Blind channel equalization and carrier frequency offset estimation for MC-CDMA systems using guard interval redundancy and excess codes

It is well known that multipath channel and carrier frequency offset (CFO) destroy the orthogonality between subcarriers in the downlink of MC-CDMA and degrade the system performance. In this paper, a new low complexity algorithm is proposed to iteratively estimate the CFO and the channel equalizer using only the guard interval (GI) redundancy, or excess codes (EC) or both GI and EC without additional pilots. Simulations illustrate and compare the performance efficiency of these schemes in different scenarios.     

A subband approach to channel estimation and equalization for DMT and OFDM systems

Cyclic prefix (CP) is commonly used for channel equalization of discrete multitone (DMT) and orthogonal frequency-division multiplexing (OFDM) systems. This is often done in conjunction with a time-domain equalizer (TEQ) for reducing the capacity overhead caused by the CP. However, the use of TEQ greatly increases the computational cost, and is unable to eliminate the need for the CP. In this paper, we propose a subband approach to channel estimation and channel equalization for DMT and OFDM systems. This approach involves splitting the received signals into a number of frequency bands (called subbands), and estimating a constant parameter in each subband. The subband approach is conceptually simple, requires no CP, is much more numerically efficient than the TEQ method, and gives compatible or better estimation errors than the CP-based methods.     

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     

An enhanced joint equalization algorithm for full-duplex DMT transceivers

An objective function incorporating the factors that affect the system throughput is proposed, and an enhanced joint time-domain equalization algorithm is developed for full-duplex discrete multitone (DMT) transceivers. This algorithm is able to shorten the channel and echo impulse responses using a single filter while keeping the system throughput high.     

Efficient handoff policy without guard channels for mobile radio telephone systems

Presented in this letter is a prioritised pushout handoff policy without guard channels for increasing the total grade of service (GOS) in a mobile radio telephone system. Blocking probability and waiting time distribution for each call are obtained, and a boundary for two handoff policies between with and without guard channels is found.<>     

Time-domain channel estimator based on cyclic correlation for OFDM systems with guard interval

Channel impulse response （CIR） can be estimated on the basis of cyclic correlation in time-domain for orthogonal frequency division multiplexing （OFDM） systems, This article proposes a generalized channel estimation method to reduce the estimation error by taking the average of different CIRs. Channel impulse responses are derived according to the different starting points of cyclic correlation. In addition, an effective CIR length estimation algorithm is also presented. The whole proposed methods are more effective to OFDM systems, especialiy to those with longer cyclic prefix. The analysis and the simulation results verify that the mean square error performance is 4-5 dB better than the conventional schemes under the same conditions.     

Fast bit-loading algorithm for DMT systems

In this paper, a novel solution to bit-loading algorithm for Discrete MultiTone modulation (DMT) systems is proposed. Simulation results, using the carrier serving area (CSA) loops in an ADSL environment impaired by crosstalk, show that the proposed algorithm converges more rapidly than the Campello algorithm, which is the fast algorithm that can be efficiently implemented in practice among those proposed in the past.     

Fractionally spaced linear and DF Mimo equalizers for multitone systems without guard time

WhenOFDM or multitone modulation is used, the dispersiveness of the channel is usually circumvented by means of the technique of guard time. As an alternative it is possible not to use a guard time but design receivers able to equalize theisi andici. In the present paper we show how to design fractionally spaced linear or decision-feedback Mimo equalizers for multitone systems without guard time. Full complexity and reduced complexity solutions are investigated. The design is made for anmmse criterion and the steady-state performance is analysed. It is shown that Mimo equalization outperforms the technique of guard time, that fractionally spaced devices are very effective and the complexity reduction produces a performance degradation which is negligible compared to full complexity fractionally spaced devices, especially when the processing rate is larger than the symbol rate.     

Turbo multiuser detection for coded DMT VDSL systems

In recent years, iterative processing techniques with soft-in/soft-out components have received considerable attention. Such techniques, based on the so-called turbo principle, are exemplified through turbo decoding, turbo equalization, and turbo multiuser detection. Turbo multiuser detection is applied to a discrete multitone (DMT) very-high-rate digital subscriber line system to combat crosstalk signals and to obtain substantial coding gain. The proposed iterative DMT receiver is shown to achieve an overall 7.0 dB gain over the uncoded optimum receiver at a bit error rate of 10^-7 for a channel with severe intersymbol interference and additive white Gaussian noise and with one dominant crosstalk signal. Impulse noise is detrimental to the proposed scheme but can be overcome through erasure decoding techniques, as is shown by example     

