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.

Finite-state Markov Model for Lognormal,Chi-square (Central), Chi-square (Non-central), and <Emphasis Type="Italic">K</Emphasis>-distributions

This paper formulates a finite-state Markov channel model to represent received signal-to-noise (SNR) ratios having lognormal, K-distribution, chi-square (central) and chi-square (non-central) distributions in a slow fading channel. The range of the SNRs is partitioned into a finite number of states following earlier works in literature. Performance measures like level crossing rates, steady-state probabilities, transition probabilities, and state-time durations are derived, and numerical results are plotted and discussed for the FSMC models for all the distributions.

Diversity Combining and SNR Estimation for Turbo-Coded Frequency-Hopped Spread-Spectrum in Partial-Band Interference and Fading Channels

We compare different combinations of the repetition diversity order L and code rate R for turbo-coded Frequency-Hopped Spread-Spectrum (FH/SS) communication systems in the presence of fading and partial-band Gaussian interference. For a fixed overall channel code rate R/L we show that using the lowest code rate and no repetition diversity always performs better than using a higher code rate and some repetition for both coherent and non-coherent schemes. We then propose a simple maximum-likelihood-based method for signal-to-noise-ratio (SNR) estimation in Non-Coherent Binary Frequency Shift Keying (NCBFSK) without training symbols. Except for impractically small hop sizes of 8 bits or less we obtain performance virtually equal to that of perfect SNR knowledge but with much less complexity than iterative schemes previously proposed. For the case of Coherent Binary Phase Shift Keying (CBPSK) we derive the Expectation Maximization (EM) estimate of the SNR without training symbols and iteratively feed the estimator with the extrinsic information from the turbo decoder. The performance for CBPSK is near that of perfect SNR knowledge for hop sizes of 64 bits or more. Unlike previously proposed methods for CBPSK the EM estimate of SNR does not require knowledge of the noise and interference variance, received bit energy, or the fading channel model.

The Adaptive PSAM Design in Cross-Layer

The purpose is to design the pilot symbols efficiently and adaptively in communication systems to guarantee the minimum channel estimation error and the required quality of service (QoS). The various adaptive schemes about pilot symbols assisted modulation (PSAM) are obtained in terms of the pilot-insertion frequency and the power arrangement. The pilot-insertion frequency is redefined according to the sampling theory and the characteristics of fading channels, such as the level crossing rate (LCR) and the average fading duration (AFD). The power arrangement is optimized by maximizing the average spectral efficiency (ASE) in cross-layer design system. Besides, the ASE function in such a system is modified taking the imperfect channel state information caused by channel estimation into considered. The closed-form expression of modified ASE is present in this paper. From both the theoretical analysis and simulation points of view, a suitable scheme about PSAM design is obtained for cross-layer design.

Single-Carrier Single-Block Differential Space–Frequency Coding Transmission Over the Frequency-Selective Fading Channels

In this paper, a single-carrier single-block differential space-frequency block coding scheme for multiple input multiple output frequency-selective fading channels is proposed. In the proposed scheme, an alternative constant modulus single-carrier transmission is adopted, which significantly mitigates the sensitivity to the nonlinear distortion while having comparable lower complexity to the orthogonal frequency division multiplexing modulus. Based on this, subgrouping the signal transmit matrix through the block matrix method and fatherly differential space-frequency complex orthogonal coding on each subblocks, it not only transmits the differentially encoded signal matrix within one symbol block periods regardless of the number of transmit antennas, but also achieves the available spatial and frequency diversities without the requirement of multichannel estimation at the receiver. In the proposed scheme, it is only required that the fading channels keep approximately constant within each subblock during one symbol block transmission period, and thus can be more robust and effective to combat the channel rapidly fading with even lower bit error ratio. Theoretical analysis and corroborating simulation under various channel conditions shows that, our proposed scheme yields superior performance to previously proposed differential schemes.

Adaptive Bit Loading and Transmit Diversity for Iterative OFDM Receivers

In this paper, we consider a transmission system employing orthogonal frequency division multiplexing with bit-interleaved coded modulation and perfect channel state information at both transmitter and receiver. An adaptive bit loading scheme in combination with cyclic delay diversity and discontinuous Doppler diversity is proposed at the transmitter and iterative demapping and decoding at the receiver. The loading procedure minimizes the bit-error rate at the decoder output, and the transmit diversity schemes mitigate channel correlations. We analyze the iterative receiver with extrinsic information transfer charts and present the achievable gains.

A Decorrelating-MRC Based Space–Time Multiuser Receiver for Time-Hopping UWB System

In this paper, we propose a two-stage linear multiuser detector (LMD) for ultra wideband (UWB) multiple single-input multi-output (M-SIMO) system and multipath fading environment. Time-hopping (TH) and antipodal pulse amplitude modulation (PAM) are employed for the multiple access system. The decorrelating detector is first employed at the front end of each receive antenna to eliminate the multi-user interference (MUI), then a set of maximum-ratio-combiners (MRC) are proceeded to maximize the signal-to-noise power ratio (SNR) for each user. Since the channel information is crucial for the Decorrelating-MRC (D-MRC) receiver, we develop a subspace-based blind M-SIMO channel estimation method. The effect of channel estimation error on system performance is extensively evaluated. It is also verified from the analytical and numerical results that by exploiting both spatial and temporal diversities, the D-MRC receiver dramatically improves system performance even without additional coding. Moreover, we demonstrate that both the D-MRC receiver and subspace-based blind channel estimator are computationally feasible and near-far resistant.

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.

