nsists of allocat- ing a complete OFDM symbol for pilot transmission. Thus, an estimation of the channel frequency response can be obtained by least squares (LS), minimum mean square error (MMSE), or similar methods. This approach is a suitable strategy for slow time-varying channels. An alternative is comb type pilot arrangement, shown in Figure 3.34b. In this technique, pilot subcarriers are transmitted only in some frequencies, but continu- ously in time. For the rest of the frequencies, the channel frequency response is obtained by linear (López-Martínez et al. 2007, 987), second-order (Hsieh and Wei 1998, 217), or other interpolation methods. The last strategy (Figure 3.34c) is a combination of the former methods, where pilot subcarriers are spaced in both time and frequency. In general, the © 2009 by Taylor & Francis Group, LLC Radio-Interface Physical Layer time spacing between symbols with pilots is given by channel time variability- that is, the coherence time of the channel. On the other hand, frequency spac- ing between pilot subcarriers is determined by the frequency selectivity of the channel, which can be quantified by its coherence bandwidth (Pätzold 2002; Goldsmith 2005). 3.4.2.1 Channel Estimation in OFDMA DL In LTE downlink, cell-specific downlink reference signals are transmitted on one or several antenna ports of the eNodeB in order to facilitate the demodulation of the OFDMA signal at the UE. The structure of this reference signal was presented in Section 3.1.2 and corresponds to a time-frequency spaced pilot arrangement scheme (see Figure 3.34c). Thus, two-dimensional time and frequency interpola- tion is required at the receiver. For MIMO transmission, the time-frequency allocation of reference signals on each antenna is targeted to avoid interfering with each other (see Figure 3.3). Therefore, MIMO channel estimation may be addressed as the estimation of M X N individual channels, where M is the number of transmit antennas and N is the number of receive antennas. This way, channel estimati