e of relatively high corruption due to time dispersion not covered by the complexity. cyclic prefix. As already mentioned, this implies that, although the amount of time dispersion typically in- In case of pre-filtering or pre-coding, linear processing creases with the cell size, beyond a certain cell size there is applied to the sequence of modulation symbols is often no reason to increase the cyclic prefix further as before OFDM modulation. DFT-spread-OFDM can the corresponding power loss would have a larger neg- be seen as one kind of pre-filtering. ative impact, compared to the signal corruption due to In case of selective scrambling, the coded-bit the residual time dispersion not covered by the cyclic sequence to be transmitted is scrambled with prefix. a number of different scrambling codes. Each One situation where a longer cyclic prefix may be scrambled sequence is then OFDM modulated and needed is in the case of multi-cell transmission using SFN the signal with the lowest peak power is selected for (Single-Frequency Network), as further discussed in transmission. After OFDM demodulation at the Section 4.4.11. receiver side, descrambling and subsequent decoding Thus, to be able to optimize performance to different is carried out for all the possible scrambling environments, some OFDM-based systems support sequences. Only the decoding carried out for the multiple cyclic-prefix lengths. The different cyclic-prefix scrambling code actually used for the transmission lengths can then be used in different transmission will provide a correct decoding result. A drawback of scenarios: selective scrambling is an increased receiver com- Shorter cyclic prefix in small-cell environments to plexity as multiple decodings need to be carried out minimize the cyclic-prefix overhead. in parallel. Longer cyclic prefix in environments with extreme time dispersion and especially in case of SFN operation. 4.4.10 OFDM as a user-multiplexing and multiple-access scheme 4.4.9 Variations in instantaneous transmission power The discu