Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
  • H04L25/03006—Arrangements for removing intersymbol interference
  • H04L25/03343—Arrangements at the transmitter end
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B7/00—Radio transmission systems, i.e. using radiation field
  • H04B7/005—Control of transmission; Equalising
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B7/00—Radio transmission systems, i.e. using radiation field
  • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
  • H04B7/022—Site diversity; Macro-diversity
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B7/00—Radio transmission systems, i.e. using radiation field
  • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
  • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
  • H04B7/06—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station
  • H04B7/0613—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission
  • H04B7/0615—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission of weighted versions of same signal
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B7/00—Radio transmission systems, i.e. using radiation field
  • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
  • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
  • H04B7/06—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station
  • H04B7/0613—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission
  • H04B7/0615—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission of weighted versions of same signal
  • H04B7/0619—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission of weighted versions of same signal using feedback from receiving side
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/24—Testing correct operation
  • H04L1/242—Testing correct operation by comparing a transmitted test signal with a locally generated replica
  • H04L1/243—Testing correct operation by comparing a transmitted test signal with a locally generated replica at the transmitter, using a loop-back
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/0202—Channel estimation
  • H04L25/0204—Channel estimation of multiple channels
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/0202—Channel estimation
  • H04L25/0224—Channel estimation using sounding signals
  • H04L25/0226—Channel estimation using sounding signals sounding signals per se
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
  • H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
  • H04L27/36—Modulator circuits; Transmitter circuits
  • H04L27/366—Arrangements for compensating undesirable properties of the transmission path between the modulator and the demodulator
  • H04L27/367—Arrangements for compensating undesirable properties of the transmission path between the modulator and the demodulator using predistortion
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/69—Spread spectrum techniques
  • H04B1/707—Spread spectrum techniques using direct sequence modulation
  • H04B1/7097—Interference-related aspects
  • H04B1/7103—Interference-related aspects the interference being multiple access interference
  • H04B1/7105—Joint detection techniques, e.g. linear detectors
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B7/00—Radio transmission systems, i.e. using radiation field
  • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
  • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
  • H04B7/06—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station
  • H04B7/0613—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission
  • H04B7/0615—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission of weighted versions of same signal
  • H04B7/0617—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission of weighted versions of same signal for beam forming
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/0202—Channel estimation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/0202—Channel estimation
  • H04L25/0212—Channel estimation of impulse response
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/0202—Channel estimation
  • H04L25/0224—Channel estimation using sounding signals
  • H04L25/0228—Channel estimation using sounding signals with direct estimation from sounding signals
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/0202—Channel estimation
  • H04L25/024—Channel estimation channel estimation algorithms
  • H04L25/0242—Channel estimation channel estimation algorithms using matrix methods
  • H04L25/0244—Channel estimation channel estimation algorithms using matrix methods with inversion
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W52/00—Power management, e.g. TPC [Transmission Power Control], power saving or power classes
  • H04W52/04—TPC
  • H04W52/38—TPC being performed in particular situations
  • H04W52/42—TPC being performed in particular situations in systems with time, space, frequency or polarisation diversity

Definitions

• the present invention generally relates to wireless communications networks, and particularly relates to generating and using loop-back signal information from remote receivers, such as mobile stations in a cellular communication network.
• Wireless communication typically involves translating information, such as digital data, into a characteristic modulation pattern that is applied to a transmitted carrier signal.
• a remote receiver obtains the transmitted information by demodulating the received carrier signal.
• the transmit-and-receive processes employed in modern digital communication systems involve significantly more complexity, such as the use of sophisticated channel encoding methods for combating signal fade, and error encoding methods for detecting and correcting errors in received data.
• a receiving device or system improves its reception performance by compensating the received signal for the transmit "channel,” which considers signal corruption effects induced by the transmission medium, and by elements of transmitter and receiver circuits, such pulse shaping filters and signal amplifiers.
• Such methods base received signal compensation on one or more "channel estimates,” which, in a mobile receiver, are frequently updated to reflect changing propagation path characteristics.
• channel estimates typically characterize the end-to- end phase and attenuation changes imparted to the transmitted information signal, and may be used to at least partially null or otherwise cancel such effects from the received signal at the receiving system.
• An enabling element of such compensation schemes involves the transmission of information known a priori at the receiving device, such as by including a training sequence within a signal transmission that is known in advance to the receiver or by transmitting a known pilot signal in conjunction with an unknown data signal.
• the receiver uses the known portion(s) of the received signal(s) to determine its channel estimates.
• the receiver estimates the transmission channel between it and the transmitting device.
• remote receivers may provide a transmitting system with loop-back signals for use by the transmitting system in estimating the downlink channels between the transmitting system and particular remote receivers. As might be guessed, more accurate channel estimates at the transmitting system enable more effective channel and interference pre-compensation.
• loop-back signals at the transmitting system such as for transmit signal pre-compensation at one more downlink signal processing elements within the wireless network
• loop-back signals at the transmitting system potentially introduces significant signal processing complications.
• the processing complications and the effectiveness with which the loop- back signals can be used to perform desired transmit signal pre-compensation depends on the methods employed for generating loop-back signals at the particular remote receivers and on the methods used for loop-back processing at the network.
• the present invention comprises a method and apparatus to transmit signals from a wireless communication network to particular remote receivers, such as to particular mobile stations within the network, which then modify the received signals in a manner unique to each receiver before loop-back transmission to the network.
• the network With knowledge of the unique modifications applied to the return signal at each receiver, and with knowledge of the signals originally transmitted by it, the network is able to separate the loop-back signals returned by the receivers. Separating the loop-back signals returned from the individual receivers enables determination of the signal propagation characteristics between the network transmitting antenna(s) and each remote receiver.
• Such characterization of the downlink channel(s) for each receiver enables precompensation of the transmit signals by the network to reduce interference at the remote receivers and/or to improve transmission power efficiency and thereby maximize received signal quality at each remote receiver.
• a fixed network of one or more transmitting stations transmits information using one or more transmitting antennas to a plurality of mobile stations.
• the mobile stations receive overlapping transmitted signals from the multiple network antennas, and each mobile station processes the received signal to obtain complex received signal samples, such as by complex downconversion, sampling and digitization of the received signal.
• the mobile station modifies the received samples in a mobile-specific manner and transmits the modified samples back to the network.
