A Proposed Receiver Structure for Optical Communication Systems that Employ Heterodyne Detection and a Semiconductor Laser as a Local Oscillator

Coherent heterodyne detection in optical communication is degraded by phase instabilities present when a semiconductor laser is used as a local oscillator. Postdetection processing that accounts for these instabilities is suggested, based on a diffusion-process model for oscillator instabilities and a maximum-likelihood sequence-estimator of information symbols modulating the received light.     

On code design for the Slepian-Wolf problem and lossless multiterminal networks

A Slepian-Wolf coding scheme for compressing two uniform memoryless binary sources using a single channel code that can achieve arbitrary rate allocation among encoders was outlined in the work of Pradhan and Ramchandran. Inspired by this work, we address the problem of practical code design for general multiterminal lossless networks where multiple memoryless correlated binary sources are separately compressed and sent; each decoder receives a set of compressed sources and attempts to jointly reconstruct them. First, we propose a near-lossless practical code design for the Slepian-Wolf system with multiple sources. For two uniform sources, if the code approaches the capacity of the channel that models the correlation between the sources, then the system will approach the theoretical limit. Thus, the great advantage of this design method is its possibility to approach the theoretical limits with a single channel code for any rate allocation among the encoders. Based on Slepian-Wolf code constructions, we continue with providing practical designs for the general lossless multiterminal network which consists of an arbitrary number of encoders and decoders. Using irregular repeat-accumulate and turbo codes in our designs, we obtain the best results reported so far and almost reach the theoretical bounds.     

The expectation-maximization algorithm for symbol unsynchronizedsequence detection

The expectation-maximization (EM) algorithm for maximizing likelihood functions, combined with the Viterbi algorithm, is applied to the problem of sequence detection when symbol timing information is not present. Although the EM algorithm is noncausal, results obtained using the algorithm on the problem of nonsynchronized sequence detection indicate that it converges most of the time in three iterations, making it both of theoretical and of practical interest. A practical algorithm based on the EM algorithm is introduced. It reduces the computational burden and improves performance by making use of timing estimates in previous observation windows     

Sequence estimation in the presence of random parameters via the EMalgorithm

The expectation-maximization (EM) algorithm was first introduced in the statistics literature as an iterative procedure that under some conditions produces maximum-likelihood (hit) parameter estimates. In this paper we investigate the application of the EM algorithm to sequence estimation in the presence of random disturbances and additive white Gaussian noise. As examples of the use of the EM algorithm, we look at the random-phase and fading channels, and show that a formulation of the sequence estimation problem based on the EM algorithm can provide a means of obtaining ML sequence estimates, a task that has been previously too complex to perform     

Joint Baud and Frame Synchronization in Direct Detection Optical Communications

The maximum-likelihood frame synchronizer was recently derived for direct-detection optical communications assuming baud synchronization. In this paper we present an extension to those results when pulse-position modulation is used and jointly derive baud and frame synchronization from slot synchronization. The optimum rule is seen to consist of a simple correlation term and a nonlinear correction term. Simulation results for the optimum rule compared to analytical and simulation results for the simple correlation rule show that equivalent performance for the simple correlation rule requires a substantial increase in signal power. To design good synchronization patterns, we use a divergence measure derived from the optimum rule. A simple algorithm is derived to test whether a given sequence is good or not.     

From few to many: illumination cone models for face recognitionunder variable lighting and pose

We present a generative appearance-based method for recognizing human faces under variation in lighting and viewpoint. Our method exploits the fact that the set of images of an object in fixed pose, but under all possible illumination conditions, is a convex cone in the space of images. Using a small number of training images of each face taken with different lighting directions, the shape and albedo of the face can be reconstructed. In turn, this reconstruction serves as a generative model that can be used to render (or synthesize) images of the face under novel poses and illumination conditions. The pose space is then sampled and, for each pose, the corresponding illumination cone is approximated by a low-dimensional linear subspace whose basis vectors are estimated using the generative model. Our recognition algorithm assigns to a test image the identity of the closest approximated illumination cone. Test results show that the method performs almost without error, except on the most extreme lighting directions     

An Asymptotically Optimal Receiver for Heterodyne Optical Communication

An incoherent receiver is derived for the heterodyne optical channel under the assumption of Phase coherence at the start of each transmitted sequence. The receiver is optimal in the limit of small bit intervals with respect to the coherence time of the laser oscillators, and reduces to that proposed by Jeromin and Chan [1] when no initial phase coherence is present. Computer simulations indicate that the small amount of performance improvement obtained by resolving an initial phase uncertainty may not justify the extra complexity needed, for the data rates currently considered.     

Blind multiuser detection in uplink CDMA with multipath fading: a sequential EM approach

We consider joint channel estimation and data detection in uplink asynchronous code-division multiple-access systems employing aperiodic (long) spreading sequences in the presence of unknown multipath fading. Since maximum-likelihood (ML) sequence estimation is too complex to perform, multiuser receivers are proposed based on the sequential expectation-maximization (EM) algorithm. With the prior knowledge of only the signature waveforms, the delays and the second-order statistics of the fading channel, the receivers sequentially estimate the channel using the sequential EM algorithm. Moreover, the snapshot estimates of each path are tracked by linear minimum mean-squared error filters. The user data are detected by a ML sequence detector, given the channel estimates. The proposed receivers that use the exact expressions have a computational complexity O(2/sup K/) per bit, where K is the number of users. Using the EM algorithm, we derive low-complexity approximations which have a computational complexity of O(K/sup 2/) per bit. Simulation results demonstrate that the proposed receivers offer substantial performance gains over conventional pilot-symbol-assisted techniques and achieve a performance close to the known channel bounds. Furthermore, the proposed receivers even outperform the single-user RAKE receiver with Nyquist pilot-insertion rate in a single-user environment.     

On the performance of a novel quasi-synchronous trellis-coded CDMA system

We introduce a novel signal set defined over a signal space that consists of L (L/spl ges/2) orthogonal planes, and a quasi-synchronous trellis-coded code-division multiple-access (TC-CDMA) system based on it. The proposed scheme makes efficient use of the available processing gain to improve power and/or bandwidth efficiency for practical multiuser interference environments. Having a multiplanar signal constellation structure, the proposed scheme provides several options for a given required data rate, which makes it better adapted to dynamic channel conditions. Analytical bounds and simulation results indicate that at practical error rates and 2 b/s/Hz the proposed scheme is approximately 1.2 dB better than a TC-CDMA system based on 8-PSK, and 3 dB better at 3 b/s/Hz compared to TC-CDMA using 16-QAM. Additionally, the proposed system is approximately 1 dB better than a multicoded system using two signature sequences per user.     

On PPM sequences with good autocorrelation properties

The problem of designing sequences of Q-ary pulse-position-modulation (PPM) symbols that have good periodic autocorrelation properties is investigated. Two cases are considered. In the first it is assumed that only slot synchronization is present and thus cyclic shifts are one slot at a time; in the second PPM symbol synchronization is present, in which case cyclic shifts are by one symbol (Q slots) at a time. In both cases, upper bounds are derived on the maximum peak-to-sidelobe distance, which are shown through a computer search to be nearly tight. When symbol synchronization is present, the bound reduces to the Plotkin bound, but it is slightly tighter in general