Design and simulation of direct spread spectrum communication terminal based on software radio

0 Introduction

Direct sequence spread spectrum communication is a type of spread spectrum communication technology. It has many advantages, such as strong anti-interference, anti-multipath fading, and anti-blocking capabilities, high spectrum utilization, good confidentiality, low interception rate, easy networking, and high-precision ranging.

This paper proposes a design scheme for a direct spread spectrum system based on software radio. Various design parameter indicators are given, and the proposed design scheme is simulated and verified.

1 Basic structure of the system

The DSSS communication terminal based on software radio adopts digital sampling of the intermediate frequency, and realizes digital signal processing such as signal spread spectrum, modulation, despreading, and demodulation by software programming. This article focuses on the specific implementation scheme of the intermediate frequency digital processing of the DSSS communication terminal. The structural block diagram of the DSSS communication terminal is shown in Figure 1.

When the signal is transmitted, the information is multiplied and spread by the spread spectrum pseudo code after source and channel coding. In order to match the spread spectrum baseband signal with the conversion rate of the subsequent DAC, the low-rate spread spectrum baseband signal must be increased to the conversion rate of the DAC by interpolation before orthogonal 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 with the carrier to realize digital modulation, and then converted into an intermediate frequency analog signal through a high-speed DAC.

When receiving the signal, the intermediate frequency analog signal is sampled by the high-speed ADC, multiplied by the local carrier, orthogonally down-converted to zero intermediate frequency, and then sent to the pseudo code synchronization loop for pseudo code capture and tracking after extraction and filtering. After pseudo code synchronization, the signal is despread and demodulated, and the corresponding channel and source decoding is performed.

2 System parameter design

The parameter constraints of direct spread communication terminals mainly include the following aspects:

(1) Transmission rate of information data: Since the direct spread communication terminal is mainly used for low-rate data communication and voice communication, and the data rate after voice coding (such as CELP and AMBE coding) is generally 2.4Kb/s, 4Kb/s, 4.8Kb/s, 8Kb/s, and 9.6Kb/s, the information rate is set to 8Kb/s, and the channel coding adopts convolution coding with a code rate of 1/2. Therefore, the data rate to be spread is 16 Kb/s.

(2) Spread spectrum pseudo code type and order: Since the designed direct spread communication terminal currently completes point-to-point communication, for the sake of simplicity, the m-sequence is used as the spread spectrum pseudo code in the direct spread communication terminal. If the length of the m-sequence is too long, it not only increases the capture 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, a compromise method is adopted, and the 11-order m-sequence is used as the spread spectrum 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 spectrum communication, reflecting the strength of the system's anti-interference ability and a measure of the degree of improvement in the signal-to-noise ratio. It is defined as the ratio of the receiver output signal-to-noise power ratio to the receiver input signal-to-noise power ratio, that is:

Where: BRF is the bandwidth after spread spectrum; Bb is the baseband data bandwidth; Rc is the pseudo code rate after spread spectrum; Rb is the baseband data rate. In this design, in order to improve the bandwidth utilization, considering the maximum bandwidth allowed, the pseudo code rate is designed to be 4.096 Mb/s. Therefore, the processing gain of the intermediate frequency digital direct spread spectrum communication terminal can be obtained to be 24 dB.

(4) Digital modulation and intermediate frequency carrier frequency: Since the DPSK signal uses a cross-product demodulation loop with a decision feedback structure, it can not only eliminate frequency deviation, but also perform differential demodulation, thus eliminating the need for carrier phase synchronization and simplifying the receiver circuit design. Therefore, DPSK is used as the digital modulation method for intermediate frequency digital direct spread communication terminals.

In the selection of intermediate frequency carrier frequency, 21.4MHz is adopted as the intermediate frequency carrier frequency of the intermediate frequency digital direct spread communication terminal.

(5) Pseudo-code synchronization circuit: For the pseudo-code capture circuit framework, the non-coherent serial capture method is used. The integral cleaning filter can be replaced by an accumulator or a matching filter. Since the direct spread communication terminal uses despreading before demodulation, the accurate carrier phase and carrier frequency cannot be obtained before despreading, so the pseudo-code tracking circuit uses a non-coherent advance delay phase-locked loop.

3 Simulation results

Since the pseudo code rate is 4.096 Mb/s, the sampling theorem shows that at least 8.192 MHz sampling frequency is required to sample the pseudo code. Considering the convenient design of the pseudo code tracking circuit delay advance phase-locked loop, a sampling rate of 16.384 MHz is used to sample the pseudo code, that is, one pseudo code is sampled at four points. Therefore, the baseband spread spectrum signal rate obtained after the information signal is spread is 16.384 Mb/s, and the DAC conversion rate is set to 81.92 Mb/s. Therefore, in order to match the data rate, the baseband spread spectrum signal needs to be interpolated, and the interpolation factor is 81.92/16.384=5. The receiving process is the reverse process of the sending process, and the extraction factor is equal to the interpolation factor, which is also 5.

In order to improve spectrum utilization and eliminate inter-code interference, a shaping filter is needed to perform shaping filtering on the spread spectrum chips. In the design of the intermediate frequency digital direct spread communication terminal, in order to save circuit resources, the shaping filter is designed to play both chip shaping and interpolation filtering roles. In order to reduce the data throughput of the filter, a multi-filter structure is used here. After the baseband spread spectrum signal is interpolated 5 times, the rate reaches 81.92 Mb/s, so the sampling frequency of the filter is 81.92 MHz. Since DBPSK modulation is used, the pseudo code rate is 4.096 Mb/s. Since the 3 dB bandwidth of the modulated signal is 4.096 MHz, the cutoff frequency of the filter only needs to be 2.048 MHz. However, in order to better filter out the signal frequency, the cutoff frequency of the filter is set to 4.096 MHz in the intermediate frequency digital direct spread communication terminal, thereby meeting both the requirements of interpolation filtering and chip shaping. Since the shaping filter is used in the transmission and reception process, the shaping filter adopts a square root raised cosine filter.

According to the set parameters, the spread spectrum modulation simulation of the direct spread communication terminal was carried out, and the Matlab simulation results of the transmitting part are shown in Figure 2.

As shown in Figure 2, after the information data is spread, its spectrum is expanded in the entire frequency band, and then multiplied by the carrier with a carrier frequency of 21.4 MHz to achieve data up-conversion (ie, DPSK modulation).

At the receiving end, after the signal is down-converted and extracted, the serial capture method is adopted. After the pseudo code is captured and synchronized, the signal can be despread. The simulated waveform after despreading is shown in Figure 3.

As can be seen from Figure 3, after the signal is processed by correlation despreading, the useful signal is despread and its power spectrum is concentrated within the information bandwidth. After the useless signal passes through the correlator, although the spectrum is greatly widened, the energy of the signal in the entire frequency band remains unchanged.

The despread signal is low-pass filtered, and the information is demodulated as shown in Figure 4. The information data obtained after the received spread spectrum signal is orthogonal down-converted, extracted and filtered, pseudo code synchronized, low-pass filtered and information demodulated is exactly the same as the transmitted information data.

4 Conclusion

This paper mainly discusses the software radio implementation structure and parameter design of the direct spread spectrum communication system in spread spectrum communication, including spread spectrum pseudo code type, spread spectrum gain, intermediate frequency selection, pseudo code synchronization circuit, etc., and simulates the scheme with Matlab. The simulation results show the feasibility of the scheme. At the same time, it shows that the design scheme has the advantages of small size, good flexibility, low power consumption, and strong scalability.