2.4.5 Orthogonal Frequency Division Multiplexing (OFDM)

Chapter 2.4.5 Orthogonal Frequency Division Multiplexing (OFDM)

Radio Frequency Integrated Circuit Design Second Edition Book
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Radio Frequency Integrated Circuit Design Second Edition Book

  • 40 Issues in RFIC Design: Noise, Linearity, and Signals2.4.5  Orthogonal Frequency Division Multiplexing (OFDM)OFDM is a designed to help improve the link performance in a radio link where there can be frequency selective fading or interference within the bandwidth of the channel. The idea is to replace a single carrier with multiple carriers (called subcar-riers), which individually handle data at lower rates. Thus, instead of having one carrier at say 64 Mbps, there can be 64 subcarriers each at 1 Mbps. Usually, the subcarriers are either a form of PSK or QAM modulation. Just as in FSK, the car-riers must be spaced at multiples of the data rate they are transmitting. This way nulls of adjacent carriers fall at other carrier center frequencies. This is the reason for “orthogonal” in OFDM, which means loosely that the individual carriers will not interfere with each other. Note that for OFDM, the bit error rate is the same as the single carrier case using the same modulation. Thus for an OFDM system using QPSK the bit error rates previously discussed still apply. The advantage with OFDM is that if interference corrupts one of the subcarriers then only the data on that subcarrier is affected while the others are not. This leads to a lower bit error rate in the presence of frequency selective interference or fading. Often in practical systems certain subcarriers have special functions. They can be used as pilot tones sometimes used to establish the phase of the signal as well as perform many other functions that aid the digital signal processor in determining what data was sent. However, OFDM signals are not constant envelope signals even when using modu-lations like QPSK, and the signal amplitude can have high peak-to-average ratio. References [1] Papoulis, A., Probability, Random Variables, and Stochastic Processes, New York: McGraw-Hill, 1984.Figure 2.22  Probability of bit error versus Es/No for 16-, 64-, and 256-QAM.