Abstract:
A novel system and method reduces the time required by a base station to re-acquire a fixed subscriber unit in a CDMA communication system by virtually locating of the subscriber units. A base station acquires subscriber units by searching only those code phases concomitant with the largest propagation delay possible in the cell, as if all subscriber units were located at the periphery of the cell. A subscriber unit which has never been acquired by the base station varies the delay between the PN code phase of its received and transmitted signals over the range of possible delays in a cell and slowly ramps-up its transmission power until it is acquired by the base station. Upon initial acquisition by the base station, the subscriber unit ceases ramping-up its transmission power, ceases varying the delay and internally stores the final value of the delay in memory. For subsequent re-acquisition, the subscriber unit adds the delay value between the PN code phase of its received and transmitted signals, making the subscriber virtually appear to be at the periphery of the cell. This permits a quick ramp-up of transmission power by the subscriber unit and reduced acquisition time by the base station.

Description:
REFERENCE TO OTHER APPLICATIONS 
     This is a continuation of application Ser. No. 08/671,068; filed Jun. 27, 1996 now Pat. No. 5,940,382. 
    
    
     CROSS REFERENCE TO RELATED APPLICATION 
     This application is being filed concurrently with an application entitled Code Division Multiple Access (CDMA) System and Method which is herein incorporated by reference as if fully set forth. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to code division multiple access (CDMA) communication systems. More particularly, the present invention relates to a CDMA communication system which utilizes virtual locating of a fixed subscriber unit to reduce the time for a base station to detect an access signal from a subscriber unit and establish a communication channel between the base station and the subscriber unit. 
     2. Description of Related Art 
     Most widely used conventional telecommunication systems require transmissions to be confined to a separate frequency or time slot. Systems using frequency division multiple access (FDMA) assign each user a specific portion of the frequency spectrum for communication. Systems using time division multiple access (TDMA) assign each user a repeating time slot to transmit the desired information. These conventional techniques require strict definition of time slots, channels and guardbands between channels in order to prevent communicating nodes from interfering with one another. 
     Channelization and guardband requirements have resulted in a tremendous inefficiency in the use of the RF spectrum. As the number of commercial applications of wireless technology increases, the need for communication systems which utilize the RF spectrum more efficiently has become paramount. 
     CDMA communication systems have had a long history of use in military applications. CDMA permits communications which are difficult to detect by enemies and offer robust communications during attempts by enemies to jam communications. In CDMA communications, each signal or communication channel is distinguished from all others in a particular frequency band by a unique pseudo noise (PN) code imprinted upon data transmitted by the transmitter. A receiver which is privy to the unique code uses the code to resolve the desired data signal channel from among many the simultaneous data signals and channels in the frequency band. 
     The features that have enabled CDMA communication systems to succeed in military applications also make CDMA communication systems well adapted for efficiently utilizing the RF spectrum. Since each subscriber unit in a CDMA communication system transmits and receives resolvable communication signals over the same frequency band, there are less stringent channelization and guardband requirements. Accordingly, the capacity of the system (the number of users able to communicate simultaneously) is significantly increased. 
     Although use of the same portion of the RF spectrum by a plurality of subscriber units increases system efficiency, each subscriber unit receives communication signals that do not have its unique code as interference. The more power that is utilized by a single subscriber unit to communicate with the base station, the more interference is presented to other subscriber units. The power from one subscriber unit may even terminate other communications if it becomes too high. Accordingly, the control of the transmission power of all subscriber units is important to maintain high quality communications throughout the system. 
     A typical CDMA communication system is shown in FIG.  1 . The system comprises a cell base station (B), and a plurality of fixed subscriber units S 1 -S 7  located at various distances from the base station. The base station constantly transmits a forward pilot signal. The subscriber units maintain epoch alignment between the forward pilot signal and their internal PN code generator such that all signals transmitted from the subscriber unit are at the same PN code phase at which the forward pilot is received. The base station receives signals from subscriber units with a code phase difference between its forward pilot signal and the received signal corresponding to the two-way signal propagation delay between the base station and the subscriber. 
