Low complexity turbo-like decision feedback equalization for broadband wireless single carrier systems

In broadband wireless communication systems, single-carrier transmission with frequency domain equalization (SC-FDE) is regarded as an alternative modulation technique to the orthogonal frequency division multiplexing (OFDM). Compared with OFDM systems, SC-FDE has similar implementation complexity and performance. However, SC-FDE with linear equalizer will suffer from serious performance deterioration under deep frequency selective fading channels. An effective method to solve this problem is to introduce non-linear decision feedback equalization (DFE) to SC-FDE. In this paper, a low complexity iterative decision feedback equalizer in the frequency domain for single- carrier systems is proposed. Based on the minimum mean square error (MMSE) criterion, a simplified parameter estimation method is introduced to calculate the coefficients of the feed-forward and feedback filters, which reduces the implementation complexity of the equalizer. Simulations show that the performance of the proposed algorithm is similar to the traditional iterative block DFE (IBDFE) under various multipath fading channels but with lower complexity.

Abstract--The LS channel estimation was modified by implementing space and time code via introducing cyclic conjugate to both the information sequence and the pilot sequence in the time-domain. The bit error rate (BER) performance of orthogonal frequency division multiplexing (OFDM) and of single-carrier transmission with frequency-domain equalization (SC/FDE) with the new channel estimator in MIMO channels is compared. Computer simulations show that the BER performance of OFDM without sub-carrier adaptive modulation and encoding is similar to SC/FDE system while the MSE of the channel estimator of OFDM system is lower when the channel Doppler frequency shift is lower in MIMO channels. The performance of OFDM becomes better than SC/FDE when Doppler frequency shift is larger.

In recent years, an Ultra Wideband (UWB) has attracted much attention in short range high speed wireless communications. Meanwhile, the single-carrier transmission with frequency domain equalization (SC-FDE) has also recently at- tracted much attention. It is reported that the SC-FDE sys- tem obtains an excellent performance as well as orthogonal frequency division multiplexing (OFDM), because the SC-FDE system has the advantage of frequency diversity gain even in strong frequency selective channels. However, in the SC-FDE system, the cyclic extension reduces the bandwidth efficiency. Thus, in the SC-FDE system, various methods to improve the bandwidth efficiency are reported. In this paper, we evaluate the improvement of bandwidth efficiency based on cyclic prefix (CP) reconstruction as a method of improving bandwidth efficiency of Ultra Wideband-Impulse Radio (UWB-IR) and Direct Sequence- Ultra Wideband (DS-UWB).

In recent years, an Ultra Wideband (UWB) system has attracted much attention in short range high speed wireless communications. Meanwhile, the single-carrier transmission with frequency domain equalization (SC-FDE) is also attracting much attention. It is reported that the SC-FDE system obtains an excellent performance as well as orthogonal frequency division multiplexing (OFDM), because the SC-FDE system has the advantage of frequency diversity gain even in strong frequency selective channels. However, in the SC-FDE system, cyclic extension reduces the bandwidth efficiency. To improve the bandwidth efficiency, we proposed UWB-IR and DS-UWB with Frequency-Domain Equalization (FDE) and Cyclic Prefix (CP) reconstruction, and showed that when the channel estimation is ideal, the proposed system achieves the good performance. Both FDE and CP reconstruction use channel gains and thus the performance is expected to depend largely on the channel estimation accuracy. In this paper, we evaluate the effect of imperfect channel estimation on the performance of UWB-IR with MMSE-FDE and CP Reconstruction. We show that CP reconstruction is effective for UWB-IR with MMSE-FDE in the IEEE UWB multipath channels, CM3, CM4.

In IEEE 802.16 broadband wireless access (BWA) systems, the uplink performance is affected by the size of the contention period in the subframe since the bandwidth request (BR) messages are sent during the period and the bandwidth for each service can be allocated only when the BR messages are accepted. In this paper, we develop a cross-layer analytical model for designing the optimal size of the contention period in IEEE 802.16 BWA systems with various classes of services. The model is validated by Monte Carlo simulation, and the results show that our cross-layer design provides a good scheme for determining the contention period size when the system performance is optimized.

