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Table of Contents CHAPTER NO.
PAGE NO. Chapter 1
1.2 Factors causing the need
Spectrum Classification in
Broader Sense 4.1 Importance Of White Space In Cognitive Radio Chapter 5
Spectrum Sharing In Cognitive
Radio Networks: ‘Underlay’ And ’Overlay’ Techniques Chapter 6
8.1 Spectrum Sensing
8.1.1 Transmitter Detection
8.1.2 Cooperative Detection
8.1.3 Intereference Detection
8.2 Spectrum Management
8.3 Spectrum Mobility
8.4 Spectrum Sharing
Pros and Cons
Practical Application And Future
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List Of Figures
real data on traffic increase
spectrum hole concept
The evolution of ‘spectrum holes’ in the spatio-temporal domain
basic cognitive cycle
classification of spectrum sensing techniques
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1. INTRODUCTION 1.1BACKGROUND The electromagnetic radio spectrum is a natural resource. Today’s wireless networks are characterized by a fixed spectrum assignment policy which is often licensed by governments of different countries. However, a large portion of the assigned spectrum is used sporadically and geographical variations in the utilization of assigned spectrum ranges from 15% to 85% with a high variance in time. The limited available spectrum and the inefficiency in the spectrum usage necessitate a new communication paradigm to exploit the existing wireless spectrum opportunistically. This new networking paradigm is referred to as NeXt Generation (xG) Networks as well as Dynamic Spectrum Access (DSA) and Cognitive Radio Networks.
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Fig.1.1-Real data on traffic increase
1.2 FACTORS CAUSING THE NEED The radio frequency spectrum involves electromagnetic radiation with frequencies between 3000 Hz and 300 GHz. Some of factors of low efficiency of the spectrum bands at an instant of time area. Some frequency bands in the spectrum are largely unoccupied most of the time; b. Some other frequency bands are only partially occupied; c. The remaining frequency bands are heavily used. Spectrum utilization can be improved significantly by making it possible for a secondary user (who is not being serviced) to access a spectrum hole unoccupied by the primary user at the right location and the time in question.
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2. COGNITIVE RADIO 2.1 HISTORY The idea of cognitive radio was first presented officially by Joseph Mitola III in a seminar at KTH, The Royal Institute of Technology in Stockholm, in 1998, published later in an article by Mitola and Gerald Q. Maguire Jr. in 1999.The term cognitive radio was coined by Joseph Mitola considering ideal context aware radios with embedded intelligence. Mitola’s vision of cognitive radios spans across all the layers of the communication protocol stack emphasizing on the need for optimum utilization of the radio resources by adopting its transmission policies and strategies. The adaptation of the local policies is based on sensing and learning the environment or by being informed about the radio environment by an information broker in the network. Page | 1
Haykin then adopted Mitola’s ideal cognitive radio concept to wireless communications by defining the corresponding communications and signal processing problems associated with cognitive radios in the lower layers of the protocol stack. The first phone call over a cognitive radio network was made on Monday 11 January 2010 in Centre for Wireless Communications at University of Oulu using CWC's cognitive radio network CRAMNET (Cognitive Radio Assisted Mobile Ad Hoc Network) that has been developed solely by CWC researchers.
2.2 DEFINITION A cognitive radio is a kind of two-way radio that automatically changes its transmission or reception parameters, in a way where the entire wireless communication network -- of which it is a node -- communicates efficiently, while avoiding interference with licensed or licensed exempt users. This alteration of parameters is based on the active monitoring of several factors in the external and internal radio environment, such as radio frequency spectrum, user behavior and network state.
2.2.1 CHARACTERISTICS From this definition, two main characteristics of the cognitive radio can be defined• Cognitive Capability- Cognitive capability refers to the ability of the radio technology to capture or sense the information from its radio environment .This capability cannot simply be realized by monitoring the power in some frequency band of interest but more sophisticated techniques are required in order to capture the temporal and spatial variations in the radio environment and avoid interference to other users. 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. •
Reconfigurability-The cognitive capability provides spectrum awareness whereas reconfigurability enables the radio to be dynamically programmed Page | 2
according to the radio environment. More specifically, the cognitive radio can be programmed to transmit and receive on a variety of frequencies and to use different transmission access technologies supported by its hardware design.