• One such exemplary modification comprises introducing a mobile-specific frequency shift to the returned samples.
• each mobile station modifies the received signal samples by applying a unique frequency shift in the form of a progressive phase rotation, the frequency shift being greater than the maximum Doppler shift caused by mobile station movement.
• the unique frequency shift applied by the mobile station may be modulo the sample rate and may leave the long-term power spectrum of the mobile station loop-back transmission unchanged.
• a frequency shifter such as one based on complex sample multiplication, may be used by the mobile station to impart the desired phase rotation to the loop-back samples.
• each mobile station modifies the received signal samples by "gating" them according to mobile-specific timing such that the loop- back signals from the mobile stations are coordinated in by time-division multiplexing across the plurality of mobile stations.
• the network receives loop-back signal information from a particular mobile station only at particularly designated times, i.e., only in unique timeslots designated for that mobile station.
• Traffic signals using mobile-unique codes may be transmitted in the periods the mobile station is not transmitting the loop-back signal, and optionally also while the loop-back signal is being transmitted.
• a pilot signal or code unique to each mobile and known to the network may be transmitted by the mobile station at any time.
• a separate pilot signal may be transmitted continuously or intermittently by the mobile station in conjunction with its mobile-specific loop-back signal.
• the mobile station may time-multiplex pilot information and loop-back signal information together, or perform some other combination of pilot and loop-back signal information.
• the mobile station may form a combined pilot and loop-back signal based on additive combining.
• the mobile station modifies the received signal samples by multiplying them with corresponding chips of a mobile- unique code.
• each mobile station uses a different code, e.g., a unique "scrambling code," and the set of codes used to generate unique loop-back signals for each of the plurality of mobile stations is orthogonal.
• the exemplary mobile station comprises a receiver to receive a signal transmitted by the network for use in loop-back signal generation, and obtain received signal samples therefrom, e.g., to obtain complex samples from the received signal.
• the exemplary receiver includes filters, amplifiers, downconverters, digitizers, and related digital processing elements as needed or desired for use in obtaining the received signal samples.
• the exemplary receiver further comprises a sample processor to generate modified signal samples based on imparting a mobile-specific characteristic to them.
• such modification may comprise imparting a characteristic frequency shift, imparting a characteristic scrambling code, and, among other alternatives, imparting a characteristic gating or other timing pattern to the received signal samples.
• the exemplary mobile station further includes a transmitter, which may comprise baseband processing circuits, modulators, carrier frequency generators, phase-locked loops, pre-amplifiers and RF power amplifiers as needed or ⁇ esire ⁇ to generate a transmit signal based at least in part on the modified signal samples.
• the transmit signal functions as a loop-back signal received by the network that includes signal samples previously transmitted by the network but modified by the mobile station according to one or more mobile-specific values, and further modified by, i.e., corrupted by, the uplink and downlink channels between the mobile station and particular ones of the network transmitting/receiving antennas.
• Uplink channel characteristics may be determined at the network based on mobile station pilot signals received at one or more network antennas, while determination of the downlink channels may be determined from the loop-back signals returned by the mobile stations.
• the mobile-specific information facilitates processing of the composite loop-back signals, i.e., the collection of loop-back signals received at each of one or more network antennas, for determination of downlink channel propagation coefficients as between individual network antennas and individual mobile stations.
• the exemplary wireless network comprises one or more network antennas and associated transceivers for transmitting signals to and receiving signals from a plurality of mobile stations.
• a given plurality of mobile stations may be treated as a group and interference cancellation or other transmit signal pre-compensation may be applied by collectively processing the individual information streams intended for transmission to mobile stations in that group.
• the present invention supports accurate determination of the downlink channels between individual network transmit antennas and individual mobile stations, i.e., the determination of downlink channel multipath propagation coefficients describing the transmission channel, which may include the effects of transmit and receive filters as well as multipath propagation effects.
• the exemplary network comprises a channel processor that includes one or more numerical processors or equivalent systems for performing exemplary signal processing functions.
• the exemplary channel processor may be included in the transmit processor named in the parent applications.
• an exemplary channel processor comprises one or more predictor circuits to generate a predicted composite loop-back signal for each received composite loop-back signal based on current uplink and downlink channel estimates associated with the corresponding network antenna.
• the channel processor comprises one or more corrector circuits to update the associated uplink and downlink channel estimates relating each network antenna to each mobile station based on prediction errors determined from the predicted and received composite loop-back signals.
• Exemplary corrector circuits include channel trackers that maintain a tracking loop for updating the collections of individual channel propagation coefficients that estimate the uplink and downlink channels between individual network antennas and individual mobile stations.
• Fig. 1 is a diagram of an exemplary wireless communication network.
• Fig. 2 is a diagram of exemplary details for the network of Fig. 1.
• Fig. 3 is a diagram of alternate exemplary details for the network of Fig. 1.
• Fig. 4 is a diagram of alternate exemplary details for the network of Fig. 1.
• Fig. 5 is a diagram of exemplary transmit signal pre-compensation.
• Fig. 6 is a diagram of an exemplary mobile station for generating mobile-specific loop-back signals.
• Figs. 7A and 7B are diagrams of exemplary uplink and downlink channel spans.
• Fig. 8 is a diagram of signal flows for actual downlink/uplink channel ordering and constructive downlink/uplink channel ordering that may be used to facilitate loop-back signal processing at the network.
• Fig. 9 is a diagram of exemplary mobile station details supporting loop-back signal generation.
• Fig. 10 is a diagram of alternate exemplary mobile station details supporting loop-back signal generation.
• Fig. 11 is a diagram of additional alternate exemplary mobile station details supporting loop-back signal generation.
• Figs. 12A and 12B are diagrams of loop-back signal flow for frequency shift- based loop-back signal generation.
• Fig. 13 is a diagram of exemplary predictor and corrector circuits in a loop-back signal processor/channel processor that support loop-back signal processing and channel estimation in the network.
• Fig. 14 is a diagram of exemplary per-mobile station predictor and corrector functional and circuit details.
• Fig. 15 is a diagram of exemplary per-mobile station predictor and corrector functional and circuit details expanded to consider two or more network antennas.
• the present invention has broad applicability to various types of wireless communication networks, and is subject to changes regarding its implementation as ⁇ ee ⁇ eo or oesire ⁇ ⁇ or a particular type or configuration of wireless communication network.