Channel equalization for block transmission systems

In a block transmission system the information symbols are arranged in the form of blocks separated by known symbols. Such a system is suitable for communication over time-dispersive channels subject to fast time-variations, e,g., the HF channel. The known reliable receiver for this system is the nonlinear data-directed estimator (NDDE). This paper presents appropriate equalization methods for this system. A nonstationary innovations representation based on Cholesky factorization is used in order to define a noise whitener and a maximum-likelihood block detector. Also block linear equalizers and block decision-feedback equalizers are derived. For each type we give the zero-forcing and the minimum-mean-squared-error versions. Performance evaluations and comparisons are given. We show that they perform better than conventional equalizers. As compared to the NDDE, the derived block decision-feedback equalizers perform better and are much less complex. Whereas the NDDE uses the Levinson algorithm to solve M/2 Toeplitz systems of decreasing order (where M is the number of symbols per block), the derived equalizers need to process only one Toeplitz system. Moreover, the Schur algorithm, proposed for Cholesky factorization allows us to further reduce the complexity     

Per-tone equalization for MIMO OFDM systems

This paper focuses on multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems with channel order larger than the cyclic prefix (CP) length. Writing the demodulating fast Fourier transform (FFT) as a sliding FFT followed by a downsampling operation, we show in this paper that by swapping the filtering operations of the MIMO channel and the sliding FFT, the data model for the temporally smoothened received signal of each individual tone of the MIMO OFDM system is very similar to the data model for the temporally smoothened received signal of a MIMO single-carrier (SC) system. As a result, to recover the data symbol vectors, the conventional equalization approach for MIMO SC systems can be applied to each individual tone of the MIMO OFDM system. This so-called per-tone equalization (PTEQ) approach for MIMO OFDM systems is an attractive alternative to the recently developed time-domain equalization (TEQ) approach for MIMO OFDM systems. In the second part of this paper, we focus on direct per-tone equalizer design and adapt an existing semi-blind equalizer design method for space-time block coding (STBC) SC systems to the corresponding semi-blind per-tone equalizer design method for STBC OFDM systems.     

Lattice-reduction aided equalization for OFDM systems

Orthogonal Frequency Division Multiplexing (OFDM) is an effective technique to deal with frequency-selective channels since it facilitates low complexity equalization and decoding. Many existing OFDM designs successfully exploit the multipath diversity offered by frequency-selective channels. However, most of them require maximum likelihood (ML) or near-ML detection at the receiver, which is of high complexity. On the other hand, empirical results have shown that linear detectors have low complexity but offer inferior performance. In this paper, we analytically quantify the diversity orders of linear equalizers for linear precoded OFDM systems, and prove that they are unable to collect full diversity. To improve the performance of linear equalizers, we further propose to use a lattice reduction (LR) technique to help collect diversity. The LR-aided linear equalizers are shown to achieve maximum diversity order (i.e., the one collected by the ML detector), but with low complexity that is comparable to that of conventional linear equalizers. The theoretical findings are corroborated by simulation results.     

Subsymbol equalization for discrete multitone systems

In conventional discrete multitone (DMT) systems, a multitap time-domain equalizer (TEQ) is used to shorten the channel length, so that the bandwidth efficiency reduction due to cyclic extension is relieved. The TEQ, however, tends to introduce spectral s which degrade the achievable signal-to-noise ratio at corresponding subchannels, thereby decreasing the bandwidth efficiency. Furthermore, computationally expensive joint TEQ initialization and optimal delay (introduced by TEQ) selection is necessary. In this paper, a novel subsymbol-equalization scheme is proposed, and is based on the observation that the high-bit-rate twisted-pair channels rarely contain a zero that is close to the unit circle. Although a delay of a fraction of a DMT symbol period is introduced, the proposed subsymbol-equalization scheme eliminates the necessity of both the channel shortening at the receiver and the cyclic extension at the transmitter. Simulation results demonstrate the computational efficiency (no TEQ) and the bandwidth efficiency of the proposed subsymbol-equalization scheme.     

