What Are MIMO, MRC, Beamforming, STBC, and Spatial Multiplexing?
Multiple input multiple output (MIMO) is an antenna system that consists of M transit antennas and N receive antennas. The MIMO technology allows spaces to become the resources used to improve performance and increases the coverage range of the wireless system.
The MIMO system generates multiple spatial flows with each antenna generating a maximum of one spatial flow. The single in single out (SISO) system sends or receives one spatial flow (one copy of signals) at a time. The MIMO technology allows multiple antennas to send and receive multiple spatial flows (multiple copies of signals) simultaneously and to differentiate the signals sent to or received from different spaces. An 802.11n device supports up to 4x4 MIMO, a maximum of four spatial flows, with a rate of up to 600 Mbit/s.
The maximal ratio combining (MRC) technology improves the signal quality of the receive end.
In MRC, the same signal from the transmit end is received by the receive end through multiple paths (multiple antennas) because the receive end receives this signal using multiple antennas. Generally, among multiple paths, there is one path providing better signal quality than the other paths. The receive end uses a certain algorithm to allocate different weights to receiving paths. For example, the receive end allocates the highest weight to the receiving path providing the best signal quality, which improves the signal quality of the receive end. When none of multiple receiving paths can provide better signal quality, the MRC technology can ensure better receive signals.
The Beamforming or Transmit Beam Forming (TxBF) technology produces the strong directional radiation pattern based on the strong correlation of the spatial channel and wave interference principle, making the main lobe of the radiation pattern adaptive to point to the wave direction. This technology improves the SNR, system capacity, and coverage range. Beamforming or TxBF is an optional feature in the 802.11n standard.
Beamforming includes explicit Beamforming and implicit Beamforming. Explicit Beamforming requires the receive end to send information about the received signal to an AP. The AP then adjusts the transmit power to the optimal value according to the signal information. This function increases the SNR of the receive end and improves the receiving capability. Implicit Beamforming allows an AP to automatically adjust the transmit power to increase the SNR of the receive end based on channel parameters without requiring the receive end to work with the AP. Currently, mainstream terminals do not support Beamforming.
Space time block coding (STBC) transmits multiple copies of one data flow in wireless communication. STBC uses many antennas to produce multiple receive versions of data, improving data transmission reliability. Among these data copies, optimal copies are combined to provide most reliable data. This redundancy increases the chance of using one or more copies of received data to correctly decode the received data. STBC combines all the copies of received signals to produce the useful data.
The MIMO technology provides the system with the spatial multiplexing gain and spatial diversity gain.
In spatial multiplexing, multiple antennas are used on the receive end and transmit end and multipath components in spatial communication are used, allowing signals to be transmitted over multiple data channels (MIMO sub-channels) in the same frequency band. This technology makes the channel capacity linearly increase with the growing number of antennas. This increase in channel capacity does not require additional bandwidth and does not consume additional transmit power. Therefore, spatial multiplexing is an efficient means to improve channel capacity and system capacity.
In spatial multiplexing, serial-to-parallel conversion is performed on the transmitted signal to produce several parallel signal flows, which are then transmitted using their respective antennas in the same frequency band simultaneously. Due to the use of multipath propagation, each transmit antenna produces a unique spatial signal for the receive end. After the receive end receives the mixed signals of data, it differentiates these parallel data flows based on the fading between different spatial channels. Spatial multiplexing requires the spacing between transmit and receive antennas to be greater than the distance, ensuring that each sub-channel of the receive end is an independently fading channel.