Contributors -Ming Chong and Santa Chowdhury (octoScope)

We are particularly grateful to Nilesh Doshi (Sr. Wireless AP Manager) forhis supervision.

Servicing many clients that are using small packets with non-Wi-Fi 6 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 (Multi-User, Multiple input, Multiple output) 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.

Cisco's Catalyst 9800 series WLC with IOS XE 17.6.1 (currently Beta) introduces futuristic Access Point scheduler design, which efficiently serves multiple clients at the same time. This is done while creating least level of sounding overhead, which in turn yields data rates close to PHY rate even in dense environment. These advancements are currently supported on Catalyst 9130 and Catalyst 9124 series Access Points. Let's first understand MU-MIMO concepts and then evaluate its performance.

Beamforming and MU-MIMO

Beamforming radio waves using an array of phased antennas has been known for decades. More recently the principles have been used to produce MU-MIMO where the concept of multiple simultaneous beams to provide independent channels for each of the users.

Similar principles apply in the audio domain where speakers can be phased to direct sound to a particular location. The idea is to adjust the phases of each speaker such that the sound adds constructively at the point where the listener is, and destructively at all other locations.

Consider a sound, S_r , played through an array of four speakers with the sound for each speaker adjusted by a phasor Q_1r through Q_4rso that the signal strength at the red listener, L_ris maximized, and the signal strength at the blue listener L_bis minimized.

Using superposition, we can take each message, impose the appropriate phase adjustment, and add the signals just before they go into the speakers. This way we can send two different messages at the same time, but each listener will hear only the message intended for them.

Note the importance of spatial separation -L_b and L_r are hearing their respective messages because the phasors were optimized to deliver each sound to their specific location. If one of the listeners moves from his position, he will no longer hear his message.

If a third person enters the picture and stands close to the speakers, he will hear the garbled sound of both messages simultaneously.

Consider this in the context of Wi-Fi where the speakers are replaced by antennas and the signal processing to control the phasors, and generate digital messages at a certain data rate, is done in the AP. Since both messages can be transmitted simultaneously one could theoretically double the aggregated data rate. The same approach can be used to service more clients simultaneously, so where is the limit? Practically, there are limits in the accuracy that the phasors can be set, there are reflections that cause "cross talk" and other imperfections that limit the gains in throughput that can be achieved.

Sniffing in the context of MU-MIMO is more complicated because of the spatial significance.  Note that placing a sniffer close to the AP will achieve the same garbled message effect we discussed earlier. The sniffer probe must be placed physically close to the device that is being sniffed, and generally one sniffer probe is required for each device.

System Overview and Test infrastructure

In this MU-MIMO test, we are using the octoScope (now part of Spirent) STACK-MAX testbed. On the infrastructure side, Cisco's Catalyst 9800 WLC running IOS XE 17.6.1 (Beta code) and Catalyst 9130 Access point is used. The C9130 AP supports up to 8