signal-to-noise and signal-to-interference domain, a time-dispersive channel corresponds to ratios can be made available, for example in small-cell a non-constant channel frequency response as illustrated environments with low traffic load or for mobile terminals in Figure 4.3-4b. This radio-channel frequency selectivity close to the cell site. will corrupt the frequency-domain structure of the Instead, to provide high data rates as efficiently as transmitted signal and lead to higher error rates for given possible in terms of required signal-to-noise and signal-to-noise/interference ratios. Every radio channel is signal-to-interference ratios, implying as good coverage as subject to frequency selectivity, at least to some extent. possible for high data rates, the transmission bandwidth However, the extent to which the frequency selectivity should be at least of the same order as the data rates to be impacts the radio communication depends on the provided. bandwidth of the transmitted signal with, in general, Having in mind that the provisioning of higher data larger impact for wider-band transmission. The amount rates with good coverage is one of the main targets for the of radio-channel frequency selectivity also depends on evolution of 3G mobile communication, it can thus be the environment with typically less frequency selectivity concluded that support for even wider transmission (less time dispersion) in case of small cells and in envi- bandwidth is an important part of this evolution. ronments with few obstructions and potential reflectors, However, there are several critical issues related to the such as rural environments. use of wider transmission bandwidths in a mobile- It should be noted that Figure 4.3-4b illustrates communication system: a 'snapshot' of the channel frequency response. As Spectrum is, as already mentioned, often a scarce and a mobile terminal is moving through the environment, expensive resource, and it may be difficult to find the detailed structure of the multi-path propagation, an