A design scheme of software radio universal signal generator is proposed, including hardware composition and software algorithm implementation. This signal generator facilitates the research and development of software radio.
Software radio is a new architecture for radio communications. At the U.S. Telecommunication System Conference in May 1992, JeoMitola first proposed the concept of software radio, which quickly attracted people's attention and began extensive and in-depth research. Specifically, the software radio is based on a programmable DSP or CPU, and connects modular and standardized hardware units in a bus manner to form a common basic hardware platform, and implements various wireless communication functions through software loading. Open architecture. It frees the communication system from the design-oriented thinking and is considered to be another breakthrough in wireless communication from analog to digital and from fixed to mobile.
In the research process of software radio, modulation and demodulation technology is an important part of the air interface of mobile communication systems. Under different cell radii and application environments, the channels of mobile communication exhibit different fading characteristics, and the modulation mode is automatically changed according to the fading condition of the mobile channel, thereby improving transmission efficiency and ensuring transmission performance. Then, a universal source is essential.
The author designed a hardware platform based on DSP+DDS structure programmable modulator, and implemented various general software algorithms for analog modulation and digital modulation on this hardware platform. When the modulation system is changed, there is no need to download the program again, and the modulation system, bit rate, and output intermediate frequency are all adjustable.
Generally, most of the waveforms output by the signal source are modulated by the periodic 01 sequence, and the output waveform is single, which can only be used as a special case of demodulating the input signal, and lacks versatility. The modulation methods used by many dedicated chips are also limited. A general-purpose multi-standard signal generator composed of DSP+DDS can realize not only analog modulation but also various digital modulations. DSP facilitates real-time processing of baseband signals, enabling high-speed modulation, while DDS features high frequency resolution, fast frequency change, continuous phase, and easy digital control. Figure 1 shows the hardware schematic of the multi-standard signal generator.
The signal generator is mainly composed of three parts: a control unit, a digital signal processor (DSP), and a quadrature digital upconverter.
The DSP adopts TI's TMS320VC5402. Its unique Harvard architecture, hardware-intensive scheme and flexible instruction system can meet the real-time processing of signals. Its high performance, low power consumption and low price make it widely used.
The quadrature digital upconverter adopts the AD9857 of AD Company. The AD9807 has a maximum operating frequency of 200MHz and an output IF frequency range of 0 to 80MHz. The AD9807 integrates a half-band filter, CIC (ascaded Integrator Comb) filter, anti-SINC filter, and high-speed 14-bit is a phase-continuous direct digital synthesizer (DSS). In this scheme, the AD9857 operates in quadrature modulation mode. Its 32-bit frequency control word allows the highest accuracy of the output frequency to be: SYSCLK (system clock) divided by 2 32.
The control unit determines which modulation system, bit rate, and output IF frequency to use.
The DSP reads in the data of the control unit, and then sends a control word to the AD9857 through the serial port. The original information data (which is a pseudo-random sequence generated by the DSP) is first subjected to encoding, modulation, and the like in the DSP to obtain a baseband signal. The baseband processing obtains the I/Q component of the orthogonal signal alternately into the AD9857, undergoes serial-to-parallel conversion, converts into two parallel I/Q data, performs interpolation and up-conversion operations, and then directly outputs the analog intermediate frequency signal through D/A conversion. Thus, the baseband processing and the intermediate frequency modulation are combined into one.
The AD9857 samples and interpolates the input digital signal, reducing the processing load of the DSP and making the overall system performance better.
Software radio is fully programmable. It adopts digital signal processing technology, and uses software to determine the functions of various parts of the radio station, including programming of wireless band, channel modulation, access mode, and data rate, on the general hardware platform of programmable control. Therefore, control and operation through the program is one of the most prominent features of software radio. The design of software algorithms is directly related to the implementation of radio software. The software radio station processes the signals in real time, so it puts high demands on the time and space complexity of the algorithm.
In order to save limited DSP computing resources, the software radio software algorithm research uses a large number of look-up table methods to improve the processing speed, usually using the waveform storage method in the modulation process. When writing a software algorithm program, as long as the modulation mode and its corresponding output state number are limited, it can be realized by means of the table method. The look-up table method avoids a lot of intermediate operations and is simple and easy. The only drawback is that it takes up a lot of storage space. Therefore, you need to create a generic table that stores the quantized 14-bit signed binary numbers. The design of the form should be simple and the different modulation methods should be met. This table can also be used to calculate the sine function by simply shifting the current phase by π/2.
In addition to a universal cosine table, corresponding modulation constellation mapping tables need to be established for different modulation modes, and a phase table library is formed according to the modulation mode. For differential phase modulation, the table is a differential phase table. After the modulation mode is determined, the current phase Φk is calculated according to the obtained symbol.
Figure 2 shows the differential phase modulation software algorithm with (π/4) DQPSK modulation as an example. The digital storage area stores a cosine function waveform sample of one cycle, and the number of sampling points of the storage area is N, and the moving step of the table is d. The original modulation is set every two bits, and the modulation constellation in Table 1 is mapped to the differential phase ΔΦk and the phase of the previous code is added to perform modulo 2π to obtain the absolute phase Φk of the current symbol, and the Φk is calculated in the cosine table. Offset address, the data of the signal is modulated according to the offset address.
Let f(i)=cos(id), where 0≤i
Then, the offset address of the current phase Φk (0 ≤ Φk < 2π) is: Φk & TImes; N/2π.
The possible values ​​of the absolute phase Φk corresponding to (π/4) DQPSK are: 0°, 45, 90°, 135°, 180°, 225°, 270°, 315°. If N=144, ie d=2.5, then the corresponding offset addresses of Φk in the cosine table are: 0°, 18°, 36°, 54°, 72°, 90°, 108°, 126°.
The hardware platform of the signal generator is realized by the PCB four-layer board design, and the software programming of the following modulation modes is completed on the basis of this platform: AM, DSB, SSB, FM, GMSK, FSK, BPSK, DBPSK, QPSK, (Ï€/4) DQPSK, 8PSK, 4-64 Star-QAM. The digital modulation mode can achieve a symbol rate of 1 MHz (that is, a bit rate of up to 2 Mbps for four-phase modulation; a bit rate of up to 5 Mbps for 32QAM modulation), a carrier frequency of up to 70 MHz, modulation mode, bit (or symbol) The rate and output intermediate frequency are adjustable.
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