3. ULTIMATE GOAL The ultimate objective of the cognitive radio is to obtain the best available spectrum through cognitive capability and reconfigurability as described before. Since most of the spectrum is already assigned, the most important challenge is to share the licensed spectrum without interfering with the transmission of other licensed users as illustrated in the figure above. The cognitive radio enables the usage of temporally unused spectrum, which is referred to as spectrum hole or white space. If this band is further used by a licensed user, the cognitive radio moves to another spectrum hole or stays in the same band, altering its transmission power level or modulation scheme to avoid interference. The underutilization of the electromagnetic spectrum leads us to think in terms of spectrum holes, for which we offer the following definition-
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“A spectrum hole is a band of frequencies assigned to a primary user, but, at a particular time and specific geographic location, the band is not being utilized by that user.” Spectrum utilization can be improved significantly by making it possible for a secondary user (who is not being serviced) to access a spectrum hole unoccupied by the primary user at the right location and the time in question.
4. SPECTRUM CLASSIFICATION IN A BROADER SENSE First let us classify the spectral usage in the spatio-temporal domain. By computing the power spectra of the received radio stimuli at a particular point and time one could broadly classify the spectra into three types as given below• • •
Black Spaces: spectra occupied by high-power ’local’ interferers. Gray Spaces: spectra occupied partially by low power interferers. White spaces: spectra free of radio frequency interferers except for ambient natural and manmade noise. It refers to frequencies allocated to a broadcasting service but not used locally.
One could clearly see that the above classification is a function in the spatiotemporal domain. Page | 2
For example, ’black’, ’gray’ and ’white’ spaces could appear and disappear back and forth at a particular location over time. Therefore it is necessary to sense and learn the radio environment in order to maximize the spectral usage opportunistically. In other words detecting ’spectrum holes’ as it is termed is quite crucial for dynamic spectrum access.
Fig.4.1-The evolution of ’spectrum holes’ in the spatio-temporal domain
4.1. IMPORTANCE OF WHITE SPACE IN COGNITIVE RADIO National and international bodies assign different frequencies for specific uses, and in most cases license the rights to broadcast over these frequencies. This frequency allocation process creates a band plan, which for technical reasons assigns white space between used radio bands or channels to avoid interference. In this case, while the frequencies are unused, they have been specifically assigned for a purpose, such as a guard band. Most commonly however, these white spaces exist naturally between used channels, since assigning nearby transmissions to immediately adjacent channels will cause destructive interference to both. In addition to white space assigned for technical reasons, there is also unused radio spectrum which has either never been used, or is becoming free as a result of technical changes. In particular, the switchover to digital television frees up large areas between about 50 MHz and 700 MHz This is because digital transmissions can be packed into adjacent channels, while analog ones cannot. This means that Page | 1
the band can be "compressed" into fewer channels, while still allowing for more transmissions.
5. SPECTRUM SHARING IN COGNITIVE RADIO NETWORKS: ‘UNDERLAY’ AND ’OVERLAY’ TECHNIQUES With cognitive radio technology the concept of ’primary users’ and ’secondary users’ of the spectrum are developed. The primary users are the incumbent users with the exclusive rights to use the spectrum at anytime and the secondary users, also known as the cognitive radio users, are the users that use the spectrum without interfering with the primary users. There are basically two spectrum sharing techniques considered for cognitive radio networks for maximizing the spectral efficiency between the primary and the secondary users. First is the ’spectrum underlay’ technique and second is the ’spectrum overlay’ technique. In the ’spectrum underlay’ method the secondary users can utilize the spectrum simultaneously with the primary users without exceeding a predefined interference level to the primary users. Secondary users in this case can share the spectrum such Page | 2
that the total interference power from the secondary users to the primary users is controlled below the interference limit set by the relevant regulatory authorities. In the ’spectrum overlay’ method the cognitive radios can identify the spectrum holes in the spatio-temporal domain and opportunistically utilize them by giving higher priority to the primary users. Whenever a primary user is not using the spectrum secondary users (cognitive radios) are allowed to transmit however when a primary user is detected in that particular band then secondary users need to immediately vacate the band by stopping transmitting in that particular band. In this sense spectrum sensing and primary user detection become a crucial functionality for reliably detecting the primary users in the environment in the spatio-temporal domain.
6. COGNITIVE CYCLE The cognitive cycle is the term describing the activities involving the intelligence of the radio device such as sensing, learning and adopting.
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Fig.6.1-Basic cognitive cycle. (The figure focuses on three fundamental cognitive tasks.)
