Effect of Carrier Frequency Offset on the Channel Capacity in Multiuser OFDM-FDMA Systems

Orthogonal Frequency Division Multiplexing (OFDM) is a very robust transmission procedure in multipath and frequency selective radio channels. A Frequency Division Multiple Access (FDMA) resource allocation technique offers the opportunity of a detailed link adaptation scheme. The combination of these transmission- and multiple access techniques in OFDM-FDMA is an ideal and very strong candidate for the downlink of future fourth generation (4G) mobile communication systems. This technical combination offers high cell capacities by exploiting the inherent multiuser diversity effect of the system. To apply OFDM-FDMA in the uplink, the time and carrier synchronization accuracy becomes very important. Non-ideal synchronization of the user signals to the carrier frequency of the base station leads to intercarrier interferences (ICI). In this paper, an analytical model for the ICI consideration in the uplink of a multiuser OFDM-FDMA based system is derived. The impact of the carrier frequency offset (CFO) on the performance of a cellular multiuser system with respect to different subcarrier allocation schemes is analyzed.

A Novel Technique for Channel Estimation and Equalization for High Mobility OFDM Systems

Orthogonal frequency division multiplexing (OFDM) technique, when used in wireless environments, is known to be robust against frequency selective fading. However, when the channel shows time selective fading, rapid variations destroy the subcarrier orthogonality and introduce inter-carrier interference (ICI). The use of ICI mitigation schemes requires the availability of channel state information (CSI) at the receiver, which is a non-trivial task in fast fading systems. In our work, we have addressed the problem of estimation of rapidly varying channels for OFDM systems. The channel is modeled using complex exponentials as basis functions and the estimation process makes use of the cyclic prefix (CP) part available in OFDM symbols as training. The system is viewed as a state space model and Kalman filter is employed to estimate the channel. Following this, a time domain ICI mitigation filter that maximizes the received SINR (signal to interference plus noise ratio) is employed for equalization. This method performs considerably well in terms of MSE as well as BER at very high Doppler spreads.

Time-varying Channel Estimation and Symbol Detection Using Superimposed Training in OFDM Systems

In mobile orthogonal frequency division multiplexing (OFDM) systems, time-varying channels result in severe intercarrier interference (ICI), and greatly degrade the system performance. So, it is necessary to estimate the accurate channel for equalization of received symbols. But, the conventional pilot-assisted channel estimation scheme consumes valuable bandwidth. In this paper, we adopt superimposed training approach for OFDM systems to estimate the time-varying channel, which is approximated by a basis expansion model (BEM). The proposed scheme is an extension of the superimposed training approach previously proposed for time-invariant channels in OFDM systems. At the same time, we employ an iterative best linear unbiased estimator (BLUE) to minimize the mean square error (MSE) of the coefficient estimates and improve the system performance. Simulation results prove the effectiveness of the proposed scheme in fast time-varying scenario.

A Novel Carrier Frequency Offset Estimation for OFDM Systems Over Time-Varying Multipath Channels

This paper introduces a novel method to estimate fractional carrier frequency offset (CFO) under time-varying multipath channels in OFDM systems based on the approximation of Basis Expansion Model (BEM). Due to the symmetrical character of the BEM basis, we propose a new design of training sequence with trailing zeros. This training sequence can be used at the receiver to estimate the frequency offset without channel estimation only in one OFDM symbol. We also compare our method with Beek’s ML estimator and Lv’s fine synchronization estimator. The numerical results demonstrate that the proposed method provides an improved performance when the training sequence has a short length.

Modified Guard Band Power Detection for Coarse Carrier Frequency Offset Recovery in OFDM System

Conventional guard band power detection (GPD) method for orthogonal frequency division multiplexing (OFDM) coarse carrier frequency offset (CFO) estimation has to be operated after the fine CFO recovery due to its sensitivity to the inter-channel interference (ICI). In the paper, modified GPD methods are presented for OFDM system to obtain more accurate estimation of CFO. The accuracy of GPD estimation can be first improved with a simple operation of power average. Then, extending the idea of Reference symbol Power Adjustment (RPA), two general patterns of sub-carrier power adjustment to improve the estimation performance under nonzero fine CFO are presented. According to complete simulations under the COST 207 multi-path fading channel, estimation performance with the presented modified GPD methods can be significantly improved and therefore can be concurrently operated with the fine carrier offset adjustment.

