MIMO Space Time Block Coding and Alamouti Codes

- overview of MIMO coding including MIMO precoding, mimo diversity coding, space time diversity coding and Alamouti codes.

In order that MIMO spatial multiplexing can be utilised, it is necessary to add coding to the different channels so that the receiver can detect the correct data.

There are various forms of terminology used including Space-Time Block Code - STBC, MIMO precoding, MIMO coding, and Alamouti codes.

Space time block codes

Space-time block codes are used for MIMO systems to enable the transmission of multiple copies of a data stream across a number of antennas and to exploit the various received versions of the data to improve the reliability of data-transfer. Space-time coding combines all the copies of the received signal in an optimal way to extract as much information from each of them as possible.

Space time block coding uses both spatial and temporal diversity and in this way enables significant gains to be made.

Space-time coding involves the transmission of multiple copies of the data. This helps to compensate for the channel problems such as fading and thermal noise. Although there is redundancy in the data some copies may arrive less corrupted at the receiver.

When using space-time block coding, the data stream is encoded in blocks prior to transmission. These data blocks are then distributed among the multiple antennas (which are spaced apart to decorrelate the transmission paths) and the data is also spaced across time.

A space time block code is usually represented by a matrix. Each row represents a time slot and each column represents one antenna's transmissions over time.

Within this matrix, Sij is the modulated symbol to be transmitted in time slot i from antenna j. There are to be T time slots and nT transmit antennas as well as nR receive antennas. This block is usually considered to be of 'length' T.

MIMO Alamouti coding

A particularly elegant scheme for MIMO coding was developed by Alamouti. The associated codes are often called MIMO Alamouti codes or just Alamouti codes.

The MIMO Alamouti scheme is an ingenious transmit diversity scheme for two transmit antennas that does not require transmit channel knowledge. The MIMO Alamouti code is a simple space time block code that he developed in 1998.

Differential space time block code

Differential space time block coding is a form of space time block coding that does not need to know the channel impairments in order for the signal to be decoded. The differential space time block codes are normally based upon the more standard space-time block codes. One block-code is transmitted from a set in response to a change in the input signal. This enables the system to work because the differences among the blocks in the set are designed to allow the receiver to extract the data with good reliability.

By Ian Poole

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