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Ever since the days when 802.11a/g grew a second antenna, we’ve had “transmit/receive diversity,” which sends the same data stream out over multiple antennas and simply lets the access point select whichever antenna is receiving the best signal. Applied to 802.11n, transmit diversity used multiple antennas to help increase range and better deal with difficult locations. This is why 11n does a generally better job than 11a/g at eliminating dead spots.
However, 802.11n equipment got another jump in intelligence with the addition of maximal ratio combining (MRC). This technology combines multiple antenna signals in such a way that strong signals are multiplied while weak signals are attenuated. The signals you want get boosted, while those you don’t have their power cut. MRC is built into all 802.11n chips.
Now, as you might expect, the receiving end can play an important role in optimizing chip-based beamforming. With 802.11a/g, access points could listen to the client and use rudimentary MRC analysis to boost power along the best-suited beam, providing a gain of roughly 1 to 2 dB. The catch here is that the access point was doing all the work. There was no active feedback coming from the 802.11a/g clients.
With “implicit beamforming,” wherein an 802.11n AP is communicating with 802.11n clients, you can have some feedback. Rather than having the access point perform all of the signal analysis, it can query the client and see if it agrees that this or that particular beam orientation is optimal. Having this limited two-way communication yields a maximum of 3 dB additional gain, but the bad news is that there are currently no products on today’s market supporting implicit beamforming.
With “explicit beamforming,” feedback between the AP and client happens much more frequently. This way, if a client moves or an antenna gets adjusted or anything happens to alter the dynamics of the signal strengths, the system is able to adapt almost instantaneously to a new, optimized configuration. Again, having the client involved in this way can yield up to a 3 dB benefit with two radios, but there are no products available today offering this capability. Hopefully this will change.