Spatial multiplexing can maximize the average transmission rate of a MIMO system, but it can only obtain a limited diversity gain. When the signal-to-noise ratio is relatively small, it may not be possible to use high-order modulation methods (such as 16QAM).
Wireless signals are frequently reflected in environments such as dense urban areas and indoor coverage, making the fading characteristics between multiple spatial channels more independent, thereby making the effect of spatial multiplexing more obvious.
Wireless signals in suburban and rural areas have less multipath components and greater correlation between spatial channels, so the effect of spatial multiplexing is much worse.
Performing space-time coding on the transmitted signal can obtain additional diversity gain and coding gain, so that high-order modulation can be used in a wireless environment with a relatively small signal-to-noise ratio, but the rate bonus brought by spatial parallel channels cannot be obtained. Space-time coding technology can also play a good role in occasions with large wireless correlation.
Therefore, in practical use of MIMO, spatial multiplexing technology is often used in combination with space-time coding. When the channel is in the ideal state or the inter-channel correlation is small, the transmitting end uses a spatial multiplexing transmission scheme, such as dense urban areas, indoor coverage, and other scenarios; when the inter-channel correlation is large, a space-time coding transmission scheme is used, such as suburban Countryside. This is also the method recommended by 3GPP in FDD systems.
When beamforming technology can obtain channel state information, it can achieve better signal gain and interference suppression, so it is more suitable for TDD systems.
Beamforming technology is not suitable for dense urban areas, indoor coverage and other environments. Due to reflection, on the one hand, the receiving end will receive signals of too many paths, resulting in poor phase superposition effect; on the other hand, a large number of multipath signals will cause DOA information estimation is difficult.