Low-complexity frequency-domain turbo equalization for single-carrier transmissions over doubly-selective channels

Abstract

Single-carrier transmissions with frequency-domain equalization have gained much interest due to their comparable complexity and performance to OFDM, which conversely suffers from a high peak-to-average power ratio. In this paper, we develop a new frequency-domain block turbo equalizer for single-carrier (SC) transmissions over doubly-selective channels. The main feature of the proposed equalizer is its low complexity, which is only linear in the block length. A comparison between SC and OFDM systems with channel coding in doubly-selective channels is also given.

New segmental turbo receiver for LTE single user uplink over double selective channels

“…The complexity is O((1 + 6(4 + 1))N log N ) = O(341N ) and the equalizer performance is illustrated by the pentagram marked lines. From the simulation results we can see that when the BER is about 1e −6 the performance of the proposed turbo equalizer is about 0.1dB better than the LDL turbo equalizer [8], 0.3dB better than the subblock turbo equalizer [5] and about 0.4 dB better than the original MMSE turbo equalizer.…”

“…1, the average bit error rate (BER) performance of the complexity-reduced frequency LDL turbo equalizers [8], the subblock turbo equalizer [5] and the proposed segmental turbo equalizers versus average receive signal to noise ratio (SNR) are compared. We first consider the medium mobility case where the UE speed is V = 485km/h.…”

“…The corresponding normalized Doppler frequency is f d = F d N T s = 0.06 with F d the absolute Doppler frequency shift and T s the symbol period (1/30.72µs). For the LDL turbo equalizer, the frequency bandwidth of H f B is set to B = 3 in accordance with [8]. By using the fast algorithm proposed in Section II, the complexity of the frequency LDL turbo equalizer is reduced from O(2N 2 log N + 6(27N + 2N log N )) = O(45350N ) to O(17N log N + 6(27N + 2N log N )) = O(481N ) and the equalizer performance is illustrated by the triangle marked lines.…”

“…In the first term in the right hand side of the second equation of (21), the matrix F H Λ H q Λ † Ωq Λ q F which is represented by Ξ q is a banded cyclic matrix with double side band width W [8]. That means the first columns of Ξ q which is represented by ξ q decays to zero within W elements from both ends and there are 2W nonzero elements altogether.…”

“…This method need several size of FFT hardware and the first LMMSE equalization is performed over the whole block which causes the performance deterioration. In [8], the frequency channel matrix of the double selective channel was assumed to be banded beside the main diagonal, and the low complexity LDL decomposition has been adopted to fast achieve the frequency equalization. Although this equalizer successfully gains the doppler diversity with low complexity, to the best of our knowledge, no method has been proposed to fast achieve the banded frequency channel matrix which becomes a heavy burden when the transmission block length is large.…”