Nokia Volte Optimization - Quality

7/24/2019 Nokia Volte Optimization - Quality

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Voice over LTE (VoLTE)Optimization

Nokia Networks


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Page 2 networks.nokia.com

Contents

1. Introduction 3

2. VoIP Client Options 5

3. Radio Network Optimization 6

4. Voice Quality Optimization 11

5. Handset Power Consumption Optimization 17

6. Summary 19

7. Further Reading 19


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

VoLTE provides a number of benets for operators and end users byenhancing LTE radio spectral eciency and oering High Denition(HD) voice quality. VoLTE can use LTE on low frequency spectrumfor good coverage, providing fast call setup times and enablingLTE data rates during the voice call. Additionally, IMS architectureis the foundation for new, innovative carrier-provided end-usermultimedia services. The main reasons for adopting VoLTE aresummarized in Figure 1.


Migrate from dual radio CDMA + LTE devicesto LTE only devices

Take benet 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.


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Deploying VoLTE requires a number of optimization steps to gain the

full benet of the technologys potential. This white paper presentsa number of aspects of VoLTE optimization in radio networks and inhandsets to provide reliable voice with good quality and low handsetpower consumption. The white paper also compares VoLTE with CSvoice and with over-the-top (OTT) VoIP solutions.

A VoLTE solution has synergies with Voice over Wi-Fi (VoWiFi), whereoperator voice can be run over Wi-Fi radio. If evolved packet core isenhanced with evolved Packet Data Gateway (ePDG), the VoWiFi canbe considered an extension of VoLTE, providing seamless handoversbetween the LTE and WiFi accesses, which also requires mobilehandset support.

The number of VoLTE deployments is increasing rapidly, with a wideselection of handsets, including iPhone 6, supporting also VoWiFi.VoWiFi specic considerations are covered in a separate VoWiFiwhite paper.

The radio network optimization solutions are considered in this paperrst, followed by end-user quality optimization and nally focusingon the optimization of handset power consumption. The contentsare summarized in Figure 2.


End user quality optimization

Terminal power consumption optimization

Radio network optimization

Fig. 2. Contents of this paper.


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2. VoIP Client OptionsVoIP over LTE can be achieved using several dierent solutions.A native VoLTE client can be integrated into the handset design,allowing optimization of the handset power consumption and oeringinterworking with legacy CS networks. The VoLTE client can alsobenet from Quality of Service (QoS) capability in the radio networkfor voice packet prioritization. It is also possible to have a non-nativeVoLTE client from third parties which runs on top of the applicationprocessor. 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 eciency:

1. Native VoLTE client integrated in the handset chip set. Thenumber of commercial handsets with native VoLTE clients isgrowing rapidly.

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

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



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kbps

SIP EFR SIP AMRWB

OTT AP1 OTT AP2 OTT AP3 VoLTE23.85 kbps

VoLTE12.65 kbps

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

Figure 3 presents the average IP user throughput, based on dierentvoice applications over a two minute call with a predened voice andsilence pattern. The measurement assumes 23% talking, 23% listeningand 54% no activity. The throughput shows the impact of the codec bitrate and the eciency of the voice activity detection. The native VoLTEwith Adaptive Multirate Wideband (AMR WB) provided an average of 10.2kbps with the codec rate of 23.85 kbps and 8.8 kbps with the codec rateof 12.65 kbps.

The third-party Session Initiation Protocol (SIP) application with AdaptiveMultirate Wideband (AMR WB) codec averaged as low as 8 kbps whenusing voice activity detection. The lower average throughput in this

case was a consequence of transmitting virtually nothing during silenceperiods, which on the other hand contributed to a signicant reductionof the Mean Opinion Score MOS. This same application with EnhancedFull rate Narrowband (EFR NB) codec generated up to 17.3 kbps, drivenby less ecient voice activity detection. The three OTT VoIP applicationsranged between 17.6 kbps to 42.8 kbps average throughput, dependingon the codec they used and the particular implementation of featuressuch as codec rate adaptation or voice activity detection.


Fig. 3. Measured throughputof dierent VoIP codecs.


