Two-User CDMA System with Despreading and Demodulation Capabilities, Providing Individual Bit Error Rates for Both Users

Performance Analysis of Two-User CDMA System in Additive Gaussian Channels

In CDMA (Code Division Multiple Access) systems, multiple users share the same frequency band through distinct spreading codes for multiple access. This system employs 15-bit M-sequences as spreading codes combined with BPSK modulation for data transmission. Implementation typically involves generating orthogonal codes using linear feedback shift registers and applying digital modulation techniques.

Signal Processing Flow Analysis Each user's baseband signal undergoes spreading using M-sequences, significantly increasing bandwidth and enhancing anti-interference capability. BPSK modulation then shifts the spread spectrum signal to the carrier frequency. At the receiver end, matched correlators perform despreading to recover the original signal. Code implementation would include correlation operations using the same spreading sequences and phase synchronization algorithms.

Time Domain and Frequency Domain Characteristics The spread spectrum signal exhibits noise-like waveforms in the time domain and broadband characteristics in the frequency domain. After demodulation, the signal passes through low-pass filters to restore the baseband signal. Due to the orthogonal nature of M-sequences, effective separation of both users' signals can be achieved through proper code design and cross-correlation minimization techniques.

Bit Error Rate Performance Under additive white Gaussian noise channels, the system's BER primarily depends on signal-to-noise ratio and multiple access interference between users. BER curves for both users can be obtained through theoretical calculations or simulation models. Thanks to spreading gain, the system can operate at relatively low SNR ratios. For ideally synchronized systems, both users' BER should approach theoretical BPSK performance. Simulation code would typically involve Monte Carlo methods with noise addition and threshold detection algorithms.

In practical systems, power differences between users and multipath effects can impact final BER performance. Implementing appropriate power control algorithms and multiuser detection techniques can further reduce bit error rates. Advanced implementations may incorporate RAKE receivers for multipath mitigation and interference cancellation algorithms.