Subband Signal Demodulation for Relayed Interference Cancellation
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Solution Overview
Problem
In asymmetrical point-to-multipoint satellite communication systems, the demodulation of remote-terminal signals is hindered by relayed interference, which is exacerbated by frequency-converter phase noise and varying link parameters, making it difficult to achieve significant interference cancellation at the hub terminal.
Innovation Solution
A method and apparatus that utilize offset-frequency conversion, match filtering, and complex equalization to produce a demodulated subband signal, reducing relayed interference by estimating and compensating for phase noise, and using a separate canceller/demodulator for each hub receiver to focus interference cancellation on the remote-terminal signal subband.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional broadband interference cancellation is used, then all remote terminal signals are processed together, but the cancellation factor is limited by frequency-converter phase noise and dynamic range requirements become excessively large
Solution Approach 1:
The patent divides the broadband interference cancellation problem into multiple narrowband subband processing tasks. Each subband corresponds to a specific remote terminal signal frequency range, allowing independent processing with reduced dynamic range requirements. The overall cancellation is achieved by combining results from all subbands.
Solution Approach 2:
The patent transforms the time-domain broadband cancellation problem into frequency-domain narrowband processing by applying Fourier transforms and processing each frequency subband separately. This dimensional transformation enables more precise interference cancellation while reducing the required dynamic range of the processing system.
2Reliability
If the hub terminal uses a large diameter antenna to receive weak remote terminal signals, then signal reception is improved, but relayed interference from the transponder becomes significantly stronger
Solution Approach 1:
The patent segments the received signal into multiple narrowband subbands, each containing a specific remote terminal signal. By processing each subband independently, the system can cancel interference specific to each frequency range more effectively, mitigating the impact of the strong relayed interference that results from using large antenna gain.
Solution Approach 2:
The patent applies local quality by tailoring the interference cancellation processing to each specific subband's characteristics. Each subband receives customized processing parameters optimized for its specific interference conditions, allowing effective cancellation despite the overall high interference level from the relayed signal.
3Adaptability or versatility
If frequency-division multiple access is used to allocate separate bandwidths for hub and remote terminals, then communication in both directions is enabled, but frequency reuse is prevented and spectral efficiency is reduced
Solution Approach 1:
The patent converts the harmful relayed interference into a useful reference signal for cancellation. By using the known transmitted signal to generate an expected interference replica, the system can subtract this replica from the received signal, enabling frequency reuse while maintaining bidirectional communication. The interference that would normally prevent frequency reuse becomes the key to achieving it.
4Measurement precision
If adaptive cancellation systems are used to handle variable link parameters, then cancellation accuracy is improved, but system complexity and computational requirements increase
Solution Approach 1:
The patent reduces computational complexity by segmenting the adaptive cancellation into independent narrowband subband processors. Each subband requires less computational resources than full-bandwidth processing, allowing adaptive algorithms to run efficiently with variable link parameters while maintaining overall system accuracy through the combination of subband results.
Data Source
AI summary
A method and apparatus at a local terminal are described for demodulating a remote-terminal signal located at a subband offset frequency in a frequency subband of relayed interference from a transponder satellite link. The demodulation of the remote-terminal signal is accomplished by transferring the digital data that produced the local-terminal transmit signal to the local-terminal receiver. The digital data is time-delayed and converted to a narrowband offset-constellation signal that cancels the relayed interference in an adaptive equalizer. Providing cancellation in the subband of the relayed interference produces larger interference cancellation factors than those obtained in conventional broadband cancellation systems. A phase-noise error signal is also generated and used to increase cancellation levels limited by phase noise generated in the satellite link frequency converters. Additionally the receiver converter frequencies are chosen such that errors in estimating the subband offset frequency do not affect cancellation in the adaptive equalizer.