     For the base station to detect a signal, it must align the phase of its receive PN code generator to the phase of the received signal, thus “acquiring” the signal. The base station can receive an access signal with any code phase difference within the range of the cell. Therefore, the base station must test all code phases associated with the range of possible propagation delays of the cell to acquire the access signal. 
     Once a communication channel is established between the base station and the subscriber unit, the transmission power of the subscriber unit is controlled by a closed loop automatic power control (APC) algorithm which prevents the power from each subscriber unit from excessively interfering with other subscriber units. During channel establishment, before the closed loop power control begins, the subscriber unit&#39;s transmission power is kept to a minimum by ramping-up from a low level and establishing the channel without the subscriber unit significantly overshooting (on the order of less than 3 dB) the minimum power necessary to operate the channel. 
     To establish a channel, each subscriber unit transmits a PN coded access signal for detection by the base station. The base station acquires the access signal and transmits a confirmation signal to each subscriber unit. The time required for the base station to acquire the access signal contributes directly to the time elapsed between a subscriber unit going “off-hook”, establishing a communication channel, connecting to the public switched telephone network (PSTN) and receiving a dial tone. It is desirable to receive a dial tone within 150 msec of detection of “off-hook”. 
     The time distribution of acquisition opportunities is shown in FIG. 2 for a typical subscriber unit located 20 km from a base station in a 30 km cell. For a base station which tests 8 code phases simultaneously at a PN rate of 12.48 MHz and a symbol rate of 64,000 symbols per second, and takes an average of 7.5 symbol periods to accept or reject a particular group of code phases, the average time to test all code phase delays within the cell is approximately 37 msec, and any one subscriber unit can only be detected during an approximately 100 μsec window during that period. Assuming that the selection of initial subscriber unit transmission power level is 15-20 dB below the proper level and a slow ramp-up rate of between 0.05 and 0.1 dB/msec, it could take 4-5 such 37 msec time periods, (or an average of approximately 200 msec,) for the base station to acquire a subscriber unit. This introduces an unacceptable delay in the channel establishment process which should be less than 150 msec. 
     Accordingly, there is a need to reduce the amount of time required for a base station to acquire a subscriber unit. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a method of reducing the re-acquisition time of a fixed subscriber unit by a base station in a CDMA communication system by utilizing virtual locating of the subscriber unit. A base station acquires subscriber units by searching only those code phases concomitant with the largest propagation delay possible in the cell, as if all subscriber units were located at the periphery of the cell. A subscriber unit which has never been acquired by the base station varies the delay between the PN code phase of its received and transmitted signals over the range of possible delays in a cell and slowly ramps-up its transmission power until it is acquired by the base station. Upon initial acquisition by the base station the subscriber unit ceases ramping-up its power and varying the delay and internally stores the final value of the delay in memory. For subsequent re-acquisition, the subscriber unit adds the delay value between the PN code phase of its received and transmitted signals, making the subscriber virtually appear to be at the periphery of the cell. This permits a quick ramp-up of transmission power by the subscriber unit and reduced acquisition time by the base station. 
     Accordingly, it is an object of the present invention to provide an improved method and system for decreasing the re-acquisition time of a fixed subscriber unit by a base station in a CDMA communication system. 
     Other objects and advantages of the present invention will become apparent after reading the description of a presently preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a prior art CDMA communication system; 
     FIG. 2 is a graph of the distribution of acquisition opportunities of the system of FIG. 1; 
     FIG. 3 is a schematic overview of a CDMA communication system in accordance with the present invention; 
     FIG. 4 is a diagram showing the propagation of signals between a base station and a plurality of subscriber units; 
     FIG. 5 is a flow diagram of the preferred embodiment of the initial establishment of a communication channel between a base station and a subscriber unit using slow initial acquisition; 
     FIG. 6 is a flow diagram of the preferred embodiment of the reestablishment of a communication channel between a base station and a subscriber unit using fast re-acquisition; 
     FIG. 7A is a diagram of the communications between a base station and a plurality of subscriber units; 
     FIG. 7B is a diagram of the base station and a subscriber unit which has been virtually located; 
     FIG. 8 is a schematic overview of a plurality of subscriber units which have been virtually located; 
     FIG. 9 is a subscriber unit made in accordance with the teachings of the present invention; 
     FIG. 10 is a flow diagram of an alternative embodiment of the initial establishment of a communication channel between a base station and a subscriber unit using slow initial acquisition; 
     FIG. 11 is a flow diagram of an alternative embodiment of the reestablishment of a communication channel between a base station and a subscriber unit using fast re-acquisition; and 
     FIG. 12 is a flow diagram of a second alternative embodiment of the initial establishment of a communication channel between a base station and a subscriber unit using slow initial acquisition. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiment will be described with reference to the drawing figures where identical numerals represent similar elements throughout. 