Per tone equalization for DMT-based systems

An alternative receiver structure is presented for discrete multitone-based systems. The usual structure consisting of a (real) time-domain equalizer in combination with a (complex) 1-tap frequency-domain equalizer (FEQ) per tone, is modified into a structure with a (complex) multitap FEQ per tone. By solving a minimum mean-square-error problem, the signal-to-noise ratio is maximized for each individual tone. The result is a larger bit rate while complexity during data transmission is kept at the same level. Moreover, the per tone equalization is shown to have a reduced sensitivity to the synchronization delay     

PERFORMANCE OF CODED OFDM VIA VARIABLE GUARD INTERVAL

This letter presents the principles of Coded Orthogonal Frequency Division Multiplexing (COFDM) system and focuses on the effect of the Guard Interval (GI) on the system performance. The role of the GI parameter period to solve the problem of interferences that occur at the receiver is discussed. A new COFDM concept with a Variable GI (VGI) is proposed in order to improve the Bit Error Rate (BER) performance in the presence of multipaths with variable delays. A series of simulations have been carried out on the BER performance as a function of GI, maximum delay spread, and Signal-to-Noise Ratio (SNR), with QPSK as modulation scheme over multipath fading and Additive White Gaussian Noise (AWGN). The results show that the optimum values of the GI parameter approximately equal to the maximum delay spread, and the proposed system with VGI provides a better performance compared with the fixed length GI.     

Layered space-time equalization for wireless MIMO systems

We investigate layered space-time equalization (LSTE) architectures for multiple-input multiple-output (MIMO) frequency-selective channels. At each stage or layer of detection, a MIMO delayed decision feedback sequence estimator (MIMO-DDFSE) is used to tentatively detect a group of selected data streams, among which a subgroup of data streams are output and are canceled from the received signals. With the proposed architectures, the numbers of the tentatively detected data streams and output data streams can be different at different LSTE stages, while the MIMO-DDFSE can also reduce to the special cases of multiple-input single-output decision feedback equalizer (MISO-DFE), MISO-DDFSE, and MIMO-DFE, allowing tradeoffs between performance and complexity. We also derive the equalizer coefficients, discuss timing recovery, and consider channel estimation. Simulation results demonstrate the performance of the proposed LSTE structures and the tradeoffs between performance and complexity of the multistage structure and the single-stage version. We also demonstrate the impact of imperfect channel estimation, imperfect interference cancellation, the number of receive antennas, filter length, and oversampling on performance.     

Electrical equalization for advanced optical communication systems

We investigate equalizers for electronic dispersion compensation (EDC) of dispersion limited optical fibre communication links in combination with different modulation formats. We show that the performance of conventional equalizers including feedforward equalizer (FFE) and decision feedback equalizer (DFE) are fundamentally limited by the nonlinearity of square-law detection of the photodiode in direct detection systems. Advanced modulation formats such as differential phase shift keying (DPSK) and optical duobinary further enhance this kind of nonlinearity and degrade further FFE/DFE performance. However, nonlinear FFE–DFE and maximum likelihood sequence estimation (MLSE) take into account the mitigation of nonlinear inter symbol interference (ISI) and hence can achieve much better performance. We show that in contrast to other modulation formats, optical single sideband modulation results in approximately linear distortions after detection and thus a simple linear FFE equalizer can achieve good compensation.     

Single-channel blind equalization for GSM cellular systems

This paper focuses on the study of blind equalization global system for mobile communications (GSM) systems using a single antenna. In order to utilize the well-known linear system model in conventional studies of blind equalization, an equivalent baseband quadrature amplitude modulation (QAM) approximation is used for the nonlinear GMSK signal in GSM systems. Since the GMSK signal in GSM has very little excess bandwidth to warrant oversampling, a derotation scheme is developed to create two subchannels for each received GMSK signal sampled at the baud rate. Linear approximation of the GMSK signal makes the traditional QAM blind equalization system model applicable for GSM. Derotation induces channel diversity without an additional antenna and reduces the number of necessary radio frequency (RF) receivers (sensors) without increasing hardware or computational costs. Several second-order statistical and higher order statistical methods of blind equalization are adopted for GSM signals     

AN IMPROVED OPTIMAL CHANNEL SHORTENING ALGORITHM FOR DMT MODULATION SYSTEMS

In Discrete Multi-Tone (DMT) modulation systems, the well-known technique to overcome the Inter-Carrier Interference (ICI)/Inter-Symbol Interference (ISI) caused by the inadequate Cyclic Prefix (CP) length is to use a Time-Domain Equalizer (TDE) at the receiver front-end. An algorithm used to calculate the coefficients of the optimal shortening Time Domain Equalizer (TDE) was given by Melsa. However, this algorithm requires that the length of the TDE must be smaller than or equal to the memory length of the target impulse response. This paper modifies this algorithm and makes it not only fit for calculating the coefficients of the TDE with arbitrary length, but also have a much less computational time.