Jorge Martinez-Bauset

Optimal admission control in cognitive radio networks

We study a cognitive radio system in which the spectrum handover technique is applied by secondary users when they have to vacate a channel due to a primary user arrival. In order to limit the forced termination probability of secondary users a fractional guard channel reservation scheme is applied to give priority to spectrum handovers over new arrivals. We show that, contrary to what has been suggested, the reservation parameter cannot be adequately adjusted as a result of maximizing the throughput of secondary users. Instead we propose and explore two alternative configuration methods. The proposed methods are based on optimization problems that target the existing trade-off between blocking new sessions of secondary users and dropping ongoing ones. Additionally in our numerical experiments we identify some interesting and counterintuitive phenomena.

Comments on "analysis of cognitive radio spectrum access with optimal channel reservation"

We claim that some of the expressions, results and the conclusion of X. Zhu et al., (2007) are not correct. The correct expressions, results and conclusion are discussed in this letter.

Modeling and Characterization of Spectrum White Spaces for Underlay Cognitive Radio Networks

Cognitive Radio (CR) networks are envisaged as the key technology to realize dynamic spectrum access and solving the scarcity of radio spectrum. Having a temporal characterization of the spectrum white spaces in the primary network is a key element for studying and designing radio resource management mechanisms in CR networks. In that sense, most of the studies in the literature rely on an ON-OFF model with exponentially distributed on and off times. The usage of that model, however, is principally based on its analytical tractability. In this paper we propose a versatile Markovian model for the duration of the spectrum white spaces. Our model builds on a simple model of the channel holding time (CHT) in the primary network and then matrix-analytic techniques are applied to derive and analyze the duration of the white spaces. Despite its simplicity, the proposed approach is proven to be able to model very accurately scenarios where the CHT distribution is of a more complex type not amenable to mathematical analysis. Our numerical results show that the duration of the white spaces exhibits a low sensitivity to the distribution of the channel holding time beyond the mean.

Comments on "Call Blocking Probability and Bandwidth Utilization of OFDM Subcarrier Allocation in Next-generation Wireless Networks"

We claim that the Markovian model proposed by J.-C. Chen and W.-S.E. Chen (2006) is not suitable for the intended physical system. A more adequate model is proposed, which additionally offers important advantages that are discussed in this letter.

Measurement-based analysis of the performance of several wireless technologies

Wireless technologies have rapidly evolved and are becoming ubiquitous. An increasing number of users attach to the Internet using these technologies; hence the performance of these wireless access links is a key point when considering the performance of the whole Internet. In this paper we present a measurement-based analysis of the performance of an IEEE 802.16 (WiMAX) client and an UMTS client. The measurements were carried out in a controlled laboratory. The wireless access links were loaded with traffic from a multi-point videoconferencing application and we measured three layer-3 metrics (one-way-delay, IP-delay-variation and packet loss ratio). Additionally we estimate the performance of a WiFi and Ethernet client as a reference. Our results show that Ethernet and WiFi have comparable performances. Both the WiMAX and the UMTS links exhibited an asymmetric behavior, with the uplink showing an inferior performance. We also assessed the causes of the discretization which appears in the jitter distributions of these links.

Cognitive radio networks with elastic traffic

We model and evaluate an infrastructure based cognitive radio network where impatient unlicensed or secondary users (SUs) offer elastic traffic. The interference created by SUs to the licensed users is analyzed when the secondary network uses two different spectrum access schemes, the conventional random access scheme and a new scheme we refer to as access with preference. To further control the interference, a limit is set to the number of channels SUs have access to. With this new constraint, the abandonment probability, the mean delay and throughput of the SUs degrades significantly. To improve the QoS perceived by the active SUs, we define and evaluate an admission control scheme for the SUs.

