Spatial multiplexing

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Multiplex
techniques
Circuit mode
(constant bandwidth)
TDM · FDM · WDM
Polarization multiplexing
Spatial multiplexing (MIMO)
Statistical multiplexing
(variable bandwidth)
Packet mode · Dynamic TDM
FHSS · DSSS · OFDMA
Related topics
Channel access methods
Media Access Control (MAC)
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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

\mathbf{N_s}=\min(N_t,N_r)

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).

Contents

[edit] Encoding

[edit] Open-loop approach

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

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

where \mathbf{x} = [x_1, x_2, \ldots, x_{N_t}]^T is the N_t\times 1 vector of transmitted symbols, \mathbf{y,n} are the N_r\times 1 vectors of received symbols and noise respectively and \mathbf{H} is the N_r\times N_t matrix of channel coefficients.

[edit] Closed-loop approach

In a closed-loop MIMO system the input-output relationship with a closed-loop approach can be described as

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

where \mathbf{s} = [s_1, s_2, \ldots, s_{N_s}]^T 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.

A precoding matrix \mathbf{W} is used to precode the symbols in the vector to enhance the performance. The column dimension Ns of \mathbf{W} can be selected smaller than Nt which is useful if the system requires N_s (\neq N_t) streams because of several reasons. Examples of the reasons are as follows: either the rank of the MIMO channel or the number of receiver antennas is smaller than the number of transmit antennas.

[edit] History

  • Single-user MIMO
  • Multi-user MIMO: Samsung, Qualcomm, Ericsson, TI, Huawei, Philipse, Lucent-Alcatal, Freescale, et al.
    • PU2RC allows the network to allocate each antenna to the different user which is not considered in single-user MIMO scheduling. Instead of a physical antenna, the network can transmit a user date through a codebook based spatial beam, i.e., a virtual antenna. The efficient user scheduling such as pairing spatially distinguishable users with codebook based spatial beams are additionally used for the simplification of wireless networks in terms of additionally required wireless resource and complex protocol modification.
  • Enhanced multiuser MIMO
    • Employ advanced decoding techniques
    • Employ advanced precoding techniques

[edit] See also