Orthogonal Frequency Division Multiplexing

Orthogonal Frequency Division Multiplexing (OFDM) as a Viable Modulation Scheme for Broadband Wireless Communication

OFDM Basics:

Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier transmission technique, which divides the available spectrum into many carriers, each one being modulated by a low rate data stream. OFDM is similar to FDMA in that the multiple user access is achieved by subdividing the available bandwidth into multiple channels that are then allocated to users. However, OFDM uses the spectrum much more efficiently by spacing the channels much closer together. This is achieved by making all the carriers orthogonal to one another, preventing interference between the closely spaced carriers.

The basic idea of OFDM is to divide the available spectrum into many narrowband, low-data-rate carriers, or subcarriers. To obtain high spectral efficiency, the frequency responses of the sub-carriers are overlapping and orthogonal, hence the name orthogonal frequency-division multiplexing. Each narrowband subcarrier can be modulated using various modulation formats in which binary or quadrature phase shift keying (BPSK or QPSK) and quadrature amplitude modulation (QAM) are commonly used.

Since the modulation rate on each subcarrier is very low, each subcarrier experiences flat fading in a multipath environment and is easy to equalize. The need for equalization can be eliminated by using differential QPSK (DQPSK) modulation, wherein the data is encoded as the difference in phase between the present and previous symbol in time on the same subcarrier. Differential modulation improves performance in environments where rapid changes in phase are possible, as is the case in power line communications. OFDM can be implemented equally well, with coherent (non-differential) modulation and demodulation to maximize the signal-to-noise ratio performance of the system. This approach is preferred for performance-oriented systems, as are likely to be found in future wireless networking systems operating at greater than 20-Mbit/second for distributing digital video in the home.

OFDM modulation is generated using a fast Fourier transform (FFT) processor. First, data is encoded in the frequency domain onto individual subcarriers. For example, with DQPSK modulation, 2 bits are encoded onto each subcarrier. The number of subcarriers is one of the design parameters for the system. An inverse FFT (IFFT) is performed on the set of frequency subcarriers, converting to the time domain and producing a single OFDM symbol. The length of time for the OFDM symbol is equal to the reciprocal of the subcarrier spacing (the second critical design parameter) and is generally a very long time compared with the data rate. The final signal-processing step is to create a cyclic prefix, which is a copy of the last part of the symbol that is added to the beginning of the symbol before transmission.

The basic transmitter and receiver structure in the simple sense is very simple and is summarized in the figure below: