This invention relates generally to wireless communication systems, and more particularly to wireless communication systems using a base transceiver station and remote transceivers, wherein both the base transceiver station and the remote transceivers have multiple antennas and signal processing capabilities.
Wireless communication is becoming an increasingly common form of communication, and the demand for wireless service continues to grow. The sources of demand include cellular mobile communication networks, wireless local area computer networks, wireless telephone networks, wireless cable TV, multi-user paging systems, high frequency modems, and more. Current implementations of these communication systems are all confined to limited frequency bands of operation either by practical considerations or by government regulation. As the capacity of these systems has been reached, demand for more service is met by allocating more frequency spectrum to the particular application and by utilizing the allocated spectrum more efficiently. In light of the basic physical principle that transmission of information requires bandwidth, the fundamental limitations of a finite amount of practically usable spectrum present a substantial barrier to meeting an exponentially increasing demand for wireless information transmission.
Conventional wireless communication systems attempt to solve the problem of high demand by using different multiple access schemes, the most common being frequency-division multiple access (FDMA), time-division multiple access (TDMA), and code-division multiple access (CDMA). All current systems employ FDMA, wherein the available frequency bandwidth is sliced into multiple frequency channels and signals are transmitted over the different channels simultaneously.
Current wireless systems also use TDMA, wherein multiple users share a common frequency channel by doing so at different times. Typically, analog data such as voice is digitized, compressed, then sent in bursts over an assigned frequency channel in assigned time slots. By interleaving multiple users in the available time slots, the number of simultaneous users of the system is increased.
CDMA allows multiple users to share a common frequency channel by using coded modulation schemes. The technology involves preprocessing the signal to be transmitted by digitizing it, modulating a wideband coded pulse train, and transmitting the modulated coded signal in the assigned channel. Multiple users are given distinct codes which decoders in the receivers are programmed to detect.
Another scheme for increasing the capacity of a wireless communication system is spatial division multiple access (SDMA), as discussed by Roy, III et al. in U.S. Pat. No. 5,642,353. SDMA exploits the spatial separation of a number of users to serve the users within the same conventional channel (that is, within the same time slot in the case of TDMA, frequency slot in the case of FDMA, and code in the case of CDMA). In this case, efficient exploitation of the spatial dimension to increase capacity requires the ability to separate a number of users simultaneously communicating on the same channel at the same time in the same local area (or cell).
The above mentioned separation of user up-link and down-link signals can be based on the direction of arrival (DOA) of the individual signals, as described in U.S. Pat. No. 5,828,658 by Ottersten et al. The DOA should be estimated accurately enough to enable the separation. If the users are close to each other, or if the signals are scattered many times, the DOA estimates are inaccurate. In these cases the SDMA technique fails because the separation of the signals is impossible.
As described in U.S. Pat. No. 5,592,490 by Barratt et al., the SDMA separation of signals can also be based on the transmit and receive spatial signatures. The transmit spatial signature characterizes how the remote terminal receives signals from each of the antenna array elements at the base station. The receive spatial signature characterizes how the base station antenna array receives signals from a particular remote terminal. The base station uses these spatial signatures to form multiple beams simultaneously so that each beam maximizes signal reception for one remote terminal. Whereas the receive spatial signatures can be determined by the remote user upon reception, the transmit spatial signatures must be known prior to transmission. Feedback from the remote terminals is necessary to enable computation of the transmit spatial signatures.
FIG. 1 shows the operation of SDMA downlink, considering two remote terminals for example. During downlink, information is transmitted from a base transceiver station (BTS) to the remote terminals. (During uplink, information is transmitted from the remote users to the base transceiver station.) The BTS must have knowledge of the spatial signatures prior to transmission. An accurate estimate of the spatial signaturesxe2x80x94or more generally, knowledge of the channels between the BTS and the remote terminalsxe2x80x94is necessary to enable SDMA communication. As the accuracy of the channel estimate (or spatial signature estimate) deteriorates, SDMA communication becomes prone to error. In the extreme case when channel knowledge is absent, SDMA is impossible.
In present SDMA systems, the base station has multiple antennas, and each remote terminal has one antenna. Processing is carried out at the base station during both uplink and downlink operation. These SDMA systems require accurate channel knowledge, and this knowledge can only be gained by recording how signals sent from the base station are attenuated and phase-shifted by the time they are received remotely. This information, recorded at the remote units, must be sent back to the base station so that the channels may be computed by data processors. By the time this feedback and computation has occurred, the channel will have changed. (Wireless communication channels are constantly changing since the remote users, as well as the objects from which their signals are reflected, are in general moving.) Therefore, present wireless systems cannot reliably estimate the transmit spatial signatures accurately enough to make SDMA practical.
