|Publication number||USRE43494 E1|
|Application number||US 11/981,508|
|Publication date||Jun 26, 2012|
|Filing date||Oct 31, 2007|
|Priority date||Dec 2, 1999|
|Also published as||US6961363|
|Publication number||11981508, 981508, US RE43494 E1, US RE43494E1, US-E1-RE43494, USRE43494 E1, USRE43494E1|
|Inventors||Dinesh Kashinath Anvekar, Manika Kapoor|
|Original Assignee||Wistron Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (27), Classifications (7), Legal Events (1)|
|External Links: USPTO, USPTO Assignment, Espacenet|
1. Field of the Invention
The present invention generally relates to time division duplex indoor wireless communication networks and, more particularly, to a frequency look-ahead, packet/slave scheduling scheme and master/slave link characterization using a link state history table in the master unit in order to account for channel and system characteristics.
2. Background Description
Bluetooth™ is a computing and telecommunications industry specification that describes how mobile phones, computers, personal digital assistants (PDAs), and other devices can interconnect using a short range wireless connection. Each device is equipped with a microchip transceiver that transmits and receives in the frequency band of 2.45 GHz. Each device will have a unique 48-bit address from the IEEE 802 standard. Connections are one-to-one, and the maximum range is ten meters. Data can be exchanged at a rate of one megabits per second (Mbs) and up to two Mbs in the second generation of the technology. The five founding companies of the Bluetooth™ Special Interest Group (SIG) are Ericsson, IBM, Intel, Nokia, and Toshiba. Additional information may be had by reference to the Web site www.bluetooth.com and an article by Andrew Seybold entitled “Bluetooth Technology: The Convergence of Communications and Computing”, reprinted from Andrew Seybold's Outlook, May 1998, on the World Wide Web at www.gsmdata.com/artblue.htm.
Indoor wireless networks based on standards such as Bluetooth™ use frequency hopping to combat the problem of interference from sources such as microwave ovens and cordless telephones, which also use frequencies in the same band. In practical environments, in addition to active interfering sources, there can also be objects such as water fountains and racks of bottles with water content which absorb much of the radiation in the 2.45 GHz band and obstruct communication between master and slave units in the vicinity. Therefore, a master unit needs to detect such problems in communication and take necessary actions to prevent loss of packets during the periods of interference.
In the current Bluetooth™ standard, due to frequency hopping, the carrier frequency used in consecutive time slots is a different one of several different frequencies within the 2.45 GHz band of frequencies. Therefore, an interference in sub-bands centered around one of these frequencies will only affect communication during that time-slot in which the frequency sub-band is used. Further, in the Bluetooth™ standard, a packet can occupy one, three or five time slots, and in the case of multiple size packets, the same frequency as fixed for the first time slot is used. Because of this, it is possible to mask the effect of an interference by transmitting a packet of appropriate size. For example, if it is known that there is high chance of interference in one of the second through fifth time slots, and very low probability of the first time slot being bad, it is possible to skip the frequencies corresponding to second through fifth time slots by transmitting a five time-slot packet instead of one or three time-slot packets.
The characterization of link between any slave unit and the master can be done by the master unit based on the receipt or otherwise of acknowledgments received from the slave. Alternatively, all the slaves can record the number of times they detect good packet headers sent by the master to any slave. This information can be transmitted from the slaves to the master at periodic intervals of time. The master can use this information along with frequency look-ahead to determine the next slave for communication and also the appropriate packet size.
Several methods and schemes to combat the effect of interference in cellular wireless communication systems have been proposed. For example, a Fast Fourier Transform (FFT) based adaptive interference cancellation method has been proposed in U.S. Pat. No. 5,612,978 to Blanchard et al for “Method and Apparatus for Real-time Adaptive Interference Cancellation in Dynamic Environments”. The method allows relatively fast changes in the interference environment to be tracked and rejected.
In the U.S. Pat. No. 5,541,954 to Emi et al. for “Frequency Hopping Communication Method and Apparatus Changing a Hopping Frequency as a Result of a Counted Number of Errors”, a technique of changing the hopping frequency based on the number of errors encountered on a given frequency is proposed. This scheme is suitable in systems where the hopping frequency can be changed at any time.
