For rapid deployment, Internet of Things (IoT) technology operating in nonterrestrial networks (NTNs) usually follows 3GPP specifications. The 3GPP aims to establish unified specifications for global narrowband IoT (NB-IoT) operations. This article investigates a unified random access (RA) process for an NB-IoT protocol seamlessly operating between NTNs and terrestrial networks without global navigation satellite system (GNSS) connectivity. For high accuracy and backward compatibility, the common Doppler shift is employed for pre- and post-compensations at a narrowband physical RA channel (NPRACH) receiver; meanwhile, a common propagation delay is employed for pretime advance (Pre-TA) and RA opportunity (RAO) postponement at the NPRACH receiver on the satellite. Derivations of joint maximum likelihood estimation result in a bank of matched filters (MFs), whose outputs are employed to test for the presence of user equipment (UE). The squared magnitudes of the symbol-level MF (SLMF) outputs are coherently accumulated to improve the signal-to-interference-plus-noise ratio performance of the proposed NPRACH detector, instead of the conventional extension of the length of the MF impulse response. The residual frequency error and excess timing delay are detected at half of the subcarrier spacing and half of the symbol duration, respectively, via a grid search. The false alarm and miss probabilities are derived in the presence of a high Doppler frequency, long differential delay, random initial phase errors, and different channel gains through which the preambles of multiple UEs propagate. Computer simulations confirm the theoretical analyses and show that the proposed technique performs effective RA tasks with low miss probabilities.

This paper proposes a rake receiver to convey a bank of symbol-level correlators (SLCs) on each arm for joint maximum likelihood estimation (JMLE) of residual frequency error (RFE) and residual time error (RTE) after narrowband physical random access channel (NPRACH) preamble detection is completed. The proposed technique exploits multi-rate signal processing to decimate (i.e., downsample) the received signal for reducing subsequent computational complexity and to interpolate (i.e., upsample) in the frequency domain to enhance the accuracy of RFE estimation. Coherent accumulation of the squared magnitudes of SLC outputs further improves the signal-to-interference-plus-noise ratio (SINR). The proposed approach offers flexibility and practicality, enabling highly accurate estimation of RFE and RTE over extended ranges beyond one subcarrier spacing and beyond one symbol duration, respectively, while achieving precision at the fractional levels of a subcarrier spacing and a symbol duration. Simulation results confirm effectiveness of the proposed technique.

Owing to limited radio resources (RRs), nonorthogonal pilot sequences are usually used in time-division duplex massive multiple-input multiple-output communications, resulting in pilot contamination (PC) problems that are detrimental to practical maximum-likelihood channel estimation (CE). Minimum mean-square error (MMSE) CE, which requires a priori covariance matrices, has become preferable. Conventional studies exploited multicell cooperation to overcome PC problems and calculated sample covariance matrices on extra RRs to replace the covariance matrices required for MMSE CE. This paper proposes a novel CE technique based on multistep Kalman filtering (KF) to effectively overcome PC problems without the need for multicell cooperation or extra RRs. To account for both spatial and temporal correlations, the wide-sense stationary uncorrelated scattering channel model is statistically characterized to develop signal and observation models of the KF. The proposed KF-based CE technique works iteratively in pilot and data-aided (DA) modes to not only achieve MMSE CE but also address the outdated-CE problem. The covariance matrix of the channel vector between the UE of interest and the serving base station can be replaced by a sample covariance matrix that is iteratively evaluated during the DA mode. The sum of the covariance matrices of the channels through which all cochannel interference (CCI) UEs propagate is replaced by a single sample covariance matrix that is iteratively evaluated during the pilot mode. The simulation results show that the proposed technique achieves pilot decontamination without extra requirements or computational power for individual sample covariance matrices. This results in a low mean-square error, close to that obtained with complete a priori knowledge of all covariance matrices of the desired channel and all CCI channels.

This paper proposes a novel technique to effectively detect superimposed preambles transmitted via narrowband Internet-of-Things (NB-IoT) physical random access channels (NPRACHs). Because the quasi-stationarity of these channels conserves local phase coherence over a few symbols, a symbol-level matched filter (SLMF) is used to effectively reduce out-band noise and interference without excessively accumulating phase increments occurring with the cross-correlation operations. The squared norms of the SLMF outputs are accumulated to test for the presence of a user equipment (UE). The false alarm and miss probabilities in the presence of frequency selectivity, time selectivity and different channel gains through which the preambles of multiple UEs propagate are rigorously derived. To reduce intercarrier interference (ICI), which is mainly caused by different carrier frequency offsets (CFOs), a forward error reduction method based on interference regeneration and cancellation (IRnC) is studied. The IRnC method effectively reduces the miss probability but is not able to reduce the false alarm probability because the ICI cannot be perfectly regenerated and completely canceled. In addition, a closed-loop CFO compensation method is proposed to effectively maintain low false alarm and miss probabilities by sending quantized normalized CFO (NCFO) estimates back to the corresponding UEs via random access response (RAR) and reducing the CFO accordingly at the transmission end. Computer simulations confirm the theoretical analyses and show that the proposed technique achieves low false alarm and miss probabilities.

This paper proposes a novel optimum combiner that raises the time-averaged signal-to-interference-plus-noise ratio (SINR) for maximal-ratio combining (MRC) reception over spatially correlated Nakagami-m fading channels. By exploiting the principal component analysis (PCA) with eigenvalue decomposition (EVD) of the covariance matrix analytically evaluated offline, a complete set of orthonormal basis functions can be obtained. A decorrelation process analyzes and then projects the received signal into the space spanned by the basis functions. In accordance with the theorem of irrelevance, a principal component selection combining (PCSC) method is proposed to remove components in a few dimensions in which SINRs are considered low to raise the resulting time-averaged SINR on the subsequent MRC reception. The proposed technique also avoids noise enhancement occurring with MRC reception in the scenario where noises on different branches are correlated. The SINR distribution, level-crossing rate (LCR) and average fade duration (AFD) are derived. Based on a novel scattering model interpretation, a simulator consisting of spatially correlated Nakagami-m fading channels is developed according to the analytically evaluated covariance matrix. Computer simulations show that the proposed optimum combiner not only decreases interference and noise from the irrelevant subspace to achieve higher time-averaged SINR and lower AFD but also significantly reduces the complexity required for subsequent signal processing.

In this paper, we study an amplify-and-forward relay network with energy harvesting (EH) source and relay nodes. Both nodes can continuously harvest energy from the environment and store it in batteries with finite capacity. Additionally, the source node is capable of transferring a portion of its energy to the relay node through a dedicated channel. The network performance depends on not only the energy arrival profiles at EH nodes but also the energy cooperation between them. We jointly design power control and transfer for maximizing the sum rate over finite time duration, subject to energy causality and battery storage constraints. By introducing auxiliary variables to confine the accumulated power expenditure, this non-convex problem is solved via a successive convex approximation approach, and the local optimum solutions are obtained through dual decomposition. Also, when channels are quasi-static and the power control values of the source (relay) node are preset to a constant, a monotonically increasing power control structure with the time is revealed for the relay (source) node with infinite battery capacity. Computer simulations are used to validate the theoretical findings and to quantify the impact of various factors, such as EH intensity at nodes and relay position on the sum rate performance.

Human dynamics are investigated for deployment of picocell in cellular network in this paper. Human dynamics may lead to variation of channel condition, e.g., moving fast or slow, driving in a mega city or in a countryside may change channel conditions. The data delivery throughput depends on user distribution, commuting trajectory, and Internet access regularity. Effectiveness of frequency reuse assisted from picocell deployment is affected by human dynamics. By applying human dynamics to a network management strategy, the effectiveness of frequency reuse can be improved in practice. Pricing strategy and resource management can be reconsidered according to users' loyalty with respect to a service provider. Novel pricing strategies based on user's request-to-receive ratio are also proposed. Computer simulations verify the improvements obtained using the proposed methods.

