Range extension within a communication system

Receivers (201, 202) within a base station (101) receive a transmission from a remote unit (114). The received signal is delayed a first amount by first delay circuitry (203), and a second amount by second delay circuitry (204). During despreading, a delayed input signal is despread with a Pseudo-Random (PN) code. The system time utilized by the PN generators (213, 214) is delayed a third time period by third and fourth delaying circuitry (215, 216).

FIELD OF THE INVENTION 
 The present invention relates generally to communication systems and, in 
 particular, to extending the range of a base site in a cellular 
 communication system. 
 BACKGROUND OF THE INVENTION 
 In current Code Division, Multiple Access (CDMA) communication systems, 
 receivers (channel elements) restrict a search and demodulation window to 
 512 chips. More particularly, a remote unit accessing a CDMA communication
 system can have a round-trip delay of no more than 416 micro seconds (512 
 chips), or equivalently, a maximum distance of 62 kilometers (km) from the
 base station. Remote units with a larger delay than 512 chips will not be 
 within a base station's search window, and will not be acquired by the 
 base station. 
 Although the above restriction on round-trip delay is adequate for most 
 urban areas, there exists locations where a maximum cell size of greater 
 than 62 km is desired. For example, along coastal areas, or within very 
 sparsely populated locations, it may be economically unfeasible to have 
 small cell sizes. Therefore a need exists for a base station that has an 
 extended cell size. Additionally, it would be beneficial if existing CDMA 
 equipment can be inexpensively modified to provide such coverage.

DETAILED DESCRIPTION OF THE DRAWINGS 
 To address the need for a base station having an extended cell size, a 
 method and apparatus for extending the cell size of a base station is 
 provided. Receivers within a base station receive a transmission from a 
 remote unit. The received signal is delayed a first amount by first delay 
 circuitry, and a second amount by second delay circuitry. During 
 despreading, a delayed input signal is despread with a Pseudo-Random (PN) 
 code. The system time utilized by the PN generators is delayed a third 
 time period by third and fourth delaying circuitry. As a result of 
 delaying system time, the PN sequence utilized to despread the delayed 
 signals will not repeat on the even second of system time, but will begin 
 repeating a time period after every even second of system time. By 
 delaying the signal input into a receiver, as well as delaying system time
 utilized by the PN generator, allows base stations to receiving 
 transmissions from remote units outside the normal 512 chip window. 
 Additionally, existing base stations can be inexpensively modified in 
 accordance with the preferred embodiment by the addition of delay 
 circuitry as described above. 
 The present invention encompasses an apparatus for range extension within a
 communication system. The apparatus comprises a first receiver comprising 
 a first input signal delay having a received signal as an input and 
 outputting the received signal delayed a first time period. The receiver 
 additionally comprises a first despreader having the received signal 
 delayed the first time period as an input and outputting a first despread 
 signal, wherein the first despreader utilizes a spreading code that is 
 delayed a second time period with respect to system time. 
 The present invention additionally encompasses a method for range extension
 within a communication system. The method comprises the steps of receiving
 a first signal and delaying the first signal a first time period to 
 produce a first delayed signal. The delayed signal is despread utilizing a
 spreading code that has been delayed a second time period in order to 
 receive signals transmitted within a first range. The method additionally 
 comprises the steps of receiving a second signal and delaying the second 
 signal a third time period to produce a second delayed signal. The second 
 delayed signal is despread utilizing the spreading code that has been 
 delayed the second time period in order to receive signals transmitted 
 within a second range. 
 The present invention additionally encompasses a method for range extension
 within a Code Division, Multiple Access (CDMA) communication system. The 
 method comprises the steps of receiving a first signal transmitted from a 
 remote unit and delaying the first signal a first time period to produce a
 first delayed signal. In this embodiment of the invention the first time 
 period is equal to a time period selected from the group consisting of 0, 
 N, 2N, . . . , (K-1)N, where N is a maximum search and demodulation range 
 of the second receiver and K is the maximum round-trip delay divided by N,
 rounded up to a nearest integer. The first delayed signal is despread 
 utilizing a Pseudo-Random (PN) code that has been delayed a second time 
 period in order to receive signals transmitted within a first range. The 
 method also comprises the steps of receiving a second signal transmitted 
 from the remote unit and delaying the second signal a third time period to
 produce a second delayed signal. In this embodiment of the present 
 invention the third time period is equal to a time period selected from 
 the group consisting of 0, N, 2N, . . . , (K-1)N. Finally, the second 
 delayed signal is despread utilizing the PN code that has been delayed the
 second time period in order to receive signals transmitted within a second
 range. 
