The existing wireless mobile communication systems provide several types of services and mostly depend on channel coding to overcome any inferiority of channels. However, due to the increasing demands, for example for a high-quality multimedia services, in which users can communicate with anyone regardless of time and place, the existing services have evolving into data-oriented services. Accordingly, there is a high demand for next generation wireless transmission technology for transmitting the larger amount of data at a lower error rate. In particular, it is very important to transmit data at a high rate in a link in which the amount of required data is large.
For the next generation wireless communication, various antenna systems have been proposed. For example, a Multiple-input multiple-output (MIMO) system increases spectrum efficiency through all of transmission antennas without excessive use of a frequency bandwidth. Generally, MIMO is classified into Space-Time Coding (STC), Diversity, Beam Forming (BF), and Spatial Multiplexing (SM) according to the transmission structure and scheme of a transmitter, all of which provide high data rate and reliability.
A MIMO system adopts multiple antennas or array antenna to transmit/receive data in the transmitter and receiver. Multiple antennas are provided in different spatial positions, with different fading features, thus the received signals of adjacent antennas can be approximated as uncorrelated entirely as long as the spacing between adjacent antennas for transmitting/receiving signals in the MIMO system is large enough. The MIMO system takes full advantage of the spatial characteristics of multipath for implementing space diversity transmission and reception.
FIG. 1 illustrates an exemplary and simplified MIMO system 100 constructed by M Tx antennas 103 and N Rx antennas 104. As mentioned earlier, the antenna spacing between the Tx antennas and Rx antennas in the MIMO system in FIG. 1 is generally big enough, to guarantee the spatial un-correlation of signals. As FIG. 1 shows, in the transmitter, MIMO architecture unit 101 first transforms a channel of data stream into M channels of parallel sub data streams; then, multiple access transform unit 102 performs multiplex processing; finally, the corresponding M Tx antennas 103 transmit the signal simultaneously into the wireless channels. The MIMO architecture unit 101 can adopt any one of the MIMO processing methods, such as STTC (Space Time Trellis Code), space-time block code, space-time Turbo code, BLAST code and etc. While multiple access transform unit 102 can implement TDD, FDD or CDMA.
At the receiver site N Rx 104 antennas receive the broadcasted signals, which are transformed by multiple access inverse transform unit 105, performing multiple access demultiplexing processing, and provided to a MIMO detection unit 106.
Usually, a MIMO antenna system with nR receive and nT transmit antennas operating in a frequency non-selective channel is described by the following matrix representation:y=Gx+z  (1)
Wherein y is the nR×1 received signal vector, G is the nR×nT MIMO channel response, z is the independent and identically distributed elements Additive White Gaussian Noise (AWGN) at the receiver with individual variance of σ2z and x is the nT×1 transmitted signal vector with a certain power constraint.
The best performance of such a system is achieved when the channel response is known to the transmitter so that the transmit signals can be designed accordingly. This is disclosed, for example in G. G Raleigh & J. M. Cioffi, “Spatio-temporal Coding For Wireless Communication” IEEE trans. On Comm. Vol. 46, no. 3 Mar. 1998, pp. 357-366, and K. C. Zangi and L. G Krasny “Capacity achieving Transmitter and Receiver Pairs for MISO Channels” IEEE Transaction on Wireless Communications Vol. 2 No. 6 Nov. 2003, pp 1204-1216.
In many cases, the channel response is only known to the receiver through the reference signals sent by the transmitter on a forward link and therefore requires being explicitly fed back to the transmitter on a reverse link. Such a feedback may sometimes be a significant overhead especially for a configuration with a large number of antennas. It may also require a substantial amount of computational power.
Conventionally, the entire channel matrix G is fed back from the receiver. For the case of two transmit antennas and one receiver antenna, for example, the close-loop mode in 3rd Generation Project Plan (3GPP) feeds back one phase factor optimized for the frequency selective channel to adjust one of the transmitter antennas.