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Making the most of multi-user MIMO

MU-MIMO requires sizable physical distance between the clients, as well as the access point, for spatial diversity, which may limit its usefulness.
Read time 4min 10sec

In a previous Industry Insight, I discussed the importance of Orthogonal Frequency Division Multiple Access (OFDMA) for WiFi 6 and how it results in better frequency reuse, reduced latency and increased efficiency.

In this chapter I will look at the other WiFi 6 multi-user technology: MU-MIMO (multiple-input multiple-output).

Much like OFDMA, MU-MIMO allows for multiple user communications downlink from an access point (AP) to multiple clients during the same transmission opportunity. However, as opposed to partitioning the frequency space, MU-MIMO instead takes advantage of the fact that APs have multiple radios and antennas. A MU-MIMO access point transmits unique modulated data streams to multiple clients simultaneously (see Figure 1). The goal is to improve efficiency by using less airtime.

Figure 1: MU-MIMO – multi-user multiple-input multiple-output.
Figure 1: MU-MIMO – multi-user multiple-input multiple-output.

A five-number syntax is sometimes used when describing MU-MIMO radio capabilities. In a MU-MIMO system, the first number always references the transmitters, and the second number references the receivers. The third number represents how many unique single-user streams of data can be sent or received.

The fourth number references how many multiple-user (MU) streams can be transmitted. A fifth number is used to represent a MU-MIMO group or how many MU-MIMO clients are receiving transmissions at the same time.

WiFi 6 allows for simultaneous use of both MU-OFDMA and MU-MIMO, but this is not expected to be widely implemented.

For example, when a MU-MIMO-capable AP operates using 4×4:4:4:4, four unique spatial streams would be destined to four independent MU-MIMO-capable clients (see Figure 1).

However, when a MU-MIMO-capable AP operates as a 4×4:4:4:2 MU-MIMO AP, two unique spatial streams would be destined to one 2x2:2 client and the other two spatial streams would be destined for a different 2x2:2 client (see Figure 2).

Figure 2: Downlink MU-MIMO – 4×4:4:4:2
Figure 2: Downlink MU-MIMO – 4×4:4:4:2

So how would this work if there are 20 MU-MIMO clients associated to the WiFi 6 AP? The AP makes the decision as to which clients receive the downlink MU-MIMO transmissions and which clients are assigned to the MU-MIMO client group.

For example, four clients could receive spatial streams simultaneously in the first downlink transmission and then four different clients might receive spatial streams simultaneously in the next downlink transmission.

Downlink MU-MIMO was first introduced in the second generation of 802.11ac radios. However, very few MU-MIMO-capable 802.11ac (WiFi 5) clients currently exist in the marketplace, and the technology has rarely been used in the enterprise. Figure 3 displays the maximum client capabilitiesview within the ExtremeCloud IQ management platform. In this example, less than 10% of the clients support MU-MIMO.

Figure 3: Maximum client capabilities
Figure 3: Maximum client capabilities

A key difference between WiFi 5 (802.11ac) MU-MIMO and WiFi 6 MU-MIMO is how many MU-MIMO clients communicate with an AP at the same time. WiFi 5 is limited to a MU-MIMO group of only four clients. WiFi 6 is designed to support up to 8x8:8 MU-MIMO in both downlink and uplink, which allows it to serve up to eight users simultaneously and provide significantly higher data throughput.

Remember that downlink MU-MIMO will be available in WiFi 6 radios. Support for uplink MU-MIMO will not be available in the first generation of WiFi 6 radios.

WiFi 6 clients support downlink MU-MIMO; however, MU-MIMO requires spatial diversity. Because of this, physical distance between the clients is necessary (see Figure 4). Even if all WiFi clients support MU-MIMO, the majority of modern-day enterprise deployments of WiFi involve a high density of users and devices that is not conducive for MU-MIMO conditions.

MU-MIMO requires sizable physical distance between the clients, as well as the AP, for spatial diversity, which may limit its usefulness.

Figure 4: Spatial diversity – MU-MIMO
Figure 4: Spatial diversity – MU-MIMO

Almost all indoor WLANs are high-density environments because there are so many users and so many devices. Many of the users want to connect to an enterprise WLAN with as many as three or four WiFi devices. Most high-density environments consist of multiple areas where roaming is also a top priority.

The required spatial diversity simply does not exist within the bulk of indoor enterprise WiFi high-density deployments, as depicted in Figure 5.

Figure 5: High-density enterprise WiFi deployment
Figure 5: High-density enterprise WiFi deployment

A very good use case for MU-MIMO is point-to-multipoint (PtMP) bridge links between buildings (see Figure 6). The spatial diversity that is required for MU-MIMO exists in this type of outdoor deployment. Bridge links require high bandwidth, which MU-MIMO can deliver with a PtMP deployment.

Figure 6. Point-to-multipoint (PtMP) bridge links
Figure 6. Point-to-multipoint (PtMP) bridge links

What is the difference between MU-OFDMA and MU-MIMO?

So how do the two separate multi-user technologies that WiFi 6 has to offer (MU-OFDMA and MU-MIMO) compare? Table 1 compares these two technologies.

Table 1: Comparison of MU-OFDMA and MU-MIMO
Table 1: Comparison of MU-OFDMA and MU-MIMO

MU-MIMO would theoretically be a favourable option in very low client density, high-bandwidth application environments where large packets are transmitted.

Conclusion

WiFi 6 allows for simultaneous use of both MU-OFDMA and MU-MIMO, but this is not expected to be widely implemented. Do not confuse OFDMA with MU-MIMO. OFDMA enables multi-user access by subdividing a channel. MU-MIMO enables multi-user access by using different spatial streams.

Andre Kannemeyer

National chief technical officer (CTO) at specialist distributor Duxbury Networking.

Andre Kannemeyer is national chief technical officer (CTO) at specialist distributor Duxbury Networking.

Based in the Cape Province, he has been with the company for 20 years and has extensive experience in the IT industry, particularly within the networking space.

Kannemeyer is a passionate, entrepreneurial and tech-savvy technologist with proven technical leadership in his interactions with all Duxbury Networking customers and partners.

As national CTO, he is responsible for looking at new trends and technologies that Duxbury could bring onboard to the benefit of the company’s customers, as well as ensuring the company continues to be a leader in the networking arena.

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