On system capacity and coverage improvements by linear multiuser detection for UMTS

The main objective of this paper is to investigate asynchronous linear multiuser detection (MUD) in mixed service UMTS networks in terms of system capacity as well as power reduction at mobile stations. This especially requires us to theoretically describe the interaction of linear multiuser detection with radio resource management concepts like power control and load control for asynchronous multirate transmissions. For system capacity assessments we apply a two-stage power control multiuser detection algorithm where the users are detected by a minimum mean square error (MMSE) block detection. By detailed system level simulations we evaluate the UMTS load control parameter noise rise and also show corresponding system capacity bounds for different service mixes. It is important to note that for typical UMTS services the signal-to-noise operating points of the MMSE detector are very low (less than 0 dB). These low SINR operating points severely limit multiuser detection performance gains. Compared to simple single-user matched filtering the MMSE leads to fairly modest capacity gains of up to 60% dependent on the service mix assumed.

Abstract. An expanded effort is under the way to support the evolution of UMTS (Universal Mobile Telecommunication System). Apart from delivering high data rates, future UMTS releases will also require to provide high network performance in terms of system capacity, low radiated power, and high coverage. Well promising performance-enhancing technologies are smart antennas as well as multiuser detection. Although these new radio technologies have recently been subject to intense research, main UMTS network integration aspects with their specific constraints have been neglected in many cases. Especially the interaction with UMTS radio resource control being required to meet Quality of Service (QoS) constraints has to be included to assess the applicability of these technologies for UMTS. In this paper, we study the interaction of beamforming concepts as well as multiuser detection with load and power control. We also work out UMTS specific constraints like signal-to-interference-plus-noise ratio (SINR) operating points, pilot power pollution or channel estimation, all strongly limiting network performance. Results are shown for capacity gains and power reduction for all beamforming concepts of interest as well as linear multiuser detection schemes. The results show that fix as well as user-specific beamforming significantly improves network performance gains in downlink. In uplink multiuser detection indicates fairly modest system capacity gains, while it reduces tremendously mobile station power.

A signal-to-interference-noise ratio (SINR) constrained power control for code division multiple access (CDMA) and minimum mean square error (MMSE) multiuser detection is investigated. Two scenarios of a CDMA wireless local area network (WLAN) with MMSE multiuser detection are considered, i.e., the centralized power control for the uplink of a cellular WLAN, and the decentralized power control for an ad- hoc WLAN. It is proved that in both scenarios the interference between terminals is standard, so that the convergence of the proposed iterative power control (IPC) algorithm is guaranteed. Assuming asymptotically large number of communicating terminals and asymptotically large processing gain, it is shown that the power control becomes independent of particular spreading sequences, and thus, the random spreading sequences can be assumed. Furthermore, the transmitting powers can be readily adapted while the terminals join or leave the network. Numerical examples confirm that the simple IPC algorithm presented in this paper is very effective for cellular as well as ad-hoc CDMA WLAN's with MMSE multiuser detection for small to medium traffic loads and various SINR's requirements.

Beamforming (BF) with multiple receiving antennas significantly improves the bit error rate (BER) performance for a DS-CDMA system in a flat Rayleigh fading environment. However, BF fails to suppress multiple access interference (MAI) completely, which results in an irreducible BER floor. Furthermore, soft-decision Viterbi decoding (VD) with BF further lowers the BER floor. The BER performance difference between the minimum mean square error (MMSE) linear multiuser detector (MUD) and zero forcing (ZF) MUD increases with the eigenvalue spread of the code correlation matrix. BF with MUD performs better than MUD alone, and BF+VD is much more effective at combating MAI than MUD+VD for low to moderate SNRs. In this regime, the performance of BF+VD approaches that of BF+MUD+VD, which clearly offers the best performance for all SNRs.

