Voice Service on LTE

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Prepared by: Date: Document :Dr. Irina Cotanis, Anders Hedlund October 2012 NT12-13122., Rev. 1.0

© Ascom (2012) All rights reserved. TEMS is a trademark of Ascom. All other trademarks are the property of their respective holders.

Voice Service Over LTE Networks(VoLTE) and the Implications forTesting

An Ascom Network Testing White Paper

By Dr. Irina Cotanis and Anders Hedlund



© Ascom (2012) Document :NT12-13122., Rev. 1.0 2(28)

Contents

1 Voice as One of the Many LTE Services .................. 4

2 An In-Depth Look Into LTE Voice ServiceSolutions ..................................................................... 5

2.1 Circuit Switched Fallback (CSFB) Solution ............................5

2.2 Voice over IMS (VoLTE) Solution.............................................6

2.3 Over the Top (OTT) Voice Solution ..........................................8

3 What Is LTE’s Signature on Voice ServiceQoE? ........................................................................... 9

3.1 Traditional Sources Impacting Voice QoE ............................ 10

3.1.1 Coding and Error Concealment ................................................. 10 3.1.2 Voice Enhancement Devices (VED).......................................... 10 3.1.3 Devices (phones) and Clients ................................................... 11 3.1.4 Network-Centric Factors ........................................................... 11

3.2 Subscriber Experience View on VoIP vs. PSTN VoiceQuality ..................................................................................... 12

4 What Does It Take to Ensure High VoLTEQoE? ......................................................................... 12

4.1 Device Implemented Jitter-Buffer and Time Scaling ............ 13

4.2 Network-Centric View ............................................................. 13

4.2.1 Protocol Optimization of the Radio Network .............................. 15 4.2.2 Physical Layer Optimization ...................................................... 16

4.3 Customer Experience Perspective and the Significanceof On-Device Testing .............................................................. 17

4.3.1 QoE Evaluation Metrics ............................................................ 18 4.3.2 On-Device Testing .................................................................... 18

5 LTE Voice Quality Testing Scenarios .................... 19

5.1 VoLTE Troubleshooting ......................................................... 19

5.1.1 Radio Interface Aspects ............................................................ 20 5.1.2 Beyond the Radio Interface ...................................................... 24

5.2 Circuit Switched Fallback Testing Scenario Results ........... 25




6 Conclusions ............................................................. 27

7 References ................................................................ 28






This paper discusses the different LTE voice service solutions as well asaspects of the key performance evaluation metrics that must be consideredwhen implementing them. It takes an in-depth look into the challenges that

accompany the delivery of high quality of experience (QoE) LTE voiceservices, as well as what is required to cope with these challenges. Thepaper concludes with several examples of LTE voice servicetroubleshooting that can help carriers efficiently provide voice service atexigent QoE levels, consequently easing the all-IP migration for the voiceservice that still accounts for more than 70% of their revenue.

2 An In-Depth Look Into LTE Voice ServiceSolutions

For several years, the 3GPP and other wireless industry forums [4], [5], [6]evaluated various voice service solutions that could optimally meet therequirements imposed by the integration of voice within a data-orientednetwork, such as LTE. Two 3GPP standardized solutions proved to befeasible: Circuit Switch Fallback (CSFB) [7] and Voice over InternetProtocol (VoIP) over IMS (or One Voice or Voice over LTE – VoLTE) [8].CSFB is seen as an interim and transitional solution until IMS technology isfully deployed for wireless capabilities so it can then reliably offer completemobile voice support.

In addition, the deployment of packet switched (PS) voice (VoIP), and theevolution of smartphones and broadband services, made it possible forthird-party Over the Top (OTT) voice solutions (e.g., Skype and Viber) to beoffered wirelessly over LTE, as well as 3G.

2.1 Circuit Switched Fallback (CSFB) Solution

To support voice service similar to 2G and 3G networks, LTE voice serviceneeds to enable mobile-originated and mobile-terminated voice and videotelephony calls. Therefore, mobile devices with an integrated telephonyclient that camps on to the LTE radio access network can either originate orterminate calls by performing a ‘fallback’ to the legacy 2G/3G network. Thissolution is known as Circuit Switched Fallback (CSFB). The user equipment(UE) will not even camp on to LTE unless the core network provides asuitable voice capable service such as CSFB. Therefore, the speech pathused in an actual established call is made via legacy radio accesstechnology rather than via LTE. Once a CSFB call has been completed, the

UE moves back to LTE coverage, if available, or continues camping on tothe 2G/3G cell. This high-level network architecture is presented in Figure1.

3GPP Release 8 of the LTE standard already specifies means to fall backto a circuit switched voice service in GSM or WCDMA or CDMA network, ifavailable in the same coverage area. The specification also allows for SMSto be carried along with voice. To achieve the fallback, the CSFBfunctionality requires the availability of the SG interface, between theMobility Management Entity (MME) and the Mobile Switch Centre (MSC)server, to enable it to provide circuit-switched paging to the LTE side, as

well as combined Evolved Packet Core (EPC) and international mobile




subscriber identity (IMSI) attach and detach. These functionalities involveupdates of network elements such as the MSC as well as the Gs (MSC – Serving GPRS Support Node, or SGSN) interface. Besides these required

network changes, the terminals need to support the CSFB solution.

Figure 1

By design, the CSFB solution does not allow LTE functionality during voicecalls and generates interruption of an ongoing data connection. In addition,it has weak support for multilayer networks (e.g., femto cells). The CSFBsolution does have minimal flexibility to be integrated with broadband voiceand multimedia services (e.g., presence, instant messaging, contentsharing) defined by the GSMA in the Rich Communication Suite Enhanced(RCSe) [9] for LTE offerings.

As one would expect, because of the extensive signaling required to set upthe call, the CSFB solution comes with longer call setup times that couldsignificantly degrade the user experience. The call setup time alsoincreases with a change to another network. Estimated values show anincrease of 1.5 seconds in call setup time, regardless of call origination.Some results of a live CSFB scenario are presented in section 5.2.

Therefore, the evaluation of the CSFB solution’s performance requirestesting related to registration (e.g., MME translation of the Tracking AreaIndicator [TAI] of the LTE domain to the MSC Local Area Indicator [LAI] ofthe 2G/3G domain) as well as the call setup; the latter potentially having asignificant negative impact on the QoE of the voice service. In addition,evaluation of how much an incoming CSFB call impacts an ongoing userdata session is important in understanding the overall QoE.

2.2 Voice over IMS (VoLTE) Solution

The VoLTE solution initiated by the GSMA [8] is based on IMS technologyas defined by the 3GPP. The high-level architecture is presented in Figure2. LTE radio access does not support direct connectivity to the circuit-switched core network and services, but rather radio is connected to an

Evolved Packet Core (EPC) that provides IP connectivity for the end user

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services and interworking toward existing circuit-switched networks. Theconnectivity is achieved using the IMS-based new infrastructure: theTelephony Application Server (TAS), which is designed to ensure seamless

service migration by using the MSC as the direct platform for TAS.

Figure 2

VoLTE also defines a set of new interfaces (e.g., between the user'sequipment and the operator ’s network, the Home and Visited Networkduring roaming, and the networks of the two parties making a call).

On the network side, besides the new infrastructure and interfaces, VoLTEstandardization needs to address a series of functionalities required by theintegration within the LTE and the 2G/3G legacy networks. The subscribers’ requirement for a seamless, anytime and anywhere call makes mobility andhandover to a non-LTE radio access technology (RAT, e.g., GSM, CDMA,WCDMA) one of the most important functionalities. This is achieved by

using the Single Radio Voice Call Continuity (SRVCC) [10] function. Otherfunctionalities address optimal routing of bearers for voice calls whencustomers are roaming, commercial frameworks and provisioningcapabilities for roaming and interconnect, as well as security and fraudthreat audit to prevent hacking and unauthorized entry into any area withinthe network.

On the terminal side, the phone needs to have VoIP client software loadedto provide the VoLTE functionality, which can be implemented at theapplication layer of the phone’s protocol stack, in the form of an app. In thiscase, the client’s features – such as time scaling of the voice signal whichregards the jitter-buffer handling – can be controlled and tuned for improved

voice service quality by the phone vendors. The VoLTE functionality alsocan be embedded in the phone’s chipset, in which case the modem-basedclient’s features are set by the chipset vendor (allowing less flexibility forcontrol and tuning). Details on the importance of the time-scaling feature onthe QoE of the voice service are presented in section 3.1.3.