This paper proposes a correlated Nakagami-rn fading simulator incorporating phase components. A phase difference distribution is exploited to evaluate the covariance matrix of the random vector whose elements are complex-valued, correlated channel weights. Several uncorrelated Jakes' fading simulators are then employed to synthesize correlated Nakagami-m fading channels accordingly to maintain the Clarke's spectrum. Envelope profiles, probability density functions of the envelopes and the phase differences, autocovariance functions and power spectral density are simulated to compare with the theoretical analyses. Simulation results exhibit the effectiveness of the proposed spatially correlated, complex-valued Nakagami-m fading simulator.

A perturbation-based power allocation and multirelay selection approach is proposed for multiple-input multiple-output relay networks in multipath fading channels. In this approach, the relays are partitioned into two groups according to Lagrangian multipliers of power constraints. The power allocation for the relays is perturbed by increasing the power for the potential relay's group, while decreasing the power of the relays in the other group. An optimization framework is then formulated as a tradeoff between the relay selection and the mean square error performance degradation. Computer simulations are used to demonstrate the performance.

Energy harvesting and wireless energy transfer are capable of relieving the battery limitation of wireless devices. In this paper, an amplify-and-forward relay network (AF-RN) is considered, where an energy harvesting source node communicates with a destination node through an energy harvesting relay node. To further improve the performance, the relay is allowed to harvest energy from the radio frequency (RF) signals sent by the source node through a dedicated energy control channel. A joint power control and energy transfer scheme is investigated with the goal of maximizing the achievable sum rate by a deadline subject to energy causality constraints. The problem is challenging in that the objective function is non-convex, and a successive convex approximation approach is proposed to achieve the optimal power control and energy transfer solution. Finally, numerical examples are given to demonstrate the effectiveness of our proposed algorithm.

This paper investigates a disintegrated channel estimation technique required to accomplish the spatial diversity supported by cooperative relays. The relaying strategy considered herein is a filter-and-forward (FF) relaying method with superimposed training sequences for separately estimating the backhaul and access channels. To reduce inter-relay interference, a generalized filtering technique is proposed and investigated thoroughly. Unlike the interference suppression method commonly used in the conventional FF relay networks, the generalized filtering matrix essentially multiplexes the superimposed training sequences from different relays to the destination by time-division multiplexing, frequency-division multiplexing, and code-division multiplexing methods. The Bayesian Cramér-Rao lower bounds (BCRBs) for this channel estimation problem are derived as the estimation performance benchmarks. The mean square errors (MSEs) of the disintegrated channel estimates are also derived. Finally, the improvements offered by the proposed technique are verified by comprehensive computer simulations in conjunction with the calculations of the BCRBs and the MSEs derived in this paper.

This paper proposes an initial synchronization architecture for the sector search process in third-generation partnership project (3GPP) long-term evolution (LTE) communications. The proposed system, consisting of three techniques, takes intercell interference, intercarrier interference and multipath fading into consideration with assistance from inherent diversity. The outage and detection probabilities are derived by taking multisector diversity into account in the coarse timing alignment. The long-lag differential correlator can achieve approximately 4.7-dB signal-to-noise ratio (SNR) gain in primary synchronization sequence (PSS) acquisition probability. Joint estimation of the residual timing error and the fractional frequency offset is achieved by evaluating the short-lag autocorrelation at an orthogonal frequency division multiplexing (OFDM) symbol duration. Mean-square errors obtained via simulations are compared with the modified Cramér-Rao lower bounds derived here. The proposed technique can achieve approximately 6.9-dB SNR gain when it is compared with that assisted from the synchronization signalling. Joint detection of the integral frequency offset (IFO) and the sector identification (SID) is achieved by exploiting a frequency-domain (FD) matched filter (MF) to deal with frequency selectivity. Differential detection on a segmental FD MF is considered in frequency-selective environments. Computer simulations are conducted to verify that the proposed technique can achieve high IFO and SID detection probabilities.

