Spread Spectrum GMSK Signal Inter-Chip Interference Reduction
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Solution Overview
Problem
Spread spectrum GMSK signals suffer from intra-symbol interference (ISI) and inter-chip interference (ICI) due to the widening of the correlation function, which degrades tracking performance in GNSS systems compared to BPSK modulated signals.
Innovation Solution
The use of different spread spectrum codes for pilot and data channels, where pre-modulation chips are assigned to In-Phase and Quadrature channels such that no more than two consecutive chips generated with the same code are transmitted in the same channel, effectively isolating and reducing ICI.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If GMSK modulation is used for spread spectrum signals, then spectrum efficiency is improved, but inter-chip interference increases due to widened correlation function
Solution Approach 1:
The patent divides the spread spectrum signal into separate in-phase (I) and quadrature (Q) components, each modulated independently with GMSK. This segmentation allows the correlation functions of I and Q channels to be separated in time, preventing their overlap and reducing inter-chip interference while maintaining the spectrum efficiency benefits of GMSK modulation
Solution Approach 2:
The patent introduces a temporal dimension to the problem by offsetting the correlation functions of the I and Q channels in time. The in-phase channel uses even chip intervals while the quadrature channel uses odd chip intervals, creating a time-based separation that eliminates inter-chip interference without sacrificing the spectral efficiency of GMSK modulation
2Reliability
If larger GMSK filter is used to reduce ISI, then intra-symbol interference is reduced, but inter-chip interference increases due to wider correlation function
Solution Approach 1:
By segmenting the signal into I and Q components with temporally separated correlation functions, the patent allows each component to use optimal filter sizes for reducing ISI without their correlation functions overlapping to create ICI. This segmentation resolves the contradiction between filter size and interference types
Solution Approach 2:
The patent applies preliminary time-offsetting to the correlation functions of I and Q channels before transmission. This preliminary action ensures that even with larger filters that produce wider correlation functions, the separated time intervals prevent inter-chip interference while allowing each channel to benefit from reduced intra-symbol interference
Data Source
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AI summary
The invention relates to improvements of spread spectrum GMSK (Gaussian Minimum Shift Keying) signals, particularly to method for generating a spread spectrum Gaussian Minimum Shift Keying, GMSK, signal comprising obtaining a sequence D(t) of data symbols for a data channel, obtaining at least one first spread spectrum code comprising a first sequence CD(t) of spread spectrum chips for the data channel, obtaining at least one second spread spectrum code comprising a second sequence CP(t) of spread spectrum chips for a pilot channel, generating a sequence r(t) of pre-modulation chips by combining the sequence D(t) of data symbols for the data channel with the spread spectrum chips of the first sequence CD(t) of the at least one first spread spectrum code and data symbols for the pilot channel with the spread spectrum chips of the second sequence CP(t) of the at least one second spread spectrum code to a combined sequence of chips, assigning the chips of the combined sequence of chips to the transmission channels In-Phase I and Quadrature Q of the spread spectrum GMSK signal so that not more than two consecutive pre-modulation chips generated with the same spread spectrum code are transmitted in the same transmission channel I or Q, and performing GMSK modulation g(t) using the sequence of pre-modulation chips to generate a spread spectrum GMSK signal s(t).