This involves understanding three aspects of the cyclea. Spectrum Sensing: A cognitive radio monitors the available spectrum bands, captures their information, and then detects the spectrum holes. b. Spectrum Analysis: The characteristics of the spectrum holes that are detected through spectrum sensing are estimated. c. Spectrum Decision: A cognitive radio determines the data rate, the transmission mode, and the bandwidth of the transmission. Then, the appropriate spectrum band is chosen according to the spectrum characteristics and user requirements.
7. CLASSIFICATION Depending on the set of parameters taken into account in deciding on transmission and reception changes, and for historical reasons, we can distinguish certain types of cognitive radio. The main two are • •
Full cognitive radio (Mitola Radio): in which every possible parameter observable by a wireless node or network is taken into account. Spectrum sensing cognitive radio: in which only the radio frequency spectrum is considered.
Also, depending on the parts of the spectrum available for cognitive radio, we can distinguish: •
Licensed band cognitive radio: in which cognitive radio is capable of using bands assigned to licensed users, apart from unlicensed bands, such as U-NII band or ISM band . Page | 4
Unlicensed band cognitive radio: which can only utilize unlicensed parts of radio frequency spectrum.
Most of the research work is focused on ‘spectrum sensing cognitive radio’ particularly in TV bands. The essential problem is in designing of high quality spectrum sensing devices and algorithms for exchanging spectrum sensing data between nodes.
8. MAIN FUNCTIONS 8.1. SPECTRUM SENSING It involves detecting the unused spectrum and sharing it without harmful interference with other users. It is an important requirement of the cognitive radio network to sense spectrum holes. Detecting primary users is the most efficient way to detect spectrum holes. Spectrum sensing techniques can be classified into three categories:
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Fig.8.1-Classification of spectrum sensing technique
8.1.1.TRANSMITTER DETECTION Cognitive radios must have the capability to determine if a signal from a primary transmitter is locally present in a certain spectrum. There are several approaches proposed: • Matched Filter Detection- Matched filtering is known as optimal method for detection of primary users when the transmitted signal is known. It is a linear filter designed to maximize the output signal to noise ratio for given input signal. It is obtained by correlating a known signal, with an unknown signal to detect the presence of the known signal in the unknown signal. This is equivalent to convolving the unknown signal with a time-reversed version of the signal. Convolution is at the heart of matched filters. The end result comes in the form of a graph which peaks at the point where the two images are most similar. The matched filter is the optimal linear filter for maximizing the signal to noise ratio (SNR) in the presence of additive white stochastic noise. • Energy Detection- Energy detection is a non coherent detection technique in which no prior knowledge of pilot data is required. • Cyclostationary Feature Detection- Cyclostationary feature detection is a method for detecting primary user transmissions by exploiting the cyclostationary features of the received signals. A signal is said to be cyclostationary, if its autocorrelation is a periodic function of time t with some period.
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8.1.2. COOPERATIVE DETECTION It refers to spectrum sensing methods where information from multiple cognitive radio users are incorporated for primary user detection. Multi-path fading is a very important factor which make spectrum sensing less reliable and it cannot be ignored. It is difficult to reducing the effective error rate in a multipath fading channel. The improvement in SNR cannot be made by higher transmit power or additional bandwidth because this is against the requirements of next generation systems .Before transmitting any signal, cognitive radio should estimate the power spectral density of the radio spectrum so as to check which bands are in use and which bands are not utilized. However, there might be another user of that spectrum behind the next building, transmitting to a tower on the hill. Because the building is between the users, the cognitive radio user does not receive another user signal and so concludes the spectrum is unoccupied. The “hidden terminal problem” must be overcome to ensure that primary users of a band are protected from interference. The solution to this hidden terminal problem is cooperative spectrum sensing technique. For contemplation of recent advances in cooperative spectrum sensing, readers are referred to Cognitive radio cooperative spectrum sensing occurs when a group or network of cognitive radios share the sense information they obtain with each other. Cooperation allows achieving the robustness against severe fading without drastic requirements on individual radios. Cooperative sensing rely on the variability of signal strength at various locations is made. It is expected that a large network of cognitive radios with sensing information exchanged with each other would have a better chance of detecting the primary user compared to individual spectrum sensing. The operation of this technique can be performed as follows:• Every CR calculates its own local spectrum sensing measurements independently through sensing channels and then makes a binary decision (1or 0) on whether the PU is present or not. • All of the CRs forward their decisions to a common receiver through reporting channels. • The common receiver fuses the CR decisions using some fusion logic (Xoring or ORing) and makes a final decision to infer the absence or presence of the PU. 8.1.3. INTERFERENCE BASED DETECTION Interference is typically regulated in a transmitter-centric way, which means interference can be controlled at the transmitters. However, interference actually takes place at the receivers.
