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Precoding is a generalization of beamforming to support multi-stream (or multi-layer) transmission in multi-antenna wireless communications. In conventional single-stream beamforming, the same signal is emitted from each of the transmit antennas with appropriate weighting (phase and gain) such that the signal power is maximized at the receiver output. When the receiver has multiple antennas, single-stream beamforming cannot simultaneously maximize the signal level at all of the receive antennas.〔G.J. Foschini and M.J. Gans, (On limits of wireless communications in a fading environment when using multiple antennas ), Wireless Personal Communications, vol. 6, no. 3, pp. 311–335, 1998.〕 In order to maximize the throughput in multiple receive antenna systems, multi-stream transmission is generally required. In point-to-point systems, precoding means that multiple data streams are emitted from the transmit antennas with independent and appropriate weightings such that the link throughput is maximized at the receiver output. In multi-user MIMO, the data streams are intended for different users (known as SDMA) and some measure of the total throughput (e.g., the sum performance or max-min fairness) is maximized. In point-to-point systems, some of the benefits of precoding can be realized without requiring channel state information at the transmitter, while such information is essential to handle the inter-user interference in multi-user systems.〔D. Gesbert, M. Kountouris, R.W. Heath Jr., C.-B. Chae, and T. Sälzer, (Shifting the MIMO Paradigm ), IEEE Signal Processing Magazine, vol. 24, no. 5, pp. 36-46, 2007.〕 Precoding in the downlink of cellular networks, known as network MIMO or coordinated multipoint (CoMP), is a generalized form of multi-user MIMO that can be analyzed by the same mathematical techniques.〔E. Björnson and E. Jorswieck, (Optimal Resource Allocation in Coordinated Multi-Cell Systems ), Foundations and Trends in Communications and Information Theory, vol. 9, no. 2-3, pp. 113-381, 2013.〕 ==Precoding in Simple Words == Precoding is a technique which exploits transmit diversity by weighting information stream, i.e. the transmitter send the coded information to the receiver in order to the pre-knowledge of the channel. The receiver is a simple detector, such as a matched filter, and does not have to know the channel side information. This technique will reduce the corrupted effect of the communication channel. For example you are sending the information and it will pass through the channel, , and add Gaussian noise, . The received signal at the receiver front-end will be ; The receiver will have to know the information about and . It will suppress the effect of by increasing SNR, but what about ? It needs information about the channel, , and this will increase the complexity. The receiver (mobile units) has to be simple for many reasons like cost or size of mobile unit. So, the transmitter (the base station) will do the hard work and predict the channel. Let us call the predicted channel . The received signal will be . If your prediction is perfect, and and it turns out to be the detection problem in Gaussian channels which is simple. Why do they call it "coding"? It is a preprocessing technique that performs transmit diversity and it is similar to equalization, but the main difference is that you have to optimize the precoder with a decoder. Channel equalization aims to minimize channel errors, but the precoder aims to minimize the error in the receiver output. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Precoding」の詳細全文を読む スポンサード リンク
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