Introduction to IEEE 802.11nIEEE 802.11n uses several techniques to increase throughput 1. MIMO PHY 2. Wider Channels 3. Aggregation and block ack at MAC layer MIMO stands for multiple input/multiple output and implies multiple simultaneous spatial data streams propagated over multiple antennas. IEEE 802.11n mandates 1 and 2 spatial streams while 3 and 4 spatial streams are optional. Cyclic shift diversity is a multiplexing technique to transmit a signal from multiple antennas and is equivalent to delay diversity (where a linear delay is applied on the chains; a chain is an analog front end + antenna) in non-OFDM systems. The signal is cyclically and progressively shifted with the number of antennas. When multiple streams are used, each stream is transmitted over all the antennas. Beamforming requires channel knowledge between transmitter and receiver while STBC (Space- Time Block Coding) doesn't require channel knowledge. Frame aggregation: A-MSDU aggregates several subframes into a single MPDU. Each subframe is typically an ethernet frame. A-MSDU can have a max size of 8KB. MPDU is a 802.11 frame with 802.11 header. A-MPDU aggregates several independent MPDUs. It is really bursting of MPDUs one after the other. Each MPDU is independent and can be retransmitted independently. A-MPDU can have a max size of 64KB. Block Ack follows 802.11e spec. Can be immediate or delayed acknowledgement. 802.11n modifies block ack by compressing the block ack bitmap from 128 bytes to 8 bytes and allows upto 63 unacked frame outstanding at any time. Also, immediate block ack support is mandatory and delayed block ack support is optional. |
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