Date Added: Nov 2010
Common WLAN standards defined by IEEE operate in the ISM (Industrial, Scientific and Medical) bands, i.e. 2.4 GHz and 5.2 GHz. OFDM (Orthogonal Frequency Division Multiplexing) is applied to overcome Inter-Signal Interference (ISI). The transmission runs in a frame mode. Numerous Modulation and Coding Schemes (MCS) are provided, which are switched by the transmitter adaptively, according to the channel condition. The new specification of WLAN systems has introduced many techniques to improve data rate in the physical layer. Apart from modification of the OFDM symbol (52 subcarriers dedicated for data transmission instead of 48 in 802.11a/g, shorter guard interval), two groups of methods can be distinguished: with backward signaling and without it.