Principles of OFDM - IEEE Potentials

communications is reliably and efficiently transmitting information signals over imperfect channels. One successful approach to achieving high-speed data transmission is multicarrier modulation (MCM), often also called multitone modulation . The principle of MCM is to divide the transmission channel into a number of orthogonal subchannels or subcarriers. This channel partitioning method attempts to construct a set of parallel subchannels that are largely independent. Channel partitioning consists of modulation and demodulation using suitable basis functions. Unitary transforms, that is orthonormal bases, are usually chosen so as to preserve the energy and independence of individual samples transmitted over subchannels. A block diagram of a generic multicarrier modulation scheme is shown in Fig. 1. An incoming data stream is divided into several parallel streams and each is modulated using a subcarrier. The amount of energy and information carried on each subchannel can be optimized for efficient transmission depending on the channel. If each subchannel is excited with independent information, then detection can be done at the receiver by processing the output samples obtained independently on each subchannel. The origins of this idea can be traced to the Collins Kineplex system created about four decades ago, where the principle of transmitting a data stream by dividing it into several parallel streams, each modulated by a subcarrier, was applied. In a single channel, serial data transmission system, symbols are transmitted sequentially, with each data symbol occupying the entire available bandwidth. Single channel communication is usually far more susceptible to inter symbol interference than multicarrier modulation. In multitone modulation, since the entire channel bandwidth is divided into several narrow subbands, the frequency response over each individual subband is relatively flat. Each subchannel occupies only a small fraction of the original bandwidth. Since the system’s data throughput is the sum of the throughputs of all the subchannels, the data rate per subchannel in a multicarrier system is only a fraction of the data rate of a conventional single carrier system having the same throughput. This feature allows for system designs supporting high data rates while maintaining symbol durations much longer than the memory of the channel. As a result, we avoid complex