Voice over LTE (VoLTE) Optimization

Nokia Networks

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Contents

1. Introduction 32. VoIP Client Options 53. Radio Network Optimization 64. Voice Quality Optimization 115. Handset Power Consumption Optimization 176. Summary 197. Further Reading 19

1. IntroductionMobile operator networks carry tens of billions of minutes of voice traffic every day, the majority of it using Circuit Switched (CS) voice in 2G and 3G technologies. Most LTE operators serve voice calls via so-called CS-fallback function, while several operators have already made the switch to Voice over LTE (VoLTE), providing a carrier grade Voice over IP (VoIP) solution, built on IP-Multimedia Sub System (IMS) architecture.

VoLTE provides a number of benefits for operators and end users by enhancing LTE radio spectral efficiency and offering High Definition (HD) voice quality. VoLTE can use LTE on low frequency spectrum for good coverage, providing fast call setup times and enabling LTE data rates during the voice call. Additionally, IMS architecture is the foundation for new, innovative carrier-provided end-user multimedia services. The main reasons for adopting VoLTE are summarized in Figure 1.

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Migrate from dual radio CDMA + LTE devicesto LTE only devices

Take benefit of low band LTE forextended coverage Enhance voice quality with wideband codec

Improve spectral efficiency

Provide faster call setup time

Enable simultaneous voice and LTE data

Prepare evolution to LTE only deploymentsProvide evolution from voice to rich nextgeneration IMS services

Fig. 1. Motivations for VoLTE.

Deploying VoLTE requires a number of optimization steps to gain the full benefit of the technology’s potential. This white paper presents a number of aspects of VoLTE optimization in radio networks and in handsets to provide reliable voice with good quality and low handset power consumption. The white paper also compares VoLTE with CS voice and with over-the-top (OTT) VoIP solutions.

A VoLTE solution has synergies with Voice over Wi-Fi (VoWiFi), where operator voice can be run over Wi-Fi radio. If evolved packet core is enhanced with evolved Packet Data Gateway (ePDG), the VoWiFi can be considered an extension of VoLTE, providing seamless handovers between the LTE and WiFi accesses, which also requires mobile handset support.

The number of VoLTE deployments is increasing rapidly, with a wide selection of handsets, including iPhone 6, supporting also VoWiFi. VoWiFi specific considerations are covered in a separate VoWiFi white paper.

The radio network optimization solutions are considered in this paper first, followed by end-user quality optimization and finally focusing on the optimization of handset power consumption. The contents are summarized in Figure 2.

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End user quality optimization

Terminal power consumption optimization

Radio network optimization

Fig. 2. Contents of this paper.

2. VoIP Client OptionsVoIP over LTE can be achieved using several different solutions. A native VoLTE client can be integrated into the handset design, allowing optimization of the handset power consumption and offering interworking with legacy CS networks. The VoLTE client can also benefit from Quality of Service (QoS) capability in the radio network for voice packet prioritization. It is also possible to have a non-native VoLTE client from third parties which runs on top of the application processor. Also, any general VoIP application, such as Skype or Viber, will work over LTE radio, but without QoS support.

This paper compares three voice options for quality and efficiency:

1. Native VoLTE client integrated in the handset chip set. The number of commercial handsets with native VoLTE clients is growing rapidly.

2. Non-native VoLTE clients. Third party applications which can register to IMS (IP Multimedia Subsystem) and establish VoLTE call using QoS (QCI1). Examples include Bria and CSipSimple.

3. Over-the-Top (OTT) VoIP applications such as Skype, Facetime and Viber.

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3. Radio Network OptimizationThe success rate and the retainability of a VoLTE call must match and exceed the level provided by CS connections. The key performance indicators in the radio network are setup success rate, handover success rate and call completion success rate. Network optimization also aims at reducing the required bandwidth for voice and maximizing the capacity. The network optimization includes parameter optimization and feature activation, like header compression, TTI (Transmission Time Interval) bundling and QoS. These features help to make VoLTE call reliability, quality and efficiency significantly higher than with OTT VoIP applications.

Figure 3 presents the average IP user throughput, based on different voice applications over a two minute call with a predefined voice and silence pattern. The measurement assumes 23% talking, 23% listening and 54% no activity. The throughput shows the impact of the codec bit rate and the efficiency of the voice activity detection. The native VoLTE with Adaptive Multirate Wideband (AMR WB) provided an average of 10.2 kbps with the codec rate of 23.85 kbps and 8.8 kbps with the codec rate of 12.65 kbps.