 Turning now to the drawings, where like numerals designate like components,
 FIG. 1 is a block diagram of communication system 100 in accordance with 
 the preferred embodiment of the present invention. In the preferred 
 embodiment of the present invention, communication system 100 utilizes a 
 CDMA system protocol as described in Cellular System Remote unit-Base 
 Station Compatibility Standard of the Electronic Industry 
 Association/Telecommunications Industry Association Interim Standard 95-B 
 (TIA/EIA/IS-95B), which is incorporated by reference herein. (EIA/TIA can 
 be contacted at 2001 Pennsylvania Ave. NW Washington D.C. 20006). In 
 alternate embodiments communication system 100 may utilize other analog or
 digital cellular communication system protocols such as, but not limited 
 to, the Narrow band Advanced Mobile Phone Service (NAMPS) protocol, the 
 Advanced Mobile Phone Service (AMPS) protocol, the Global System for 
 Mobile Communications (GSM) protocol, the Personal Digital Cellular (PDC) 
 protocol, or the United States Digital Cellular (USDC) protocol. 
 Communication system 100 includes sectorized cell 115, and base station 
 101 which is suitably coupled to antennas 108-113. Although not shown, one
 of ordinary skill in the art will recognize that base station 101 is 
 additionally coupled to necessary infrastructure equipment such as 
 Centralized Base Station Controllers (CBSCs), Mobile Switching Centers 
 (MSCs), and the like. 
 In the preferred embodiment of the present invention maximum cell range is 
 increased by increasing the range for specified sectors 102-107 only. 
 Thus, in the preferred embodiment of the present invention each sector 
 102-107 of cell site 115 has a maximum and minimum operational range. A 
 first sector (e.g., sector 104) may receive remote units within a first 
 region (e.g., 0-62 km from base station 101), while an adjacent sector 
 (e.g., sector 105) will receive only remote units within a second region 
 (between 62 km and 124 km from base station 101). Additionally, as will be
 described below, the maximum cell radius of cell 115 is increased without 
 requiring a redesign of the existing infrastructure equipment's receivers.
 FIG. 2 is a block diagram of base station 101 in accordance with the 
 preferred embodiment of the present invention. As shown base station 101 
 comprises first receiver 201 and second receiver 202. Although only two 
 receivers are shown, one of ordinary skill in the art will recognize that 
 typical base stations comprise many receivers. For example, the Motorola 
 J-CDMA Base Station (SC4840) contains twelve channel cards with 24 
 receivers residing on each card, resulting in a total of 288 receivers. 
 FIG. 2 additionally shows receivers 201 and 202 having a single antenna 
 port with antennas 111 and 112 as inputs, however, one of ordinary skill 
 in the art will recognize that multiple antennas may be input into 
 receivers 201 and 202 in order to gain diversity benefits. In the 
 preferred embodiment of the present invention receiver 201 and receiver 
 202 have input antennas (111 and 112, respectively) originating from 
 differing sectors (104 and 105, respectively). 
 As shown, base station 101 comprises input signal delay circuitry 203-206, 
 and each receiver comprises system time delay circuitry 215, 216, 
 despreading circuitry 207, 208, demodulating circuitry 209, 210, decoding 
 circuitry 211, 212, and PN generators 213, 214. Base station 101 
 additionally comprises clock 217 to provide system time, and controller 
 219. In the preferred embodiment of the present invention delay circuitry 
 203-206 is a standard digital delay line, and delay circuitry 215-216 is a
 counter which produces a delayed version of the original system reference 
 pulse. Both of these delay circuits serve to delay signals input into the 
 circuitry for a finite period of time. 
 Prior to describing operation of base station 101, the following text and 
 equations are provided to show derivation of the time delays utilized by 
 delay circuitry 203-206 and 215-216. 
