Low-complexity Frequency-domain Equalizer Research Articles

Swarm Intelligence Based MMSE Frequency Domain Equalization for MIMO Systems

The automatic upgradation of equalizer weights in channel equalization demands a low-complexity, highly accurate estimation of recovery at the minimum possible time. The low-complexity frequency domain equalization improves the minimum mean square error (MMSE) of the equalization process. Adding the superiority of particle swarm optimization (PSO) to the equalizer coefficient selection process enhances the MMSE. This work proposes frequency-domain channel equalization along with a modified PSO (MPSO) as an adaptive algorithm for equalizer weight selection in MIMO systems. The simulation results validate the performance with the time domain linear and decision feedback equalizer structures for BPSK and QAM systems. The parameters are carefully selected by analyzing MMSE thoroughly under timevarying channel conditions.

Common CP-OFDM Transceiver Design for Low-Complexity Frequency Domain Equalization

Common cyclic prefix-orthogonal frequency-division multiplexing (CCP-OFDM) groups multiple OFDM symbols and uses one CP to protect them in slow time-variant channels. However, due to this particular design of CCP-OFDM systems, complex time-domain estimation algorithms are used to estimate and equalize the channel, such as maximum likelihood (ML) estimators. In this letter, we propose a novel CCP-OFDM transceiver design enabling low-complexity frequency-domain channel estimation and equalization. Simulations under frequency selective channels have validated the effectiveness of the proposed scheme in terms of computational complexity, normalized mean squared error (NMSE), and bit error rate (BER).

A New Exact Low-Complexity MMSE Equalizer for Continuous Phase Modulation

This letter introduces a new low-complexity frequency-domain equalizer for continuous phase modulations (CPM). The derivation of a fractionally spaced representation for circular block-based CPM leads, without any approximation, to a simple yet efficient frequency-domain equalization. The proposed equalizer is compared to the state-of-the-art approaches. Simulation results show the equivalence in terms of performance with a lower or similar complexity.

Single-Carrier Frequency-Domain Equalization With Index Modulation

Motivated by the recent index modulation (IM) concept in the spatial, frequency, and space-time domains, we propose a novel broadband single-carrier (SC)-based IM (SC-IM) scheme as well as its low-complexity frequency-domain equalization (FDE) algorithm. The proposed FDE-aided SC-IM scheme outperforms the conventional FDE-aided SC scheme, while achieving a comparable low level of detection complexity. In addition, in order to reduce the inter-channel correlation arising from the activated symbol indices, we propose a symbol mapping algorithm, which further improves the achievable error-rate performance of the SC-IM scheme. Our simulation results demonstrate the performance advantage of the proposed FDE-aided SC-IM scheme over the conventional FDE-aided SC scheme.

Asymptotically optimal low-complexity SC-FDE in data-like co-channel interference

In this paper, the design of a low-complexity linear equalizer is considered for single-carrier (SC) block transmission in the presence of inter-symbol interference (ISI) and data-like co-channel interference (CCI). Unlike the linear minimum mean-squared error (LMMSE) frequency-domain equalizer (FDE) designed to suppress ISI only, the LMMSE FDE suffers from high computational complexity due to the CCI component in the signal correlation matrix. Motivated by the fact that the double Fourier transform of the autocorrelation function of a wide-sense cyclostationary process consists of impulse fences with equal spacing, a low-complexity FDE is proposed that approximates the frequency-domain correlation matrix of the CCI plus noise by a block matrix with diagonal blocks. It is shown that the proposed FDE is asymptotically optimal in the sense that the average mean-squared error (MSE) converges to that of the LMMSE FDE as the block length tends to infinity. It is also shown that the proposed FDE is more numerically stable than the LMMSE FDE when the receive filter is matched to the transmit filter and its output is over-sampled to better capture the cyclostationarity of the CCI. Discussions and numerical results include SC block transmission systems with unique word instead of cyclic prefix, and systems with multiple receive antennas.

Generalized precoded block-spread CDMA

A novel block-spread code-division multiple access (BS-CDMA) framework is presented whereby user-specific, channel-independent precoding is employed along with special spreading codes to ensure multi-user interference free reception with maximum user load. This framework is presented in a general form, from which other BS-CDMA techniques can be derived. Finally, it is shown that specific precoder and spreading code designs facilitate low-complexity frequency-domain equalization at the receiver.

Shifted Known Symbol Padding for Efficient Data Communication in a WLAN Context

To allow for a computationally efficient equalization scheme for the frequency-selective transmission channels encountered in wireless local area network (WLAN) applications, cyclic prefix (CP) block transmission schemes have been proposed, such as single-carrier CP (SC-CP) and multi-carrier CP (MC-CP) transmission, also known as orthogonal frequency division multiplexing (OFDM). In this letter, however, we focus on the known symbol padding (KSP) transmission scheme. In this scheme known padded sequences can be exploited for synchronization as well as for channel estimation. However, to simultaneously allow for low-complexity frequency-domain equalization and accurate channel estimation within the KSP context, a modified KSP scheme is proposed, namely shifted KSP (S-KSP). Comparing different block transmission schemes in the WLAN context, the S-KSP scheme is shown to offer a very good performance.

Perfect equalization for DMT systems without guard interval

We propose a new, low-complexity frequency-domain equalizer for discrete multitone (DMT) systems, which, in the absence of a guard interval, utilizes existing redundancy in the frequency-domain to completely eliminate intersymbol and interchannel interference. A perfect reconstruction condition is derived for the noise-free case leading to a sparse equalizer matrix structure. It is furthermore shown that under realistic scenarios minimum mean square error adaptation of the equalizer coefficients allows for nearly perfect reconstruction already for a much smaller amount of redundancy than indicated by the perfect reconstruction condition. The new equalization scheme has at least the same potential compared with traditional DMT while offering new degrees of freedom for designing short-latency DMT systems.

A new equalizer for multitone systems without guard time

We propose a new, low-complexity frequency-domain equalizer, which, in the absence of a guard interval, utilizes redundancy in the frequency domain to completely eliminate intersymbol and interchannel interference. Simulation results show that the new equalization scheme has at least the same potential compared to conventional DMT/OFDM while offering the shortest possible latency at a reasonable complexity enhancement.

Training sequence versus cyclic prefix-a new look on single carrier communication

Orthogonal frequency-division multiplexing (OFDM), with the help of a cyclic prefix, enables low complexity frequency domain equalization, but suffers from a high crest factor. Single carrier with cyclic prefix (SC-CP) has the same advantage with similar performance, but with a lower crest factor and enhanced robustness to phase noise. The cyclic prefix is overhead, so we put more information in it by implementing this cyclic prefix as a training sequence (TS). This new training aided frequency domain equalized single carrier (TASC) scheme offers us additional known symbols and enables better synchronization and (potentially) channel estimation, with the same performance as SC-CP.