The aim of the paper is to compare multicarrier and single carrier modulation schemes for radio communication systems. In both cases the fast Fourier transform (FFT) and its inverse are utilized. In the case of OFDM (orthogonal frequency division multiplexing), the inverse FFT transforms the complex amplitudes of the individual subcarriers at the transmitter into the time domain. At the receiver the inverse operation is carried out. In the case of single carrier modulation, the FFT and its inverse are used at the input and output of the frequency domain equalizer in the receiver. Different single carrier and multicarrier transmission systems are simulated with time-variant transfer functions measured with a wideband channel sounder. In the case of OFDM, the individual subcarriers are modulated with fixed and adaptive signal alphabets. Furthermore, a frequency-independent as well as the optimum power distribution are used.

In this paper, we consider minimum bit-error rate (BER) transceiver design for multiple-input multiple-output (MIMO) decode-and-forward (DF) relay systems employing single-carrier transmission with frequency-domain equalization (SC-FDE). The problem is formulated as the minimization of the end to-end (e2e) BER subject to a joint source and relay transmit power constraint. Since the e2e-BER is highly non-convex in terms of the complex matrix optimization variables, solving the optimization problem directly is challenging. By resorting to an upper bound on the e2e-BER and by assuming an optimal sum power budget splitting for the source and relay, we show that the problem can be reduced to the optimization of two equivalent point-to-point MIMO systems. This enables us to derive the optimal eigen-structure of the precoders and the matrix optimization problem simplifies into a convex power allocation problem involving real scalar variables. Primal decomposition is further applied to solve the resulting convex problem in a layered manner, where closed-form solutions are obtained for the inner subproblems. Simulation results are provided to confirm the BER performance of the proposed transceiver design for SC-FDE based MIMO DF relay systems.

Using a part of a general methodology for population analysis, developed recently in Lazov and Lazov [1], and relying on the fundamental concepts of system information i and system entropy $$S=E\left( i \right) $$S=Ei, this paper promotes a methodology for information and capacity analysis of broadband wireless access (BWA) systems. A BWA system consists of a base station (BS) and a group of M users in its coverage area, with N simultaneously active users, $$0\le N\le M$$0≤N≤M, working in point-to-multipoint mode. As in [1], we model this system as family of birth-death processes (BDPs), with size $$M+1$$M+1, in equilibrium, indexed by the system utilization parameter $$\rho $$ź, ratio of its primary birth and death rates, $$\rho =\lambda /\mu $$ź=ź/μ. We evaluate the BWA system information and entropy, and full system capacity, and then, assuming the same Gaussian distribution for the arrival traffic at BS from any user, system capacity (as a function of the system information) and its mean value, and mean normalized square deviation of the system capacity from its linear part. We compare the information of empty $$\left( {N=0} \right) $$N=0 and full $$\left( {N=M} \right) $$N=M system with system entropy, and further, system mean capacity with full system capacity, as functions of parameter $$\rho $$ź. The developed methodology is illustrated on families of BDPs with truncated geometrical, truncated Poisson and Binomial distributions as their equilibrium ones, which model the information linear, Erlang loss and Binomial BWA systems, respectively.

The application of frequency domain equalization techniques makes single carrier modulation a potentially valuable alternative to OFDM, especially in regard to its robustness to RF implementation impairments. In this overview paper we survey recent results on single carrier systems with frequency domain equalization (SC-FDE), including comparisons and compatibilities with OFDM, and extensions via decision feedback equalization and overlap-save processing.

The transmission of digital TV pictures has become important topic. For terrestrial broadcasting, a multi-carrier system consisting of a large number of orthogonal carriers (orthogonal frequency division multiplexing (OFDM)) has been proposed. In geographical areas where the cable TV (CATV) distribution system is already widely spread, the CATV distribution network is the main candidate for broadcasting digital TV signals. As the impulse response duration of the CATV transmission channel is much shorter than the delay spread of the terrestrial broadcast channel and the linear distortion is much smaller, it is not evident that OFDM is still the preferred modulation for a CATV environment. The authors have previously pointed out that a single-carrier (SC) QAM system with ideal decision-feedback equalization (DFE) is about 2 dB better than OFDM, when transmitting digital TV signals over the CATV network. The present paper investigates the performance of M/sup 2/-QAM and M-PAM VSB single-carrier (SC) systems. The authors compute the degradation in SNR (as compared to a non-distorting channel) when using either ideal decision feedback equalization (with adaptive filtering) or reduced-complexity feedback equalization (with fixed filtering). They show that the performance of the reduced-complexity equalizer is sensitive to a timing offset (both QAM and VSB) and a carrier offset (VSB only). This indicates that the reduced-complexity equalizer should be used in combination with a synchronizer that carefully controls the phase of the carrier generator (VSB) and sampling clock (VSB and QAM).

