Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail
Reexamination Certificate
2000-04-06
2002-04-23
Trost, William (Department: 2683)
Telecommunications
Transmitter and receiver at same station
Radiotelephone equipment detail
C455S101000, C455S115200, C455S517000, C370S277000, C370S342000, C370S343000, C370S347000, C370S442000
Reexamination Certificate
active
06377819
ABSTRACT:
FIELD OF THE INVENTION
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.
BACKGROUND
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 signatures—or more generally, knowledge of the channels between the BTS and the remote terminals—is 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.
OBJECTS AND ADVANTAGES
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.
SUMMARY
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 T
1
. . . T
R
on the same conventional channel, wherein R≦N.
Information signals s
1
. . . s
R
are simultaneously transmitted from the base transceiver station to remote transceivers T
1
. . . T
R
, respectively. The base transceiver station comprises processing means for selecting R discrimination vectors V
1
. . . V
R
, 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
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 i
th
remote transceiver T
i
receives an M-component signal vector Z
i
t
Gesbert David J.
Paulraj Arogyaswami J.
Sebastian Peroor K.
Iospan Wireless, Inc.
Lumen Intellectual Property Services
Nguyen Simon
Trost William
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