Performance analysis of access class barring for handling massive M2M traffic in LTE-A networks

The number of devices that communicate through the cellular system is expected to rise significantly over the coming years. But cellular systems, such as LTE-A, were designed to handle human-to-human traffic. Hence, they are not suitable for managing massive machine-to-machine communications. Therefore, additional congestion control methods must be developed and evaluated. Up to date, access class barring (ACB) and extended access barring (EAB) methods are the preferred solutions for reducing congestion in the access channels of the evolved NodeB. These methods are based on restricting the access of certain classes of UEs, so the system capacity is not exceeded. Due to the high complexity of the LTE-A system, evaluating its performance is not straightforward. Specifically, a large number of variables, coexistent mechanisms, and test scenarios make it difficult to identify the network parameters that enhance performance. In this paper, we analyze the ACB method in highly congested environments. For this, we evaluate the effect of ACB parameters (barring rates and barring times) by means of several key performance indicators (KPI) such as delay, energy consumption (preamble transmission attempts required) and success probability. We observed that ACB is appropriate for handling sporadic congestion intervals in LTE-A networks.

Saturation Throughput in a Heterogeneous Multi-Channel Cognitive Radio Network

In this paper we consider a cognitive radio network in a heterogeneous multi-channel scenario where channels are different in terms of achievable bit rates. Upon arrival licensed users (PUs) will occupy first the channels with higher achievable rates. As a result, secondary users (SUs) will see a set of channels which are different not only in their physical capacities but also in the activity profile of PUs. Consequently, from the point of view of the effective throughput that SUs can obtain in each channel there exists a trade-off between the rate they can obtain in a certain channel and the amount of time they can use it. We develop a model that allows to solve this trade-off by computing the maximum effective throughput that SUs can achieve in each channel.

Resource management for Macrocell Users in hybrid access femtocells

The constant evolution of mobile-phone traffic demands for novel networking solutions especially focused on indoor environment. In this context, the use of femtocells, i.e., cells with very limited coverage area, has been proposed. In this paper, a femtocell network with hybrid access control mode is considered. The activity profile of the Femtocell Users (FUs) is modeled to compute the maximum achievable throughput and the consumed energy per successfully transmitted data bit by the Macrocell Users (MUs), depending on which set of channels are operated in open access mode, i.e., which channels can be used by MUs. Thus, it is identified how many and which channels must be operated in open access mode, depending on the physical capacities of the channels and the amount of time these channels are not occupied by FUs. The results motivate the need for novel resource management schemes which can dynamically adapt the set of open access channels to the network conditions.

Dynamic spectrum sharing in cognitive radio networks using truthful mechanisms and virtual currency

Abstract In cognitive radio networks, there are scenarios where secondary users (SUs) utilize opportunistically the spectrum originally allocated to primary users (PUs). The spectrum resources available to SUs fluctuates over time due to PUs activity, SUs mobility and competition between SUs. In order to utilize these resources efficiently spectrum sharing techniques need to be implemented. In this paper we present an approach based on game-theoretical mechanism design for dynamic spectrum sharing. Each time a channel is not been used by any PU, it is allocated to SUs by a central spectrum manager based on the valuations of the channel reported by all SUs willing to use it. When an SU detects a free channel, it estimates its capacity according to local information and sends the valuation of it to the spectrum manager. The manager calculates a conflict-free allocation by implementing a truthful mechanism. The SUs have to pay for the allocation an amount which depends on the set of valuations. The objective is not to trade with the spectrum, but to share it according to certain criteria. For this, a virtual currency is defined and therefore monetary payments are not necessary. The spectrum manager records the credit of each SU and redistributes the payments to them after each spectrum allocation. The mechanism restricts the chances of each SU to be granted the channel depending on its credit availability. This credit restriction provides an incentive to SUs to behave as benefit maximizers. If the mechanism is truthful, their best strategy is to communicate the true valuation of the channel to the manager, what makes possible to implement the desired spectrum sharing criteria. We propose and evaluate an implementation of this idea by using two simple mechanisms which are proved to be truthful, and that are tractable and approximately efficient. We show the flexibility of these approach by illustrating how these mechanisms can be modified to achieve different sharing objectives which are trade-offs between efficiency and fairness. We also investigate how the credit restriction and redistribution affects the truthfulness of these mechanisms.