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8.2. SPECTRUM MANAGEMENT It refers to the capturing the best available spectrum to meet user communication requirements while not creating undue interference to other (primary) users. Cognitive radios should decide on the best spectrum band to meet the quality of service requirements over all available spectrum bands, therefore spectrum management functions are required for Cognitive radios. These management functions can be classified as: • spectrum analysis • spectrum decision The practical implementation of spectrum management functions is a very complex and multifaceted issue in itself, given that it has to address a mixture of technical and legal requirements. An example of the former is choosing appropriate sensing threshold to detect other users, while the latter is exemplified by the need to meet the rules and regulations set out for radio spectrum access in international (ITU Radio Regulations) and national (Telecommunications Law, etc.) legislation. 8.3. SPECTRUM MOBILITY It is defined as the process when a cognitive radio user exchanges its frequency of operation. Cognitive radio networks target to use the spectrum in a dynamic manner by allowing the radio terminals to operate in the best available frequency band, maintaining seamless communication requirements during the transition to better spectrum. 8.4. SPECTRUM SHARING It means providing the fair spectrum scheduling method. One of the major challenges in open spectrum usage is the spectrum sharing. It can be regarded to be similar to generic media access control (MAC) problems in existing systems.
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9. PROS AND CONS 9.1 DISADVANTAGES • • • • • • •
High power consumption Security Cost Software reliability Keeping up with higher data rates Both the subscriber and the base unit should be a software defined radio Still at a research level-involves development of information collection and modeling, decision process, learning process and hardware support. • Regulatory concerns • Loss of control • Fear of undesirable adaptations 9.2 ADVANTAGES • • • • •
Better performance Better and efficient use of the same spectrum Better connectivity More number of users can be added without slowing down of network Provision for easy upgrades Page | 3
• Adjusts operations to meet the quality of service required by the application for the signal environment • Opens more avenues-high speed internet in rural areas and high speed videoconferencing • Interference management • Minimum routing cost maintenance
10. PRACTICAL APPLICATION AND FUTURE USE
Cognitive radio can sense its environment and without the intervention of the user can adapt to the users communication needs while conforming to government rules. Conceptually, the amount of spectrum is infinite, practically for propagation and other reasons it is finite because of the desirability of certain portions of the band. Even the spectrum which is assigned is far from being 100% utilized, hence efficient use of the spectrum is a growing concern. CR offers a solution to this problem. A CR can intelligently detect whether any portion of the spectrum is in use or not, and can temporarily latch into or out of it without interfering with the transmissions of other users thereby efficiently utilizing spectrum. According to Dr.Bruce Fette (2004), "Some of the radio's other cognitive abilities include determining its location, sensing spectrum use by neighboring devices, changing frequency, adjusting output power or even altering transmission parameters and characteristics. All of these capabilities, and others yet to be realized, will provide wireless spectrum users with the ability to adapt to real-time spectrum conditions, offering regulators, licenses and the general public flexible, efficient and comprehensive use of the spectrum".
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The phenomenal success of the unlicensed band in accommodating a range of wireless devices and services has led the FCC to consider opening further bands for unlicensed use. In contrast, the licensed bands are underutilized due to static frequency allocation. Realizing that CR technology has the potential to exploit the inefficiently utilized licensed bands without causing interference to incumbent users; the FCC released the Notice of Proposed Rule Making to allow unlicensed radios to operate in the TV broadcast bands. For instance, the IEEE 802.22 working group formed in November/2004 is equipped with the task of defining the air interface standard for Wireless Regional Area Networks based on CR sensing for the operation of unlicensed devices in the spectrum allocated to TV service.
1. Ian F. Akyildiz, Won-Yeol Lee, Mehmet C. Vuran, Shantidev Mohanty- NeXt generation/dynamic spectrum access/cognitive radio wireless networks: A survey available at www.science direct.com 2. Simon Haykin,IEEE-Cognitive Radio: Brain Empowered Wireless Communications 3. Nisha Yadav and Suman Rathi.-A Comprehensive Study of Spectrum Sensing Techniques In Cognitive Radio 4. Sithamparanathan Kandeepan, Gianmarco Baldini, Radoslaw Piesiewicz-UWB Cognitive Radios . 5. www.wikipedia.org 6. MIT open courseware-www.ocw.mit.edu
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