As we all know, the radio spectrum is a valuable natural resource. Although the electromagnetic spectrum below 3000GHz is theoretically called the radio spectrum, due to technical limitations, humans currently only divide the use frequency band from 9kHz to 400GHz. In fact, military frequency bands are concentrated below 40GHz, while civilian frequency bands are mainly concentrated below 3GHz. With the rapid development of mobile communication technology, new wireless communication services emerge in an endless stream, the number of users continues to increase, and spectrum resources become increasingly scarce. In order to solve this problem, a variety of advanced modulation techniques, coding techniques, multi-antenna techniques, link adaptation and other new technologies have been invented successively. These technologies have improved the channel capacity from different angles and achieved good results. However, due to the Shannon limit, it is impossible to increase the channel capacity indefinitely. Although the radio spectrum can be reused, for a certain frequency point or frequency band, it is limited in a certain time domain and space domain, and cannot be reused. In stark contrast to the shortage of spectrum resources is the extremely low utilization rate of the existing spectrum.
Figure 1 shows the 0-6GHz spectrum utilization tested by the University of California, Berkeley. The actual measurement results show that in the global licensed frequency band, even in the 300MHz to 3GHz frequency band with good signal propagation characteristics and very tight demand, the spectrum utilization rate is less than 6%; in the 3-4GHz frequency band, the spectrum utilization rate is reduced to 0.5%; Above 4GHz, the frequency utilization is lower. Therefore, how to effectively share spectrum resources and fully improve spectrum utilization has become an urgent problem to be solved. In this context, Cognitive Radio (CR, Cognitive Radio) technology proposes a new solution—improving the utilization efficiency of wireless spectrum through Dynamic Spectrum Sharing (DSS, Dynamic Spectrum Sharing).
Application of Cognitive Radio in Military Communication
3.1 Increase the capacity of the communication system
The shortage of wireless spectrum is not only prominent in the civilian field, but also in the military field. Especially under the conditions of modern warfare, a variety of electronic devices are set up intensively in a limited area, which will make the spectrum resources extremely tight. Moreover, with the upgrading of civilian radio equipment and the sharp increase in the number of users, the demand for spectrum is also increasing. Some organizations in some countries have applied to allocate part of the military spectrum to civilian use. This trend will undoubtedly further aggravate the shortage of military radio spectrum resources. CR can dynamically utilize spectrum resources, which can theoretically improve spectrum utilization by dozens of times. Therefore, even if CR is partially adopted, the capacity of the entire communication system can be greatly improved.
3.2 Improve spectrum management efficiency
Battlefield spectrum management is a very important subject, and the military of all countries attaches great importance to the research of this issue. However, at present, the battlefield spectrum allocation is basically carried out in the form of fixed frequency allocation. From the actual combat situation, this plan is not completely successful. On the one hand, this allocation scheme not only leads to low utilization of spectrum resources, but also easily causes electromagnetic interference within the system or between friendly forces; on the other hand, this allocation scheme requires a lot of time for spectrum planning before the battle begins ; In addition, once the communication frequency is determined, it cannot be changed no matter what happens in the combat state. because
Therefore, in the modern warfare where the battlefield situation is changing rapidly, the fixed spectrum allocation scheme is easy to delay the fighters. CR can sense the electromagnetic environment of the battlefield in the area where it is located, and automatically detect the availability of the required bandwidth and spectrum. Therefore, the allocation of spectrum resources can be completed quickly with the help of CR, and the communication frequency can also be automatically adjusted during the communication process. It not only improves the speed of networking, but also improves the electromagnetic compatibility of the entire communication system.
3.3 Improve the anti-interference EOD jammer ability of the system
Anti-jamming capability is an important indicator for measuring communication equipment under the conditions of modern warfare, and it is also an important guarantee for winning the war. Traditional channel anti-jamming technologies mainly include spread spectrum, frequency hopping, time hopping and related technologies derived therefrom. CR not only has the above anti-interference ability, but also suppresses interference by adjusting the beam direction due to its use of position sensing technology, combined with DBF technology. CR not only improves the anti-interference ability, but also can reduce the transmit power and improve the anti-interception ability. Cognitive radio has advanced machine learning capabilities, which can learn and analyze interference, so that it can choose appropriate anti-jamming strategies (choose appropriate communication channel, modulation method, transmit power, frequency hopping pattern, etc.) to actively avoid interference . In addition, because the working frequency band of CR is very wide, it also increases the difficulty of interference.
3.4 Provide RF signal jammer Electronic Countermeasures
The traditional method of electronic countermeasures is to first detect the electromagnetic environment of the battlefield through battlefield radio detection, and then communicate the detected situation to the electronic countermeasures force through the battle communication network, and the troops responsible for the electronic countermeasures task will carry out interference. This method not only requires a lot of manpower and material resources, but also requires close cooperation with the troops responsible for electromagnetic environment reconnaissance and electronic countermeasures. Therefore, the period from reconnaissance to implementation of interference is long, which is easy to delay fighters. CR can quickly and accurately identify friend or foe by sensing the characteristics of the electromagnetic spectrum of the battlefield. It can quickly release or avoid interference while conducting electromagnetic spectrum reconnaissance, which is not available in traditional radio.
3.5 Enhance system interconnectivity
At present, various branches of our army are equipped with a large number of radio stations of different models. These radio stations have different working frequencies, transmit power, modulation methods, etc., and cannot achieve interconnection and interoperability, which has become an important factor restricting the joint operations of the three armed forces. cell phone scrambler jammer can cover a wide frequency band, and use software to realize the baseband processing of the signal, the IF modulation and the generation of the RF signal waveform. By independently loading different software, a CR can communicate with both shortwave radio stations, ultra-short wave radio stations, and even satellites. It is precisely because CR can autonomously learn network communication protocols and services,
Improve the interoperability and interconnectivity of the system.
In addition to the above functions and advantages, CR also provides positioning and environment perception functions, which have the advantages of being less susceptible to civil radio interference and quicker networking, which are irreplaceable advantages of traditional radio.
4 Opportunities and challenges for military cognitive radio
CR is considered to be the direction of the next generation of communication development. Since the CR technology can significantly improve the utilization efficiency of the wireless spectrum, it has attracted great attention in the industry and has developed rapidly in recent years. However, CR still faces many challenges from laboratory to practical and military use:
(1) Most of the current research stays at the physical layer, and there are few researches on CR networking, network topology, network protocols, etc.;
(2) Fast spectrum sensing and signal identification technology needs further research;
(3) Fast and effective environmental perception and how to effectively utilize environmental perception information;
(4) Link re-establishment and maintenance technology when CR performs dynamic frequency adjustment;
(5) The design of the CR terminal is complex, requiring a wide-band, high-sensitivity RF front-end, fast and efficient digital signal processing algorithms, and a robust and reliable hardware and software design that meets military standards;
(6) Machine learning and how to make better use of computer language to make the CR network more intelligent and more in line with the characteristics and requirements of military communications;
(7) Compatibility between CR and active equipment. The cost of CR terminals is high, and even for military use, it is impossible to equip troops on a large scale in a short period of time. This requires that CR can cover most of the frequency bands, modulation methods and frequency hopping methods of our military's active communication equipment;
(8) Miniaturization and low power consumption design of the terminal.
