Voice over IP (VoIP)
While the data traffic and the data revenues are increasing, the voice service still makes the majority of operators' revenue. Therefore, LTE is designed to support not only data services efficiently, but also good quality voice service with high efficiency. As LTE radio only supports packet services, the voice service will also be Voice over IP (VoIP), not Circuit Switched (CS) voice. This chapter presents the LTE voice solution including voice delay, system performance, coverage and inter-working with the CS networks. First, the general requirements for voice and the typical voice codecs are introduced. The LTE voice delay budget calculation is presented. The packet scheduling options are presented and the resulting system capacities are discussed. The voice uplink coverage challenges and solutions are also presented. Finally, the LTE VoIP inter-working with the existing CS networks is presented.
13.2 VoIP Codecs
GSM networks started with Full rate (FR) speech codec and evolved to Enhanced Full Rate (EFR). The Adaptive Multi-Rate (AMR) codec was added to 3GPP Release 98 for GSM to enable codec rate adaptation to the radio conditions. AMR data rates range from 4.75 kbps to 12.2 kbps. The highest AMR rate is equal to the EFR. AMR uses a sampling rate of 8 kHz, which provides 300–3400 Hz audio bandwidth. The same AMR codec was included ...