In today’s market, 2×2 APs are widely available, and several suppliers promote them as a less expensive, high-density deployment-capable alternative to the 4×4. On paper, 2×2 APs can theoretically offer a 2×2 client the same throughput numbers as a 4×4 one, which makes them more desirable.
The actual query is, in practice, do they realistically offer a 2×2 client the same throughput numbers? I’ve always known by heart that, a 4×4 AP has about 3dB of beam-forming gain on the downlink because of the two extra chains, and 3dB of gain on the uplink because of the MRC gain of the extra chains. That sounds wonderful, until you have to explain to someone why they should choose the more expensive 4×4 AP over the less expensive 2×2 AP when the manufacturer has assured them that the 2×2 AP can manage high density installations and can offer the same throughput numbers to a 2×2 client.
I start seeking for scientific evidence to back up my argument that in real life scenarios, a 4×4 AP is superior to a 2×2. During my research, I stumbled across Wes Purvis’ outstanding talk from the 2018 WLPC in Phoenix. Wes went on to demonstrate that a 4×4 had a gain over a 3×3 AP of roughly 2.4dB on the downlink and 1dB on the uplink. In multi-client scenarios, this resulted in a 15% increase in data rate and a 10% increase in throughput.
I was content with what I had discovered up until this point, but not quite. After all, this demonstrates that a 4×4 is superior to a 3×3, but not superior to a 2×2. What if the odd number of antennas is to blame for the 3×3’s subpar performance? The question might seem absurd or strange to us wireless engineers, but it is an illustration of the kind of inquiry non-technical management might ask you in an effort to comprehend why you selected the more expensive 4×4 AP.
As a result, I carried out more research and discovered this fantastic Matlab work that described how to demonstrate the advantages of beam-forming from a 4×4 AP to a 2×2 client over using techniques such spatial expansion.
I hypothesized that by just changing the number of transmitting antennas from 4 to 2, I could use the same illustration to demonstrate how beam-forming from a 4×4 AP to a 2×2 client is superior to beam-forming from a 2×2 AP to a 2×2 client. I executed the code twice under identical setup conditions, first for the case of 4×4=>2×2 and a second time for the case of 2×2=>2×2.
The setup evaluated a 2 spatial streams transmission using MCS4 on a 20MHz BW and TGac channel model with a Model-B delay profile from the AP to a client at a distance of 100 meters.
One can clearly see from the above results that the 4×4 AP had a lower EVM values than the 2×2 AP. This would lead to improved SNR values in real-world scenarios, which in turn would enable the 4×4 AP to shift gears and switch to a higher MCS rate.