Adopting the IMS-based specifications allows the VoLTE solution to beintegrated with the suite of applications that will become available on LTEthrough the IMS core. A variety of services can run seamlessly, rather thanhaving several disparate applications operating concurrently. The GSMAdefined the multimedia communication suite (RCS) to run both over LTEand other networks such as 2G/3G. It covers multimedia services in threeareas: rich address book, rich messaging, and rich call. As part of the richcall, RCS includes the voice service, regardless of whether it is realized on

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circuit-switched or packet-switched networks. When using packet-switchedvoice, RCS-e (RSC enhanced) aligns with the packet-switched voiceprotocols as described in the GSMA recommendation [8], making VoLTE

the base for packet-switched voice in RCS. In this way, RCS-e and VoLTE jointly provide a comprehensive set of communication services for the LTEenvironment, from basic voice to a full set of rich multimedia services [22].

At a high level, the implementation of the VoLTE solution as well as itsfunctionality might appear straightforward. However, there are manynetwork- and terminal-related issues that are expected to impact VoLTEQoE, especially the vagaries of the radio access network where time delaysand propagation anomalies add considerably to the complexity of deliveringhigh-quality voice services.

Radio access aspects of VoLTE are mainly related to the Radio LinkControl (RLC) mode functionality (acknowledge (ACK) or un-acknowledge

(U-ACK) mode) as well as the handover performance, especially for theSRVCC operation. The key role for achieving a high VoLTE QoE is playedby the optimization of the VoIP protocol stack configuration, such as ascheduling scheme (e.g., semi-persistent scheduling), VoIP bearer QualityClass Indicator (QCI, e.g., QCI =1) as well as the usage of TransmissionTime Interval (TTI) bundling ensuring a more continuous transmission and,therefore, shorter end-to-end delay. Non-RF-related aspects of VoLTEpertain to the terminals, such as client implementation and jitter-bufferhandling, as well as issues related to the voice enhancement devices(VEDs), such as echo and gain control (see section 3.1). All of theserepresent potentially serious impediments to delivering and maintaininggood VoLTE QoE.

Details regarding these topics are presented in section 4.1, and ademonstration case for troubleshooting problems related to these aspectsis in section 5.1. Section 3.1 describes in more detail the impact of terminaland network performance on VoLTE service QoE.

2.3 Over the Top (OTT) Voice Solution

Similar to VoLTE, the third-party OTT solution relies on new infrastructure(Telephony Application Server) and, therefore, comes with a series ofattractive advantages. These include being free from MSC legacy continuityand IMS complexity, as well as being technically viable on LTE and

WCDMA networks. Integration with presence, support for nontraditionalvoice apps, and the availability of app stores ensuring an easy userinstallation, represent very attractive features which rival the ones offeredby RCS-e.

However, the OTT solution comes with important technical challenges thatcan significantly impact the subscriber’s voice QoE.

First, unlike VoLTE, the OTT solution does not benefit from a voicededicated bearer and VoIP optimized protocol stack. Therefore, it is

expected that OTT voice will be delivered on a non-Guaranteed Bit Rate




(GBR) bearer with something like a. Quality Class Indicator (QCI) =7 [11]. Itwill more likely be a dynamic scheduler scheme that ensures optimal radioresources for each transmission depending on the radio conditions and

load instead of an optimized VoIP scheduler (e.g., semi-persistent) anddedicated bearer (e.g. QCI=1 [11]). Third-party voice service providers haveno control over these QoS aspects in the wireless network, and thus theycannot ensure a good QoE under all load situations. This issue couldpossibly be resolved by installing logic in the network that would ensureQoS for data streams that are recognized as belonging to an external voiceservice to which the user is subscribed. However, standardized work isneeded on this topic.

Second, third-party OTT calls cannot be handed over to a circuit-switched2G/3G network when a user leaves the LTE coverage area since the

external applications cannot easily be tied into the wireless networkinfrastructure.

Therefore, testing OTT solution performance requires a careful analysis ofthe aforementioned QoS aspects that could possibly generate poor or evenunacceptable QoE. In addition, the understanding of the OTT voice QoErequires evaluation of the behavior and performance of OTT clients thatembed proprietary error concealment schemes and adaptive bufferingtechniques on the subscriber’s device.

3 What Is LTE’s Signature on Voice Service QoE?

Traditionally, mobile voice service QoE is known to be impacted by theperformance of codecs, voice enhancement devices (VEDs) that are bothnetwork- and terminal-implemented, user device characteristics, and last – but not least – the network.

During the past decade, the following trends caused the rapid technologyevolution from 2G to today’s 4G/LTE networks:

Maintaining and improving wireless voice service QoE

Increasing voice capacity

Minimizing CAPEX/OPEX costs

Adding many more wireless services, e.g., data, Web, video

As expected, technical complexity along with the constraints andrequirements that come with the 4G wireless ecosystem bring with themmore challenges for delivering, assuring and maintaining voice serviceQoE, both for circuit-switched and packet-switched networks. Sophisticatedspeech processing techniques implemented in new codecs, voiceenhancement devices and terminals have been designed to cope withcomplex network conditions (e.g., radio conditions, traffic load). However,the complexity of the signal processing made codecs and VEDs morevulnerable to performance artifacts, and therefore made them moresusceptible to significant degradation of voice service QoE.




Therefore, understanding the evolution of the speech technology within the4G/LTE network environment is very important for the evaluation andcontrol of the voice service’s quality. Also, understanding the interaction

between different components impacting the voice QoE is significant fordetecting and troubleshooting their individual contribution on the overallperceived quality. This section discusses aspects related to the speechtechnology and to the network evolution that impact the voice QoE.

3.1 Traditional Sources Impacting Voice QoE

3.1.1 Coding and Error Concealment

Coding and error concealment techniques are moving quickly fromnarrowband (NB) voice to high-definition (HD) quality, wideband (WB) andeven super WB (SWB) for some OTT voice application such as Skype and

Google Talk. New codecs supporting these bandwidths and high bit rates,whether standardized (like Adaptive Multi-Rate-Wide Band+, EnhancedVariable Rate Codec, Evolved Voice Service, Advance Auto Coding) orproprietary like SILK (Skype), need to ensure a broad range ofcompression levels while ensuring high speech quality in error-freeconditions. However, a high level of compression removes almost all of theredundancy in the speech signal, which in turn leaves voice qualitysensitive to transmissions errors. Although complex error concealmentschemes are implemented in these codecs to reconstruct the signal at thereceiving side, they are prone to errors especially in high transmission errorenvironments such as the OFDM radio environment of LTE radio accesstechnology. In these cases, the reconstructed speech frames can exhibit anartificial, robotic sound.

3.1.2 Voice Enhancement Devices (VED)

Voice enhancement devices, or VEDs [12], [13] such as noise reduction(NR), automatic gain control (AGC), and echo cancellation (EC), aredesigned to maintain, and even increase, the voice quality in conditionsprone to noise, level variation, or echo. However, they come with their ownunique set of challenges that impact QoE. Noise reduction techniques thatdo not properly balance between noise and speech periods, or thatcompletely remove the natural background noise, can drastically degrade

voice quality. Similarly, AGCs that are too aggressive or too slow candeteriorate the speech signal, resulting in a further perception ofdegradation. An LTE voice service that offers superior speech bandwidths,such as wideband and super wideband, is more sensitive to noise reductionand AGC design. This is a result of the fact that speech degradations withinlarger bandwidths are more acutely perceived than those in narrowbandscenarios. Acoustic and hybrid echo cancellers are designed forattenuations up to -45dB and for delays longer than 200ms, values well-known to be perceived as annoying by subscribers. The high non-linearityand time variance of the LTE radio environment challenges the echoestimation and, thereby, can result in an annoying echo either not beingremoved or compensated. In addition, specific scenarios of VoIP could




exhibit a larger range of delays and/or attenuations than the ones the echocanceller was designed to intervene with and compensate for.

3.1.3 Devices (phones) and Clients

Commonly used phones impact the voice quality due to time-variant lineardistortions, such as spectral shaping, and/ or non-linear distortions likemicrophone and transducer interfaces and reverberations caused byhands-free set-ups at acoustical interfaces. Today’s smartphones designedfor HD voice, and with technologically advanced acoustical interfaces, areexpected to have less impact on voice quality. However, LTE requirementsfor high bandwidth efficiency (to support a multitude of data and multimediaservices while coexisting with voice delivered on PS – IMS support) drovethe necessity of adaptive buffering schemes. These buffering schemes canuse various time-scaling or speech-frequency re-sampling algorithms to

cope with challenging network behaviours affected by packet loss anddelays such as inter-RAT handovers and IP congestions. Time scaling canbe either stretching (under sampling) or compressing (over sampling) thespeech signal, depending on the rate with which it comes from the bufferand/or if the buffer is over-run or empty [19].

There are two main categories of time-scaling algorithms: with speech-pitch preservation or without ([14]); each exhibiting different trade-offsbetween performance and speech processing complexity. The impact ofthe algorithm’s performance on the overall speech quality is determined bythe distribution of the time scaling and its frequency of occurrence withinthe speech sample, as well as its length. All these characteristics are givenby the network behavior (e.g., packet loss, variable delay), which requiresdifferent levels and distributions of algorithmic error compensation.

The time-scaling algorithms are not standardized, leaving open thepossibility of various performances and, therefore, of different QoE trends.The algorithms are implemented in the VoIP client that supports the voiceservice in the network. The clients can be either software clients asapplications on the device, or modem-based implemented in the phone’schip. Therefore, the clients’ performance of coping with network behaviorsis client- and chipset vendor-dependent and phone-dependent.

3.1.4 Network-Centric Factors

These factors affecting the LTE voice service quality emerge from variousnetwork’s radio frequency (RF) and non-RF characteristics described inmore detail in section 4. Those that have immediate impact generateinterruptions or loss and delay. Not cancelled packet loss could generateperceived interruptions, whether caused by reasons such as non-ACK RLCmode of the VoIP dedicated bearer or IP congestion, and uncompensatedhandover (inter-RAT). Delay – especially when it is variable length andrandomly distributed during speech, rather than at the beginning of speech – is very annoying to subscribers. These scenarios could be caused byuncompensated handover delays (LTE intra-RAT) or uncompensated RLCretransmissions in scenarios for which the VoIP bearer is deployed using ACK RLC mode. VoLTE might also show the delays and interruptions dueto the IMS technology solutions that cope with mobility and unreliable radio




conditions, all still under standardization evolution. An important role is alsoplayed by Discontinuous Transmission (DTX), which provides reduced RFinterference and, therefore, satisfies bandwidth efficiency requirements and

capacity constraints more drastically needed in LTE. Network DTX inconjunction with Voice Activity Detection codec-based schemes could betoo aggressive and cause significant speech front-end time clipping,strongly impacting the perceived voice quality.

3.2 Subscriber Experience View on VoIP vs. PSTN VoiceQuality

LTE’s signature on voice service quality also comes with the humanperception dimension; that is, subscribers comparing the experience of theVoIP quality and additionally VoLTE (VoIP over IMS) quality against thecircuit switched (CS) voice quality they have experienced for the past 20

years.

PSTN/CS voice service with a dedicated 56k (or 64k) time slice allocatedfor each channel/circuit is governed by highly optimized 2G/2.5G/3Gnetworks. In addition, CS voice service benefits from well established andstandardized codecs with highly efficient error concealment and rateadaptation techniques encoding both NB and WB, as well as enhancedspeech processing procedures. All these factors contribute to raise voiceservice quality to levels that are known to satisfy subscribers, and this iscategorized as providing a mean opinion score (MOS) of 4.2MOS to4.4MOS for the entire call’s length.

VoIP service is supported by packet switching that was not originally

designed for real-time sessions, such as voice and video traffic, and/ormobility. VoIP requires the call to go through various transformations, suchas encoding/decoding at low and adaptive bit rates, changes in routingduring the call, packets out of sequence or lost, delays, and buffering/jitterdelays. To compensate for all of these challenges while sustaining thecustomer experience, new protocols such as Real-time Transport Protocol(RTP), Real-Time Transport Control Protocol (RTCP), and SessionInitiation Protocol (SIP), as well as new QoS strategies and policies such asMultiprotocol Label Switching (MPLS) have been developed. In addition,new codecs, with a large variety of rates and even variable rates, andmultiple bandwidths, from narrowband to super wideband with complexerror concealment techniques, create the foundation for a high quality voiceservice, if provided at pre-established service level agreements (SLAs).

Therefore, given a dedicated bandwidth, minimum delay, HD voice andefficient QoS policies such as MPLS, it is expected that the quality of VoIPservice will soon meet, and actually surpass, the voice service QoE thatsubscribers are already used to and expect.

4 What Does It Take to Ensure High VoLTE QoE?

In the previous section are discussed aspects related to the speechtechnology and to the network evolution that impact the voice QoE. Thissection discusses the procedures required for ensuring a high QoE as wellas what, and how, measurements, evaluation and troubleshooting need to




be performed in order to achieve an accurate subscriber perception of theVoLTE service.