In this paper, we focus on designing energy-efficient power allocation schemes to improve the energy efficiency (EE) in multiuser multi-carrier two-way relay networks which are able to not only balance the EE of the two-way links but also ensure the quality-of-service (QoS). Specifically, the proposed design framework attempts to maximize a ratio of the spectral efficiency (SE) over the total network power consumption under a total power constraint as well as a signal-to-interference plus noise power ratio (SINR) constraint. The original problem is indeed non-convex, and we prove the convexity of the problem after a series of convex transformation. An iterative approach is proposed to find the local optimal solution of the original problem for achieving the maximum EE. Simulation results are provided to demonstrate the tradeoff between the EE and the SE.

Estimations of time error and frequency offset upon initial synchronization are investigated in this paper as components of the sector search process in third-generation partnership project (3GPP) long-term evolution (LTE) communications. The proposed system considers intercell interference, inter-carrier interference and multipath fading by exploiting various diversity. A coarse timing alignment is achieved by the acquisition of a primary synchronization sequence. The coarse timing alignment extracts autocorrelation at a long lag of one half of a frame length from the decimated signal at a lower sampling rate by exploiting a differential correlator. Estimations of residual time error and fractional frequency offset are achieved by evaluating the short-lag autocorrelation at an OFDM symbol duration. The estimations utilize 70 cyclic prefixes that are pre-stored in the buffer employed in the aforementioned differential correlator. The estimations can effectively overcome the time variation of the channel that occurs as a result of a Doppler spread. Comprehensive computer simulations are conducted to verify that the proposed technique can effectively overcome inter-cell interference, time-selectivity, and multipath fading, thereby achieving low estimation errors.

Energy efficiency is a growing concern for the future wireless networks as energy consumption becomes a global environment problem. In this paper, an energy-efficient power control scheme is investigated for achieving the maximum energy efficiency in multiuser two-way balancing relay networks. We formulate the design problem as a ratio of the spectral efficiency over the entire energy consumption of the network under a total power constraint. An optimal power control scheme is proposed to iteratively improve the efficiency and finally reach the globally optimal solution. Compared with a heuristic scheme where the total available power is equally distributed among all nodes, the proposed scheme can dramatically improve not only the energy efficiency but also the spectral efficiency.

The rapid growth of diversified applications has led to significant increase in data traffic and energy consumption in wireless networks. Hence, the energy efficiency becomes one of critical performance indices for designing next-generation wireless networks. In this paper, an optimization framework of power allocation is investigated for maximizing energy efficiency in multiuser relay networks. Under a total power constraint, we formulate the design problem with the objective as the ratio of the spectral efficiency over the entire power consumption of the network. An optimal energy-efficient power allocation algorithm is proposed for the source and the relay nodes to approach the maximum efficiency in an iterative manner. Compared with a heuristic scheme where the total available power is equally allocated to all nodes, the proposed optimal power allocation algorithm can dramatically improve energy efficiency with a slight loss in spectral efficiency.

Cooperative networks recently became attractive because they can achieve spatial diversity. Multiple decode-and-forward (DF) relay nodes result in multiple carrier frequency offsets (CFOs) because each relay node owns its local oscillator. This paper presents a novel semi-blind multiple-CFO estimator for DF OFDM cooperative networks. A procedure is designed to effectively derive the semi-blind multiple-CFO estimator for relay nodes. Each relay node occupies its own subchannels, and its CFO falls in a nonoverlapped spectrum. The CFO mapping can simply match the corresponding relay nodes. In addition, the proposed method can reduce hardware and computational complexity. Semi-blind random sequences are derived from the characteristics of the signal matrix in the multiple signal characterization (MUSIC) algorithm. Comprehensive simulations show that random sequences can replace periodic training sequences. Furthermore, some information can be conveyed by random sequences to increase spectral efficiency.

