Wideband space-time communication with implicit channel feedback

We consider wideband communication (e.g., using OFDM) over a typical cellular "downlink," in which the base station may have several antenna elements, while the mobile has 1 or 2 antenna elements. The following are our major findings: (a) Implicit channel feedback regarding the covariance matrix for the downlink space-time channel can be obtained, without any power or bandwidth overhead, by suitably averaging uplink channel measurements across frequency. This approach applies to both Time Division Duplex (TDD) and Frequency Division Duplex (FDD) systems. The covariance feedback can be used to obtain better performance on the downlink, at lower encoding and decoding complexity, compared to standard space-time coding. (b) The conventional design without channel feedback is to space the transmit antennas far enough apart so as to ensure uncorrelated responses. However, when implicit feedback is available, much better performance is obtained by significantly smaller antenna spacing, optimized such that the number of dominant eigenmodes of the channel matches the (small) number of receive antenna elements.