Multipath Parameter Estimation from OFDM Signals in Mobile Channels

We study multipath parameter estimation from orthogonal frequency division multiplex signals transmitted over doubly dispersive mobile radio channels. We are interested in cases where the transmission is long enough to suffer time selectivity, but short enough such that the time variation can be accurately modeled as depending only on per-tap linear phase variations due to Doppler effects. We therefore concentrate on the estimation of the complex gain, delay and Doppler offset of each tap of the multipath channel impulse response. We show that the frequency domain channel coefficients for an entire packet can be expressed as the superimposition of two-dimensional complex sinusoids. The maximum likelihood estimate requires solution of a multidimensional non-linear least squares problem, which is computationally infeasible in practice. We therefore propose a low complexity suboptimal solution based on iterative successive and parallel cancellation. First, initial delay/Doppler estimates are obtained via successive cancellation. These estimates are then refined using an iterative parallel cancellation procedure. We demonstrate via Monte Carlo simulations that the root mean squared error statistics of our estimator are very close to the Cramer-Rao lower bound of a single two-dimensional sinusoid in Gaussian noise.Comment: Submitted to IEEE Transactions on Wireless Communications (26 pages, 9 figures and 3 tables

Non-Gaussian Behaviour of Extrinsic Log-Values

non-Gaussian behaviour of extrinsic Log-values (L-values) during iterative turbo-decoding process is investigated. I. Summary Extrinsic L-values are exchanged between the constituent log-APP decoders during each iteration of the turbo decoding process. It is this transfer of information that is critical in understanding the convergence behavior. In particular, Gaussian models have been used extensively. However, statistical tests on experimental data suggest that the Gaussian assumption is invalid. Fig. 1 plots the average extrinsic L-values skewness squared (β1) versus kurtosis (β2) at each decoder iteration while decoding a Berrou-Glavieux-Thitimajshima (BGT) Parallel Concatenated Convolutional Code (PCCC) [1]. Under the Gaussian assumption (β1, β2) would be a single fixed point at (0, 3) on Fig. 1. However, the measured (β1, β2) of the extrinsic L-values at each decoder iteration never exactly corresponds to Gaussian (unless by chance) but rather evolves along various points. A system of distributions that better reflects the (β1, β2) evolution is the Pearson family of density functions [2]. These distributions are solutions to the differential equation d ln f(x) d

Estimation of channel parameters and background irradiance for free-space optical link

Free-space optical communication can experience severe fading due to optical scintillation in long-range links. Channel estimation is also corrupted by background and electrical noise. Accurate estimation of channel parameters and scintillation index (SI) depends on perfect removal of background irradiance. In this paper, we propose three different methods, the minimum-value (MV), mean-power (MP), and maximum- likelihood (ML) based methods, to remove the background irradiance from channel samples. The MV and MP methods do not require knowledge of the scintillation distribution. While the ML-based method assumes gamma–gamma scintillation, it can be easily modified to accommodate other distributions. Each estimator’s performance is compared using simulation data as well as experimental measurements. The estimators’ performance are evaluated from low- to high-SI areas using simulation data as well as experimental trials. The MV and MP methods have much lower complexity than the ML-based method. However, the ML-based method shows better SI and background-irradiance estimation performance

Multi-user decoder assisted code-acquisition in CDMA systems

This paper focuses on interference cancellation techniques to improve performance of code-acquisition in codedivision multiple access systems. We propose a decoder-aided technique with parallel interference cancellation to overcome multiple access interference in order to detect weak users in a near-far environment. In particular, we consider the case where interference is cancelled after a certain number of multiuser decoder iterations. Time offset of weak user is estimated using a single user synchronization algorithm with interference cancelled received signal, treating residual interference as noise. The probability of missed detection is considered as a measure of performance. We analyze the performance improvement of codeacquisition with number of decoder iterations using the extrinsic information transfer chart of the decoder.

Decoder-aided synchronization for multiuser CDMA systems

This paper addresses the problem of frame synchronization in code division multiple access systems in the presence of high multiple access interference. Already acquired user signals are regenerated using soft information from the respective decoders and subtracted from the received signal to mitigate multiple access interference. We consider partial cancellation of interference after few decoder iterations for fast acquisition of remaining weak users. Hence the effect of residual interference after cancellation is considered in the derivation of the time offset estimator. We consider both data-aided and non-data-aided scenarios. Our approach uses linear pre-processing (whitening filter) on the interference cancelled signal for further performance improvement. Analytical expressions for the missed detection probability of a given user are derived. These analytical results are verified via Monte-Carlo simulations. We compare conventional code acquisition to our whitening filter method within the parallel interference cancellation framework.