An initial synchronization technique based on novel estimations of the time error and carrier frequency offset (CFO) is investigated in this paper to operate in frequency-selective fading environments. Based on motivation from statistical derivations, a novel estimator is proposed by embedding matched filters (MFs) into the RAKE fingers to approach the modified Cramer-Rao lower bounds (MCRLBs). Meanwhile, a dual chirp signal is proven to have the ability to decorrelate the performances between the time-error and the CFO estimators. By taking advantage of pseudo-noise (PN) MFs, the individual channel tap-weighting coefficients can be extracted from the interpath interference on a path-by-path basis. The proposed technique is then built to approach the MCRLBs by taking advantage of the maximum ratio combining (MRC) criterion. In practice, the proposed technique can significantly outperform a conventional initial synchronization technique that is not assisted by the diversity in terms of higher probabilities of burst acquisition and lower mean-square errors (MSEs) on the time-error and CFO estimations over multipath fading channels. Comprehensive computer simulations were conducted to verify the improvements achieved using the technique that is statistically derived in this paper.

The deployment of relays has been regarded as an effective means to improve the performance of the conventional wireless networks. The achievable performance generally depends on the availability of channel state information (CSI) at the relay and the destination nodes. The requirement of the dual-hop CSIs at the destination node, however, results in high computational complexity and unrealistic assumption for mobile relay networks, especially when the source-to-relay channel fluctuates rapidly due to the mobility of the source node. In this paper, we design an amplify-and-forward (AF) mobile relay network in which the instantaneous CSI of the source-to-relay link is unknown to the destination node. A composite hypothesis testing problem is formulated to derive the optimal detector. Unlike the existing approaches, the designed mobile relay network does not require excessive signalling overhead for obtaining the instantaneous CSI of the source-to-relay link at the destination while offering a significant performance improvement over the conventional wireless networks without the help from relays. The obtained performance even approaches that of the relay networks with full CSIs. When considering both implementation complexity issues and performance enhancement, the proposed detection scheme provides an alternative way of developing a low-complexity mobile relay networks.

This paper jointly designs power allocation and two-dimensional (2-D) equalizers for multiple relay nodes in a distributed multiple-input multiple-output (MIMO) relay network. Based on the minimum mean-square error (MMSE) criterion, 2-D temporal-and-spatial equalizers are investigated at relays with a total equalizer power constraint for equalizing-and-forwarding the signals from the source to the destination in multipath fading channels, which has not been addressed in the existing literature. A bisection algorithm is proposed to obtain the optimal solution by utilizing the Karush-Kuhn-Tucker (K.K.T.) conditions. With the proposed 2-D equalizers, the distributed MIMO relay network can not only effectively mitigate the inter-symbol interference (ISI) and multiple-antenna interference (MAI) but also achieve the spatial and multipath diversity gains. Simulation results show that the proposed scheme can provide a substantial performance gain in terms of the bit error rate (BER) as compared with the conventional one-tap equalizer scheme for distributed MIMO relay systems.

This paper considers an equalize-and-forward (EF) strategy for two-hop multiple-input multiple-output (MIMO) relay networks in multipath fading channels, where the relay nodes and the destination only know its respective backward channel state information (CSI) knowledge, and each node is equipped with multiple antennas for transmitting, receiving or forwarding signals. For such a relay network, the inter-symbol interference (ISI) and multiple-antenna interference (MAI) are two detrimental effect to degrade the bit error rate (BER) performance. In order to compensate for the interference problem, we design temporal-and-spatial (TS) equalizers to assist in forwarding and decoding the signals at the relay nodes and the destination node, respectively. Based on the minimum mean square error (MMSE) criterion with a total power constraint, an optimization framework is formulated to find the optimal TS equalizers with the backward CSI knowledge, and an iterative algorithm using the Karush-Kuhn-Tucker (K.K.T.) conditions is investigated to achieve the optimal solution. With these optimal TS equalizers, the MIMO relay network can not only effectively mitigate the interference but also provide both the spatial and multipath diversity gains. Simulation results indicate the effectiveness of the proposed algorithm, yielding a significant improvement on the BER performance.

