Felipe A. Cruz-Pérez

Random-access control mechanisms using adaptive traffic load in ALOHA and CSMA strategies for EDGE

In this paper, three random access control mechanisms based on the well-known Slotted ALOHA, NP-CSMA, and 1P-CSMA protocols are presented. The basic idea is to limit the number of transmissions and retransmissions at high traffic loads in order to minimize collisions while keeping system stability. A new medium-access control protocol called Adaptive Traffic Load (ATL) is proposed. With ATL, all users are assigned authorization of transmission probabilities that vary according to the prevailing average traffic conditions in the system. ATL ensures that the system throughput is kept constant at its maximum value regardless of the traffic load. A mathematical analysis to calculate the probability density function of the access delay in the ATL protocol under the assumption of infinite user population is also presented. Mean access delay follows increases exponentially with respect to the traffic load when conventional random access protocols are used. However, it follows a linear function with respect to the traffic load when ATL is used. The average traffic load of the system is an input of the ATL protocol in order to assign access authorization probabilities to all users attempting to access the network. A simple algorithm for traffic load estimation based on the probability of finding empty slots in the system within an estimation period is proposed in this paper to asses the average traffic load. In the numerical evaluations of ATL, the Enhanced Data for GSM Evolution (EDGE) system is considered as a case study. For high arrival rates, channel utilization can be low in EDGE even if the system has sufficient capacity to serve incoming data users. A mathematical analysis of ATL-Slotted ALOHA as well as ATL-CSMA is presented. In the case of ATL-CSMA, system throughput varies according to the cell size. Hence, the ATL protocol is evaluated in picocell, microcell and macrocell environments as recommended by ITU-R. Also, the performance of EDGE is evaluated in terms of average data rate and packet delay for both S-ALOHA and ATL S-ALOHA considering long range dependent traffic type.

Differentiated Backoff Strategies for Prioritized Random Access Delay in Multiservice Cellular Networks

In this paper, a novel prioritization mechanism for random access strategies in cellular networks is proposed. The proposed prioritized random access strategy sets different retransmission probabilities to users, depending on their priorities. First, a mathematical analysis method for simultaneously evaluating throughput and access delay, considering an infinite population model and different retransmission policies, is developed. Most of the previous related research has been done, considering both finite population and saturation conditions where all the nodes in the system always have a packet ready to be transmitted, and the transmission queue of each station is assumed to be always nonempty. This assumption is a good approximation for a local area network working at full capacity. However, in a cellular system, the assumptions of a finite population and every node in a cell always having a packet to transmit are not very realistic. Here, the analytical results consider a Poisson arrival process-which is more suitable for the traffic model in a cellular system-for the users in the cells. Then, considering slotted ALOHA as a random access strategy and the developed analysis method, the case where two different types of service exist (low-and high-priority users) is mathematically analyzed. With the proposed prioritized random access strategy, the average access delay achieved for the high priority users is always lower than that for the low-priority users. Additionally, a mathematical expression for the throughput is derived with and without service differentiation. Finally, two approaches for finding the optimum retransmission probabilities are developed; in the first approach, the number of backlogged packets is required, whereas a simpler and efficient alternative method requires only the knowledge of the mean new packet arrival rate. For the performance analysis, four of the most common backoff algorithms are considered: (1) Uniform (UB); (2) Binary Exponential (BEB); (3) Negative Exponential (XB); and (4) Geometric (GB).

Call level performance analysis for multiservices wireless cellular networks with adaptive resource allocation strategies

In this paper, a novel mathematical approach to evaluate the performance of adaptive or flexible resource allocation (FRA) strategies with uniform quality of service (QoS) provisioning in multiservices wireless cellular networks, in terms of the call blocking probabilities and transmission delay, is proposed. FRA strategies improve channel utilization by dynamically adjusting user transmission rates. Based on the available cell capacity, the bandwidth offered to users can be adjusted in accordance with the elasticities of their service types. When an FRA strategy provides similar QoS to all the calls of the same service type, it is known as an FRA with uniform QoS provisioning. In multiservices wireless cellular networks, calls can be identified as belonging to one of four service classes (i.e., conversational, streaming, interactive, and background). Transmission delay is one of the most important performance measurements of interactive and background service classes. Transmission delay, however, has not been addressed in previous studies on FRA with uniform QoS provisioning either in conjunction with interactive or background service classes. This is because such studies have been based on the Markov property of the negative exponential distribution of the service time which lacks time delay information. The analytical approach in this paper is based on the fact that in FRA strategies with uniform QoS provisioning, calls of all service types tend to use an average number of resources (i.e., the number of resources available for a service type divided by the number of active users of that service type). This feature facilitates the assessment of the mean service time and the number of resources allocated to a call during its lifetime and, consequently, a calls transmission delay. The study also considers the fact that the probability density function (pdf) of the normalized transmission delay is almost a symmetrical function and has low variance. The accuracy of the proposed mathematical analysis is then corroborated by means of semianalytical methods and by discrete event computer simulation.

