COGNITIVE RADIO: A REVIEW

COGNITIVE RADIO: A REVIEW

Pooja Ameta, Nidhi Jain, K. Mahima,

M. Tech Scholars, Department Of Electronics and Communication Shrinathji Institute of Technology and Engineering, Nathdwara

Pujaameta21@gmail.com, nidhijain745@gmail.com, k.mahima88@gmail.com

Abstract – The frequency spectrum bandwidth used in modern wireless systems is limited while the number of wireless systems is rapidly increasing. In order to reduce the spectrum scarcity, secondary systems can opportunistically access the temporarily unused licensed bands of primary systems which are known as spectrum holes or white spaces, by altering their transmitting parameters so that the interference is minimum to primary user. Cognitive radio is the exciting technologies that offer new approaches to the spectrum usage. Cognitive Radio provides a tempting solution to spectral crowding problem by introducing the opportunistic usage of frequency bands that are not heavily occupied by their licensed users. This paper survey intdoduce fundamental of cognitive radio technology, architecture of a cognitive radio network and its applications are first introduced.

Keywords: Cognitive radio, Cognitive radio Network, Primary User, Secondary User, Spectrum Sensing

1. INTRODUCTION

   The concept of CR was first proposed by Dr. Joseph Mitola in 1999.Cognitive radio technology is the key technology that enables a network to use spectrum in a dynamic manner. The term, cognitive radio, can be formally defined as follows:

   A Cognitive Radio is a radio that can change its transmitter parameters based on interaction with the environment in which it operates [1].

   Since a cognitive radio operates as a secondary user which does not have primary rights to any pre-assigned frequency bands, it is necessary for it to dynamically detect the presence of primary users. From this definition, two main characteristics of the cognitive radio are defined as follows [2], [3]:

   • Cognitive capability: Cognitive capability refers to the ability of the radio technology to capture or sense the information from its radio environment. Through this capability, the portions of the spectrum that are unused at a specific time or location can be identified. Consequently, the best spectrum and appropriate operating parameters can be selected.

   • Re-configurability: The cognitive capability provides spectrum awareness whereas re-configurability enables the radio to be dynamically programmed according to the radio environment. More specifically, cognitive radios can be programmed to transmit and receive over a broad range of frequencies and to use different transmission access technologies supported by their hardware.

     The cognitive radio enables the usage of temporally unused spectrum (figure 1), which is referred to as spectrum hole or white space. Spectrum holes are frequency bands left unused by a primary used for a certain amount of time. In order to exploit

     the spectrum holes, the cognitive radio can adjust its transmission parameters.

     Figure 1 spectrum holes used by CR user

2. COGNITIVE RADIO CYCLE

   The main functions of cognitive radio are Spectrum Sensing, Spectrum mobility, Spectrum management and Spectrum Sharing. The cognitive radio technology will enable the users (a) to determine which portions of the spectrum is available and detect the presence of licensed users when a user operates in a licensed band (spectrum sensing), (b) to select the best available channel (spectrum management), (c) to coordinate access to this channel with other users (spectrum sharing), and (d) to vacate the channel when a licensed user is detected (spectrum mobility)[1]. Figure shows a Cognitive radio cycle [4].

   • Spectrum Sensing: As the first and most important function of a cognitive radio, it is the process of detecting unused portions of spectrum in order to use them opportunistically.

   • Spectrum management: Once the spectrum holes are detected, the cognitive radio must then have the ability to choose the channel that suits its communication requirements.

   • Spectrum mobility: Since the cognitive radios are given lower priority, they should be able to suspend their communication in case a licensed user comes back and seamlessly move onto another vacant channel.

   • Spectrum sharing: In a network there must a scheduling

     algorithm involved to ensure that all the cognitive radios get a fair chance to use the spectrum [1].

     Figure 2 Cognitive Radio Cycle

     Since the cognitive (unlicensed) users utilize the licensed band, they must detect the presence of licensed (primary) users in a very short time and must vacate the band for the primary users.

