Abstract:
A system and method for dynamically calibrating a base station in a wireless communication system, is presented herein. In accordance with an embodiment of the invention, the system includes a base station for transmitting, receiving, and processing communication signals and a wireless communication device for communicating with the base station. The wireless communication device is configured to determine its location (e.g., using GPS information), to detect the arrival time of a first signal transmitted from the base station, and to calculate a line-of-sight (LOS) delay corresponding to the LOS distance between the wireless communication device and the base station. The LOS distance calculation is based on the base station location information and the wireless communication device location information. The base station measures a round trip delay (RTD) corresponding to the delay incurred by the first signal and a delay incurred by a second signal transmitted from the wireless communication device back to the base station in response to the first signal. The base station then determines a base station timing calibration error based on the LOS delay, the first signal arrival time, and the RTD, and dynamically calibrates the base station timing to compensate for the base station timing calibration error.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims priority to U.S. Provisional application Ser. No. 60/239,318, filed Oct. 10, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates in general to wireless communications systems and, in particular, to system and method for dynamically calibrating base station timing.  
           [0004]    2. Description of Related Art and General Background  
           [0005]    Calibrating and maintaining proper timing is an important concern in communication systems. This is particularly true in wireless communications operating under Code Division Multiple Access (CDMA) schemes. CDMA is a digital radio-frequency (RF) channelization technique, defined in Telecommunications Industry Association/Electronics Industries Association Interim Standard-95 (TIA/EIA IS-95), entitled “MOBILE STATION-BASE STATION COMPATIBILITY STANDARD FOR DUAL-MODE WIDEBAND SPREAD SPECTRUM CELLULAR SYSTEM”, published in 1993. Other aspects of CDMA communication systems are defined in well-known standards, such as, for example, TLA/EIA IS-97, TIA/EIA IS-98, cdmaOne, cdma2000, and wideband CDMA (WCDMA) standards.  
           [0006]    Wireless communication systems employing CDMA technologies assign a unique code to communication signals and apply pseudorandom noise (PN) modulation to spread these communication signals across a common wideband spread spectrum bandwidth. In particular, the communication signals are modulated with PN sequences to spread the signals over a wide bandwidth. CDMA systems employ two short PN code sequences (i.e., “I” and “Q”) and one long PN code sequence. The short PN codes are used for quadrature spreading and have unique offsets serving as identifiers for a cell or a sector. At the WD  110  receiver, the received spread spectrum signal is despread in order to recover the original data. As long as the WD  110  receiver has the correct code, it can successfully detect and select its communication signal from the other signals concurrently transmitted over the same bandwidth. The encoding/decoding, modulation/demodulation, and spreading/despreading processes depend on accurate timing for synchronization and proper system operation.  
           [0007]    [0007]FIG. 1 (Prior Art) illustrates a simplified block diagram of CDMA wireless communication system  100 . System  100  allows mobile station or wireless communication device (WD)  110  to communicate with an Interworking Function (IWF)  108  via a base station (BS)  106 . The IWF  108  serves as a gateway between the wireless network and other networks, such as the Public Switched Telephone Network (PSTN) and wireline packet data networks providing Internet- or Intranet-based access. WD  110  communicates with BS  106 , which is associated with a geographic cell or sector, via the wireless interface U m  on the reverse link transmission path. BS  106  is configured to process the communication signals from WD  110 .  
           [0008]    On the forward link transmission path, BS  106  communicates with WD  110  via the wireless interface U m . During forward link transmissions, each BS  106  is capable of transmitting information-bearing signals as well as control signals, such as pilot signals. Pilot signals are used to identify the BS  106  best suited to accommodate reverse link transmissions. Pilot signals also provide a time and coherent phase reference to enable WD  110  to obtain initial system synchronization and facilitate coherent demodulation on the forward link. All pilot signals are subjected to the same PN spreading code but with a different code phase offsets to enable WD  110  to distinguish between different pilot signals, thereby identifying the originating BS  106 .  
