Inter-symbol Interference Cancellation in Wireless Receivers
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Solution Overview
Problem
In coded spread spectrum communication systems, inter-symbol interference (ISI) and other forms of interference significantly degrade communication quality, leading to errors, reduced capacity, and decreased data rates, especially in multipath environments where signals spread out and overlap, causing leakage between subchannels and between different base stations.
Innovation Solution
A generalized interference-cancelling receiver is designed to mitigate intra-channel, inter-channel, and inter-symbol interference using a decision-feedback mechanism, incorporating a post-cursor ISI synthesis and cancellation module, iterative interference cancellation, and feedback mechanisms to estimate and subtract ISI signals, employing weighting modules, channel emulators, and stabilizing step sizes to improve symbol estimates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If interference cancellation techniques are applied to mitigate intra-channel and inter-channel interference, then communication quality improves, but inter-symbol interference (ISI) remains unaddressed and continues to degrade performance
Solution Approach 1:
The interference cancellation process is segmented into distinct stages: first cancelling intra-channel interference between orthogonal subchannels, then cancelling inter-channel interference from other base stations, and finally addressing inter-symbol interference through iterative cancellation. This segmentation allows each interference type to be handled by specialized processing blocks, improving overall effectiveness.
Solution Approach 2:
The system performs preliminary interference cancellation for intra-channel and inter-channel interference before addressing inter-symbol interference. By removing these interference components first, the subsequent ISI cancellation operates on a cleaner signal, improving convergence and reducing the complexity of the cancellation process.
2Reliability
If iterative interference cancellation is used to address multiple interference types, then overall interference reduction improves, but computational complexity increases
Solution Approach 1:
The iterative cancellation process is segmented into separate processing blocks for different interference types (intra-channel, inter-channel, and inter-symbol interference). Each block processes specific interference components independently, allowing for optimized computation and reducing the overall complexity compared to a monolithic approach.
Solution Approach 2:
The system employs a limited number of iterative cancellation passes rather than exhaustive processing. By performing a fixed number of iterations (e.g., 2-3 passes) to cancel different interference types, the system achieves sufficient interference reduction without the exponential complexity growth that would result from continuous or excessive iteration.
3Reliability
If decision-feedback mechanisms are applied to cancel post-cursor ISI, then reception quality improves, but feedback loop stability becomes challenging
Solution Approach 1:
The system implements a decision-feedback mechanism where symbol decisions are made and then fed back to reconstruct and subtract the post-cursor ISI from the received signal. This feedback loop continuously refines the ISI cancellation by using the latest symbol decisions to improve subsequent cancellations, enhancing reception quality while maintaining stability through careful loop design.
Solution Approach 2:
The system performs preliminary cancellation of intra-channel and inter-channel interference before implementing the decision-feedback loop for post-cursor ISI cancellation. This preliminary processing stabilizes the feedback loop by removing other interference sources that would otherwise corrupt the symbol decisions and destabilize the feedback mechanism.
Data Source
AI summary
An interference-canceling receiver is configured for cancelling inter-symbol interference due to both inter- and intra-channel interference in coded, multiple-access, spread spectrum transmissions that propagate through frequency-selective communication channels. The receiver mitigates the effects of post-cursor inter-symbol interference using feedback of previously estimated symbols, and mitigates pre-cursor inter-symbol interference using an interference canceller, such as an iterative interference canceller.