Toni Huovinen

Independent component analysis using successive interference cancellation for oversaturated data

In this paper, advanced interference cancellation strategies are considered and applied in DS-CDMA uplink receivers. Namely, we introduce a new successive interference cancellation (SIC) scheme in which blind source separation (BSS) techniques based on independent component analysis (ICA) are considered. Our main emphasis is on the oversaturated data in which situation easily arises for example in DS-CDMA systems when the number of active connections/users is high compared to number of chips per bit being used, that is, a highly loaded system. Proposed receiver structures combine the main benefits of pure BSS/ICA and SIC methods: (i) inherent mitigation of various types of interference sources by BSS/ICA, (ii) robustness against parameter estimation errors due to BSS/ICA, (iii) greatly improved interference suppression capability due to novel combination of SIC ideology and ICA for oversaturated data. The last item in the list can also be seen as a way to somewhat circumvent the ‘more sources than sensors’—problem, which is a challenging problem in BSS area. Numerical experiments with DS-CDMA uplink data are given to illustrate the achieved gains in capacity and bit-error-rate performance compared to conventional successive and parallel interference cancellation (PIC) schemes as well as more an advanced LMMSE-PIC receiver and variants of BSS/ICA techniques alone. Copyright

Performance Evaluations for Multiuser CQI Enhancements for LTE-Advanced

In LTE-Advanced, multiuser MIMO has been identified as a key technique for increasing system spectral efficiency in closed-loop MIMO. Transmission parameters like precoding and transmission rate are adapted based on finite rate feedback from the users. Switching between single user and multiuser transmission modes is possible without higher layer signaling, which means that the feedback should be designed to sustain efficiently both single user and multiuser transmissions. In this paper, performance of LTE-Advanced is evaluated with system level simulations. The focus is on studying the viability of an additional multiuser specific channel quality feedback. The baseline in the comparison is single stream single user feedback consisting of a preferred precoding vector and a channel quality feedback. This baseline is compared to feedback schemes that aim at improving the multiuser performance by an additional multiuser-specific CQI. Also, the gain from a user specific scaling for the channel quality feedback performed by the base station is evaluated.

Blind Diversity Reception and Interference Cancellation using ICA

In this paper, we consider blind diversity reception and interference rejection in multi-antenna communications context, in terms of maximizing the output signal-to-interference-and-noise ratio (SINR). More specifically, we demonstrate that independent component analysis (ICA), although originally designed for noise-free linear models, is able to provide essentially the best possible output SINR among all linear transformations of received data in noisy linear models. In particular, our experiments indicate that one of the most widely applied ICA algorithms, equivariant adaptive source identification (EASI) algorithm, is, in practice, identical with SINR maximizing generalized eigenfilter in terms of SENR, even though it does not use explicit knowledge of the channel states and noise statistics. We also show that, in a special case of interference-free (that is, noise only) system, the EASI algorithm attains the greatest diversity gain blindly, i.e., performs as a blind maximal ratio combiner (MRC).

Enhanced Differential Correlation Method for the Acquisition of Galileo Signals

In this paper, we analyze an enhanced differential non-coherent integration method for the acquisition stage of a BOC-modulated CDMA signal in the new European Galileo satellite navigation system. This method, denoted by DN2, offers an improved suppression of any temporally uncorrelated interferers. Background noise is naturally the most obvious source of temporally uncorrelated interference, but, for example, in CDMA systems, also a multiaccess interference is inherently very weakly time-correlated. Here, we carry out a comprehensive comparison between DN2 method, the conventional non-coherent integration, and the traditional differential non-coherent integration methods. The correlation methods are compared and analyzed in fading channels, taking into account both ambiguous BOC and sideband based unambiguous technique (such as Betz&Fishman method). We also propose a combined algorithm, which chooses the best correlation method based on an estimation of the coherence time of the current channel

Comparison of nonlinear interference and blind source separation techniques in the DS-CDMA uplink

In this paper we compare the performance of conventional nonlinear interference cancellation and blind source separation (BSS) techniques. Namely, serial interference cancellation (SIC). M-stage parallel interference cancellation (PIC) and BSS based on independent component analysis (ICA) are employed in the uplink of direct-sequence code division multiple access (DS-CDMA) system. Existing studies on BSS/ICA in DS- CDMA have mainly concerned blind interference cancellation, which have been argued by the ability of BSS to deal with unknown (interfering) signals while demodulating a specific user. The performance of BSS/ICA techniques compared to conventional single user detection and blind (linear) multiuser detectors are quite well understood. In this paper we highlight the effectiveness of BSS/ICA against conventional nonlinear interference cancellation schemes. The systems with and without additional external interference like adjacent channel interference or jamming, are considered. Numerical experiments are given to evaluate the performance in different settings. They indicate clear performance gains in comparison to SIC and PIC with just a few stages when no jamming is present. However, even with a moderate level of additional jamming, BSS/ICA can outperform both the nonlinear schemes. The results thus show how the blindness of BSS/ICA makes it a robust method in the presence of intentional/unintentional out-of-cell interference.

On the Analytic Performance of Differential Spread Spectrum Code Acquisition

Spread spectrum code acquisition methods based on differential correlation are interesting alternatives to conventional non-coherent acquisition. Differential correlation can be seen simply as a correlation between two consecutive outputs of a coherent integrator. A differential acquisition variable is then obtained by averaging these correlation variables which results in an attenuation of any time-uncorrelated (or weakly time-correlated) interference components in a received signal. An existing analysis on the differential methods is mainly numeric or at least approximative in its nature. In this paper, we carry out rigorous analytic performance comparison of differentially coherent and conventional, non-coherent acquisition in terms of probabilities of detection and false alarm. First, we derive an exact probability distribution for the differential method from which we, then, evaluate the needed probabilities. In addition, we give a theoretical discussion about differentially noncoherent code acquisition which provides more reliable detection under residual frequency errors. Finally, we also give numerical examples to illustrate a suitability of the differential methods in Galileo-satellite positioning system