METHOD AND APPARATUS FOR PROVIDING A REFERENCE SIGNAL FROM TIME DIVISION MULTIPLEXED PILOT DATA

Abstract: Techniques for generating a reference signal based on time-multiplexed pilot data. A modulated signal is received (FIG. 7) and processed to provide received samples, which are then further processed to provide pilot symbols. A reference signal is generated based on the available pilot symbols (722a, 722b). The reference signal comprises a sequence of "predicted" pilot symbols representative of future estimates of one or more characteristics (e.g., the phase and amplitude) of one or more carrier signals used to generate the modulated signal (FIG's 5A-5E). The reference signal can be generated using a number of techniques. For example, the reference signal can be generated by (e.g., linearly) extrapolating pilot symbols from prior time intervals (FIG. 5B), curve fitting the pilot symbols (FIG. 5C), or (normal or ensemble) averaging (unweighted or weighted) pilot symbols (FIG's 5D, 5E). For the W-CDMA standard, the processing to generate the pilot symbols typically includes decovering the received samples with a channelization code to provide channelized symbols (720a, 720b), and multiplying the channelized symbols with a particular pilot symbol pattern. The pilot symbols from each slot may also be filtered to generate one or more filtered pilot symbols.

Description: METHOD AND APPARATUS FOR PROVIDING A REFERENCE SIGNAL FROM TIME DIVISION MULTIPLEXED PILOT DATA BACKGROUND OF THE INVENTION I. Field of the Invention The present invention relates to data communication. More particularly, the present invention relates to a novel and improved method and apparatus for providing a reference signal from time division multiplexed (TDM) pilot data. II. Description of the Related Art A modern day communication system is required to support a variety of applications. One such communication system is a code division multiple access (CDMA) system that supports voice and data communication between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Patent No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS," and U.S. Patent No. 5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING WANEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM," both assigned to the assignee of the present invention and incorporated herein by reference. A CDMA system is typically designed to conform to one or more standards. One such first generation standard is the "TIA/EIA/IS-95 Terminal- Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System," hereinafter referred to as the IS-95 standard and incorporated herein by reference. IS-95 compliant CDMA systems are able to transmit voice data and (albeit not efficiently) packet data. A newer generation standard that can more efficiently transmit packet data is offered by a consortium named "3 rd Generation Partnership Project" (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214, which are readily available to the public. The 3GPP standard is hereinafter referred to as the W-CDMA standard and incorporated herein by reference. The W-CDMA standard defines a channel structure capable of supporting a number of users and designed for efficient transmission of packet data. In accordance with the W-CDMA standard, a user at a terminal is typically assigned a "dedicated" channel on the downlink and the uplink for the duration of the call. The downlink refers to transmission from the base station to the terminal, and the uplink refers to transmission from the terminal to the base station. The dedicated channel can be used for voice communication and may also be used for transmission of packet data. In accordance with the W-CDMA standard, the dedicated channel has a time division multiplexed (TDM) structure in which a data transmission is partitioned into radio frames, with each radio frame having a duration of 10 msec and including 15 slots. Each slot is further partitioned into a number of fields used to carry packet data and other information. For example, one field in each slot is used for pilot data associated with the particular dedicated channel, one or more fields in each slot are used for packet data, and other field(s) may be used for signaling information. At the transmitter unit, the packet, signaling, and pilot data is used to modulate an inphase and quadrature carrier signals. At the receiver, the time-multiplexed pilot data is processed and used to generate a "reference" signal having phase and amplitude that approximate that of the received (quadrature) carrier signals. The reference signal is then used to coherently demodulate the received signal to recover the packet and signaling data. The pilot data is only available for a fraction of each slot, and the reference signal is estimated for the time periods in which pilot data is not present. The reference signal is typically generated at the receiver using pilot data that periodically appears for a short period of time. The accuracy of the generated reference signal directly impacts the performance of the demodulation process, which in turn determines the performance of the communication system. Moreover, if the reference signal can be "predicted", the demodulation can be performed in real-time, thereby reducing or eliminating the need to temporarily store the received data. Thus, techniques that can be used to accurately and efficiently generate reference signals based on time-multiplexed pilot data, such as that used for the dedicated channel in the W-CDMA system, are highly desirable. SUMMARY OF THE INVENTION The present invention provides techniques to generate reference signals representative of future estimates (i.e., predictions) of the phase and amplitude of the carrier signals used to generate a modulated signal in the communication system. Each reference signal is generated based on currently available pilot data (e.g., from prior slots). An aspect of the invention provides a method for generating a reference signal based on time-multiplexed pilot data. In accordance with the method, a modulated signal is received and processed to provide received samples, which are then further processed to provide pilot symbols. A reference signal is generated based on the available pilot symbols. The reference signal comprises a sequence of "predicted" pilot symbols representative of future estimates of one or more characteristics (e.g., the phase and amplitude) of one or more carrier signals used to generate the modulated signal. The reference signal can be generated using a number of techniques. For example, the reference signal can be generated by (e.g., linearly) extrapolating pilot symbols from prior slots, curve fitting the pilot symbols, (normal or ensemble) averaging the (unweighted or weighted) pilot symbols, or some other technique. Another aspect of the invention provides a receiver unit operative to process a physical channel in a CDMA communication system. The receiver unit includes a receiver and at least one demodulator element. The receiver receives