Ann-Louise Johansson

A matrix-algebraic approach to successive interference cancellation in CDMA

In this paper, we describe linear successive interference cancellation (SIC) based on matrix-algebra. We show that linear SIC schemes (single stage and multistage) correspond to linear matrix filtering that can be performed directly on the received chip-matched filtered signal vector without explicitly performing the interference cancellation. This leads to an analytical expression for calculating the resulting bit-error rate which is of particular use for short code systems. Convergence issues are discussed, and the concept of /spl epsiv/-convergence is introduced to determine the number of stages required for practical convergence for both short and long codes.

Successive interference cancellation in multiple data rate DS/CDMA systems

Analyses a successive interference cancellation (IC) scheme for M-ary QAM modulation in direct sequence code division multiple access (DS/CDMA) systems. The performance under Rayleigh fading is analysed for single modulation systems and systems employing either of two different methods, mixed modulation or parallel channels, for handling multiple data rates. The authors show that the successive IC scheme for higher modulation together with mixed modulation or parallel channels give a considerable increase in performance and flexibility compared to single modulation systems employing a conventional detector.

Linear group-wise successive interference cancellation in CDMA

In this paper, we consider a matrix-algebraic approach to linear group-wise successive interference cancellation. As for both successive and parallel cancellation, it is shown that the general linear group-wise scheme corresponds to a one-shot linear matrix filtering. Furthermore, it is shown that if the general structure converges, it converges to the decorrelator. Four structures are considered for the group detector, namely the matched filter, the parallel canceller, the decorrelator and the MMSE detector, and the equivalent one-shot filters are derived. Conditions for convergence related to the grouping are studied and it is shown that group detection with a decorrelator, which has conventional successive interference cancellation as a special case, always converges. Through numerical examples, it is illustrated that the performance for all the structures behaves like the performance for the linear successive canceller where a minimum BER is reached before convergence to the performance of the decorrelator occurs. The matched filter group canceller achieves the lowest minimum BER but also has the slowest convergence.

On Multirate DS-CDMA Schemes with Interference Cancellation

This paper investigates interference cancellation (IC) in direct-sequence code-division multiple access (DS-CDMA) systems that support multiple data rates. Two methods for implementing multiple data rates are considered. One is the use of mixed modulation and the other is the use of multicodes. We introduce and analyze a new approach that combines these multiple data rate systems with IC. The cancellation in the receiver is performed successively on each user, starting with the user received with the highest power. This procedure can in turn be iterated, forming a multistage scheme, with the number of iterations set as a design parameter. Our analysis employs a Gaussian approximation for the distribution of the interference, and it includes both the AWGN and the flat Rayleigh fading channel. The systems are also evaluated via computer simulations. Our analysis and simulations indicate that the IC schemes used in mixed modulation or multicode systems yield a performance close to the single BPSK user bound and, consequently, give a prospect of a considerable improvement in performance compared to systems employing matched filter detectors.

Multistage interference cancellation in multirate DS/CDMA on a mobile radio channel

A novel hybrid interference cancellation (IC) scheme is proposed for the uplink of multirate direct-sequence code division multiple access (DS/CDMA) systems communicating over mobile radio channels. The hybrid IC scheme includes both non-decision directed and decision directed IC, joined together with a modified RAKE combiner for utilization of the diversity. The performance is evaluated via computer simulations for two multiple data rate schemes, mixed modulation and parallel channels, in a multipath environment. Both the case of known channel parameters and channel estimation using pilot symbols are considered. An analytical performance assessment is also given using a Gaussian approximation.

One-shot filtering equivalence for linear successive interference cancellation in CDMA

We consider a matrix-algebraic approach to linear successive interference cancellation (SIC). It is shown that both single- and multi-stage linear SIC schemes correspond to a one-shot linear matrix filtering that can be performed directly on the received chip-matched filtered received vector without explicitly performing the interference cancellation. This description allows for determining the bit error rate analytically. Also eigenvalue conditions for convergence of the multi-stage scheme are derived and the concept of /spl epsiv/-convergence introduced for determining the number of stages necessary for practical convergence.

Multi-stage interference cancellation in multi-rate DS/CDMA systems

We show that the one-stage IC scheme for Mlevel rectangular QAM can easily be extended to a multi-stage IC scheme. The performance of different single modulation systems, with equal throughput, is compared with systems employing conventional detectors and the single user bounds. In the analysis we consider two methods for handling multiple data rates, mixed modulation and parallel channels. These methods are analysed under the assumption of Rayleigh fading. The results show a considerable improvement in both performance, close to the single user bound, and capacity compared with systems employing a conventional detector.

Convergence of Linear Successive Interference Cancellation in CDMA

In this paper we consider a matrix-algebraic approach to linear successive interference cancellation (SIC). It has been shown that both single- and multi-stage linear SIC schemes correspond to a one-shot linear matrix filtering. Eigenvalue conditions on the resulting linear filter for convergence of the multistage scheme are considered and the concept of e-convergence is introduced for determining the number of stages necessary for practical convergence. It is observed that the BER for the users does not generally reach its minimum when the scheme converges, hence, optimal performance is achieved after a limited number of cancellation stages, which does not have to be the same for different users.

Multistage interference cancellation with decision directed channel estimation in multirate DS/CDMA on a mobile radio channel

A combination of a hybrid interference cancellation (IC) scheme and decision directed channel estimation is proposed for the uplink of multirate direct-sequence code division multiple access (DS/CDMA) systems communicating over mobile radio channels. The hybrid IC scheme includes both non-decision directed and decision directed IC, joined together with a modified RAKE combiner for utilization of the diversity. The channel estimator is a decision directed minimum mean square error estimator that utilizes the fading correlation. The performance is evaluated via computer simulations for a multiple data rate scheme, parallel channels, in a multipath environment.

On multirate DS/CDMA with interference cancellation for wireless multimedia applications

Second generation digital mobile telephone systems have been in public use for several years. Research is now focusing on third generation systems. These are generally referred to as personal communications networks (PCN), universal mobile telecommunication systems (UMTS) or future public land mobile telecommunication systems (FPLMTS). Different modulation schemes supporting multiple data rates in a direct sequence code division multiple access (DS/CDMA) system are studied. A successive multistage interference cancellation scheme for the considered modulations is described. Bit error probability on both AWGN and Rayleigh fading channels are presented both with single user detection and with multistage interference cancellation. It is shown that DS/CDMA is very flexible in supporting multiple data rates for wireless multimedia applications and that significantly improved performance may be obtained using interference cancellation.