8x8:8 versus 4x4:4 - More is Not Better!

Lately, the Wi-Fi 6 technology question I have been getting asked the most often is, "Which is better? 8x8:8 access points or 4x4:4 access points?" Several WLAN vendors are selling Wi-Fi 6 access points that have a 2.4 GHz radio that is 4x4:4 and a 5 GHz radio that is 8x8:8. These WLAN vendors are forging marketing pitches for 8x8:8. Eight must be better than four — right?

First of all, the PoE power requirements of 8x8:8 access points are substantial. In some cases, these 8x8:8 access points will require 31 watts of power or more, which means that even PoE Plus power will not be sufficient. While some of the 8x8:8 APs can use the 25 Watts provided by 802.11at (PoE Plus), there is often some downgrade functionality such as loss of USB, BLE radio, and so on. Regardless, these very expensive 8x8:8 access points will also be an enormous drain on any PoE power budget.

Theoretically, an 8x8:8 access point could modulate data on all eight radio chains to a single client, which would result in some substantially high data rates. The problem is that there will never be any 8x8:8 mobile client devices due to the drain on battery life. Figure 1 depicts Wi-Fi client information in the Network 360 monitoring view of ExtremeCloud™ IQ. We look at the probe request and association requests to determine the Maximum Client Capabilities of your Wi-Fi client population. As you can see in Figure 1, we live in a world where the bulk of Wi-Fi client devices are 2x2:2. In the cloud, we can then compare the client capabilities to both the real-time and historical operations of your client population. Depending on existing conditions, the 2x2:2 clients will often downgrade to 1x1:1 communication.

_{Figure 1}

So, is there any advantage of an 8x8:8 AP over a 4x4:4 AP? Yes, throughput gains due to MU- MIMO functionality. An 8x8:8 AP could modulate two independent streams of data each to four Wi-Fi 6 2x2:2 clients that support downlink MU-MIMO. Also, an 8x8:8 AP could transmit downlink a single and unique modulated stream of data to each of eight Wi-Fi 6 clients, simultaneously. While this sounds good in theory, MU- MIMO requires spatial diversity. Wi-Fi 6 clients support MU-MIMO; however, the majority of modern-day enterprise deployments of Wi-Fi involve a high density of users and devices that is not conducive for MU-MIMO conditions. To function, MU-MIMO requires spatial diversity. In other words, there must be a sizable physical distance between all the clients, as well as the AP, as shown in Figure 2.

_{Figure 2}

A good use case for MU-MIMO is an outdoor point-to-multipoint (PtMP) bridge link. The spatial diversity required for MU-MIMO exists in this type of outdoor deployment. But once again, almost all indoor WLANs are high-density environments because there are so many users and devices. Most users want to connect to an enterprise WLAN with as many as three or four Wi-Fi devices. Furthermore, high-density environments consist of multiple areas where roaming is also a top priority. MU-MIMO rarely works well with mobile clients, and the required spatial diversity simply does not exist within the bulk of indoor enterprise Wi-Fi high-density deployments.

The significant gains realized by Wi-Fi 6 are a result of OFDMA, a multi-user technology that holds the most promise for efficiency improvements. You might hear some marketing hype that 8x8:8 APs will support more clients than a 4x4:4 AP. That is a complete falsehood; regardless of the number of radio chains and regardless of stream count, all Wi-Fi 6 APs will support the same number of OFDMA clients during a transmission opportunity (TXOP).

Although the theoretical MU-MIMO gains sound enticing, the reality is that an 8x8:8 access point offers no real advantage over a less expensive 4x4:4 Wi-Fi 6 access point.

Portions of this blog were excerpted from the free E-Book, "Wi-Fi 6 & 6E for Dummies."