3. COGNITIVE RADIO NETWORK

   The components of the cognitive radio network architecture, as shown in Figure. 3 can be classified in two groups as the primary network and the cognitive network. Primary network is referred to as the legacy network that has an exclusive right to a certain spectrum band. While, cognitive network does not have a license to operate in the desired band.

   A secondary network refers to a network composed of a set of secondary users with/without a secondary base station. Secondary users can only access the licensed spectrum when it is not occupied by a primary user. The opportunistic spectrum access of secondary users is usually coordinated by a secondary base station, which is a fixed infrastructure component serving as a hub of the secondary network. Both secondary users and secondary base stations are equipped with CR functions. If several secondary networks share one common spectrum band, their spectrum usage may be coordinated by a central network entity, called spectrum broker [5]. The spectrum broker collects operation information from each secondary network, and allocates the network resources to achieve efficient and fair spectrum sharing.

   Figure 3 Cognitive Radio Network

   A primary network is composed of a set of primary users and one or more primary base stations. Primary users are authorized to use certain licensed spectrum bands under the coordination of primary base stations. Their transmission should not be interfered by secondary networks. Primary users and primary base stations are in general not equipped with CR functions. Therefore, if a secondary network shares a licensed spectrum band with a primary network, besides detecting the spectrum white space and utilizing the best spectrum band, the secondary network is required to immediately detect the presence of a primary user and direct the secondary

   transmission to another available band so as to avoid interfering with primary transmission.

4. SPECTRUM SENSING

   Spectrum sensing technique can be categorized into two types. They are: Direct and Indirect Techniques. Direct Technique is also called as frequency domain out in which estimation is carried out directly from signal approach. Where as in Indirect Technique (also called as time domain approach), in this technique estimation is performed using autocorrelation of the signal. Another way of classification depends on the need of spectrum sensing as stated below.

   Where H0 = Absence of User. H1 = Presence of User.

   The block diagram for the energy detection technique is shown in the Figure 4. In this method, signal is passed through band pass filter of the bandwidth W and is integrated over time interval. The output from the integrator block is then compared to a predefined threshold. This comparison is used to discover the existence of absence of the primary ser. The threshold value can set to be fixed or variable based on the channel conditions.

   y(k) = n(k) H0 y(k) = h * s(k) + n(k) H1

   Figure 4 Spectrum Sensing Technique

   1. Spectrum Sensing for Spectrum opportunities

      1. Primary transmitter detection: Based on the received signal at CR users the detection of primary users is performed. This approach includes matched filter (MF) based detection, energy based detection, covariance based detection, waveform based detection, cyclostationary based detection, Primary Transmitter Detection etc .

      2. Cooperative and collaborative detection: The primary signals for spectrum opportunities are detected reliably by interacting or cooperating with other users, and the method can be implemented as either centralized access to spectrum coordinated by a spectrum server or distributed approach implied by the spectrum load smoothing algorithm or external detection.

   2. Spectrum Sensing for Interference Detection

   1. Interference temperature detection: In this approach, CR system works as in the ultra wide band (UWB) technology where the secondary users coexist with primary users and are allowed to transmit with low power and are restricted by the interference temperature level so as not to cause harmful interference to primary users.

   2. Primary receiver detection: In this method, the interference and/or spectrum opportunities are detected based on primary receiver's local oscillator leakage power

   1. Primary Transmitter Detection: In this we are going to discuss about few primary transmitter detection techniques. They are:

      1. Energy Detection: In this technique there is no need of prior knowledge of Primary signal energy.

         Figure 5 Block Diagram Of Energy Detector

         Where y (k) is the sample to be analyzed at each instant k and n (k) is the noise of variance 2. Let y(k) be a sequence of received samples k{1, 2.N} at the signal detector, then a decision rule can be stated as,

         H0 if > v H1 if < v

         Where = E |y(k)|2 the estimated energy of the received signal and v is chosen to be the noise variance 2.

