Application of Software Radio in CDMA Reception

Introduce the method of software radio in CDMA receiving, and design the realization scheme of CDMA receiver based on the idea of ​​software radio.
Keywords: software radio, CDMA, digital down conversion, DSP

1 Introduction Software Radio (Software Radio) is a new revolution in radio communications. Its core technology is to replace the original narrow-band receiver with a wide-band wireless receiver, to bring the wide-band analog / digital and digital / analog converters as close to the antenna as possible, and to use as many software functions as possible for the radio's functions. The emergence of software radio has transformed radio technology from an era dominated by hardware to an era dominated by software.
Due to the technical limitations of broadband antennas, high-speed A / D and DSP, the conditions for realizing an ideal software radio platform are not yet available. Therefore, the current research on software radio is focused on the research of the above key technologies on the one hand; on the other hand, it is more under the existing technical conditions to study how to maximize the versatility and software radio Flexibility, reflect the software and general design ideas into specific applications.
After the concept of software radio was proposed, it attracted wide attention in various fields. The research of software radio is not only limited to the original military field, it is becoming a hot spot in the field of civil communications, especially in the field of mobile communications. Due to the rapid development of communication technology and the continuous emergence of new standards and standards, software radio technology has important application value in mobile communication systems. The base station equipment in the mobile communication system is less affected by power consumption and volume, so it is easy to use software radio technology to realize some of its functions under the existing technical conditions.
The CDMA cellular mobile communication system has attracted widespread attention due to its advantages of high spectrum utilization rate, simple frequency planning, low transmit power, good anti-multipath fading performance, soft handover across areas, and compatibility with analog systems. And the history of the development of personal communications has had an important impact. Almost all experts believe that CDMA will be the most important multiple access method for wireless communications in the next century. In addition to the advantages of the general software radio system, the use of software radio technology in CDMA reception has the following special advantages:
(1) The CDMA system uses code patterns to distinguish different channels, so it can use 1 cell multiplexing, that is, all cells use the same carrier frequency, all mobile users share the same frequency band, and the RF / IF processing module does not require ideal software The radio system must have the ability to process the entire frequency band and flexibly extract the required bandwidth signal.
(2) The CDMA system is a system with limited capacity. Any method of reducing interference can increase the capacity of the system, and the CDMA system based on software radio is easier to adopt the latest anti-interference technology and algorithm to improve the anti-interference performance of the receiving equipment , Making the entire system easy to upgrade and expand capacity.
2 The ideal software radio for receiver design uses broadband multi-band antennas, high-speed A / D, D / A converters and high-speed DSP / CPU to complete all the functions of traditional radio stations. Restricted by the level of development of existing hardware, it is currently impossible to realize a true software radio. Especially for the CDMA system, its wideband characteristics increase the difficulty of subsequent DSP processing. However, since all users in the CDMA system share the same frequency band, the requirements on the radio frequency front end and A / D devices are greatly reduced. The digital down-converter is used to obtain the baseband signal of the received signal from the sampled data, so that the DSP only needs to process the baseband spread spectrum signal. In the processing of the baseband DSP, when the synchronous acquisition is completed, the processing of the broadband spread spectrum signal by the DSP includes only despreading operations, and the despread signal can be reduced by decimation filtering to reduce the signal rate. The rate of the data signal is calculated.

CDMA receiving block diagram based on software radio, as shown in Figure 1. It is mainly composed of radio frequency front end, digital down converter, DSP processing board, interface module and system controller. The radio frequency front end receives the signal, completes the spectrum shift from the radio frequency to the intermediate frequency, the orthogonal decomposition of the signal, and the band-pass filtering. Under the condition of satisfying the band-pass sampling theorem, the A / D sampling rate can be taken as twice the chip rate to reduce the requirements for A / D devices and subsequent digital signal processing modules. The system controller mainly completes the initial setting of the system parameters, converts the data sent by the demodulation of the DSP processing board into the required format and sends it to the interface module. The interface module provides an external data interface.
Figure 2 is a block diagram of the digital down converter. The intermediate frequency digitized input signal passes through digital orthogonal mixing, and then passes through variable parameter orthogonal filtering to extract the required frequency band. Then pass low-pass filtering and decimation filtering to adjust the signal timing to generate two orthogonal baseband signals with a sampling rate of one chip rate. The control and status module provides an interface with the system controller, receives the control word from the system controller and completes the status setting of the digital downconverter.

The digital down-converter can use a dedicated digital down-converter device or a programmable logic device. The requirement for digital down-conversion is to have a certain flexibility and programmability, for example, the frequency of the local oscillator can be adjusted, the sampling rate can be flexibly converted, the timing adjustment function, and the programmable baseband filter, etc. In addition to completing the initial configuration of the system and providing the interface between the demodulated data and the interface module, the system controller can also be used to complete some real-time calculations because of the rapid development of CPU processing speed.
The key to realizing the CDMA base station receiving unit based on software radio is whether the processing power of the baseband DSP can complete the calculation of the baseband signal in real time. At present, the processing capacity of the single-chip C6X has reached 1600Mips, and many companies have launched DSP processing boards and corresponding development tools dedicated to the realization of software receivers, such as CPCI / C6400 of BlueWave Systems and Barce lona / Barcelona67 of Spectrum. . The current narrow-band CDMA system only needs to use the above-mentioned high-speed DSP processing board, and it can completely complete the operation of high-speed spread spectrum signals such as PN code acquisition, spread spectrum signal generation, and despreading. The use of DSP to realize the deinterleaving, channel decoding, speech decoding and other operations in the baseband low-speed data processing process has been widely researched and applied.
In order to provide high-speed data transmission and a wide range of multimedia services, most third-generation mobile communication systems use broadband CDMA technology to provide multiple services such as voice, data, and image. For wideband CDMA systems, the current DSP devices are still difficult to process high-speed spread spectrum signals. Because the signal processing of this part of the module is relatively simple and relatively fixed, you can consider using FPGA to implement operations such as local PN code generation, PN code acquisition and despreading, and send the despread low-speed signal to the DSP for processing. Atmel's AT40K series and Xilinx's XC6200 series FPGAs now have the capability of dynamic reconfiguration, so the modules implemented with FPGA devices maintain a certain degree of flexibility and upgradeability. In fact, the use of a mixed architecture of DSP and FPGA to implement a software radio system has become an important direction of software radio research.
Due to the limitations of the current device level, software radio technology can only be better played in the baseband processing part of the mobile communication system, but this does not mean that this technology has no other effect. On the one hand, software radio technology and traditional methods can be used to develop the experimental system, and the problems that should be paid attention to in software radio technology are discovered and solved in practice. On the other hand, we should go beyond the limitations of the device level, conduct in-depth theoretical research, propose new solutions and good algorithms, and lay a solid foundation for the wide application of software radio.
3 Conclusion This paper proposes a program to implement CDMA reception using software radio technology, analyzes the functions of each module that constitutes the system, and discusses the feasibility of implementing the above program under the existing technical conditions.
Most of the third-generation mobile communication systems designed to meet future personal communication requirements use broadband CDMA technology. By adopting the method of combining the DSP device and the FPGA device, a wideband CDMA system based on software radio can be realized to a certain extent, making it possible to realize a future personal communication system in the true sense.

1 Guo Tiyun, Yang Jiawei, Li Jiandong. Digital mobile communications. Beijing: People's Posts and Telecommunications Press, 1995
2 Sun Lixin, You Xiaohu, Zhang Ping. The third generation mobile communication technology. Beijing: People's Posts and Telecommunications Press, 2000
3 Zoran KosTIc. DigitalSignalProcessors in Cellular RadioCommunicaTIons. IEEE Comm-M ag. , December, 1997
4 Mark Cummings. FPGA in the Software Radio. IEEEComm-Mag. , Feb. , 1999
5 Jhong Sam Lee. CDMA System Engineer Manual. Beijing: People's Posts and Telecommunications Press, 2001

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