           [0009]    As noted above, proper CDMA system  100  operation requires accurate timing. For example, in accordance with IS-95 and IS-97 standards, each BS 106 is required to use a time base reference from which all time-sensitive transmission components, including pilot PN code sequences and frames, are to be derived. Each BS  106  time base reference is required to be synchronized to CDMA system time. Benefits of synchronized BSs  106  include, for example, improved hand-off speed and reliability, enhanced initial system acquisition (i.e., cell search) speed, increased handset (e.g. WD  110 ) stand-by time, and improved reliability and power economy due to common channel hand-off operations.  
           [0010]    CDMA system time may employ a Global Positioning System (GPS) time base, which may be synchronized with a Universal Coordinated Time (UTC) reference. GPS and UTC may differ by up to a few seconds to compensate for the number of leap year seconds corrections added to UTC since Jan. 6, 1980. BSs  106  are further required to radiate pilot PN code sequences within ±3 μs of CDMA System Time and all CDMA channels radiated by BSs  106  are required to be within ±1 μs of each other. The rate of change for timing corrections may not exceed  {fraction (1/8)} PN chip ( 101.725 ns) per 200 ms.  
           [0011]    Moreover, in accordance with IS-95 and IS-98 standards, each WD  110  is required to use a time base reference used to derive timing for the transmit chip, symbol, frame slot, and system time. During steady-state conditions, each WD  110  is also required to have a timing reference within ±1 μs of the time of the earliest arriving multipath component being used for demodulation, as measured at the WD  110  antenna connector. In addition, if WD  110  time reference correction is needed, then it is to be corrected no faster than  {fraction (1/4)} PN chip ( 203.451 ns) in any 200 ms period and no slower than  {fraction (3/8)} PN chip ( 305.18 ns) per second.  
           [0012]    These stringent timing requirements are necessary because of the interdependence between BS  106  and WD  110  timing. FIG. 2 illustrates the timing relationship at various points within system  100 . The start of CDMA System Time is Jan. 6, 1980, 00:00:00 UTC, which corresponds to the start of GPS time, indicated as GPS time stamp zero (GPS TS-0). Because, as noted above, each BS  106  time base reference is to be synchronized to CDMA system time, GPS provides an absolute time reference and each BS  106  transmission includes a GPS time stamp. For convenience, GPS TS-0 will be used heretofore to demonstrate the timing relationships between BS  106  and WD  110 .  
           [0013]    As indicated in FIG. 2, the interval denoted by reference numeral A 1 , demonstrates the trailing portions of the PN codes sequences conveyed by the pilot signals transmitted by BS  106  during forward link transmissions, prior to the start of CDMA System Time. The notation 0 (n)  denotes a portion of the PN code sequences, which comprise n consecutive zeros. The initial state of the long PN code sequence is configured with a “1” at the most significant bit (MSB), followed by 41 consecutive “0”s. Similarly, the initial state for both, the I and Q short PN code sequences are configured with a “1” at the MSB, followed by 15 consecutive “0”s.  
           [0014]    Interval A 2  demonstrates the beginning portions of the pilot PN codes sequences transmitted by BS  106  to WD  110  at GPS TS-0. It is to be noted that BS  106  is synchronized with the absolute time reference provided by GPS in order to transmit pilot signals at exactly 2 second intervals (i.e., even second marks). Even second marks are generally divided into twenty-five 80 ms. periods for CDMA frame boundary timing. Moreover, for the Paging Channel, Forward Traffic Channel, Reverse Traffic Channel, and Access Channel, the 80 ms. period is divided into four 20 ms. frames. For the Sync Channel, the 80 ms. period is divided into three ≈26.66 ms. frames. The pilot PN sequence repeats every ≈26.66 ms. and the ≈26.66 ms. frame boundaries coincide with the pilot PN sequence rollover points, which are offset in the forward CDMA channel to identify the transmitting sector of BS  106 .  
           [0015]    Interval B 3  indicates the reception, by WD  110 , of the pilot PN code sequences after a one-way forward link transmission delay (Δ fl ). The forward link transmission delay Δ fl  may include delays attributable to the line-of-sight (LOS) propagation delay (Δ LOS ) between BS  106  and WD  110 , as well as BS  106  and WD  110  processing and hardware delays (Δ bf , Δ wf , respectively) associated with processing the forward link transmissions.  
