Review the origin, concept and characteristics of software radio, and introduce its basic structure and some implementation techniques in detail.
First, the origin of software radioSoftware Radio originally originated from military communications. Military radios are generally designed for a specific purpose and have a single function. Although some radios have similar basic structures, their signal characteristics vary widely, such as working frequency bands, modulation methods, waveform structures, communication protocols, coding methods, or encryption methods. These differences greatly limit the interoperability between different stations and bring difficulties to coordinated operations. Similarly, civil communication also has interoperability problems. For example, the existing mobile communication systems have different standards and frequencies, and cannot be interoperable and compatible, which brings great inconvenience to people engaged in transnational business, tourism and other activities. In order to solve the problem of interoperability of wireless communications, the military of various countries has actively explored. May 1992, at the United States Telecom System Conference. At the IEEE NaTIonal Telesystems Conference, MITRE's JoeMitola first explicitly proposed the concept of software radio.
Second, software radio concept and characteristics The so-called software radio, that is, the modulation waveform of its path is determined by software, that is, software radio is a wireless communication design that uses software to achieve physical layer connection. The core of software radio is to make broadband A/D and D/A as close as possible to the antenna and realize as many radio functions as possible by software. The central idea is to implement software programming through a standardized and modular general hardware platform. An open architecture with multi-channel, multi-level and multi-mode wireless communication capabilities. With software radio technology, a mobile terminal can be used unimpeded between different systems and platforms.
The main advantage of software radio is its flexibility, which can be easily added by adding software modules. In software radio, such as channel bandwidth, modulation and coding, can be dynamically adjusted to accommodate changes in network standards and environments, network traffic loads, and user needs. Software radio has a strong openness. Due to the standardized and modular structure, its hardware can be updated or expanded with the development of devices and technologies, and software can be upgraded as needed.
Software radio has driven the development of programmable hardware, expanding its programming capabilities and increasing its flexibility. Today's wireless communication devices, including mobile phones, use DSP, but DSP software is mostly solidified in the device, and the DSP hardware is dedicated. If the DSP hardware is more generalized and its software can be loaded by wired or wireless means, then one device can work under different standards, frequency bands and protocols. When a user carries a software radio to another country, the software radio can receive and download the communication standard of the area from the air, and then use the regional communication standard to run its own software radio. It brings great convenience to people.
1. Broadband/multi-band antenna and RF module
Broadband/multi-band antennas and RF modules are an irreplaceable hardware gateway for software radios. Software radio requires that the antenna cover all frequency bands and can be programmed with its functions and parameters. Intelligent antenna technology can be used.
The smart antenna is also called an adaptive array antenna, and is composed of an antenna array, a beamforming network, and a beamforming algorithm. It adjusts the weighted amplitude and phase of each array element signal by satisfying a certain criterion algorithm, and then adjusts the shape of the antenna array to achieve the purpose of enhancing the required signal and suppressing the interference signal. Smart antennas can also be explained by the concept of space division multiplexing (SDMA), which uses the difference in the direction of incidence of the signal to distinguish the signals of the same frequency and the time slot, thereby achieving the purpose of multiplying the capacity of the communication system. The smart antenna has the functions of suppressing noise, automatically tracking signals, and forming a digital beam by using an intelligent spatiotemporal processing algorithm. At present, smart antenna technology is becoming more and more perfect. China Telecom Science and Technology Xinwei Company has launched a synchronous CDMA system with smart antennas, and the smart antenna of American Midea Communication Company has also begun to be put on the market.
The RF section includes two parts: preamplification and power output. The RF transmitter and receiver consist of a common platform and multiple RF transmitter modules. The operating frequency band should be wide enough and set by digital frequency synthesis technology. It should be capable of multi-carrier operation for each standard. The transmitter includes multiple high power amplifiers that require high linearity.
2. Analog to digital conversion part
Digitization is the foundation of software radio, and analog signals must be sampled and converted to digital signals for processing in software. An important feature of the software radio architecture is that the A/D and D/A are as close as possible to the RF front. The location of the A/D and D/A devices in the software radio is critical and directly reflects the software operability of the software radio system. In order to reduce the analog link and adapt to the intricate electromagnetic environment, A/D devices are required to have moderate sampling frequency, high working speed, wide working bandwidth and large dynamic range. When designing a radio system, the performance indicators based on the selection of the analog-to-digital device are: signal-to-noise ratio, conversion sensitivity, no-split dynamic range, nonlinear error, intermodulation distortion, full-power analog input bandwidth, and so on.
Limitations of A/D device performance and spectral aliasing and quantization errors introduced during sampling can adversely affect the performance of software radio stations, but this effect is still lacking in quantitative analysis.
3. High speed digital signal processor
DSP is the basic device necessary for software radio, and it is the soul and core of it. The system digitizes the received signal at the radio frequency or intermediate frequency (IF), and flexibly implements wideband digital filtering, direct digital frequency synthesis, digital up-down conversion, modulation and demodulation, error coding, signaling control, source coding and addition through software programming. Decryption function. Upon reception, the signal from the antenna is RF processed and transformed, digitized by the wideband A/D, and then subjected to various signal processing required by the programmable DSP module, and the processed data signal is sent to the multi-function user terminal. When transmitting, the data is transmitted through the antenna through a reverse process similar to the process of receiving the signal. It can be seen that the flexibility, openness and compatibility of software radio are mainly realized by a general hardware platform and DSP software centered on digital signal processing.
The current DSP can not meet the requirements of the radio in terms of function or performance. It is difficult to directly process the broadband RF or IF signal with a single DSP. The digital RF technology can be used to process the broadband RF or IF signal, and then Use DSP to complete various signal processing functions. The composition of the digital frequency conversion is similar to that of the analog frequency conversion, including the digital mixer, the digitally controlled oscillator and the low-pass filter. The difference is that the digital frequency conversion uses orthogonal mixing. Digital frequency conversion has the advantages of carrier frequency and digital filter coefficient programmability, non-linear distortion, good frequency response characteristics and low cost.
Because software radio has many advantages that the existing wireless communication system does not have, it has broad application prospects. At present, software radio has developed rapidly at home and abroad. The US Department of Defense has completed the second phase of the "Speakeasy Project" and applied it in the field of electronic warfare; the ACTSFIRST project of the European Community and the University of Rutgers in the United States have separately studied the application of software radio to the third generation mobile communication system; Radio technology has been incorporated into the national “863†high-tech development plan. At present, the core of the second-generation synchronous orbit space measurement and control equipment program being researched and developed in China is the introduction of software radio technology.
With the development of wireless networks, various wireless communication architectures and design specifications have emerged. Future seamless multi-mode networks require radio terminals and base stations with flexible RF bands, channel access modes, data rates, and application functions. Software radio can improve service quality through flexible resilience; it also simplifies hardware composition and quickly adapts to emerging standards and management methods.
It can be foreseen that with the rapid development of modern computer software and hardware technology and microelectronic technology, software radio technology will be developed faster and more perfect in the 21st century and put into application.
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