     A communication network  10  embodying the present invention is shown in FIG.  3 . The communication network  10  generally comprises one or more base stations  14 , each of which is in wireless communication with a plurality of fixed subscriber units  16 . Each subscriber unit  16  communicates with either the closest base station  14  or the base station  14  which provides the strongest communication signal. The base stations  14  also communicate with a base station controller  20 , which coordinates communications among base stations  14  and between base stations  14 . The communication network may also be connected to a public switched telephone network (PSTN)  22 , whereupon the base station controller  20  also coordinates communication between the base stations  14  and the PSTN  22 . Preferably, each base station  14  communicates with the base station controller  20  over a wireless link, although a land line may also be provided. A land line is particularly applicable when a base station  14  is in close proximity to the base station controller  20 . 
     The base station controller  20  performs several functions. Primarily, the base station controller  20  provides all of the overhead, administrative and maintenance (OA&amp;M) signaling associated with establishing and maintaining all of the wireless communications between the subscriber units  16 , the base stations  14 , and the base station controller  20 . The base station controller  20  also provides an interface between the wireless communication system  10  and the PSTN  22 . This interface includes multiplexing and demultiplexing of the communication signals that enter and leave the system  10  via the base station controller  20 . Although the wireless communication system  10  is shown employing antennas to transmit RF signals, one skilled in the art should recognize that communications may be accomplished via microwave or satellite uplinks. Additionally, the functions of a base station  14  may be combined with the base station controller  20  to form a master base station. The location of where these base station controller functions are performed is not central to the present invention. 
     Referring to FIG. 4, the propagation of certain signals in the establishment of a communication channel  18  between a base station  14  and a plurality of subscriber units  16  is shown. The forward pilot signal  20  is transmitted from the base station  14  at time to, and is received by a subscriber unit  16  after a propagation delay Δt. To be acquired by the base station  14  the subscriber unit  16  transmits an access signal  22  which is received by the base station  14  after a further propagation delay of Δt. Accordingly, the round trip propagation delay is 2 Δt. The access signal  22  is transmitted epoch aligned to the forward pilot signal  20 , which means that the code phase of the access signal  22  when transmitted is identical to the code phase of the received forward pilot signal  20 . 
     The round trip propagation delay depends upon the location of a subscriber unit  16  with respect to the base station  14 . Communication signals transmitted between a subscriber unit  16  located closer to the base station  14  will experience a shorter propagation delay than a subscriber unit  16  located further from the base station  14 . Since the base station  14  must be able to acquire subscriber units  16  located at any position within the cell  30 , the base station  14  must search all code phases of the access signal corresponding to the entire range of propagation delays of the cell  30 . 
     It should be apparent to those of skill in the art that the establishment of a communication channel between a base station  14  and a subscriber unit  16  is a complex procedure involving many tasks performed by the base station  14  and the subscriber unit  16  which are outside the scope of the present invention. The present invention is directed to decreasing the re-acquisition time of a fixed subscriber unit  16  by a base station  14  during the re-establishment of a communication channel. 
     Referring to FIG. 5, the tasks associated with initial acquisition of a subscriber unit  16  by a base station  14  in accordance with the preferred embodiment of the present invention are shown. When a subscriber unit  16  desires the establishment of a channel  18  with a base station  14  with which it has never established a channel, the subscriber unit  16  has no knowledge of the two-way propagation delay. Accordingly, the subscriber unit  16  enters the initial acquisition channel establishment process. 