4.1 Device Implemented Jitter-Buffer and Time Scaling

Real-time services for packet-switched networks have to have a bufferhandling variations in the delays in the bit-pipe. Delays are introduced byscheduling, cross traffic, retransmissions to cope with errors, and handoveramong others. Typically these buffers (referred to as jitter-buffers) contain80-100ms of buffered end-user data such as speech. Large delays causedata to be consumed from the buffer at a faster pace than new data isadded. In this case the jitter-buffer could run empty and cause speechdegradation if not compensated for. However, in addition to this scenario,the VoLTE service could experience cases in which speech comes athigher bit rates than the play out rate mainly due to the traffic optimization

techniques implemented in the scheduler. Therefore, the jitter-buffer mayoverflow, if it is not compensated for. As mentioned in section 3.1, theempty or over-flown buffer is compensated for by using time scaling. Timescaling of 40-80ms using different techniques is not perceived asnoticeable degradation by the end user.

The jitter-buffer is very tightly integrated within the real-time applicationsuch as the VoLTE client. Feeding the jitter-buffer with data at the samepace as it’s consumed is the main task for the lower layers of the radioaccess network. Therefore, the performance of these network layers willsignificantly impact VoLTE QoE. Too many packets lost or received out ofsequence, or long delays, can generate error levels and patterns which the

jitter-buffer’s time scaling scheme can either not cope with or plays backthe speech signal with annoyingly perceived stretch and compressionlevels.

4.2 Network-Centric View

End-to-end VoLTE services involve several protocol layers, as shown inFigure 3, for a mobile-to-mobile setup. The protocol stack (Figure 3) can bedivided into two parts, with the lowest three layers belonging to the radioaccess network (RAN), and the higher layers travelling though the corenetwork to the other calling party.




Figure 3

Figure 4

Therefore, achieving the best possible VoLTE QoE requires theoptimization of the VoLTE protocol stack. Details are presented in sections4.2.1, 4.2.2 and 4.2.3. The process (Figure 4) is complex due to the factthat the VoLTE protocol stack’s configuration needs to be performed in acompletely different way than typical non-real-time service usage in order to

support VoLTE in an appropriate manner. Several network nodes and thesubscriber device (or user equipment – aka UE) need to be involved. Inaddition, per-service type configuration and optimization of the VoLTEprotocol stack is required. Otherwise, it is likely that the voice will travelthrough a bit-pipe configured for a completely different type of service,resulting in poor end-user-perceived quality, and non-optimal utilization ofthe network investments resulting in a waste of precious frequencyspectrum.

In addition, the voice service delivered on devices travelling through thenetwork and experiencing constant serving cell changes require handoversthat should be less frequent and when taking place successfully performed

in the shortest possible time. This is because handovers cause delays




impacting the quality of voice service. To achieve these kinds of handovers,radio optimization needs to be performed differently than other serviceswhich are less sensitive to long delays than real-time voice services.

Handovers to 2G/3G networks also require smooth SRVCC functionoperability.

4.2.1 Protocol Optimization of the Radio Network

4.2.1.1 Radio Link Control (RLC)

The highest layer in the RAN part of the stack, the RLC layer, can be set upin two different modes for voice; acknowledged (ACK) and un-acknowledged (U-ACK). The third existing mode is transparent and it isused for signaling broadcast-like system information.

The RLC acknowledge mode ensures an error-free radio interface since theerroneous blocks from the Medium Access Control layer are retransmittedby the RLC layer. However, the price paid for an error-free air interface canresult in delays caused by the retransmissions.

In LTE, the MAC layer handles retransmissions with a very short delay(within 10ms in most cases). In this case, it is better to leave the eventualremaining block errors (residual errors from the MAC layer) to be handledby the error concealment mechanism in the UE-based VoIP client, ratherthan introducing additional delays in the RLC layer.

The delay values larger than the jitter-buffer in the voice client will leave theclient with no speech to decode resulting in degraded speech quality. The

jitter-buffer consists of 20ms speech frames delivered from the RTP layer.The jitter-buffer is typically around 80 to 100ms and could be dynamicallyadjusted to the level of variations measured on the received packets fromthe RTP layer.

Therefore, for a real-time, delay-sensitive service such as VoLTE, the RLClayer should be set up in unacknowledged mode. A sequence numberingmechanism is generally used to ease the RLC packet handling and ensurean efficient, more reliable unacknowledged mode.

4.2.1.2 The MAC Layer

In the MAC layer, a set of parameters can be optimized and severalfeatures can be used to improve the performance of a VoLTE service. Themost important one relates to how the radio resources are optimally sharedamong all the end users in a cell. This function is accomplished by thescheduler in various ways.

