RF Analog Impairments Modeling for Communication Systems Simulation: Application to OFDM-based Transceivers

3.2 WLAN and Mobile WiMAX PHY Overview

3.2.1 WLAN: Standard IEEE 802.11a/g

“WiFi” is a brand name used by the WiFi Alliance (industry consortium) certifying the interoperability between the WLAN systems specified by the IEEE 802.11 standard (IEEE, 1999, 2003). WLAN systems target high data-rate wireless communications for local networking (coverage radius of several tens of meters). The 802.11a and the 802.11g flavors are similar because they use the same OFDM modulation; the difference is the band (carrier frequency), unlicensed 5 GHz U-NII for 11a and 2.4 GHz ISM (industrial, scientific, and medical) for 11g. The channel bandwidth is 20 MHz and 48 subcarriers are used to transmit data. Four additional subcarriers are dedicated to pilots, the DC subcarrier is unused, and 11 subcarriers are blanked to provide guard bands (six on the left and five on the right), giving a total number of 64 subcarriers for the IFFT/FFT size. Pilot subcarriers are only modulated using binary phase shift keying (BPSK) whereas data can be transmitted using quadrature phase shift keying (QPSK), 16-QAM or 64-QAM depending on the channel conditions. A convolutional channel coding is specified with three coding rates: 1/2, 2/3, and 3/4.

The IFFT/FFT sampling frequency being 20 MHz and the number of subcarriers 64, the subcarrier spacing is then 20 MHz/64 = 312.5 kHz, giving a useful OFDM symbol duration of 3.2 µs. By adding a cyclic prefix of one fourth of the symbol length, that is, 0.8 µs, we obtain a ...