Quality of Service in Cognitive Radio Network : Issues and challenges

Cognitive radio (CR) is a novel technology to resolve the issue of underutilization of wireless spectrum. There exists number of challenges and issues in designing and implementation of the cognitive radio. Extending qualityof-service (QoS) enabled applications to CR network is even more difficult task due to non-availability of the dedicated allocation of idle spectrum. CR imposes peculiar and unique challenges to guarantee quality of service of diverse flows in contrast to other wireless networks. This paper identifies the issues and challenges of QoS provisioning in cognitive radio networks. Oriental Journal of Computer Science and Technology Journal Website: www.computerscijournal.org ISSN: 0974-6471, Vol. 11, No. (1) 2018, Pg. 34-39


Introduction
The spectrum is a range of electromagnetic radiations that enables wireless communication and is managed by government.Day by day, the number of new wireless users and applications are increasing at a very rapid pace and particularly last decade has seen tremendous growth of both.Wireless users worldwide have reached 3.2 billion and were predicted to increase by 100 folds by 2013 1 .New applications demand additional allocation of spectrum which is not available and has created artificial scarcity of new allocation of spectrum.The first utilization survey reported that the allocated spectrum remained idle most of the time 2 and is also function of geographical location and time.Other spectrum occupancy measurements conducted have revealed that the average utilization in New York City 3 was at 5.2% while in Chicago 4 at 17.4%.Several other countries spectrum occupancy measurement studies, such as Spain 5 , Singapore 6 , Germany 7-8 , New Zealand 9 and United Kingdom 10 also confirmed that spectrum is mostly under-utilized at this moment.The solution to decrease the under-utilization of the allocated spectrum is the cognitive radio (CR).CR is defined as an intelligent device 11 that is fully aware of its environment and takes decisions with an aim to realize maximum and efficient utilization of wireless spectrum.Based on the information extracted from the external world, CR alters its transmission parameters in order to adapt to the time varying environment 12 .

Cognitive Radio
The main purpose of the cognitive radio (CR) is to make dynamic access to the idle spectrum a reality in order to have communication.A typical scenario consists of CRs, those co-exist with the primary users (PUs).PU has the first priority to use the spectrum while CRs can use the spectrum opportunistically when the PU is not using the spectrum at that time.Main functions of CR are classified as under.

Spectrum Sensing
Spectrum sensing is the important function of the CR.The CR has the capability to sense the radio environment in order to find idle channels known as spectrum hole and use those idle channels for its transmission.The sensing operation should also have the ability to detect the presence of the PU accurately when the later starts its communication again 13 .

Spectrum Management
Spectrum management helps in acquiring the best idle hole for the transmission of CR among large number of available holes found through the operation of sensing.Based on the availability of the spectrum, the suitable hole is allocated to the CR.

Spectrum Mobility
Spectrum mobility is referred as changing the frequency band during data transmission due to presence of PU on that band.The CR has to switch to another idle hole which is not used by the PU at that time in order to continue its transmission 14 .As soon as the PU needs the current frequency band, the CR has to terminate its transmission and free the presently occupied hole for the PU functioning 15 .

Spectrum Sharing
Spectrum sharing is an important functionality of CR as it coordinates the traffic between CRs and PUs.It is a challenging task as it requires high degree of cooperation, understanding and coordination between the two.

Quality of Service
Over the last decade, the development of wireless communication technology has shown exponential increase and wireless services have evolved from traditional voice service to a wide range of multimedia services.The bandwidth requirement varies from application to application.Hence, providing QoS to these applications according to their requirements of bandwidth is a critical and challenging task.QoS is the performance level of a service offered by the network to the user in order to achieve deterministic behavior by proper utilization of the network resources.The meaning of QoS changes as per the requirements of the application field.It is the process by which the performance and reliability of a network is controlled.It is a complex process and the modeling of QoS is a tough and difficult task.ISO 9000 defined QoS as "the degree to which a set of inherent characteristics fulfils requirements".ITU-T [16][17] defined QoS as "the collective effect of service performance which determines the degree of satisfaction of a user of the service".IETF 18 considers QoS as "the ability to segment traffic or differentiate between traffic types in order to enable the network to treat certain traffic flows differently from others".The authors of 19 identified three types of QoS such as Intrinsic QoS, Perceived QoS and Assessed QoS.Intrinsic QoS depends on the performance of the network and is described in terms of objective parameters such as delay, jitter, loss etc. Perceived QoS is the quality perceived by the users and is measured by their average opinion.Assessed QoS refers to the willingness of the users to continue the use of the specific service.

