WCQPSK Modulation for Simulcast Delay Spread
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Solution Overview
Problem
Simulcast radio communication systems experience significant signal degradation due to delay spread, leading to high bit error rates, especially in P25-compatible systems, which are challenging to mitigate with existing technologies.
Innovation Solution
Implementing a raised cosine filter with a roll-off factor of 1 to maintain the phase characteristic of the modulated carrier signal within ten degrees of the desired symbol phase for over 50% of the symbol period, and using a wide eye pattern-CQPSK (WCQPSK) modulation scheme with increased bandwidth and roll-off factor to reduce bit error rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If simulcast transmission is used to cover large geographic areas, then communication coverage is improved, but delay spread increases causing signal degradation
Solution Approach 1:
The patent changes the modulation scheme from conventional P25 C4FM or CQPSK to wide eye pattern-CQPSK (WCQPSK) with modified parameters including increased bandwidth and adjusted roll-off factor. This parameter change allows the system to tolerate delay spreads of over 170 microseconds while maintaining acceptable bit error rates, thus preserving signal quality across large geographic coverage areas.
2Reliability
If delay spread is reduced by adjusting site placement and antenna configuration, then bit error rate is improved, but system complexity and installation difficulty increase
Solution Approach 1:
Rather than requiring precise physical adjustments of site placement and antenna configuration, the patent achieves bit error rate improvement through parameter changes in the modulation scheme. The WCQPSK scheme with increased bandwidth and modified roll-off factor provides robustness against delay spread without requiring complex system configuration or difficult field adjustments.
3Adaptability or versatility
If conventional P25 modulation schemes are used, then compatibility with existing systems is maintained, but bit error rate increases with delay spread
Solution Approach 1:
The patent uses differential CQPSK modulation with modified parameters (increased bandwidth, adjusted roll-off factor) that maintain compatibility with existing P25 system infrastructure while achieving superior performance. The WCQPSK scheme can tolerate delay spreads of over 170 microseconds before reaching 5% bit error rate, compared to conventional schemes that fail at much lower delay spreads, thus improving reliability without sacrificing adaptability.
Data Source
AI summary
A method and associated apparatus for generating a modulated carrier signal with reduced bit error rates based on a plurality of data symbols. A plurality of data symbols are received (702), and a digital input signal is generated based on the plurality of data symbols. The digital input signal are filtered to produce a digital output signal (710) including a phase characteristic. The phase characteristic of the digital output signal (710) remains close to the desired symbol phase for substantial portion of the symbol period. A carrier signal is modulated using the digital output signal to produce the modulated carrier signal.