Number of antennas on AP – bellwethers of WLAN technology generation
Early 802.11b APs used to have 1 antenna on them, which later became 2 in the 802.11g/a era, which now have become 3 or 6 in the current 802.11n era. So why do number of antennas keep changing as WLAN technology advances to every next generation.
Early 802.11b APs – single band antenna
The 802.11b operates in 2.4 GHz band and needed antenna tuned to 2.4 GHz to operate. One antenna did the job.
802.11g/a APs – two single band antennas or two dual band antennas for diversity
The 802.11g operates in 2.4 GHz band, while 802.11a operates in 5 GHz band. Some 802.11g/a APs operated with one antenna tuned to 2.4 GHz band and another tuned to 5 GHz band. So they needed two antennas.
In another configuration, each antenna was dual band, i.e., tuned simultaneously to 2.4 GHz and 5 GHz, but still two of them were used to achieve receiver diversity to fight multipath. Multipath results in temporal and spatial variation of the received signal strength. By having two antennas, if signal strenght at any given time is low at one antenna, it is likely that it is high at the other antenna at the same time. So by dynamically switching to antenna with stronger signal strength, it is possible to improve the received signal strength. This technique is also called as “switched diversity”. There is still only one radio receiver chain which is fed by signal from stronger antenna.
802.11n APs – six single band antennas MIMO or three dual band antennas MIMO
Switched diversity is not fully optimal as it throws away signal from weaker antenna. The idea of MIMO is to not throw away any signal, but combine all of them in a certain way (in technical jargon “combine coherently”) to achieve maximum signal strength advantage. For this one needs to introduce separate radio receiver chain for each antenna and then combine the signal in each radio receiver chain using advanced signal processing techniques.
So 802.11n AP with three dual band antennas has three radio receiver chains, one behind each antenna. Similarly, 802.11n AP with six single band antenna has one radio receiver chain behind each pair of 2.4GHz/5GHz antenna. Well, one could also do MIMO with two dual band antennas by having separate radio chain behind each antenna, though most of the AP vendors have chosen to not use such configuration.
802.11n APs – eight single band antennas MIMO or four dual band antennas MIMO
As MIMO becomes mainstream, it will not be long before we start seeing APs with eight single band or four dual band antennas. Of course there will be additional cost associated with extra radio chain and signal processing associated with combing signal from extra radio chain.
