Hybrid Digital-Analog transmission for wireless acoustic sensor networks

A very crucial task of a Wireless Acoustic Sensor Network (WASN) is the wireless transmission of the acoustic signal to a central fusion node. The positioned microphones experience different radio channel qualities, since they may be, e.g., positioned at different distances to the fusion node. The design of the coding and modulation scheme is usually governed by the worst-case radio channel quality. Mostly, limited complexity of the sensors and/or latency constraints do not allow for a feedback channel and adaptation. The conventional solution employs purely digital transmission using a quantizer in the audio encoder with the unavoidable quantization noise which limits the end-toend SNR even with increasing radio channel quality. No advantage from good radio channels can be taken. In the fusion node, an audio signal processing algorithm (fusion algorithm) combines the received microphone signals and the output quality depends on the SNR of each received microphone signal. Using purely digital transmission, the quality of output is limited due to the quantization noise. In this work, the use of Hybrid Digital-Analog (HDA) transmission systems for WASNs is proposed and the transmission quality is analyzed. The benefit is that the quality of the output of the fusion node improves with increasing radio channel qualities. It is shown that for fusion algorithms whose output quality is governed by the SNR of the individual received microphone signals, the HDA system supersedes purely digital transmission for all radio channel qualities. Especially for WASN, with many transmitters which may not be able to adapt to the radio channel quality, HDA transmission systems can show their full potential.