e 4.4-3. It consists of a bank of No The use of a relatively large number of narrowband complex modulators, where each modulator corresponds subcarriers. In contrast, a straightforward multi- to one OFDM subcarrier. carrier extension as outlined in Figure 4.3-5 would In complex baseband notation, a basic OFDM signal typically consist of only a few subcarriers, each with x(t) during the time interval mT < t < (m + 1)T can a relatively wide bandwidth. As an example, thus be expressed as a WCDMA multi-carrier evolution to a 20 MHz overall transmission bandwidth could consist of four (sub) carriers, each with a bandwidth in the order of (4.4.1) 5 MHz. In comparison, OFDM transmission may imply that several hundred subcarriers are transmit- where xk(t) is the kth modulated subcarrier with fre- ted over the same radio link to the same receiver. quency fk = k.Af and alm), in general complex, is the Simple rectangular pulse shaping as illustrated in modulation symbol applied to the kth subcarrier during Figure 4.4-1a. This corresponds to a sinc-square- the mth OFDM symbol interval, i.e. during the time shaped per-subcarrier spectrum, as illustrated in interval mT ,<t<(m+1)Tu. OFDM transmission is thus Figure 4.4-1b. block based, implying that, during each OFDM symbol Tight frequency-domain packing of the subcarriers interval, Nc modulation symbols are transmitted in par- with a subcarrier spacing Af = 1/T, where Tu is the allel. The modulation symbols can be from any modula- per-subcarrier modulation-symbol time (see tion alphabet, such as QPSK, 16QAM, or 64QAM. Communications Engineering Desk Reference; ISBN: 9780123746481 Copyright © 2007, Erik Dahlman, Stefan Parkvall, Johan Skold and Per Beming. Published by Elsevier Ltd 2001. All rights reserved CHAPTER 4.4 OFDM transmission Af 1/T j2nfot j2nfft X1(t) am am S-> P aNc-1 XNc-1(t) Figure valid for time interval Figure 4.4-3 OFDM modulation. Figure 4.4-2 OFDM subcarrier spacing. r(m+1)T The number of OFDM subcarriers can range from less r(m+1)T than one hundred to sever