802.11n-2009 (HT-Clasue 20) amendment was ratified on September 2009, essentially a list of enhancement to improve the performance & throughput of 802.11a/g standards.

PHY Layer enhancements
1. Spatial Multiplexing (SM) – Tx multiple radio signal stream at the same time.
2. Transmit Beamforming (TxBF) – allow transmitter using multiple antenna to focus the Tx in the best direction of a receiver (Rx)
3. Space Time Block Coding (STBC) – Improve the reliability of data transfer by transmitting different copy of the data stream from different antenna.
4. Antenna Selection (ASEL) – increase signal diversity by dynamically
5. Low Density Parity Check (LDPC) –
6. Channel Bonding –
7. Maximal Radio Combining (MRC) –
8. Use of MIMO (Multiple Input & Multiple Ouput) technology.

MAC layer enhancements
1. Frame Aggregation
2. Block Acknowledgement
3. Power Save Multi-Poll (PSMP)
4. Reverse Direction (RD) Protocol
5. Reduces Inter Frame Spacing (RIFS)

MIMO
– Require the use of multiple radios & antennas called “radio chains”.
– Transmitting multiple streams of data with spatial multiplexing for high throughput
– make use of multipath
– TxBF can be used in MIMO system to steer beams & provide greater range & throughput.

Radio Chains
– conventional radio transmit & receive using RF signals by using single-input single-output (SISO)
– MIMO consist of multiple radio chains
– MIMO system is characterized by the number of transmitters & receivers.
eg 2×3 MIMO would consist of 3 radio chains with 2 transmitters & 3 receivers.
3×3 MIMO would consist of 3 radio chains with 3 transmitters & 3 receivers.

HT Channels
1. 20MHz non-HT & HT Channels
– 802.11a/g, each 20MHz OFDM channel contain 64 subcarriers
– each subcarrier 312.5KHz wide.
– First 6 & last 5 subcarriers are null because they act as guard band for the channel
– center subcarrier is also null & called the direct conversational(DC) subcarrier.
– This leaves 52 subcarriers (64-12).
– Out of 52, 48 transmit data while 4 are used as pilot tones for dynamic calibration between Tx & Rx.
– OFDM use convolution coding & forward error correction.

– HT clause 20 also use OFDM technology & have the capability of using either 20MHz or 40MHz channel.
– 20MHz channels  used by HT radio use extra 4 carriers to carry data with small guard interval.
– so HT channel can carry a little more data than non-HT OFDM channel.
– HT 20MHz OFDM channel has 56 subcarriers.
– 52 of them carry data while 4 subcarriers used as pilot tones.

2. 40 MHz Channels

– 40MHz HT channel use 114 subcarriers
– 108 subcarriers transmit data & 6 subcarriers are used as pilot tones
– each 40MHz channels used by HT radios are essentially 2x 20MHz bonded together.
– One 20MHz channel termed as “Primary” & the other 20MHz termed as “Secondary”
– Primary & Secondary 20MHz should be adjacent 20MHz channels.

– Primary field indicate the primary channel
– A postive or negative offset indicates whether the secondary channel is one channel above or one channel below the primary channel.
– standard 20MHz HT channel reserves some freqency at the top and bottom of the channel to avoid interference with adjacent channel.
– when two 20MHz are bonded together, there is no need to reserve this bandwidth at bottom of the higher CH & at the top of the lower CH.
– therefore an HT 40MHz spectral space add two more subcarriers, given total of 114 subcarriers instead of 112.

MCS – Modulation & Coding Scheme.
– non-HT OFDM(802.11a/g) defines data rates 6, 9,12,18,24,36,48,54 Mbps
– HT data rates defined based on different factors, modulation, number of spatial streams, channel size & guard interval.
– 77 modulation coding schemes eixst for both 20MHz & 40 MHz channels.- 8 mandatory MCS for 20MHz HT channels as shown below.

Below table shows the data rates when using 4 spatial streams (max no of SS defines in 802.11n standard)

Below table shows the MCS for a 40MHz channel using one spatial stream.

Below table show the MCS for a 40MHz channel using 4 spatial stream.

Full index of MCS Rates –MCS Index – 802.11n and 802.11ac ( courtesy of @keithRParsons – Wireless LAN Professional )

HT PHY
There are 3 different preamble use in 802.11n
1. non-HT Legacy
– use of short & long training symbols (L-STF, L-LTF) used for synchronization
– OFDM symbol consist of 12 bits
– header contain the signal field, which indicates the time needed to transmit the payload of non-HT PPDU (which is MPDU)
– support of non-HT legacy format is mandatory in 802.11n radios.
– non-HT format effectively is the same format used by legacy 802.11a or 802.11g

2. HT Mixed
–  include non-HT short & long training symbols along with L-SIG that can be decoded by legacy 802.11a/g radios.
-non-802.11n receivers will not be able to read rest of the frame, but length field in  the legacy section allow them to know how long the medium is going to be busy.
– HT mixed format is also considered mandatory & transmission can occur in both 20MHz and 40MHz channel.
– When 40MHz in use, all broadcast traffic must be sent on a legacy 20MHz channel.

3. HT Greenfield
– This preamble is not compatible with legacy 802.11a/g radios.
– it can use both 20MHz or 40 MHz channels.

Below diagram show the 3 different PHY types used in 802.11n

WiFi Certified 802.11n
– All wifi certified 802.11n devices must support WMM (QoS) & WPA2 security mechanism|
– AP should support two spatial streams (mandatory)
– Client devices are only required to Tx & Rx at least one spatial stream.
– A-MSDU & A-MPDU in receive mode is mandatory
– Block Acknowledgement is mandatory.
– Several other optional features added in the standard.

References
1. CWAP Official Study Guide – Chapter 10

Related Posts

1. CWAP – HT Control Field
2. CWAP – HT Capabilities Information Element
3. CWAP – HT Operations Information Element.