The third-party Session Initiation Protocol (SIP) application with Adaptive Multirate Wideband (AMR WB) codec averaged as low as 8 kbps when using voice activity detection. The lower average throughput in this case was a consequence of transmitting virtually nothing during silence periods, which on the other hand contributed to a significant reduction of the Mean Opinion Score MOS. This same application with Enhanced Full rate Narrowband (EFR NB) codec generated up to 17.3 kbps, driven by less efficient voice activity detection. The three OTT VoIP applications ranged between 17.6 kbps to 42.8 kbps average throughput, depending on the codec they used and the particular implementation of features such as codec rate adaptation or voice activity detection.

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Fig. 3. Measured throughput of different VoIP codecs.

The throughput measurements shown in Figure 3 include full IP headers. VoLTE throughput requirements on the radio interface can be further reduced with Robust Header Compression (ROHC,) which improves spectral efficiency. ROHC runs between the base station and the handset. Figure 4 illustrates the benefit of RoHC - the header size is reduced from 40 bytes to 5 bytes, which is relevant for voice traffic since the voice packets are small. For example, an AMR 23.85 kbps voice packet is 60 bytes and AMR 12.65 kbps just 32 bytes. The header can be larger than the voice packet if header compression is not used. Therefore, activation of header compression is essential for maximizing VoLTE capacity.

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Fig. 4. Benefit from header compression.

VoLTE uplink performance in the weak signal can be enhanced with TTI bundling which allows the handset to repeat the same transmission in four consecutive 1 ms TTIs. TTI bundling makes the uplink more robust and enhances coverage by 4 dB. The benefit of TTI bundling in the weak signal is shown in Figure 5. The uplink Block Error Rate (BLER) is reduced from 73% to 9% with TTI bundling. The low BLER maintains good voice quality and avoids unnecessary retransmissions which eat up substantial radio resources. TTI bundling is switched on only when the handset hits the edge of the coverage area. TTI bundling runs between the base station and the handset.

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Fig. 5. Benefit of TTI bundling in uplink quality.

A high success rate for VoLTE calls requires reliable control channel transmission, which can be obtained by optimized channel coding in the radio interface to protect signaling quality. The high success rate also requires that interference levels and excessive signaling can be controlled in highly loaded cells. Figure 6 shows an example VoLTE call setup success rate in a live network including tens of millions of VoLTE calls during the measurement period. The success rate exceeds 99.7%, showing that excellent VoLTE availability can be achieved even in the loaded network. The network carried tens of billions of packet calls at the same time.

Another important factor for VoLTE is reliable mobility. The handover success rate can be optimized with RF planning and new features included in Nokia iSON (Self Organizing Network) such as Mobility Robustness Optimization. This enables radio level connection re-establishment in the event of handover failure.

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VoLTE call setup success rate100

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Fig. 6. VoLTE call setup success rate.

When the handset leaves the LTE coverage area, the VoLTE connection can be handed over to a CS connection in a 3G or 2G network. This procedure is called Single Radio Voice Call Continuity (SRVCC) or enhanced SRVCC (eSRVCC). If LTE coverage is extensive, the SRVCC probability is low. It is beneficial to have a low SRVCC probability to gain full benefit of VoLTE capabilities and to minimize any potential loss in voice quality, or even call drops which may be caused by a radio leg change. The SRVCC probability also depends on the parameter settings, like minimum Reference Signal Received Power (RSRP), which define the threshold when the LTE network initiates SRVCC. Figure 7 shows the probability of SRVCC in an example network. The SRVCC probability is reduced by 7% to below 3% by network optimization. More than a million SRVCC attempts are included in this graph.

SRVCC functionality is available for QCI1 connections but not for OTT VoIP. If an OTT VoIP connection loses LTE coverage, the call continues in the 3G or 2G network as a VoIP connection over a best effort data connection, not as a CS call. VoIP in 3G can provide reasonable voice quality in low loaded networks, but VoIP does not work in practice on a 2G network. Also, the connection break during the inter-system handover is substantially longer for best effort data than with SRVCC for QCI1 (less that 300ms according 3GPP standard).

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Percentage of VoLTE Calls Using SRVCC

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Fig. 7. Probability of VoLTE call using SRVCC.

