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We consider Least-Squares (LS) downlink channel estimation for LTE OFDMA receivers. The 3GPP Long Term Evolution (LTE) project aims at continuing the competitiveness of the 3GPP standard reached with HSPA. OFDMA was chosen as multiple-access scheme for the downlink and 10 MHz bandwidth is the minimum capability for User Equipments (UE). In such a context, an ill conditioning problem of the classical LS approach was found due to the un-excitation of a large portion of available sub-carriers of the LTE OFDMA symbol (due to the guard band). Two methods are investigated and compared to solve this problem: the first solution is regularization and the second one is down-sampling of the channel impulse response. The latter solution proves to perform better and allows some computational savings.

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The attractiveness of OFDM decreases with the rising of inter-carrier interference in quickly time-varying channels. Classical OFDM low-complex detection is impaired and more elaborated techniques are required to mitigate the need for full matrix equalization. We present here a fresh approach to this subject, introducing novel fast-converging iterative techniques based on preconditioning. Moreover, we interpret windowing under a new perspective in association with the Basis Expansion Modeling of the time-varying channel. We discuss the complexity of the proposed methods, showing that they are still linear to the OFDM block size. We conclude by illustrating their competitive performance by means of numerical simulations.

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