With the rapid development of digital signal processing (DSP) technology based on very large scale integrated circuits (VLSI), people can gradually get rid of the technical limitations and develop a full-dimensional space that can be arbitrarily accessed regardless of the electromagnetic spectrum without time and space. Equipment with limited power, etc. This paper introduces a new type of adaptive spectrum access technology that can achieve the above functions. It can use frequency resources more flexibly, and solves the spectrum usage problem of software radio (SDR) to some extent. The use of resources is more effective.
The adaptive spectrum access technology introduced in this paper is a new type of frequency assignment and usage system developed on the basis of software radio. The system is more efficient and flexible than traditional frequency assignment and use. Since the formulation of policies has a certain lag compared to the development of technology, the current spectrum policy may not be supported for this system. But we are mainly considering the technical aspects in the text.
1, design background The design of previous wireless communication systems was based on static. The so-called static means that after the frequency band is assigned by the spectrum management department, the communication system can only work on the assigned proprietary frequency band. Use a proprietary single modulator/demodulator, channel protocol, and more. Even multi-band phones switch between only a few predefined bands. This design is usually based on harsh conditions and does not fully utilize the RF spectrum. The ideal software radio can dynamically adapt to changes in any part of the spectrum environment, such as modulation, coding, channel protocol, bandwidth, etc., and can adapt to real-time changes, accessing the spectrum (multidimensional spectrum space) in a multi-dimensional space, Maximize spectrum utilization.
Static design not only reduces the utilization of the RF spectrum, but also affects it in other ways. For example, in a mobile communication system, users can transmit data when the system is idle, but this does not improve the data transmission rate and frequency band utilization. If the system can automatically sense the spectrum environment, adjust its own transmission in real time or use the idle frequency band outside the original designated frequency band, which will definitely change the spectrum utilization greatly, so that users can obtain bandwidth outside the rated range without Need to add extra overhead. The high degree of flexibility of software radios removes the limitations of static design. This has led to continuous research and development, and adaptive spectrum access technology is one of them.
In reality, due to limitations in technology and spectrum assignment systems, the use of any spectrum is not 100% continuous, which results in a huge waste of spectrum resources. In theory, aside from the above limitations, the RF spectrum should be free to use and distribute. This has not been possible in the past, but with the rapid development of radio technology from digital signal processing to antenna technology, it is technically possible to adaptively adjust itself according to the spectrum environment and then access the spectrum space in the entire range. may. This is the adaptive spectrum access technology. For what is adaptive spectrum access technology, just as there is no strict definition of software radio so far, there is no clear definition of adaptive spectrum system, but this system should at least be able to perform the following functions:
Ability to sense the radio spectrum environment.
Self-transmission control based on measurements and other prior information in an autonomous, random, real-time environment.
Adjust multiple transmission parameters, including (but not limited to) frequency, power, modulation scheme, signal timing, data rate, coding rate, and antenna.
Can work in a cooperative mode network system or non-cooperative mode system environment (that is, random application of spectrum).
Thus, the system work is divided into three parts:
(1) Firstly, the system monitors the radio frequency channel in real time, and perceives those unoccupied idle channels (the black square is the main user of the spectrum);
(2) According to the spectrum environment, create a radio frequency transmission waveform compatible with the spectrum environment on the idle channel, and then use multiple subcarriers for data transmission (the subcarriers may or may not be adjacent to each other as shown by the black and white lines in the figure) Adjacent);
(3) Repeat steps 1 and 2.
It can be seen that the adaptive access to the idle spectrum has a lot to do with the activity of the primary users on the channel. By using this technology, we can get the following benefits:
(1) Improved spectrum access and usage. By adjusting the transmission method, the adaptive system can utilize the unused spectrum even if it changes over time;
(2) Maintaining the quality of service in a changing environment;
(3) Adjust the radiated power to reduce or maintain the ability to interfere with other systems.
Compared with the prior art, adaptive spectrum access technology is a technology based on dynamically selecting a spectrum in a multi-dimensional spectral space. The so-called multi-dimensional spectrum space refers to frequency, time, space, power, signal, etc. (not necessarily satisfying the orthogonal condition). The determination of these parameters is important for the system because it has a guiding role in the development and development of spectrum use policies, but its more fundamental role is the responsibility and rights to use the spectrum. From Figure 1, we can see that the main technology in adaptive spectrum access is Dynamic Frequency Selection (DFS).
