Efficient spectrum utilization technology based on cognitive radio
Abstract: Based on cognitive radio technology (CR), introduces technologies in dynamic spectrum sensing, spectrum management and spectrum sharing. Cognitive radio equipment can use this type of technology to dynamically manage spectrum and improve spectrum utilization.
Keywords: cognitive radio, spectrum sensing, spectrum management, spectrum sharing
Spectrum UTIlizaTIon EffecTIvely Technology Based on CogniTIve Radio
Liu Qingfeng, Chen Jinying, Li junting, Zuo Youfu
(1. College of Information Engineering of Chengdu University Of Technology;)
Abstract: In this paper, we propose Spectrum Sensing, Spectrum Managing and Spectrum sharing based on Cognitive Radio. Cognitive Radio facilities can take advantage of those technologies to dynamically manage spectrum to use spectrum utilization effectively.
Key words: Cognitive radio, Spectrum Sensing, Spectrum Managing, Spectrum sharing
1 Introduction The wireless spectrum is a precious natural resource, and its allocation and utilization are usually determined by the Radio Regulations Department. At present, countries around the world adopt principles and schemes based on static (fixed) frequency band allocation. A frequency band is generally only used by one wireless communication system independently. Different wireless communication systems use different frequency bands without mutual interference. This static wireless spectrum management method is simple and effective to avoid mutual interference between different wireless communication systems. However, this also makes the current spectrum resources increasingly scarce. In November 2002, the US Federal Communications Commission (FCC) released a report submitted by the Spectrum-Policy Task Force to effectively manage US spectrum resources. The report clearly states: “In many frequency bands, the access to spectrum is a more important issue than the scarcity of the spectrum itself, largely because of regulations restricting access to potential users on these spectrums.†This is indeed the case. From January 2004 to August 2005, the spectrum company surveyed the spectrum usage of the US 30-300,000 MHz frequency band and found that the average utilization rate of the frequency band was only 5.2%, of which the highest utilization rate in New York was only 13.1% The lowest rate is the frequency band allocated to radio astronomy, with a utilization rate of only 1%. It can be seen that users in congested frequency bands cannot access the static spectrum management of other idle frequency bands, which greatly limits the spectrum utilization rate. For this reason, increasing spectrum utilization has become an issue of increasing concern.
2 Overview of Cognitive Radio J Mitola of the Swedish Royal Institute of Technology on the basis of software radio in 1999, proposed the concept of cognitive radio (CR, Cognitive Radio), hoping to use this new technology, flexible and effective dynamic management of spectrum The use of resources improves frequency utilization. CR is by definition a smart wireless communication system that can sense the surrounding environment, use the "understand-build" method to obtain information from the surrounding environment, and change transmission parameters such as transmission power, carrier frequency, modulation method, etc. in real time To adapt to changes in the operating environment. In principle, CR can automatically change its own transmission and access parameters through interactive perception of the wireless communication environment in which it works, and dynamically reuse available spectrum. This interactive perception includes passive spectrum sensing or the process of active communication and negotiation with other spectrum users based on the cognitive radio itself, which can only learn inference capabilities. By optimizing its own transmission and reception characteristics to adapt to the surrounding wireless environment, CR's wireless link layer and network layer can effectively transmit information in a limited signal space in an optimized manner, which is an increasingly crowded wireless communication system Realizing efficient use of spectrum resources with devices, coexistence, compatibility and interaction show a bright future.
Academia and IEEE standardization organizations are increasingly interested in CR technology, and call it the "next big event" in the future of wireless communications.
3 Key technologies for the efficient use of spectrum by cognitive radio
The ultimate goal of CR is to obtain the best available spectrum through spectrum sensing, spectrum management and sharing. Since most of the spectrum has been allocated, the biggest challenge is how to share the spectrum with legal users without interfering with it. CR refers to spectrum holes or blank sections that are not being used temporarily. If this frequency band is subsequently used by legal users, CR may transfer to another spectrum hole or continue to use this frequency band as appropriate.
3.1 The spectrum-aware cognitive radio can sensitively know the changes of the surrounding environment, and find the spectrum holes through the spectrum-aware function to adapt it to the surrounding communication environment. Since the first user network has no obligation to change its structure to share spectrum with the CR network, CR can only independently and reliably detect legal users through continuous spectrum sensing, which is a core function of cognitive radio. The most effective detection method is to detect within its communication range and is accepting the legal use of data. Here, the legal user may also be referred to as the first user, and the CR user is accordingly referred to as the second user.
When performing detection, different types of first users have different perceptual sensitivity requirements. For example, the sensitivity of a television receiver is worse than that of a GPS receiver, and a television broadcast signal is easier to detect than a GPS signal. Therefore, the sensitivity of the CR should exceed the sensitivity of the first user receiver to avoid hidden terminal problems. This is the key reason why spectrum sensing is very challenging. It is necessary to meet the sensitivity requirements of each type of first user and have an additional margin of 30-40db. The reason for this margin is: CR is difficult to The measurement of the channel between the user's transmitter and receiver can only be based on the measurement of the local channel condition made by the probe of the first user's transmitter.
Because the radio environment changes at any time, but also because of the different types of first users, different propagation losses, interference and other factors, the realization of the spectrum sensing function requires a high degree of CR flexibility. The main challenge in design comes from how to define the RF structure to achieve a compromise between linearity, sampling frequency, accuracy, and power. In this way, the technology of digital signal processing can be applied to the perception function, which also promotes the research of signal processing to reduce the simulation period for large broadband scenarios, especially broadband amplifiers, mixers and analog-to-digital converters. Challenging requirements.