 Assume that N is the maximum search and demodulation range of a receiver, 
 and K is the number of search windows that are necessary for covering the 
 desired range. K is equal to the maximum round-trip delay divided by N, 
 rounded up to the nearest integer (i.e., K=Round_up((maxRTD/N)). For 
 example, if a maximum search and demodulation range of a receiver is 512 
 chips (62 km), and it is desired that base station 101 be able to receive 
 calls having delays of 826 chips (100 km), then N=512 and K=2 
 (Round_up(826/512)). In the preferred embodiment of the present invention 
 system time delay circuitry 215 and 216 delay system reference time by a 
 time period equal to (K-1)N. 
 In the preferred embodiment of the present invention signals input into 
 receivers 201-202 are delayed for a period of time prior to being 
 despread. The period of time that a particular signal is delayed is 
 dependent upon a particular range that the receiver wishes to cover. In 
 the preferred embodiment of the present invention these delays are integer
 multiples of N, up to (K-1)N (i.e., 0, N, 2N, . . . , (K-1)N). Using the 
 above example, with N=512 and K=2, there exist two distinct input signal 
 delays utilized by base station 101 (0 and 512 chips). Therefore, when 
 K=2, system time is delayed by N, and receivers having no delay perceive a
 signal transmitted with an 826 chip offset, as being transmitted with a 
 326 chip offset. In other words, actual PN offsets of (K-1)N to KN appear 
 to be offset only 0 to N for the non-delayed baseband input. 
 It should be noted that receivers having delays of jN receive remote units 
 having PN offset ranges of (NK-N-jN) to (NK-jN). Therefore, any remote 
 unit with a range between (NK-N-jN) and (NK-jN) will be received by the 
 receiver delayed by jN, and will not be received by receivers having 
 different input signal delays since the signal will lie outside of the 
 receiver's search window. This is illustrated in Table 1 for N=512, K=3, 
 and system time delayed 1024 chips. 
 TABLE 1 
 Illustration of perceived chip offsets at varying distances 
 from a base station. 
 Perceived offset Perceived offset Perceived offset 
 for remote unit for remote unit for remote unit 
 transmitting transmitting be- transmitting be- 
 Input between 0-62 km tween 63-124 km tween 125-186 km 
 signal (0-511 chip (512-1023 (1024-1535 
 delay offset) chip offset) chip offset) 
 0 chips -1024 to -513 chips -512 to -1 chips 0 to 511 chips 
 512 chips -512 to -1 chips 0 to 511 chips 512 to 1023 chips 
 1024 chips 0 to 511 chips 512 to 1023 chips 1024 to 1535 chips 
 As illustrated in Table 1, when system time is delayed 1024 chips, 
 receivers having input signal delays of zero chips will perceive remote 
 units between 125 and 186 km as having chip offsets of between 0 and 511. 
 Likewise, receivers having input signal delays of 512 chips, will perceive
 remote units between 63 and 124 km as having chip offsets between 0 and 
 511. Finally, receivers having input signal delays of 1024 chips will 
 perceive remote units between 0 and 62 km as having chip offsets between 0
 and 511. All receivers will be unable to receive remote units whose 
 transmissions are perceived to be outside the 0 to 511 chip window. 
 The above-described base station is capable of receiving transmissions from
 remote units outside the normal 512 chip window. In fact, the 
 above-described base station can receive transmissions from remote units 
 that are delayed up to KN chips. Additionally, existing base stations can 
 be inexpensively modified in accordance with the preferred embodiment by 
 the addition of delay circuitry as described above. 
 FIG. 3 is a flow chart showing operation of the base station of FIG. 1 in 
 accordance with the preferred embodiment of the present invention. The 
 logic flow begins at step 300 where receivers 201-202 determine an antenna
 and delay window that it chooses to demodulate (e.g., antenna 111 with 
 first delay 203, and antenna 112 with second delay 204). In particular, in
 the preferred embodiment of the present invention each receiver may choose
 to demodulate signals from any antenna coupled to base station 101. 