Some of the most basic systems based on adaptive spectrum access technology have been operating in unlicensed bands for many years (for example, cordless telephone systems operating at around 45 MHz must use an automatic channel selection mechanism to avoid establishing on occupied channels). link). Compared to earlier adaptive spectrum access technologies, more advanced adaptive capabilities include various methods of data rate changes, coding rates, etc., adopted by the second and third generation mobile communication standards.
In recent years, technological advances, especially in the sense of spectrum environment and networks, have formed two adaptive spectrum access technologies. The first type is often referred to as the type of opportunity to look for. The frequency band region where no other system operates does react to the changed radio frequency environment by adaptively changing its own transmission.
In this way of working, the system can work in a non-cooperative manner with other systems, potentially increasing spectrum utilization. Such a system can also share information about the spectrum environment with other similar systems, preventing other users from being interfered by the adaptive system.
This type of opportunistic adaptive spectrum access assumes that the spectrum is not always occupied in multidimensional spectral space, especially in frequency and time. Moreover, this type is only suitable for access to only one adaptive spectrum access device in the current environment.
Another type of adaptive spectrum access is the ability to use shared information or operations to access the spectrum of the multidimensional spectral space in a cooperative manner. This situation inevitably exists in a network with multiple adaptive spectrum access devices. Since each adaptive spectrum access device in the network is an independent node, the adaptive spectrum access behavior is independently performed in the same radio environment, which inevitably causes conflicts in spectrum usage, so Do some necessary negotiation before doing normal communication:
(1) Sharing spectrum patterns among the same device users;
(2) Optimize the shared spectrogram associated with the electromagnetic environment.
Different from the first type mentioned above, the interference mode conflict is avoided by the negotiation mode and the shared spectrum information, so that the multi-dimensional spectrum space is accessed in a coordinated manner, and the spectrum utilization rate is greatly improved.
In future communication systems, one or both of the above two methods may be used simultaneously, so they should not be viewed in opposition to each other. In the first way, since there is no competition with the same device, the use restriction is small, the requirements for the electromagnetic environment and the device are low, and the use is flexible. As can be seen from the second method, since there is competition between multiple identical adaptive devices, negotiation between the same devices is inevitable, which increases the difficulty in use and the complexity of device design. Consider the interference problem with the main spectrum users. These are the directions that need to be studied in the future.
We describe a software-based radio-based adaptive spectrum access technology, which emerged as a result of the rapid development of digital signal processing and a challenge to traditional spectrum access methods. It should be noted that the above adaptive behavior is only a rough adaptive behavior, which requires people to continuously carry out research and improvement. The goal is to create communication systems with higher adaptive capabilities, so future advanced adaptive behaviors should probably include the following:
Formulation of relevant regulations for the use of spectrum in conjunction with this technology.
Limit access and the range of spectrum space that can be accessed (band, time, location, direction, etc.).
Access to the spectrum space under certain conditions (eg, in specific areas, specific signal indications, etc.).
Access negotiated using the dimension of the spectrum space (eg, access to the C-band at time A of time A).
dynamic. The system can decide which criteria and behaviors to use to share information and make requests based on its own measurements of the spectrum environment, not based on pre-defined behavior.
It can be seen that the future adaptive spectrum access is quite intelligent. In order to achieve the above objectives, the following aspects are the focus of our research in the following aspects:
(1) Digital signal processing
Improved A/D, D/A converter with higher sample rate and resolution.
An application specific integrated circuit (ASIC) with lower power consumption and higher processing speed.
Low power consumption, high processing speed, programmable FPGA, DSP, microprocessor, etc.
(2) Radio frequency equipment
Antennas with adaptive techniques (eg dynamic antenna beam, interference cancellation, multi-antenna/channel technology).
Power amplifiers, low noise amplifiers, filters, etc.
(3) Other related electronic technologies
The emergence of new technologies will inevitably bring about the impact on the current spectrum use policy, so that the spectrum management department should re-establish the regulations and policies that are compatible with these technologies, which are people who need to spend a lot of energy to solve in the future.
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