• CDMA Code Division Multiple Access
• WCDMA Wideband CDMA
• one or more other embodiments have applicability to networks employing Time Division Multiple Access (TDMA) techniques, such as IS-136 or GSM- based networks.
• TDMA Time Division Multiple Access
• Fig. 1 illustrates an exemplary wireless communication network 10.
• network 10 comprises one or more transceiver stations 12 (shown as 12-1 , 12-2, . .
• exemplary interface/routing entities might include Mobile Switching Centers (MSC) and other Circuit-Switched Core Network (CSCN) entities as needed, and/or Packet Data Serving Nodes (PDSNs) and other Packet Switched Core Network (PSCN) entities as needed.
• MSC Mobile Switching Centers
• CSCN Circuit-Switched Core Network
• PDSNs Packet Data Serving Nodes
• PSCN Packet Switched Core Network
• LUU-.4J une or more of the parent applications provide exemplary details for transmit processor 18 as regards the use of the included numeric processor 20 in transmit signal pre-compensation operations.
• other figures herein provide at least some exemplary details, but it should be understood that transmit pre-processing may be implemented in a variety of different ways, and that the loop-back signal details of the present invention are not dependent on the details of transmit pre-processing operations. Rather, the present invention is properly viewed as an enabling element for transmit pre-processing as it supports the accurate and efficient generation of downlink channel estimates that underlie transmit pre-processing operations.
• the focus of this disclosure properly is on the loop-back/channel processor 22, that is included in the transmit processor 18 in an exemplary embodiment.
• each transceiver station 12 includes one or more network transmit antennas transmitting signals to some or all of the mobile stations 16-1 through 16-M. Similarly, signals transmitted from each mobile station 16 are received at some or all of the transceiver stations 12. Thus, in a nominal case, each mobile station 16 receives a composite signal comprising combined signals from some or all transceiver stations 12, and each transceiver station 12 receivers a composite signal comprising combined signals from some or all of the mobile stations 16-1 through 16-M.
• transceiver stations 12 as illustrated include network antennas for wirelessly transmitting to and receiving from mobile stations 16, but one or more embodiments of the present invention contemplates implementation of separate transmit and receive stations that may or may not use co-located receiving/transmit antennas.
• downlink and uplink channels nominally exists between each transceiver station 12 and each mobile station 16.
• the downlink channel from transceiver station 12-1 to mobile station 16-1 is denoted as C11
• the uplink cnannel for the same pair is denoted as C'11 , where the first digit indicates the mobile station number (i.e., mobile station 1, 2, 3, 7) and the second digit represents the network antenna number (i.e., transceiver station 1 , 2, 3, ).
• the downlink channel from network antenna 2 to mobile station 3 would be designated as C32.
• C ' 32 may be represented as a complex polynomial characterizing the multipath propagation between the corresponding network antenna and mobile station.
• network 10 may use knowledge of the downlink channels to pre-compensate its transmissions to the mobile stations 16 for reducing interference at the mobile stations 16, for increasing transmit power efficiency, or for some advantageous combination thereof.
• the downlink channels pose a more complicated estimation problem.
• downlink channel estimates may be obtained from loop-back signal information transmitted to network 10 by the mobile stations 16.
• the present invention provides novel loop-back signal generation and processing wherein each mobile station 16 generates a mobile-specific loop-back signal that enables efficient processing of the composite loop-back signals received by network 10 for separation of per-mobile loop-back signal information and downlink channel estimation therefrom.
• the network 10 transmits known signals from its one or more transceiver stations, which signals are received and processed by the mobile stations 16 for generation of mobile- specific loop-back signals that are transmitted back to the network 10 for use in downlink channel estimation. [0029] hig.
• ⁇ illustrates a more detailed exemplary network 10, wherein the interface/routing entities 14 comprise a MSC 30 and an associated Base Station Controller 32, and wherein the transceiver stations 12 comprise base stations 34 and associated transmit/receive antennas 36.
• An exemplary base station 34 comprises transmitter resources 38, including modulators 40 and power amplifiers (PAs) 42, RF generating circuits 44, and receiver resources 46.
• PAs power amplifiers
• Fig. 2 is thus representative of an exemplary communication network 10 disclosed in one or more of the parent applications for transmitting downlink signals using multiple network antennas (36-1, 36-2, ...) to multiple remote stations (mobile stations 16-1, 16-2, ...) in which the signals transmitted are preconditioned by transmit processor 18 based on knowledge of the downlink propagation paths.
• Such operation is, as noted above, enhanced by the loop-back signal generation and processing of the present invention, which provide improved processing methods that enable network 10 to acquire and maintain the needed downlink propagation path characteristics.
• Fig. 3 illustrates an application of the present invention wherein one network antenna 36 supports a plurality of mobile stations 16 based on a CDMA implementation of network 10.
• transmit processor 18 includes a matrix precombiner 50 that pre-processes symbol streams for transmission to the mobile stations 16 based on symbol-rate processing only, using knowledge of the CDMA spreading codes used in coder 52 and on the estimated downlink channel characteristics, to minimize interference between signals destined for different mobile stations 16.
• Fig. 4 illustrates yet another application supported by the present invention, in which two network antennas are collocated at the same site and can, for example, just be a dual-polarized antenna element on network antenna 36. With that configuration, information is preconditioned for transmission by the two antennas using chip rate processing to independently optimize transmissions from the network 10 to each mobile station 16 independently.
• the systems of Figs. 3 and 4 both require downlink channel information to optimize their transmissions to the individual mobile stations 16.
• Fig. 4 illustrates a potential information deficiency problem that arises in determining estimates for two downlink channels from only one loop-back signal.
• one or more of the parent applications provided exemplary solutions for solving such deficiency problems.
• Fig. 5 essentially illustrates a combination of the network embodiments illustrated in Figs. 3 and 4, in which transmission is optimized both by chip rate processing to optimize efficiency given multiple network transmit antennas and by symbol rate processing to pre-cancel interference between information symbols intended for different ones of the mobile stations 16.
• the parent application entitled “Mobile Station Loopback Signal Processing” disclosed two different loop-back methods intended respectively for optimizing transmit preconditioning at the chip-rate level and for optimizing transmit preconditioning at the symbol-rate level.
