Interference avoidance and suppression in code -division multiple -access systems
The first part of this report examines an interference suppression technique for multicarrier (MC) CDMA systems. The proposed technique exploits carrier offsets between users, which arise because of inaccuracies of oscillators or Doppler shifts. The suppression capability comes from oversampling the received signal in the frequency domain. It is shown that the proposed technique is robust in severe near-far interference environments. In addition, the system performance can easily be found using the derived signal-to-interference ratio and the Conditional Gaussian Approximation (CGA). The second part of the report examines the structure of asynchronous direct sequence (DS) CDMA systems with space-time codes. It is shown that the MMSE combining solution for joint space-time decoding and interference suppression must be done in a specific way to ensure the received signal has a pseudo-correlation matrix equal to zero. A simple technique is proposed to provide both multiple-access interference suppression and transmit diversity. Finally, a new technique for interference avoidance in asynchronous DS-CDMA systems is proposed. The user capacity of such systems is shown to improve significantly if users are allowed to control their signature sequences to avoid interference from other users. The degree to which the user capacity increases is a function of the chip waveform and the delay profile of each user. The effective dimension of a chip waveform is characterized and optimal delay profile sets are determined based on properties of specific chip waveforms.
Lehnert, Purdue University.
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