【Practical Exercise】Simulation of a MATLAB-based Direct Sequence Spread Spectrum BPSK Modulation Communication System

# 2.1 The Fundamental Principle of Spread Spectrum Communication

Spread spectrum communication is a modulation technique that expands a narrowband signal into a wider frequency band to enhance its resistance to interference. Its basic principle involves modulating the original information signal with a high-rate pseudorandom sequence (PN sequence), thereby dispersing the signal's energy across a broader frequency spectrum.

**Spread Spectrum Gain:** The interference resistance of spread spectrum communication is measured by the spread spectrum gain, which is defined as the ratio of the processing gain of the spread signal to that of the unspread signal. Spread spectrum gain is proportional to the spread factor (the ratio of the PN sequence rate to the information signal bit rate), meaning that the larger the spread factor, the greater the spread spectrum gain.

**Interference Resistance:** The interference resistance of spread spectrum communication is primarily evident in the following aspects:

- **Resistance to Narrowband Interference:** Since the energy of the spread signal is spread across a wider frequency band, narrowband interference can only affect a part of the signal, not causing a significant impact on the entire signal.
- **Resistance to Multipath Interference:** The broadband characteristics of the spread signal can mitigate the effects of multipath interference, as signals from different paths will arrive at the receiver at different times, and the spread spectrum receiver can recombine these signals using correlation techniques.
- **Resistance to Interception:** The broadband characteristics of the spread signal and the unpredictability of the pseudorandom sequence make it very difficult to intercept and demodulate the spread spectrum signal.

# 2. Direct Sequence Spread Spectrum BPSK Modulation Theoretical Foundations

### 2.1 The Fundamental Principle of Spread Spectrum Communication

Spread spectrum communication is a technique that transmits signals by expanding a narrowband signal over a broader frequency spectrum. Its basic principle is to modulate the original information signal with a so-called spread spectrum code, which is a pseudorandom sequence, resulting in a spread signal. The bandwidth of the spread spectrum code is much wider than that of the original signal, thus the spread signal occupies a broader frequency band in the frequency domain.

Spread spectrum communication has the following advantages:

- Strong resistance to interference: Due to the wider bandwidth of the spread signal, the energy of interference signals is dispersed over a broader frequency spectrum, reducing the impact of interference on the signal.
- High confidentiality: The spread spectrum code is a pseudorandom sequence known only to legitimate receivers, effectively preventing unauthorized interception.
- Strong multi-access capability: Different users can communicate using different spread spectrum codes, thus achieving multi-access.

### 2.2 Direct Sequence Spread Spectrum BPSK Modulation Principles

Direct sequence spread spectrum BPSK modulation is a spread spectrum communication technology where the spread spectrum code is a pseudorandom binary sequence (PRBS). BPSK modulation is a binary phase shift keying modulation technique where binary "0" and "1" are represented by carrier phase shifts of 0° and 180°, respectively.

The direct sequence spread spectrum BPSK modulation process is as follows:

1. **Generate a Pseudorandom Binary Sequence (PRBS):** A PRBS is a binary sequence generated by a pseudorandom number generator, possessing good statistical properties such as long period, low autocorrelation, and low cross-correlation.
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