Filter bank-based multicarrier (FBMC) systems based on offset QAM (FBMC/OQAM)have recently attracted increased interest in several applications due to theirenhanced flexibility, higher spectral efficiency, and better spectralcontainment compared to conventional OFDM. They suffer, however, from aninter-carrier/inter-symbol interference that complicates signal processingtasks such as channel estimation. Most of the methods reported thus far rely onthe assumption of (almost) flat subchannels to more easily tackle this problem,addressing it in a way similar to OFDM. However, this assumption may be oftenquite inaccurate, due to the high freq. selectivity of the channel and/or thesmall number of subcarriers employed to cope with frequency dispersion in fastfading. In such cases, severe error floors are exhibited at medium to high SNRvalues, which cancel the advantage of FBMC over OFDM. Moreover, the existingmethods provide estimates of the subchannel responses, most commonly in thefrequency domain. The goal of this paper is to revisit this problem through analternative formulation that focuses on the estimation of the channel impulseresponse itself and makes no assumption on the degree of frequency selectivityof the subchannels. The possible gains in estimation performance offered bysuch an approach are investigated through the design of optimal (in the MSEsense) preambles, of both the full and sparse types, and of the smallestpossible duration of only one pilot FBMC symbol. Existing designs for flatsubchannels are then shown to result as special cases. Longer preambles,consisting of two consecutive pilot FBMC symbols, are also analyzed. Thesimulation results demonstrate significant improvements from the proposedapproach for both mildly and highly frequency selective channels. Most notably,no error floors appear anymore over a quite wide range of SNR values.
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