     The subscriber unit  16  selects a low initial power level and zero code phase delay, (epoch aligning the code phase of the transmitted access signal  22  to the code phase of the received forward pilot signal  20 ), and commences transmitting the access signal  22  while slowly (0.05-0.1 dB/msec) ramping-up transmission power (step  100 ). While the subscriber unit  16  is awaiting receipt of the confirmation signal from the base station  14 , it varies the code phase delay in predetermined steps from zero to the delay corresponding to the periphery of the cell  30 , (the maximum code phase delay), allowing sufficient time between steps for the base station  14  to detect the access signal  22  (step  102 ). If the subscriber unit  16  reaches the code phase delay corresponding to the periphery of the cell  30 , it repeats the process of varying the code phase delay while continuing the slow power ramp-up (step  102 ). 
     In order to acquire subscriber units  16  desiring access, the base station  14  continuously transmits a forward pilot signal  20  and attempts to detect the access signals  22  from subscriber units  16  (step  104 ). Rather than test for access signals  22  at all code phase delays within the cell  30  as with current systems, the base station  14  need only test code phase delays centered about the periphery of the cell  30 . 
     The base station  14  detects the access signal  22  (step  106 ) when the subscriber unit  16  begins transmitting with sufficient power at the code phase delay which makes the subscriber unit  16  appear to be at the periphery of the cell  30 , thereby “virtually” locating the subscriber unit  16  at the periphery of the cell  30 . The base station  14  then transmits a signal to the subscriber unit  16  which confirms that the access signal  22  has been received (step  108 ) and continues with the channel establishment process (step  110 ). 
     Once the subscriber unit  16  receives the confirmation signal (step  112 ), it ceases the ramp-up of transmission power, ceases varying the code phase delay (step  114 ) and records the value of the code phase delay for subsequent re-acquisitions (step  116 ). The subscriber unit  16  then continues the channel establishment process including closed-loop power transmission control (step  118 ). 
     On subsequent re-acquisitions when a subscriber unit  16  desires the establishment of a channel  18  with a base station  14 , the subscriber unit  16  enters the re-acquisition channel establishment process shown in FIG.  6 . The subscriber unit  16  selects a low initial power level and the code phase delay recorded during the initial acquisition process, (shown in FIG.  5 ), and commences continuously transmitting the access signal  22  while quickly (1 dB/msec) ramping-up transmission power (step  200 ). While the subscriber unit  16  is awaiting receipt of the confirmation signal from the base station  14 , it slightly varies the code phase delay of the access signal  22  about the recorded code phase delay, allowing sufficient time for the base station  14  to detect the access signal  22  before changing the delay (step  202 ). The base station  14  as in FIG. 5, transmits a forward pilot signal  20  and tests only the code phase delays at the periphery of the cell  30  in attempting to acquire the subscriber units  16  within its operating range (step  204 ). The base station  14  detects the access signal  22  when the subscriber unit  16  transmits with sufficient power at the code phase delay which makes the subscriber unit  16  appear to be at the periphery of the cell  30  (step  206 ). The base station  14  transmits a signal to the subscriber unit  16  which confirms that the access signal  22  has been received (step  208 ) and continues with the channel establishment process (step  210 ). 
     When the subscriber unit  16  receives the confirmation signal (step  212 ) it ceases power ramp-up, ceases varying the code phase delay (step  214 ) and records the present value of the code phase delay for subsequent re-acquisitions (step  216 ). This code phase delay may be slightly different from the code phase delay initially used when starting the re-acquisitions process (step  202 ). The subscriber unit  16  then continues the channel establishment process at the present power level (step  218 ). If a subscriber unit  16  has not received a confirmation signal from the base station  14  after a predetermined time, the subscriber unit  16  reverts to the initial acquisition process described in FIG.  5 . 
     The effect of introducing a code phase delay in the Tx  20  and Rx  22  communications between the base station  14  and a subscriber unit  16  will be explained with reference to FIGS. 7A and 7B. Referring to FIG. 7A, a base station  160  communicates with two subscriber units  162 ,  164 . The first subscriber unit  162  is located 30 km from the base station  160  at the maximum operating range. The second subscriber unit  164  is located 15 km from the base station  160 . The propagation delay of Tx and Rx communications between the first subscriber unit  162  and the base station  160  will be twice that of communications between the second subscriber unit  164  and the base station  160 . 
