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Toward a Gigabit Wi-Fi Nirvana: 802.11ac and 802.11ad

By Matthew Gast, director of product management, Aerohive Networks, special to Network World
February 14, 2011 05:04 PM ET

Network World - This vendor-written tech primer has been edited by Network World to eliminate product promotion, but readers should note it will likely favor the submitter's approach.

Today's existing state-of-the art wireless LAN can achieve 300 Mbps using 802.11n with two spatial streams. Future developments will deliver three- and four-stream speeds of up to 600 Mbps. But the 802.11 working group has set its sights on a more ambitious milestone: 1 Gbps throughput.

After considering several approaches for getting to gigabit speeds, the 802.11 WG settled on two related approaches, and formed two task groups to produce future gigabit standards: 802.11ac and 802.11ad. While both groups share the same goal, the approaches taken are different because the groups have fundamentally different purposes.

ROUNDUP: 802.11n Wi-Fi essentials for enterprise IT

Fundamentally, all wireless LAN standards depend on access to radio spectrum. 802.11ac will be designed for use at frequencies under 6 GHz, which in practice refers to the existing radio spectrum available today in the 2.4 GHz and 5 GHz bands used by 802.11a/b/g/n. Therefore, an important component of the work in Task Group AC will be to design backward-compatibility mechanisms to peacefully coexist with existing networks.

Higher data rates in 802.11ac are supported by a set of familiar techniques. Once again, the speed will be supported by well-understood OFDM techniques, another bump up in the size of radio channels, and MIMO. Advances in both chip manufacturing technology and processing power have also made it possible to use more sensitive coding techniques that depend on finer distinctions in the received signal as well as more aggressive error correction codes that use fewer check bits for the same amount of data.

Wider radio channels support higher speeds. Just as 802.11n provided a leap in speed by doubling channel width from 20 MHz to 40 MHz, 802.11ac provides a bump in throughput with still-wider 80 MHz channels. At 80 MHz, channel layout once again becomes a challenge, even in the relatively expansive 5 GHz spectrum. Manufacturers will need to adapt automatic radio tuning capabilities to offer higher-bandwidth channels only where necessary to conserve spectrum.

Increasing data rates through efficiency is an important goal of every new 802.11 standard. One common measure of efficiency is the number of megabits transmitted per megahertz of spectrum (Mbps/MHz). 802.11 began life at 0.1 Mbps/MHz, and current 802.11n standards have pushed that figure to 7.5 Mbps/MHz. Several efficiency enhancements are on the drawing board for 802.11ac, and the most interesting of these is multi-user MIMO (MU-MIMO).

MU-MIMO builds on the beamforming capabilities of 802.11n and enables the simultaneous transmission of different data frames to different clients. Correctly using MU-MIMO requires that vendors develop spatial awareness of clients and sophisticated queuing systems that can take advantage of opportunities to transmit to multiple clients when conditions are right.

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