Physical Layer (PHY)
IEEE 802.11a PHY specification

Products conforming to the 802.11a standard will let WLANs achieve data rates as high as 54 Mbps. IEEE 802.11a devices operate in the 5 GHz frequency range. It is from this higher frequency that the standard gains some of its performance. The rest comes from the combination of the coding and modulation techniques used.

802.11a moved to a wider frequency (5 GHz) in part to get the higher speeds, but also to avoid interference issues in the more crowded 2.4 GHz band. In addition to 802.11b WLANs, HomeRF LANs, Bluetooth devices, cordless phones, and even microwave ovens all operate in the 2.4 GHz band.

The benefits of using the 5 GHz spectrum are offset somewhat by the lack of backwards compatibility with the current generation of 802.11b LANs, because the frequencies do not match. Many vendors are addressing this problem by producing dual-mode products that actually contain two radios, one that operates in the 2.4 GHz range, and one that operates in the 5 GHz range.

Orthogonal Frequency Division Multiplexing (OFDM)
The IEEE 802.11a standard uses orthogonal frequency division multiplexing, a technique that divides a communications channel into a number of equally spaced frequency bands. OFDM uses multiple subcarriers, of which there are 52, spaced 312.5 KHz apart. Data is sent on 48 carriers simultaneously, with each subcarrier carrying a portion of the user data. Four subcarriers are used as pilots. Subcarriers are orthogonal (independent) of each other.

The time to transmit each bit increases in proportion to the number of carriers. This makes the system less sensitive to multipath interference, a major source of distortion. Figure shows the different coding and modulation schemes that are used by 802.11a, along with the corresponding data rates.