Multiple-input multiple-output (MIMO) signal processing combined with orthogonal frequency division multiplexing (OFDM) is a widely accepted solution to achieve the high bit rates that new communication standards require in frequency-selective wireless channels. In recent years, a new frequency-domain equalized (FDE) cyclic-prefixed single-carrier (CPSC) block transmission system has been proven to achieve similar performance to OFDM for coded systems with the same overall complexity, avoiding its main drawbacks and becoming a candidate for future wireless standards. A FDE CPSC spatial multiplexing system is analyzed and compared to OFDM. The implementation of different frequency-domain MIMO equalization and detection schemes is evaluated. Simulation-based performance results prove the efficiency of FDE CPSC based spatial multiplexing systems and highlight that these transmission schemes can outperform OFDM in uncoded and high coding rate MIMO systems, as it has been shown in the literature for the SISO case.

A fair and efficient service flow management architecture for IEEE802.16 broadband wireless access (BWA) systems is proposed for TDD mode. Compared with the traditional fixed bandwidth allocation, the proposed architecture adjusts uplink and downlink bandwidth dynamically to achieve higher throughput for unbalanced traffic. A deficit fair priority queue scheduling algorithm is deployed to serve different types of service flows in both uplink and downlink, which provides more fairness to the system. Simulation results show the proposed architecture can meet the QoS requirement in terms of bandwidth and fairness for all types of traffic.

Spatial Modulation (SM), is a new multiple antennas transmission technique for broadband wireless communications. In this paper, SM is firstly combined with single-carrier (SC) transmission systems for the merit of low peak average power ratio (PAPR) over conventional SM-orthogonal frequency division multiplexing (OFDM). Furthermore, for SM-SC systems, different detection techniques such as linear frequency-domain equalization (FDE) detection, decision feedback equalizer (DFE) detection and maximum likelihood detection (MLD) employing QR decomposition and M algorithm (QRM-MLD) techniques are explored to investigate the system performance. Finally, a comparison of the bit error rate (BER) performance between SM-SC and clipped SC-OFDM systems is given to show the merits of SM-SC systems.

An introduction of multi-user multiple-input multiple-output (MU-MIMO) to distributed antenna-based small-cell networks is considered a promising approach toward 5G mobile networks to enhance the sum throughput. However, the inter-symbol interference (ISI) caused by the channel frequency-selectivity, the inter-antenna interference (IAI), and the inter-user interference (IUI) degrade the MU-MIMO downlink throughput. This paper proposes two joint transmit-and-receive (Tx/Rx) filtering schemes (called BD-SVD and MMSE-SVD) for MU-MIMO downlinks using single-carrier (SC) transmission and orthogonal frequency-division multiplexing (OFDM) transmission. In BD-SVD, IUI is removed by block diagonalization (BD) at the transmitter side, and then, the equivalent channel after BD is transformed into IAI-free eigenmodes using singular value decomposition (SVD). On the other hand, in MMSE-SVD, BD is not used and the channel is directly transformed into eigenmodes. IUI and IAI are suppressed by a minimum mean square error (MMSE)-based precoding at the transmitter assuming that each receiver does eigenmode reception. Furthermore, in the case of SC downlink, ISI is suppressed by applying MMSE-based Tx power allocation (PA) and Rx frequency-domain equalization (FDE) to each eigenmode. Numerical results show that BD-SVD and MMSE-SVD achieve a higher sum throughput than conventional MMSE precoding.

Relay is considered as a method to ensure capacity and coverage in the broadband wireless access (BWA) system. This paper proposes a type of MAC layer simulator architecture for the relay enhanced BWA networks, and then addresses the issue on the network topology when relay is introduced. Furthermore, downlink co-operative communication in such relay scenario is discussed on. The results show that the proposed relay enhanced BWA system outperforms the single-hop one on spectrum efficiency.