A general modem design scheme combining SoPC and CORDIC algorithm

“Software Defined Radio (SDR) is to construct an open, standardized, and modular general hardware platform. It uses software to complete functions such as working frequency band, modem type, data format, encryption mode, and communication protocol, and enables broadband The A/D converter and the D/A converter are as close as possible to the antenna, and the various functions of the radio station are realized by the use of upgradeable and reconfigurable application software, and a new generation of highly flexible and open wireless communication system has been developed.The CORDIC (Coordinate Rotation Digital Compute) algorithm is a series of successively decreasing, and operation

“

1 Introduction

Software Defined Radio (SDR) is to construct an open, standardized, and modular general hardware platform. It uses software to complete functions such as working frequency band, modem type, data format, encryption mode, and communication protocol, and enables broadband The A/D converter and the D/A converter are as close as possible to the antenna, and the various functions of the radio station are realized by the use of upgradeable and reconfigurable application software, and a new generation of highly flexible and open wireless communication system has been developed. The CORDIC (Coordinate Rotation Digital Compute) algorithm uses a series of successively decreasing reciprocating deflection angles related to the calculation base to approximate the final desired rotation angle. The algorithm can take into account accuracy, speed and hardware complexity. , Without the multiplier that takes up a lot of chip resources, the CORDIC algorithm combined method to design the modem, can meet the software radio performance requirements, and has important applications. Therefore, here is a design scheme for a general-purpose modem based on SoPC.

2 CORDIC algorithm principle

The principle of CORDIC algorithm can be derived by vector rotation.

3 General purpose modem based on SoPC

Software radio requires multiple modulation and demodulation methods to be implemented by running different software on a common hardware platform, which requires the establishment of a common model for signal modulation and demodulation. In contemporary wireless communication, in theory, various communication signals can be realized by orthogonal modulation methods. Therefore, the use of orthogonal modulation can establish a unified model, which is suitable for software radio implementation.

The effective solution to realize the universal modulator is to use the CORDIC algorithm circle. The CORDIC algorithm is used in the rotation mode to realize the coordinate transformation of (R, θ) → (X, Y).

To achieve amplitude modulation, the signal A

The digital down-conversion receives the data from the A/D converter, and obtains the baseband signal after orthogonal digital conversion and low-pass filtering, that is, it is divided into the in-phase component and the quadrature component of I and Q channels. Like signal modulation, demodulation also extracts information through one or more parameters of amplitude, frequency, and phase. Then the universal demodulator must first calculate the amplitude A(n) and phase from the I and Q baseband signals after digital downconversion? (N), then calculate the frequency f(n) through the phase, and finally demodulate the information bit stream through the amplitude, frequency, and phase information.

The demodulator contains 2 CORDIC modules, 3 FIFO (first in first out) modules and 2 RISC (reduced instructions) CPU modules. CORDICl completes frequency offset compensation; CORDIC2 has two functions: phase correction and amplitude discrimination and phase discrimination; RISC CPU1 is used to judge symbols; RISC CPU2 is used for frequency offset estimation, phase offset estimation, bit synchronization and amplitude judgment threshold estimation. The CORDIC module is implemented by hardware description language (HDL) programming, and the CPU is customized by SoPC BUILDER for the NIOS soft core CPU, and the entire system is implemented on a programmable FPGA.

CORDICl uses the rotation mode to receive the frequency offset estimation value from CPU2, and correct the frequency offset of the signal. The output is:

CORDIC2 uses the vector mode to receive the phase deviation estimation value from CPU2 to correct the signal, and complete the coordinate conversion (x, y) → (R, θ) to calculate the amplitude A(n) and phase? (N), realize the signal amplitude and phase discrimination function, the output is:

Customize the CPU interface and peripherals according to actual needs, and finally carry out software programming and debugging through NIOSⅡIDE (integrated development environment), which is convenient for system development. Take frequency modulation as an example to simulate a general-purpose modem.

In order to simulate and test the entire modem, combine the modems, first modulate the binary system signal and then demodulate it. The simulation waveform is shown in Figure 5. It can be seen from Figure 5 that the demodulated baseband signal is basically the same as the original baseband signal.

4 Conclusion

The method of combining SoPC technology and CORDIC algorithm is adopted to realize the universal modem. The CORDIC algorithm only needs simple addition and shift operations, and does not require multipliers that take up a lot of chip resources. When implementing NCO (digital controlled oscillator), it saves a lot of ROM than the look-up table method. Make it easy to implement on FPGA. In signal demodulation, only the amplitude and phase are calculated from the I and Q baseband signals after digital down-conversion, and then the frequency is calculated from the phase, and the signal information is demodulated from these amplitudes, phases and frequencies. Use the coordinate transformation function of the CORDIC algorithm to calculate the amplitude and phase to realize the signal amplitude and phase discrimination function. SoPC’s software and hardware co-design solution is a new trend of system-level design, applying SoPC technology to the field of communication software radio. Give full play to the software radio on the open hardware platform through software programming to realize the various functions of the communication system, facilitate the upgrade of various functions and systems of the software radio, and make full use of the reconfigurability of FPGA, which also reflects the completion of software in the software radio The essential characteristics of as many radio functions as possible. Therefore, it is feasible to use the method of combining SoPC technology and CORDIC algorithm to realize the software radio general modem, which reduces the cost and also reflects the flexibility of the software radio technology.