802.11ac also has taken the opportunity to simplify one promising 802.11n feature: Most access points use omnidirectional antennas that direct radio energy equally in all directions no matter where the intended receiver of a transmission may be located. With the multiple-antenna arrays required for 802.11n, access points gained the capability of selectively focusing radio energy in a particular direction using a technology called explicit beamforming. By taking a measurement of the radio channel between two devices, it was possible for an AP to determine how to transmit a signal to focus the energy. 802.11n had multiple ways of performing the explicit measurement, with the unfortunate side effect that none was ever widely adopted. 802.11ac has simplified beamforming by settling on one protocol for explicit measurement.
Even as preparations are underway for the first wave of 802.11ac devices, momentum is building for a second wave following closely behind. An additional bump in channel width to 160MHz is possible, though the availability of such wide channels is subject to regulatory developments. 160MHz channels offer multi-gigabit throughput, but at significant costs in terms of battery life and limited spectral availability.
Beamforming is a key to increasing the efficiency of the radio channels used by 802.11ac. The 802.11ac standard includes up to eight spatial streams (compared to just four in 802.11n today). However, 802.11ac specifies that a maximum of four streams are available to a single client device.
In the first wave of 802.11ac devices, this restriction does not come into play, but it offers interesting possibilities for the future. In Multi-User MIMO (MU-MIMO), the AP can divide its transmit streams between multiple devices. As an example, say that an AP allocates four of six transmit streams to a laptop performing a high-speed file transfer, and each of its two remaining streams to two tablet devices. All three streams are active simultaneously, and can have different beamforming steering information applied.
With the rise of smartphones and tablets, the existence of pervasive wireless LANs is just assumed to exist by most users. The continued development of robust, high-speed technologies such as 802.11ac will meet the needs of users for the next several years. [Also see: "Wi-Fi client surge forcing fresh wireless LAN thinking"]
Matthew Gast is a product manager at Aerohive. Praveen Mehrotra is a senior engineer at Aerohive and a participant in the development of the Wi-Fi Alliance VHT5 certification program.
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