N-GEN Communications Web-based Seminars ©NGCS

Seminar - An Overview of Communications Technologies

Module 9 - Wireless Systems and Technologies                                                                                 Slide 12 of 80                                                                                                    ________________

    Orthogonal Frequency Division Multiplexing (OFDM)

                                    Divide Available Spectrum into Many Sub-carriers 

                                    Sub-carriers Overlapping and Orthogonal (right angle)

                                    High Speed data (100 M bps or higher rate) carried over
                                     sub-carriers

                                    Each Sub-carrier modulated to carry data at low rate

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Orthogonal Frequency Division Multiplexing is a coding rather than a multiplexing scheme. It is a method of using many carrier waves instead of one, and using each carrier wave to carry part of the information.

OFDM modulation is the simultaneous transmission of a large number of narrow-band carriers, known as sub-carriers, each modulated with low data rate, but with the sum total aggregating to a very high data rate. Each sub-carrier experiences flat fading in a multi-path environment and is easy to equalize. The need for equalization is also eliminated by using Differential Quadrature Phase Shift Keying (DQPSK) modulation where the data is encoded as the difference in phase between the present and previous symbol in time, on the same sub-carrier.

The second scheme uses the coherent modulation scheme, which is preferred for performance orientated systems, like point to multi-point licensed radios, where a higher bit rate per cycle is most desirable.

B bits per sub-carrier per symbol
QPSK, B = 2 bits
16-QAM = 4 bits
64-QAM, B = 6 bits