Abstract:
A method for estimating the earliest signal arrival in a wireless communication system is presented. The system includes a base station that transmits a plurality of pilot signals and a mobile station configured to receive a plurality of signals corresponding to the pilot signals. The mobile station includes a receiver containing a searcher correlation mechanism and at least one finger correlation mechanism. The mobile station receiver detects the arrival times and energy levels of the received signals and constructs a searcher histogram and a finger histogram associated with each pilot signal. Each of the searcher and finger histograms represents an arrival time distribution of samples corresponding to the received signals. The mobile station receiver processes samples contained within each of the searcher histograms and the finger histograms to generate a plurality of estimated early signal arrivals. The earliest signal arrival is determined from the plurality of estimated early signal arrivals.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to wireless communications systems and, in particular, to system and method for accurately estimating the earliest arrival of CDMA radio signals, either in the forward or reverse links. 
     2. Description of Related Art and General Background 
     Efforts are underway to augment wireless communications systems by adding the capability to locate the position of a particular mobile station (MS). The Federal Communications Commission (FCC) has promulgated a regulation directed to this capability (Docket No. 94-102, third report and order adopted Sep. 15, 1999, released Oct. 6, 1999). This regulation requires wireless carriers adopting hand-held position location solutions to locate the position of a mobile station making an emergency 911 call to within 50 meters for 67% of calls (and to within 150 meters for 95% of calls) by October 2001. 
     In satisfying this requirement, one approach to determining the position of a MS may be to use the available information at the base stations (BSs) and MSs of a wireless communication system, operating under Code Division Multiple Access (CDMA) schemes. CDMA is a digital radio-frequency (RF) channelization technique that is defined in the 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 July 1993 and herein incorporated by reference. Wireless communication systems employing this technology assign a unique code to each different communication signal and apply pseudonoise (PN) modulation to spread these communication signals across a common wideband spread spectrum bandwidth. As long as the receiving apparatus in a CDMA system has the correct code, it can successfully detect and select its signal of interest from the other signals concurrently transmitted over the same bandwidth. 
     FIG. 1 (Prior Art) illustrates a simplified block diagram of CDMA wireless communication system  100 . System  100  allows MS  110 , typically comprising mobile terminal equipment (TE2 device  102 ) and a wireless communication device (MT2 device  104 ) to communicate with an Interworking Function (IWF)  108 . 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. MS  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 MS  110 . BS  106  may also include, or be associated with, position processing capabilities (e.g., Position Determination Entity (PDE) server mechanisms). 
     On the forward link transmission path, BS  106  communicates with MS  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 have a plurality of uses, one of them is to identify the BS  106  best suited to accommodate reverse link transmissions. As such, pilot signals are instrumental in determining which BS  106  to “hand-off” the reverse link transmission to in order to seamlessly maintain communications as the MS  110  travels across different cells or sectors of cells. Pilot signals also provide a time and coherent phase reference to enable MS  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 MS  110  to distinguish between different pilot signals coming from different sectors or base stations. Each BS  106  may transmit up to 6 different pilot signals with 6 different PN offsets. Use of the same pilot signal code allows MS  110  to find system timing synchronization by conducting a search through all pilot signal code phases of the same code. 
     As is well known, signal transmissions traveling across air interface U m  may be subject to multipath propagation. As such, MS  110  may first receive a direct (i.e., line-of-sight (LOS)) signal corresponding to the forward link signal transmitted by BS  106 , followed by time-delayed and attenuated versions of the same signal due to multipath. There may be situations where the first LOS signal is not received and only the multipath components are present. MS  110  may determine the time of arrival (TOA) and energy of all received pilot signals to identify the earliest useable received pilot signal. 
     To determine the TOA of the received pilot signals, MS  110  may count and store the number of chips (or fractions thereof) of PN code sequences (i.e., PN chips) that lapse from a reference while the signals were received. MS  110  may then identify the earliest received pilot signal by detecting which pilot signal was received after the smallest number of lapsed PN chips. The reference (or zero arrival time) may in general be an arbitrary mark: because of this, isolated TOA measurements cannot be used directly in position determination algorithms. There is the need of at least two TOA measurements corresponding to pilots coming from different geographical points to overcome this arbitrary error. For instance, by subtracting said two measurements, we get a measurement proportional to the difference between the radial distances of the mobile to the two origins: the common error induced by the ambiguity in the zero timing falls out in the subtraction. 
