VoLTE Capabilities. RAN Optimization Requirements

Table of Contents

1 Introduction

2 Migrating to IMS-based VoLTE: Initial considerations
2.1 Benefits of IMS-based VoLTE
2.2 Positioning VoLTE

3 VoLTE Parameter Optimization
3.1 Robust Header Compression (RoHC)
3.2 Transmission Time Interval (TTI) Bundling
3.3 Discontinuous Reception (DRX)
3.4 Dedicated Bearers
3.5 Semi-Persistent Scheduling (SPS)

4 Conclusion

Bibliography

List of Figures

List of Abbreviations

1 Introduction

The increasing gap among capacity and demand poses an important call for novel network technologies to allow mobile operators to improve performance (from the end-users’ point of view) on a cost-effective basis. In fact, Voice over Long Term Evolution (VoLTE) is a key component for an innovative set of base services defined for all-IP networks: the objective relies on making such new services as accessible as voice and SMS are nowadays, while also offering a flexible interaction with Internet applications.

Indeed, LTE focus on a (rather flat) all-IP access technology aiming at delivering a bandwidth-efficient method of carrying multiple types of user traffic at the same time. In other words, the capability of deploying Voice over IP (VoIP) services, while also supporting high-rate data throughputs, characterizes one of the critical drivers for the development to LTE.

Within this context, the IP Multimedia Subsystem (IMS) and Session Initiation Protocol (SIP) are essential technologies for deploying VoIP in an LTE setting. Nevertheless, the LTE RAN features are ultimately responsible for creating ‘value added’ services in relation to VoIP. Therefore, this paper illustrates in the second chapter the advantages of the IMS-based VoLTE approach as well as its (strategic) positioning compared to initial methods and over-the-top (OTT) providers.

Furthermore, the third chapter explains that optimization of radio features and parameters is required to offer reliable VoLTE connections based on high success rates. This is especially important for highly-competitive markets like the Austrian telecommunication sector: annual network performance tests (conducted by the P3 Group for example) may unmask a mobile operator’s (network) vulnerabilities and critically influence its customer’s preference and the likely churn rate. Finally, conclusions are drawn based on the key findings described in this paper.

2 Migrating to IMS-based VoLTE: Initial considerations

The need for bandwidth derived from devices and subscribers has been constantly increasing for many years. Actually, the data volume transferred by mobile networks is doubling approximately every year (Ericsson Mobility Report 2013) and the quantity of connected machine-to-machine (M2M) devices is estimated to exceed 50 billion by 2020. Still, one of the key performance indicators like the average revenue per user (ARPU) is decreasing in many markets.

As a result, to take advantage of mobile-broadband opportunities (while at the same time increasing profits/benefits for business and consumers) remains the focus of every operator’s activities. In this sense, LTE networks are able to carry mobile broadband with huge data capacity and a minor latency level. Though, since there is no circuit-switched voice domain in LTE (Fig. 1), the telecom industry has implemented a universally interoperable IP-based voice and video calling solution for LTE: VoLTE, which further facilitates the evolution of innovative communication services.

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Figure 1: Circuit and packet domains. LTE within the Evolved Packet System (EPS) [1]

Furthermore, over-the-top (OTT) communication solutions like Skype (as well as other app providers) have influenced the way users assess a particular service based on VoIP. Nonetheless, a completely satisfactory user experience cannot be offered by OTT solutions due to missing QoS measures or the lack of handover mechanisms to the circuit-switched (CS) network. In addition, there is no guaranteed emergency support or extensive interoperability of services among diverse OTT services and devices. Thus, the readiness of subscribers to use a service that does not offer security, quality, flexibility or even mobile-broadband coverage clearly influence the adoption of OTT services in a negative manner.

2.1 Benefits of IMS-based VoLTE

On the contrary, VoLTE operators have an attractive competitive positioning: they can deliver mobile HD voice, implement video and messaging as well as converge with the web via Web Real Time Communications (WebRTC). Moreover, (VoLTE) operators can work in partnership with application providers and supply the best user experience on a low cost basis, since they can translate application developers’ solutions onto innovative communication services. Besides, a quick customization of features for strategic activities (as the mobile healthcare industry could be) becomes feasible and effective.

