Multi-carrier modulation( MCM) is a modern modulation technique that was developed several decades ago and is becoming increasingly important in practice, especially in WLANs according to IEEE 802.11a, 802 .15.3 and 802.16a. Whereas in the classic single-carrier method a carrier frequency is transmitted with a frequency band, in the multicarrier method the transmission channel is divided into several subchannels with the same bandwidth and each subchannel is assigned to its own carrier frequency.
In principle, each subchannel is modulated with the same modulation method and has the same transmission rate. However, the subchannels can also be modulated with different modulation methods to counteract impairments caused by interference. In this way, the individual subchannels are optimally utilized.
Multicarrier methods include the multitone method( DMT) and frequency division multiplexing. In frequency division multiplexing, a safety frequency band is inserted between the individual subcarriers to prevent mutual interference between the frequency bands. In orthogonal multicarrier techniques, such as those used in orthogonal frequency division multiplexing( OFDM), the guard frequency band is omitted, and the subcarriers are closer together and even overlap.