and processes a modulated signal, and provides received samples indicative of the data transmitted on the physical' channel. Each demodulator element includes a data processing unit and a pilot processing unit. The data processing unit processes the received samples to provide channelized symbols. The pilot processing unit processes the channelized symbols to provide pilot symbols, and also generates a reference signal based on the available pilot symbols. The reference signal comprises a sequence of predicted pilot symbols representative of future estimates of one or more characteristics (e.g., phase and amplitude) of one or more carrier signals used to generate the modulated signal. The reference signal can be generated based on the available pilot symbols and using extrapolating, curve fitting, averaging, weighted averaging, or ensemble averaging, or other techniques. Various aspects, embodiments, and features of the invention are described in further detail below. BRIEF DESCRIPTION OF THE DRAWINGS The features, nature, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein: FIG. 1 is a diagram of a spread spectrum communication system that supports a number of users; FIG. 2 is a simplified block diagram of an embodiment of the signal processing for a downlink physical channel; FIG. 3 is a block diagram of a modulator, a transmitter, and antennas that support transmission of the downlink physical channel over one or two antennas; FIG. 4A is a diagram of a frame format and a slot format for the dedicated physical channel as defined by the W-CDMA standard; FIG. 4B is a diagram of a portion of a particular data transmission on the dedicated physical channel; FIGS. 5A through 5E are diagram respectively illustrating the use of interpolation, extrapolation, curve fitting, weighted averaging, and ensemble averaging to generate a reference signal; FIG. 6 is a block diagram of an embodiment of a portion of a receiver unit that can be used to receive and demodulate a physical channel; and FIG. 7 is a block diagram of an embodiment of a finger element that can be used to implement one finger element of a rake receiver shown in FIG. 6. DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS FIG. 1 is a diagram of a spread spectrum communication system 100 that supports a number of users. System 100 provides communication for a number of cells 102a through 102g, with each cell 102 being serviced by a corresponding base station 104. Various terminals 106 are dispersed throughout the system. In an embodiment, each terminal 106 communicates with one or more base stations 104 on the downlink and uplink at any given moment, depending on whether the terminal is in soft handoff. System 100 may be designed to support one or more CDMA standards, such as the IS-95 standard, the W- CDMA standard, other standards, or a combination thereof. As shown in FIG. 1, base station 104a transmits data to terminals 106a and 106j on the downlink, base station 104b transmits data to terminals 106b and 106j, base station 104c transmits data to terminal 106c, and so on. In FIG. 1, the solid line with the arrow indicates a transmission from the base station to the terminal. A broken line with the arrow indicates that the terminal is receiving the pilot signal, but no user-specific data transmission, from the base station. The uplink communication is not shown in FIG. 1 for simplicity. In certain transmission modes in the W-CDMA standard, a terminal can receive multiple transmissions from multiple antennas of a single base station for certain physical channel types such as the downlink dedicated physical channel (DPCH). As shown in FIG. 1, terminal 106a receives multiple transmissions from base station 104a, terminal 106d receives multiple transmissions from base station 104d, and terminal 106f receives multiple transmissions from base station 104f . FIG. 2 is a simplified block diagram of an embodiment of the signal processing for a downlink physical channel. At the transmitter unit, data is sent, typically in packets, from a data source 212 to an encoder 214. Encoder 214 performs a number of functions, depending on the particular CDMA system or standard being implemented. Such encoding functions typically include formatting each data packet with the necessary control fields, cyclic redundancy check (CRC) bits, and code tail bits. Encoder 214 then encodes one or more formatted packets with a particular coding scheme, performs rate matching of the encoded packet (e.g., by repeating or puncturing the symbols), and interleaves (or reorders) the symbols within the packets. For the W-CDMA standard, the interleaved data is then segmented into radio frames, further segmented into physical channel, interleaved, and then mapped to one or more physical channels. The processing performed for a downlink physical channel is described in further detail in the W-CDMA standard (e.g., Document No. 3G TS 25-212) and the processing performed for a forward link traffic channel is described in further detail in the IS-95 standard. The channel data designated for each physical channel conforms to a particular frame format defined for that physical channel. The channel data for the dedicated physical channel in the W-CDMA standard includes packet data, signaling data, and pilot data, or a combination thereof, and the frame format for the dedicated physical channel is described in further detail below. The channel data for each physical channel is provided to a modulator (MOD) 216 and may be scrambled with a scrambling sequence (for the IS-95 CDMA system), covered with a channelization code, and spread with spreading codes (e.g., the PN codes). The spreading with the spreading codes is referred to as "scrambing" by the W-CDMA standard. The channelization code can be an orthogonal variable spreading factor (ONSF) code (for the W- CDMA system), a Walsh code (for the IS-95 CDMA system), or some other orthogonal code, again depending on the particular CDMA system or standard being implemented. The spread data is then provided to a transmitter (TMTR) 218 and quadrature modulated, filtered, and amplified to generate one or more downlink signals. The downlink signal(s) are transmitted over the air from one or more antennas 220. The downlink processing is described in further detail in the IS-95 and W-CDMA standards. At the receiver unit, the downlink signal(s) are received by an antenna 230 and routed to a receiver (RCNR) 232. Receiver 232 filters, amplifies, (quadrature) demodulates, samples, and quantizes the received signal. The digitized samples are then provided to a demodulator (DEMOD) 234 and despreaded (or descrambled) with despreading codes and, for each physical channel being processed, channelized with a channelization code. For the IS-95 CDMA system, the despread samples are descrambled with a descrambling code after the channelization. The dispreading, descrambling, and channelization codes used at the receiver unit correspond to the codes used at the transmitter unit. The demodulated data is, then