• network 10 For optimizing at the chip-rate level, network 10 must have current knowledge of the involved downlink channels, which knowledge may be provided, for example, by looping back the mobile station's raw (unprocessed) received signal samples obtained from a network-transmitted signal.
• a mobile station 16 can alternatively loop-back its RAKE receiver soft symbol output values, i.e. it can loop-back partially processed signal values obtained from a received network-transmitted signal. In such cases, the mobile station 16 may re-spread the soft symbol values using a mobile-unique code that enables the network 10 to separate the loop-back signals it receives from different mobile stations 16.
• the separation of a small number of overlapping mobile station loop-back signals may be achieved by having an equal number of small network antennas. Such an arrangement would obviate the need for applying mobile- specific information to each loop-back signal otherwise needed for loop-back signal separation at the network 10.
• the number of overlapping mobile station loop-back signals can greatly exceed the number of network antennas.
• each loop-back signal is a function of only one network-transmitted signal per network antenna, then the sum of the loop-back signals received back at the network 10 is also a function only of that number of independent signals, and cannot then be separated into a greater number of mobile station loop-back signals in a unique fashion.
• the present invention contemplates a loop-back signal generation and processing method and apparatus that provides efficient loop-back signal processing.
• one or more embodiments of the present invention provide exemplary loop-back signal generation and processing wherein the processing increases in complexity only in proportion to the number of involved mobile stations 16.
• FIG. 6 illustrates an exemplary mobile station 16 for generating a mobile-specific loop-back signal in support of downlink channel estimation by network 10.
• Mobile station 16 comprises an antenna assembly (antenna) 60, a switch/duplexer 62, a receiver 64, a transmitter 66, a loop-back sample processor 68, a baseband processor 70, a system processor 72, and a user interface (Ul) 74.
• antenna assembly antenna assembly
• switch/duplexer 62 a receiver 64
• a transmitter 66 a transmitter 66
• a loop-back sample processor 68 a baseband processor 70
• system processor 72 system processor
• Ul user interface
• portions of the receiver 64 and transmitter 66 may oe included in tne oaseoan ⁇ processor tu, wnicn itself may comprise one or more Digital Signal Processors (DSPs) and/or other programmable logic devices, such as Field Programmable Gate Arrays (FPGAs) or Application Specific Integrated Circuits (ASICs).
• DSPs Digital Signal Processors
• FPGAs Field Programmable Gate Arrays
• ASICs Application Specific Integrated Circuits
• baseband processor 70 may comprise one or more digital processing circuits that, together, perform selected receive operations, such as equalization or RAKE combining for receiver 64, and selected transmit operations, such as digital encoding and modulation.
• sample processor 68 depicts an exemplary functional block that may be implemented as a hardware function, a software function, or some combination thereof, and may be integrated with the baseband processor 70.
• the network 10 transmits a signal from each of one or more network antennas, such that a particular mobile station 16 receives a combination of such signals as a received signal.
• Receiver 64 processes that received signal, such as by filtering, amplifying, downconverting, and sampling to provide sample processor 68 with received signal samples to be used in loop-back signal generation.
• Sample processor 68 applies some mobile-specific modification to these received signal samples, which are then processed for transmission back to the network 10 in a loop- back signal transmitted by mobile station 16.
• Such processing may involve encoding of the modified signal samples by baseband processor 70, possibly together with pilot and/or uplink traffic information, and subsequent modulation onto a RF carrier signal by transmitter 66.
• Fig. 7A illustrates an exemplary downlink channel "span" as between a particular network antenna 36 and a particular mobile station 16.
• the exemplary downlink channel to be estimated includes the effects of selected elements in the transmit and receive signal paths at the involved network transmit station 12 and at the moDiie station 16, as well as the more obvious effects of multipath over-the-air signal propagation associated with actual transmission to the mobile station 16.
• the exemplary downlink channel span is defined as being from the sample inputs of transmitter filter 80 in transceiver station 12 to the sample outputs of sampler 90 in the receiver circuits of mobile station 16.
• the exemplary uplink channel is defined as an "end-to-end" channel that includes the mobile station's transmit circuit path, the over-the-air propagation paths, and the transceiver station's receive circuit path.
• the exemplary uplink channel spans from the sample inputs of filter 91 in mobile station 16, to the sample outputs of receive signal sampler 85 in transceiver station 12.
• other channel definitions may be used as needed or desired.
• the "loop" channel represents the round-trip channel definition from network-to-mobile-back-to-network.
• the loop channel is the product of the downlink and uplink channels defined above.
• Fig. 8 illustrates that the loop channel is unchanged by imagining a reversed ordering of the uplink and downlink channels. In that reversed case, the intermediate samples arising between the reversed uplink and downlink channels do not correspond with samples that arise with the actual channel ordering.
• these imagined intermediate samples may be calculated at network 10 by its passing known downlink samples through a model of the uplink channel as determined by network 10 from the pilot codes added to or sent in conjunction with the mobile station's loop-back signal, i.e., as if the samples had been input to transmit filter 91 of mobile 16.
• the output represents an estimated loop-back signal, which may be processed with the actual loop-back signal to determine modeling errors, i.e., channel estimation errors.
• the loop-back signals; oeing processed by the network are composite loop-back signals representing the summation of many individual loop-back signals received from different mobile stations 16. For example, if the individual loop-back signals are each a function only of the same originally transmitted signals, it is only possible to determine a combination for the downlink channels and not possible to separate that information into particular downlink channel estimates for each mobile station 16.
• Fig. 9 illustrates an exemplary embodiment of mobile station 16 in which the sample processor 68 generates mobile-specific sample values by applying a mobile specific code to samples obtained from a network- transmitted signal.
• the exemplary sample processor comprises a complex multiplier 100 and a complex adder 102.
• these functional representations may be realized as part of baseband processor 70 and/or other digital processing circuits available in mobile station 16 rather than as separate processing elements.
• the mobile-unique codes can be selected from a set of orthogonal codes of finite length, which are repeated, such as complex-scrambled Walsh codes.
• the mobile station 16 receives a mix of base station transmissions at antenna 60, which is coupled to downconverter 92 of receiver 64 through switch/duplexer 62. If simultaneous receive/transmit capability is desired, then element 62 includes duplex filter arrangements to maintain receive/transmit signal isolation. For half-duplex operation, as is common with contemporary digital communication standards, element 62 may comprise a selective receive/transmit switch.
