On Cognitive Radio
This article analyzes some of its key technologies through the introduction of the concept, background, features, and principles of cognitive radio (CR), and explains its spectrum management ideas and development status. Finally, it mentions it and software radio (SDR) relationship. The research believes that CR and SDR technologies should be linked at this stage. After their respective technologies are relatively mature, SDR technology will be integrated into CR to achieve a true CR.
[Keywords] CR; key technologies; SDR
[Abstract] This essay introduces the definiTIon, background, characterisTIc, principle of CogniTIve Radio, CR, and we analyze its several key technologies, besides, we talk about its management ideas, the present situaTIon of development, and in the end, we connect it with SDR. This essay point that now we should connect CR with SDR, and we could merge SDR into CR when technology develops and realize real CR.
[Key words] CR; key technologies; SDR
Introduction In modern society, wireless spectrum has become an indispensable precious resource. Numerous studies by the Federal Communications Commission (FCC) have shown that some unlicensed bands such as industrial, scientific, and medical bands and the band around 2 GHz used for land mobile communications are overcrowded, while some bands are often idle. So people thought that if there is a system, it can automatically perceive the spectrum environment, find the spectrum hole (the frequency band that is not used by the main user temporarily) and use it, it can greatly improve the spectrum utilization rate. Based on this idea, people have proposed cognitive radio.
1 Cognitive Radio Concept The concept of Cognitive Radio (CR) originated from the groundbreaking work of Dr. Joseph Mitolo in 1999 [1]. It can adaptively adjust the internal communication mechanism and change specific wireless operating parameters (such as power, carrier modulation and coding, etc.) in real time through learning, understanding, etc. to adapt to the external wireless environment and autonomously find and use idle spectrum. It can help users choose the best and most suitable service for wireless transmission, and can even delay or initiate transmission based on existing or upcoming wireless resources.
2 The cognitive ability of cognitive radio features to the environment: This feature is the premise of the establishment of CR technology. Only on the basis of environmental perception and detection can spectrum resources be used. The main function of spectrum sensing is to monitor a certain range of frequency bands and detect spectrum holes.
Learning ability and adaptability to environmental changes: This feature reflects the intelligence of CR technology. When encountering the main user signal, it can actively retreat as soon as possible and switch freely between spectrum holes.
High reliability of communication quality: The system is required to achieve highly reliable communication at any time and any place, accurately determine the time, location, frequency band [2] and other information of the main user signal, adjust its own parameters in time, and improve communication quality.
Reconfigurability of system function modules: CR devices can be dynamically programmed according to the spectrum environment, or can be designed through hardware to support different transceiver technologies. Parameters that can be reconstructed include: operating frequency, modulation method, transmit power, and communication protocol.
3 The principle of cognitive radio The principle of cognitive radio is shown in Figure 1. It can be seen from the figure that CR equipment perceives, detects, and analyzes the surrounding environment. This detection and perception is omnidirectional, and it also has comprehensive information such as terrain and weather. Understand. It can also be drawn from this figure that CR is a highly intelligent device and should contain an intelligent transceiver. With enough artificial intelligence, it can draw on past experience to respond to the actual situation. Past experience includes understanding of dead zones, interference, and usage patterns. Its learning ability is the real reason that makes it move from concept to application. 
Figure 1: Schematic diagram of cognitive radio. When CR users find spectrum holes and use the spectrum resources of authorized users [3], they must ensure that its communication does not affect the communication of authorized users. Once the frequency band is used by the main user, There are two ways to respond to CR: one is to switch to other idle frequency band communication; the other is to continue to use this frequency band, change the transmission frequency or modulation scheme, to avoid interference with the main user.
4 Key technologies of cognitive radio
4.1 Spectrum detection technology Spectrum detection is to monitor the spectrum gap that CR can use in a complex environment. The following figure 2 is the actual operation diagram: 
Figure 2: The actual spectrum detection diagram In the above figure, the left part is the processing of the environment in which it is located. Mastering various information of the environment is the prerequisite for correct judgment of spectrum information. Integrate various technologies.
The processing of the environment mainly considers the seamless access, security, privacy, data learning ability, tracking ability, dynamic data management, positioning, and perception of CR devices. The following mainly introduces seamless access.
Seamless access ensures the accuracy of CR in a dynamic environment. There are two main methods to achieve it: a waveform-based method and an environment-aware method. The former is equipped with standard waveforms in the positioning system and waveform converter, which can be called at any time when needed. The SPACE project in Europe uses this method. The model consists of a positioning system, a waver, an algorithm selector, and sensors. The second method is to perceive the parameters of the channel environment and select the correct algorithm accordingly. In an unknown environment, the positioning analysis algorithm based on RSS has been widely adopted. This algorithm comprehensively analyzes the positioning parameters of CR devices and the loss parameters of devices in the channel environment, and ensures seamless access through the sensor output signal. The latter method is less complex than the previous method, and CR is considered to have two methods.
