802.11n throttles up WLAN throughput

802.11n throttles up WLAN throughput


The drive for higher throughput in wireless LANs has pushed the IEEE to develop 802.11n, a new version of the 802.11 standard that promises throughput in excess of 100M bit/sec in 20-MHz to 40-MHz of bandwidth. This standard would permit very-high-speed interconnection of wireless devices over distances of 300 feet or more.

Current IEEE 802.11a/g WLANs operate with raw data rates up to 54M bit/sec, but the actual throughputs are generally no more than 20M bit/sec. Although fine for many applications, interconnection devices with higher data rates, particularly HDTV and streaming video, can require throughputs of 100M bit/sec and higher. Achieving this throughput within the unlicensed channel of 20 MHz requires improvements in the physical layer (that is, the raw data rate) and the efficiency of the media access control (MAC) layer, such that the throughput is closer to the raw data rate.

One technique to increase MAC efficiency involves aggregating packets so the data is sent in longer units, decreasing the overhead of the packet preambles.

The most practical method to increase the raw data rate is a technique called multiple input/multiple output (MIMO). MIMO uses multiple transmit and receive antennas to create multiple spatial channels between a transmitter and receiver. In the multi-path environment in which WLANs operate, by using, for example, four transmit and four receive antennas, data rate can be quadrupled within the same bandwidth, using the same transmit power.

At an IEEE standard meeting last September, four complete and 28 partial proposals for 802.11n were presented. The proposals will be evaluated and converged in a final proposal during subsequent meetings that will occur every two months. The two complete proposals with most of the support are the WWise and TGn Sync plans. These two proposals include a common two transmit, two receive antenna mode in 20-MHz channels.

These two proposals differ in many aspects, though, including their preambles and degree of packet aggregation. For example, TGn Sync uses longer data unit lengths (about eight to 32 times longer) with longer preambles (up to two times longer for more robustness) than WWise.

In addition, the other complete and partial proposals contain a variety of techniques, such as transmit beam forming, each with advantages and disadvantages. The 802.11n proposals are similar enough that reaching a compromise proposal appears feasible. A number of optional modes might need to be included, though.

Some proposed modes include four transmit/receive antennas in 2-MHz to 20-MHz channels for data rates in excess of 500M bit/sec, showing the power of MIMO to provide extremely high data rates.

Even though the standard is not expected to be completed until 2007, so-called pre-n equipment is being marketed, and more companies are expected to introduce pre-n gear before the standard is approved. Although the final standard will most likely differ from these products, they use the term pre-n because they use MIMO (which is included in the two main proposals and will likely be in the final standard) and achieve data rates much higher than 802.11a/g.

This certainty also permits an 802.11n analog radio frequency front end to be designed well before the standard is approved, with the much-shorter-development-cycle digital circuitry/software finalized with the standard. 802.11n could cost more than twice that of 802.11a/g, because the multiple RF chains required with MIMO have not seen the dramatic cost reductions of digital circuitry.

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