4. Voice Quality OptimizationThe voice quality depends heavily on the voice codec sampling rate and the resulting audio bandwidth. An AMR Narrowband (NB) codec provides audio bandwidth of 80-3700 Hz, while an AMR Wideband (WB) extends the audio bandwidth to 50-7000 Hz. Furthermore, handset acoustics may limit the maximum bandwidth provided by the speech codecs. Terminal acoustic requirements can be found in 3GPP TS 26.131. The bandwidth is illustrated in Figure 8. The CS connections can use either AMR-NB or AMR-WB, while VoLTE in practice always uses AMR-WB. The AMR-WB data rate for CS connection ranges from 6.6 kbps to 12.65 kbps, while the VoLTE connection can use data rates up to 23.85 kbps, enhancing the quality of the connection compared to HD voice in CS networks.

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Fig. 8. Audio bandwidth of narrowband and wideband AMR.

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AMR - NB 80 - 3700 Hz

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AMR - WB50 - 7000 Hz

The average user opinion of a system’s speech quality can be presented with MOS. Latency also has an influence on the perceived quality. Traditionally, in network quality testing, absolute category rating (ACR), listening-only tests, defined in the ITU-T P.800 Recommendation, have been used to collect subjective responses of the performance of the telephone transmission system by listening to spoken material and by giving the judgments on a five-point scale, ranging from 1 (poor) to 5 (excellent). The averaged results have been presented as a mean opinion score (MOS).

In addition to subjective opinion, the abbreviation MOS is generally used for scores that originate from objective models. Algorithms have been developed for objective assessment of speech quality in telecommunication applications, estimating subjective listening tests performed in accordance with the ACR method. Objective methods are often more convenient than subjective methods. The latest standardized algorithm, Perceptual Objective Listening Quality Assessment (POLQA, or ITU-T P.863) has two operational modes: a narrowband (NB) mode and a super wideband (SWB) mode. The SWB mode can handle up to 14 kHz audio frequencies.

Figure 9 presents the average MOS for different voice applications. VoLTE with AMR-WB 23.65 kbps provided a MOS of 3.9.The reference CS narrowband call provided a score of 2.9 in the POLQA SWB scale in good radio conditions, while the non-native SIP client with NB EFR codec scored only slightly lower with 2.7. The same SIP client with an AMR-WB codec configuration provided a score of 3.0. The score of this and other third party SIP clients could be increased to 3.4 or 3.6 by tweaking some optional functionality such as deactivating voice activity detection. However, this caused an increase in the power consumption and throughput requirements as the application would transmit a constant data stream regardless of whether the speaker was talking or silent. The OTT VoIP applications scored between 4.1 and 4.2 in the POLQA SWB scale using proprietary codecs, quite close to the native VoLTE client.

Future VoLTE voice quality can be further improved with the new super wideband (SWB) and full band (FB) codecs, which will able to cover all the voice and audio bandwidths. 3GPP has defined a new speech/audio codec in Release 12 called Enhanced Voice Services (EVS) codec, which includes SWB and FB modes in addition to NB and WB modes. That codec will make it possible for VoLTE to match and beat the voice quality of all OTT clients.

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Fig. 9. Voice quality with different voice applications.

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Next, we present voice quality measurements for VoLTE and for one OTT VoIP as a function of network loading. VoLTE uses QoS Class Identifier (QCI) 1, while OTT VoIP is carried as non-guaranteed data traffic. A LTE base station is able to identify VoLTE calls and prioritize VoLTE in the packet scheduler. Figure 10 shows MOS measurements as a function of cell loading. Two cases are studied: good radio conditions with the signal level better than -80 dBm and bad radio conditions with the signal level at -110 dBm. The voice quality for VoLTE remains stable regardless of the loading, while the quality of OTT VoIP decreases as a function of loading. The OTT VoIP call also frequently drops during high loading. There is some difference in the voice quality between good and bad radio conditions but the VoLTE quality remains stable, also under bad radio conditions.

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Fig. 10. Voice quality with different loadings in good and bad radio conditions.

Mouth-to-ear delay as a function of loading is shown in Figure 11. The delay for VoLTE remains constant for all different load cases, demonstrating the benefit of QoS and Nokia Smart Scheduler in the base station. The delay of OTT VoIP increases as a function of simultaneous data loading in the cell.

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Fig. 11. Mouth-to-ear delay with different loadings.