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Figure 2: VoLTE’s strategic value for operators [3]

Needless to say, VoLTE plays an important role in empowering all-IP communications in the 4G LTE network. As a result, VoLTE will enable operators to:

- Generate attractive communication services by merging mobile voice with video, the web and social networking
- Improve customer experience by delivering data and HD voice simultaneously (which also contributes to offload legacy infrastructure). In addition, the current ‘fragmented’ communications landscape that relies on various (rich) media approaches becomes more harmonized.

For sure, VoLTE is not the only method by which all operators will initially launch voice service in the short-run. Yet, it embodies the ideal approach if competitive advantages and their related (business) risks are considered.

2.2 Positioning VoLTE

Considering that there is no other well-specified, wide accepted or well-supported solution offered, operators will have no other choice than adopting VoLTE. This is true even if there are some provisional methods to convey voice in LTE devices (this is useful to operators aiming at selling out smartphones previous to the proper VoLTE’s market maturity). As shown in Fig. 3, some approaches would usually rely on current circuit-switch 2G and 3G networks or, in a rather simple way, on app providers.

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Figure 3: VoLTE compared to other preliminary approaches for voice in LTE devices [3]

Particularly, the Circuit-Switch Fallback (CSFB) and the dual-radio methods on which CDMA operators rely (in some cases known as Simultaneous Voice-and-LTE, SVLTE) are convenient to the extent that current telephony services are reused. However, these are not indeed LTE solutions and, as a result, are not subject to suitable development. Likewise, OTT methods are attractive since they run over IP and support, on the one hand, richer experiences. On the other hand, important features like interoperability and 2G/3G integration/handover are (with the OTT approach) definitely at stake.

Furthermore, CSFB voice is characterized by the limited service since a voice call (originating or terminating) leads to a fallback to legacy 2G/3G service. This implies the restriction to CS voice to the subscriber, including slower 3G mobile data services or its complete loss during a fallback to 2G. Nevertheless, operators who temporarily choose CSFB should critically reflect on the invested resources due to:

- Communication experience for end-users might be at stake: during voice calls CSFB subscribers would be downgraded from 4G LTE data service to 3G HSPA+ or 2G, losing data services completely.
- Boundaries are imposed on innovation: New services that work on all-IP networks cannot be realized (such as video calling)
- Increased OPEX and risk: the economic and technical challenges that rise with the operation and maintenance of 4G LTE and legacy 2G/3G networks in parallel should not be underestimated.

Despite the fact that HD voice is being gradually implemented in 3G networks, 4G LTE networks would be more present in the years to come along with more capable devices and network elements. In terms of call setup time, CSFB requires 4 seconds (around 1 additional second compared to the 3G’s required time) and approximately twice the setup time necessary for a VoLTE’s call (idle to idle state, see [5]).

3 VoLTE Parameter Optimization

Deploying VoLTE requires a number of optimization steps to gain the full benefit of the technology’s potential. Ideally, the success rate and the retainability of a VoLTE call must exceed the level provided by CS connections.

Basically, network optimization involves the activation of features and optimization of parameters such as:

- Robust Header Compression reduces the bandwidth associated with the headers used to transport relatively small encoded audio packets
- Transmission Time Interval (TTI) Bundling overcomes the limitation of using short (1ms) TTIs at cell boundaries

These features contribute to make a VoLTE call reliable while still providing more efficiency compared to OTT VoIP applications. Special attention is paid, therefore, to decreasing the necessary bandwidth for voice and get the most out of the capacity. By doing so, the handover, setup and call completion success rates could be further improved.

In terms of handset power consumption, the focus lies on the one hand on the enhancement of the handset architecture: the integration of VoLTE into the chip set or the sleep mode available to the application processor fall into this category. On the other side, radio features like the Discontinuous reception (DRX) intended to conserve the UE’s battery life during a VoLTE conversation complement the (handset) power saving solutions. Other valuable drivers are:

- Dedicated Bearers allow for the prioritization of VoLTE audio packets over all other best-effort traffic
- Semi-Persistent Scheduling (SPS) reduces the complexity and overhead of the continuous allocation of DL/UL physical layer resource blocks to transport the audio traffic

Additional (optimization) features in the field of speech quality, depending primarily on the voice codec sampling rate and the derived audio bandwidth, will be discussed separately.

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