Cognitive Radio

The growth in wireless communications initiated by the variety of wire- less applications and systems being used nowadays has introduced an important challenge for the adaption of new spectrum allocation and utilization methods. The scarcity of licensed spectrum bands, con-

Section 1.4. Cognitive Radio 12

trolled by governmental policies, makes such challenges even more dif- ficult and complicated. However, the utilization of radio spectrum has been found to be not optimal and remains unutilized most of the times. Hence, Mitola [36] proposed cognitive radio (CR) to optimize the de- mand for the radio spectrum and utilize the unutilized licensed spec- trum. CR arose as a promising solution to spectral crowding problem by allowing the unlicensed (secondary) user to opportunistically utilize the frequency bands that are not heavily occupied by the licensed (pri- mary) user. Figure 1.5 below illustrates the basic CR behavior to train itself about the radio environment. The main two challenges to the suc- cess of cognitive radio include the primary user (PU) detection and the transmission opportunity exploitation [37] [38]. The detection of PU (spectrum sensing) leads to the detection of spectrum holes that can be defined as the unoccupied spectrum bands that are licensed to the PU. The cognitive users have the ability to exploit the opportunity of transmission (spectrum sharing) to improve their performance without causing any interference to the PU. Moreover, unlicensed users should posses the ability of measuring, sensing, learning, and the awareness of the surrounding radio environment for the reliable detection of the spectrum in order to check for the presence and activity of PU’s [37] as well as adapting itself for the exploitation of unused spectrum. This concept has recently been developed to be fully aware of the surround- ing environment and the primary users, and thereby further improve the efficiency of spectrum utilization [39]. Recently, several IEEE 802 standards for wireless systems have considered cognitive radio systems such as IEEE 802.22 standard [40] and IEEE 802.18 standard [41]. Cognitive radio classification can be performed according to the utiliza-

Section 1.4. Cognitive Radio 13

Figure 1.5. Key concepts in cognitive radio spectrum sensing

tion of spectrum band, the spectrum availability, or the interference as shown in Figure 1.6. The classification of CR based on the utilization of spectrum band can be divided into ideal CR and spectrum sensing CR [1]. Ideal CR is supposed to be aware of the operating parameters of all radios in its environment and can intelligently decide and use any unutilized spectrum band. Spectrum sensing CR, on the other hand, is supposed to notice and observe the spectrum bands of the PU before transmission [2]. Moreover, depending on the availability of spectrum bands, CR can be divided into licensed or unlicensed band CR [1] [2]. In the first category, i.e. licensed band CR, the cognitive system is able to utilize the spectrum band assigned to the licensed users, whereas, the unlicensed band CR is allowed only to utilize the unlicensed part of the spectrum bands that are available for secondary users only. There- fore, there is no need for the CR to sense the entire spectrum before the secondary user (SU) use the channel. An example of unlicensed band CR is IEEE.802.19 [42].

In CR, a secondary user (SU) can only borrow spectrum if it does not generate interference to the PU. Generally, there are two common ap-

Section 1.4. Cognitive Radio 14

proaches to avoid interference namely spectrum overlay (interference avoidance), and spectrum underlay (interference control) [1–6]. In the

Figure 1.6. Classification of cognitive radio [1–6]

underlay approach or interference control approach (Figure 1.7(a)), SU utilizes the radio spectrum at the same time with the PU provided that its transmission power is below the noise floor of the PU in order to avoid any interference to the PU. The advantage of this approach is the ability of the SU to transmit at any time without performing the sensing operation for detecting spectrum holes. However, the power control techniques related to this approach are likely to have a large degree of complexity. In the overlay approach (Figure 1.7(b)), the SU can access the detected spectrum holes only and utilize the unoccu- pied spectrum for transmission causing no interference to the PU. This approach enjoys interference-free communication between the primary and secondary users. However, this approach requires the continuous sensing of spectrum holes which is a difficult task due to the range of potential modulation schemes as well as the problem of hidden termi- nals, not to mention the fast time required for sensing and detecting these unoccupied spectrum bands [1]. The work introduced in this

Section 1.4. Cognitive Radio 15

P ow er

(a) Overlay spectrum approach

P ow er

(b) Underlay spectrum approach Figure 1.7. Approaches for avoiding harmful interference

thesis related to this subject assumes an overlay approach during the transmission of the cognitive system, whereas, the sensing and detec- tion operation is outside of the scope of the research.

The inherited fading phenomena of wireless channels limits the ser- vice reliability and coverage of the wireless communication systems. A potential solution to this problem is by using cooperative relaying net- works which is a technique exploited by a relay network to cope with the challenges of cognitive radio [43] [44] [45]. Cognitive technology can be deployed over the intermediate wireless relay nodes in order to achieve reliable communication and coordinate the spectrum sharing among primary and secondary users. Moreover, cooperative cognitive relaying is about fully utilizing the dynamic spectrum through a num- ber of cognitive relay nodes in order to achieve reliable communication and seamless transmission without causing any interference to primary users when they are present.

At the end of this thesis, the combination between a cooperative re- lay network and cognitive radio where the intermediate relay nodes are

Section 1.5. Challenges and Motivations of the proposed research work 16

equipped with cognitive radios will be presented. This network will be referred to as a cooperative cognitive relay network considering spec- trum sharing and transmission robustness.