Written by: Christina Birkehammar, Stefan Bruhn, Peter Eneroth, Karl Hellwig and Stefan Johansson
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The ongoing evolution of wireless communication systems and mobile phones has given rise to a variety of compelling mobile applications (music player, camera, game console) and services (mobile internet, mobile TV, and so on). Likewise, many services have evolved significantly in order to satisfy user demands. In contrast, from a user perspective, voice telephony has not changed noticeably since mobile telephony was still very young. Notwithstanding, voice service has continued to evolve. Significant milestones include the introduction of the enhanced full-rate codec (EFR) and, later, the adaptive multirate (AMR) voice codec, which increased voice quality and boosted channel error robustness and capacity. The narrowband AMR (AMR-NB) codec, which supports the bandwidth of traditional telephony, is now widely deployed in GSM/EDGE and UMTS systems. It is also the codec of choice for the forthcoming multimedia telephony service for IMS (MTSI) standard from 3GPP.
As one would expect, AMR also continues to evolve. The new wideband AMR (AMR-WB) codec, whose voice frequency band is twice that of AMR-NB, enables telephony services with true, natural voice quality, clearly outperforming other existing mass-market telephony services, including those used for wireline telephony.
User and operator benefits of improved voice quality
Results of laboratory listening tests A MUSHRA listening test carried out by Ericsson Research showed that users clearly prefer AMR-WB voice over uncoded narrowband voice. (MUSHRA: multi-stimulus test with hidden reference and anchor, ITU-R recommendation BS.1534.) In particular, AMR-WB is superior to the 64kbps pulse-code-modulation (PCM) coding of the public switched telephone network (PSTN).
On error-free channels, AMR-WB (even in its lowest mode at 6.6 kbps) outperformed every AMR-NB mode up to 12.2kbps (Figures 1-2). And on error-prone GSM/EDGE and UTRAN channels under normal operating conditions, AMR-WB was favored over AMR-NB. It should be noted, however, that these results are only indicative of an artificial lab environment and testing with short voice samples. The results might thus be inconclusive when comparing voice service based on AMR-WB with that based on AMR-NB. As a consequence, to compile more evidence, Ericsson and T-Mobile International also conducted a joint consumer trial in Germany.
Consumer trial in live network
The joint consumer trial ran for four weeks during April and May 2006. Ericsson and T-Mobile equipped 150 end users with special AMR-WB mobile phones in order to study the effects of improved voice quality on end-user perception and behavior. The results of the trial are very encouraging (Figure 3). More than 70% of the participants perceived a distinct improvement in voice quality – they found that they could more easily place and complete calls in noisy environments, and reported that the improved voice quality created a greater sense of privacy, discretion and comfort. Accordingly, Ericsson anticipates that AMR-WB will lead to positive changes in calling patterns, generating substantially more mobile traffic, both in terms of number and duration of calls.
Initially, while AMR-WB terminal penetration is low, operators can boost income and strengthen their brands by providing special AMR-WB services with high voice quality, such as conference calls and announcements. They can also drive information services, ring-back signals, enhanced automatic voice recognition, and improved voice mail. In addition, they might offer AMR-WB telephony to limited user groups or as an enterprise solution. During the consumer trial, for instance, business users indicated that they have high expectations regarding voice quality, in particular because improved voice quality will help them to communicate important figures, reduce expenses, and leave a positive impression.
AMR-WB can also help operators to cut costs, for example, by reducing the cost of acquiring subscribers. A more satisfying voice service should also help reduce helpdesk costs. The AMR-WB phone and consumer trial typifies the planned activities of Ericsson’s Network Solution Area Characteristics unit (Business Networks), whose aim is to continuously enhance the end-to-end performance of Ericsson services and applications.