Another related invention can be found in U.S. Pat. No. 5,323,447 to Gillis et al. for “Apparatus and Method for Modifying a Frequency Hopping Sequence of a Cordless Telephone Operating in a Frequency Hopping Domain”, in which the hopping sequence is modified for a cordless telephone. Substitute alternative communication channels are identified and then substituted for those communication channels experiencing interference. This is done without disrupting the communications between the handset unit and its associated base unit.
In systems in which the hopping frequency is fixed depending on the frequency hopping sequence corresponding any given pico-cell, there is need for other methods to combat the problem of interference. In U.S. Pat. No. 5,570,352 to Pöyhönen for “Digital Cellular Network/System with Mobile Stations Communicating with Base Stations Using Frequency-Hopping and Having Enhanced Effect of Interference Diversity”, frequency hopping utilizing interference diversity is presented.
In U.S. Pat. No. 5,659,879 to Dupuy for “Method of Covering Shadow Areas in a Cellular Mobile Radio and Radio Booster for Implementing this Method”, the problem of covering shadow areas by using radio boosters is addressed. To cover the shadow areas, a radio signal received by a radio booster from the base transceiver station on a basic frequency is re-transmitted to a mobile station on a translated frequency different from the basic frequency and associated with the latter by a translation law.
It is therefore an object of the present invention to provide a method to combat the effect of interfering sources in frequency hopping based indoor wireless networks.
According to the invention, a method of combating the problem of interference from external sources and shadowing objects in indoor pico-cellular wireless networks utilizes frequency look-ahead and short-term history about channel state with reference to different mobile units within a pico-cell. The method monitors the states of master-slave wireless communication links through values recorded in link counters. Based on the recorded values, an appropriate slave is scheduled and the suitable packet size chosen to overcome the effect of interfering sources if any in the pico-cell.
The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of preferred embodiments of the invention with reference to the drawings, in which:
In the current Bluetooth™ indoor wireless networking standard, the master unit and slave units in any given pico-cell use the same frequency hopping sequence to communicate with each other. In each unit, there is a frequency selection unit (FSU) which produces a frequency index for one of several possible frequencies. The inputs to the frequency selection unit are the address bits of the pico-cell master and the current clock bits in each unit. When a master or slave has to transmit or receive information over the wireless medium, the frequency index is used to synthesize the appropriate frequency in the frequency hopping sequence of the pico-cell. As all the units in a pico-cell use the same address of the master for determining the frequency in any slot, the same hopping sequence is used by all of them. However, as the clocks of the different units are not synchronized, each unit uses a corresponding offset with reference to the clock of the master.
Each time slot in the Bluetooth™ standard occupies 625 micro-seconds. The master always begins transmission on an even time slot, and a slave always begins transmission on an odd time slot. A slave unit can transmit to the master on an up-link slot only if the master had transmitted to that slave on the preceding down-link slot. The frequencies used on adjacent time slots by master or slave units are chosen to be different based on the frequency hopping sequence for the pico-cell.
A schematic representation of hopping frequencies used during consecutive time slots is shown in
A preferred embodiment of the invention comprises frequency look-ahead to know at any point of time the frequencies that will be used during the future time slots by the master and slave units. A frequency selection unit FSU) as shown in block diagram form in
In the present invention, an additional FSU is used to find the frequencies corresponding to future time slots, as shown in block diagram form in
The process is shown in the flowchart shown in
While look-ahead for sequential time slots is implied in this flowchart, the master can in general look-ahead for future time slots in any arbitrary order by just providing the clock bits corresponding to the required time slots.
Another feature of the invention is maintaining the information about link state with reference to each frequency of operation and each active slave. In the present invention, this information is contained in a counter corresponding to each slave-frequency pair. The master unit maintains a table of counters as shown in Table 1. In Table 1, “C(i,j)” represents the counter for link state of frequency fj for slave i.
Link State History Table of Counters
. . .
. . .
. . .
. . .
. . .
. . .
. . .
. . .
Here, seven rows corresponding to seven slaves have been shown because, in each pico-cell a maximum of seven active slaves are allowed at any time. However, the number of rows in the table during any period of time is equal to the number of active slaves in the pico-cell during that period. Also, the number of columns in the table is equal to the number of hopping frequencies used in the system. In the current standard, the number of hopping frequencies is 79 (or 23 when operating in some countries of the world).
