Fig. 1: Schematic image showing the electromagnetic radio spectrum
The electromagnetic radio spectrum is a natural resource licensed and allocated by the government for use by transmitters and receivers. A spectrum is a collection of wavelengths of radio frequency waves that are available for wireless communication services. These wavelengths are divided and their brackets (called 'bands') are allocated for various purposes such as GSM/HSPA, 3G, 4G, TV, Wi-Fi, defense communication, broadcasting, mobile satellites, aeronautical satellites, etc. . The electromagnetic spectrum is a resource that cannot be exhausted, but it can be overcrowded in some bands more than others due to this static allocation. The bands most used by telecommunications operators tend to be overcrowded, while some of the other bands, such as broadcast TV or radio, are mostly empty. This causes inefficient use of spectrum, leading to poor signal quality on crowded bands, while many of the undercrowded bands go unused. Underutilization of this electromagnetic spectrum leads to what we call “Spectrum Holes” (a band of frequencies assigned to a primary user, but at a specific time and place is not being used by that user).
To make spectrum usage more efficient, a secondary user, who cannot be served by their band, can access a spectrum gap at the correct time and geographic location. This type of dynamic allocation can be done by “Cognitive Radio” which uses a Software Defined Radio (SDR) for efficient spectrum allocation by using spectrum holes in one band to compensate for overcrowding in another band. In this article we will define a cognitive radio and why we need it and delve into the technology involved. making it work.
History of Cognitive Radio
The need for the development of a cognitive radio has been contributed by many factors, the main one being the constant increase in the need for the radio spectrum with the increase in wireless communication at high communication speeds and stronger signals. In 2000, an explosion occurred at a fireworks factory in the Netherlands, in which 23 people died, injured many people and destroyed many properties. During this crisis, emergency hotlines faced severe communication problems and caused further deterioration and destruction. The same problem was faced during 9/11 in the USA in 2001. These devastating incidents led intellectual and academic societies to work towards developing a better management system for the scarce resource that is the electromagnetic spectrum. In this sense, the United Kingdom took a first step in 2002 through a report by Professor Cave that detailed the possibility of selling spectrum according to the required bandwidth. Many developments and research have been done in the field of communications. The actual word 'Cognitive Radio' was coined by Joseph Mitola in 1999 at a wireless communications seminar on the same at the Royal Institute of Technology, Sweden.
CR Technology
Cognitive radio includes four main functional blocks
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Spectrum Detection
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Spectrum Management
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Spectrum sharing and
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Spectral Mobility
Figure 2: Diagram explaining the concept of cognitive radio
Spectrum Sensing will determine spectrum availability and presence of licensed users (also known as primary users). Spectrum management will predict how long spectrum gaps will remain open for use by unlicensed users (also called secondary users or CR users). Spectrum Sharing aims to allocate spectrum gaps equitably among secondary users, keeping in mind the cost of usage. Spectrum Mobility aims to ensure and maintain seamless communications requirements during the transition to a lighter spectrum. The spectrum sensing function is the most crucial for establishing a cognitive radio network. There are some techniques used for spectrum detection, which are; Primary transmitter detection, cooperative detection and interference detection. The reason why spectrum detection is the most crucial task is that there are many uncertainties connected while picking up the signals to find the gaps in the band, such as channel uncertainty, noise uncertainty, detection interference limit, etc. while solving the problem of spectrum sensing in cognitive radio networks.
Figure 3: Cognitive Radio Network Cycle
Spectrum sensor
Therefore, the biggest challenge in spectrum sensing is that secondary users have to detect the presence of primary users in the licensed spectrum and exit the frequency band as quickly as possible if the corresponding primary radio emerges, so that the primary user does not face any problem. interference. This is the first and most important step in implementing the CR system. Spectrum detection techniques are classified into 3 main types, transmitter detection (non-cooperative detection), cooperative detection and interference based testing. Below are descriptions of the first two detection techniques.
Transmitter Detection
In transmitter detection, there is an additional classification into Power Detection, Combined Filter Detection and Cyclostationary Feature Detection. The power detection method detects the primary signal based on the detected power. The signal passes through a bandpass filter and is integrated over a period of time. The output is tested against a predefined threshold. This reveals whether a primary user is present or absent. A matching filter is a linear filter and is used when the secondary user has prior knowledge of the primary user's signal. It provides more accuracy at the cost of being more complex.
Figure 4: Block diagram showing the power detection detection process
Figure 5: Block diagram explaining corresponding filter detection
Cooperative Techniques
Higher sensitivity requirements for the CR user if multiple CR users cooperate in channel detection. Therefore, it will be shared and compared among multiple CR users to detect a signal
Fig. 6: Comparison of accuracies versus complexities between various CR technologies
Spectrum Management
It captures the best available spectrum to meet the secondary user's requirements while simultaneously creating no interference to the primary user and then transmits within the spectrum range that is allocated through the spectrum sensor. There are often multiple options presented by the spectrum sensing function, but it is the function of the spectrum management system to decide which option is the best choice. Spectrum management is carried out in two main steps
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Spectrum analysis (comparing the options)
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Spectrum Decision (Choosing the best option)
Future of Cognitive Radio
The success of the unlicensed band in accommodating a wide range of wireless devices and services has inspired the FCC (Federal Communications Commission) in the US to open up more unlicensed bands. The US telecommunications authority has realized the capability of CR technology and hopes to open unlicensed bands that will exploit inefficiently used licensed bands since primary users are not interfered with. An example would be the use of radios to operate in TV transmission bands. Although TRAI (Telecom Regulatory Authority of India) still does not use CR networks due to concerns over certain security issues such as; Protection against protocol attacks, application attacks, unauthorized user introduction, unauthorized access to system data, denial of service (DoS) attacks, and distributed denial of service (DDoS). However, a lot of research and funding is being directed towards Cognitive Radio to further resolve the serious problem of Spectrum overcrowding, so that India is now becoming a global hub for Information Technology and Services.