• the network-transmitted signal is received at antenna 60 and passes to receiver 64, where an exemplary received signal is converted into a stream of complex samples at a rate of one complex sample per chip.
• the complex samples having an I- or real part and a Q- or imaginary part are combined with a mobile-specific code sequence C(nT) which can be a complex code sequence comprising a real code sequence C](nT) and an imaginary code sequence C q (nT), and the combining can, for example, be complex multiplication using multiplier 100.
• a mobile-specific uplink pilot and optionally uplink traffic can be combined in complex adder 102 with the modified loop-back signal samples for transmission to the network 10. Such combination may be obtained by linearly adding the complex pilot/traffic sequence to the modified signal samples in adder 102.
• Combination with traffic/pilot can alternatively take place before multiplier 405 so that the traffic/pilot signal is also multiplied with the mobile-specific code C(nT).
• the combined loop-back, pilot and uplink traffic signal samples would then be applied to an upconverter 104 in transmitter 66, which can comprise low-pass filtering to turn the sample stream into a continuous-time waveform and a quadrature modulator to impress the signal on an uplink carrier signal for transmission.
• the exemplary transmitter 66 further includes a power amplifier (PA), such as a linear PA, for amplifying the carrier signal for transmission to network 10 via antenna 60.
• PA power amplifier
• a more preferred exemplary method for generating mobile-unique modifications in loop-back signal generation is one which does not invalidate the interchange of uplink and downlink channels illustrated in Fig. 8.
• One such method is time-division multiplexing, in which each mobile station 16 transmits its loop-back signal in an allocated timeslot, and blanks loop-back transmission in other timeslots.
• each mobile station 16 transmits a gated ⁇ oop-oac ⁇ signal according to mooiie- specific timing.
• the allocated timeslots are arranged to recur with a frequency adequate to track the time-varying downlink channels. In certain circumstances, this limitation may involve restricting the benefits of loop-back based precompensation to mobile stations 16 whose rate of movement is below some practical upper limit.
• Fig. 10 illustrates an exemplary sample processor 68 in mobile station 16 for implementing time-division multiplexing of loop-back signals.
• Mobile station receive and transmit operations are essentially as described above, but the received signal samples obtained from the network-transmitted signal are now modified by applying a mobile- specific gating operation to them.
• sample processor 68 gates the loop-back return of the received signal samples by enabling their return transmission only in mobile-specific timeslots.
• Selector 110 thus selects the loop-back signal samples to be applied to transmitter 66 only when controlled to do so by a slot timing control signal.
• network 10 determines or otherwise assigns one or more mobile-speoific parameters to each mobile station 16 in a particular group of mobile stations 16. Such "generating values" may be transmitted to an individual mobile station during call setup, or using other available channels, such as common control or paging channels used for transmission of control signaling from network 10 to mobile stations 16. Thus, network 10 may "define" a plurality of mobile stations 16 based on, for example, which transceiver stations 12 are (or will be) used to serve them, and then assign mobile- specific generating values to individual mobile stations 16 as needed.
• Slot-timing information is representative of such a generating value, and thus network 10 might transmit slot timing information to each mobile station 16, such that each mobile station ⁇ t> gates its ⁇ oop-oac ⁇ signal according to moone-speciTic timing, it should be noted that pilot and/or uplink traffic may be transmitted by a mobile station 16 during the periods when it is not looping back network-transmitted signal samples from sample processor 68.
• Such a gating selection process can also be equated to using a code C(nT) in Fig ' . 9 at is zero when T is not within the mobile station's allocated loop-back timeslot and non-zero only in the allocated loop-back slot(s).
• Selector 110 may, as shown, select uplink traffic and/or pilot signals to be transmitted at other times.
• uplink traffic + pilot signals can be transmitted continuously and the loop-back signal selected to be combined with the uplink traffic + pilot information only in the allocated loop-back slots.
• the timeslot method avoids the challenges arising from loop-back signal separation processing at the network 10.
• disadvantages might arise where a large number of mobile stations 16 are required to transmit loop-back signals.
• Fig. 11 illustrates an exemplary sample processor 68 in mobile station 16 that generates a mobile-specific loop-back signal by applying a mobile- specific frequency shift to received signal samples. While similar to the mobile-specific sample coding of Fig. 9, the sample processor 68 in this embodiment applies a mobile- specific frequency shifting sequence to received signal samples. More particularly, a shift generator 112 imparts a frequency shifting sequence to received samples based on one or more mobile-specific generating values that uniquely define the shifting sequence applied by the mobile station. It should be understood that the term "unique sequence" broadly includes different mobile stations 16 using the same sequence or sequence pattern but at different offsets, etc.
• An exemplary frequency shifting sequence produced by frequency shift generator 112 is a sequence of complex numbers of constant amplitude and progressively increasing phase. Such a sequence is described for successive instants
• the frequency shift dw is a multiple of 2 ⁇ VMT
• the sequence repeats in a finite number M of periods of T and may be pre- computed and stored in memory. If all frequency shifts used by all mobiles are multiples of the basic frequency 2 ⁇ /MT , then all frequency shift sequences can be produced ⁇ rom tne same pre-computed values stored in memory. For example, a shift sequence for 5d is produced by selecting stored values from memory addresses sequentially incremented by 5, modulo-M.
• Network 10 generates transmit signal samples at a base station 34 and transmits them through downlink channel 200, which may be defined, as discussed above, to comprise the transmit spectral shaping filter, the multipath propagation channel and the receive filter. Received samples at receiver 64 of a particular mobile station 16 are then frequency shifted modulo the sample frequency by multiplying by a frequency shift sequence in multiplier 100.
• W e i2 ⁇ /M
• M the total number of mobile stations 16 in a particular group of interest, such as a particular plurality of mobile stations looping back to network 10 on the same CDMA channel frequency.
• the modulo frequency shift has the effect that frequency components at a frequency f more than F s /2 above the channel center frequency are wrapped around to appear below the center frequency by (f - F s ) instead.
• the shifted frequency samples are then transmitted by the mobile station 16 though the uplink channel 202, which may be defined, as described above, to comprise mobile station transmit filtering, the uplink multipath channel, and base station receive filtering.