In this contribution we propose and study a novel multiuser detection (MUD) scheme for multicarrier direct-sequence code-division multiple-access systems employing both time (T)-domain and frequency (F)-domain spreading, which are referred to as the TF/MC DS-CDMA systems. Specifically, a reliability-aided MUD scheme is proposed, which consists of a linear MUD and a so-called L-level maximum-likelihood (ML)-MUD. The linear MUD is either a joint TF-domain linear MUD or constituted by two linear MUDs, one of which is operated in the T-domain and the other one in the F-domain. The L-level ML-MUD is a reduced ML-MUD, which only searches in a space of size 2L in order to find the optimum solutions for the L most unreliable data bits detected by the linear MUD. In this contribution the bit error rate (BER) performance of the TF/MC DS-CDMA using the reliability-aided MUD is investigated, when communicating over additive white Gaussian noise (AWGN) channels or over frequency-selective Rayleigh fading channels. Our simulation results show that the reliability-aided MUD is capable of significantly outperforming the corresponding linear MUD. It can be shown that, provided that the signal-to-noise ratio (SNR) is sufficiently high, the reliability-aided MUD can readily obtain several decibels of SNR gain over the linear MUD at the cost of slight or moderate increase of the detection complexity.

Novel concepts are introduced for finding the relationship between multiuser detection (MUD) and multiuser transmission (MUT), so that the study in MUT can benefit from the well-built theory in MUD. Our study shows that, for any given linear MUD scheme, there exists a counterpart linear MUT scheme, which can be readily designed from the original linear MUD. Based on our observations, a range of MUT algorithms are derived in correspondence with the well-known linear processing schemes in the fields of MUD and array processing, including minimum mean-square error (MMSE), minimum variance distortionless response (MVDR), minimum power distortionless response (MPDR) and maximum signal-to-interference-plus-noise ratio (MSINR), respectively. Furthermore, the physical principles behind the considered MUT schemes are explained with the aid of the concepts built in this contribution. In order to gain insight into the effect of noise power on the achievable performance of a MUT scheme, a noise-suppression factor is introduced to those MUT schemes, such as MMSE, requiring the knowledge of noise variance. Our study shows that the achievable performance of the MUT is generally loosely depended on the noise-suppression factor. This finding may result in extremely low-complexity wireless transceivers in time- division duplex (TDD) cellular systems, where the uplink MUD and downlink MUT can be implemented based on one adaptive signal processing scheme.

This paper analyzes the performance of space-time decision feedback equalization (STDFE) assisted multiuser detection (MUD) for multiple-antenna space division multiple access (SDMA) systems to improve system capacity over dispersive fading channels. The MUD-STDFE receiver consists of a bank of matched filters and symbol-spaced feedforward filters (FFF) that both spatially and temporally whitens noise and the precursive parts of multiple access interference (MAI), followed by a bank of causal feedback filters (FBF) to cancel postcursive ISI and MAI for each user. The algorithm for computing this multi-dimensional FFF and FBF coefficients is presented, based on minimum mean-squared error (MMSE) optimality criterion. The error probability is estimated accurately and efficiently using the Gauss quadrature rule (GQR). The proposed MUD-STDFE receiver is capable of improving the performance significantly, separating the users and offering a significant performance gain relative to linear MMSE detection in an SDMA system.

In this paper we consider cell-free (CF) massive MIMO (MaMIMO) systems, which comprise a very large number of geographically distributed access points (APs) serving a much smaller number of users. We exploit channel sparsity to tackle pilot contamination, which originates from the reuse of pilot sequences. Specifically, we consider semi-blind methods for joint channel estimation and data detection. Under the challenging assumption of deterministic parameters, we determine sufficient conditions and necessary conditions for semi-blind identifiability, which guarantee the non-singularity of the Fisher Information Matrix (FIM) and the existence of the Cramer-Rao bound (CRB). We propose a message passing (MP) algorithm which determines the exact channel coefficients in the case of semiblind identifiability. We show that the system is identifiable if the Karp-Sipser algorithm yields an empty core. Additionally, we propose a Bayesian semi-blind approach which results in an effective algorithm for joint channel estimation and multi-user detection. This algorithm alternates between channel estimation and linear multi-user detection. Numerical simulations verify the analytical derivations.

This paper investigates the effect of other-cell multiple-access interference (MAI) on some suboptimal multiuser detectors such as decorrelating multiuser detector, parallel interference canceller and adaptive minimum mean-squared error (MMSE) multiuser detector. Based on the Gaussian approximation for other-cell MAI, the bit-error rate (BER) of each detector is derived. Both analytical results and simulations indicate that the performance degradation due to other-cell MAI is significant. It is also observed that the other-cell MAI introduces more errors to multiuser detectors that require prior information on users' signature waveforms.

