o nearest frequency reuse: 4 cell radii 5.5 cell radii Figure 5.1-18 Examples of cellular coverage plans for 4 and 7 nonoverlapping channels. Wireless local area networks CHAPTER 5.1 input assign to subcarriers Figure 5.1-19 802. Convolutional encoder. To allow for varying conditions, many different com- amplitudes over the time corresponding to a single binations of code rate and subcarrier modulation are OFDM symbol shown at the top right of the figure. allowed. These are shown in Table 5.1-2. Recall that At the bottom right we show the combined cumulative BPSK transports one bit per (subcarrier) symbol, QPSK distribution function for the signal power, averaged over two bits, and SO on up to 64 quadrature-amplitude- the packet length, for a packet at 54 and 6 Mbps. The modulation (QAM), which carriers 6 bits per symbol. combined cumulative distribution function is a graphic The product of the bits per symbol of the modulation display of the frequency with which the instantaneous and the rate of the convolutional code gives the number power in the signal exceeds the average by a given value. of bits each subcarrier contributes to each OFDM The 54-Mbps packet instantaneous power is greater than symbol; multiplication by 48 provides the num- 9 dB above the average around 0.1% of the time. The ber of bits transported per OFDM symbol. 6-Mbps packet displays an 8-dB enhancement in power Note that to avoid bursts of bit error from disruption with this frequency of occurrence. Although that may of a few neighboring subcarriers, the output bits from the not seem like much, recall that the spectral mask (Figure code are interleaved-that is, distributed in a pseudo- 5.1-16) requires that the transmitted signal 20 MHz away random fashion over the various subcarriers. from the carrier must be reduced by more than 28 dB. A Several views of a simulated OFDM frame are shown in high-power signal may become distorted, resulting in Figure 5.1-20. The frame consists of a preamble and spectral components far from the carrier. If su