MU-MIMO – A Practical Example
While OFDMA (orthogonal frequency-division multiple access) increases the channel access efficiency for a large number of clients, MU-MIMO (Multi-User, Multiple input, Multiple output) creates separate spatial channels for each client and boosts aggregated network throughput.
The mathematics behind MU-MIMO is complicated and often difficult to understand. This whitepaper uses a simple audio analogy to help the reader visualize what is happening in MU-MIMO channels without reverting to matrices and equations. This understanding helps the reader appreciate the meaning of spatial diversity and helps in the interpretation of results from a real life MU-MIMO test run where we successively connect a number of clients and observe the increases in throughput.
Servicing a large number of clients that are using small packets with legacy Wi-Fi is inefficient because the overheads incurred by the preamble and other mechanisms tend to dominate. OFDMA is ideally suited for this scenario because it divides up the channel and services up to 37 users (for 80MHz bandwidth) simultaneously, which amortizes the overhead. OFDMA improves system efficiency, but it does not necessarily improve throughput.
MU-MIMO creates spatially distinct separate channels between the transmitter and each of a small number of receivers such that each receiver hears only the information intended for itself, and not the information intended for other receivers. This means that the transmitter can, by superposition, transmit to a few receivers simultaneously, increasing the aggregate throughput by a factor equivalent to the number of receivers being serviced.
In this whitepaper we will discuss how MU-MIMO can be achieved in a relatively small anechoic chamber with proper antenna placement, how multiple stations help achieve the best MU-MIMO gain, and why spatial separation is key to testing.