Initially, all the counters are reset to zero, and during transmissions the counter values are changed depending on the success or failure of a packet transmission between the master and a slave. Counter C(i,j) and C(i,k) corresponding to slave “i” and frequencies fj and fk are updated as shown in the flowchart of
As shown in
At the time of deciding about transmission to a slave “i”, if counter C(i,j) value is less than or equal to a threshold value TTRANSMIT, a packet transmission is made to the slave. However, if counter C(i,j) value is greater than TTRANSMIT, then no transmission is sent to slave “i” at frequency fj during the scheduled time slot for the slave, and the counter C(i,j) is incremented by one. After the increment operation, if value of counter C(i,j) is greater than a threshold TRESET, the counter is reset to zero. The counters C(i,j) start with an initial value of zero, and with progressive communications between the master and the slaves, assume different values depending on the relative positions of the mobile units and the nature and location of interfering sources. The two counter thresholds TTRANSMIT and TRESET are useful to distinguish between transient and persistent errors on a link. If a master-slave link on a certain frequency is affected with very short-term interference, then before the threshold TTRANSMIT is exceeded, the master gets an acknowledgment for one of its packets sent to the slave, and the corresponding counter is reset. However, if the inference is persistent, with repeated transmission failures, the threshold TTRANSMIT is soon reached and no further attempts to transmit to the slave at that frequency are made for (TRESET−TTRANSMIT) scheduled time slots for the slave. Thus, for each master-slave link TTRANSMIT consecutive attempts are allowed and thereafter (TRESET−TTRANSMIT) period of back-off time is provided. During this back-off period, other slaves can be scheduled for transmission. The values of TTRANSMIT and TRESET are fixed depending on the desired number of transmission attempts at any frequency and the required back-off time.
Another feature of the invention comprises the determination of the next slave for transmission and the packet size to be used. The procedures for this are embodied in flowcharts shown in
The first processing loop shown in
After look-ahead of the transmission frequency fj, the master selects a slave for transmissions as per the flowchart embodied in
Packet Size Selection
Continuing the description of the flow chart, the process next goes to
As indicated in the flowcharts of
Another embodiment of the invention is an alternative method of maintaining short term link state history which involves transfer of link state information between the different slaves and the master. Every active slave is required to monitor the transmissions from the master in expectation of a packet being sent to it. If a packet is not meant for a slave, then it can go to sleep state until the next packet transmission from the master. In this alternative method of link state history maintenance, slave “i” maintains a “goodness” counter GC(i,j) for each frequency fj. The counter GC(i,j) is incremented every time the slave “i” successfully receives a valid packet header from a master transmission on frequency fj. However, if an error in receiving packet header is detected the goodness counters are unaltered. An error in receiving a master's packet header can occur if there is interference in the master's transmission frequency or if the even numbered time slot is being used by another slave to transmit a three or five sized packet. The counter is also incremented when a packet transmitted by the slave “i” on frequency fj is successfully acknowledged by the master.
In this method, in response to periodic requests by the master, the active slaves in the pico-cell send the values of the goodness counters to the master. After successfully transmitting the goodness counter values, each slave resets its goodness counters to zero for link state monitoring during the next period of communication. The values of the counters received from all the slaves are compiled by the master to form a link state history table of goodness counters as shown in Table 2 for use during the next scheduling period. In Table 2, GC(i,j) is the goodness counter for the link state of frequency fj for slave i.
Link State History Table of Goodness Counters
. . .
. . .
. . .
. . .
. . .
. . .
. . .
. . .
As the number of bits in the counter is to be limited to minimize the overhead in transmitting the information to the master, the counters GC(i,j) are allowed to count up to the maximum value and stay there until reset. The value of any counter GC(i,j) received by the master indicates the relative goodness of the link between the master and slave “i” on the frequency fj; the higher the count value, the better the link. In this method, as the slaves listen to all the master transmissions and record the successful transmissions, the monitoring of interference on different frequencies occurs more frequently, and therefore results in better characterization of the link states.
