Spatial multiplexing

From Wikipedia, the free encyclopedia

Spatial multiplexing is a transmission technique in MIMO wireless communication to transmit independent and separately encoded data signals, so called streams, from each of the multiple transmit antennas. Therefore, the space dimension is reused, or multiplexed, more than one time.

If the transmitter is equipped with Nt antennas and the receiver has Nr antennas, the maximum spatial multiplexing order (the number of streams) is

Ns = min(Nt,Nr)

if a linear receiver is used. This means that Ns streams can be transmitted in parallel, leading to a Ns increase of the spectral efficiency (the number of bits per second and per Hz that can be transmitted over the wireless channel).

In a MIMO system with Nt transmitter antennas and Nr receiver antennas, the input-output relationship can be described as

\mathbf{y}=\mathbf{HWx}+\mathbf{n}

where \mathbf{x} is the N_s\times 1 vector of transmitted symbols, \mathbf{y,n} are the N_r\times 1 vectors of received symbols and noise respectively, \mathbf{H} is the N_r\times N_t matrix of channel coefficients and \mathbf{W} is the N_t\times N_s linear precoding matrix.

Linear precoding implies that a precoding matrix W is used to precode the symbols in the vector to enhance the performance. The column dimension Ns of W can be selected smaller than Nt which is useful if the channel can not support Ns streams.

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