Analysis and optimization of pilot symbol-assisted Rake receivers for DS-CDMA systems

The effect of imperfect channel estimation (CE) on the performance of pilot-symbol-assisted modulation (PSAM) and MRC Rake reception over time- or frequency-selective fading channels with either a uniform power delay profile (UPDP) or a nonuniform power delay profile (NPDP) is investigated. For time-selective channels, a Wiener filter or linear minimum mean square error (LMMSE) filter for CE is considered, and a closed-form asymptotic expression for the mean square error (MSE) when the number of pilots used for CE approaches infinity is derived. In high signal-to-noise ratio (SNR), the MSE becomes independent of the channel Doppler spectrum. A characteristic function method is used to derive new closed-form expressions for the bit error rate (BER) of Rake receivers in UPDP and NPDP channels. The results are extended to two-dimensional (2-D) Rake receivers. The pilot-symbol spacing and pilot-to-data power ratio are optimized by minimizing the BER. For UPDP channels, elegant results are obtained in the asymptotic case. Furthermore, robust spacing design criteria are derived for the maximum Doppler frequency

Semiblind Channel Estimation and Data Detection for OFDM Systems With Optimal Pilot Design

This paper considers semiblind channel estimation and data detection for orthogonal frequency-division multiplexing (OFDM) over frequency-selective fading channels. We show that the samples of an OFDM symbol are jointly complex Gaussian distributed, where the mean and covariance are determined by the locations and values of fixed pilot symbols. We exploit this distribution to derive a novel maximum-likelihood (ML) semiblind gradient-descent channel estimator. By exploiting the channel impulse response (CIR) statistics, we also derive a semiblind data detector for both Rayleigh and Ricean fading channels. Furthermore, we develop an enhanced data detector, which uses the estimator error statistics to mitigate the effect of channel estimation errors. Efficient implementation of both the semiblind and the improved data detectors is provided via sphere decoding and nulling-canceling detection. We also derive the Cramér-Rao bound (CRB) and design optimal pilots by minimizing the CRB. Our proposed channel estimator and data detector exhibit high bandwidth efficiency (requiring only a few pilot symbols), achieve the CRB, and also nearly reach the performance of an ideal reference receiver

Blind Receiver Design for OFDM Systems Over Doubly Selective Channels

We develop blind data detectors for orthogonal frequency-division multiplexing (OFDM) systems over doubly selective channels by exploiting both frequency-domain and time-domain correlations of the received signal. We thus derive two blind data detectors: a time-domain data detector and a frequency-domain data detector. We also contribute a reduced complexity, suboptimal version of a time-domain data detector that performs robustly when the normalized Doppler rate is less than 3%. Our frequency-domain data detector and suboptimal time-domain data detector both result in integer least-squares (LS) problems. We propose the use of the V-BLAST detector and the sphere decoder. The time-domain data detector is not limited to the Doppler rates less than 3%, but cannot be posed as an integer LS problem. Our solution is to develop an iterative algorithm that starts from the suboptimal time-domain data detector output. We also propose channel estimation and prediction algorithms using a polynomial expansion model, and these estimators work with data detectors (decision-directed mode) to reduce the complexity. The estimators for the channel statistics and the noise variance are derived using the likelihood function for the data. Our blind data detectors are fairly robust against the parameter mismatch

On Multiple Symbol Detection for Diagonal DUSTM Over Ricean Channels

This letter considers multiple symbol differential detection for multiple-antenna systems over flat Ricean-fading channels when partial channel state information (CSI) is available at the transmitter. Using the maximum likelihood (ML) principle, and assuming perfect knowledge of the channel mean, we derive the optimal multiple symbol detection (MSD) rule for diagonal differential unitary space-time modulation (DUSTM). This rule is used to develop a sphere decoding bound intersection detector (SD-BID) with low complexity. A suboptimal MSD based decision feedback DD (DF-DD) algorithm is also derived. The simulation results show that our proposed MSD algorithms reduce the error floor of conventional differential detection and that the computational complexity of these new algorithms is reasonably low

Power delay profile and noise variance estimation for OFDM

In this letter, we present cyclic-prefix (CP) based noise-variance and power-delay-profile estimators for Orthogonal Frequency Division Multiplexing (OFDM) systems. Signal correlation due to the use of the CP is exploited without requiring additional pilot symbols. A heuristic estimator and a class of approximate maximum likelihood (ML) estimators are proposed. The proposed algorithms can be applied to both unitary and non-unitary constellations. These algorithms can be readily used for applications such as minimum mean-square error (MMSE) channel estimation

Generalized feedback detection for spatial multiplexing multi-antenna systems

We present a unified detection framework for spatial multiplexing multiple-input multiple-output (MIMO) systems by generalizing Heller’s classical feedback decoding algorithm for convolutional codes. The resulting generalized feedback detector (GFD) is characterized by three parameters: window size, step size and branch factor. Many existing MIMO detectors are turned out to be special cases of the GFD. Moreover, different parameter choices can provide various performance-complexity tradeoffs. The connection between MIMO detectors and tree search algorithms is also established. To reduce redundant computations in the GFD, a shared computation technique is proposed by using a tree data structure. Using a union bound based analysis of the symbol error rates, the diversity order and signal-to-noise ratio (SNR) gain are derived analytically as functions of the three parameters; for example, the diversity order of the GFD varies between 1 and N. The complexity of the GFD varies between those of the maximum-likelihood (ML) detector and the zero-forcing decision feedback detector (ZFDFD). Extensive computer simulation results are also provided

Differential modulation for two-way wireless communications: a perspective of differential network coding at the physical layer

This work considers two-way relay channels (TWRC), where two terminals transmit simultaneously to each other with the help of a relay node. For single antenna systems, we propose several new transmission schemes for both amplify-and-forward (AF) protocol and decode-and-forward (DF) protocol where the channel state information is not required. These new schemes are the counterpart of the traditional noncoherent detection or differential detection in point-to-point communications. Differential modulation design for TWRC is challenging because the received signal is a mixture of the signals from both source terminals. We derive maximum likelihood (ML) detectors for both AF and DF protocols, where the latter can be considered as performing differential network coding at the physical layer. As the exact ML detector is prohibitively complex, we propose several suboptimal alternatives including decision feedback detectors and prediction-based detectors. All these strategies work well as evidenced by the simulation results. The proposed protocols are especially useful when the required average data rate is high. In addition, we extend the protocols to the multiple-antenna case and provide the design criterion of the differential unitary space time modulation (DUSTM) for TWRC

Optimal Bandwidth and Power Allocation for Sum Ergodic Capacity under Fading Channels in Cognitive Radio Networks

This paper studies optimal bandwidth and power allocation in a cognitive radio network where multiple secondary users (SUs) share the licensed spectrum of a primary user (PU) under fading channels using the frequency division multiple access scheme. The sum ergodic capacity of all the SUs is taken as the performance metric of the network. Besides all combinations of the peak/average transmit power constraints at the SUs and the peak/average interference power constraint imposed by the PU, total bandwidth constraint of the licensed spectrum is also taken into account. Optimal bandwidth allocation is derived in closed-form for any given power allocation. The structures of optimal power allocations are also derived under all possible combinations of the aforementioned power constraints. These structures indicate the possible numbers of users that transmit at nonzero power but below their corresponding peak powers, and show that other users do not transmit or transmit at their corresponding peak power. Based on these structures, efficient algorithms are developed for finding the optimal power allocations.Comment: 28 pages, 6 figures, submitted to the IEEE Trans. Signal Processing in June 201

Role of proteolipid protein (PLP/Dm20) and polyunsaturated fatty acids in normal and pathological central nervous system