     Referring to FIG. 7B, after an added delay value  166  is introduced into the Tx PN generator of the second subscriber unit  164  the propagation delay of communications between the first subscriber unit  162  and the base station  160  will be the same as the propagation delay of communications between the second subscriber unit  164  and the base station  160 . Viewed from the base station  160 , it appears as though the second subscriber unit  164  is located at the virtual range  164 ′. 
     Referring to FIG. 8, it can be seen that when a plurality of subscriber units S 1 -S 7  are virtually relocated S 1 ′-S 7 ′ to the virtual range  175 , the base station must only test the code phase delays centered about the virtual range  175 . 
     Utilizing the present invention, a subscriber unit  16  which has achieved a sufficient power level will be acquired by the base station  14  in approximately 2 msec. Due to the shorter acquisition time, the subscriber unit  16  can ramp-up at a much faster rate, (on the order of 1 dB/msec), without significantly overshooting the desired power level. Assuming the same 20 dB power back-off, it would take the subscriber unit  16  approximately 20 msec to reach the sufficient power level for detection by the base station  14 . Accordingly, the entire duration of the re-acquisition process of the present invention is approximately 22 msec, which is an order of magnitude reduction from prior art re-acquisition methods. 
     A subscriber unit  200  made in accordance with the present invention is shown in FIG.  9 . The subscriber unit  200  includes a receiver section  202  and a transmitter section  204 . An antenna  206  receives a signal from the base station  14 , which is filtered by a band-pass filter  208  having a bandwidth equal to twice the chip rate and a center frequency equal to the center frequency of the spread spectrum system&#39;s bandwidth. The output of the filter  208  is down-converted by a mixer  210  to a baseband signal using a constant frequency (Fc) local oscillator. The output of the mixer  210  is then spread spectrum decoded by applying a PN sequence to a mixer  212  within the PN Rx generator  214 . The output of the mixer  212  is applied to a low pass filter  216  having a cutoff frequency at the data rate (Fb) of the PCM data sequence. The output of the filter  216  is input to a codec  218  which interfaces with the communicating entity  220 . 
     A baseband signal from the communicating entity  220  is pulse code modulated by the codec  218 . Preferably, a 32 kilobit per second adaptive pulse code modulation (ADPCM) is used. The PCM signal is applied to a mixer  222  within a PN Tx generator  224 . The mixer  222  multiplies the PCM data signal with the PN sequence. The output of the mixer  222  is applied to low-pass filter  226  whose cutoff frequency is equal to the system chip rate. The output of the filter  226  is then applied to a mixer  228  and suitably up-converted, as determined by the carrier frequency Fc applied to the other terminal. The up-converted signal is then passed through a band-pass filter  230  and to a broadband RF amplifier  232  which drives an antenna  234 . 
     The microprocessor  236  controls the acquisition process as well as the Rx and Tx PN generators  214 ,  224 . The microprocessor  236  controls the code phase delay added to the Rx and Tx PN generators  214 ,  224  to acquire the forward pilot signal  20 , and for the subscriber unit  200  to be acquired by the base station  14 , and records the code phase difference between these PN generators. For re-acquisition the microprocessor  236  adds the recorded delay to the Tx PN generator  224 . 
     The base station  14  uses a configuration similar to the subscriber unit  16  to detect PN coded signals from the subscriber unit  200 . The microprocessor (not shown) in the base station  14  controls the Rx PN generator in a similar manner to make the code phase difference between Rx PN generator and the Tx PN generator equivalent to the two-way propagation delay of the subscriber unit&#39;s  16  virtual location. Once the base station  14  acquires the access signal  22  from the subscriber unit  16 , all other signals from the subscriber unit  16  to the base station  14  (traffic, pilot, etc.) use the same code phase delay determined during the acquisition process. 
     It should be noted that although the invention has been described herein as the virtual locating of subscriber units  16  at the periphery of the cell  30  the virtual location can be at any fixed distance from the base station  14 . 
     Referring to FIG. 10, the tasks associated with initial acquisition of a “never-acquired” subscriber unit  16  by a base station  14  in accordance with an alternative embodiment of the present invention are shown. The subscriber unit  16  continuously transmits an epoch aligned access signal  22  to the base station  14  (step  300 ) when the establishment of a channel  18  is desired. While the subscriber unit  16  is awaiting the receipt of a confirmation signal from the base station  14 , it continuously increases the transmission power as it continues transmission of the access signal  22  (step  302 ). 