• this type of modulo frequency rotation has the advantage that the mobile station's transmitted power spectrum remains unchanged, i.e., it remains centered on the allocated channel and not shifted into any adjacent channel.
• the uplink and downlink channels may thus still be interchanged at network 10 for advantageous loop-back signal processing as described above without affecting the loop-back samples providing the uplink channel estimate considers the frequency rotation applied by the mobile station 16.
• This equivalent signal flow path is shown in the Fig. 12B, which essentially reproduces the arrangement shown in Fig. 12A.
• the reproduced frequency-rotated uplink filtering is located at the network receive processing site, e.g., a base station 34.
• the output of the reproduced, frequency-rotated uplink when input to the network's estimated downlink channel, produces an estimate of the actual loop-back signal.
• the network 10 may implement the following exemplary method:
• Frequency rotate the base-transmitted samples pass these frequency rotated samples through the uplink channel estimated in (1), and then frequency de-rotate the output from the estimated uplink channel to obtain "reference samples” representative of loop-back sample values that have passed through a frequency shifted uplink channel but that have not passed through the downlink channel;
• derotator 120 After frequency de-rotating actual received loopback samples in, for example, derotator 120 or other such de-rotation circuits, correlating the de-rotated actual loopback samples with the reference samples computed in (2) to estimate the downlink channel. It may also be realized that the de-rotation, which is applied both to the actual loop-back samples received by the network 10 in (3) and to the reference samples computed in (2), can be omitted without affecting the results of correlation.
• the loop-back signal received by network 10 from a particular mobile station 16 comprises modified sample values obtained by the mobile station 16 from a previous network-transmitted signal that is known to the network 10.
• network 10 further has knowledge of the mobile-specific modification applied to those received signal samples at the mobile station 16, it can pass locally-stored (or re-generated) copies of those same sample values (but without any channel corruption) through a locally stored model of the frequency-rotated uplink channel to obtain the reference samples.
• these reference samples differ from the actual loop-back signal samples received at network 10 primarily in that they have not passed through the downlink channel, passing such reference samples through the network's estimated downlink channel and then correlating or otherwise comparing them to the actual loop- back signal samples reveals errors in the network's downlink channel estimate.
• the reference samples further differ from the actual samples to the extent that the network's uplink channel estimate is in error, but since the uplink channel can be estimated directly based on received pilot information from the mobile station, the uplink channel models at network 10 tend to be quite accurate.
• each mobile station 16 uses frequency shifts that differ from any other mobile station 16 (in a given group of mobile stations 16) by a multiple of the reciprocal of the correlation averaging time in Step (3) above, their loop-back signals appear as uncorrelated interference and do not result in the network's estimation of downlink channels for each mobile station 16 being biased.
• the downlink channels for all mobile stations 16 may be jointly estimated by network 10, e.g., estimated by loop- back/channel processor 22, without ambiguity. Joint estimation compensates for the interference between mobile loop-back signals either by simultaneous (parallel) estimation or by successive estimation in which interference of already processed loop- back signals is subtracted before processing the next signal.
• Exemplary successive joint estimation is carried out in descending order of signal strength, and may be iterated to improve the first estimations.
• a preferred realization of joint (parallel) estimation is developed in the Appendix, included herein, which allows the required hardware and/or software to be separated into an identical element or block which is repeated for each mobile station's loop-back signal. Such separate elements operate largely independently. Such an approach is significantly simpler than using matrix methods, which methods imply heavy cross coupling between channel estimations for different mobile stations 16.
• the Appendix gives exemplary mathematical formulations for jointly estimating all coefficients for all channels at the same time, leading to the derivation of an optimum functional implementation as illustrated in Fig. 13.
• a base station transmitted signal sample stream T is applied to a set of models of the entire loop path, with one model for each mobile station 16 that is transmitting loop-back signals to the network 10, or at least for a particular group of mobile stations 16 looping back on, for example, the same CDMA channel(s).
• An exemplary model set comprises predictor circuits 130, including a predictor 132 for each mobile station 16, the collective outputs of which are coupled to corrector circuits 134 through a summing circuit that subtracts the actual composite loop-back signal received at a given network antenna 36 from the combination of predicted sample streams generated by predictor circuits 130.
• An exemplary arrangement of corrector circuits 134 comprises individual correctors 136 for each mobile station 16 of interest.
• the exemplary predictor 132 for each mobile station 16 provides for the following operations:
• each predictor 132 represent the loop-back sample stream predicted for a particular mobile station 16 based on current network-stored estimates of the uplink and downlink channels relating that mobile station 16 to a particular network antenna 36. These predicted samples streams are then summed together and the actual composite loop-back signal received at the antenna 36 is subtracted from them, yielding an error signal (error sample stream) that is processed by the corrector circuits 134. [0071]
• the error sample stream is non-zero due to noise, any mobile station transmissions that are not modeled, and errors in the network's uplink and downlink models.
• the error sample stream can be applied to decoders (not shown) for mobile station signals that are not using loop-back, so that these signals can be decoded after subtraction of all loop-back interference.
• the loop-back signals do not interfere with conventional traffic from mobile stations 16, even from those not participating in loop-back.
• the error sample stream can also be multiplied with the uplink reference signals and downlink reference signals derived in the Appendix to determine the amount of error in each coefficient of the uplink and downlink polynomials for each mobile station 6.
• These errors may be used immediately, on a sample-by-sample basis, to update the channel polynomials, but since such updating might require high-speed processing, the errors may also be averaged over a suitable period, with the resultant average errors used to update the channel polynomials less frequently.
• the errors may be averaged over 256 chips, which averaging imposes a delay of 66.66 ⁇ s at an exemplary chip rate of 3.84 Mega-chips-per-second (Mcps).
• a delay of this amount is small enough to allow the channel changes due to reasonable mobile station speeds to be tracked.
• the tracking rate and accuracy depends on how the channel coefficients are updated using the average errors dumped every 66 uS from the corrector circuits 134.
• An exemplary update method employs a second-order tracker that also estimates rate-of- change of channel coefficients.
• the exemplary second-order tracker tracks a constant rate-of-change of channel coefficients with no static error.