provided to a decoder 236 that performs the inverse of the functions performed at encoder 214 (e.g., the de-interleaving, decoding, and CRC check functions). The decoded data is provided to a data sink 238. A controller 240 can direct the operation of demodulator 234 and decoder 236. A memory unit 242 couples to demodulator 234 (and possibly also to controller 240, as indicated by the dashed line) and, in certain modes of operation, can be used to store intermediate results, or data, from demodulator 234. The hardware, as described above, supports transmissions of packet data, messaging, voice, video, and other types of communication on the downlink. A bi-directional communication system also supports uplink transmission from the terminal to the base station. However, the uplink processing is not shown in FIG. 2 for simplicity. FIG. 3 is a block diagram of a modulator 300, a transmitter 302, and antennas 304 that support transmission of a physical channel over one or two antennas on the downlink. The processing units shown in FIG. 3 can be used to support the "no Transmit Diversity" mode defined by the W-CDMA standard in which data transmission occurs over one antenna and the "Transmit Diversity" mode in which data transmission occurs over two antennas. The Transmit Diversity mode is also referred to as the "space time block coding transmit antenna diversity" (STTD) mode in the W-CDMA standard. The data for the physical channel (i.e., the channel data) is provided to a STTD encoder 310 that generates STTD encoded data for each of the antennas used to transmit the channel data. The operation of STTD encoder 310 is described in further detail in U.S. Patent Application Serial No. 09/655,609, entitled "METHOD AND APPARATUS FOR PROCESSING A PHYSICAL CHANNEL WITH PARTIAL TRANSPORT FORMAT INFORMATION," filed on the same date herewith and assigned to the assignee of the present invention and incorporated herein by reference. The STTD encoded data for each antenna is provided to a respective channelizer 320 that "covers" the data with a channelization code assigned to the physical channel to generate "channelized" data. In the W-CDMA system, the same channelization code is used for both antennas in the STTD mode. Within channelizer 320, the STTD encoded data is provided to an I/Q demultiplexer (DEMUX) 322 that demultiplexes the data into inphase (I) and quadrature (Q) data. The I data and Q data are provided to respective multipliers 324a and 324b and covered (i.e., multiplied) with the channelization code, C d , assigned to the physical channel. Multipliers 324a and 324b perform covering using the channelization code in similar manner as the covering performed with Walsh code in the IS-95 CDMA system. Channelization is described in further detail in the IS-95 and W-CDMA standards and in the aforementioned U.S. Patent Application Serial No. 09/655,609 filed on the same date herewith. For the IS-95 CDMA system, Walsh codes having a fixed length of 64 chips are used to cover the traffic channels, with each traffic channel having a variable but limited data rate (e.g., < 32 Kbps). For the W-CDMA system, OVSF codes having a variable length are used to cover the traffic channels. The length of the OVSF code varies from 4 to 512 chips and is dependent on the data rate of the physical channel. OVSF codes are described in further detail in the W-CDMA standard and the aforementioned U.S. Patent Application Serial No. 09/655,609 filed on the same date herewith. The covered Q data from multiplier 324b is provided to a multiplier 326 and multiplied with the complex symbol, j, to generate the imaginary part of the channelized data. The real part from multiplier 324a and the imaginary part from multiplier 326 are combined by an adder 328 to provide the complex channelized data. The channelized data for each antenna is then scrambled (i.e., spread) with a complex scrambling code, PN, by a multiplier 328 and scaled with a weight factor, G, by a multiplier 330. The weight factor, G, is used to adjust the transmit power of the physical channel. The scrambled and weighted data from multiplier 332b comprises the processed data for the physical channel. The processed channel data for each antenna is provided to a respective combiner 334 that also receives the processed data for other physical channels to be transmitted from that antenna. Each combiner 334 combines the processed channel data received for the physical channels and generates output data for the associated antenna. The composite data for each antenna is further multiplied with a complex weight factor, W, by a multiplier 336. As specified in the W-CDMA standard, the weight factor is used for phase adjustment in closed loop mode 1 and for phase /amplitude adjustment in closed loop mode 2. Closed loop modes 1 and 2 are two operating modes of the STTD mode. The data from each multiplier 336 is provided to a respective transmitter 302 that converts the data into an RF modulated signal, which is then transmitted from a respective antenna 304. FIG. 4A is a diagram of a frame format and a slot format for the dedicated physical channel as defined by the W-CDMA standard. Generally, a different frame format is defined by the W-CDMA standard for each type of physical channel such as the downlink dedicated channel (DPCH), the downlink shared channel (DSCH), and so on. The data to be transmitted on each physical channel is partitioned into radio frames, with each radio frame including 15 slots labeled as slot 0 through slot 14. Each slot is further partitioned into one or more fields used to carry packet, signaling, and pilot data, or a combination thereof. As shown in FIG. 4A, for the dedicated physical channel, a slot 410 includes a first data (Datal) field 420a, a second data (Data2) field 420b, a transmit power control (TPC) field 422, a transport format combination indicator (TFCI) field 424, and a pilot field 426. Data fields 420a and 420b are used to send packet data for the dedicated physical channel. Transmit power control field 422 is used to send power control information to direct the receiver unit to adjusts its transmit power on the uplink either up or down to achieve the desired performance while minimizing interference. Transport format combination indicator field 424 is used to send information indicative of the format (e.g., the bit rate, channelization code, and so on) of a shared physical channel associated with the dedicated physical channel. Pilot field 426 is used to send pilot data for the dedicated physical channel. Table 1 lists some of the slot formats defined by the W-CDMA standard version V3.1.1 for the dedicated physical channel. Each slot format in Table 1 defines the length (in number of bits) of each field in the slot. As shown in Table 1, the bit rate of the dedicated physical channel can vary between a range of values (e.g., from 15 Kbps to 1920 Kbps) and the number of bits in each slot varies correspondingly. One or more of the fields described above may be omitted (i.e., length = 0) for some