     To detect subscriber units which have never been acquired, the base station  14  transmits a forward pilot signal  20  and sweeps the cell by searching all code phases corresponding to the entire range of propagation delays of the cell (step  304 ) and detects the epoch aligned access signal  22  sent from the subscriber unit  16  after the transmission has achieved sufficient power for detection (step  306 ). The base station  14  transmits a signal to the subscriber unit  16  (step  308 ) which confirms that the access signal  22  has been received. The subscriber unit  16  receives the confirmation signal (step  310 ) and ceases the increase in transmission power (step  312 ). 
     The base station  14  determines the desired code phase delay of the subscriber unit  16  by noting the difference between the Tx and Rx PN generators  224 ,  214  after acquiring the subscriber unit  16 . The desired code phase delay value is sent to the subscriber unit  16  (step  316 ) as an OA&amp;M message, which receives and stores the value (step  318 ) for use during re-acquisition, and continues with the channel establishment process (steps  322  and  324 ). 
     Referring to FIG. 11, an alternative method of fast re-acquisition in accordance with the present invention is shown. When a communication channel must be reestablished between the subscriber unit  16  and the base station  14 , the subscriber unit  16  transmits the access signal  22  with the desired code phase delay as in the preferred embodiment. 
     With all of the previously acquired subscriber units  16  at the same virtual range, the base station  14  need only search the code phase delays centered about the periphery of the cell to acquire the access signals  22  of such subscriber units  16  (step  330 ). Thus, a subscriber unit  16  may ramp-up power rapidly to exploit the more frequent acquisition opportunities. The subscriber unit  16  implements the delay the same way as in the preferred embodiment. The base station  14  subsequently detects the subscriber unit  16  at the periphery of the cell (step  336 ), sends a confirmation signal to the subscriber unit (step  337 ) and recalculates the desired code phase delay value, if necessary. Recalculation (step  338 ) compensates for propagation path changes, oscillator drift and other communication variables. The base station  14  sends the updated desired code phase delay value to the subscriber unit  16  (step  340 ) which receives and stores the updated value (step  342 ). The subscriber unit  16  and the base station  14  then continue the channel establishment process communications (steps  344  and  346 ). 
     Note that the alternative embodiment requires the base station to search both the code phase delays centered on the periphery of the cell to re-acquire previously acquired subscriber units and the code phase delays for the entire cell to acquired subscriber units which have never been acquired. 
     Referring to FIG. 12, the tasks associated with initial acquisition of a never-acquired subscriber unit  16  by a base station  14  in accordance with a second alternative embodiment of the present invention are shown. In the embodiment shown in FIG. 10, when a never-acquired subscriber unit  16  is acquired the access signal  20  remains epoch aligned to the forward pilot signal  20 . In this embodiment, the base station  14  and subscriber unit  16  change the code phase alignment of the access signal  22  from epoch aligned to delayed, (by the code phase delay), to make the subscriber unit  16  appear at the periphery of the cell. This change is performed at a designated time. 
     Steps  400  through  418  are the same as the corresponding steps  300  through  318  shown in FIG.  10 . However, after the base station  14  sends the desired delay value to the subscriber unit  16  (step  416 ) the base station  14  sends a message to the subscriber unit  16  to switch to the desired delay value at a time referenced to a sub-epoch of the forward pilot signal  20  (step  420 ). The subscriber unit  16  receives this message (step  422 ), and both units  14 ,  16  wait until the switchover time is reached (steps  424 ,  430 ). At that time, the base station  14  adds the desired delay value to its Rx PN operator (step  432 ) and the subscriber unit  16  adds the same desired delay value to its Tx PN generator (step  426 ). The subscriber unit  16  and the base station  14  then continue the channel establishment process communication (step  428 ,  434 ). 
     Although the invention has been described in part by making detailed reference to the preferred and alternative embodiments, such detail is intended to be instructive rather than restrictive. It will be appreciated by those skilled in the art that many variations may be made in the structure and mode of operation without departing from the spirit and scope of the invention as disclosed in the teachings herein.