Showing papers by "David Tse published in 1999"

Linear multiuser receivers: effective interference, effective bandwidth and user capacity

Abstract: Multiuser receivers improve the performance of spread-spectrum and antenna-array systems by exploiting the structure of the multiaccess interference when demodulating the signal of a user. Much of the previous work on the performance analysis of multiuser receivers has focused on their ability to reject worst case interference. Their performance in a power-controlled network and the resulting user capacity are less well-understood. We show that in a large system with each user using random spreading sequences, the limiting interference effects under several linear multiuser receivers can be decoupled, such that each interferer can be ascribed a level of effective interference that it provides to the user to be demodulated. Applying these results to the uplink of a single power-controlled cell, we derive an effective bandwidth characterization of the user capacity: the signal-to-interference requirements of all the users can be met if and only if the sum of the effective bandwidths of the users is less than the total number of degrees of freedom in the system. The effective bandwidth of a user depends only on its own SIR requirement, and simple expressions are derived for three linear receivers: the conventional matched filter, the decorrelator, and the MMSE receiver. The effective bandwidths under the three receivers serve as a basis for performance comparison.

Optimal sequences, power control, and user capacity of synchronous CDMA systems with linear MMSE multiuser receivers

Abstract: There has been intense effort in the past decade to develop multiuser receiver structures which mitigate interference between users in spread-spectrum systems. While much of this research is performed at the physical layer, the appropriate power control and choice of signature sequences in conjunction with multiuser receivers and the resulting network user capacity is not well understood. In this paper we will focus on a single cell and consider both the uplink and downlink scenarios and assume a synchronous CDMA (S-CDMA) system. We characterize the user capacity of a single cell with the optimal linear receiver (MMSE receiver). The user capacity of the system is the maximum number of users per unit processing gain admissible in the system such that each user has its quality-of-service (QoS) requirement (expressed in terms of its desired signal-to-interference ratio) met. This characterization allows one to describe the user capacity through a simple effective bandwidth characterization: users are allowed in the system if and only if the sum of their effective bandwidths is less than the processing gain of the system. The effective bandwidth of each user is a simple monotonic function of its QoS requirement. We identify the optimal signature sequences and power control strategies so that the users meet their QoS requirement. The optimality is in the sense of minimizing the sum of allocated powers. It turns out that with this optimal allocation of signature sequences and powers, the linear MMSE receiver is just the corresponding matched filter for each user. We also characterize the effect of transmit power constraints on the user capacity.

A framework for robust measurement-based admission control

Abstract: Measurement-based admission control (MBAC) is an attractive mechanism to concurrently offer quality of service (QoS) to users, without requiring a priori traffic specification and on-line policing. However, several aspects of such a system need to be dearly understood in order to devise robust MBAC schemes, i.e., schemes that can match a given QoS target despite the inherent measurement uncertainty, and without the tuning of external system parameters. We study the impact of measurement uncertainty, flow arrival, departure dynamics, and of estimation memory on the performance of a generic MBAC system in a common analytical framework. We show that a certainty equivalence assumption, i.e., assuming that the measured parameters are the real ones, can grossly compromise the target performance of the system. We quantify the improvement in performance as a function of the length of the estimation window and an adjustment of the target QoS. We demonstrate the existence of a critical time scale over which the impact of admission decisions persists. Our results yield new insights into the performance of MBAC schemes, and represent quantitative and qualitative guidelines for the design of robust schemes.

A time-scale decomposition approach to measurement-based admission control

Abstract: We propose a time-scale decomposition approach to measurement-based admission control (MBAC). We identify a critical time-scale T/spl tilde//sub h/ such that: (1) aggregate traffic fluctuation slower than T/spl tilde//sub h/ can be tracked by the admission controller and compensated for by flow admissions and departures; (2) fluctuations faster than T/spl tilde//sub h/ have to be absorbed by reserving spare bandwidth on the link. The critical time-scale is shown to scale as T/sub h///spl radic/n, where T/sub h/ is the average flow duration and n is the size of the link in terms of number of flows it can carry. A MBAC design is presented which filters aggregate measurements into low and high frequency components separated at the cutoff frequency 1/T/spl tilde//sub h/, using the low frequency component to track slow time-scale traffic fluctuations and the high frequency component to estimate the spare bandwidth needed. The analysis shows that the scheme achieves high utilization and is robust to traffic heterogeneity, multiple time-scale fluctuations and measurement errors. The scheme uses only measurements of aggregate bandwidth and does not need to keep track of per-flow information.

Performance of linear multiuser receivers in random environments

Abstract: We study the signal-to-interference (SIR) performance of linear multiuser receivers in random environments, where signals from the users arrive in "random directions". Such random environment may arise in a DS-CDMA system with random signature sequences, or in a system with antenna diversity where the randomness is due to channel fading. Our results show that for both the decorrelator and the MMSE receiver, and the SIR distribution is asymptotically Gaussian in a system with large number of users. We compute closed-form expressions for the asymptotic means and variances for both receivers. Simulation results are presented to verify the accuracy of the asymptotic results for finite-sized systems.

Linear multiuser receivers for multipath fading channels

Abstract: In this paper we study the impact of channel estimation errors on the performance of linear multiuser receivers, as well as the channel estimation problem itself. Of particular interest are the scalability properties of the channel and data estimation algorithms: what happens to the performance as the system bandwidth and the number of users (and hence channels to estimate) grows? Our main results involve asymptotic expressions for the signal-to-interference ratio of linear multiuser receivers in the limit of large processing gain, with the number of users divided by the processing gain held constant.

Trade-offs of performance and single chip implementation of indoor wireless multi-access receivers

Abstract: The performance and computational complexity of five multi-access receivers are compared. A methodology is then presented for making area and power estimates of these algorithms for both software programmable DSP and dedicated direct mapped architectures. With this methodology and by using experimental data from previous designs, the feasibility of implementation of the multi-access receivers can be determined.

Multimedia CDMA wireless network design: the link layer perspective

Abstract: Multimedia traffic sources with tight latency constraint, arising in sessions such as data query, image and video transmissions, can be very bursty and are inefficient to be serviced with a dedicated high-speed link. On wireline networks, bursty traffic is statistically multiplexed to fully utilize the link capacity. In light of recent proposals for wideband CDMA (WCDMA) to service multimedia data, there is a similar need to develop wireless network architectures with flexible bandwidth allocation to facilitate statistical multiplexing. To address this issue, we compared the throughput performance of two promising WCDMA configurations, high speed CDMA (HS-CDMA) and multi-code CDMA (MC-CDMA). HS-CDMA assigns each user a single code with small spreading gain to enable a high transmission rate when it is needed. In contrast, MC-CDMA employs codes with a large spreading gain but permits a user to acquire more than one code. As expected, the throughput of a configuration depends on the receiver structure as well as the operation scenarios like power and SNR constraints. When matched filter receivers are applied, HS-CDMA fares better in most occasions. When multi-user receivers are used, both configurations deliver the same throughput except the situation when the total power is bounded.

Asymptotic performance analysis of linear multiuser receivers in multipath fading channels

Abstract: A linear multiuser receiver for a particular user in a CDMA network gains potential benefits from knowledge of the channels of all users in the system. In fast multipath fading environments we cannot assume that the channel estimates are perfect and the inevitable channel estimation errors will limit this potential gain. We study the impact of channel estimation errors on the performance of linear multiuser receivers, as well as the channel estimation problem itself. Our main results involve asymptotic expressions for the signal-to-interference ratio of linear multiuser receivers in the limit of the large processing gain, with the number of users divided by the processing gain held constant.