At present, there are many methods of spectrum detection research: matched filter method, energy detection method, specific detection method of cycle balance, etc.
Matched filter method: If the CR user has known various information of the main user signal in advance, this method can get the best effect in the additive Gaussian noise channel, which can maximize the signal-to-noise ratio of the received signal. Its disadvantage is that CR needs the information of the main user, which is suitable for the spectrum environment where the main user information is relatively known, such as the UHF TV frequency band that everyone is already familiar with.
Energy detection method: The energy detection method is applicable to the environment where the main user information is not known. For this method, the FCC has proposed a new measurement concept of the interference temperature limit. The channel with the interference temperature below it is a suitable spectrum hole. Its advantage is simple operation, the disadvantage is that the interference temperature limit is more difficult to set, and its threshold value depends on the specific environment [4].
Cyclic balance specific detection method: In communication, useful signals are generally modulated by artificial periodic signals and have potential periodicity, but noise does not have this feature. This method is used to detect this feature and is more suitable for noisy environments. Its disadvantage is that the algorithm is complex and computationally intensive, and the detection time is relatively long.
4.2 Spectrum management in cognitive radio At this stage, some people divide the spectrum into three levels: the first level: strict management and non-interference; the second level: can be interfered by unauthorized users to a certain extent; the third level : Unlimited use. At present, CR's spectrum management ideas have not reached a consensus. Good management ideas should reflect the advantages of CR and improve spectrum utilization to meet the needs of modern society. At present, some standardization organizations have formulated a series of standards to promote the development of the technology in various application scenarios. Various CR-based spectrum management ideas and management rules are still under study.
5 The status quo of cognitive radio development At present, CR is mainly in the primary stage, and various theories and technologies are in research and exploration, but it has received attention from all walks of life, and many well-known scholars and institutions have invested in its research and launched many targets. This is an important research project. The most interesting thing is the work of the IEEE802.22 working group, which has developed technical standards for broadband wireless access using idle TV bands. This is the first IEEE technical standardization activity to introduce the concept of cognitive radio. Radio Knowledge Description Language (RKRL) also came into being. The main goal of CR in the near future is to improve the spectrum utilization rate. Researches predict that the spectrum utilization rate will increase by 3% -10% [5]. Its long-term goal is to better integrate with various technologies to meet the increasing frequency requirements of users. At present, cognitive radio technology is hot, and its application prospects are promising. There are reports that wireless LAN products with cognitive functions will come out in the next year or two, but to truly implement CR technology, key technical issues including spectrum detection technology, adaptive spectrum resource allocation technology and wireless spectrum management technology need to be solved.
6 Relationship between Cognitive Radio and Software Radio Cognitive radio technology is a new intelligent wireless communication technology developed on the basis of software radio technology (SDR). It is an extension of SDR. It enables SDR from pre-defined protocols. The blind performer transformed into an intelligent agent in the radio field. The most basic form of CR is a hybrid technology that includes SDR, where SDR is applied to spread-spectrum communications. SDR focuses on the implementation of signal processing in radio systems; CR refers to wireless systems that can perceive changes in the operating environment Adjust the system working parameters accordingly [6]. In this sense, CR is a higher-level concept that includes not only signal processing, but also high-level functions for reasoning and planning based on corresponding tasks, policies, rules, and goals. This article believes that at this stage, we can use the advantages of SDR and CR to let SDR mainly complete background work, such as signal processing and corresponding algorithms. CR intelligently selects spectrum holes based on the results of SDR processing. Can improve efficiency. When the CR detects a spectrum hole, it sends the relevant information of the spectrum to the SDR terminal, such as the information of the main user: modulation mode, timing, packaging format, etc. The SDR analyzes the processing, and at the SDR terminal, fast processing ensures the CR ’s real-time. After the technology is mature, you can consider the comprehensive use of various technologies to expand the CR function and integrate SDR transfer into the CR to achieve system optimization.
Conclusion The introduction of cognitive radio technology provides a strong technical support for solving the shortage of spectrum resources, realizing the dynamic management of spectrum and improving the utilization rate of spectrum. It provides very strong support for the full utilization of spectrum resources [7], and it will become the next wave of impactful technological innovation. At this stage, people are researching and exploring various technologies to achieve CR. This article believes that it can be combined with SDR first. After the technology is mature, CR functions can be expanded to make excellent CR products.
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