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The throughput measurements shown in Figure 3 include full IP

headers. VoLTE throughput requirements on the radio interface canbe further reduced with Robust Header Compression (ROHC,) whichimproves spectral eciency. ROHC runs between the base stationand the handset. Figure 4 illustrates the benet of RoHC - the headersize is reduced from 40 bytes to 5 bytes, which is relevant for voicetrac since the voice packets are small. For example, an AMR 23.85kbps voice packet is 60 bytes and AMR 12.65 kbps just 32 bytes. Theheader can be larger than the voice packet if header compression isnot used. Therefore, activation of header compression is essential formaximizing VoLTE capacity.


4540

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5

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Header compression

Headersize

(byte)

Original header size

Downlink header size

Uplink header size

Fig. 4. Benet from header compression.


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VoLTE uplink performance in the weak signal can be enhanced with TTI

bundling which allows the handset to repeat the same transmissionin four consecutive 1 ms TTIs. TTI bundling makes the uplink morerobust and enhances coverage by 4 dB. The benet of TTI bundling inthe 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 maintainsgood voice quality and avoids unnecessary retransmissions which eatup substantial radio resources. TTI bundling is switched on only whenthe handset hits the edge of the coverage area. TTI bundling runsbetween the base station and the handset.


9.0

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PUSCH BLER comparison

BLER(in%)

With TTI bundling

Without TTI bundling

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


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A high success rate for VoLTE calls requires reliable control channel

transmission, which can be obtained by optimized channel coding in theradio interface to protect signaling quality. The high success rate alsorequires that interference levels and excessive signaling can be controlledin highly loaded cells. Figure 6 shows an example VoLTE call setup successrate in a live network including tens of millions of VoLTE calls during themeasurement period. The success rate exceeds 99.7%, showing thatexcellent 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 handoversuccess rate can be optimized with RF planning and new features includedin Nokia iSON (Self Organizing Network) such as Mobility Robustness

Optimization. This enables radio level connection re-establishment in theevent of handover failure.


VoLTE call setup success rate100

98

96

94

92

90

(%)

Three-month period

Fig. 6. VoLTE call setup success rate.


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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. Thisprocedure is called Single Radio Voice Call Continuity (SRVCC) orenhanced SRVCC (eSRVCC). If LTE coverage is extensive, the SRVCCprobability is low. It is benecial to have a low SRVCC probability togain full benet of VoLTE capabilities and to minimize any potentialloss in voice quality, or even call drops which may be caused by a radioleg change. The SRVCC probability also depends on the parametersettings, like minimum Reference Signal Received Power (RSRP), whichdene the threshold when the LTE network initiates SRVCC. Figure 7shows the probability of SRVCC in an example network. The SRVCCprobability 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 OTTVoIP. If an OTT VoIP connection loses LTE coverage, the call continuesin the 3G or 2G network as a VoIP connection over a best eort dataconnection, not as a CS call. VoIP in 3G can provide reasonable voicequality in low loaded networks, but VoIP does not work in practiceon a 2G network. Also, the connection break during the inter-systemhandover is substantially longer for best eort data than with SRVCCfor QCI1 (less that 300ms according 3GPP standard).


Percentage of VoLTE Calls Using SRVCC

Three-month period

10%

9%

8%

7%

6%

5%

4%

3%

2%

1%

0%

Fig. 7. Probability of VoLTE call using SRVCC.


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4. Voice Quality OptimizationThe voice quality depends heavily on the voice codec sampling rateand the resulting audio bandwidth. An AMR Narrowband (NB) codecprovides audio bandwidth of 80-3700 Hz, while an AMR Wideband (WB)extends the audio bandwidth to 50-7000 Hz. Furthermore, handsetacoustics may limit the maximum bandwidth provided by the speechcodecs. Terminal acoustic requirements can be found in 3GPP TS26.131. The bandwidth is illustrated in Figure 8. The CS connectionscan use either AMR-NB or AMR-WB, while VoLTE in practice always usesAMR-WB. The AMR-WB data rate for CS connection ranges from 6.6kbps to 12.65 kbps, while the VoLTE connection can use data rates up

to 23.85 kbps, enhancing the quality of the connection compared toHD voice in CS networks.


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

1 2 3 4 5 6 7 kHz

AMR - NB

80 - 3700 Hz

0.10.05

AMR - WB

50 - 7000 Hz

The average user opinion of a systems speech quality can bepresented with MOS. Latency also has an inuence on the perceivedquality. Traditionally, in network quality testing, absolute categoryrating (ACR), listening-only tests, dened in the ITU-T P.800Recommendation, have been used to collect subjective responses ofthe performance of the telephone transmission system by listeningto spoken material and by giving the judgments on a ve-point scale,ranging from 1 (poor) to 5 (excellent). The averaged results have beenpresented as a mean opinion score (MOS).

In addition to subjective opinion, the abbreviation MOS is generallyused for scores that originate from objective models. Algorithms

have been developed for objective assessment of speech qualityin telecommunication applications, estimating subjective listeningtests performed in accordance with the ACR method. Objectivemethods are often more convenient than subjective methods. Thelatest standardized algorithm, Perceptual Objective Listening QualityAssessment (POLQA, or ITU-T P.863) has two operational modes: anarrowband (NB) mode and a super wideband (SWB) mode. The SWBmode can handle up to 14 kHz audio frequencies.


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Figure 9 presents the average MOS for dierent voice applications.

VoLTE with AMR-WB 23.65 kbps provided a MOS of 3.9.The referenceCS narrowband call provided a score of 2.9 in the POLQA SWB scalein good radio conditions, while the non-native SIP client with NB EFRcodec scored only slightly lower with 2.7. The same SIP client with anAMR-WB codec conguration provided a score of 3.0. The score ofthis and other third party SIP clients could be increased to 3.4 or 3.6by tweaking some optional functionality such as deactivating voiceactivity detection. However, this caused an increase in the powerconsumption and throughput requirements as the application wouldtransmit a constant data stream regardless of whether the speakerwas talking or silent. The OTT VoIP applications scored between 4.1and 4.2 in the POLQA SWB scale using proprietary codecs, quite closeto the native VoLTE client.

Future VoLTE voice quality can be further improved with the new superwideband (SWB) and full band (FB) codecs, which will able to cover allthe voice and audio bandwidths. 3GPP has dened 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 voicequality of all OTT clients.


3G CS

AMR-NB

SIP EFR SIP

AMR-WB

OTT AP1 OTT AP2 OTT AP3 VoLTE

4.0

3.5

3.0

4.5

2.5

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1.5

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MOS

Fig. 9. Voice quality with dierent voice applications.


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S

Effective number of non-GBR data users

OTT VoIP fails

to work

OTT VoIP-Good OTT VoIP-Bad VoLTE-Good VoLTE-Bad

Next, we present voice quality measurements for VoLTE and for one

OTT VoIP as a function of network loading. VoLTE uses QoS ClassIdentier (QCI) 1, while OTT VoIP is carried as non-guaranteed datatrac. A LTE base station is able to identify VoLTE calls and prioritizeVoLTE in the packet scheduler. Figure 10 shows MOS measurementsas a function of cell loading. Two cases are studied: good radioconditions with the signal level better than -80 dBm and bad radioconditions with the signal level at -110 dBm. The voice quality forVoLTE remains stable regardless of the loading, while the quality ofOTT VoIP decreases as a function of loading. The OTT VoIP call alsofrequently drops during high loading. There is some dierence in thevoice quality between good and bad radio conditions but the VoLTEquality remains stable, also under bad radio conditions.


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


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Mouth-to-ear delay as a function of loading is shown in Figure 11.

The delay for VoLTE remains constant for all dierent load cases,demonstrating the benet of QoS and Nokia Smart Scheduler inthe base station. The delay of OTT VoIP increases as a function ofsimultaneous data loading in the cell.


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OTT VoIP fails

to work

OTT-Good OTT-Bad VoLTE-Good VoLTE-Bad

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


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The call setup time is an important factor aecting the quality perceived

by the end user. Figure 12 shows how VoLTE can signicantly improvethe call establishment time compared to the legacy CS systems. Thetotal time for call establishment depends on many factors such aswhether the handsets were initially in RRC Idle or RRC Connectedstate when the call was originated and the latency experienced by thesignaling network. The laboratory measurements show a VoLTE callsetup time of 0.9 2.2 s, while the delay in the eld is slightly higherdepending on the operator network and transport architecture. Thecorresponding typical CS call setup time is four seconds and with CSFallback (CSFB) at both ends, approximately six seconds.


idle - idle(eld)

idle - idle connected -connected

3G idle -idle

CSFB - CSFB

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Call setup time

VoLTE CS voice

Fig. 12. Call setup time measurements.


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To minimize the call setup time, an operator can apply a special paging

strategy for incoming VoLTE calls. VoLTE paging logic is executed in aMobility Management Entity (MME), based on the Evolved Packet Core(EPC) bearer information from the Serving Gateway in the DownlinkData Notication (DDN) message. The IMS network may assist thisprocedure by marking packets related to the incoming voice calls witha special Dierentiated Services Code Point (DSCP) value.

Figure 13 shows an example of paging response delay distribution. Amore aggressive timer value for paging a re-try timer could be appliedfor VoLTE to minimize the paging response delay, for example twoseconds instead of a typical value of several seconds. In addition, MMEcould be congured to execute VoLTE paging immediately for the

whole Tracking Area instead of rst paging the last known cell.


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


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5. Handset Power ConsumptionOptimization

Device battery life is one of the most relevant factors for smartphoneusers. It can be extended by using advanced features in the radionetwork and by optimized handset design. The two main factorsaecting the mobile handsets power consumption are considered tobe availability of radio optimization features such as DiscontinuousReception (DRX) and the handsets implementation of the voice client.

The power optimization solutions are shown in Figure 14.


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.


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The voice application can run in the handset application processor,

which however needs a lot of power. Power consumption can beminimized by integrating a VoLTE client to the modem processor.The idea of DRX is to use sleep mode in the handset between packetreceptions. The VoLTE voice packets arrive every 20 ms while thetransmission time in LTE is just 1 ms. Such bursty transmission andreception allows the use of power saving sleep mode even betweenvoice packets. The DRX activation and parameter congurations arecontrolled by the base station. The DRX can be congured in sucha way that two voice packets are transmitted simultaneously, whichincreases the packet arrival period to 40 ms. Figure 15 illustrates theinstantaneous handset power consumption during a VoLTE call whenthe person is listening and not talking. The short power peaks happenmostly every 40 ms during the reception of the voice packet, while thepower consumption is considerably lower between the packets.


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Fig. 15. Discontinuous reception (DRX) during VoLTE call.

The measured current consumption of dierent voice applicationsis shown in Figure 16. VoLTE power consumption can be reduced by80% with DRX. 121 mA has been measured on a handset, which iseven slightly below the 125 mA measured on the same codec on 3GCS using the same handset. The power consumption of the OTT VoIPapplication is substantially higher at 248 mA.


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3G CSAMR-NB

3G CSAMR-WB

noDRX DRX on with different settings OTT VoIP

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

6. SummaryVoLTE is being deployed globally, supported by new VoLTE-capablehandsets. VoLTE can enhance an operators network eciency andimprove end-user performance in areas such as voice quality andsetup times. This paper illustrates that optimization of radio featuresand parameters is needed to provide reliable VoLTE connections withhigh success rates and low drop rates. The live network results showthat excellent key performance indicators can be obtained for VoLTE inoptimized networks.

The measurements also show that VoLTE oers a number of benetscompared to OTT VoIP applications, including robust performance andhigher 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

Nokia VoWiFi white paper
http://networks.nokia.com/sites/default/files/document/nokia_volte_white_paper.pdfhttp://networks.nokia.com/sites/default/files/document/nokia_vowifi_white_paper.pdfhttp://networks.nokia.com/sites/default/files/document/nokia_vowifi_white_paper.pdfhttp://networks.nokia.com/sites/default/files/document/nokia_volte_white_paper.pdf


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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.

NokiaNokia Solutions and Networks OyP.O. Box 1FI-02022Finland

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

Product code C401-011925-WP-201509-1-EN

Nokia Solutions and Networks 2015

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