802.11n PHY and MAC Layer Improvements

Physical Layer Improvements

1. Higher Modulation Rates – from 54 Mbps to 58.5 Mbps
2. Improved Forward Error Correction – from 58.5 Mbps to 65 Mbps
3. Short Guard Interval – from 65 Mbps to 72.2 Mbps
4. Channel Bonding, 40 MHz Channels – from 72.2 Mbps to 150 Mbps
5. MIMO – from 150Mbps to 600Mbps

MAC Layer Improvements

1. Aggregation of Packets
2. Block Acknowledge
3. Lower Overhead: Reduced Interframe Space
4. Backwards Compatibility
5. Power Save Modes

• Spatial Multiplexing (SM) power save – static and dynamic

• Power Save Multi-Poll (PSMP)

802.11 Beamforming

Beamforming technology allows the antennas and controlling circuitry to focus the transmitted RF signal only where it is needed, unlike the omnidirectional antennas people are used to. Beamforming is PHY layer procedure.

Two types of beam forming:

1.    In implicit beamforming, the upstream wireless channel is measured by the beamformer, and the measurement used to derive the parameters for subsequent downstream beam formed transmission.

2.    Explicit beamforming, requires the downstream channel to be  measured at the receiver, or beamformee, and relayed back to the transmitter, or beamformer. The beamformer uses the measured channel information to derive the transmit beamforming parameters.

Implicit beamforming has the advantage that the beamformee does not need to measure and send the channel state information to the beam former. However, 11n standard implicit beamforming requires a calibration exchange between the beamformer and beamformee, which can complicate the transceiver design.

Since 802.11n allowed multiple forms of beamforming and both ends of a wireless link need to use the same beamforming method to produce real benefit.

Sounding frames are known pattern of RF symbols sent from each antenna.

Following is the sequence of events that take place as part of Transmit Beamforming to get reports from clients.

1.    AP sends an announcement that its going to send out a sounding frame containing data to be evaluated by client.

2.    AP sends the data in the Null data packet.

3.    All the clients supporting TxBF receive it as start responding to the report request by sending back a Compressed V-Matrix to the AP.

4.    Based on the feedback received by the AP from clients, it re-calibrates the phase shift for each of the transmitted signal from each antenna so that the signal strength reaches its maximum at client

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802.11n Vs 802.11ac

802.11n Versus 802.11ac
IEEE Standard	802.11n	802.11ac
Ratification	2009	Jan-2014 (Wave-1)
Frequency Band	2.4GHz and 5 GHz	5 GHz
PHY Rates	65 Mbps – 600 Mbps	250 Mbps – 6.5 Gbps
Technology	OFDM	OFDM
Modulation	Up to 64 QAM	Up to 256 QAM
Channel Widths	20, 40 MHz	20, 40, 80 MHz (mandatory) 160 MHz, 80+80 MHz (optional)
Spatial Streams	1 to 4	1 to 8 total Up to 4 per client
MU-MIMO	No	Yes
Green Field	Yes	No
Implicit Beamforming	Yes	No
Explicit Beam Forming	Yes	Yes
Single Stream (1X1) Max Client Data rate per radio	150 Mbps	450 Mbps
Three Stream (3X3) Max Client Data rate per radio	450 Mbps	1.3 Gbps (80 MHz, Wave 1)
Max A-MPDU size	65,535 octets	1,048,575 octets