ch compo- simulated (random) data. The spectrum of the output nents were comparable in size with the main signal and signal is about 16-MHz wide, in good agreement with the occurred only 1% of the time, their intensity would be 20 simple estimate given above. Recall that a signal in time dB below the main signal. Infrequent power peaks con- can be described by its in-phase and quadrature compo- tribute significantly to the output spectrum when distor- nents, I and Q. The I and Q amplitudes for the frame are tion is present. This problem of a high ratio of peak power shown at the lower left. The frame begins with a set of to average power represents one of the important disad- simplified synchronization characters, which use only 12 vantages of using OFDM modulations, because it forces subcarriers, and are readily visible at the left of the image. transmitters to reduce their average output power and The remainder of the preamble and data frame appear to requires higher precision and linearity from receivers than vary wildly in amplitude. This apparently random varia- is the case for simpler modulations. tion in the signal is also shown in the close-up of I and Q Recall that the OFDM symbol is constructed using an inverse fast Fourier transform (FFT), with the reverse operation being performed on the received signal to re- cover the subcarriers. In the case of 802.1 a 64-entry Table 5.1-2 802.11a Modulations and code rates. FFT involving some hundreds of arithmetic operations Modulation Code rate Data rate (Mbps) must be performed at fairly high precision every 4 usec, followed by a trellis decode of the resulting 48 data symbols. The rapid progress of digital integrated circuit scaling has permitted such a relatively complex compu- tational system to be implemented inexpensively in stan- dard silicon complementary metal-oxide semiconductor (CMOS) circuitry, a remarkable achievement. 16QAM 16QAM 5.1.3.7 802.11g PHY 64QAM The great disadvantage of the 802.11a PHY is its in- 64QAM compatibility with the older classi