L.B. Milstein

Spectral-encoded UWB communication systems: real-time implementation and interference suppression

This paper considers a particular implementation of an ultra-wideband communication system that uses spectral encoding as both the multiple-access scheme and the interference-suppression technique. The main advantage of this technique is that the transmitted signal spectrum can be conveniently shaped to suppress narrowband interference and to not cause noticeable interference to overlaid systems. An extensive analysis of a possible implementation of this system by using surface acoustic-wave models is presented, and general expressions for the system performance are obtained. Numerical results show that a significant improvement in the system performance is obtained when the proposed interference-suppression method is used.

Multilevel coding with low-density parity-check component codes

We design multilevel coding (MLC) schemes with low-density parity-check (LDPC) codes as component codes at each level. We develop a method to analyze the performance of an LDPC code at any level as the codeword length goes to infinity, even if the equivalent binary-input component channels are not symmetric. By joint optimization of code rates and degree distributions, the optimized irregular LDPC codes at each level are capable of achieving reliable transmission at signal-to-noise ratios (SNR) very close to the capacity of the additive white Gaussian noise (AWGN) channel as the codeword length tends to infinity. Simulation results show that the optimized LDPC codes also perform very well at moderate codeword lengths.

The effects of narrowband interference on UWB communication systems with imperfect channel estimation

The proposed spectrum-overlay scenario for ultra-wideband systems brings new technical challenges that must be considered during the system design. In this scenario, the receiver has to perform functions, such as multipath combining and channel estimation, on a large number of low-energy multipath components in the possible presence of strong in-band interference. This paper proposes an analytical formulation that allows for the analysis of the channel estimation in the presence of narrowband interference. It is shown that the affect of narrowband interference is exacerbated by channel estimation errors, and that the number of training bits used for channel estimation is strongly dependent on whether or not interference suppression is employed. It is also shown that sources of degradation, such as imperfect channel estimation and the presence of narrowband interference, impose a practical limit on the number of multipath components that should be combined.

On the performance of turbo coding for the land mobile channel with delay constraints

Turbo coding for a Rayleigh fading channel is considered in a scenario where an externally imposed delay constraint exists. The interactions between the delay constraint, the rate of variation of the fading channel, and the block size and complexity of the turbo codes are investigated by simulation. Furthermore, the choice of dimensions of the block interleaver is addressed.

Narrowband Interference Rejection in PN Spread Spectrum Communication Systems using Decision Feedback Filters

In this paper. the performance of direct sequence (DS) spread spectrum systems using decision feedback interference suppression filters is analyzed. Analytical expressions for the filter output powers are obtainied, and their parameter dependence is quantified. In addition, under the assumption of perfect decision feedback. analytical expressions for the bit-error rates are developed and compared to simulation results. Finally, a comparison of two different decision feedback filter realizations shows the advantage of filtering the interference and noise only, excluding the desired signal.

Tradeoffs of source coding, channel coding and spreading in CDMA systems

We consider a CDMA system consisting of an image source coder, a convolutional channel coder, an interleaver, and a direct sequence spreading module. With different allocations of bandwidth to source coding, channel coding and spreading, the system is analyzed over a frequency selective Rayleigh fading channel. The performance of the system is evaluated using the cumulative distribution function of peak signal-to-noise ratio. We show that, among other things, given a fixed channel coding rate, allocating more bandwidth to source coding allows higher maximum image quality. At the same time, the probability of achieving this high quality is small. Allocating more bandwidth to spreading decreases the number of source information bits transmitted, thus limiting the achievable image quality, but the probability of achieving this maximum quality is high.

Detection with spatial diversity using noisy channel estimates in a correlated fading channel

Usually, diversity reception, either with or without perfect channel state information (CSI), is studied with independent fades. In this paper, the condition for independence is relaxed, and the effect of fade correlation on both the diversity gain and the accuracy of channel estimation for a direct sequence spread spectrum (DSSS) system employing pilot-aided, coherent, spatial-diversity reception under the most general form of correlated Rayleigh fading is assessed.

Implementation of a DS-Spread Spectrum Intercept Receiver with an Adaptive Narrow-Band Interference Exciser using Transform Domain Processing and Time Weighting

This paper describes detection of DS-BPSK signals in the presence of narrowband interference (CW) where transform domain processing is employed for detection and excision of the narrowband interference. The receiver rejects the interference by performing a real-time Fourier transformation followed by tracking and excision. This, in turn, is followed by inverse transformation and energy detection. To further improve the narrowband interference excision, the received signal is time weighted. Using weighting together with excision, one obtains about 12 dB additional reduction of the interference power at the output of the receiver over and above what one would achieve by using the system without weighting.

Anti-jamming properties of a DS-CDMA equalization filter

An adaptive tapped delay line equalizer is described which operates in a DS-CDMA receiver. This structure, without modification, can be applied to make the system insensitive to spectral inefficiency of the transmitted pulse shape (by rejecting multiple-access noise), to act as a RAKE in the presence of multipath interference, and to reject narrowband interference. The latter property is the main focus of this paper; this technique is compared to previously published methods of narrowband noise rejection.<<ETX>>

WLC36-6: Multicode MIMO for High Data Rate Mobile Ad-hoc Networks

In a mobile ad-hoc network scenario, where communication nodes are mounted on moving platforms (like jeeps, trucks, tanks, etc.), use of V-BLAST requires that the number of receive antennas in a given node must be greater than or equal to the sum of the number of transmit antennas of all its neighbor nodes. This limits the achievable spatial multiplexing gain (data rate) for a given node. In such a scenario, we propose to achieve high data rates per node through multicode direct sequence spread spectrum techniques in conjunction with V-BLAST. In the considered multicode V-BLAST system, the receiver experiences code domain interference (CDI) in frequency selective fading, in addition to space domain interference (SDI) experienced in conventional V-BLAST systems. We propose two interference cancelling receivers that employ a linear parallel interference cancellation approach to handle the CDI, followed by conventional V-BLAST detector to handle the SDI, and then evaluate their bit error rates.