The CNS myelin sheath is an extension of the oligodendrocyte plasma membrane and is composed of 80% lipid and 20% protein. Myelin formation is a very sophisticated and highly conserved process, any disturbance of myelin protein or lipid composition has a strong impact on myelin biogenesis and myelin phenotype, which often results in CNS myelin pathology. The work presented here is divided into two parts: A. An investigation of the role of proteolipid protein (PLP/Dm20) Cys residues in myelin structure and function, and B. The role of PLP/Dm20 and polyunsaturated fatty acids (PUFAs) in CNS myelin morphology and CNS physiology. A. PLP is highly conserved and is the most abundant protein of CNS myelin. Also, point mutations in PLP are known to cause a variety of mild to severe PMD/SPG2 dysmyelinating leukodystrophies both in human and in mice. Some PLP point mutations are directly correlated to the perturbed plasma membrane trafficking of PLP/Dm20 in oligodendroyctes and correlate with the severity of PMD. PLP and its isoform Dm20 have 14 and 12 Cys residues, respectively, and are involved in post-translational S-acylation and the formation of two disulfide bridges. The work presented within this thesis furthers the understanding of the role of the Cys residues in PLP/Dm20. We analysed PLP/Dm20 Cys residue function by replacing Cys residues in PLP/DM20 with Ser residues (i.e., Mut-PLP/Dm20). In silico analyses predicted that the Mut-PLP and Dm20 have impaired transmembrane structure and reduced hydrophobic properties, supported by our in vivo and in vitro analysis which revealed perturbed cell surface targeting of the mutant proteins. Furthermore, in order to understand the role of Mut-PLP in CNS at early embryonic stages, a transgenic mouse mutant is being generated via pronuclear injection of a transgene composed of a Nestin enhancer and MBP promoter regulatory elements along with Mut-PLP cDNA, on a PLP negative genomic background. In addition, to understand the role of di-sulfide bridges in PLP; a knock-in mouse mutant is being generated in which the disulfide bridges of PLP/Dm20 have been deleted. B. PLP is an integral myelin membrane protein, and PUFAs are constituents of phospholipids, which are critical structural components of the CNS myelin membrane. In this thesis, we have expanded the understanding of the significant role of PLP and PUFAs and their impact in the CNS by phenotyping the plp-/-fads2-/- double knock-out mouse model (DM). The DM has been generated and validated on a genomic, RNA and protein level confirming lack of PLP and Fads2 which is a key enzyme for endogenous PUFAs synthesis. The DM exhibited a profound behavioral cataleptic state with mild tremor and seizures starting from 5 months age, and had a significantly reduced life span, compared to the single mutants (plp-/- and fads2-/-) and wild-type counterparts. In the brain and CNS myelin of the DM mice, PUFAs analysis revealed only partially depleted docosahexanoic acid (DHA) levels (40-50%) suggesting that DHA is highly regulated and/or is compensated for depleted PUFAs and was retained tenaciously in membrane structures of the CNS. Intriguingly, an unusual PUFA, eicosatrienoic acid (20:3) was detected, which may substitute for the observed depletion of arachidonic acid. Further analyses of the gene expression profile in the DM brain revealed no significant differences among myelin membrane proteins, desaturases and other myelin specific proteins, thus suggesting no alteration of these proteins in the absence of PLP and PUFAs. The DM mice showed a massive alteration in morphology of the CNS myelin manifesting as hypomyelination and a loss of myelin compaction around axons. In retinal electron micrographs, the retinal pigment epithelium (RPE) region showed no major significant morphological alterations. Retinal function, as assessed by ERG, revealed a decrease in a-wave signals suggesting defects in visual functions. Behavioral physiology studies, as assessed by the rota-rod task and Morris water maze and other similar tasks, showed partial but significant defects in neuromotor coordination/functions. The Morris water maze also revealed significant defects in cognitive ability (i.e., spatial acquisition/learning and in the reference memory). Alterations in behavioral physiology and the reduced lifespan in DM mice may be correlated with the disrupted myelin morphology and hypomyelination of CNS