Spectrum Handoff Mechanism in the Framework of Mobility Management in Cognitive Radio Networks

Introduction

Since some tens of years, the field of wireless telecommunications tends to take a more and more important in the current companies, both in economic terms and in terms of technological advances. Unfortunately, this spectacular development of wireless technologies is threatened as the majority of the spectrum, which represents the physical media for the wireless transmission, has already been allocated to existing systems. In 2002, the body of regulation and spectrum management in the United States, the Federal Communications Commission (FCC), has created a group of reflection before work on the shortage of frequency resources (Coase, 1959). The preliminary measures carried out by this group highlighted the need for more flexibility in the management of the spectrum. Indeed, some frequency bands are used intensively, others barely or not at all used. At a time and place given, by browsing through the spectrum fully, it is quite possible to find a band of frequencies not used by its owner. It is therefore clear that the shortage of frequencies is only artificial and can be regulated by a new policy of access more flexible.

The objective of the cognitive radio (CR) is to enable terminals, using different standards of communications, to coexist on the same frequency bands. On the one hand, bands of fixed frequencies are assigned to primary users (PU). This allows the PUs to communicate in priority. On the other hand, secondary users (SU) are allowed to communicate on the frequency bands of the PU in a non-priority. In effect, the SU must interfere with the least possible communications of PUs. There are several methods for a secondary user to access the radio frequency (RF) spectrum. These can be classified into three different techniques of access to the spectrum of the PU: The technique interweave, the overlay technique and the technique underlay (Ho-Van, 2014). The technical interweave is to operate during a given time and at a given location the frequency bands not used by the PU. The techniques overlay and underlay allow, with respect to them, to an SU to use a frequency band occupied by a PU. In this context, the SU must guarantee a level of minimum interference on the signal of the PU.

The mobility aspect is very important in the cognitive radio networks (CRNs) and its taking into account requires (1) to adapt the mechanisms of anticipation of the handoff to the context of the CR or well (2) to propose specific solutions to this new type of networks. The change of the operating frequency band of the SUs is a task related to the management of the spectrum mobility which makes it possible to guarantee the continuity of transmission of SU without interruption. When a PU decides to resume its licensed radio channel in use by the SU, the SU must switch to another frequency band deemed inactive over a given period to guarantee the quality of service (QoS) of one or the other. The mechanism of this change in the frequency band is known as spectrum handoff. The spectrum handoff process for the secondary system is much more based on the PU and SU action models. Therefore, the probabilistic spectrum handoff study that takes into account both the PU and SU elements is fundamental to the performance evaluation of the CRN.

The channel maintenance time by the PU is a key element in analyzing the performance of spectrum handoff mechanism in a CRN. The accessibility of the channels for the SU depends on the appearance of PU, which can be characterized by the residual time of the holes of the spectrum. In general, there are three distinct conditions for spectrum transfer: 1) when an SU fails connection because of mobility, 2) when the available spectrum does not support the prior requirements, 3) when the PU activity on the licensed spectrum is detected. We distinguish several spectrum handoff techniques in the CRN namely: Pure proactive handoff, pure reactive handoff, hybrid handoff, non-handoff, but our study is focused on the technique of proactive handoff. It is important to underline that the proactive handoff technique considerably reduces the detection time, this is due to the fact that the instant broadband detection is not feasible in this type of handoff. This has the advantage of significantly reducing the total service time and the delay of handoff in the network. Anyway, the problem is that the channels targeted preselected cannot be accessed when the interrupt event occurs. Unlike other spectrum handoff techniques, the proactive handoff is considered the most accurate and reliable in terms of delay reduction and collision. The multiple handoff processes result in a degradation of the QoS of the SUs while increasing the total service time the handoff delay.