of the slot formats. In accordance with the W-CDMA standard, each dedicated physical channel is associated with its own pilot information. The dedicated pilot supports beam forming to provide improved directivity for data transmissions, which improves performance and reduces interference. The phase of a modulated signal can (and typically does) vary from one beam-formed signal to another. Thus, each dedicated physical channel is provided with its own pilot. As defined by the W-CDMA standard, the pilot data for each slot comprises a particular symbol pattern (i.e., a defined sequence of complex symbols). At the transmitter unit, the pilot data is used to modulate the inphase and quadrature carrier signals. Because the pilot data is of a known pattern, the receiver unit can process the received signal, recover the pilot data, and estimate the phase and amplitude of the received carrier signals. The pilot data can thus be used to generate a "reference" signal at the receiver unit, which is then used to coherently demodulate the packet and signaling data transmitted on the dedicated physical channel. The reference signal can also be used to coherently demodulate other physical channels such as the shared physical channel. Table 2 lists the pilot symbol patterns defined by the W-CDMA standard for the no diversity antenna. Referring to Table 1, the pilot field can have one of four different lengths ranging from 2 bits to 16 bits. Also, in accordance with the W-CDMA standard, a different pilot symbol pattern is used for each of the 15 slots of a particular radio frame. Thus, four sets of 15 pilot symbols patterns are defined and are shown in Table 2. Each pilot symbol pattern includes 1, 2, 4, or 8 complex symbols used to modulate the inphase and quadrature carrier signals at the transmitter. The W-CDMA standard defines two STTD operating modes. In the closed loop mode 1, orthogonal pilot symbol patterns are used for the two transmit antennas. The pilot symbol patterns shown in Table 2 are used for the first (non-diversity) antenna and the pilot symbol patterns shown in Table 3 are used for the second (diversity) antenna. In the closed loop mode 2, the pilot symbol patterns shown in Table 2 are used for both antennas. At the transmitter unit, pairs of data bits in the Datal, TPC, TFCI, Data2, and Pilot fields are grouped into complex symbols that are then used to modulate the inphase and quadrature carrier signals. Because the pilot data is of a known symbol pattern, the receiver unit can process the pilot data and determine the phase and amplitude of the received carrier signals during the time interval in which pilot data is valid. The pilot data is time multiplexed and not available during the transmission of the packet and signaling data. However, the phase and amplitude of the carrier signals are needed at these times to coherently demodulate the data in the Datal, Data2, TPC, and TFCI fields. FIG. 4B is a diagram of a portion of a particular data transmission on the dedicated physical channel. A number of slots are shown, with each slot including a combination of data fields 420a and 420b, TPC field 422, TFCI field 424, and pilot field 426. Pilot data is transmitted at the end of each slot, as designed by pilot fields 426a through 426e in FIG. 4B. The pilot data in pilot fields 426 are used to generate a reference signal that is then used to demodulate the packet and signaling data transmitted between the pilot fields. A number of methods can be used to coherently demodulate packet and signaling data that is time multiplexed with pilot data. In some methods, the packet and signaling data for the current slot (z) is temporarily stored until the pilot data P(i) for the current slot (which is transmitted at the end of the slot in the W-CDMA system) is received and processed (with old pilot data) to generate the reference signal. The packet and signaling data for the current slot is then retrieved and demodulated using the generated reference signal. For these methods, the pilot data P(ϊ) at the end of the current slot (i) is used to generate "past" estimates of the phase and amplitude of the carrier signals, and the data needs to be temporarily stored until the pilot data is received and processed. In some other methods, the pilot data P(i - 1) at the end of the prior slot (ϊ-1) is processed (typically along with prior pilot data, e.g., P(i — 2) ) to generate the reference signal that is then used to demodulate the packet and signaling data transmitted in the current slot (i). For these methods, the pilot data in prior slots is used to generate "future" estimates of the phase and amplitude of the carrier signal. These methods allow for real-time processing of the received data in the current slot (i), and the data storage requirement may be substantially reduced or possibly eliminated. Various techniques can be used to generate future estimates of the phase and amplitude of the carrier signals, as described in further detail below. FIG. 5A is a diagram illustrating the use of interpolation to generate the reference signal. Interpolation is used to generate "past" estimates of the phase and amplitude of the received carrier signals. Pilot data 510a and 510b is received at slots (i) and (z ' -l), respectively, and processed to generate the phase and amplitude of the reference signal for the time period between the received pilot data 510a and 510b (as represented by a dashed line 520). Packet and signaling data received between pilot data 510a and 510b is temporarily stored until pilot data 510a is received and processed. The stored data is then retrieved and demodulated with the generated reference signal. This method requires storage of one or more slots of packet and signaling data until pilot data 510a at the end of slot (i) is received and processed. FIG. 5B is a diagram illustrating the use of extrapolation to generate the reference signal. Extrapolation can be used to generate "future" estimates of the phase and amplitude of the received carrier signals. Pilot data 510b and 510c is received at slots (z ' -l) and (z ' -2), respectively, and processed to generate the phase and amplitude of the reference signal (as represented by a dashed line 522) for the time period after pilot data 510b, e.g., between pilot data 510b at slot (z ' -l) and the pilot data at slot (i). Packet and signaling data received after pilot data 510b can be demodulated approximately in real-time with the generated reference signal. This method can reduce or eliminate the storage requirement for packet and signaling data. FIG. 5C is a diagram illustrating the use of curve fitting to generate the reference signal. Curve fitting can be used to generate past and/ or future estimates of the phase and amplitude of the received carrier signals. As shown in FIG. 5C, pilot data 510b, 510c, and 510d is received at slots (z ' -l), (z-2), and (z- 3), respectively, and processed using a curve fitting technique to generate the phase and amplitude of the reference signal (as represented