Frequency-Offset QAM for GFDM Out-of-Band Emission Control
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Solution Overview
Problem
Current GFDM systems face challenges in mitigating intercarrier and intersymbol interference, especially in time-frequency dispersive channels, and struggle to achieve low out-of-band radiation while maintaining good time-frequency localization.
Innovation Solution
A multicarrier radio transmitter using Frequency-Offset QAM modulation, where a frequency shift of half a subcarrier bandwidth is applied to adjacent subcarriers, along with pulse shaping and cyclic prefixes/suffixes, to reduce interference and out-of-band emissions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional OFDM is used in ideal AWGN channels, then intersymbol interference and intercarrier interference are eliminated, but the system cannot cope with time-frequency dispersive channels and asynchronicities
Solution Approach 1:
The patent applies Offset-QAM modulation which changes the timing parameter by offsetting the imaginary part of symbols by half a symbol duration relative to the real part. This parameter change enables the system to achieve good time-frequency localization and cope with time-frequency dispersive channels while maintaining interference mitigation capabilities
Solution Approach 2:
The patent introduces dynamic pulse shaping filters that can adapt to channel conditions. The use of time-shifted OQAM with adjustable offset parameters allows the system to dynamically adjust its time-frequency localization properties to match varying channel characteristics and asynchronicity levels
2Reliability
If good time-frequency localization is achieved in GFDM systems, then interference mitigation improves, but out-of-band radiation increases
Solution Approach 1:
The patent applies different pulse shaping filters to different parts of the signal spectrum. By using tailored pulse shaping functions for different subcarriers or signal regions, the system achieves good time-frequency localization where needed while controlling out-of-band radiation in other regions through localized filter design
Solution Approach 2:
The patent employs asymmetric pulse shaping filters rather than symmetric ones. This asymmetry allows for optimized time-frequency localization characteristics that can achieve good interference mitigation performance while better controlling spectral leakage and out-of-band radiation through non-uniform energy distribution in the time-frequency plane
Data Source
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AI summary
Generalized Frequency Division Multiplexing (GFDM) can use a conventional QAM modulation but the required good time-frequency localization can result in distorted reconstruction of complex-valued symbols sent at Nyquist rate. To mitigate this problem the use of Offset-QAM has been proposed as solution, in particular with a non-symmetric conjugate-root pulse-shaping filter. However, the use of time shifting in conventional Offset-QAM reduces the effect of using guard sub-symbols to achieve low out-of-band radiation. Therefore the invention proposes to use a frequency-domain Offset-QAM, i.e. the real and imaginary part are shifted against each other in frequency by half a subcarrier bandwidth, which in turn allows the use of guard symbols, i.e. a cyclic prefix and/or a cyclic suffix of complex valued samples, to achieve low out-of-band emission.