Multi-carrier modulation

Special modulation scheme that stands some what apart from others are multi-carrier modulation scheme(MCM),orthogonal frequency division multiplexing (OFDM)or coded OFDM (COFDM) that are used in contxt of European digital radio system DAB and the WLAN standards ,IEEE 802.11a and hiperLan2 .The main attraction of MCM is its good ISI mitigation property.As explained,higher bit rates aare more vulnerable to ISI . MCM splits the high bit rate stream into many lower bitrate streams,each stream being sent using an independent carrier frequency. If for example,n symbols/s have to be transmitted, each subcarrier transmits n/c symbols/s with c being the number of subcarriers. One symbol could for example represent 2 bit as in QPSK .DAB ,foe example ,uses in between 193 and 1538 of these sub-carriers. The physcial layer of HiperLan2 and IEEE 802.11a uses 49 subcarriers for data.
In this there is actually the superposition of orthogonal frequencies.The maximum of one subcarrier frequency appers exactly ata frequency where all other sub-carriers are equal to zero.
Using this scheme , frequency selective fading only influences some sub-carriers and not the whole signal - an additional benefit on MCM. Typically ,MCM transmits the symbol with guard spces between single symbols or groups of symbols .This helps the receiver to handle multi-path propagation .OFDM is a special method of implementing MCM using orthogonal carriers.Computationally , it is a very efficeint algorithm based on fast fourier transform (FFT) for modulation or demodulation.