Interference Cancellation in Variable Codelength CDMA Systems
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
CDMA systems face interference limitations due to multipath propagation and multiple-access interference, which degrade communication quality, reduce capacity, and increase energy consumption.
Innovation Solution
The development of methods and receivers for processing multi-rate CDMA signals using orthogonalizing codes of different lengths, specifically employing analysis and synthesis techniques to identify and cancel multiple-access interference, utilizing digital signal processors and specific circuitry to handle high-rate and low-rate subchannels, and implementing interference cancellation algorithms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If spread-spectrum techniques with orthogonalizing codes are used to allow multiple users to share the same frequency band and time interval, then frequency utilization efficiency is improved, but multiple-access interference increases due to multipath propagation destroying code orthogonality
Solution Approach 1:
The patent converts the harmful multiple-access interference into a beneficial signal by using it as input to an interference cancellation algorithm. The algorithm estimates the interference signal from orthogonal codes and subtracts it from the received signal, thereby improving signal quality and enabling better frequency utilization in multipath environments.
Solution Approach 2:
The patent applies preliminary interference cancellation before final signal detection. By estimating and removing interference from orthogonal codes in advance, the system prepares a cleaner signal for subsequent processing, improving the accuracy of signal detection and maintaining code orthogonality despite multipath effects.
2Productivity
If interference cancellation is applied to suppress multiple-access interference, then system capacity is increased, but computational complexity and processing requirements increase
Solution Approach 1:
The patent segments the interference cancellation process into distinct stages: correlation with orthogonal codes, estimation of interference signals, and subtraction from received signals. This segmentation allows for efficient implementation using specialized hardware components for each function, reducing overall computational complexity while maintaining high system capacity.
Solution Approach 2:
The patent uses locally generated replica codes that are copies of the transmitted orthogonal codes to correlate with and estimate interference signals. This copying approach enables efficient interference estimation without requiring complex signal processing, as the replica codes can be generated deterministically from known code sequences.
3Adaptability or versatility
If orthogonalizing codes of different lengths are used to support varied data rates, then adaptability is improved, but maintaining orthogonality becomes more difficult due to variable spreading factors
Solution Approach 1:
The patent dynamically adjusts the spreading factor for different users and data rates while maintaining orthogonality through careful code selection from the Walsh-Hadamard tree. The system adapts code lengths to support varied data rates while ensuring that selected codes remain orthogonal, even as spreading factors change to accommodate different traffic requirements.
Solution Approach 2:
The patent applies different code lengths and spreading factors to different users and channels based on their specific requirements. By assigning appropriate code qualities (lengths and orthogonality properties) to different signal streams, the system maintains overall orthogonality while supporting varied data rates through localized code allocation strategies.
Data Source
AI summary
A receiver employs low-rate processing to synthesize the effect of high-rate interference in a received multi-rate signal. Each high-rate subchannel is analyzed on its low-rate descendents to produce symbol estimates for each low-rate symbol interval. The symbol estimates are applied to low-rate descendent subchannels, which are then combined to synthesize the effects of the high-rate interference. An interference canceller processes the synthesized interference with the received signal for producing an interference-cancelled signal. Alternatively, analogous steps may be applied at high-rate to analyze, synthesize, and cancel the effects of low-rate interference in a multi-rate signal.