Some details about CCK and OFDM.

Complementary Code Keying (CCK) is a modulation scheme used with wireless networks (WLANs) that employ the IEEE 802.11b specification. In 1999, CCK was adopted to replace the Barker code in wireless digital networks.

A complementary code contains a pair of finite bit sequences of equal length, such that the number of pairs of identical elements (1 or 0) with any given separation in one sequence is equal to the number of pairs of unlike elements having the same separation in the other sequence. A network using CCK can transfer more data per unit time for a given signal bandwidth than a network using the Barker code, because CCK makes more efficient use of the bit sequences.

Wireless networks using the 802.11b specification employ CCK to operate at data speeds of up to a theoretical maximum of 11 Mbps in the radio-frequency (RF) band at 2.400 GHz to 2.4835 GHz. Networks using the 802.11g specification employ CCK when operating at 802.11b speeds. At higher speeds (up to a theoretical maximum of 54 Mbps), 802.11g WLANs use a more sophisticated modulation scheme called orthogonal frequency division multiplexig (OFDM). This is the modulation method used by 802.11a WLANs in the RF band at 5.725 GHz to 5.850 GHz.

 

Orthogonal frequency-division multiplexing (OFDM) is a method of digital modulation in which a signal is split into several narrowband channels at different frequencies. The technology was first conceived in the 1960s and 1970s during research into minimizing interference among channels near each other in frequency.

In some respects, OFDM is similar to conventional frequency-division multiplexing (FDM). The difference lies in the way in which the signals are modulated and demodulated. Priority is given to minimizing the interference, or crosstalk, among the channels and symbols comprising the data stream. Less importance is placed on perfecting individual channels.

OFDM is used in European digital audio broadcast services. The technology lends itself todigital television, and is being considered as a method of obtaining high-speed digital data transmission over conventional telephone lines. It is also used in wireless local area networks.

Also see frequency-division multiplexing (FDM), time-division multiplexing (TDM), and multi-carrier modulation (MCM).