introduction
Direct sequence spread spectrum communication is one of the spread spectrum communication technologies. It has anti-interference, anti-multipath fading, and strong anti-blocking capabilities, as well as high spectrum utilization, good confidentiality, low interception rate, easy networking, and high precision. Ranging and many other advantages.
This paper presents a design scheme of a direct-spread system based on software radio. Various design parameter indexes are given, and the proposed design scheme is simulated and verified.
1 Basic system structure
The direct-spread communication terminal based on software radio adopts the digital sampling of the intermediate frequency, and the digital signal processing of signal spreading, modulation, despreading and demodulation is realized by software programming. This article focuses on the specific implementation scheme of IF digital processing of direct-spread communication terminals. The structure block diagram of the direct communication terminal is shown in Figure 1.
When the signal is transmitted, the information is multiplied and spread with the spreading pseudo-code after being encoded by the source and channel. In order to match the spread-spectrum baseband signal with the conversion rate of the following DAC, the low-rate spread-spectrum baseband signal must be increased to the DAC conversion rate by interpolation before quadrature modulation. The interpolated data passes through a shaping filter to eliminate inter-symbol interference and high-frequency image interference. The interpolated and filtered spread-spectrum baseband signal is multiplied by the carrier to achieve digital modulation, and then converted into an intermediate-frequency analog signal by a high-speed DAC.
When the signal is received, the intermediate frequency analog signal is sampled by the high-speed ADC, multiplied by the local carrier and orthogonally down-converted to zero intermediate frequency. After decimation and filtering, it is sent to the pseudo code synchronization loop for pseudo code acquisition and tracking. After the pseudo codes are synchronized, the signal is despread and demodulated and the corresponding channels and sources are decoded.
Figure 1 Block diagram of the direct communication terminal
2 System parameter design
The direct communication terminal parameter constraints mainly have the following aspects:
(1) Transmission rate of information data: Because the direct communication terminal is mainly used for low-rate data communication and voice communication, and the current data rate after voice coding (such as CELP, AMBE coding) is generally 2! 4 Kb / s, 4 Kb / s, 4! 8 Kb / s, 8 Kb / s, 9! 6 Kb / s. Therefore, the information rate is set to 8 Kb / s in the selection of the information rate, and the channel coding adopts convolutional coding with a code rate of 1/2. Therefore, the data rate to be spread is 16 Kb / s.
(2) Type and order of spreading pseudo-code: Since the designed direct-spreading communication terminal currently completes point-to-point communication, for simplicity, m-sequence is used as the spreading pseudo-code in the direct-spreading communication terminal. If the length of the m sequence is too long, it not only increases the acquisition time of the receiver but also increases the complexity of the receiver structure. If the length of the m sequence is too short, the anti-interference ability of the intermediate frequency digital direct-spread communication terminal is weakened. Therefore, the compromise method is adopted, and the 11th order m sequence is used as the spreading pseudo code of the intermediate frequency digital direct-spread communication terminal.
(3) Spread-spectrum processing gain: Spread-spectrum gain is an important parameter of direct-spread communication, which reflects the strength of the system's anti-interference ability, and is a measure of the improvement of the signal-to-noise ratio, which is defined as the output signal-to-noise power ratio of the receiver The ratio of the input signal-to-noise power ratio of the receiver, namely:
Among them: B RF is the bandwidth after spreading; Bb is the baseband data bandwidth; Rc is the pseudo code rate after spreading; Rb is the baseband data rate. In this design, in order to improve the utilization rate of the frequency band, considering the maximum bandwidth allowed, the rate of the pseudo code is designed to be 4.096 Mb / s. Therefore, it can be obtained that the processing gain of the intermediate frequency digital direct-spread communication terminal is 24 dB.
(4) Digital modulation and intermediate frequency carrier frequency: Because the DPSK signal uses a cross-product frequency discrimination loop with a decision feedback structure, it can not only eliminate the frequency offset, but also perform differential demodulation, thereby eliminating the need for carrier phase synchronization and simplifying reception Machine circuit design. Therefore, DPSK is adopted as the digital modulation method of the intermediate frequency digital direct-spread communication terminal.
In the selection of the intermediate frequency carrier frequency, 21.4 MHz is adopted as the intermediate frequency carrier frequency of the intermediate frequency digital direct-spread communication terminal.
(5) Pseudocode synchronization circuit: For the pseudocode capture circuit framework, the non-coherent serial capture method is used. The integral cleaning filter can be replaced by an accumulator or a matched filter. Since the direct-spread communication terminal uses despreading and then demodulation, accurate carrier phase and carrier frequency cannot be obtained before despreading, so the pseudocode tracking circuit uses a non-coherent early delay phase-locked loop.
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