This paper proposes a new scheme coupling power control with a minimum outage probability multiuser detector. The resultant iterative algorithm is conceptually simple and finds the minimum sum transmission power of all users with a set of outage probability constraints. Bound on the outage probability expression are found that extend a previous result that did not include receiver noise. These bounds are used to create a suboptimal scheme coupling power control and a MMSE multiuser detector. This new problem becomes a variant of an existing problem where outage probability constraints are first mapped to average SIR threshold constraints. Simulation results are presented showing the closeness of the two schemes and speed of convergence.

Space time trellis coded-orthogonal transmit diversity (STTC-OTD) scheme was proposed in S. Thirukkmaran and K.K. Pang (2004) and S. Thirukkmaran and K.K. Pang (2005) and its performance when applied to a CDMA system using a single user detector (SUD) was analyzed. It was therein shown that significant performance gain could be achieved using this scheme. Although this scheme requires extra spreading codes, the performance gains could more than offset the loss in cell capacity. In this paper our contribution is to analyze the performance of STTC-OTD scheme with a linear minimum mean squared error multiuser detection (LMMSE-MUD) receiver. We also provide, for comparison, the performance of the standard STTC scheme with a LMMSE-MUD receiver. It is shown that the performance of the STTC-OTD scheme with LMMSE-MUD receiver outperforms both STTC-OTD scheme with single user detection and STTC scheme with LMMSE-MUD.

This paper studies linear multiuser detection for code-division multiple-access (CDMA) with continuous-time constant envelope and continuous phase. The restriction to continuous phase is found to cause negligible degradation of multiuser efficiency when detected by means of linear multiuser detectors. This holds for both classical linear multiuser detection as well as for multistage detection based on universal weight design.

This paper is focused on the cross-layer design problem of joint multiuser detection and power control for energy-efficiency optimization in wireless data networks through a game-theoretic approach. Unlike previous studies in this area, which have considered the application of game theory to achieve energy-efficiency in idealized synchronous code division multiple access systems, more realistic assumptions including system asynchronism, the use of band-limited chip-pulses, and the multipath distortion induced by the wireless channel are explicitly incorporated in the development of this paper. Several noncooperative games are proposed wherein each user may vary its transmit power and uplink receiver in order to maximize its utility, which is defined here as the ratio of data throughput to transmit power. In particular, the case in which a linear multiuser detector is adopted at the receiver is considered first, and then, the more challenging case in which a nonlinear decision feedback multiuser detector is employed is considered. Via large system analysis (LSA), a decentralized implementation of the power allocation game requiring very little prior information on the interference background is proposed. LSA is also used to compare the energy efficiency of several linear multiuser detectors, and to obtain the optimal (i.e., utility-maximizing) length of the training sequence of each data frame. Numerical results show the effectiveness of the proposed solutions, as well as a very satisfactory agreement of the LSA-based analysis with simulation results obtained for systems with finite (and not so large) numbers of users.

Multiuser detection (MUD) of signals occupying the same time/frequency slot has been shown to provide significant increases in the capacity of a single cell or coverage area. In a cellular system, however, increasing the number of users per cell also produces increased interference from other cells. Compensation by increasing the co-channel cell spacing partially offsets the capacity gain due to MUD, and the net effect on system capacity then rests on the detection method, such as maximum likelihood (ML) or minimum mean-squared error (MMSE). This paper is the first to analyze the net capacity gains due to ML-MUD in time division multiple access (TDMA), and the first to compare ML-MUD with MMSE-MUD on this basis. We demonstrate that ML-MUD with M cochannel users per cell and four base station antennas gives a net system capacity that is M times greater than that of single-user TDMA system with two antennas. In contrast, we show that MMSE-MUD gives only a modest net capacity increase, and that the system capacity may actually decrease with more than one or two users per cell, making it poorer than the original single-user system.

Multiuser detection of time-hopping PPM UWB system in the presence of multipath fading