**4. References**

92 Mobile Networks

For G.723.1, the following polynomial was generated, where x represents the level of packet loss and y represents the level of impairment (Ie). Figure 14, shows a graph of the observed

For G.723.1, the following polynomial was generated where x represents the level of packet

IP Multimedia Subsystem (IMS) is very important due to the critical role it plays in the Next

In this chapter we provide a theoretical model that can be used by operators and network designers to determine the effects of introducing IMS to their networks in term of bandwidth usage needed to establish IMS session. The inputs of this model are the required number of Calls or Sessions per Second, Network losses, SIP Messages size, Number of Network hops and number of ringing times. The output of this model is the bandwidth

y = 0.084x3 - 0.74x2 + 5.2348x + 15 (48)

Fig. 12. G.729A Polynomial Fit

results versus the curve fit.

Fig. 13. G.723.1 Polynomial Fit

needed to insert IMS in the network.

**3.6 Conclusions** 

loss and y represents the level of impairment (Ie):

Generation Network (NGN) of the Fixed and Mobile Networks.


Juan J. Alcaraz, Mario Torrecillas-Rodríguez, Luis Pastor-González

**Dynamic Spectrum Access in Cognitive** 

Cognitive radio refers to a set of technologies aiming to increase the efficiency in the use of the radio frequency (RF) spectrum. Wireless communication systems are offering increasing bandwidth to their users, therefore the spectrum demand is becoming higher. However, RF spectrum is scarce and operators gain access to it by a licensing scheme by which public administrations assign a frequency band to each operator. Currently, this allocation is static and inflexible in the sense that a licensed band can only be accessed by one operator and their clients (licensed users). However, it is a known fact that while some RF bands are heavily used at some locations and at particular times, many other bands remain largely underused FCC (2002). This is, in fact, a classical property of tele-traffic systems, *i.e.* traffic intensity is highly variable during a day. The consequence is a paradoxical situation: while the spectrum scarcity problem hinders the development of new wireless applications, there are large portions of

Cognitive radio provides the mechanisms allowing unlicensed (or secondary) users to access licensed RF bands by exploiting spectrum opportunities. Cognitive radio is based on software-defined radio, which refers to a wireless communication system that can dynamically adjust transmission parameters such as operating frequency, modulation scheme, protocol and so on. It is crucial that this opportunistic access is performed with the least possible impact on the service provided to licensed users. Therefore, cognitive users should implement algorithms to detect the spectrum use (*spectrum sensing*), identify the spectrum holes (*spectrum analysis*) and decide the best action based on this analysis (*decision making*). Once the decision is made, the cognitive user performs the *spectrum access* according to a medium access control (MAC) protocol facilitating the communication among unlicensed

Dynamic spectrum access (DSA) refers to the mechanism that manages the spectrum use in response to system changes (e.g. available channels, unlicensed user requests) according to certain objectives (e.g. maximize spectrum usage) and subject to some constraints (e.g. minimum blocking probability for licensed users). DSA can be implemented in a centralized or distributed fashion. In the former one, a central controller collects all the information required about current spectrum usage and the transmission requirements of secondary users in order to make the spectrum access decision, which is generally derived from the solution of some optimization problem. In distributed DSA unlicensed users make their own decisions autonomously, according to their local information. Compared to centralized DSA, this

unoccupied spectrum (*spectrum holes* or spectrum opportunities).

users with minimum collision with other licensed and unlicensed users.

**1. Introduction**

and Javier Vales-Alonso

*Spain*

**5**

*Universidad Politécnica de Cartagena*

**Radio: An MDP Approach** 