• Fig. 14 Exemplary details for processing an individual loop-back signal for a particular mobile station 16 are illustrated in Fig. 14. The illustrated arrangement may be implemented as part of loop-back signal/channel processor 22 within transmit processor 18, but also might be at least partially implemented in individual network transceiver stations 12, e.g., base stations 34, for corresponding network antennas 36. Regardless, the transmit signal T being transmitted from a particular network antenna 36 is applied to delay memory 140, which represent modeled downlink propagation delays relating transmissions from that antenna 36 to a particular mobile station 16.
• Delayed samples are tapped out at each delay corresponding to a multipath ray to be tracked; the ray- corresponding samples are weighted with downlink channel coefficients in weighting block 144 and added.
• the output of weighter/adder 144 is what the mobile station 16 is estimated to receive based on the network's current downlink channel estimate for that mobile station 16.
• the mobile-specific frequency shift that the mobile station 16 will apply to the loop-back signal is then applied in phase rotator (complex multiplier) 150, which multiplies successive values of the predicted samples by increasing powers of W m .
• the frequency shifted signal is then combined with the mobile-specific pilot code Pm(i) in summing circuit 162 in any one of the ways previously disclosed, i.e. linear addition, time-multiplexing etc.
• the loop-back/pilot combination which is what the mobile station 16 would transmit back to the network 10, is then applied to delay • memory 142, which represents currently estimated uplink propagation delay for the mobile station 16.
• Delayed samples from this memory are tapped out corresponding to uplink multipath rays to be tracked, with the ray-corresponding samples weighted using uplink channel coefficients and added in weighter/adder 146.
• the output of weighter/adder represents what the network anticipates receiving from the mobile station 16, denoted as the mth mobile station. Summing this signal stream in adder 164 with the output of all other predictors 132 for the other mobile stations 16 that are transmitting loop-back signals back to network 10 yields the composite loop-back signal the network expects to receive on antenna 36.
• the error sample stream is non-zero if there are errors in any network- estimated channel coefficient.
• the sample stream input from summing node 162 to uplink delay memory 142 is used as a reference sample stream to be correlated with the error sample stream in correlator 160.
• Correlator 160 correlates with the uplink reference signal delayed by each delay corresponding to a coefficient to be tracked.
• Each correlation involves multiplying an error sample by conjugate of a delayed reference sample and accumulating the result.
• the result may be accumulated over a period of 256 samples, for example, so that a subsequent tracker circuit 156 for the uplink channel need only manage values at 1 /256th of the chip rate.
• An exemplary implementation for coefficient tracker 156 is configured to act as a tracking loop for a corresponding channel coefficient; thus, a channel tracker for tracking the uplink (or downlink) channel for a particular mobile station, would employ as many coefficient trackers 156 as there were channel coefficients.
• An exemplary embodiment of the coefficient-tracking loop implemented by coefficient tracker 156 uses a Proportional-Integral (PI) controller implementation.
• PI Proportional-Integral
• the corresponding error correlation is accumulated (integrated) in a first accumulator to produce the integrated (I) signal.
• the I output is then weighted by a factor ⁇ and added to the non-integrated or proportional error signal P weighted by a factor ⁇ .
• ⁇ and ⁇ determine the tracking loop bandwidth and damping, and there may be motivations to choose these differently for different mobile stations, depending on their speed, and to choose them in dependence on the mean value of the coefficient being tracked, so that, for example, weaker rays are filtered more than stronger rays.
• the weighted sum of the P and I terms is then integrated in another accumulator to produce an updated coefficient value, thus creating a second-order servo system for tracking each coefficient.
• the downlink coefficients for the downlink channel to the mobile station 16 are tracked in an analogous way, except that exemplary tracking of the downlink channels takes advantage of the previously discussed interchangeability of the uplink and downlink channels.
• the base station transmit sample stream T is first frequency shifted in phase rotator (complex multiplier) 148 and passed through the estimated uplink channel block 152 using the same uplink channel coefficients as used in weighter/adder 146. This action generates a sample stream, which, if passed through the downlink channel, should yield the loop-back signal.
• the output sample stream of channel block 152 is correlated with the error sample stream in correlator 158 to isolate errors in the downlink channel coefficients, which are each then subject to an associated channel coefficient tracker 154 as described above to produce continually updated estimates of the downlink channel coefficients for the mobile station 16.
• one or more DSPs and/or other processors or processing devices may be used to implement the functional circuits illustrated in Figs. 13 and 14, and to implement variations thereof, as needed or desired.
• all of the coefficient trackers 154 and 156 implemented for each mobile station 16 may share a common DSP, as the processing rate for each is 1/256th of the exemplary chip rate.
• correlators 158 and 160 may be implemented in dedicated processing resources, such as in a dedicated ASIC and/or FPGA, as the typical processing rates are high (equal to the chip rate, for example).
• the delay memories 140 and 142, along with weighting/adding units 144 and 146 function as Finite Impulse Response (FIR) filters that may be advantageously implemented in dedicated hardware.
• FIR Finite Impulse Response
• Fig. 15 illustrates estimation of uplink and downlink channels for multiple network transmitting and receiving antennas, i.e., for a coherent macrodiversity application as described in the parent applications.
• the signals T1 and T2 are the transmit sample streams transmitted by respective network antennas 36 (denoted as Antenna 1 and Antenna 2) and are, in an exemplary embodiment, Gaussian-noise-like signals that are the sum of all mobile-directed CDMA signals.
• Predictor(m) predicts what network antennas 1 and 2 will receive looped back from a particular mobile station, denoted as mobile(m), base on: - applying the downlink channel polynomial Dm1(z) to T1 and Dm2(z) to T2 in FIR filters 140A and 140B, respectively, and summing the result in summing circuit 144 to predict what mobile(m) will receive; - applying mobile-specific frequency shift used by mobile(m) in phase rotator
• Um1 (z) and Um2(z) (from 142A and 142B) using FIR filters 146A and 146B to determine what will be received from mobile(m) at network antennas 1 and 2, respectively;
• correlators 160A and 160B correlating Error Signal 1 and Error Signal 2 with the predicted mobile loop-back signal in correlators 160A and 160B to determine how much of the error is related to errors in respective uplink channel coefficients (the correlators multiply shifts of the error signals with the conjugate of the predicted loopback signal and average over some suitable period such as 256 chips);
• uplink channel trackers 156A and 156B which correct the coefficients for uplink channel polynomials (Um1(z) and Um2(z)) for the respective network receive antennas;
• the downlink reference signals are determined using the principle of interchangeability of the diversity combined uplink, now Um1 x Um1 # + Um2 x Um2 , and the respective downlink.