Frequency error, non‐ideal channel estimation (CE) and inefficient seamless road side unit (RSU) service are critical issues occurring in conventional systems that are completely specified by current IEEE 802.11p standards. This study investigates novel techniques for achieving accurate frequency offset compensation and effective CE and RSU selection for handover in signal‐overlapping areas. Recently, communications using data‐aided orthogonal frequency‐division multiplexing (DA‐OFDM), such as pseudo‐random‐postfix OFDM and time‐domain‐synchronous OFDM (TDS‐OFDM), have been actively studied because of their higher effectiveness, efficiency and better transmission quality. This study consists of three parts: (i) derivation of the maximum likelihood estimation for DA‐OFDM, (ii) design of an accurate RSU selection scheme and (iii) implementation of a superior TD CE technique. Performance comparisons between the conventional and the proposed techniques are conducted through comprehensive computer simulations.

In this paper, a novel technique is proposed to address the joint sampling timing acquisition for baseband and broadband power-line communication (BB-PLC) systems using Orthogonal-Frequency-Division-Multiplexing (OFDM), including the sampling phase offset (SPO) and the sampling clock offset (SCO). Under pairwise correlation and joint Gaussian assumption of received signals in frequency domain, an approximated form of the log-likelihood function is derived. Instead of a high complexity two-dimension grid-search on the likelihood function, a five-step method is employed for accurate estimations. Several variants are presented in the same framework with different complexities. Unlike conventional pilot-assisted schemes using the extra phase rotations within one OFDM block, the proposed technique turns to the phase rotations between adjacent OFDM blocks. Analytical expressions of the variances and biases are derived. Extensive simulation results indicate significant performance improvements over conventional schemes. Additionally, effects of several noise models including non-Gaussianity, cyclo-stationarity, and temporal correlation are analyzed and simulated. Robustness of the proposed technique against violation of the joint Gaussian assumption is also verified by simulations.

This paper proposes a novel channel estimation technique for Single Carrier Frequency Division Multiple Access (SC-FDMA) communications operating on the Long Term Evolution (LTE) uplink over time-varying multipath channels. The proposed method keeping the block-type pilot arrangement (BTPA) can improve the performance of the conventional channel estimation (CE) technique with no assistance from apriori knowledge of the channel statistics. In addition, the proposed method based on a sliding windowing can mitigate the interferences effectively with low computational complexity. Extensive computer simulations verify the superiority and improvements achieved by the proposed technique.

The primary challenge in the current IEEE 802.11p draft standard is the weak performance in multipath environment. Hence a time-domain (TD) and frequency-domain (FD) channel estimation techniques that can enhance the performance of channel estimation are proposed in this paper. The TD technique makes a little modify in current standard to use the symbol prefix to assist channel estimation and obtain remarkable improvement. In addition, the FD pseudo-pilot technique change nothing in the standard and can obtain adequate improvement. Furthermore, this paper also discuss how to employ the preamble to improve the FD channel estimation performance. The channel models adopted for simulation are referencing HIPERLAN/2 and a modified exponential model.

A channel estimation (CE) technique that uses a postfixed pseudo-random sequence padded with zero samples is investigated to extend pseudo-random-postfix orthogonal-frequency-division-multiplexing (PRP-OFDM) communications onto mobile applications. The proposed technique takes advantage of pseudo-noise sequences periodically inserted after each OFDM block within the original guard interval (GI) in conventional zero-padded OFDM (ZP-OFDM) or within the original cyclic prefix (CP) in conventional CP-OFDM. This allows accurate estimation of the channel impulse response. In addition, the proposed technique takes advantage of the null samples padded after the PN sequences for reducing inter-symbol interference occurring with the CEs and with the overlap-add information detection in the conventional PRP-OFDM. The proposed technique successfully applies either (1) a least-squares (LS) CE with assistance from decision-directed data or (2) an approximate LS CE method onto PRP-OFDM-like communications for the purpose of improving CE performance on rapid time-varying channels. Computer simulations in conjunction with statistical analysis assess the performance evaluation of the proposed technique and show that it can achieve lower mean-square estimation errors and lower error probabilities in mobile environments.