     To compensate for the effects of multipath propagation, CDMA systems, such as system  100 , employ rake receivers, which process and combine the direct and multipath versions of the forward link pilot signal to generate a better received signal. FIG. 2 (Prior Art) depicts a high-level functional block diagram of a MS  110  receiver  200 , including a rake receiver demodulator  225  for coherently demodulating the forward link signals received by MS  110 . As indicated in FIG. 2, the radio-frequency/digital converter modulo  205  downconverts and digitizes the received signal from the antenna/producing digital samples. The digital samples are supplied to a rake receiver demodulator  225 , which includes a searcher  215 . 
     Searcher  215  is configured to search for signals by sweeping across the samples that are likely to contain multipath signal peaks in steps of one or half-PN chip increments. Searcher  215  then assigns finger correlators  210 A-C to the stronger multipath signals. Each finger correlator  210 A-C locks onto their assigned multipath signal, coherently demodulates the signal, and continues to track the signal until the signal fades away or the finger correlator  210 A-C is reassigned by searcher  215 . The demodulated outputs of finger correlators  210 A-C are then combined by combiner  220  to form a stronger received signal. 
     Given the ability to detect the TOA of forward link signals, CDMA systems may, at least in theory, exploit these capabilities to extract MS  110  location information. As noted above, MS  110  is capable of determining the TOA of the received multipath components. 
     As noted above, the promulgated FCC regulation requires the location of a MS to within 50 meters for 67% of calls. A limitation of current CDMA systems is their inability to estimate TOAs with the necessary resolution to comply with the location requirements. For example, counting lapsed PN sequences to within a tolerance of a PN chip to determine the earliest received pilot signal, is of no consequence in establishing a communications link with the closest BS. However, given the fact that a PN chip corresponds to approximately 800 ns., which translates into a radial distance of 240 meters, such a tolerance clearly fails to comply with the location requirements. 
     Furthermore, since the LOS signal may not be the strongest signal arriving at the receiver, isolating that first arriving signal will not be a trivial task. Note that using a multipath delayed signal for ranging information will have an inherent error due to the extra delay. 
     Another limitation of current CDMA systems is the effect of time offset jittering on finger correlators of rake receivers. As noted above, the searcher in a MS rake receiver detects the strongest forward link receive signals and assigns a finger correlator to track and coherently demodulate one of the detected signals. However, due to the resolution on the hardware, finger correlators may experience jitter as they attempt to track their assigned signal. The resolution of finger correlators are typically ⅛ of a PN chip, which translates to jittering jumps of approximately 24 meters. Cumulatively, such effects may compromise the accuracy of the ranging information. 
     Accordingly, what is needed is a system and method capable of accurately estimating the earliest arrival of CDMA forward and reverse link signals. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the need identified above by providing a novel system and method capable of accurately estimating the earliest arrival of forward and reverse link CDMA signals. 
     Although the description will be done for the forward link case where the receiver is the mobile station and the transmitters are the base stations, the method and apparatus of the present invention apply the same in the reverse link case where the base station acts as receiver and the mobile station is the transmitter. 
     System and methods consistent with the principles of the present invention as embodied and broadly described herein include a base station, or group of base stations, that transmit a plurality of pilot signals and a mobile station configured to receive a plurality of signals corresponding to one of the transmitted pilot signals. The mobile station includes a receiver containing a searcher correlating mechanism and at least one finger correlating mechanism. For each different pilot signal, the mobile station receiver detects the arrival times and energy levels of the multipath signals corresponding to said pilot and constructs a searcher histogram and a finger histogram representing an arrival time distribution of samples. The mobile station receiver processes the samples contained within searcher histogram and finger histogram to generate an estimate of the TOA for the first received multipath component of each pilot. At that point, the mobile station can choose to report all the results (one per pilot) to another entity (base station , PDE , . . . ),or if it has the knowledge of which PN pilot sequences are transmitted from which base stations, further process the measurements, reporting only one measurement per base station, corresponding to the smallest TOA of the pilots belonging to that base station. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 (Prior Art) is a block diagram illustrating a conventional CDMA wireless communication system. 
     FIG. 2 (Prior Art) is a block diagram depicting a conventional CDMA rake receiver demodulator. 
     FIG. 3A is a flow-chart illustrating a process for estimating the earliest arrivals of CDMA signals, constructed and operative in accordance with an embodiment of the present invention. 
