oding, the receiver should within the OFDM time-frequency grid, as illustrated in multiply bk with the complex conjugate of Hk, as illus- Figure 4.4-12. Using knowledge about the reference trated in Figure 4.4-11, This is often expressed as a one- symbols, the receiver can estimate the frequency-domain tap equalizer being applied to each received subcarrier. channel around the location of the reference symbol. The reference symbols should have a sufficiently high density 4.4.6 Channel estimation and in both the time and the frequency domain to be able to provide estimates for the entire time/frequency grid in reference symbols case of radio channels subject to high frequency and/or time selectivity. As described above, to demodulate the transmitted Different algorithms can be used for the channel es- modulation symbol ak and allow for proper decoding of timation, ranging from simple averaging in combination Frequency-domain channel Receiver HNG-1 NNc-1 HNG-1 bNc-1 aNc-1 N-C-1 Figure 4.4-11 Frequency-domain model of OFDM transmission/reception with 'one-tap equalization' at the receiver. OFDM transmission CHAPTER 4.4 over a frequency-selective channel, certain modulation symbols may be fully confined to a frequency band with very low instantaneous signal strength as illustrated in Figure 4.4-13b. Thus, the individual modulation symbols will typically not experience any substantial frequency Reference symbol diversity even if the channel is highly frequency selective over the overall OFDM transmission bandwidth. As Figure 4.4-12 Time-frequency grid with known reference a consequence, the basic error-rate performance of symbols. OFDM transmission over a frequency-selective channel is relatively poor and especially much worse than the basic error rate in case of a single wideband carrier. with linear interpolation to Minimum Mean-Square However, in practice channel coding is used in most Error (MMSE) estimation relying on more detailed cases of digital communication and especially in case of knowledge of the channel 