corresponding increase in the symbol time Tu as Assuming a sufficiently large cyclic prefix, the linear a consequence. However, this will increase the sensitivity convolution of a time-dispersive radio channel will Copy and insert insertion (IFFT) Tu + Tcp (N samples) samples) Direct path Reflected path Integration interval for demodulation of direct path Figure 4.4-9 Cyclic-prefix insertion. CHAPTER 4.4 OFDM transmission Transmitter Receiver Noise Channel demod. insertion (IFFT) removal (FFT) aN, -1 DNc-1 HNG-1 NNc-1 Channel response for symbol aj ni : Noise imposed on symbol aj aNc-1 bnc-1 Figure 4.4-10 Frequency-domain model of OFDM transmission/reception. appear as a circular convolution during the demodulator the transmitted information at the receiver side, scaling integration interval . The combination of OFDM with the complex conjugate of the frequency-domain modulation (IFFT processing), a time-dispersive radio channel tap Hk should be applied after OFDM de- channel, and OFDM demodulation (FFT processing) can modulation (FFT processing) (see Figure 4.4-11). To be then be seen as a frequency-domain channel as illustrated able to do this, the receiver obviously needs an estimate of in Figure 4.4-10, where the frequency-domain channel the frequency-domain channel taps H, HNc-1 taps H, HNc- can be directly derived from the The frequency-domain channel taps can be estimated channel impulse response. indirectly by first estimating the channel impulse The demodulator output bk in Figure 4.4-10 is the response and, from that, calculate an estimate of Hk. transmitted modulation symbol ak scaled and phase However, a more straightforward approach is to estimate rotated by the complex frequency-domain channel tap Hk the frequency-domain channel taps directly. This can be and impaired by noise Nk. To properly recover the trans- done by inserting known reference symbols, sometimes mitted symbol for further processing, for example data also referred to as pilot symbols, at regular intervals demodulation and channel dec