Iterative frequency-domain equalization for single-carrier systems in doubly-dispersive channels

Abstract

Frequency-domain equalization (FDE) offers an attractive alternative to time-domain equalization in systems that communicate over large-delay-spread channels. Traditionally, FDE leverages the fact that time-domain convolution is equivalent to frequency-domain multiplication and the fact that time/frequency conversion is efficiently handled by the fast Fourier transform (FFT). In doubly dispersive channels, i.e., quickly varying large-delay-spread channels, the traditional FDE methods fail due to the time-varying property of the channel. Here we present a new FDE that is based on Doppler channel shortening, soft iterative interference cancellation, and block decision feedback. Numerical simulations show that the proposed technique outperforms the well-known FIR-MMSE-DFE in both performance and complexity. 1 I. INTRODUCTIONIn systems that communicate over large-delay-spread channels, the use of time-domain equalization (TDE) leads to expensive receivers. For example, North American terrestrial digital television is plagued by delay spreads on the order of hundreds of symbol intervals, requiring time-domain equalizers with hundreds of coefficients. Frequency-domain equalization (FDE) offers an attractive alternative. FDE leverages the fact that circular convolution in the time domain can be accomplished by pointwise multiplication in the frequency domain, and the fact that transformation to/from the frequency domain can be efficiently accomplished using the FFT algorithm. Roughly speaking, the processing complexity required for TDE is linear in the channel delay spread while for FDE it is logarithmic in the delay spread. Thus, FDE can lead to significant savings over TDE for long channels.FDE is the principle idea behind orthogonal frequency division multiplexing (OFDM) [1] and single-carrier cyclicprefix (SCCP) modulation [2]. Both OFDM and SCCP systems transmit data in blocks separated by guard intervals. The guard prevents inter-block interference, thereby simplifying receiver processing. The use of a cyclic-prefix (CP) guard makes the channel's dispersion act as a cyclic (rather than linear) convolution, implying that deconvolution can be accomplished through pointwise frequency-domain multiplication. When guards are not included, FDE can still be accomplished using overlap-add/save FFT algorithms (see, e.g., [3]).The previously mentioned FDE techniques assume a delayspread channel whose impulse response varies negligibly over

Frequency Domain Turbo Equalization for Vestigial Sideband Modulation with Punctured Trellis Coding

“…As a result of this overlap, the overall computational complexity of FDTE scheme increases by the factor N/(N − ND). A similar block-overlap technique was applied in [15,16]. For our performance comparison, we used the three propagation models summarized in Table 1.…”

A frequency domain equalization algorithm for fast time-varying fading channels

“…For example, using the basis extension model (BEM), the authors in [6] presented iterative FDE with widely linear filtering. In [7], the windowing technique was utilized to transform the channel matrix into a banded form. Based on this simplified model, the iterative equalization was applied.…”

A novel receiver architecture for single-carrier transmission over time-varying channels

“…This implies that it is more practical to assume a banded FD channel matrix 1 . Many equalizers exploit this banded (rather than diagonal) structure to lower the complexity, e.g., the block linear zero-forcing (ZF) equalizer in [7], the block linear minimum mean square error (MMSE) equalizer in [8], [9], the iterative serial MMSE equalizer in [10]- [12], the maximum likelihood (ML) equalizer in [13]- [15], etc. It can be imagined that to enhance the equalization performance, especially at a moderate to high signal-to-noise ratio (SNR), the band approximation error must be reduced as much as possible.…”

“…Since we need not to enforce a diagonal FD channel matrix but a banded one, a reduced-order FIR filter with just a single tap could be adequate. Such a filter is referred to as a receiver window in [9]- [12], [15].…”