Spatial Diversity

Spatial diversity is a MIMO technique that improves SNR by transmitting replicas of the same data stream from multiple antennas, but with some form of coding or time delay to ensure that the signals are combinable at the receiver. The multiple versions of the signal provide robustness against channel fades because the various transmission paths undergo different fading. The receiver benefits from these replicas and sees a diversity gain. Specifically, a reliable communications link might still be possible when one or more paths experience deep fading, but other paths do not experience deep fading.

Transmit Diversity

Transmit diversity is a spatial diversity method used in multiple-input single-output (MISO) systems, which have multiple transmit antennas and a single receive antenna. If the transmitter has no knowledge of the channel information, you can use transmit diversity schemes to transmit multiple versions of the transmitted data out of different antennas. While transmitting multiple versions of the data does not increase throughput, combining the redundant signals at the receiver results in a higher SNR for the received signal.

For an example that demonstrates use of transmit diversity in a flat-fading Rayleigh channel, see Introduction to MIMO Systems. This example focuses on transmit diversity in lieu of traditional receive diversity. It also provides a measure of the performance degradation when the channel is imperfectly estimated at the receiver, compared to the case of perfect channel knowledge at the receiver.

STBC Diversity

Space-time block coding (STBC) diversity is a transmit diversity method that uses multiple transmit antennas to precode a single data stream on multiple transmit chains, with each transmit chain transmitting a different precoded version of the data stream. The receiver combines multiple blocks together to form the original signal with higher SNR. A popular 2-by-1 MISO system uses Alamouti coding, which is an orthogonal space-time block code (OSTBC). The ability of OSTBC to achieve the full (maximum) spatial diversity order and its use of the computationally simple symbol-wise maximum-likelihood (ML) decoding makes it an attractive spatial diversity technique.

The comm.OSTBCEncoder and comm.OSTBCCombiner System objects and OSTBC Encoder and OSTBC Combiner blocks offer a variety of specific codes (with different rates) for up to four transmit and eight receive antenna systems. The encoder maps symbols to multiple antennas in the transmitter, while the combiner extracts the soft information per symbol by using the received signal and the channel state information at the receiver.

For an example that demonstrates OSTBC diversity, see Concatenated OSTBC with TCM. This example shows an OSTBC signal concatenated with trellis-coded modulation (TCM) transmitted over a 2-by-1 MIMO channel.

Delay Diversity

Delay diversity is a transmit diversity method that varies the time domain delay of a signal transmitted out of different transmit antennas. This scheme has the effect of creating its own multipath component, guaranteeing that the receiver sees a delay-dispersive signal even if no multipath is present. This scheme also reduces the possibility of destructive interference reducing SNR at the receiver. A receiver can take advantage of the multiple signals through proper equalization.

For an example that demonstrates delay diversity, see PDSCH Port 5 UE-Specific Beamforming (LTE Toolbox). This example uses cyclic delay diversity (CDD).

Receive Diversity

Maximum ratio combining (MRC) reception uses multiple receive links to increase the received SNR and improve the link performance. A receive link is defined as the link from a transmit antenna to a receive antenna. A SIMO system has one transmit antenna and multiple receive antennas. For an example that demonstrates spatial diversity in the receiver, see Introduction to MIMO Systems.