System-Level Analysis of Mobile Cellular Networks Considering Link Unreliability

Traditionally, system-level performance evaluation of mobile wireless communication networks has been addressed by only considering resource insufficiency, whereas the effect of an unreliable wireless channel has largely been ignored because of the complexity that its inclusion entails. To fill this void, a general analytical model for the system-level performance evaluation of mobile wireless networks taking into account both resource insufficiency and link unreliability is proposed in this paper. The effect of link unreliability is captured through the appropriate probabilistic characterization of the ldquounencumbered call-interruption time.rdquo Additionally, useful functional relationships between the call-interruption processes of the proposed analytical model and some system-level parameters that can easily be obtained from statistics collected at base stations (BSs) are derived. For the sake of generality, the involved time variables (i.e., cell dwell time, unencumbered call-interruption time, and channel holding time) are considered generally distributed. Then, general and easily computable mathematical expressions for many useful performance metrics under more realistic considerations are obtained. The analytical model proposed here is able to provide new and important insights into the dependence of system performance on link unreliability. Such understanding of this teletraffic engineering issue is vital for planning, designing, dimensioning, and optimizing mobile cellular networks for present and future wireless communication systems beyond the third generation.

Call admission and code allocation strategies for WCDMA systems with multirate traffic

In this paper, call admission and code allocation schemes are proposed to provide service differentiation in the forward link of wideband code-division multiple-access (WCDMA) systems. In particular, this paper proposes multiple leaf code reservation (MLCR) schemes, where different numbers of orthogonal variable spreading factor (OVSF) leaf codes (i.e., codes of the lowest layer of the OVSF code tree) are reserved to differentiate users with different bandwidth requirements. Leaf codes are only reserved for as long as the call admission process lasts. Once the decision of whether a new request is admitted or not has been made, a Code Dereservation procedure is carried out to increase flexibility in the code assignment phase. The performance of these MLCR strategies with/without code reassignments is then evaluated. Analysis shows that MLCR schemes are also useful in improving fair access among different traffic classes. In addition, perfect fair access among requests with different data rates can be achieved when code reassignments are jointly employed with the proposed OVSF-code reservation schemes.

Performance evaluation of mobile wireless communication systems with link adaptation

The performance of key quality-of-service metrics for mobile cellular systems with link adaptation is evaluated by means of a teletraffic analysis. To our knowledge, no similar analysis considering link adaptation exists in the literature. In particular, novel mathematical expressions for inter-cell handoff call arrival rate, intracell handoff failure, and forced termination probabilities are derived.

Teletraffic Model for the Performance Evaluation of Cellular Networks with Hyper-Erlang Distributed Cell Dwell Time

Abstract-In this paper, a teletraffic model to analyze wireless mobile cellular networks with hyper-Erlang distributed cell dwell time is developed. We demonstrate that the residual cell dwell time is also hyper-Erlang distributed with a greater number of stages. More important, it is shown that the phases on each stage of the hyper-Erlang distributed cell dwell and residual cell dwell times have the same mean permanence time. This fact allows us to make our teletraffic model computationally tractable by keeping track in a single state variable all the calls (new and handed off) in a phase (of any stage) with both the same mean permanence time and order within the stages. For this feature, it is also shown that the, so called, global cell dwell time (for new and handoff calls) can be represented by a Coxian model. This Coxian model can be the mixture of Coxian probability distributions. The teletraffic model proposed in this paper represents a step toward the development of a general, analytical, and computationally tractable modelling tool for the performance evaluation of mobile wireless communication networks under more realistic considerations.

A new EDGE medium access control mechanism using adaptive traffic load slotted ALOHA

We propose a new medium access control mechanism for EDGE. This strategy employs the slotted ALOHA random multiple access based on the traffic load of the system. Slotted ALOHA reaches the maximum throughput of 36% at a normalized traffic load of 1 packet per slot transmission time. Adaptive traffic load S-ALOHA keeps the maximum throughput for normalized offered load higher than one, by assigning a probability to transmit to each user in the system. In this way, we maintain the throughput constant at the maximum value for all normalized traffic loads bigger than 1 packet per time slot and we keep the blocking probability lower than the original slotted ALOHA protocol. In particular, we consider the EDGE frame structure and slot duration but this is a general medium access control mechanism that can be employed by all S-ALOHA protocols with a minimum modification.

On the Functional Relationship between Channel Holding Time and Cell Dwell Time in Mobile Cellular Networks

In this paper, under the assumption that the unencumbered service time is exponentially distributed, a novel algebraic set of general equations that examines the relationships between cell dwell time (CDT) and residual cell dwell time as well as between cell dwell time and channel holding time (CHT) are derived. This work includes relevant new analytical results and insights into the dependence of CHT characteristics on the CDT distribution. For instance, it is found that when CDT is Coxian or hyper-exponentially distributed, CHT is also Coxian or hyper-exponentially distributed, respectively.

Performance analysis of adaptive resource allocation strategies with service time dependence on the allocated bandwidth

In this paper, a novel approach to mathematically analyse the performance of adaptive resource allocation strategies with uniform quality of service (QoS) provisioning, when session service time depends on the allocated bandwidth, is proposed. Our proposed approach is based on the fact that in adaptive resource allocation strategies with uniform QoS provisioning all active sessions of each service type tend to utilise a bandwidth close to the average. The transmission delay experienced by sessions, compared to an optimal transmission operating at the maximum bandwidth requested is a characteristic related performance measurement that is also addressed in this work.