           [0016]    Interval C 3  indicates that WD  110  aligns the timing of the reverse link transmissions with the timing of the received forward link transmissions. This may be achieved by taking into account the well-known forward link processing and hardware delays Δ wf  of WD  110 , the well-known reverse link processing and hardware delays Δ wr  of WD  110 , and compensating for the delays by advancing the timing of the reverse link transmissions to correspond to the forward link timing at the antenna connector of WD  110 .  
           [0017]    Finally, interval D 4  indicates the reception, by BS  106 , of the reverse link signals conveyed by WD  110 , after a one-way reverse link transmission delay (Δ rl ). The reverse link transmission delay Δ rl  may include delays attributable to the line-of-sight (LOS) propagation delay (Δ LOS ) between BS  106  and WD  110 , as well as BS  106  and WD  110  processing and hardware delays (Δ br , Δ wr , respectively) associated with processing the reverse link transmissions.  
           [0018]    As noted above, the forward and reverse link WD  110  hardware/processing delays Δ wf , Δ wr  are generally well-known and stable. In order to ensure that such delays are accounted for, WD  110  may be specifically calibrated in advance for such purposes. However, unlike Δ wf , Δ wr , BS  106  forward and reverse link hardware/processing delays Δ bf , Δ br  are subject to change and may be difficult to measure. BSs  106  are not configured identically and, depending on traffic statistics, urban densities, frequent RF tuning, and system resources, each BS  106  may be equipped with a variety of system components, each of which have their own delay characteristics. Unless these delays are calibrated and compensated for, there is no certainty that signals at selected reference points have the requisite timing.  
           [0019]    Considerable effort and human resources are required to determine and calibrate the delays due to BS  106  components. In many cases, services may be shut down and numerous technicians may be tasked to adequately assess and effect such calibrations.  
           [0020]    Moreover, BSs  106  are frequently being modified and upgraded to provide better service or compensate for faulty equipment. Because each component, as noted above, manifests certain delay characteristics, each modification, whether it be a new cable, a new component, or antenna repositioning, requires the re-assessment and re-calibration of BS  106  delays.  
           [0021]    Clearly, BS  106  delay determination and calibration is time and task intensive, requiring substantial economic and manpower resources. Accordingly, what is needed is a system and method for dynamically calibrating base station timing.  
         SUMMARY OF THE INVENTION  
         [0022]    The present invention addresses the need identified above by providing a novel system and method capable of dynamically calibrating base station timing.  
           [0023]    System and methods consistent with the principles of the present invention as embodied and broadly described herein include a base station for transmitting, receiving, and processing communication signals and a wireless communication device for communicating with the base station. The wireless communication device is configured to determine its location, to detect an arrival time of a first signal transmitted from the base station, and to calculate a line-of-sight delay corresponding to a line-of-sight distance between the wireless communication device and the base station. The line-of-sight distance is based on the base station location information and the wireless communication device location information. The base station measures a round trip delay corresponding to a delay incurred by the first signal and a delay incurred by a second signal transmitted from the wireless communication device back to the base station in response to the first signal. The base station then determines a base station timing calibration error based on the line-of-sight delay, the first signal arrival time, and the round trip delay, and dynamically calibrates the base station timing to compensate for the base station timing calibration error. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 (Prior Art) is a block diagram illustrating a conventional CDMA wireless communication system.  
         [0025]    [0025]FIG. 2 is a timeline diagram depicting the timing relationships between a base station and a wireless communication device.  
         [0026]    [0026]FIG. 3A is a functional block diagram depicting a CDMA wireless communication system capable of base station calibration, constructed and operative in accordance with the present invention.  
         [0027]    [0027]FIG. 3B is a functional diagram illustrating the delay relationships between a base station and a wireless communication device.  
         [0028]    [0028]FIG. 3C is a flow-chart illustrating a process for calibrating a base station, constructed and operative in accordance with another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    The following detailed description refers to the accompanying drawings that illustrate embodiments of the present invention. Other embodiments are possible and modifications may be made to the embodiments without departing from the spirit and scope of the invention. Therefore, the following detailed description is not meant to limit the invention. Rather the scope of the invention is defined by the appended claims.  