4.2.1.3 Scheduling

A VoLTE service can be scheduled with higher priority than other non-real-time services and prioritization between different services is possible if thescheduler knows which type of services each user runs. This is referred toas a “QoS-aware scheduler,” which is a feature of the scheduler, rather

than a configuration of the MAC protocol itself.




4.2.1.4 Semi-Persistent Scheduling

The MAC layer allows for the possibility of persistent scheduling. However,the VoLTE service follows a known pattern (typically 20ms blocks with a

limited size) and each radio block does not have to be assigned uniquely.Therefore, it is possible to reserve and dedicate a part of the resources to aparticular end-user service. Called a Semi-Persistent Scheduling (SPS)configuration, it reduces the signaling overhead significantly and therebythe load on the Physical Dedicated Control Channel (PDCCH).

4.2.1.5 DRX Configuration

In addition, the VoLTE-specific pattern allows configuring a discontinuousreception scheme for the UE in order to save battery life; the UE can be putin sleep mode when there is nothing to receive.

4.2.1.6 MAC Retransmissions

Whenever RLC runs in U-ACK mode, it is likely that more than the networkdefault setting of four retransmissions is needed. The LTE MAC canconfigure the Hybrid Automatic Repeat reQuest (HARQ) retransmissionhandling mechanism so that it performs better for voice services.

4.2.2 Physical Layer Optimization

4.2.2.1 Overhead

Keeping as small an overhead as possible, and consequently maximizingtransport block size, is critical for the LTE physical layer which needs toensure low latency. This is performed based on the physical layeradaptation to the radio quality, which is itself based on the channel stateinformation (CSI) feedback from the UE and the eNodeB measurements onUE reference signals. However, this is not optimal for low bit-rate servicessuch as voice.

As an example, let’s assume a scenario in which the UE reports the highestpossible Channel Quality Indicator (CQI=15), and that a 20MHz channelwith 100 resource blocks per TTI (1ms) is available at the LTE cell to whichthe UE connects. In a 100 Radio Blocks (RB) configuration, there is alwaysa minimum of 4 RBs allocated to a communication link. The typical situationis now for the eNodeB to select 64QAM modulation and a very largetransport block size – see Table 1, [20]. A new block with speech every20ms means 2984/0.02 = 149kbit/s, which is much higher than the MAClayer payload for the voice service (typically only 20 to 30kbit/s)

Therefore, in the trade-off between overhead and payload governed by theCQI and the LTE cell configuration respectively, the modulation scheme isthe key to optimizing VoLTE service.




Table 1

4.2.2.2 Frequency Hopping

Utilizing frequency diversity by using optimal parts of the spectrum over thetime domain is possible in LTE. In the downlink, the scheduler can use thesub-band CQI feedback from the UE. For the uplink, methods for both inter-TTI and intra-TTI hopping exists, and the sounding signals from the UE canbe used to detect optimal parts of the spectrum for specific UEs.

This also helps in optimizing the VoLTE service.

4.2.2.3 VoLTE Setup for Protocols Above the Radio AccessNetwork

The protocols above the radio layer are controlled mainly by the EPSSession Management and the EPS Evolved Packet System, which sets upa bearer context describing the characteristics of the service qualityrequirements. The requirements of VoLTE service on delay, bandwidth,and priority make the use of the QCI configuration 1 suitable for theappropriate performance to be provided [7].

The Packet Data Convergence Protocol includes functions such asencryption, header compression, and sequence numbering. The headercompression is very important in order to keep the protocol overhead at aminimum level. The Robust Header Compression (RoHC) profile (e.g.,0x0001 or 0x0101) is recommended for VoLTE services [15].

4.3 Customer Experience Perspective and the Significanceof On-Device Testing

The network elements and configurations that ensure an optimal VoLTEservice delivery are critical for understanding why VoLTE servicedegradation happens. Equally important is accurately evaluating if the voiceservice quality degradation happened at a statistically significant rate,when, and how much it affected the customer experience. Cost-efficientlyanswering these questions requires the most accurate QoE assessmentmetric, such as the latest ITU-T P.863 standard (POLQA algorithm) as wellas a statistically significant processing of the metric’s output. In addition,the test design and setup for the quality evaluation needs to closelyemulate the real subscriber scenario.




