Precoding: Difference between revisions

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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 [[space-division multiple access|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.<ref name=gesbert>D. Gesbert, M. Kountouris, R.W. Heath Jr., C.-B. Chae, and T. Sälzer, [http://dx.doi.org/10.1109/MSP.2007.904815 Shifting the MIMO Paradigm], IEEE Signal Processing Magazine, vol. 24, no. 5, pp. 36-46, 2007.</ref> 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.<ref name=fnt2013>E. Björnson and E. Jorswieck, [http://kth.diva-portal.org/smash/get/diva2:608533/FULLTEXT01 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.</ref>

==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 matched filter, and does not have to know the channel side information. This technique will reduce the corrupted effect of communication channel.

For example you are sending information, s

and it will pass through the channel, h

and Gaussian noise, n

The received signal at receiver front-end will be, r=sh+n;

The receiver will have to know the information about h and n. It will supress the effect of n by increasing SNR but what about h? It need information about the channel, h and this will increase the complexity. The receiver, mobile units, have to be simple for many reasons like cost, size of mobile unit. So, the transmitter, base station, will do the hard work and predicts the channel.

Let call the predicted channel, h_est

and for a system with precoder the information will be coded, s/h_est

and the received signal will be, r=(h/h_est)s+n

if your prediction is perfect, h_est=h, and r=s+n, 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 you have to optimize precoder with a decoder. Channel equalization aims to minimize channel errors, but precoder aims to minimize the error in the receiver output.

==Precoding for Point-to-Point MIMO Systems ==