In general, spectrum sensing technology can be summarized as transmitter detection, cooperative detection and interference-based detection, as shown in Figure 1. 
Figure 1 Classification of spectrum sensing technology
3.2 Spectrum management The unused frequency bands detected by the cognitive radio through the spectrum sensing function may be scattered in a wide frequency domain including licensed and unlicensed frequency bands, and they have different dynamic spectrums such as center frequency and bandwidth. Characteristics, and can be used for different times. Therefore, the most important problem of spectrum management is to design an effective and efficient adaptive strategy for utilizing spectrum. For example, assuming that there is a set of available spectrum holes, dynamic spectrum management can choose a more effective modulation strategy or select another available spectrum hole to improve communication when CR cannot meet user requirements (such as FER requirements cannot be met) Reliability mainly includes two aspects: spectrum analysis and spectrum judgment. First, through spectrum analysis, the spectrum characteristics of the available spectrum holes in the CR network in different time periods are summarized. Once all the available spectrum characteristics are analyzed, the CR should select the appropriate operating band for the current transmission according to the QoS requirements .
3.3 Spectrum sharing It is generally accepted that the modulation strategy applicable to CR is Orthogonal Frequency Division Multiplexing (OFDM). This is because of the flexibility and computational effectiveness of OFDM. Obviously, with the change of time, spectrum holes can be used to come and go, OFDM constantly adjusts its carrier frequency, as shown in Figure 2. 
Figure 2 Dynamic spectrum sharing process
Figure 2 depicts the spectrum sharing strategy at T1, T2, T3 (T1
Figure 3 Classification of spectrum sharing technologies
Structure-based spectrum sharing technology can be divided into centralized spectrum sharing and distributed spectrum sharing. The former solution is mainly a centralized unit to control the spectrum allocation and access process. Each distributed node in the network collects the spectrum information of its own detection and sensing to the centralized control unit, and it draws the spectrum allocation map. The latter solution is mainly used in situations where a centralized structure cannot be constructed. Accordingly, in this case, each distributed node participates in spectrum allocation. Sharing technology based on spectrum allocation behavior can be divided into cooperative spectrum sharing and non-cooperative spectrum sharing. The cooperative spectrum sharing solution considers that the node's behavior will affect other nodes. In other words, each node's perceptual gains will be shared with other nodes, and the spectrum allocation algorithm will also take this information into account. Centralized solutions can be viewed as cooperative, and there are also distributed cooperative solutions. The non-cooperative spectrum sharing is different from the cooperative scheme. The non-cooperative scheme only considers the behavior of its own nodes, so this scheme is also called "selfish". The non-cooperative scheme may result in a reduction in spectrum utilization, but in practical applications, it requires the lowest communication from other nodes. The spectrum sharing based on the access technology is divided into Overlay spectrum sharing and Underlay spectrum sharing. Overlay spectrum sharing is specifically that a cognitive node uses a section of unused spectrum to access the network. In this case, the interference to the first user is minimal. Underlay spectrum sharing utilizes the spectrum spreading technology of the cellular network. Once the spectrum allocation projection is obtained, the cognitive node starts to transmit. On this spectrum, the first user treats the transmission of the cognitive node as noise. Therefore, this solution requires complex spectrum spreading technology. Compared with the Overlay spectrum sharing scheme, Underlay spectrum sharing can utilize a wider bandwidth. The theoretical study of spectrum access in CR networks reveals the trade-offs to be considered when designing spectrum access protocols. For example, the advantage of the cooperative scheme is that it can improve spectrum utilization and fairness. However, if the cost such as frequent information exchange between users is considered, its advantage is not very obvious; for example, Overlay technology The focus is on spectrum holes. Underlay technology must be combined with dynamic expansion technology to avoid interference with the first user. If the balance between system complexity and performance is considered, hybrid technology should be given priority.
4 Conclusion In traditional wireless communication systems, the allocation of spectrum is fixed. Due to the suddenness of the communication process, the utilization of these spectrums is very low. On the other hand, with the rapid development and wide application of wireless communication and multimedia, wireless spectrum resources are becoming increasingly tense, and how to improve spectrum utilization becomes an urgent problem to be solved [6]. CR technology can realize automatic transmission of different frequencies on the basis of communication quality, improve spectrum utilization, and can also use it to launch low-cost 4G services.
references:.
[1] Xia Jinxiang, Fan Pingzhi. Problems, Opportunities and Countermeasures Faced by Radio Spectrum Utilization [J]. China Radio, 2005 (5).
[2] Zhang Xiao, Deng Jianguo, etc. Cognitive radio: see the pin and expand the spectrum [J]. Modern Communications. 2006 (6).
[3] Tan Xuezhi, Jiang Jing, Sun Hongjian. Research on Cognitive Radio Spectrum Sensing Technology [J]. Communication Technology, 2007 (3).
[4] Zhou Xiaofei, Zhang Honggang. Principles and applications of cognitive radio [M]. Beijing: Beijing University of Posts and Telecommunications Press, 2007 (3).
[5] Zou Feifei, Yang Zhen. A dynamic spectrum access MAC scheme based on cognitive radio [J]. Modern Electronics, 2007 (5).
[6] Menon R, Buehrer RM, Reed J. Outage probility based comparison of underlay and overlay spectrum sharing techniques. In Proc. IEEE DySPAN 2005.
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