Background Technology & Concepts
The MIMO Space Time Diversity system utilises the propagation property that radio signals travelling via different paths will by definition arrive at a remote receiver at different times. In normal radio reception, this effect generally degrades radio channel performance and is known as multi-path often resulting from the familiar "multi-path" fading and distortion effects.
In MIMO systems, the multi-path effects are used to improve radio channel performance. The MIMO concept utilises multiple antennas in both the transmit and receive locations to not only increase the radio channel throughput but to also improve the received signal quality.
The multiple receive antennas improve the quality of the received signal in the following two ways:
i) Firstly combining the received signals from multiple antennas is similar to increasing what is known as the "effective aperture." This can be likened to having a larger receive antenna area, which has the effect of sampling a greater area of space to find the required radio signals. The additional signal gain over a known reference antenna is commonly referred to as aperture gain.
ii) Secondly by providing what is known as "receiver diversity". In a typical receiver environment where the signal is undergoing a particular type of fading, that fading is typically unique to a particular location at that point in time. A separate antenna placed an appropriate distance away from the first antenna will probably not be subject to the same faded signal at that same point in time. By intelligently combining the outputs from multiple antennas the signal quality can be dramatically improved and thus the receiver can be provided some protection from this type of fading. This is very important in data transmission as deep signal fades may mean unrecoverable dropped bits of information.
Having multiple transmit antennas allows the transmission of different information at the same frequency on each "virtual channel" which allows much more information to be sent simultaneously and translates to a much higher effective data throughput. Using multiple antennas in the system effectively enables the transmission and recovery of information across space and time.
An environment where the signal is more "richly scattered" provides a better throughput and enables the system to perform in non line of sight conditions for short distances and near line of sight (NLos) for reasonable distances. The system performance relies on having multiple channels of information arriving at multiple receiver antennas positioned in such a way that diversity reception is possible.