Constant Envelope OFDM Phase Modulation - Dr. James R. Zeidler

71.2SubcarrierOverallSpectrum magnitude, |S(f)/TB|10.80.60.40.20-2024681012141618Normalized frequency, fT BFigure 1.5: Subcarrier and overall spectrum. (N = 16; |I 0,k | = 1, for all k)For single carrier each symbol occupies the entire signal bandwidth, while for multicarrierthe bandwidth is split into many frequency bands (also referred to as frequency bins).Notice that the multicarrier signal transmits the N data symbols in parallel over multiplecarriers each centered at (f c + k/T B ) Hz, k = 0, 1, . . . , N − 1.By properly designing the subcarrier spacing, each frequency bin is made frequencynonselective.The wideband frequency-selective channel is converted into N contiguousnarrowband frequency-nonselective bins. Figure 1.6 shows 18 bins in the range[−0.9, −0.78] MHz for the N = 300 OFDM system over the channel in Figure 1.2(b).Notice that the channel gain per bin varies over a 15 dB range. The OFDM modulationcan be optimized for the channel by sending more bits in frequency bins with high gainand fewer bits in frequency bins with low gain. This technique, known as bit loading,requires a fairly stable channel, one that can be accurately measured. For this reason,bit loading is more common in wireline systems and stationary wireless systems than inwireless systems with high mobility.Frequency selectivity is the frequency-domain dual of intersymbol interference. Transmittingthe single carrier signal over the 2 MHz channel results in a frequency-selectiveresponse. For OFDM, the overall channel is frequency-selective but for each bin the chan-