Challenges of Qos Provisioning in CR
The resources available to CRs in cognitive radio are highly dynamic and the QoS should be met without violating the priority right of PUs.QoS provisioning in cognitive radio is a challenging and highly difficult task.The requirement of QoS depends upon the nature of flows and the type of the networks.Cognitive radio imposes unique challenges for providing QoS and the identified issues for QoS provisioning are described as under.

Resource Identification
The bandwidth of wireless networks is scarce than that of wire-line networks due to the physical limitation of wireless media.But the problem becomes even worse in cognitive radio as there is no fixed and dedicated allocation of spectrum for its functioning.CR has to find the idle hole to be used for its operation which is not being used by the PU at that time 20,21,22 .Therefore, the number of idle holes available at any instant is a function of time and varies with the activities of the primary users.Transmission of the CR is allowed only if idle holes are available.If no idle hole is found, blackouts may occur which introduce a peculiar challenge for QoS provisioning 23,24 .

Wireless Channel Characteristics
The wireless is a broadcast medium.The radio waves suffer from several impairments such as attenuation, thermal noise, interference, multipath fading and shadowing etc. 25,26 during propagation through the wireless medium from transmitter to receiver 27,28 .These factors increase the variability of the some of the parameters responsible for providing the QoS.In cognitive radio, due to switching of operating channel, these impairments also change which results in change of channel characteristics.As a result, this puts additional challenges for QoS provisioning.

Sensing Duration
Detection of the primary users is performed through spectrum sensing and sensing duration is the critical parameter for QoS provisioning in cognitive radio 29,30,31 .During sensing phase, data transmission is stopped.Sensing time is the function of interference in the channel and may increase with increase in interference 32 .As a result, this creates the varying jitter between the data packets.The sensing time should be of such duration that it is able to scan the whole spectrum and at the same time it doesn't affect the QoS of the application running over the CR.Therefore, maintaining a trade-off between the two is a challenging task 24,33,34 .

Handoff Management
Due to the arrival of the primary user on the channel presently occupied by the CR, there is a need to change the operating channel.Therefore, the spectrum mobility imposes additional delay due to instantaneous sensing of the spectrum in order to find idle channels to continue the ongoing communication.The movement from one frequency channel to another imposes further challenges of change in channel characteristics and may also introduce variations in the bandwidth and noise levels.The previously existing channel conditions will not be valid in the new operating frequency channel.As a result, there will be change in certain parameters (such as delay, jitter, SNIR, loss etc.) and those parameters have strong influence on QoS provisioning 23,33,35 .

Handoff Rate
Experimental studies have shown that cost of channel switching is significant in wireless mesh networks 36 and channel switching causes non-negligible interruption in the connectivity among nodes which can be of the order of 10 seconds 37 .In cognitive radio, frequent channel switching due to spectrum mobility takes place which will make the situation even worse and will result in QoS degradation of the CRs.Hence, reduction in switching (or handoff) rate is a critical and challenging task.

Interference Management
Sensing phase is followed by data transmission phase in cognitive radio.Interference 38 in the communication channel is introduced due to missed detection which occurs due to low signal-to-noiseratio (SNR) or hidden terminal problem of the primary user.As a result, collision of data packets takes place between CR and the PU which increases the loss rate of the transmission and at the same time increases the interference to the primary user.As a result, there will be decrease in the QoS.Therefore, maintenance of the assured level of QoS becomes a challenging task 11,[23][24][39][40] .

Conclusion
The resource availability is highly dynamic in CR and thus, providing the QoS as per the requirements of the flow is a challenging issue.This paper introduced the challenges of QoS provisioning in the CR and the identified issues for the QoS provisioning.The identified issues are; resource identification, wireless channel characteristics, sensing duration, handoff management, handoff rate and interference management.These issued needs to be resolved before quality of service provisioning becomes reality in CR networks.