A channel estimation (CE) technique that uses a postfixed pseudo-random sequence padded with zeros is investigated in this paper to extend pseudo-random-postfix orthogonal-frequency-division-multiplexing (PRP-OFDM) communications onto mobile applications. The proposed technique takes advantage of pseudonoise (PN) sequences periodically inserted after each OFDM block within the original guard interval (GI) in conventional zero-padded OFDM (ZP-OFDM) or within the original cyclic prefix (CP) in conventional CP-OFDM. This allows accurate estimation of the channel impulse response (CIR). In addition, the proposed technique takes advantage of the null samples padded after the PN sequences for reducing inter-symbol interference (ISI) occurring with the CEs and the information detection in the conventional PRP-OFDM. The proposed technique successfully applies either (1) a least-squares (LS) CE with assistance from decision-directed (DD) data or (2) approximate LS (ALS) CE method onto PRP-OFDM-like communications for the purpose of improving CE performance on rapid time-varying channels. Computer simulations show that the proposed technique can achieve lower error probabilities in mobile environments.

A least-squares (LS) channel estimation (CE) technique for mobile OFDM communications over a rapidly time-varying frequency-selective fading channel is investigated in this paper. The proposed technique keeping the comb-type pilot arrangement can achieve low error probability by accurately estimating channel impulse response (CIR) and effectively tracking rapid CIR variations. In addition, the LS CE technique proposed here is conducted in the time domain (TD); meanwhile, a generic estimator is, thus, performed serially block by block without assistance from a priori channel information and without increasing computational complexity. By taking advantage of linearly frequency-modulated (LFM) or pseudorandom signals that are transceived for the purpose of sounding pilot subchannels, the proposed LS channel estimator (CE) can inherently perform pseudo-noise (PN) matched filtering (MF) to suppress multipath interference (MPI) caused by frequency-selective fading and inter-carrier-interference (ICI) resulting from data subchannels. The dual optimality of the LFM and PN pilot symbols is verified for both time-domain (TD) and frequency-domain (FD) CEs. Furthermore, the proposed technique also exhibits good resistance against residual timing errors occurring with the DFT demodulation. Extensive computer simulations in conjunction with statistical derivations show the superiority of the proposed technique.

It is well-known that in order to enhance system performance, channel estimation in coherent OFDM systems is necessary and crucial. In practice, lots of coherent OFDM systems transmit pilot symbols over some of the subcarriers for the purpose of estimating channel attenuation, and also insert cyclic prefixto avoid inter-carrier and inter-symbol interference. In this paper, a pseudo noise (PN) sequence combined with zero padding is inserted in the postfix of each OFDM symbol to estimate the channel impulse response. The major advantage of the proposed channel estimation techniques is the ability of estimating and, then, tracking the channel variations. These proposed channel estimation algorithms include least-square technique with decision-directed data-assistance (DDLS), least-square (LS) estimation and maximum-likelihood (ML) scheme with different observation windows. The proposed techniques are thus very appropriately used in presence of high mobility with large Doppler spreads. Some comparative simulations are given to illustrate the superior performance of the proposed channel estimation techniques.

Orthogonal frequency division multiplexing (OFDM) technique is effective and powerful in high data rate digital transmission due to its spectral efficiency, robustness in multipath propagation environments and ability to cope with intersymbol interference. Channel estimation is a crucial problem in coherent OFDM systems, and the various estimation techniques with particular pilot arrangements have been investigated recently. In this paper, a two-dimensional pilot-aided channel estimation technique is developed for wireless OFDM communications under severe frequency-selective fading channels. By applying least-square (LS) estimation for the leading pilot symbol in time domain, the channel statistics can be obtained and preserved. And then, LS estimation is also applied for the following regular pilot tones in the OFDM data symbol. Based on the variation of the two LS estimates in the time domain, a novel two-dimensional pilotaided channel estimation algorithm is presented. From the simulation results, the proposed channel estimation scheme has superior performance in the severe frequency-selective fading channels, especially in high SNR. Even if the mobile speed rises, the proposed algorithm still achieves excellent performance.