 Additionally, each antenna has multiple signal delays that may be chosen 
 by receivers 201-202, depending upon an amount of range extension desired 
 for base station 101. In the preferred embodiment of the present invention
 multiplexers within each receiver 201-202 choose an antenna and a 
 particular delay from all possibilities of antenna/delay combinations. For
 purposes of this example it is assumed that receiver 201 chooses antenna 
 111, having delay 203 as an input, and receiver 202 chooses antenna 112, 
 having delay 204 as an input. 
 Continuing, at step 301 receivers 201 and 202 receive a transmission from 
 remote unit 114. At step 303 the received signal is delayed a first amount
 by circuitry 203, and a second amount by circuitry 204. As discussed 
 above, these delays are integer multiples of N, up to (K-1)N (i.e., 0, N, 
 2N, . . . , (K-1)N). Additionally, delay circuitry 203 and 204 may exist 
 after demodulating the received signal into its baseband components, or 
 may simply serve to delay the received radio-frequency (RF) signal prior 
 to demodulation. 
 At step 305 the delayed signals are output from delay circuitry 203 and 
 204, and enter despreaders 207 and 208 respectively. At step 307 
 despreaders 207 and 208 utilize standard CDMA despreading techniques to 
 despread the delayed signals. More particularly, during despreading, the 
 delayed input signal is despread with a Pseudo-Random (PN) code. The PN 
 code is a 32,768 bit sequence that repeats exactly 75 times every 2 
 seconds with a chip rate of 1.2288 MegaChips per second. In standard CDMA 
 systems the PN sequence is synchronized to repeat on every even second of 
 system time. Although in the preferred embodiment of the present invention
 the spreading code utilized is a PN code, one of ordinary skill in the art
 will recognize that other spreading codes may be utilized as well. At step
 307 the system time utilized by PN generators 213 and 214 is delayed a 
 third time period equal to (K-1)N chips by circuitry 215 and 216, 
 respectively. As a result of delaying system time, the PN sequence 
 utilized to despread the delayed signals will not repeat on the even 
 second of system time, but will begin repeating (K-1)N chips after every 
 even second of system time. 
 Finally, at step 309 the despread data is demodulated by demodulator 209 
 and decoded by decoder 211. In the preferred embodiment of the present 
 invention despreading and demodulation operations are standard CDMA 
 demodulating/decoding operations described in detail in IS-95B. 
 As described above, receivers having input signal delays of jN receive 
 remote units having PN offset ranges of (NK-N-jN) to (NK-jN) when system 
 time is delayed by (K-1)N. Therefore, any remote unit with a range between
 (NK-N-jN) and (NK-jN) will be received by the receiver delayed by jN, and 
 will not be received by receivers having different input signal delays 
 since the signal will lie outside of the receiver's search window. The 
 result is a base station capable of receiving transmissions from remote 
 units outside the normal operating window. Additionally, the 
 above-described base station can receive transmissions from remote units 
 that are delayed up to KN chips with receivers having input signal delays 
 of jN. Additionally, existing base stations can be inexpensively modified 
 in accordance with the preferred embodiment by the addition of delay 
 circuitry as shown. 
 Also note that in the preferred embodiment each receiver is a RAKE receiver
 having multiple despreaders. A multiplexer is provided for each despreader
 to select an antenna and delay. The output of the despreaders are 
 separately demodulated, combined together, then decoded. Each receiver, 
 therefore, has the ability to demodulate received signals spread across 
 multiple windows. 
 The descriptions of the invention, the specific details, and the drawings 
 mentioned above, are not meant to limit the scope of the present 
 invention. For example, in alternate embodiments of the present invention 
 a cells radius can be expanded or contracted by adjusting the delay times 
 for circuitry 203, 204, 215, and 216 accordingly. It is envisioned in such
 an embodiment that controller 219 utilizes clock 217 (system time) and 
 dynamically adjusts all delay circuitry to utilize an appropriate delay. 
 For example, during a first time period (e.g., day time hours) controller 
 may delay system time by 1024 chips in order to extend the base station's 
 range to 186 km, and during a second time period (e.g., night time hours) 
 controller may delay system time by 512 chips in order to extend the base 
 station's range to 124 km. It is the intent of the inventors that various 
 modifications can be made to the present invention without varying from 
 the spirit and scope of the invention, and it is intended that all such 
 modifications come within the scope of the following claims and their 
 equivalents.