     FIGS. 3B,  3 C depict histograms generated by an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
     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. 
     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. 
     FIG. 3A is a high-level flow diagram illustrating process  300 , constructed and operative in accordance with an embodiment of the present invention. As indicated in block B 360 , process  300  first establishes, at searcher  215  of MS rake receiver  200 , threshold levels for the minimum energy of pilot signals (E min ) to be processed, the minimum number of samples occurring for any bin (T min ) of a searcher histogram (to be described below). Threshold levels E min , and T min  will be used to discriminate between pilot signals, reflected multipath signals, noise, etc. and are, therefore, selected in a manner that ensures the processing of valid pilot signals. 
     In block B 362 , MS receiver  200  detects, for a particular BS (denoted BS j ), the relative TOA i  and energy level E i  for each signal P i  received by searcher  215  of MS rake receiver  200 . As noted above, each BS j  may transmit up to 6 different pilot signals and for a given PN offset corresponding to a particular pilot signal, searcher  215  sweeps across samples corresponding to received signals in order to detect signal peaks. For a given PN offset, searcher  215  may detect signal peaks that comprise a corresponding LOS pilot signal, reflected versions of the pilot signal, and noise. Upon detecting signal peaks, searcher  215  measures the peaks and produces two values, one indicating when the signal arrives (TOA), and one indicating the energy of that signal (E). As noted above, the calculation of TOA may be achieved by counting and storing the number of PN chips that lapse while each signal was received. 
     In block B 364 , MS receiver  200  discards from further processing, any signal containing an energy level E that is less than threshold level E min . By discarding signals with energy levels E less than E min , process  300  ensures that the TOA estimation is from a valid pilot signal. 
     For each PN offset, MS receiver  200 , in block B 366 , constructs a searcher histogram  390 A for the undiscarded signals P m -P n  based on their corresponding TOA m -TOA n . As is well known, a histogram depicts the distribution of a collection of values over a predefined interval. In this case, searcher histogram  390 A is constructed by collecting samples of signals having signal strengths above threshold level E min  over a search period corresponding to a particular PN offset. For a BS j  that transmits 3 pilot signals, MS receiver  200  may construct 3 separate searcher histograms  390 A- 390 C. 
     An exemplary searcher histogram  390 A for a particular PN offset is illustrated in FIG.  3 B. The horizontal axis represents relative TOA m  of an undiscarded signal P m , measured in bins (from earliest −36 to latest 15.7), and the vertical axis represents the number of samples occurring at the relative TOA m . Generally, the stronger the signals, the higher the number of occurrences within the bins: weak signals will be discarded more often by the E min  threshold. Each bin is configured to represent a fraction of a PN chip, which depends on the resolution of the hardware. In an exemplary implementation, a bin is equivalent to ⅛ of a PN chip. As indicated in FIG. 3B, searcher histogram  390 A contains three signal peaks A, B, C, as evidenced by three bins having the highest number of occurrences occurring at relative TOAs of −28, −16 and −4. 
     As noted above, for each PN offset, searcher  215  assigns a finger correlator  210 A to a signal to track and process the corresponding samples in order to demodulate the signal. After finger correlators  210 A- 210 C have been assigned to the strongest signal peaks (e.g., peaks A, B, C) by searcher  215 , MS receiver  200 , in block B 368 , constructs a finger histogram  395 A for all the assigned signals P m -P n . Much like searcher histogram  390 A, for a BS j  transmitting three pilot signals, process  300  may construct three separate finger histograms  395 A- 395 C. 
     An exemplary finger histogram  395 A is shown in FIG.  3 C. Although finger histogram  395 A is similarly constructed to searcher histogram  390 A, it is to be noted that finger histogram  395 A depicts the distribution of the assigned signals P m -P n  with a higher resolution than searcher histogram  390 A. As such, finger histogram  395 A is more accurate than searcher histogram  390 A and may indicate groups of signal peaks as the finger correlators  210 A- 210 C track pilot signals P m -P n . These group signal peaks are symptomatic of the jittering effects noted above. As illustrated in FIG. 3C, finger histogram  395 A contains a first significant group of peaks A′, proximately disposed at relative TOA−28, a second significant group of peaks B′, proximately disposed at relative TOA−17.5, and a third significant group of peaks C′, proximately disposed at relative TOA−2.9. 