Maximum-Likelihood Estimator for Coarse Carrier Frequency Offset Estimation in OFDM Systems

Orthogonal frequency division multiplexing (OFDM) systems are more sensitive to carrier frequency offset (CFO) compared to the conventional single carrier systems. CFO destroys the orthogonality among subcarriers, resulting in inter-carrier interference (ICI) and degrading system performance. To mitigate the effect of the CFO, it has to be estimated and compensated before the demodulation. The CFO can be divided into an integer part and a fractional part. In this paper, we investigate a maximum-likelihood estimator (MLE) for estimating the integer part of the CFO in OFDM systems, which requires only one OFDM block as the pilot symbols. To reduce the computational complexity of the MLE and improve the bandwidth efficiency, a suboptimum estimator (Sub MLE) is studied. Based on the hypothesis testing method, a threshold Sub MLE (T-Sub MLE) is proposed to further reduce the computational complexity. The performance analysis of the proposed T-Sub MLE is obtained and the analytical results match the simulation results well. Numerical results show that the proposed estimators are effective and reliable in both additive white Gaussian noise (AWGN) and frequency-selective fading channels in OFDM systems.

Joint Carrier-Frequency Offset and Channel Estimation for MIMO-OFDM Transmission

In this paper, we consider a multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) system operating over frequency-selective fading channels. We propose a novel scheme for joint carrier-frequency offset (CFO) and channel estimation based on the expectation–maximization (EM) algorithm. Furthermore, the Cramer-Rao bounds (CRBs) for both CFO and channel estimators are exploited to evaluate the performance of the proposed scheme. Computer simulations show that the proposed algorithm achieves almost ideal performance compared with the CRBs for both channel and frequency offset estimations.

On the MIMO Channel Capacity Predicted by Kronecker and Müller Models

This paper presents a comparison between the outage capacity of MIMO channels predicted by the Kronecker and Müller models as a function of the number of scatterers, transmit and receive antennas. The Müller model is based on the singly-scattered rays between arrays of transmit- and receive antennas, while the Kronecker model considers only double scattering. The channel capacity predictions by the Müller model were observed to be higher than those by the Kronecker model. Moreover, Müller model is simpler since it is characterized by fewer parameters, and accounts for frequency selective fading whilst the Kronecker model is valid only for frequency flat fading.

Convolutional Multiplexing for Multicarrier Transmission: System Performance and Code Design

The paper presents a new multiplexing scheme, called convolutional multiplexing (CM), to achieve high diversity gain and high spectrum efficiency for OFDM-based systems. In this scheme, data symbols are spread onto several subcarriers by the convolutional spreader. Spectrum efficiency can be improved by two approaches: high order modulation and multi-code multiplexing. With the best spreading codes searched out, the multi-code multiplexing OFDM-CM system performs better in AWGN channel, but may lose diversity gain in frequency selective fading channels. On the other hand, the single-code spreading OFDM-CM system with high order modulation can achieve the maximum diversity order. Simulation results show that the multi-code convolutional multiplexing perform better than code-division multiplexing (CDM) for OFDM-based systems.

Blind Joint Symbol Detection and DOA Estimation for OFDM System with Antenna Array

An analysis of the received signal of Orthogonal frequency division multiplexing (OFDM) system with array antennas shows that the received signal has trilinear model characteristics. Trilinear decomposition-based joint symbol detection and direction of arrival (DOA) estimation for OFDM system with antenna array is proposed in this paper. The simulation results reveal that the symbol detection performance of the proposed algorithm is very close to the post-FFT receiver with perfect channel state information; DOA estimation performance is very close to least squares method, and even this algorithm supports small sample sizes. Finally this algorithm does not require the channel fading information, DOA and training sequence or pilot information, so it has blind characteristics.

Outage Performance of OSTBC in Double Scattering MIMO Channels

In this letter, taking into account realistic propagation environments in the presence of double scattering, we analyze the outage probability of orthogonal space-time block coding (OSTBC) for an arbitrary finite number of transmit and receive antennas. Closed-form expressions for the outage probability of OSTBC over Rayleigh-fading multiple-input multiple-output (MIMO) channels with double scattering are presented. Finally we give some numerical results to verify our analysis.

Uplink Channel Estimation for Multi-user OFDM-based Systems

In this paper we propose a simple, yet flexible and efficient, channel estimator for the uplink in broadband orthogonal frequency division multiplexing (OFDM) systems. The processing is performed in the time-domain, by extracting the Channel’s Impulse Response (CIR) for each user from a joint training signal. In this OFDM system, the pilot sequence we advocate, where all users share the same pilot sub-carriers, consists of one OFDM-symbol endowed with time-shifted properties per user, which isolates each user’s CIR and is robust against multi-user interference. The feasibility of our approach is substantiated by system simulation results obtained using BRAN-A broadband mobile wireless channel model.