The call setup time is an important factor affecting the quality perceived by the end user. Figure 12 shows how VoLTE can significantly improve the call establishment time compared to the legacy CS systems. The total time for call establishment depends on many factors such as whether the handsets were initially in RRC Idle or RRC Connected state when the call was originated and the latency experienced by the signaling network. The laboratory measurements show a VoLTE call setup time of 0.9 – 2.2 s, while the delay in the field is slightly higher depending on the operator network and transport architecture. The corresponding typical CS call setup time is four seconds and with CS Fallback (CSFB) at both ends, approximately six seconds.

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Fig. 12. Call setup time measurements.

To minimize the call setup time, an operator can apply a special paging strategy for incoming VoLTE calls. VoLTE paging logic is executed in a Mobility Management Entity (MME), based on the Evolved Packet Core (EPC) bearer information from the Serving Gateway in the Downlink Data Notification (DDN) message. The IMS network may assist this procedure by marking packets related to the incoming voice calls with a special Differentiated Services Code Point (DSCP) value.

Figure 13 shows an example of paging response delay distribution. A more aggressive timer value for paging a re-try timer could be applied for VoLTE to minimize the paging response delay, for example two seconds instead of a typical value of several seconds. In addition, MME could be configured to execute VoLTE paging immediately for the whole Tracking Area instead of first paging the last known cell.

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Fig. 13. Paging response delay distribution in live network.

5. Handset Power Consumption Optimization

Device battery life is one of the most relevant factors for smartphone users. It can be extended by using advanced features in the radio network and by optimized handset design. The two main factors affecting the mobile handset’s power consumption are considered to be availability of radio optimization features such as Discontinuous Reception (DRX) and the handset’s implementation of the voice client.

The power optimization solutions are shown in Figure 14.

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Handset architectureoptimization

• VoLTE integrated to the chip set• Application processor can enter sleep mode

Radio featuresincluding DRX

• Discontinuous reception (DRX)• Modem activity from 100% to below 50%

Fig. 14. Mobile handset power saving solutions.

The voice application can run in the handset application processor, which however needs a lot of power. Power consumption can be minimized by integrating a VoLTE client to the modem processor. The idea of DRX is to use sleep mode in the handset between packet receptions. The VoLTE voice packets arrive every 20 ms while the transmission time in LTE is just 1 ms. Such bursty transmission and reception allows the use of power saving sleep mode even between voice packets. The DRX activation and parameter configurations are controlled by the base station. The DRX can be configured in such a way that two voice packets are transmitted simultaneously, which increases the packet arrival period to 40 ms. Figure 15 illustrates the instantaneous handset power consumption during a VoLTE call when the person is listening and not talking. The short power peaks happen mostly every 40 ms during the reception of the voice packet, while the power consumption is considerably lower between the packets.

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The measured current consumption of different voice applications is shown in Figure 16. VoLTE power consumption can be reduced by 80% with DRX. 121 mA has been measured on a handset, which is even slightly below the 125 mA measured on the same codec on 3G CS using the same handset. The power consumption of the OTT VoIP application is substantially higher at 248 mA.

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Fig. 16. Handset power consumption with different voice applications, showing 50% lower power consumption for VoLTE than for OTT VoIP.

6. SummaryVoLTE is being deployed globally, supported by new VoLTE-capable handsets. VoLTE can enhance an operator’s network efficiency and improve end-user performance in areas such as voice quality and setup times. This paper illustrates that optimization of radio features and parameters is needed to provide reliable VoLTE connections with high success rates and low drop rates. The live network results show that excellent key performance indicators can be obtained for VoLTE in optimized networks.

The measurements also show that VoLTE offers a number of benefits compared to OTT VoIP applications, including robust performance and higher voice quality during congestion and lower power consumption. Reliability is obtained by using QoS features in the radio network and by optimizing power consumption with the DRX feature.

7. Further Reading• Nokia VoLTE white paper:

http://nsn.com/sites/default/files/document/nsn_volte_white_paper.pdf

• Nokia VoWiFi white paper: (to be published 4Q2014)

PublicNokia is a registered trademark of Nokia Corporation. Other product and company names mentioned herein may be trademarks or trade names of their respective owners.

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Visiting address: Karaportti 3, ESPOO, Finland Switchboard +358 71 400 4000

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© Nokia Solutions and Networks 2014

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