Understanding the 802.11n Physical Layer

11n Introduction

This section of the WLAN tutorial covers:

• What is WLAN?
• WLAN standards - 11a, 11b, 11g, 11n, 11ac
• 11a WLAN Physical layer
• 11b WLAN Physical layer
• 11n WLAN Physical layer
• WLAN 802.11-ac
• WLAN 802.11-ad
• WLAN MAC layer
• Difference between 11a, 11b, 11g, 11n
• WLAN router providers
• WLAN providers

This document describes the WLAN 802.11n wireless networking standard, introduced by IEEE. It covers various frames transmitted by an 11n compliant device and the 11n physical layer.

It is the successor to previous standards, i.e., 802.11a and 802.11g, and supports the features supported by these legacy standards. The main goal for 11n is to increase the data rate, accomplished by increasing the bandwidth to 40 MHz (from the legacy systems’ 20 MHz) and supporting multiple streams. The standard supports a maximum of 4 streams.

11n Frame Structure

In an 802.11n system, the WLAN OFDM system supports three physical layer modes: Legacy mode, Mixed Mode, and Green Field Mode.

In Legacy mode, the bandwidth supported is 20MHz, and a 64-point IFFT is used. It supports legacy 11a and 11g systems. L-SIG is structured as per the SIGNAL field defined in the IEEE 802.11a standard.

HT mode is supported in both Green Field and Mixed modes. 40MHz bandwidth and 128-point IFFT are employed here. The HT-SIG field is described in the IEEE 802.11n-2009 standard.

Legacy Mode

In the legacy mode, frames are transmitted in the legacy IEEE 802.11a or 11g OFDM format.

Legacy Frame = L-STF (2 sym) + L-LTF (2 sym) + L-SIG (1 sym, 4 microsec) + Data symbols

Mixed Mode

In Mixed Mode, a frame is composed of both legacy preambles/header as well as 11n compatible preambles/header (HT), as shown below. This is done so that the frame can be decoded by legacy 11a/g devices.

Mixed Mode Frame = L-STF (2 sym) + L-LTF (2 sym) + L-SIG (1 sym, 4 microsec) + HT-SIG (2 sym) + HT-STF (1 sym) + HT-LTFs (4 microsec per LTF) + Data symbols

Green Field Mode

In Green Field mode, the legacy preamble, HT preamble, and Header are incorporated as shown below.

Green field Frame = L-STF (2 sym) + HT-LTF1 (2 sym) + HT-SIG (1 sym) + HT LTFs (4 microsec per LTF) + Data symbols

802.11n Physical Layer

802.11n Physical Layer Transmitter

The transmitter consists of: Scrambler, Encoder parser, FEC encoder, Stream parser, Interleaver, Constellation mapper, Space time block encoder, Spatial mapper, IDFT, Cyclic shift insertion, Guard insertion, Windowing and packet formation.

Scrambler

The scrambler scrambles the data to prevent long sequences of zeros or ones. Visit standard section 20.3.10.2 for more information.

Encoder Parser

The encoder parser de-multiplexes the scrambled bits among NES (number of FEC encoders) FEC encoders, in a round-robin manner.

FEC Encoder

The FEC encoder encodes the data to enable error correction. An FEC encoder may include a binary convolution encoder followed by a puncturing device or an LDPC encoder.

Stream Parser

The stream parser divides the outputs of the encoders into blocks that are sent to different interleaver and mapping devices. Bits to the input of these interleavers are called spatial streams.

Interleaver

If BCC encoding is to be used, the interleaver interleaves the bits of each spatial stream (changes the order of bits) to prevent long sequences of adjacent noisy bits from entering the BCC decoder.

Constellation Mapper

The constellation mapper maps the sequence of bits in each spatial stream to constellation points (complex numbers).

Space-Time Block Encoder

Constellation points from Nss spatial streams are spread into Nsts (space time streams) using a space time block code, whereby Nss is less than Nsts. Read section 20.3.10.8.1 for Space-Time Block Coding (STBC).

Spatial Mapper

The spatial mapper maps space time streams to transmit chains. This may include one of the following:

• Direct mapping: Constellation points from each space time stream are mapped directly onto the transmit chains (one-to-one mapping).
• Spatial expansion: Constellation points multiplied by a matrix will expand the points to produce the input for all the transmit chains.
• Beamforming: Similar to spatial expansion, where a vector of constellation points from all the space time streams is multiplied by a matrix of steering vectors.

IDFT

Inverse Discrete Fourier Transform (IDFT): converts a block of constellation points to a time-domain block.

Cyclic Shift (CSD) Insertion

CSD insertion of the cyclic shifts prevents unintentional beamforming. There are three cyclic shift types:

• A cyclic shift specified per transmitter chain with the values defined in Table n61 (Cyclic shift for the non-HT portion of the packet). A possible implementation is shown in Figure n63 (Transmitter block diagram for the non-HT portion and the HT signal field of the HT mixed-format packet).
• A cyclic shift specified per space time stream with the values defined in Table n62 (Cyclic shift values of HT portion of the packet). A possible implementation is shown in Figure n64 (Transmitter block diagram for the green field format packet and HT portion of the mixed format packet except HT signal field).
• A cyclic shift Mcsd (K) may be applied as a part of the spatial mapper; see 20.3.10.10.1 (Spatial mapping).

When beamforming is not used, it is sometimes possible to implement the cyclic shifts in the time domain.

Guard Interval Insertion

The guard insertion module extracts the last few samples of the OFDM symbol and appends them at the beginning.

Windowing

Windowing smooths the edges of each symbol to increase spectral decay.

Packet Formation

The packet is formed as per the frame structure defined in the section above.

REFERENCES

IEEE 802.11n-2009 Standard