It is therefore a primary object of the present invention to provide a system and method for wireless communication that allows multiple users to share the same time slot, frequency slot, and code, even in the absence of accurate transmit channel knowledge. It is a further object of the present invention to provide a wireless communication system wherein signal processing is distributed between the base transceiver station and the remote transceivers.
The present invention has the advantage of providing a system and method of multiple access that is reliable on both downlink and uplink, even when transmit channels are unknown or rapidly changing.
A wireless communication system comprises a base transceiver station and remote transceivers having multiple antennas. Each of the remote transceivers comprises M remote antennas, wherein M is a number greater than 1. The base transceiver station comprises N base station antennas, wherein N is a number greater than 1. The base transceiver station services R remote transceivers T1 . . . TR on the same conventional channel, wherein Rxe2x89xa6N.
Information signals s1 . . . sR are simultaneously transmitted from the base transceiver station to remote transceivers T1 . . . TR, respectively. The base transceiver station comprises processing means for selecting R discrimination vectors V1 . . . VR, each of the discrimination vectors having N components. The base transceiver station computes an N-component transmission signal vector U as follows:   U  =            ∑              i        =        1            R        ⁢          xe2x80x83        ⁢                  V        i            ⁢                        s          i                .            
The transmission signal vector U is transmitted from the base transceiver station, preferably one component of U per base station antenna.
The ith remote transceiver Ti receives an M-component signal vector Zi through its M remote antennas, one component of Zi per antenna. The ith remote transceiver computes a reconstructed signal yi from the received signal vector Zi.
In a preferred embodiment, the discrimination vectors V1 . . . VR are selected to be linearly independent, preferably orthogonal. In another preferred embodiment, the vectors V1 . . . VR are selected to optimize an efficiency of transmission of information signals s1 . . . sR to remote transceivers T1 . . . TR, respectively. The efficiency is measured, for example, by the strength of the ith information signal si at transceiver Ti, or by the interference due to si at remote transceivers other than Ti.
Remote transceiver Ti computes reconstructed signal yi using either a linear or a nonlinear relationship between yi and Zi. When a linear relationship is used, remote transceiver Ti selects an M-component signature vector Wi, and computes yi according to yi=Wi*xc2x7Zi. Vector Wi is preferably selected to maximize a signal quality parameter xcfx81i that measures the quality of reconstructed signal yi. Signal quality parameter xcfx81i is typically defined using an Mxc3x97N channel matrix Hi that models a channel between the base transceiver station and remote transceiver Ti. In some embodiments, signal quality parameter xcfx81i is a signal to interference ratio, equal to             "LeftBracketingBar"                        W          i          *                ⁢                  H          i                ⁢                  V          i                    "RightBracketingBar"        2    /            ∑              j        ≠        1              ⁢                            "LeftBracketingBar"                                    W              i              *                        ⁢                          H              i                        ⁢                          V              j                                "RightBracketingBar"                2            .      
During uplink, transceiver Ti transmits a remote information signal sixe2x80x2 by selecting an M-component remote processing vector Wixe2x80x2 and transmitting the product Wixe2x80x2sixe2x80x2, one component per antenna. The vector Wi is preferably selected to optimize a remote transmission quality parameter.
The base transceiver station receives an N-component base station received signal vector X during uplink. The N components of vector X correspond to the N base station antennas. The base transceiver station selects N-component signature vectors V1xe2x80x2 . . . VRxe2x80x2, and computes base station reconstructed signals u1 . . . uR corresponding to signals sent from transceivers T1 . . . TR, respectively, according to the formula: ui=Vixe2x80x2*xc2x7X. The signature vectors V1xe2x80x2 . . . VRxe2x80x2 are preferably selected to optimize base station reception quality parameters.
Some embodiments of the present system and method employ time filtering, and some embodiments use frequency filtering. In these embodiments, the transmission signal vector U is computed using p-component information vectors S1 . . . SR and Nxc3x97p discrimination matrices V1 . . . VR as follows:   U  =            ∑              i        =        1            R        ⁢          xe2x80x83        ⁢                  V        i            ⁢                        S          i                .            
Upon receiving signal vector Zi, remote transceiver Ti uses signal vector Zi to compute a reconstructed signal vector Yi. In case of time filtering, the reconstructed signal vector Yi has only one component and it is computed from pxe2x80x2 consecutive signal vectors Zi. pxe2x80x2 is a function of time of flight (ToF) difference between multipaths. In case of frequency filtering (for example when Orthogonal frequency Division Multiplexing [OFDM] is used), the reconstructed signal vector Yi has p components computed from each signal vector Zi.
The wireless system of the present invention operates consistently better than prior art SDMA systems, even when the base transceiver station or remote transceiver lacks accurate transmit channel data. The improvement occurs because (1) the multiple antennas of the remote transceivers are used advantageously, and (2) the remote transceivers possess signal processing capabilities, so the remote transceivers do not need to wait for the base transceiver station to perform all of the processing.