Outage Probability Constraint Research Articles

Practical Multiuser Diversity With Outdated Channel Feedback

Inspired by the information theoretic results concerning multiuser diversity, we address practical issues in implementing multiuser diversity in a multiple access wireless setting. Considering a channel-assigning strategy that assigns the channel only to the user with the best instantaneous SNR , our emphasis is on the effects of channel feedback delay in downlink transmissions. A finite set of M-ary quadrature amplitude modulation (M-QAM) constellations is adopted and a constant transmit power is assumed in this practical multiuser adaptive modulation scheme. Based on the closed-form expressions for average bit error rate (BER) and average data rate, we illustrate the impact of channel feedback delay on the achievable multiuser diversity gain with the number of users. Simple and accurate asymptotic approximations are also provided in the limit of large numbers of users. Focusing on different applications, we propose two optimization criteria for the switching thresholds, based on either an average BER, or an outage probability constraint. Two novel constant power, variable rate M-QAM schemes that are less sensitive to feedback delay are proposed using the optimal switching thresholds, which are derived to maximize the average data rate subject to these two constraints, respectively. To obtain a certain degree of fairness among the users, we also consider a fair channel-assigning strategy that assigns the channel to only the user with the greatest normalized SNR.

On call admission and performance evaluation for multiservice CDMA networks

Call admission control and performance evaluation in wireless communication networks are essential for system design and operation. The network capacity and admissibility of imperfectly power-controlled multiservice code division multiple access (CDMA) system are studied in this paper. A generalized capacity formula considering traffic activity is derived, based on which a handoff prioritized call admission policy is presented and evaluated using the K-dimensional birth-death process model. System performance in terms of blocking probabilities, resource utilization and average system throughput are demonstrated and discussed in detail as the traffic intensity increases. It is also shown that the system parameters such as outage probability constraint and power control error have great impact on system capacity and performance. Since this work is based on the imperfect power control model, it is more appropriate to the design of practical multiservice CDMA networks.

Service outage based power and rate allocation

This article combines the concepts of ergodic capacity and capacity versus outage for fading channels, and explores variable-rate transmissions under a service outage constraint in a block flat-fading channel model. A service outage occurs when the transmission rate is below a given basic rate. We solve the problem of maximizing the expected rate subject to the average power constraint and the service outage probability constraint. When the problem is feasible, the optimum power policy is shown to be a combination of water filling and channel inversion allocation, where the outage occurs at a set of channel states below a certain threshold. The service outage approach resolves the conflicting objectives of high average rate and low outage probability.

Optimal policies for handoff and channel assignment in networks of LEO satellites using CDMA

In this paper, we formulate the combined handoff and channel assignment problems in a CDMA LEO satellite network as a reward/cost optimization problem. The probabilistic properties of signals (channel fading as function of satellite elevation angle) and of the traffic in the footprints are used to formulate a finite-horizon Markov decision process. The optimal policy is obtained by minimizing a cost function consisting of the weighted sum of the switching costs and blocking costs of traffics subject to a bit-error-rate or outage probability constraint. A backward induction algorithm is applied to derive the optimal policy. Performance of the optimal policy and the direct threshold policy are compared.