Adaptive Max SNR Packet Scheduling for OFDM Wireless Systems

In this paper, we consider scheduling and resource allocation for a downlink in a wireless OFDM system. If each broadcast sub channel is allocated to a user according to max SNR selection, optimal system throughput is obtained for the cost of a significant loss in fairness among users. As a solution to resolve this issue and in an attempt at reaching a compromise between fairness and throughput, we propose to add to the max SNR scheme a weak control based on user QoS requirements. In this work, user latency between two successive channel accesses is considered as a parameter for the control. The feedback of quantized channel state information (CSI) is proposed to reduce the feedback burden. Performance analysis of the proposed scheme has been presented to illustrate the capacity-fairness-feedback trade-off of the considered scheme compared to max SNR and proportional fair algorithms used as benchmark.

Comments and Further Results on ‘Performance Evaluation of a New BPSK-OFDM Timing Estimation Algorithm’

This comment first points out some inaccurate formulae and irrelevant comparisons between Wen’s algorithm and Beek’s one. Then, the correct formulae are provided, the limitations of these two estimators are discussed and some investigations on the local maximum in timing estimation are also presented. Both the simulation and the analysis demonstrate that the timing precision of Wen’s algorithm highly depends on the length of cyclic prefix (CP) and the channel fading rate. On the other hand, Beek’s algorithm will work better under a large CP length and in fast fading channels.

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.

Directional Diversity of Smart Antenna in LAS CDMA Systems

The performance of code division multiple access (CDMA) systems can be affected by small scale fading such as Rayleigh fading channel. In this paper, the application of smart antenna and Large Area Synchronous CDMA (LAS CDMA) systems, which introduce directional diversity channel, is presented. A novel interference cancellation scheme through dynamic space code (DSC) algorithm is briefly described. The directional diversity can be realized from the directional gain of smart antenna system. It can be found that when the number of elements in smart antenna is increase the directional gain of antenna system is much higher than single antenna. The system performance analysis in term of error probability is compared between traditional and LAS CDMA systems in both single and smart antenna systems. From the performance analysis it is found that CDMA system is more susceptible to multipath fading channel than interferences from existing users.

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.

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.

A Channel based Fair Scheduling Scheme for Downlink Data Transmission in TD-CDMA Networks

High speed data transmission in wireless networks demands better radio resource management schemes. A fair packet scheduling is proposed for downlinks of a cellular TD-CDMA system for delay-tolerant applications. It is based on the combined consideration of channel conditions, required throughput and achieved average throughput. A system dependent parameter is introduced to control the maximum achievable date rate as well as the degree of fairness. Through analysis and simulation, we study the tradeoff between system throughout (i.e., efficiency) and individual throughput (i.e., fairness). The relative performance between the proposed scheme and the traditional schemes is evaluated through simulation to confirm the analytical observations. The sensitivity of the system tolerance factor towards efficiency and fairness is also investigated.

Design and Performance Evaluation of Space–time Receivers with Dynamically Adjustable Tap Delays

The delay elements of a conventional two-dimensional space–time receiver are fixed. The rather inflexible design is inappropriate since the received signals among the antenna branches are correlated due to limited spacing, and the fading environment is time-varying. In this paper we propose a dynamic finger assignment strategy for a space–time receiver, where the delay taps are adjusted dynamically, based on the theory of finite projective planes, to improve the signal-to-noise ratio in correlated antenna branches. The proposed finite projective plane (FPP) based finger assignment scheme maximizes the collected energy by dynamically setting the tap delays depending on the correlation among antenna branches and the delay spread of the channel. In the performance evaluation, we demonstrate that the FPP based finger assignment scheme can improve the signal-to-noise ratio significantly in comparison to the conventional mesh-grid configuration when the correlation among antenna branches is high and the channel dispersion is relatively long in comparison to the overall span of the fingers. However, the gain is moderate if the channel dispersion ratio is short, where the gain of reduced correlation among antenna branches must compensate for the reduced signal-to-noise ratio due to the missed signals.

Performance of MIMO System Through Nakagami Fading Channel with Arbitrary Fading Parameters

In this paper, the analytical and simulation results of the bit error rate (BER) performance for a multiple-input multiple-output (MIMO) system with an arbitrary number of transmit and receive antennas are developed for the uplink transmission. The fading channel is assumed to follow Nakagami-m distribution with correlation among branches. The BER is expressed in terms of Lauricella’s multivariate hypergeometric function for both independent and correlated antenna branches for BPSK system.

Low-Complexity PAPR Reduction of MC-SS Signals in PAN

A peak-to-average power ratio (PAPR) reduction scheme with low complexity is proposed for the multicarrier spread spectrum (MC-SS) system in personal area network (PAN). Traditional clipping and filtering scheme requires a high oversampling rate to meet the emission mask requirements. This would cause high power consumption for mobile PAN devices in personal network. To solve the problem, upsampling is introduced between clipping and filtering in this paper to reduce the oversampling rate. A simplified implementation structure is also derived for the proposed scheme. Simulation results show that its complexity is about 65% of the conventional scheme while achieving satisfying performance.

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.

Spectrum Efficiency Evaluation for MRC Diversity Schemes Under Different Adaptation Policies Over Generalized Rayleigh Fading Channels

In this paper, closed-form expressions for the capacities per unit bandwidth for Generalized Rayleigh fading channels are derived for optimal power adaptation, constant transmit power, channel inversion with fixed rate, and truncated channel inversion adaptation policies. The closed-form solutions are derived for the single antenna reception (without diversity combining) and MRC diversity reception cases. Optimal power adaptation policy provides the highest capacity over the other adaptation policies both with and without diversity combining. Truncated channel inversion policy suffers a large capacity penalty relative to the optimal power adaptation policy as the number of degrees of freedom is increased. However, with increase in diversity, the capacity penalty for the truncated channel inversion policy decreases. Capacity gains are more prominent for channel inversion with fixed rate policy as compared to the other adaptation policies.