An experiment with real-time data transmission over global scale mobile voice channel

This paper presents a details of experiment for real-time transmission of low bit rate data over global scale mobile voice channel. The emphasize is on high availability and low delay variations, rather then channel capacity. Since 3G and LTE technologies are still not predominantly deployed on global scale, the experiment is performed on GSM channel. Although this technology provides low bit rate data transmission by GPRS, voice traffic has higher priority than data traffic and hence data transmission is not suitable for interactive applications due to large delay variations. In order to enable data transmission over voice channel, a special type of modem that would be robust to speech compression algorithm is necessary. We have designed special modulation scheme whose symbols are combination of carrier frequency and anvelope changes. Both simulation and experimental results are presented. It is shown that cascading of certain amount of standard codecs, like EFR or AMR, is not sufficient for real world scenario simulation, so additional processing, like noise reduction, should be also included.