For original paper see IEEE Trans. Commun., vol.46, p.1516-24 (1998 November). In the original paper, the authors presented a noncoherent tracking loop with diversity and multipath interference cancellation to improve the loop performance on frequency-selective fading channels. However, the effects of imperfections in multipath interference cancellation have been dropped out in the statistical analyses, and only evaluated by using computer simulations. The present authors show that such effects can change the error characteristic, at least in the mean sense, and thus make the analyses of jitter performance improper.

Differentially coherent detection techniques for PN code acquisition in direct-sequence spread-spectrum (DS/SS) receivers are designed and analyzed in this letter. The full-period cross correlation (FPC) can be modeled as deterministic for certain types of PN sequences whose autocorrelation functions have ideal shapes the same as those of maximum length shift registration (MLSR) sequences. This model is exploited here to simplify the design and performance evaluation of the proposed detection techniques. Such techniques can be confidently employed as alternatives to noncoherent detectors because of their attractive improvements in the signal-to-noise ratio (SNR) and, thus, are very suitable for commercial code-division multiple-access (CDMA) systems, which often operate in severely noisy environments. The results of statistical analyses show that the proposed techniques can provide rapid acquisition and improved performance in SNR.

The pull-in capability of a modified code tracking loop (MCTL) for direct-sequence spread-spectrum communication is investigated on a frequency-selective fading channel. By taking advantage of the inherent diversity, the modified code tracking loop is embedded into a RAKE receiver to avoid the problem of drift and flutter of error signals caused by multipath interference; thus, a robust pull-in capability can be provided. Furthermore, a multipath interference regeneration and cancellation technique is also proposed to improve the error characteristics of the proposed technique. Analytical expressions of the error characteristics are derived and then confirmed by means of extensive computer simulation results. The very attractive behaviors obtained using the proposed technique are verified.

A modified fully digital pseudonoise code tracking loop is proposed for direct-sequence spread-spectrum communication. By taking advantage of the inherent diversity, a modified code tracking loop is embedded into a RAKE receiver in order to avoid problems caused by unstable locked points of error signals. Such unsteadiness of locked points often occurs with a conventional code tracking loop because the error signals may be randomly biased by multipath fading. Thus, a robust pull-in capability can be provided over a time-variant fading channel where multiple propagation paths are arbitrarily correlated. Furthermore, an effective multipath interference regeneration and cancellation technique is also proposed to improve the error characteristics of the proposed technique. Analytical expressions of the error characteristics and error signals are derived and then confirmed by means of extensive computer simulation results. In addition, several simulation results for the timing jitter and the mean time to lose lock are also presented. The very attractive behavior obtained using the proposed technique is verified.

Improved blind equalisation techniques based on a modified constant modulus algorithm (CMA) are proposed. This algorithm can not only accomplish blind equalization and carrier phase recovery simultaneously, but also choose a nonlinear gain with a least-squares algorithm to avoid the gradient noise amplification problem and achieve improved stability and robustness without increasing the computation complexity. It can be switched over to a decision-directed equalization scheme once the error level is reasonably low to obtain higher convergence speed and lower residual intersymbol interference as well. Extensive computer simulation results have verified the analysis and indicated the very attractive behavior of the proposed algorithm.

A modified fully-digital code tracking loop is proposed in this paper for direct-sequence spread-spectrum signaling on a frequency-selective fading channel. A data-modulated channel estimator is used to cope with the time-varying Rayleigh fading effect and the data modulation effect, and extract the desired error signal from each path independently in the multipath environments. By taking advantage of the inherent diversity with the maximal ratio combining (MRC) or a proposed Even/odd maximal ratio combining (EMRC) technique, this modified code tracking loop can avoid the problem due to the drift or flutter effects of the error characteristics, and provide better performance on frequency selective fading channels. Extensive computer simulation has verified the analysis and indicated very attractive behavior of the proposed digital tracking loop.