802.11n HT OFDM Overview

The 802.11n-2009 wireless LAN Local Area Network: A communications network that serves users within a local geographical area, typically over distances of around 100m. Wireless LANs use wireless communicaitons to network devices so there is no need for data cabling. standard provides Higher Throughput (HT high throughput) rates. These higher rates are achieved by a combination of MAC (Medium Access Control: In most wired and wireless networks, a mthod is used to control how and when a device can transmit data over the communications link. This is the network's Medium Access Control scheme. The MAC protocol operates within the Data Link layer (Layer 2) of the ISO OSI 7 layer Model. The IEEE 802.11 standard, for example, specifies the MAC protocol for sharing of the wireless medium, packet formats, addressing, eror detection and recovery following errors. and PHY Physical Layer layer enhancements. The enhancements include:

Reducing the per-packet overhead in the MAC layer.
Allowing multiple MAC packets to be combined into a single PHY-layer burst.
Allowing (optionally) a more efficient LDPC low-density parity check encoder.
Allowing (optionally) a shorter guard interval (cyclic prefix) on the data symbols.
Increasing the number of subcarriers used in the default 20 MHz Megahertz: A unit of frequency equal to one million hertz or cycles per second. physical channel.
Providing an (optional) 40 MHz mode.
Using MIMO Multiple Input, Multiple Output: A physical layer (PHY) configuration in which both transmitter and receiver use multiple antennas. techniques to broadcast multiple data streams over a single frequency channel.

By combining these techniques, the goal is to increase the usable data transfer rate by a factor of 10.

Signal Characteristics

The 802.11n standard uses OFDM Orthogonal Frequency Division Multiplexing: OFDM employs multiple overlapping radio frequency carriers, each operating at a carefully chosen frequency that is Orthogonal to the others, to produce a transmission scheme that supports higher bit rates due to parallel channel operation. OFDM is an alternative tranmission scheme to DSSS and FHSS. modulation to transmit all data. It defines three operating modes:

Non-HT or Legacy Mode. In this mode the signal is transmitted in legacy 802.11a/g format.

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HT-mixed. In this mode, the burst has an initial legacy-compatible preamble, followed by an HT preamble and then HT data.

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HT-greenfield. This mode does not use a legacy-compatible preamble, it has only an HT preamble followed by HT data. The resulting burst preamble is slightly shorter than a HT-mixed preamble, so overall data transmission rates in this mode is slightly more efficient.

In HT mode, the burst preamble includes an HT-SIG High-Throughput SIGNAL field field that is somewhat similar to the SIGNAL symbol in 802.11a/g. HT-SIG is transmitted using the most robust data rate (BPSK Binary phase shift keying - A type of phase modulation using 2 distinct carrier phases to signal ones and zeros.), and it encodes information about how the rest of the data will be transmitted. This information includes the total data length, the data modulation format, the number of data streams, the signal bandwidth, and whether a shortened guard interval is used.

All operating modes use OFDM to transmit the data. For 20 MHz bandwidth signals, Legacy Mode uses 48 data subcarriers and 4 pilot subcarriers, while HT modes use 52 data subcarriers and 4 pilot subcarriers. This gives 20 MHz HT mode slightly more throughput than Legacy Mode. For 40 MHz bandwidth signals, there are 108 data subcarriers and 6 pilot subcarriers. This gives 40 MHz HT mode more than twice the throughput of 20 MHz HT mode.

MIMO

One of the biggest changes from 802.11a/g that 802.11n brings is Multiple Input Multiple Output (MIMO) transmission. MIMO requires a transmitter to consist of multiple antennas, with separate signals on each antenna. Similarly, MIMO requires a receiver to have multiple antennas. Finally MIMO requires that the transmission channels between the transmitters and receivers be "different". When the transmission channels are "different enough," signal processing can be used by the receiver to recover the multiple original transmitted signals.

OFDM signals are a good choice to use for MIMO transmission, because the transmission channels for an individual subcarrier can generally be characterized by a single complex matrix. Additionally, WLAN WLAN - Wireless Local Area Network: A system that includes the distribution system (DS), access points (APs), and portal entities. It is also the logical location of distribution and integration service functions of an extended service set (ESS). A WLAN system contains one or more APs and zero or more portals in addition to the DS. is a good choice for MIMO transmission because it is typically used in environments where there are many reflections, which help ensure that the transmission channels are "different enough".

802.11n demodulation uses the known HT preamble to characterize the transmission channels. The output of this characterization is the MIMO channel matrix for each subcarrier. This matrix is used to convert the multiple received signals (each of which may have a mixture of the original transmitted data streams) into separated data streams. These separated data streams can then each be demodulated similarly to a single-channel 802.11a signal.

In order to demodulate a MIMO signal, the MIMO channel matrix must be inverted. This inversion can be done when the transmission channels are "different enough". When the transmission channels are too similar, the matrix becomes ill-conditioned and inversion becomes difficult. The Condition Number of a matrix is a standard measure of how well conditioned the matrix is. The larger the Condition Number, the more ill-conditioned the matrix is. As a rule of thumb, if the Condition Number is larger than the SNR Signal-to-Noise Ratio of the signal, the matrix inversion will probably not be accurate enough and MIMO demodulation will fail.