A Peek Ahead at 802.11n: MIMO-OFDM

A Peek Ahead at 802 11n MIMO OFDM

802.11 task group N (TGn) has an interesting goal. Most IEEE task groups focus on increasing the peak throughput, making data fly as fast as possible during the time it is being transmitted. TGn's goal is to achieve 100 Mbps net throughput, after subtracting all the overhead for protocol management features like preambles, interframe spacing, and acknowledgments. Although the goal is 100 Mbps net throughput, the final proposal seems certain to blow past that number, and offer many times that throughput in maximum configurations. There are two roads to 100 Mbps: improve the efficiency of the MAC, increase the peak data rate well beyond 100 Mbpsor both.

Six complete proposals were made to the group creating the eventual 802.11n, but support has coalesced around two main proposals, from groups named TGnSync and WWiSE (short for "World-Wide Spectrum Efficiency"). Both camps have chip-makers. Atheros, Agere, Marvell, and Intel are part of TGnSync; Airgo, Broadcom, Conexant, and Texas Instruments are the core of WWiSE. However, quite a few manufacturers of electronic devices that might use 802.11 (Cisco, Nokia, Nortel, Philips, Samsung, Sanyo, Sony, and Toshiba) have also become part of the effort, and they are disproportionately represented in TGnSync.

At a very high level, both proposals are similar, though they differ in the emphasis on increasing peak data rates versus improving efficiency. Each of them makes use of multiple-input/multiple-output (MIMO) technology in several configurations and provides for backwards compatibility with installed systems in the same frequency band. Both support operation in the current 20 MHz channels, with provisions to use double-width 40 MHz channels for extra throughput.

As the standards war is fought across the globe at IEEE meetings, a "pre-N" access point has already hit the streets, based on Airgo's chipset. Purchasing it well before the standards process is underway is a roll of the dice. When most "pre-G" products were brought to market, the task group had begun to work in earnest on a single proposal. TGn is currently in the "dueling proposal" stage right now, and there is no guarantee that an early device will be firmware upgradeable to the final 802.11n standard. 802.11n is likely to be the last chance to standardize a PHY this decade. Developing a standard is as much political engineering as technical engineering. IEEE rules require that a proposal get a 75% supermajority vote before becoming the basis for a standard. As this book went to press, TGnSync was garnering a clear majority of support, but was still falling short of the necessary 75%. I expect that features from competing proposals will be incorporated into the working document to bring the vote count to the necessary level. As a result, this chapter describes both of the main competing proposals. Although TGnSync will probably be the basis for the 802.11n specification, some horse trading will likely result in a few WWiSE features being incorporated.

This chapter describes both the WWiSE and TGnSync proposals. The final standard will have some resemblance to both of them, and will likely pick and choose features from each. Fortunately, many basic concepts are shared between the two. As you read this, keep in mind that the proposals themselves may have changed quite a bit since the drafts upon which this chapter was based were written.