ming. For high-mobility users, uplink timing has to be adjusted continuously. This fact may even cause a residual misalignment because UE position when the timing advance is computed by the eNodeB differs from UE position when the timing advance adjustment reaches the UE. In this scenario, more advanced reception techniques may be required in the uplink. 3.4.2 Channel Estimation It is well known that one of the main benefits of OFDM is its inherent immunity against multipath effects. Thanks to the decomposition of the transmitted signal on orthogonal subcarriers along the available bandwidth, it is possible to consider the frequency selective channel as approximately flat for every subcarrier. This assump- tion allows for easily performing the channel equalization process in the frequency domain by means of multiplications by constant values. In the general case, mobile channels are considered to be time variant and fre- quency selective. This fact requires a dynamic estimation of the channel frequency response at the receiver. The most extended strategy for channel estimation in OFDMA relies on the use of pilot subcarriers, distributed along the transmission bandwidth, whose transmitted values are known at the receiver (Coleri et al. 2002, 223). This strategy allows an easy estimation of the channel frequency response for this set of frequencies. © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology Frequency Pilot subcarrier Data subcarrier (a) Frequency continuous-time spaced pilot allocation (Block type) Frequency Pilot subcarrier Data subcarrier (b) Time continuous-frequency spaced pilot allocation (Comb type) Frequency Pilot subcarrier Data subcarrier (c) Time spaced-frequency spaced pilot allocation Figure 3.34 Pilot arrangement strategies in OFDM. When pilot subcarriers are allocated in the time-frequency domain of an OFDMA system, different possibilities exist, as depicted in Figure 3.34. The first strategy (Figure 3.34a)-namely, block type pilot arrangement-co