ink to the same receiver. By transmitting a consequence, the bandwidth outside of which the M signals in parallel over the same radio link, the overall WCDMA theoretical spectrum equals zero is approxi- data rate can be increased up to M times. At the same mately 4.7 MHz (see right part of Figure 4.3-6). time, the impact in terms of signal corruption due to For a straightforward multi-carrier extension of radio-channel frequency selectivity depends on the WCDMA, the subcarriers must thus be spaced Narrowband transmission Wideband multi-carrier transmission RF modulation RF modulation Spectrum shaping Channel coding, Spectrum Channel coding, modulation, etc. shaping modulation, etc. fc 3/2Af Spectrum shaping fc 1/2Af Spectrum shaping fc + 1/2Af Spectrum shaping fc + 3/2Af Figure 4.3-5 Extension to wider transmission bandwidth by means of multi-carrier transmission. CHAPTER 4.3 High data rates in mobile communications 3.84 MHz -10.0 -20.0 BW=2.f2fr(1-a)-4.7z -30.0 -40.0 Frequency (MHz) Figure 4.3-6 Theoretical WCDMA spectrum. Raised-cosine shape with roll-off a = 0.22. approximately 4.7 MHz from each other to completely transmission), due to the higher importance of high avoid inter-subcarrier interference. It should be noted power-amplifier efficiency at the mobile terminal. though that a smaller subcarrier spacing can be used with The main advantage with the kind of multi-carrier only limited inter-subcarrier interference. extension outlined in Figure 4.3-5 is that it provides a very A second drawback of multi-carrier transmission is smooth evolution, in terms of both radio equipment and that, similar to the use of higher-order modulation, the spectrum, of an already existing radio-access technology parallel transmission of multiple carriers will lead to to wider transmission bandwidth and a corresponding larger variations in the instantaneous transmit power. possibility for higher data rates, especially for the down- Thus, similar to the use of higher-order modulation, link. In essence this kind of multi-carrier