Taking full advantage of mobility in wireless communications is a complex and ever-evolving issue with the problem of fading ― random attenuation and phase instabilities due to multipath components from signal obstacles (e.g., walls, rain, mountains, brush, etc.). When the user is mobile, updates are triggered often because of the changes in topology and decay in received signal strength. These updates change the channel state information (CSI) at the receiver and/or transmitter ― instantaneous channel gains that enable either the receiver (open loop MIMO) or both the receiver and transmitter (closed loop MIMO) to update its transmission/reception strategy. It is beyond the scope of this article to dive into stochastic modeling that goes into realizing a multiple MIMO data streams although there is a plethora of research covering this topic.
There are generally three different diversity techniques to alleviate the issue of fading: frequency diversity, time diversity, and space diversity. As shown in Table 1, most diversity techniques are a function of the coherence bandwidth or coherence time. This parameter expresses the bandwidth over which two signals will experience similar fading environments. For instance, in the case of a signal in deep fade with a signal with a coherence bandwidth of 1.25 MHz, shifting 1 MHz away from the carrier frequency will likely maintain the deep fade environment. Moving beyond the 1.25 MHz bandwidth yields a higher likelihood of a different fading environment.