Читать книгу Spatial Multidimensional Cooperative Transmission Theories And Key Technologies - Lin Bai - Страница 51

If the transmitter has only partial channel information, it is also possible to exploit the gain of the array. The complete channel information at the transmitter requires high-speed feedback links at both ends of the receiver and transmitter to ensure that the latter can continuously obtain channel state information. On the converse, exploiting the statistical property of the channel at the transmitter or the quantitative description of the channel requires only a lower rate feedback link.

Precoding techniques usually combine a multi-mode beamformer that can spread codewords in orthogonal directions of the channel distribution-related direction with a constellation shaper, or more simply, with a power allocation algorithm. There are many similarities with various eigenmode transmissions, while the difference is that the eigenbeam is based on the statistical property of matrix H rather than its instantaneous value.

Similarly, the antenna selection technique may also only depend on partial channel information, namely the first- and second-order statistics of the matrix H to select the transmitting or receiving antenna. Intuitively, they are selected from the antenna pair with the lowest selective correlation ratio. Such techniques do not minimize transient error performance metrics, but minimize the average error probability.

By quantization precoding, the generalization of antenna selection is to derive limited feedback from the transmitter. This technique depends on the selection of codebooks in the precoding matrix, which is a finite set of precoders. It can be designed offline and is known to both the transmitter and the receiver. The receiver estimates the best precoder as a function of the current channel and then feeds the index of the best precoder back into the codebook.