         [0030]    It will be apparent to one of ordinary skill in the art that the embodiments as described below may be implemented in many different embodiments of software, firmware, and hardware in the entities illustrated in the figures. The actual software code or specialized control hardware used to implement the present invention is not limiting of the present invention. Thus, the operation and behavior of the embodiments will be described without specific reference to the actual software code or specialized hardware components. The absence of such specific references is feasible because it is clearly understood that artisans of ordinary skill would be able to design software and control hardware to implement the embodiments of the present invention based on the description herein.  
         [0031]    Moreover, the processes associated with the presented embodiments may be stored in any storage device, such as, for example, non-volatile memory, an optical disk, magnetic tape, or magnetic disk. Furthermore, the processes may be programmed when the system is manufactured or via a computer-readable medium at a later date. Such a medium may include any of the forms listed above with respect to storage devices and may further include, for example, a carrier wave modulated, or otherwise manipulated, to convey instructions that can be read, demodulated/decoded and executed by a computer.  
         [0032]    [0032]FIG. 3A is a simplified functional block diagram of CDMA communications system  300 , constructed and operative in accordance with an embodiment of the present invention. System  300  comprises BS  106  and WD  110  and is capable of determining the location of a WD  110  and the exact time of day by employing GPS functionality. Such capabilities are well known. For example, one well-known technique includes a GPS-equipped WD  110 , which measures the ranges to a plurality of GPS satellites  310 A- 310 B having known locations at the time the measurements are made. Other techniques employ a combination of GPS functionality and a plurality of BSs  106  to triangulate the location of WD  110 . Still other techniques for identifying the location of WD  110  have been disclosed, for example, in U.S. Patent Application Nos. 6,058,338 and 6,081,229, commonly owned by the assignee of the present application and herein incorporated by reference.  
         [0033]    As indicated in FIG. 3A, WD  110  comprises antenna subsystem  110 A for transmitting CDMA signals to, and receiving CDMA signals from, BS  106 . Antenna subsystem  110 A is coupled to an antenna connector  110 B, which serves as the reference point from which WD  110  delays are measured. Antenna connector  110 B is coupled to a Radio Frequency (RF) subsection  110 C, which is, in turn coupled to an Intermediate Frequency (IF) subsection  110 D. As illustrated in FIG. 3A, along the receive path, RF subsection  110 C is configured to down-convert the CDMA RF signals received from antenna subsystem  110 A and supply the down-converted signals to IF subsection  110 D. Conversely, along the transmit path, RF subsection  110 C is configured to up-convert the IF signals received from IF subsection  110 D and supply the up-converted signals to antenna subsystem  110 A for transmission.  
         [0034]    IF subsection  110 D is coupled to microprocessor  110 E, which processes the received IF signals to extract payload information as well as formatting the payload information in a form suitable for IF subsection  110 D. Microprocessor  110 E is also coupled to a GPS receiver  110 F, configured to receive absolute timing information from GPSs  310 A- 310 C in order to determine the exact location of WD  110 , as noted above. It will be appreciated that subsections  110 C,  110 D and GPS receiver  110 F may include demodulators, power control devices, filters, deinterleavers, decoders, time/frequency units, and other conventional circuitry that, for the purposes of illustration, have been omitted.  
         [0035]    During forward link transmissions, the aggregate forward link WD  110  processing/hardware delay incurred by the respective components along the transmit path is denoted as Δ wf . Similarly, during reverse link transmissions, the aggregate reverse link WD  110  processing/hardware delay incurred by the respective components along the receive path is denoted as Δ wr . WD  110  forward and reverse link delays Δ wf , Δ wr  are known and are typically compensated for. As noted above, WDs 110 may be specifically calibrated in advance to ensure proper WD  110  operation. Because WDs  110  are user-end devices, requiring very little in the way of modifications due to system upgrades, delays Δ wf , Δ wr  are generally stable.  