A Novel Adaptive Detection Scheme based on Parallel Interference Cancellation for V-BLAST System

In this paper, in order to reduce the computational complexity of the detector of vertical Bell Laboratories layered space-time (V-BLAST) system over time-varying channels, an adaptive detection scheme is proposed based on parallel interference cancellation (PIC). The presented scheme has three units, which are primary adaptive detector, detection ordering determiner, and adaptive PIC detector. The proposed scheme can employ many of known adaptive algorithms for detection of V-BLAST system. In this paper, we present computational complexity of the proposed scheme using LMS, RLS, APA and AMSER adaptive algorithms and evaluate its performance with numerical simulations.

Improved Fractional Fourier Transform Based Receiver for Spatial Multiplexed MIMO Antenna Systems

In this work the performance of a Fractional Fourier transform (FrFT) based Minimum Mean Squared Error receiver for MIMO systems with space time processing over Rayleigh faded channels is presented. The proposed receiver called Optimum FrFT based MIMO receiver (OFMR) shows improved performance outperforming the simple MMSE receiver in Rayleigh faded channel.

A New Linear Group-Wise Parallel Interference Cancellation Detector

In this paper, a new linear group-wise parallel interference cancellation (LGPIC) detector is proposed. Four different group-detection schemes are derived, namely, the linear group matched filter PIC (LGMF-PIC) detector, the linear group decorrelator PIC (LGDEC-PIC) detector, the linear group minimum mean square error PIC (LGMMSE-PIC) detector and the linear group parallel interference cancellation weighted PIC (LGPIC-PIC) detector. The convergence behavior of the proposed detector is analyzed and conditions of convergence are derived. Finally, extensive simulations regarding the convergence behavior and the effect of the grouping on the convergence behavior of the proposed LGPIC detector are conducted.

Cognitive Radio Dynamic Access Techniques

The ever growing demand for wireless services has placed enormous burden on valuable resources such as spectral bandwidth. This has resulted in a major rethinking in resource allocation policies culminating in an explosion of research activity in the field of Cognitive Radio (CR) towards optimum resource usage. In this tutorial paper the physical layer design and transmission techniques for CR in the context of efficient spectrum utilization are discussed. Spectrum sensing as the key element of CR awareness is described. Orthogonal frequency division multiplexing (OFDM) as a spectrally efficient modulation scheme is discussed and the rationale for its use in the CR system is explained. Spectrum pooling for efficient use of spectrum is studied and the role of adaptive OFDM techniques in this method is highlighted. Wavelet basis function as a replacement of Fourier transform in OFDM is evaluated. MIMO system as an added value to the CR performance is described. Adaptive Waveform and beamforming as alternative techniques in CR are reviewed.

Sub-Carrier and Band Hopped Orthogonal Frequency Division Multiple Access (OFDMA) Systems

This article proposes a novel way of grouping users in an orthogonal frequency division multiplexing (OFDM) communication link based on predefined criteria. The total available spectrum is divided into a number of bands equal to the number of groups. For efficient time-frequency resource allocation, sub-carrier and band hopping are used simultaneously. Under some constraints, sub-carriers can also be assigned to different users based on known channel characteristics using dynamic sub-carrier allocation. Sub-carrier and band hopping are used for mitigating the frequency selectivity of the wireless channel and for minimizing and avoiding interference in the system. The proposed scheme is equally applicable to both downlink and uplink.

An Expression for the Average Quantization Error

The quantization error for MIMO (multiple-input multiple-output) downlink channels is known to be the minimum of independent and identically distributed beta random variables. In this note, an exact expression is derived for the average quantization error. Computational issues relating to its correctness, usage and approximations are discussed.

Cognitive Radio with Single Carrier TDCS and Multicarrier OFDM Approach with V-BLAST Receiver in Rayleigh Fading Channel

This article presents the performance comparison of TDCS and OFDM based cognitive radio for MIMO system using VBLAST receiver architecture to reconstruct the transmitted data. The interference avoidance performance in terms of BER and bitrate are improved by adding multiple antennas to the system and the use of V-BLAST technique at the receiver. The results show the most promising interference avoidance technique combined with MIMO V-BLAST architecture to be applied in the CR system.

Robust Transmission Over Fast Fading Channels on the Basis of OFDM-MFSK

We present an OFDM-based transmission scheme which is suitable for robust transmission in fast fading environments, where a reliable channel estimate is impossible or very difficult to obtain. Our scheme is based on the combination of noncoherently detected M-ary frequency shift keying (MFSK) and orthogonal frequency division multiplexing (OFDM). Noncoherent detection of OFDM-MFSK allows an arbitrary phase choice for all subcarriers in the transmitter. One possibility to exploit this degree of freedom is to choose the subcarrier phases such, that the peak-to-average power ratio (PARR) is reduced. A second possibility is to use the subcarrier phases to transmit additional data. This can be done by differentially modulating the subcarriers that are occupied by the OFDM-MFSK scheme. Both possibilities do not affect the robustness of the underlying noncoherently detected OFDM-MFSK modulation.