Wireless Speech Transmission Using Silent-Interval Carrier Switching
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Solution Overview
Problem
Wireless communication devices operating at 6.25 kHz bandwidth face challenges in matching the transmit spectral mask 'Mask E' due to significant degradation of the S/N ratio and increased adjacent channel interference, particularly with the conventional analog FM scheme, which results in insufficient performance for practical use.
Innovation Solution
A wireless communication apparatus and method that includes speech data generation, FSK modulation, and sound presence/absence detection to transmit a non-modulated carrier signal during sound absence intervals, reducing FSK modulation during these intervals and using two-level FSK modulation with small shifts from the carrier frequency, thereby minimizing interference with adjacent channels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If four-level FSK modulation is used to match the transmit spectral mask, then spectral compliance is improved, but adjacent channel interference increases
Solution Approach 1:
The patent applies periodic action by detecting sound presence/absence and switching between FSK modulation and non-modulated carrier transmission during sound absence intervals. This periodic switching reduces the overall spectral occupancy and adjacent channel interference while maintaining spectral compliance during active speech periods.
Solution Approach 2:
The patent changes the modulation parameter dynamically based on sound presence/absence detection. During sound absence intervals, the system transitions from four-level FSK modulation to non-modulated carrier transmission, effectively reducing the spectral components that cause adjacent channel interference while maintaining spectral mask compliance.
2Ease of manufacture
If analog FM scheme is used, then implementation simplicity is improved, but S/N ratio degrades significantly
Solution Approach 1:
The patent replaces the analog FM scheme with digital FSK modulation. This substitution improves the S/N ratio by using digital signal processing while maintaining implementation feasibility through the addition of sound presence/absence detection and conditional modulation control.
3Reliability
If FSK modulation is applied continuously, then speech transmission quality is maintained, but interference with adjacent channels increases
Solution Approach 1:
The patent implements periodic action by continuously detecting sound presence/absence and switching modulation states accordingly. FSK modulation is applied only during sound presence intervals when speech transmission is needed, while non-modulated carrier transmission is used during sound absence intervals, reducing adjacent channel interference while maintaining speech quality during active transmission.
Solution Approach 2:
The patent applies dynamics by making the modulation state changeable based on real-time sound presence/absence detection. The system dynamically transitions between FSK modulation and non-modulated carrier transmission, optimizing both speech transmission quality and adjacent channel interference reduction according to actual speech activity.
Data Source
AI summary
When a sound presence/absence detector (12) detects a sound absence interval, a non-modulated carrier signal is outputted in an area used to transmit speech data that are included in transmission data having a frame structure and that correspond to the sound absence interval. That is, an FSK modulator (14) causes a transmitting circuit (15) to output the non-modulated carrier signal from a wireless communication apparatus (100) in the area used to transmit the speech data corresponding to the sound absence interval. In the meantime, a four-level FSK modulated wave signal is outputted in the areas other than the one of the sound absence interval. A non-modulation discriminating data for allowing the sound absence interval to be determined are included in an area of channel identifying information included in the transmission data having the frame structure, and allow a communication terminal apparatus at the receiving end to avoid any unstable operations. The present invention is suitable for narrowed bands of communication paths and reduces the affections on adjacent channels.