         [0036]    BS  106  comprises antenna subsystem  106 A for transmitting CDMA signals to, and receiving CDMA signals from, WD  110 . Antenna subsystem  106 A is coupled to an antenna connector  106 B, which serves as the reference point from which BS  106  delays are measured. As illustrated in FIG. 3A, along the receive path, RF subsection  106 C is configured to down-convert the CDMA RF signals received from antenna subsystem  106 A and supply the down-converted signals to IF subsection  106 D. Conversely, along the transmit path, RF subsection  106 C is configured to up-convert the IF signals received from IF subsection  106 D and supply the up-converted signals to antenna subsystem  106 A for transmission.  
         [0037]    IF subsection  106 D is coupled to microprocessor  106 E, which processes the received IF signals to extract payload information as well as formatting the payload information in a form suitable for IF subsection  106 D. Microprocessor  106 E is also coupled to a GPS receiver  106 F, configured to receive absolute timing information from GPSs  310 A- 310 C in order to generate timing and frequency references for proper CDMA system operation. It will be appreciated that subsections  106 C,  106 D and GPS receiver  106 F may include demodulators, power control devices, filters, deinterleavers, decoders, time/frequency units, and other conventional circuitry that, for the purposes of illustration, have been omitted. Furthermore, BS  106  may include additional functionality to assist in determining the position of WD  110  (i.e., Position Determination Entity (PDE) server mechanism).  
         [0038]    During forward link transmissions, the aggregate forward link BS  106  processing/hardware delay incurred by the respective components along the transmit path is denoted as Δ bf . Similarly, during reverse link transmissions, the aggregate reverse link BS  106  processing/hardware delay incurred by the respective components along the receive path is denoted as Δ br .  
         [0039]    [0039]FIG. 3B illustrates the relationships between the various delays encountered in system  300 . As noted above, system  300  is capable of determining the location of WD  110  (i.e., X w , Y w , and Z w  coordinates). The location of the antenna radiating center of BS  106  (i.e., X b , Y b , and Z b  coordinates) is also known. Therefore, as indicated in FIG. 3B, the LOS distance (d Los ) between WD  110  and BS  106  may be determined by equation (1):  
           d   Los ={square root}{overscore (( X   b   −X   w ) 2 +( Y   b   −Y   w ) 2 +( Z   b   −Z   w ) 2  )}  (1)  
         [0040]    Consequently, the LOS delay ALOS incurred by a signal propagating across dLos may be determined by equation (2):  
         Δ LOS   =d   LOS   /c   (2)  
         [0041]    where c is the speed of light (i.e.˜3×10 8  m/sec.).  
         [0042]    As noted above, during forward link transmissions, BS  106  transmits a pilot signal to WD  110 , which provides a timing reference. By virtue of its ability to determine its own location and exact time of day, WD  110  is capable of determining A LOS  information. In addition, as indicated in FIG. 2, WD  110  exploits the well-known WD  110  forward link hardware/processing delays Δ wf  to align the reverse link transmissions with the forward link transmissions. Therefore, as indicated by FIG. 3B, the one-way forward link transmission delay (Δ fl ) detected by WD  110 , which represents the total delay encountered in the forward link between BS  106  and WD  110  may be determined by equation (3):  
         Δ fl =Δ wf +Δ LOS +Δ bf    (3)  
         [0043]    where: Δ wf  represents the forward link WD  110  hardware/processing delay, Δ LOS  represents the LOS delay, and Δ bf  represents the BS  106  forward link hardware/processing delays due to calibration error. As will be described below, WD  110  is capable of measuring the forward link transmission delay Δ fl  and, by virtue of knowing the WD  110  forward link hardware/processing delays Δ wf  and Δ LOS , it is possible to determine, via equation (3), the value of the BS  106  forward link hardware/processing delays Δ bf .  
         [0044]    It is to be noted that, as indicated by FIG. 3B, the one-way reverse link transmission delay (Δ rl ), which captures the total delay encountered during the reverse link between WD  110  and BS  106 , may be represented by equation (4):  
         Δ rl =Δ wr +Δ LOS +Δ br    (4)  
         [0045]    where: Δ wr  represents the reverse link WD  110  hardware/processing delay, Δ LOS  represents the LOS delay, and Δ br  represents the BS  106  reverse link calibration error due to BS  106  hardware/processing delays.  
