent/JP2789388B2/en not_active Expired - Fee Related
- 1990-05-11 AR AR90316837A patent/AR247636A1/en active
- 1990-05-17 MX MX020750A patent/MX174520B/en unknown
- 1990-05-24 TR TR90/0547A patent/TR25382A/en unknown
- 1990-05-24 PT PT94149A patent/PT94149B/en not_active IP Right Cessation
- 1990-05-25 EP EP90305752A patent/EP0399845B1/en not_active Expired - Lifetime
- 1990-05-25 CN CN90103799A patent/CN1018690B/en not_active Expired
- 1990-05-25 DE DE69024533T patent/DE69024533T2/en not_active Expired - Fee Related
- 1990-05-25 AT AT90305752T patent/ATE132676T1/en active
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1992
- 1992-05-26 US US07/890,320 patent/US5740530A/en not_active Expired - Fee Related
-
1997
- 1997-12-27 HK HK97102628A patent/HK1000980A1/en not_active IP Right Cessation
Also Published As
| Publication number | Publication date |
|---|---|
| EP0399845A3 (en) | 1992-08-05 |
| AR247636A1 (en) | 1995-01-31 |
| CN1047594A (en) | 1990-12-05 |
| KR920702136A (en) | 1992-08-12 |
| US5740530A (en) | 1998-04-14 |
| KR960009925B1 (en) | 1996-07-24 |
| HK1000980A1 (en) | 1998-05-15 |
| JPH04505527A (en) | 1992-09-24 |
| EP0399845A2 (en) | 1990-11-28 |
| AU635742B2 (en) | 1993-04-01 |
| DE69024533T2 (en) | 1996-08-01 |
| WO1990014730A1 (en) | 1990-11-29 |
| PT94149B (en) | 1996-12-31 |
| DE69024533D1 (en) | 1996-02-15 |
| JP2789388B2 (en) | 1998-08-20 |
| ATE132676T1 (en) | 1996-01-15 |
| EP0399845B1 (en) | 1996-01-03 |
| MX174520B (en) | 1994-05-23 |
| AU5643090A (en) | 1990-12-18 |
| PT94149A (en) | 1992-02-28 |
| CN1018690B (en) | 1992-10-14 |
| TR25382A (en) | 1993-03-01 |
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