      2. Matched Filter:

      Figure 6 Block Diagram of Matched Filter

      Where H0 = Absence of User. H1 = Presence of User.

      A matched filter (MF) is a linear filter designed to maximize the output signal to noise ratio for a given input signal. When

      secondary user has a priori knowledge of primary user signal, matched filter detection is applied. Matched filter operation is equivalent to correlation in which the unknown signal is convolved with the filter whose impulse response is the mirror and time shifted version of a reference signal. The operation of matched filter detection is expressed as:

      Y[n] = h[n-k] x[k]

      Where x is the unknown signal (vector) and is convolved with the h, the impulse response of matched filter that is matched to the reference signal for maximizing the SNR. Detection by using matched filter is useful only in cases where the information from the primary users is known to the cognitive users.

   2. Cooperative Techniques

      1. Decentralized Uncoordinated Techniques: In uncoordinated techniques Cognitive Radio will independently detects the channel and will vacate the channel when it finds a primary user without informing the other users. So these are not advantageous when compared to coordinated techniques.

      2. Centralized Coordinated Techniques: Here in this

         technique we have Cognitive Radio controller. When one Cognitive Radio detects the presence of primary user then it intimates the Cognitive Radio controller about it. Then that controller informs all the Cognitive radio users by broadcast method.

      3. Decentralized Coordinated Techniques: This type of coordination implies building up a network of cognitive radios without having the need of a controller. Various algorithms have been proposed for the decentralized techniques among which are the gossiping algorithms or clustering schemes, where cognitive users gather to clusters, auto coordinating themselves. The cooperative spectrum sensing raises the need cost sensor node close to a primary user's receiver in order to detect the local oscillator (LO) leakage power emitted by the RF

      Spectrum Sensing Technique	Advantage	Disadvantage
      Matched	Optimum	Requires full primary
      Filtering	performance	signal
      		knowledge, high power
      		consumption
      		and implementation
      		complexity
      Energy	Low	Vulnerable to noise
      Detection	complexity,	uncertainty
      	no primary
      	knowledge
      	required
      Cyclo	Robust to	High computational
      Feature	interference	complexity,
      Detection	and	vulnerable to
      	noise	sampling clock offsets
      	uncertainty	and
      		model uncertainties,
      		long
      		observation time

      Table1 Summary of main spectrum sensing Technique

      front end of the primary user's receiver which are within the communication range of CR system users. The local sensor then reports the sensed information to the CR users so that they can identify the spectrum occupancy status. We note that this method can also be used to identify the spectrum opportunities to operate CR users in spectrum overlay.

      2) Interference Temperature Management: The basic idea behind the interference temperature management is to set up an upper interference limit for given frequency band in specific geographic location such that the CR users are not allowed to cause harmful interference while using the specific band in specific area. Typically, CR user transmitters control their interference by regulating their transmission power (their in CRNs. There is an inherent requirement to distinguish between PUs and CRUs. Therefore, authentication can be considered as one of the basic requirements for CRNs. In the infrastructure based CRNs, where the primary and secondary BSs are connected to a wired backbone network, it may be

      for a control channel, which can be implemented as a dedicated frequency channel or as an underlay UWB channel

   3. Interference Based Detection: In this section, we present interference based detection so that the CR users would operate in spectrum underlay (UWB like) approach.

   1) Primary Receiver Detection: Primary receiver emits the local oscillator (LO) leakage power from its RF front end while receiving the data from primary transmitter. It has been suggested as a method to detect primary user by mounting a low

   out of band emissions) based on their locations with \respect to primary users. This method basically concentrates on measuring interference at the receiver. The operating principle of this method is like an UWB technology where the CR users are allowed to coexist and transmit simultaneously with primary users using low transmit power that is restricted by the interference temperature level so as not to cause harmful interference to primary users[7].