Slave Selection Based on Goodness Counters
A flowchart for selection of a slave for packet transmission is shown in
In the process shown in
Packet Size Selection Based on Goodness Counters
As shown in
Second Level Look-Ahead
Generally, a single time slot packet transmission occurs within the first time period Tt, out of the total time slot period of Tts, as shown in
As shown in
While the invention has been described in terms of preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.
|Cited Patent||Filing date||Publication date||Applicant||Title|
|US5276686 *||Oct 17, 1991||Jan 4, 1994||Kabushiki Kaisha Toshiba||Mobile radio communication system having mobile base and portable devices as a mobile station|
|US5394433 *||Apr 22, 1993||Feb 28, 1995||International Business Machines Corporation||Frequency hopping pattern assignment and control in multiple autonomous collocated radio networks|
|US5425031 *||Nov 29, 1993||Jun 13, 1995||Nec Corporation||Mobile radio communication system|
|US5455959 *||Aug 19, 1994||Oct 3, 1995||Alcatel Network Systems, Inc.||System for collecting from masters information independently collected from associated slaves in shelves of a telecommunications terminal|
|US5588005 *||Jun 7, 1995||Dec 24, 1996||General Electric Company||Protocol and mechanism for primary and mutter mode communication for asset tracking|
|US5781582 *||May 4, 1995||Jul 14, 1998||Interwave Communications International Ltd.||Frequency agile transceiver with multiple frequency synthesizers per transceiver|
|US5914947 *||Apr 5, 1995||Jun 22, 1999||Kabushiki Kaisha Toshiba||Mobile radio communication system which allocates spread codes mobiles which are able to determine the distance between a mobile and a base station|
|US5936211 *||Dec 17, 1997||Aug 10, 1999||Lg Industrial Systems Co., Ltd||Elevator control system|
|US6014406 *||Apr 24, 1996||Jan 11, 2000||Hitachi, Ltd.||Frequency-hopped wireless communication system and mobile wireless terminal|
|US6028853 *||Jun 6, 1997||Feb 22, 2000||Telefonaktiebolaget Lm Ericsson||Method and arrangement for radio communication|
|US6031864 *||Sep 16, 1997||Feb 29, 2000||International Business Machines Corporation||Method and system for controlling the time occupation of signalling frequencies in a frequency hopping system|
|US6094426 *||Feb 22, 1999||Jul 25, 2000||Nokia Mobile Phones Limited||Method for scheduling packet data transmission|
|US6212221 *||Mar 11, 1998||Apr 3, 2001||Brother Kogyo Kabushiki Kaisha||Communication apparatus|
|US6223048 *||Dec 15, 1998||Apr 24, 2001||Alcatel||Method of generating a frequency-hopping sequence for radio communication, as well as radio facility and radio communication system therefor|
|US6256356 *||Sep 28, 1998||Jul 3, 2001||Sony Corporation||Resource assigning method, communication resource assigning method, and base station and terminal unit|
|US6295310 *||Jun 4, 1998||Sep 25, 2001||Mitsubishi Denki Kabushiki Kaisha||Mobile communication system|
|US6347228 *||Jun 8, 1999||Feb 12, 2002||Motorola, Inc.||Location apparatus and method in a mobile telecommunications system|
|US6351461 *||Dec 24, 1997||Feb 26, 2002||Sony Corporation||Communication method, base station and terminal apparatus|
|US6424820 *||Apr 2, 1999||Jul 23, 2002||Interval Research Corporation||Inductively coupled wireless system and method|
|US6484268 *||Feb 28, 2001||Nov 19, 2002||Fujitsu Limited||Signal transmission system having a timing adjustment circuit|
|US6501942 *||Oct 29, 1999||Dec 31, 2002||Qualcomm, Incorporated||In-building radio-frequency coverage|
|US6532228 *||Sep 24, 1999||Mar 11, 2003||Nokia Mobile Phones Limited||Open loop receiver|
|US6570857 *||Dec 15, 1998||May 27, 2003||Telefonaktiebolaget L M Ericsson||Central multiple access control for frequency hopping radio networks|
|US6587444 *||Nov 12, 1998||Jul 1, 2003||Ericsson Inc.||Fixed frequency-time division duplex in radio communications systems|
|US6603747 *||Nov 13, 1998||Aug 5, 2003||Nec Corporation||Communication control method and communication control apparatus|
|US6683886 *||Oct 19, 1999||Jan 27, 2004||Koninklijke Philips Electronics N.V.||Bluetooth communication units, wireless communication systems, wireless communication devices, bluetooth communications methods, and wireless communication methods|
|US20010040878 *||Jul 19, 2001||Nov 15, 2001||Schilling Donald L.||Channel sounding for a spread-spectrum signal|
|U.S. Classification||375/132, 375/141|
|International Classification||H04B1/713, H04B1/69|
|Cooperative Classification||H04B1/715, H04B2001/7154|