In the 802.11n system, based on the WLAN OFDM system, two new formats are defined for the PLCP (Physical Layer Convergence Protocol): the Mixed Mode and the Green Field. These two formats are called HT (High Throughput) formats. In addition to the HT formats, there is a legacy duplicate format that duplicates the 20 MHz legacy packet in two 20 MHz halves of a 40 MHz channel.
So, the 802.11n physical layer operates in one of 3 modes in the time domain: Legacy mode, Mixed Mode and Green Field Mode.
In legacy mode and HT mode, transmission is over a 20 MHz channel, and the channel is divided into 64 sub-carriers. Four pilot signals are inserted in sub-carriers -21, -7, 7 and 21. In the legacy mode, signal is transmitted on sub-carriers -26 to -1 and 1 to 26, with 0 being the center (DC) carrier. In the HT modes, signal is transmitted on sub-carriers -28 to -1 and 1 to 28.
In the 40 MHz HT transmission, two adjacent 20 MHz channels are used. The channel is divided into 128 sub-carriers. 6 pilot signals are inserted in sub-carriers -53, -25, -11, 11, 25, 53. Signal is transmitted on sub-carriers -58 to -2 and 2 to 58.
In the case of the legacy duplicate mode over 40 MHz, the same data are transmitted over two adjacent 20 MHz channels. In this case the 40 MHz channel is divided into 128 sub-carriers and the data are transmitted on carriers -58 to -6 and 6 to 58.
In the legacy mode, frames are transmitted in the legacy 802.11a/g OFDM format.
Figure 1. 80211n WLAN frame, Legacy Mode.
In the Mixed Mode, packets are transmitted
with a preamble compatible with the legacy 802.11a/g. The legacy Short
Training Sequence, the legacy Long Training sequence, and the legacy signal
description are transmitted so they can be decoded by legacy 802.11a/g
devices. The rest of the packet has a new MIMO training
sequence format. Figure 2 shows the Mixed Mode format. Click the various
parts of the figure for more details on each field.
Figure 2. WLAN frame modifications to allow for MIMO operation, Mixed Mode
In the Green Field mode, high throughput packets are transmitted without a legacy-compatible part. Figure 3 shows the Green Field format. Click the various parts of the figure for more details on each field.
Figure 3. 802.11n WLAN frame, Green Field.