by a dashed line 524) for the time period after pilot data 510b. Such curve fitting technique may be a linear regression algorithm, a non-linear regression algorithm, or any curve fitting algorithm known in the art. The linear regression algorithm minimizes the sum of the squares of the vertical distances between the received pilot data and a curve. Fewer or greater number of pilot data (i.e., <> three) can be used for the curve fitting. Packet and signaling data after pilot data 510b can be demodulated approximately at real-time with the generated reference signal. FIG. 5D is a diagram illustrating the use of weighted averaging to generate the reference signal. Weighted averaging can be used by itself or in combination with one or more other techniques to generate past and/or future estimates of the phase and amplitude of the received carrier signals. Pilot data 510b, 510c, and 510d is received at slots (z ' -l), (z ' -2), and (z ' -3), respectively, and weighted with a set of weighting factors D., D 2 , and D 3 , respectively, to generate the phase and amplitude of the reference signal (as represented by a dashed line 526) for the time period after pilot data 510b. The weighting factors may be selected to weigh recent pilot data more heavily and to give less weight to older pilot data. For example, the pilot data may be exponentially weighted such that pilot data 510b, 510c, and 510d at slots (z ' -l), (z ' -2), and (z-3) are weighted by weighting factors of 0.5, 0.25, and 0.125, respectively. Extrapolation, curve fitting, or other averaging technique can then be used to generate the reference signal based on the weighted pilot data. Packet and signaling data after pilot data 510b can be demodulated approximately in real-time with the generated reference signal. FIG. 5E is a diagram illustrating the use of ensemble averaging to generate the reference signal. Ensemble averaging can also be used by itself or in combination with one or more other techniques to generate past and /or future estimates of the phase and amplitude of the received carrier signals. As shown in FIG. 5E, pilot data 510b, 510c, 510d, and 510e are received at slots (z- 1), (z ' -2), (z ' -3), and (z ' -4), respectively, and used to generate a line (or curve) 530 that best fit the received pilot data (e.g., a line or curve that minimizes the sum of the squares of the vertical distances between the pilot data and the line or curve). The reference signal can then be generated based on line (or curve) 530. For example, the reference signal can be represented with a line 532 having the same slope as line 530 and starting from pilot data 510b (as shown in FIG. 5E), or continuing from the end of line 530, or connected to some other point. Similar to FIG. 5D, the pilot data can be weighted to give greater weight to recent pilot data and less weight to older pilot data. FIGS. 5B through 5E show some techniques that can be used to generate the reference signal comprising future estimates (i.e., "predictions") of the phase and amplitude of the received carrier signals. Other techniques can also be used to generate future estimates of the phase and amplitude and are within the scope of the present invention. In some embodiments, the phase and amplitude of the reference signal can be defined with a (complex) polynomial. For example, a first order polynomial can be used for linear interpolation and linear extrapolation. The first order polynomial for the reference signal, P(i, j) , can be expressed as: P(i, j) = ( (0 • j + a ϋ (0 / E q (!) where a (i) and a 0 (ϊ) are the coefficients for the polynomial and are applicable for slot z, the index ; ' is the time unit (e.g., in symbol periods, as described below) within slot z, and P(i, j) is a complex sequence that represents the phase and amplitude of the reference signal at time unit / of slot i. The coefficients a.(ϊ) and a 0 (i) are typically recomputed as new pilot data is received, but may be reused (e.g., extrapolated to the new slot) if the received pilot data is degraded (or not available). In an embodiment, the reference signal comprises a sequence of complex "reference symbols", one reference symbol for each symbol time. Each reference symbol identifies the instantaneous phase and amplitude of the reference signal at the particular symbol time. Referring to Table 1, each slot includes a particular number of bits, with the number being dependent on the bit rate of the slot. A pair of bits is grouped to form a complex symbol, and the number of symbols in each slot is thus half the number of bits. For demodulation, a reference symbol indicative of the instantaneous phase and amplitude of the reference signal at symbol time ; ' is used to demodulate one or more symbols received for one or more physical channels at symbol time ; ' . The number of reference symbols generated for each slot is dependent on the particular slot format, and ranges from five symbols (for 15 Kbps) up to 640 symbols (for 1920 Kbps). In equation (1), the index ; thus ranges from (0, 1, ... 4) for the lowest bit rate of 15 Kbps, up to (0, 1, ... 639) for the highest bit rate of 1920 Kbps. For linear extrapolation, the coefficients for equation (1) can be expressed as: a 0 (i) = P(i -l) . where P(i-1) and P(i- 2) are the pilot data at slots (z ' -l) and (z ' -2), respectively, which are indicative of the phase and amplitude of the received carrier signals, and T slot is the duration of a slot (e.g., T sl0l = 5, ... or 640 symbols, depending on the bit rate of the slot) . Higher order polynomials can be used for other reference signal generating techniques. For example, for a curve fitting technique that uses pilot data from four slots, a 3 rd order polynomial can be defined to express the reference signal P(i,j) . Generally, an N-th order polynomial for the reference signal can be expressed as: P(i,j) = ∑ k (i) - j k . Eq (2) k=0 The coefficients a 0 (i), a x (i) , ..., a N (ϊ) for equation (2) can be determined using any of the techniques described above (e.g., curve fitting, averaging, and so on). FIG. 6 is a block diagram of an embodiment of a portion of a receiver unit 600 that can be used to receive and demodulate a physical channel, including one transmitted from multiple transmit antennas in the STTD mode defined by the W-CDMA standard. One or more RF modulated signals from one or more transmit antennas are received by an antenna 610 and provided to a receiver (RCVR) 612 that conditions (e.g., amplifies, filters, and so on) the received signal and quadrature downconverts the conditioned signal to an intermediate frequency (IF) or baseband. Receiver 612 also samples and quantizes the downconverted inphase and quadrature signals to generate received samples that are provided to a rake receiver 620. Although a rake receiver is shown in FIG. 6 for processing the physical channel, other receiver structures and implementations can also be used and are within the scope of the present invention. In typical implementations, the received signal is sampled at a sample