Modulation Techniques
OFDM

The 802.11a and the upcoming 802.11g standards both use orthogonal frequency division multiplexing (OFDM), to achieve data rates of up to 54 Mbps. OFDM works by breaking one high-speed data carrier into several lower-speed subcarriers, which are then transmitted in parallel. Each high-speed carrier is 20 MHz wide and is broken up into 52 subchannels, each approximately 300 KHz wide . OFDM uses 48 of these subchannels for data, while the remaining four are used for error correction. Coded orthogonal frequency division multiplexing (COFDM) delivers higher data rates and a high degree of multipath reflection recovery, thanks to its encoding scheme and error correction. OFDM uses the spectrum much more efficiently by spacing the channels much closer together. The spectrum is more efficient because all of the carriers are orthogonal to one another, thus preventing interference between closely spaced carriers.

Each subchannel in the OFDM implementation is about 300 KHz wide. 802.11a uses different types of modulation, depending upon the data rate used. The 802.11a standard specifies that all 802.11a-compliant products must support three basic data rates which include the following :

  • Binary Phase Shift Keying (BPSK) – encodes 125 Kbps of data per channel, resulting in a 6,000-Kbps, or 6 Mbps, data rate.
  • Quadrature Phase Shift Keying (QPSK) – encodes to 250 Kbps per channel, yielding a 12 Mbps data rate.
  • 16-level Quadrature Amplitude Modulation (16-QAM) – encodes 4 bits per hertz, achieving a data rate of 24 Mbps.

The standard also lets the vendor extend the modulation scheme beyond 24 Mbps. Data rates of 54 Mbps are achieved by using 64-level Quadrature Amplitude Modulation (64-QAM), which yields 8 bits per cycle or 10 bits per cycle, for a total of up to 1.125 Mbps per 300-KHz channel. With 48 channels, this results in a 54 Mbps data rate. Remember that the more bits per cycle (Hertz) encoded, the more susceptible the signal will be to interference and fading and ultimately, the shorter the range unless power output is increased.

Orthogonal is a mathematical term derived from the Greek word orthos, meaning straight, right, or true. In mathematics, the word orthogonal is used to describe independent items. Orthogonality is best seen in the frequency domain, looking at a spectral analysis of a signal. OFDM works because the frequencies of the subcarriers are selected in such a way that, for each subcarrier frequency, all other subcarriers will not contribute to the overall waveform.