1. The use of different types of multi-carrier transmis- A drawback of the kind of multi-carrier evolution sion, that is transmitting an overall wider-band signal outlined in Figure 4.3-5, where an existing more as several more narrowband frequency-multiplexed narrowband radio-access technology is extended to signals, see below. One special case of multi-carrier a wider overall transmission bandwidth by the parallel transmission is OFDM transmission to be discussed transmission of M more narrowband carriers, is that the in more detail in Chapter 4.4. spectrum of each subcarrier does typically not allow for very tight subcarrier 'packing.' This is illustrated by the 2. The use of specific single-carrier transmission 'valleys' in the overall multi-carrier spectrum outlined in schemes, especially designed to allow for efficient the lower part of Figure 4.3-5. This has a somewhat but still reasonably low-complexity equalization. negative impact on the overall bandwidth efficiency of this kind of multi-carrier transmission. 4.3.3.1 Multi-carrier transmission As an example, consider a WCDMA multi-carrier evolution towards wider bandwidth. WCDMA has One way to increase the overall transmission bandwidth, a modulation rate, also referred to as the WCDMA chip without suffering from increased signal corruption due to rate, of 310 fcr = 3.84 Mchips/s. However, due to radio-channel frequency selectivity, is the use of so-called spectrum shaping, even the theoretical WCDMA multi-carrier transmission. As illustrated in Figure 4.3-5, spectrum, not including spectrum widening due to multi-carrier transmission implies that, instead of trans- transmitter imperfections, has a bandwidth that signifi- mitting a single more wideband signal, multiple more cantly exceeds 3.84 MHz. More specifically, as can be narrowband signals, often referred to as subcarriers, are seen in Figure 4.3-6, the theoretical WCDMA spectrum frequency multiplexed and jointly transmitted over the has a raised-cosine shape with roll-off a = 0.22. As same radio l