Genaro Hernandez-Valdez

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.

Joint Call and Packet Level Performance Analysis of CAC Strategies for VoIP Traffic in Wireless

Call admission control (CAC) functionality is a critical requirement for guarantee the desired quality of service (QoS) for voice calls in IP-based wireless networks. In this paper, three different CAC strategies for voice over IP (VoIP) traffic over wireless access networks with packet buffering are mathematically analyzed through a joint call and packet level discrete time teletraffic model. The admission criterion (i.e., admission threshold) of new sessions of these strategies is based on either the total number of sessions, the number of active sessions (i.e., sessions in talk-spurt periods), or the number of packets queued in the buffer. Admission threshold and buffer size can be both controlled for QoS provisioning in terms of session blocking and packet dropping probabilities. Arrivals and completions of VoIP sessions, on/off activity detection, and periodic and constant length packet generation (of active sessions) of individual VoIP sessions are modeled. The developed teletraffic analysis allows to evaluate the performance of the CAC strategies in terms of the most relevant QoS metrics of VoIP traffic at both call and packet level (i.e., session blocking and packet dropping probabilities and packet delay). Finally, the maximum traffic attained by the different studied-CAC strategies (while QoS provisioning is guaranteed) is obtained.

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.

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.

Erlang capacity in coordinated cognitive radio networks with stringent-delay applications

Due to the unpredictable nature of channel availability, supporting the quality of service (QoS) of stringent delay sensitive traffic in cognitive radio networks (CRNs) is very challenging. Stringent delay sensitive calls in CRNs are susceptible to forced termination due to the preemptive resource occupancy priority of primary users. To enable more efficient usage of the spectrum while improving the QoS experienced by secondary users (SUs), dynamic spectrum leasing (and coordinated cognitive radio) of resources has been previously proposed in the literature. In this paper, the Erlang capacity in coordinated cognitive radio networks with real-time hard delay constraint traffic as function of the rented resources is analytically calculated. For the adequate and fair performance comparison, call admission with fractional channel reservation to prioritize ongoing secondary calls over new ones is considered. From numerical results, it is observed that in CRNs exists a critical utilization factor of the primary resources from which it is not longer possible to guarantee the required QoS of SUs and, therefore, delay sensitive services cannot be even supported in cognitive radio networks. Thus, spectrum leasing can be essential for CRN operators to provide the QoS demanded by delay sensitive services. Then, the cost per capacity Erlang as function of both the utilization factor of the primary resources and the maximum allowed number of simultaneously rented channels is evaluated.

Channel holding time in mobile cellular networks with generalized Coxian distributed cell dwell time

In this paper, probability distributions of new and handoff call channel holding times in mobile cellular networks are derived under the assumption that cell dwell time has generalized Coxian distribution. It is shown that when cell dwell time has a generalized Coxian distribution, the resulting residual cell dwell time has generalized Coxian distribution as well. Furthermore, both new and handoff call channel holding times have generalized Coxian distributions of the same order (number of stages and number of phases within the stages). Additionally, it is demonstrated that the phases on each stage of the hyper-generalized Coxian distributed channel holding times have the same mean permanence time. Therefore, the global channel holding time has also generalized Coxian distribution of the same order (number of stages and number of phases within the stages) of those of the new and handoff call channel holding times. This result permits to simplify teletraffic analysis of cellular networks as a single state variable can be used to keep track of all the types of calls (new and handed off) in a phase (of any stage) with both the same mean permanence time and order within the stages. Mathematical expressions to calculate the parameters of the resulting distributions in terms of the parameters of the distributions of both cell dwell time and unencumbered service time are given. The results obtained in this work are relevant because they allow one to develop analytically and computationally tractable general teletraffic models for performance evaluation of mobile wireless networks under more realistic assumptions.

Channel holding time in mobile cellular networks with heavy-tailed distributed cell dwell time

Channel holding time is fundamental for the performance analysis/evaluation of mobile cellular networks. Channel holding time depends on both call holding time and cell dwell time. In the literature, many assumptions on cell dwell time distribution have been done and different channel holding time characteristics have been obtained. However, to our knowledge, channel holding time statistics has not been obtained under the assumption of heavy-tailed distributed cell dwell time. In this paper, this is addressed by considering Pareto, log-normal or Weibull distributed cell dwell time. Additionally, under the assumption that call holding time is exponentially distributed and cell dwell time is Pareto distributed, novel mathematical expressions for the probability distribution of the channel holding time for new and handed off calls are derived. Numerical results show the extent by which channel holding time statistics are affected by the different parameters of the cell dwell time distribution.

Queueing analysis of mobile cellular networks considering wireless channel unreliability and resource insufficiency

A queueing model for the system level performance evaluation of mobile cellular networks considering both resource insufficiency and wireless channel unreliability is proposed and mathematically analyzed. The proposed mathematical approach is based on the use of simple call interruption processes to model the effect of wireless channel unreliability. More importantly, this paper develops a system level teletraffic model for the performance evaluation considering that channel holding times for new and handed off calls are general (but not necessary identically) distributed random variables. Additionally, an approximated one-dimensional recursive approach based on the well known Kauffman-Roberts formula is proposed for the case when the channel holding time variables can be adequately characterized by Hyper-Exponential distributions. Also, the case when the channel holding time variables are Mixed-Erlang distributed, a distribution having universal approximation capability, is analyzed. Thus, our teletraffic model allows obtaining more general, realistic, and easily computable analytical results.

System Level Model for the Wireless Channel Unreliability in Mobile Cellular Networks

In this paper, a new system level approach for the wireless channel unreliability modeling is proposed. Firstly, in order to characterize the call interruption process due to the wireless channel unreliability, the Gilbert-Elliot model is used to obtain the probability distribution of the interruption time. Then, it is shown that link unreliability can be adequately modeled by considering a Poissonian call interruption process. This allows simplifying system level mathematical analysis and easily identifying the factors that mainly influence the performance of mobile cellular networks. For the characterization of the interruption process in real cellular networks, it is enough to know some few system level statistics that can be easily obtained at base stations. Then, with the proposed call interruption modeling, an elegant teletraffic analysis for the performance evaluation of mobile cellular networks considering both wireless link unreliability and resource insufficiency is developed.

Impact of the cell size and the propagation model parameters on the performance of microcellular networks

The effects of cell radius reduction and path loss model parameters on the performance of microcellular networks are studied. The performance is measured in terms of the average co-channel interference probability and reuse efficiency. The influence of reuse distance, cell size, traffic intensity, break point distance of the dual path loss law characteristics of microcells, and standard deviation of log-normal shadowed local mean on the performance parameters are investigated. Also, capacity increase due to cell radius reduction combined with dynamic channel assignment strategies (DCA) is evaluated. Our main contribution is the observation that the system performance is extremely sensitive to the propagation model parameters and depends strongly on the cell size and reuse factor. As it is expected, analytical and simulation results show that reducing the cell radius increases the cells Erlang capacity per unit area. However, this cell radius reduction implies an increase in the co-channel interference level and then a higher reuse factor may be required to keep the quality of service. On the other hand, it is shown that the system capacity increase due to the cell radius reduction is much greater in microcellular systems with DCA than in those with fixed channel assignment.