         [0046]    BS  106  is capable of measuring the round trip delay (RTD) encountered by a signal communicated from BS  106  to WD  110  and back to BS  106 . In particular, RTD encompasses the delay associated with a signal transmitted from BS  106  to WD  110  and the delay associated with a signal transmitted from WD  110  back to BS  106 , in response to the signal received from BS  106 . It is to be noted that, as shown in FIG. 2, WD  110  aligns the transmission time of its reverse link transmissions with the arrival time of the received forward link transmissions.  
         [0047]    As indicated in FIG. 3B, RTD may be determined by the sum of the one-way forward link delay Δ fl  and the one-way reverse link delay Δ rl . Thus, combining equation (3) with equation (4), yields:  
           RTD =(Δ wf +Δ LOS +Δ bf )+(Δ wr +Δ LOS +Δ br )− BTF    (5)  
         [0048]    where BTF represents a back-to-the-future counter configured to compensate for WD  110  hardware/processing delays Δ wf  and Δ wr .  
         [0049]    Taking into account the reasonable assumption that the well delays Δ wf , Δ wr  are adequately compensated for by the back-to-future counter BTF, equation (5) simplifies to:  
                   RTD   =       Δ   LOS     +     Δ   bf     +     Δ   LOS     +     Δ   br                   =       2        Δ   LOS       +     (       Δ   bf     +     Δ   br       )                     (   6   )                               
 
         [0050]    Because LOS delay Δ LOS  is a known quantity, it may be subtracted from both sides of equation (8) to yield:  
           RTD− 2Δ LOS =Δ bf +Δ br    (7)  
         [0051]    Thus, equation (7) reveals that the difference between the measured RTD and twice the Δ LOS , is the BS  106  calibration error due to the combined forward and reverse link BS  106  hardware/processing delays Δ bf , Δ br . As noted above, by measuring the forward link delay Δ fl  and employing equation (3), it is possible to determine the base station forward link processing delay Δ bf . Thus, to ensure the proper timing of system  300 , requires adequately determining forward and reverse link BS  106  calibration errors Δ bf , Δ br , and calibrating BS  106  to compensate for such delays. Conventionally, forward and reverse link BS  106  hardware/processing delays Δ bf , Δ br , are determined by physically measuring these delays, at the expense of system resources. However, as noted above, the BS  106  forward link hardware/processing delays Δ bf  may be determined by measuring the forward link delay Δ fl , factoring in the known values for the WD  110  forward link hardware/processing delays Δ wf  and Δ LOS , and using equation (3) to solve for Δ bf .  
         [0052]    Moreover, as noted above, BS  106  is synchronized with the absolute time reference provided by GPS satellites  310 A- 310 D in order to transmit pilot signals with rollover points that correspond to the ˜26.66 ms. frame boundaries. In addition, the signals received by WD  110  may be time stamped with the absolute time reference provided by GPS satellites  310 A- 310 D so as to identify when the signals were received. Thus, the arrival time of pilot signals (τ arr ) are known. Furthermore, because pilot signals are initiated by BS  106  during forward link transmissions, pilot signal arrival times τ arr  are functions of the forward link delays, in accordance with equation (3). Specifically,  
         τ arr =Δ wf +Δ LOS +Δ bf    (8)  
         [0053]    Because the pilot signal arrival time τ arr,  the forward link WD  110  hardware/processing delay Δ wf , and the LOS delay Δ LOS  are known at WD  110 , the forward link BS  106  calibration error Δ bf  may be calculated, as indicated by equation (9):  
         Δ bf =τ arr −Δ wf −Δ LOS    (9)  
         [0054]    Therefore, the forward link BS  106  calibration error Δ bf  may be determined by detecting the pilot signal arrival time τ arr , and subtracting from it the known forward link WD  110  hardware/processing delay Δ wf  and the LOS delay Δ LOS . As such, during forward link calibrations, BS  106  may be calibrated by first determining the forward link BS  106  calibration error Δ bf  based on equation (9) and then compensating for error Δ bf  by adjusting the transmission processing time of a transmitted signal, accordingly.  