• exemplary channel trackers may be comprised of the Pl-based channel coefficient trackers 154 described for tracking-loop maintenance of downlink channel coefficients in Fig. 14.
• the latest estimates of Dm1(z) and Dm2(z) produced by downlink corrector 155 are fed back to FIR filters 140A and 140B, and the latest uplink channel estimates of Um1(z) and Um2(z) from uplink corrector 157 are fed back to FIR filters 146A B, and 152A/B.
• One or more exemplary embodiments of the channel trackers 154A/B and 156A/B provide a second-order low-pass filtering function that passes the highest expected Doppler components and attenuates noise and other error components at higher frequencies.
• a typical cut-off frequency for such a low-pass function would be 200 Hz for dealing with Doppler-imposed frequency shifts on an assumed 2 GHz transmit signal carrier frequency for mobile stations 16 in vehicles moving at normal speeds.
• Error Signals 1 and 2 may be fed to non-coherent macrodiversity combiners and then decoders for mobile stations not employing loop-back transmission.
• the channel polynomials determined by the above method and apparatus can be used to implement any of the interference cancellation schemes disclosed in the parent applications.
• the adjoint of the matrix of uplink channel polynomials can be used to separate uplink signals from different mobile stations 16, including uplink traffic multiplexed with loop-back and pilot sequences.
• the downlink polynomials can be used together with models of the mobile station RAKE receivers to predict soft values obtained at RAKE receiver outputs of the mobile stations, which can then be correlated with the intended symbol steams to produce an interference coupling matrix at the symbol stream level.
• the inverse of this matrix can be applied to the symbols streams prior to CDMA encoding at the network transmitters, as disclosed in the parent applications, to cancel downlink interference.
• the present invention thus provides loop-back signal generation and processing methods and apparatus that enable a wireless communication network, such as a CDMA-based cellular radio network, to deduce both uplink and downlink channel propagation characteristics relating one or more mobile stations to one or more network transmit and receive antennas.
• a wireless communication network such as a CDMA-based cellular radio network
• Such channel characteristics may be maintained at the network for use in, transmit signal pre-compensation, and may be represented as impulse response polynomial coefficients.
• the present invention is applicable to CDMA and non-CDMA networks, and scalable to any number of remote receivers and any number of participating network antennas or antenna sites with only proportional increases in complexity.
• Matrix-based methods are disclosed in the below discussion, and such methods present exemplary details for jointly solving for all uplink and downlink channel coefficients to all mobile stations. Because of cross-interference between the loop-back signals from different mobile stations, the inverse of their autocorrelation matrix is used to separate them. However, the complexity of such a solution increases faster than the number of mobile stations squared. Moreover, the solution generally cannot be partitioned into a repetition of a basic computational elements on a per-mobile basis, thus comprising easy scalability.
• the autocorrelation matrix for the loop-back signals need not be inverted, but rather approximated to diagonal, if the uncompensated mutual interference is low enough.
• an alternative based on successive approximation methods that avoid the need for matrix inversion is also presented herein. Even so, such block-matrix solutions may still not be amenable to easy partitioning into corresponding computational elements that provide for simplified, proportional scalability.
• an interference subtraction method which, in an exemplary embodiment, is illustrated in accompanying Fig. 13. While Fig. 13 illustrates the single base station antenna case, the above discussion further develops the method for the multiple base station antennas case as might be used in a coherent macrodiversity environment.
• an overriding principle is to track changing conditions and update the network's uplink and downlink channel estimates so that the latest values of the channel estimates can be used at the network together with knowledge of the network-transmitted signals to predict what all mobile stations will loop back to the network.
• the method effectively estimates each channel tap for each mobile station after interference due to the other taps and mobile stations has been subtracted.
• channel tap denotes the channel model filter for a particular multipath propagation ray being tracked for a particular mobile station on the uplink or downlink channels.
• Mobile 1 loops back the above received samples with no frequency shift.
• Mobile 2 loops back W' C2 k T(i-k) where W is a phase shift per sample k corresponding to a frequency shift.
• the network receives the looped back samples from mobiles 1 and 2 through respective uplink channels U1 j and U2 j , giving
• X1 is the autocorrelation function of V1 and Y1 is the cross correlation function of V2 and V1.
• X1 and Y1 can also be regarded as Toeplitz matrices and the above written as:
• Wi are diagonal matrices of the phase rotation sequences representing the mobile-specific frequency shifts applied by each mobile station in generating its loop- back signal.
• the summation or integration times should be sufficiently short to avoid delays in obtaining the solution, so that the derived channel is substantially current.
• a delay of less than 0.3 ms allows satisfactory tracking of changing uplink and downlink channels for a mobile station moving at normal highway speeds.
• summation over 1024 chips could be used. This summation length would allow, for example, the determination of perhaps the eight largest downlink channel tap for eight simultaneously looping-back mobile stations.
• a preliminary procedure would be used to determine which eight of all the possible taps delays corresponding to the multipath propagation rays were the largest for each mobile station.
• Such a tap search procedure although used for other purposes, is known in RAKE receivers for decoding CDMA signals.
• a second, improved approximation may be obtained by combining the first approximation with the off- diagonal matrix elements ignored in the first solution, and subtracted from the Q vector. This is tantamount to subtraction of cross-interference. Multiplying the modified Q-vector with the reciprocal of the diagonal elements yields the second approximation. This procedure may be reiterated if desired.
• an exemplary alternate method comprises using the current estimates of the uplink and downlink channel taps, along with the known signal transmitted by a network base station to predict the composite signal sample that should be received by the network, where that composite signal is the sum of all mobile station transmissions. The error between the prediction and the actual received samples is then computed. The channel taps are then updated in proportion to the derivatives of the error with respect to each tap.
• the sequential least-squares method adjusts the channel taps U and C in the direction that would reduce Ei the most for the least change in the channel tap values, referred to as the direction of Steepest Descent.
• the adjustment does not, however, go all the way to the values that would have given a zero Ei , but only part of the way to thereby avoid "bouncing around” on noise-induced signal perturbation. This results in the solution being that which best accords the predictions with the received samples on average.

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