rate, f s , that is higher than the chip rate, f c , of the received signal. For example, the chip rate may be f c = 1.2288 Mcps for the IS-95 CDMA system (or 3.84 Mcps for the W-CDMA system) but the sample rate may be, for example, 8 times (i.e., δxchip), 16 times (i.e., lόxchip), 32 times (i.e., 32xchip), or other multiple of the chip rate. The higher sample rate allows for fine adjustment of the timing to "zoom in" on a path position. As shown in FIG. 6, rake receiver 620 includes a searcher element 622 and a number of finger elements 630a through 630n. Each of these elements receives the samples from receiver 612 and performs the tasks associated with the element or as directed by a controller 640. For example, searcher element 622 may be instructed by controller 640 (or assigned) to search for strong instances of the received signal. The strong signals may be present at different time offsets, and can be identified by searcher element 622 by processing the samples with different parameters (e.g., different PN codes, different time offsets, and so on). Searcher 622 may be designed to provide data corresponding to the searched signal or an indication of the search result to controller 640. Controller 640 assigns finger elements 630 to demodulate the strongest instances of the received signal, as determined with the assistance of searcher element 622. Each assigned finger element 630 performs demodulation of one physical channel for one instance of the received signal (i.e., a signal at a particular assigned time offset), as directed by controller 640. Each assigned finger element 630 provides recovered symbols (e.g., S A ) corresponding to the assigned instance of the received signal. The demodulation to generate the recovered symbols is described in further detail below. The recovered symbols from all assigned finger elements 630 (e.g., S A , S B , ... S N ) axe then provided to a combiner 632 and combined to provide composite symbols that are more indicative of the transmitted data. The combined symbols represent the recovered channel data, and are provided to the subsequent processing block (e.g., the decoder). The design and operation of a rake receiver for an CDMA system is described in further detail in U.S. Patent No. 5,764,687, entitled "MOBILE DEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM," and U.S. Patent No. 5,490,165, entitled "DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS," both assigned to the assignee of the present invention and incorporated herein by reference. Although not shown in FIG. 6 for simplicity, each finger element can also include a lock detector that computes a quality indicator (e.g., an average energy) of the recovered symbols at the finger element and masks the recovered symbols from the finger element if the quality indicator does not exceed a particular threshold. The masking ensures that only finger elements receiving signal of sufficient strength and reliability will contribute to the combined output, thus enhancing the quality of the recovered symbols. FIG. 7 is a block diagram of an embodiment of a finger element 700 that can be used to implement one finger element 630 in FIG. 6. The finger element is also referred to as a demodulation element. The received samples from receiver 612 are provided to a multiplier 710 and descrambled with a complex descrambling code, PN, corresponding to the scrambling code used at the transmitter unit and having a time offset assigned to the finger element. The descrambled samples include data for all physical channels in the received signal. At the transmitter unit, each physical channel is channelized with a respective channelization code (e.g., a particular Walsh code in the IS-95 CDMA system, or a particular OVSF code in the W-CDMA system) to generate data that is orthogonal to those of other physical channels. At the receiver unit, to process a particular physical channel, the descrambled samples are provided to a multiplier 712 and decovered (i.e., multiplied) with the channelization code, C d , assigned to the physical channel being processed. The output from multiplier 712 is provided to an accumulator 714 and accumulated over the length of the channelization code, C d , to generate channelized symbols. For example, if the bit rate of the physical channel is 1.92 Mbps, accumulator 714 accumulates the decovered samples from multiplier 712 over a 4-chip period to provide the channelized symbols. On the other extreme, if the bit rate of the physical channel is 7.5 Kbps, accumulator 714 accumulates the decovered samples over a 512-chip period to provide the channelized symbols. In instances where the channelization code, C d , is not known (e.g., for the shared physical channel in the W-CDMA system), a "hypothesized" channelization code can be used to process the received samples to generate intermediate results (i.e., "channelized samples") that can be stored and, upon determination of the actual channelization code used for the physical channel, further processed provide the channelized symbols. This partial processing of the physical channel is described in detail in the aforementioned U.S. Patent Application Serial No. 09/655,609filed on the same date herewith. Referring back to FIG. 4A, the channelized data from accumulator 714 include time-multiplexed packet, signaling, and pilot data. In accordance with the W-CDMA standard, for closed loop mode 1, orthogonal pilots are used for the two transmit antennas. The orthogonal pilots for the two antennas are generated by using different pilot symbol patterns, W P1 and W P2 , defined in Table 2 and Table 3, respectively. Thus, at the receiver unit, the pilot symbol patterns are removed by multiplying the channelized symbols with the same pilot symbol patterns, W P1 and W κ , by multipliers 720a and 720b, respectively. Each complex pilot symbol from each of multipliers 720a and 720b is indicative of the instantaneous phase and amplitude of the respective pilot at the particular symbol time. The pilot symbols from multipliers 720a and 720b are then provided to respective pilot processors 722a and 722b. Each pilot processor 722 receives the pilot symbols, P , for each slot and generates the reference signal, P , based on pilot symbols from previous slots. Referring to Table 1, the pilot field for a particular slot can vary from 2 bits (i.e., one symbol) to 16 bits (i.e., eight symbols). Thus, each pilot processor 722 receives one to eight pilot symbols for each slot and can filter the pilot symbols for each slot to generate one or more filtered pilot symbols. For example, each pilot processor 722 can filter (e.g., average) the pilot symbols received in each slot to generate one filtered pilot symbol representative of the phase and amplitude of the pilot for that slot. Alternatively, at the other extreme, each pilot processor 722 may not performed any filtering of the pilot symbols. Each pilot processor 722 further processes the filtered pilot to generate the reference signal, P . For example, for linear