         [0055]    Moreover, because the measured RTD is a function of both, the forward link BS  106  calibration error Δ bf  and the reverse link BS  106  calibration error Δ br , in view of the above, reverse link calibration error Δ bf  may also be determined. In particular, rewriting equation (7) to include the calculated forward link BS  106  calibration error Δ bf  set forth by equation (9) yields:  
           RTD −2Δ LOS =(τ arr −Δ wf −Δ LOS )+Δ br    (10)  
         [0056]    Solving for the reverse link BS 106 calibration error Δ br  yields:  
         Δ br   =RTD− 2Δ LOS −(τ arr −Δ wf −Δ LOS ) = RTD+Δ   wf −τ arr −Δ LOS    (11)  
         [0057]    Therefore, the reverse link BS  106  calibration error Δ br  may be determined subtracting from the measured RTD and known forward link WD  110  hardware/processing delay Δ wf , the known pilot signal arrival time τ arr  and the known LOS delay Δ LOS . As such, during reverse link calibrations, BS  106  may be calibrated by measuring both, RTD and pilot arrival time τ arr , and then calculating the reverse link BS  106  calibration error Δ br  based on equation (11).  
         [0058]    [0058]FIG. 3C is a flowchart illustrating process  350 , constructed and operative in accordance with another embodiment of the present invention. Process  350  is configured to calibrate BS  106 , based on WD  110  and BS  106  location information.  
         [0059]    As indicated in block B 355 , process  350  determines the location of WD  110  as well as the exact time of day. As noted above, the location of WD  110  may be determined by well-known means, such as, for example, equipping WD 110 with GPS functionality.  
         [0060]    In block B 360 , process  350  determines LOS delay Δ LOS  incurred by a signal propagating between BS  106  and WD  110 . As noted above in equations (1) and (2), LOS delay Δ LOS  is a function of the distance d LOS  between the BS  106  antenna radiating center location (i.e., X b , Y b , and Z b  coordinates) and the WD  110  location (i.e., x w , y w , and Z w  coordinates).  
         [0061]    In block B 365 , process  350  detects the WD  110  arrival time τ arr  of a pilot signal transmitted by BS  106 . The pilot signal arrival time τ arr  is identified by WD  110  via the absolute time reference information provided by GPS satellites  310 A- 310 D.  
         [0062]    In block B 370 , process  350  measures the RTD encompassing the delay incurred by a first signal transmitted from BS  106  to WD  110  and the delay incurred by a second signal transmitted from WD  110  back to BS  106 , in response to the first signal.  
         [0063]    In block B 375 , process  350  determines the forward BS timing calibration error Δ bf  based on Δ LOS  and τ arr . Specifically, process  350  determines the forward BS timing calibration error Abf in accordance with equation (9). Accordingly, in block B 380 , process  350  calibrates BS  106  by adjusting the transmission processing time of forward link transmissions in order to compensate for the forward BS timing calibration error Δ bf .  
         [0064]    In block B 385 , process  350  determines the reverse BS timing calibration error Δ br  based on Δ bf , Δ LOS , and τ arr . Specifically, process  350  determines the reverse BS timing calibration error Abr in accordance with equation (11). Accordingly, in block B 385 , process  350  calibrates BS  106  by adjusting the receive processing time of reverse link transmissions in order to compensate for the reverse BS timing calibration error Δ br .  
         [0065]    The foregoing description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments are possible, and the generic principles presented herein may be applied to other embodiments as well. For example, certain base stations may lack access to an absolute time reference. In such cases, the base station delays due to the forward link calibration error may contain an additional time offset, representative of the difference between the base station internal clock and an external absolute time reference. By simply combining the forward link calibration error and additional time offset, the base station timing may still be calibrated in accordance with the abovedescribed embodiments.  
         [0066]    Moreover, the invention may be implemented in part or in whole as a hardwired circuit, as a circuit configuration fabricated into an application-specific integrated circuit, or as a firmware program loaded into non-volatile storage or a software program loaded from or into a data storage medium as machine-readable code, such code being instructions executable by an array of logic elements such as a microprocessor or other digital signal processing unit.  
         [0067]    As such, the present invention is not intended to be limited to the embodiments shown above but rather is to be accorded the widest scope consistent with the principles and novel features disclosed in any fashion herein.