The way that information is sent at the physical layer is different, in fact items such as reflection and interference can be turned into an advantage instead of an issue.
2 channels are combined to make 40MHz channels rather than the 20 MHz channels of 802.11abg. 802.11n is also able to do away with the side channels which are used for protection, freeing up an additional 11 Mbps for a maximum of 119Mbps per 40MHz channel.
802.11n introduces MAC efficiency. 802.11 needs to acknowledge every frame where as 802.11n can send a number of frames and only have one acknowledgement.
MIMO - This uses multiple input and output antennas so that several frames are sent by several antennas over several paths, the frames are then recombined by the receiving antennas to optimise throughput and multipath resistance. This is known as spatial multiplexing. MIMO works when there are multiple radios on each side. However it can also provide benefits to non-802.11n single radio clients:
Transmit Beam Forming - several beams are sent from the 802.11n device to the non 802.11n client. the client can use the best one.
Maximum Radio Combining - This is similar but in the reverse direction. Multiple signals are sent from the client to the 802.11n AP in phase so that it adds strength to the signal. MRC doesn't resolve multipath in anyway, and in fact is affected by it as normal so it is nowhere near as good as MIMO.
MIMO has 3 critical advantages:
- It has better sensitivity for the stationary client when receiving, using beam forming
- It provides better sensitivity for the AP receiving by using MRC
- Both of the above translates into higher datarate
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