Tagged: Spatial Multiplexing


IEEE 802.11n was proposed in 2009 to help scale throughput of WLANs using a few different techniques known as high throughput (HT) in either the 2.4 or the 5 GHz band. 802.11n was designed to be backwards compatible with OFDM used in the 802.11g and 802.11a standards. The primary advantage of 802.11n was it’s ability to leverage multiple radios. Instead of using a single Tx/Rx radio pair (or radio chain), 802.11n devices could use multiple antennas, transmitters or receivers; a system known as multiple-input, multiple-output (MIMO). The transmitters and receivers are described in the format TxR, and 802.11n requires at least two radio chains (2×2) and supports up to a maximum of four (4×4).

In addition to the MIMO functionality, 802.11n introduced a few features to improve throughput including:

  • Channel Aggregation
  • Spatial Multiplexing (SM)
  • MAC Layer Efficiency

802.11n also introduced some features to improve the reliability of RF signals:

  • Transmit Beam Forming (TxBF)
  • Maximal-Ratio Combining (MRC)

Channel Aggregation

The 802.11n amendment first increased the 20 MHz channel throughput by increasing the number of data sub-carriers in OFDM from 48 to 52. 802.11n then goes on to allow either the use of either a single 20 MHz or a single 40 MHz channel. The aggregated channels always bond two adjacent 20 MHz channels. By bonding the channels it is able to free up the quiet space between the two original  channels for additional bandwidth. The quiet space on each end is left along to separate the 40 MHz channels. This increases the number of data sub-carriers from 52 to 108.

When the channels are aggregated it also lowers the total number of available channels. Channel aggregation shrinks the 5 GHz band from 23 non-overlapping 20 MHz channels to 11 non-overlapping 40 MHz channels. Since the 2.4 GHz band only has 3 non-overlapping channels it is not recommended nor usually attempted on that band.

Spatial Multiplexing

Channel aggregation allows for increased throughput by increasing the channel width that can be used by a single radio chain. With the advent of MIMO, the 802.11n device could have multiple radio chains waiting to be used. To further increase throughput we can multiplex, or distribute, the data across two or more radio chains, while still operating on the same channel. This is known as spatial multiplexing because the radio chains are separated by spatial diversity (they are predictably spaced out).

The spatial diversity will ultimately cause slight changes in each signal as they make their way across the free space to the receiver. If the radio signals don’t all start at the same location, then they would naturally take different paths. the 802.11n devices can also distribute the data across the multiple radio chains in a known fashion. These separate data streams can be processed as spatial streams  and can be demuxed on the receiving end. The number of spatial streams  a device can support is designated with a colon at the end of the MIMO designation. A 3×3:2 MIMO device has 3 transmitters, 3 receivers, and can support two unique spatial streams. Since not all devices in an environment may support the same amount of spatial streams, capabilities are advertised and the lowest common denominator is negotiated prior to transmitting data.

MAC Layer Efficiency

Additional improvements with 802.11n include block acknowledgement. In traditional 802.11 networks each frame of data transmitted must be acknowledged by the receiver. If no acknowledgement is received it is assumed that the receiver did not get the frame and it must be resent. Acknowledging each frames wastes communication time. With 802.11n  all the data frames can be transmitted in one burst, and only one acknowledgement is expected from the receiver. This is more efficient and helps increase throughput.

With 802.11 as OFDM symbols are transmitted they can take different paths to the receiver. If the two symbols arive two close together they can actually cause interference with each other, this is known as intersymbol interference (ISI). The 802.11 standard requires a guard interval of 800 nanoseconds between transmissions to alleviate this problem. With 802.11n devices you can configure this interval to 400 nanoseconds. Doing so will increase throughput since less time is wasted in the guard interval but it does put you at a greater risk of data corruption.

Transmit Beamforming

As data is transmitted across the multiple radio chains of a MIMO device, they will ultimately take separate paths to the receiver. To help ensure that the data arrives at the receive in the same relative time frame transmit beamforming (TxBF) is used. Transmit beamforming adjusts the phase of each signal as it leaves so that as it travels across the free space they will arrive at relatively the same time. The receiver sends back TxBF data as feedback so that the transmitter can constantly keep track of the required adjustments and send focused transmissions to each receiver dynamically.

Maximal-Ratio Combining

If you’re familiar with digital photography, you may be familiar with the concept of HDR, or High Dynamic Range. With HDR, multiple images are combined to provide a single image with the best contrast. Maximal-Ratio Combining (MRC) does something very similar with RF signals. It takes multiple received copies of a signal and combines them to provide one signal with improved Signal to Noise Ration and receiver sensitivity.