Cristian Stanciu

Fast communication: A widely linear model for stereophonic acoustic echo cancellation

The stereophonic acoustic echo, due to the coupling between two loudspeakers and two microphones, can be modelled by a two-input/two-output system with real random variables. In this paper, we recast the problem as a single-input/single-output system with complex random variables, by using the widely linear (WL) model, and propose a new distortion method that fits well in this context. In order to illustrate the behavior of this scheme, the recursive least-squares (RLS)-dichotomous coordinate descent (DCD) algorithm is used. Experimental results indicate that the RLS-DCD algorithm represents an attractive choice for this application since it has good numerical features in terms of stability and complexity.

A novel perspective on stereophonic acoustic echo cancellation

The stereophonic acoustic echo is due to the coupling between two loudspeakers and two microphones. In the classical approach, this configuration is modelled by a two-input/two-output system with real random variables. In this paper, we propose to redesign this scheme as a single-input/single-output system with complex random variables. In this framework, we illustrate the behavior of some basic adaptive algorithms and present a distortion method which is more suitable for this model.

A proportionate affine projection algorithm using dichotomous coordinate descent iterations

This paper proposes a low-complexity proportionate affine projection algorithm (PAPA) suitable for echo cancellation. It is known that proportionate-type algorithms exploit the sparseness character of the echo path in order to achieve faster convergence and tracking as compared to the conventional algorithms. The proposed algorithm results as a combination between a recently proposed “memory”-PAPA (MPAPA) and an iterative technique for solving the matrix inversion problem, i.e., the dichotomous coordinate descent (DCD) with a leading element. Simulations performed in the context of network echo cancellation indicate the good performance of the proposed algorithm.

Digital audio equalizers with flat frequency response

In this paper a design method of the digital audio equalizers with small ringing impulse responses, by using symmetric B-spline functions and spline polynomials, is presented. It is important to have a flat frequency response for the audio equalizer. The solution is to approximate the response of each frequency band by using spline functions, that insures continuous superior order derivatives. These small ringings give a natural audition. A tradeoff between frequency band independence, small ringings of the impulse response and desired frequency response, must be made.

A robust dual-path DCD-RLS algorithm for stereophonic acoustic echo cancellation

The stereophonic systems create the illusion of audio directionality using independent channels with a standard configuration consisting of two microphones and two speakers. The basic associated mathematical model used to correct the acoustic echo that affects the system has two-input/ two-output real random variables with four adaptive filters. An important simplification to this scheme is the widely linear (WL) model which uses a single-input/ single-output system with complex valued random variables (CRVs) and only one adaptive filter. In order to demonstrate the functionality, the recursive least-squares (RLS) adaptive algorithm is used. The RLS is conveniently combined with the dichotomous coordinate descent (DCD) algorithm in order to greatly reduce the matrix inversion arithmetic complexity and an aggregate of two filters (with transfer logic - TL) is used to considerably improve performance in high disturbance situations such as double talk.

Robust variable-regularized RLS algorithms

The regularization parameter is required in most (if not all) adaptive algorithms, while its role becomes very critical in the presence of additive noise. In this paper, we focus on the regularized recursive least-squares (RLS) algorithm and present a method to find its regularization parameter, which is related to the signal-to-noise ratio (SNR). Also, using a proper estimation of the SNR, we further propose a variable-regularized RLS (VR-RLS) algorithm. In addition, a low-complexity version of the VR-RLS algorithm is developed, based on the dichotomous coordinate descent (DCD) method. Due to their nature, the proposed algorithms have good robustness features against additive noise, which make them behave well in all noisy conditions. Simulations performed in the context of acoustic echo cancellation support these findings.

Optimal grouped B-spline windows for power spectral density estimation

The solution, of minimizing the effect known as spectral leakage into the sidelobe of a window function is to obtain a continuous waveform, which results in lesser sidelobe level. Conventional windows are able to control these characteristics by only one parameter. To suppress the spectral leakage, a window function in the time domain is used to shape the finite time sampled measurement data. The higher order of continuity of the of the time window function can be obtained by using grouped B-spline functions with three control parameters for power spectral density estimation.

B-spline approach for the design of quadrature mirror filters

In this paper a design method of the optimum quadrature mirror filters (QMF), by using symmetric B-spline functions, is presented. It was shown that linear phase FIR filters cannot exactly satisfy the condition that the squared amplitude frequency responses are mirror images of each other about the line ω= π /2 which has led to the name quadrature mirror flters. It is important to have a flat frequency response for the filters. The solution is to approximate the response of each frequency band by using B-spline functions, that insures continuous superior order derivatives. In the proposed method, the prototype filter is optimized with the novelty of using spline functions in the transition band.

LTE Turbo Decoding Parallel Architecture with Single Interleaver Implemented on FPGA

This paper describes the implementation on field programmable gate array (FPGA) of a turbo decoder for 3GPP Long-Term Evolution standard. Considering the high data rates required by this standard, parallel decoding architecture is used. The parallel decoding latency is reduced N times compared with the serial decoding latency, N being the parallelization factor, usually a power of 2. The decoding performances are similar for both serial and parallel schemes, when very low decoding latency is added to this theoretical parallel latency value. Taking advantage of the quadratic permutation polynomial interleaver properties, and considering some specific FPGA block memory characteristics, a novel simplified parallel decoding scheme is proposed, including only one interleaver, independently of the N value. Moreover, for the single interleaver, we propose a solution that exploits key arithmetic properties of the corresponding equation to perform the address computation in a recursive manner. The proposed method replaces divisions and multiplications by comparisons and subtractions. In addition, an even-odd merge sorting network provides correct data to all N decoding units.

Improved regularization for a low-complexity RLS algorithm

The regularization of the recursive least-squares (RLS) adaptive algorithm is a subject rarely approached in literature. The classical regularization method is implemented in the initial stages of the algorithm and has a time-limited effect on the corresponding correlation matrix. This paper proposes a new regularization technique for the RLS — dichotomous coordinate descent (DCD) algorithm, which is applicable for every filter iteration and improves the performance of the algorithm in low signal-to-noise situations. The functionality of the method is demonstrated in acoustic echo cancellation scenarios.