     In block B 370 , MS receiver  200  locates the first bin in each of the searcher histograms  390 A- 390 C having the number of occurrences greater than or equal to T min . By locating the first bin with a significant number of samples, process  300  maximizes the chances of identifying the earliest arriving pilot signals P k  for each PN offset. 
     In block B 372 , MS receiver  200  constructs a narrow window around the first bin in each of the searcher histograms  390 A- 390 C as well as constructs a narrow window around the samples in each of the finger histograms  395 A- 395 C that correspond to the first bins of the searcher histograms  390 A- 390 C. The searcher histogram  390 A- 390 C and finger histogram  395 A- 395 C windows compensate for the differences in the resolution between searcher  215  and finger correlators  210 A- 210 C, which may result in the timing misalignment of the signal. Such misalignment is indicated in FIGS. 3B and 3C, where searcher histogram  390 A demonstrates signal peaks A, B, C at respective TOAs of −28, −16 and −4 while finger histogram  395 A demonstrates signal group peaks A′, B′, C′ centered at respective TOAs of −28, −17.5, and −2.9. 
     FIGS. 3B and 3C also depict the constructed windows for a single searcher histogram  390 A and finger histogram  395 A set. The windows may be centered at a specific bin and have bin offsets equivalent to ± a fraction of a PN chip (e.g., ±½ PN chip). For example, if the bins of the searcher histograms  390 A- 390 C and finger histograms  395 A- 395 C represent ⅛ of a PN chip, the windows would span 4 bins on either side of the respective bins for a window resolution of ±½ PN chip. 
     In block B 374 , MS receiver  200  processes the sample information contained within each set of searcher histogram  390 A- 390 C and finger histogram  395 A- 395 C windows to provide a timing estimate for each of the earliest arriving pilot signals P k . In particular, for each set of searcher histogram  390 A- 390 C and finger histogram  395 A- 395 C windows, process  300  combines and averages all the samples contained within the respective windows to obtain an average TOA value (TOA_mean k ) for each of the earliest pilot signals P k . If finger histograms  395 A- 395 C do not contain samples corresponding to the first bins of searcher histograms  390 A- 390 C, MS receiver  200  simply combines and averages the samples contained within the searcher histograms  390 A- 390 C window to produce TOA_mean k . 
     In block B 376 , MS receiver  200  produces a delay index D k  for each of the estimated earliest arriving pilot signals P k  transmitted by BS j . For each of the earliest arriving pilot signals P k , delay index D k  provides a metric that accurately quantifies the delay incurred by each signal. Delay index D k  is produced by subtracting a corresponding proportionate standard deviation quantity from each of the TOA_mean k  values calculated in block B 374 . As is well known, the standard deviation is a quantity that measures the distribution (i.e., spread) of a collection of samples. Subtracting the standard deviation from TOA_mean k , minimizes the error arising from reflections, noise, or interference, thereby providing a more accurate estimation of the timing for each of the earliest arriving pilot signals P k . MS receiver  200  may then forward the delay index D k  information to BS j  to determine the first pilot signal (P F ) from all the earliest arriving pilot signals P k . Note that the preceding description assumes that the mobile stations knows which pilots come from which base stations: should the mobile station lack such knowledge, it would report all the D k  values and leave further processing to another entity. 
     In block B 378 , process  300  determines P F  by selecting the minimum of the forwarded delay indices D k (D k,min ) produced for each of the earliest arriving pilot signals P k . By definition, D k,min  corresponds to the minimal delay incurred by any of the earliest arriving pilot signals P k  corresponding a given base station BS j . Therefore, by selecting D k,min , process  300  identifies the first pilot signal P F  from all the earliest arriving pilot signals P k . 
     Because MS  110  may not possess a priori knowledge of which BS j  is transmitting which PN offset, the selection of D k,min  may be performed by BS j , or an associated PDE server (noted above), which has that knowledge. 
     Finally, in block B 380 , process  300  increments a counter and returns to block B 362  to point to a new BS j+1  in order to determine the earliest arriving pilot signal originating therefrom. If the mobile does not have the knowledge of which pilot signals correspond to which base stations, the process starting at B 362  would loop across all pilot signals (instead of across all base stations) and the final step B 378  will need to be performed somewhere else. 
     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, the invention may be implemented in part or in whole as a hard-wired 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. 
     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.