Take An In-Depth Look At IEEE 802.11ac

This white paper provides an introduction to IEEE 802.11ac documents, requirements, and the PHY, which encompasses the channel structure, frame formats, preamble fields, and data fields.

Given the success of the IEEE 802.11-2007 standard, the industry was inspired to make wireless networks perform as well as their wired brethren. One result of these efforts is the 802.11ac amendment, which offers mechanisms to increase throughput and enhance the wireless-local-area-networking (WLAN) experience. Detailing various aspects of this standard is a white paper titled, “802.11ac Technology Introduction.”

Beginning with sections on IEEE 802.11ac core documents and key requirements, the 28-page document seeks to truly provide a primer on this technology. The main requirements for IEEE 802.11ac are backwards compatibility and co-existence with IEEE 802.11a and 802.11n devices, as well as certain performance goals for single-station and multi-station throughput. IEEE 802.11ac devices must support 20-, 40-, and 80-MHz channels together with one spatial stream. The 80-MHz channel comprises two adjacent, non-overlapping 40-MHz channels. For its part, the 160-MHz channel may be formed by two 80-MHz channels, which can be adjacent or non-contiguous.

Like its predecessors, IEEE 802.11ac uses orthogonal-frequency-division multiplexing (OFDM). OFDM utilizes equally spaced subcarriers to transmit data. The number of subcarriers in the IEEE 802.11ac signal depends on the bandwidth. To make sure that all IEEE 802.11 devices can synchronize to the packet, IEEE 802.11ac sends the same preamble in each 20-MHz sub-band. Yet this results in a high peak-to-average-power ratio (PAPR), which limits power-amplifier (PA) efficiency. The subcarriers of the upper 20-MHz sub-bands are rotated to compensate for this effect.

After a section on frame formats, the white paper drills down into various preamble fields. It then discusses the IEEE 802.11ac transmitter specification. The measurement for its mask is made using a 100-kHz resolution bandwidth and a 30-kHz video bandwidth. The lowest possible mask value will be -59 dBm/MHz. To figure out whether the subcarriers have a similar amount of power, spectral flatness can be used. The average energy of a range of subcarriers is determined. The next step is to verify that no individual subcarrier’s energy in that range deviates by more than the value specified.

With the 160-MHz spectral flatness specification, an 80+80-MHz transmitter can transmit the two 80-MHz segments adjacent to each other for reception by a 160-MHz receiver. Similarly, a 160-MHz transmitter may be received by an 80+80-MHz receiver. As a result, both 80+80-MHz adjacent signals and 160-MHz signals must be considered when deriving the 160-MHz spectral-flatness test. The paper offers many details and tips like this to provide an excellent resource on IEEE 802.11ac. It ends with a discussion of clear-channel assessment, which tests the capability of an IEEE 802.11ac device to determine if a channel is free or occupied.

Rohde & Schwarz, Muehldorfstrasse 15, 81671 Munich, Germany, www.rohde-schwarz.com.

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