5. SECURITY CHALLENGES IN COGNITIVE RADIO NETWORKS

   Security and privacy have been essentia problems since the arrival of the information era. Security is essential in any network and has been relatively well studied. There is a well- defined security architecture, which consists of security attacks, security mechanisms, and security services/requirements, to define, study, and evaluate security needs in a systematic way . In the context of CRNs, the main security goals include the following:

   • Confidentiality: It prevents unauthorized disclosure of transmitted information from passive attacks, such as eavesdropping. This is achieved by employing ciphers and encrypting the data to be transmitted with a secret key which is shared only with the recipients. The encrypted data are then transmitted and only the recipients with a valid key can decrypt and read the data. This issue is even more pronounced in CRNs, where the CRU access to the network is opportunistic and spectrum availability is not guaranteed.

     • Integrity: It ensures that the transmitted information is not illegally modified. Modification includes changing, deleting, creating, delaying, or replaying transmitted messages. Integrity is extremely important in wireless networks because, unlike their wired counterparts, the wireless medium is easily accessible to intruders. A message integrity check (MIC), which is used to verify the integrity of the message by the recipient, can also be employed in CRNs.

   • Authentication: The primary objective of an authentication is to prevent unauthorized users from gaining access to protected systems. It is a necessary procedure for verifying both an entitys identity and authority. Several aspects of authentication issues should be considered when securing collaborative works

     easier to have the certificate authority (CA) or TTP connected to the wired backbone. However, in the infrastructure less CRNs with a number of CRUs dispersed over a large geographical area, providing the functionalities of a CA can be quite a challenge.

     • Nonrepudiation: It guarantees that neither the sender nor the receiver of a message is able to deny the transmission.

       In CRN setting, if malicious CRUs violating the protocol are identified, nonrepudiation techniques can be used to prove the misbehavior and disassociate/ ban the malicious users from the secondary network.

   • Access control:. In the context of CRNs, we have a unique access control requirement which could call conditional access control. It is conditional because the CRUs are authorized to transmit in licensed bands only as long as they do not interfere with PUs communications in that band. As it is difficult to pinpoint exactly which of the CRUs is responsible for harmful interference to the primaries transmission, this type of access control is hard to enforce and even more so in a distributed setting. Hence, conditional access control poses a unique challenge in dynamic spectrum access.

     • Availability: It requires the network services to be available to devices and applications via communication links. In CRNs, availability refers to the ability of PUs and CRUs to access the spectrum. For PUs, availability refers to being able to transmit in the licensed band without harmful interference from the CRUs. For the CRUs, availability refers to the existence of chunks of spectrum, where the CRU can transmit without causing harmful interference to the PUs.

     Compared with the above mentioned security problems, privacy issues have received little attention in CRNs. The definition of privacy also varies with the application scenario . In the context of CRNs, we consider the following privacy services/requirements indispensable:

   • Nonlinkability: Different communication sessions associated with the same user should not be linkable. An adversary cannot link the communication activities of a particular user together and thus establish the users profile, which contains much private information.

   • Location privacy: A devices current and past location should not be disclosed to adversaries.

   • Context privacy: An adversary should not be able to learn the exact access context information (duration, type of service request, etc.) of a user without the users prior approval or knowledge.

   • Anonymity: The identity of the origin and/or the destination of a conversation is hidden from adversaries unless it is intentionally disclosed by the user. Anonymity impacts location privacy, because as long as a user is anonymous, location privacy is preserved. Anonymity mechanisms should allow the user to use the network services while protecting the identity or other identification information from possible abuse. For keeping the user anonymous, there should not be possibility to link any parameters of the user identity with any contextbased information [8].

   1. CONCLUSION

      As the usage of frequency spectrum is increasing, it is becoming more valuable. So we need to access the frequency spectrum wisely. For this purpose we are using Cognitive Radio. In our paper we discussed about the cognitive radio concept including

      cognitive radio cycle, Network Architecture, Spectrum sensing techniques and security challenges of cognitive radio.

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