Precoding: Difference between revisions

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==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, <math>s</math>

and it will pass through the channel, <math>h</math>,

and add Gaussian noise, <math>n</math>.

The received signal at the receiver front-end will be, <math>r = sh + n</math>;

The receiver will have to know the information about <math>h</math> and <math>n</math>. It will suppress the effect of <math>n</math> by increasing SNR, but what about <math>h</math>? It needneeds information about the channel, <math>h</math>, and this will increase the complexity. The receiver, (mobile units,) havehas 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 predictspredict the channel.

Let us call the predicted channel, h_est<math>h_{\text{est}}</math>

and for a system with precoder the information will be coded,: <math>{s \over h_{\text{est}}}</math>. The received signal will be <math>r = \left(\frac{h}{h_{\text{est}}}\right) s + n</h_estmath>.

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

ifIf your prediction is perfect, h_est<math>h_{\text{est}} = h,</math> and <math>r = s + n,</math> and it turns out to be the detection problem in Gaussian channels which is simple.

whyWhy 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.

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