extrapolation, pilot processor 722 can generate the reference signal, P(i, j), for the current slot (z) based on the pilot data, P(i - 1) and P(i - 2) , for the two immediately previous slots (z ' -l) and (z-2). Each pilot processor 722 can also generate the reference signal using curve fitting, weighted averaging, ensemble averaging, or other techniques. In accordance with the W-CDMA standard, pilot data is transmitted in time-multiplexed time intervals separated by packet and signaling data. Thus, the received pilot data is filtered and processed in the manner described above to generate one or more reference signals, P γ and P 2 (depending on the operating mode), which are used to demodulate the packet and signaling data. The pilot data may also be provided to a pilot detector (DET) 738 that provides an indication of the quality of the received pilot(s). In a specific implementation, pilot detector 738 measures the power of the received pilot(s) and provides a pilot quality estimate, P Q . The reference signals, P 1 and P 2 , from pilot processors 722a and 722b and the channelized symbols, X , from accumulator 714 are provided to a data recovery element 740 that performs the necessary computations to generate the recovered symbols for the finger element. The data recovery processing is described below. In FIG. 7, multipliers 720a and 720b and pilot processors 722a and 722b comprise a portion of the pilot processing unit of the finger element. For an operating mode in which pilot data is transmitted from only one antenna, only one of the pilot processing paths is needed and the other pilot processing path can be disabled. Multiplier 712 and accumulator 714 comprise a portion of the data processing unit of the finger element. The reference signals are estimates of the carrier signals that are modulated by the data at the transmitter unit. Due to propagation delays and path loss, the amplitude and phase of the received modulated signal vary over time. These variations are also reflected in the pilot, and are detected and estimated by the reference signals. Thus, the reference signals are used to coherently demodulate the received signal. In each assigned finger element, the reference signal, P , and the channelized symbols, X , (which are estimates of the "actual" received symbols, X ), can be expressed as: P = {p 1 ,P Q ) = P l + jP Q , mTmά X = (X, ,X Q )= X, + JX Q . To recover the transmitted symbols, the following operation can be performed: + jPa Xt + JXaϊ ^ 10 ' = [føx, + P Q X Q )+ j(p,X Q - P Q X t )]• e jPil °' = [dot(P,X) + j cross(P,X)]- e JPilot where S represents the recovered symbols, e jP ' l< " is the phase of the pilot, dot(P, X) is the dot product of P and X and is equal to (PjX, + P Q X Q ), and cross(P,X) is the cross product of P and X and is equal to [P J X Q - P Q X. ). The dot and cross products scale the channelized symbols, X , from a particular finger element by the relative strength of the pilot received by that finger, which is estimated by the reference signal, P . The scaling is used to weigh the contributions from the assigned finger elements for efficient combining. Thus, the dot product performs the dual role of both phase projection and finger weighting that are characteristics of a coherent rake receiver. Dot product computation is further described in the aforementioned U.S. Patent Nos. 5,764,687 and 5,490,165. The physical channel being processed may be transmitted from one antenna or two antennas (e.g., for the STTD mode in the W-CDMA system). Moreover, the physical channel may be modulated using either BPSK modulation (e.g., for the IS-95 CDMA system) or QPSK modulation (e.g., for the W-CDMA system). For BPSK modulation (e.g., used in the IS-95 CDMA system), the cross product is a noise term and ignored, and the pilot phase is zero, i.e., g jpiiot = i + θ) . The recovered symbols, S , can thus be expressed as: = dot(P,X) For QPSK modulation (e.g., used in the W-CDMA system), the pilot phase is e Jpaot = (1 + ;') . The recovered symbols, S , can thus be expressed as: S QPSK =[(P / X / +P β β )+; ' (P / X β -P β x 7 )ll+;) = [(dot(P , X ) - cross(P, X)) + j(dot(P, Ϊ) + cross(P, X))]— In the STTD mode of the W-CDMA standard, two RF modulated signals are transmitted from two transmit antennas for one physical channel. The data transmitted from the second antenna is the same as that transmitted on the first antenna but reordered and complex conjugated to provide diversity. Thus, S A1 represents the symbol sequence {S 0 , S., S 2 , S 3 , ...} transmitted from the first transmit antenna and S^ represents the symbol sequence {-S. * , S,, * , -S 3 * , S 2 * , ...} transmitted from the second transmit antenna. Each of the RF modulated signals can experience independent and different path loss. The actual received signal, X , is a weighted sum of the two RF modulated signals, and can be expressed as: X= S A1 + βS A2 , where a and β are the path losses from the first and second transmit antennas, respectively, to the receive antenna, and X is the received symbol sequence {X 0 , X j , X 2 , X 3 , ...}. The received symbols can be expressed as: X 0 =aS 0 ~β , Eq(3) X^aS.+βs; , Eq(4) X 2 =aS 2 -βS * ,and Eq(5) Combing equations (3) through (6), the recovered symbols, S 0 and S l can be computed as: S o =K(P I X 0 +P 2 X 1 ), and where X is a scaling factor, K = 1/(|«| 2 and X represents the channelized symbols that are estimates of the actual received symbols, X . The scaling factor, K, is used to scale the recovered symbols from various assigned fingers prior to combining the symbols such that more reliable symbols are weighted more heavily. The demodulation for the W-CDMA system is described in further detail in the aforementioned U.S. Patent Application Serial No. xx/xxx, xxx [Attorney Docket No. QCPA000442] filed on the same date herewith. The processing units described herein (e.g., multipliers 710, 712, and 720, accumulators 714, pilot processors 722, data recovery element 740, controller 640, and others) can be implemented in various manners such as an application specific integrated circuit (ASIC), a digital signal processor, a microcontroller, a microprocessor, or other electronic circuits designed to perform the functions described herein. Also, the processing units can be implemented with a general-purpose or specially designed processor operated to execute instruction codes that achieve the functions described herein. Thus, the processing units described herein can be implemented using hardware, software, or a combination thereof. The memory unit can be implemented memory technologies including, for example, random access memory (RAM), Flash memory, and others. The memory unit can also be implemented with storage element such as, for example, a hard disk, a CD-ROM drive, and others. Various other implementation of the memory unit are possible and within the scope of the present invention. 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 will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. WHAT IS CLAIMED IS:

Claims: CLAIMS 1. A method for generating a reference signal based on time-multiplexed pilot data, the method comprising: receiving and processing a modulated signal to provide received samples; processing the received samples to provide pilot symbols; and generating a reference signal based on available pilot symbols, wherein the reference signal comprises a sequence of predicted pilot symbols representative of future estimates of one or more characteristics of one or more carrier signals for the modulated signal. 2. The method of claim 1, wherein the processing the received samples includes decovering the received samples with a channelization code to provide channelized symbols, and multiplying the channelized symbols with a particular pilot symbol pattern to generate the pilot symbols. 3. The method of claim 1, further comprising: filtering the pilot symbols from each time interval to generate one or more filtered pilot symbols, and wherein the reference signal is generated based on available filtered pilot symbols. 4. The method of claim 1, wherein the reference signal is generated by extrapolating pilot symbols from prior time intervals. 5. The method of claim 1, wherein the reference signal is generated by linearly extrapolating pilot symbols from two preceding time intervals. 6. The method of claim 1, wherein the reference signal is generated by curve fitting pilot symbols from prior time intervals. 7. The method of claim 1, wherein the reference signal is generated by averaging pilot symbols from prior time intervals. 8. The method of claim 1, wherein the reference signal is generated by weighting pilot symbols from prior time intervals with a set of weighting factors and averaging the weighted pilot symbols. 9. The method of claim 1, wherein the reference signal is generated by ensemble averaging pilot symbols from prior time intervals. 10. The method of claim 1, wherein the time multiplexed pilot data is available for a particular duration at each time interval. 11. The method of claim 10, wherein each time interval is associated with a particular number of pilot symbols ranging from one to eight. 12. The method of claim 1, wherein the reference signal is representative of phase and amplitude estimates of the one or more carrier signals. 13. A method for generating a reference signal based on time- multiplexed pilot data, the method comprising: receiving and processing a modulated signal to provide received samples; decovering the received samples with a channelization code to provide channelized symbols; multiplying the channelized symbols with a particular pilot symbol pattern to generate pilot symbols; filtering the pilot symbols from each time interval to generate one or more filtered pilot symbols; and generating a reference signal based on of available filtered pilot symbols using extrapolating, curve fitting, averaging, weighted averaging, or ensemble averaging, wherein the reference signal comprises a sequence of predicted pilot symbols representative of future estimates of phase and amplitude of one or more carrier signals for the modulated signal. 14. A method for demodulating a modulated signal that includes time- multiplexed pilot data, the method comprising: receiving and processing the modulated signal to provide received samples; processing the received samples to provide pilot symbols and channelized symbols; generating a reference signal based on available pilot symbols, wherein the reference signal includes a sequence of predicted pilot symbols representative of future estimates of one or more characteristics of one or more carrier signals for the modulated signal; and demodulating the channelized symbols with the reference signal to generate recovered symbols. 15. The method of claim 14, wherein the processing the received samples includes decovering the received samples with a channelization code to provide channelized symbols, multiplying the channelized symbols with a particular pilot symbol pattern to generate the pilot symbols, and filtering the pilot symbols from each time interval to generate one or more filtered pilot symbols, and wherein the reference signal is generated based on available filtered pilot symbols. 16. The method of claim 14, wherein the processing the received samples includes decovering the received samples with a channelization code to provide channelized samples, and accumulating the channelized samples over a length of the channelization code to provide the channelized symbols. 17. The method of claim 14, wherein the reference signal is generated by extrapolating, curve fitting, averaging, weighted averaging, or ensemble averaging available pilot symbols. 18. The method of claim 14, wherein the demodulating includes performing a dot product between the predicted pilot symbols in the reference signal and the channelized symbols to generate the recovered symbols. 19. The method of claim 14, wherein the demodulating includes performing a dot product between the predicted pilot symbols in the reference signal and the channelized symbols, performing a cross product between the predicted pilot symbols and the channelized symbols, and combining results of the dot product and the cross product to generate the recovered symbols. 20. A receiver unit operative to process a physical channel in a CDMA communication system, comprising: a receiver operative to receive a modulated signal and provide received samples indicative of data transmitted on the physical channel; and at least one demodulator element coupled to the receiver, each demodulator element including a data processing unit operative to receive and process the received samples to provide channelized symbols, and a pilot processing unit operative to receive and process the channelized symbols to provide pilot symbols, and to generate a reference signal based on available pilot symbols, wherein the reference signal includes a sequence of predicted pilot symbols representative of future estimates of one or more characteristics of one or more carrier signals for the modulated signal. 21. The receiver unit of claim 20, wherein each demodulator element further includes a data recovery element coupled to the data processing unit and the pilot processing unit, the data recovery element operative to receive the reference signal and the channelized symbols and generate recovered symbols. 22. The receiver unit of claim 21, further comprising: a combiner coupled to the at least one demodulator element and operative to receive and combine recovered symbols from one or more assigned demodulator elements to generate combined symbols. 23. The receiver unit of claim 20, wherein the reference signal is generated by extrapolating, curve fitting, averaging, weighted averaging, or ensemble averaging available pilot symbols.

Inventors: AGRAWAL AVNEESH, LIN JEREMY H, TERASAWA DAISUKE, AGRAWAL, AVNEESH, LIN, JEREMY, H., TERASAWA, DAISUKE
Applicants: QUALCOMM INC, QUALCOMM INCORPORATED
Application year: 2001
Main classification: H04B1
IPC codes: H04B1, H04Q7