Iterative Multi-User Interference Mitigation in CDMA Receivers
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Solution Overview
Problem
Existing CDMA receivers face challenges in effectively mitigating multi-user interference due to non-idealities in transmitter and receiver components, multipath effects, and asynchronous signal timing, which complicates the decoding process and requires computationally complex methods like interference cancellation.
Innovation Solution
A method that involves reconstructing a signal by subtracting a re-spreading of estimated symbol streams from the received CDMA signal, despreads the difference signal using despreading codes, and iteratively adjusts the estimated symbol streams to improve accuracy, reducing computational complexity and handling asynchronous signals more efficiently.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If interference cancellation is used to mitigate multi-user interference, then performance is improved, but computational complexity increases significantly
Solution Approach 1:
The patent segments the interference cancellation process into two distinct stages: a decision-directed phase that operates at symbol rate with low computational complexity, and a training phase that operates at chip rate but is limited in duration. This segmentation allows the system to achieve effective interference cancellation while avoiding the prohibitive computational complexity of continuous chip-rate processing.
Solution Approach 2:
The patent implements periodic switching between decision-directed mode and training mode. The decision-directed mode operates continuously at symbol rate for normal operation, while the training phase is activated periodically to update estimators. This periodic action maintains performance while keeping computational complexity manageable during the majority of operation time.
2Reliability
If chip-rate processing is used for interference cancellation, then interference mitigation is improved, but power consumption increases
Solution Approach 1:
The patent segments processing into decision-directed mode operating at symbol rate and training phase operating at chip rate. By limiting chip-rate processing to only the training phase rather than continuous operation, the system achieves necessary interference cancellation effectiveness while dramatically reducing overall power consumption.
Solution Approach 2:
The patent applies partial chip-rate processing only when necessary during the training phase, rather than excessive continuous chip-rate processing. This partial action provides sufficient interference mitigation to establish accurate estimators, after which the system transitions to lower-power symbol-rate processing for the majority of operation.
3Measurement precision
If orthogonal codes are used for CDMA users, then user signal separation is improved, but multi-user interference still occurs due to non-idealities
Solution Approach 1:
The patent implements feedback through decision-directed estimation, where detected symbols are fed back to update the estimators of desired and interfering signals. This feedback mechanism continuously refines the separation accuracy, compensating for the non-idealities of orthogonal codes and maintaining reliable interference rejection despite code non-orthogonality and channel effects.
4Reliability
If joint detection of all users is performed, then theoretical performance is optimized, but computational complexity becomes prohibitive
Solution Approach 1:
The patent extracts and separately estimates the desired signal and interfering signals using decision-directed estimators, rather than performing joint detection of all users simultaneously. This extraction approach achieves near-optimal performance by separately processing individual user signals while avoiding the exponential computational complexity of joint detection algorithms.
Data Source
AI summary
A method and apparatus for mitigating multi-user interference in reception of a code division multiple access (CDMA) signal is described. The CDMA signal has a plurality of spread user symbol streams, each spread by a corresponding one of a plurality of spreading codes. Estimated symbols are iteratively improved by forming a reconstructed signal from the estimated symbols, subtracting the reconstructed signal from the CDMA signal to create a difference signal, and adjusting the estimated symbols based on a combination of the difference signal and the estimated symbols.