4.3.1 QoE Evaluation Metrics

POLQA [16], an algorithm based on human perception and cognition of thevoice quality, is specifically designed to handle disruptive effects caused by

multicomponent distortions which are characteristic of the 4G ecosystem – the convergence of LTE and IMS technologies. The algorithm providesaccurate MOS estimation on standard and high-definition voice quality thatwill be challenged by LTE technology’s implementations. These includenew codecs and error concealment techniques, variable delay and timescaling, front-end speech clipping, as well as linear and nonlineardistortions characteristic to various terminals; all described in section 3.1above.

Central to understanding and cost-efficiently controlling voice servicequality is the ability to correlate voice quality with network-centric metrics.While the network dimension can be defined as described in section 4.1,

the speech-centric dimension is exposed by going beyond the MOS scoreusing QoE algorithms like POLQA. In addition to the MOS estimate, theITU-T P.863 standard provides a set of metrics to be used for diagnosingspeech quality degradation and correlating it to device performance (e.g.,codecs, VEDs, error concealment schemes such as time scaling) and, tosome extent, to the behavior of the network (e.g., packet loss, delay).

Details on the POLQA algorithm, its additional speech qualitymeasurements and their importance, and the transition to POLQA from theolder speech quality evaluation technology called PESQ (ITU-T P.862), arepresented in previous Ascom white papers available on the Ascom NetworkTesting website (www.ascom.com/networktesting.[17], [18]

4.3.2 On-Device Testing

An accurate evaluation of voice service quality requires setting up a testdesign that closely emulates the subscriber experience. It is important,therefore, to design and set up evaluation measurements usingmeasurement software that resides on the actual connected test device,rather than being part of the PC software. In this way the phone-basedVoLTE client ensures that test voice calls will be prioritized in the networkas they would with real-time service, rather than be handled on a “besteffort” basis, which is the case when the testing relies on a PC-basedVoLTE client. Therefore, a device-based VoLTE client is assigned a VoLTE

logical channel with high-priority QoS class (QCI=1). The serviceperformance will not suffer from any limitations inherent in the test setup,and it will therefore reflect the real-world subscriber experience in a waythat cannot be fully achieved with a PC-based VoLTE client. In addition, thedevice-based VoLTE client will be adapted to, and optimized for, thatparticular device, which ensures that no artificial delays are introduced inthe evaluation.

http://www.ascom.com/networktesting







5 LTE Voice Quality Testing Scenarios

5.1 VoLTE Troubleshooting

Troubleshooting such a complex setup as VoLTE requires a tool capable ofmonitoring the complete protocol stack, from the RF level up to the VoLTEapplication. Many different aspects have to be considered, from staticconfigurations of the radio network to details on the radio interface. Inaddition, the ESM (EPC Session Management) configuration, as well as theVoLTE client information, needs to be analyzed. While the radio and thecore part are network-centric, the VoLTE client information is device- andspeech-processing-centric. Figure 5 provides guidelines for finding possibleroot causes of poorly performing VoLTE service. Troubleshooting requiresthe analysis of several parameters specific to the network, and to thedevice/client, as well as the correlation of the two.

Voice service troubleshooting should be performed only if QoE statisticalmetrics (POLQA scores) show consistent and statistically significantbehavior below an acceptable level of QoE performance. [17] [18]

In addition, the distribution of QoE values over time and geographical areashould be considered. Troubleshooting local problems (over time and area)is different than severe problems over large areas, or problems that occurover a longer period of time. Severe problems of that magnitude maydictate another approach with a more thorough investigation of parametersettings of the protocol stack (see section 4.2., Figure 3, above).

The troubleshooting results must provide the perceived VoLTE service

quality values (MOS scores) as well as uncover the reasons forunacceptable QoE performance.

The VoLTE troubleshooting demo case considers the radio access network(e.g., coverage, interference, high load, bad configuration or handover) aswell as a few topics outside the radio path, such as the VoIP client.




Figure 5

5.1.1 Radio Interface Aspects

5.1.1.1 Radio Access Network Related

The channel quality indicator (CQI) measured by UE on the downlinkchannel incorporates many aspects of the signal path and has a lowest-to-highest scale of 0-15. A low value like CQI=6 indicates voice channelproblems that are more likely caused by the radio access network. Theblock error rate on the physical layer can also give an indication of RFproblems resulting in a high number of retransmissions on the MAC layer.

The transmission distribution (Figure 6) of the radio blocks should beverified against target expectations. In an unacknowledged RLC setup,more retransmissions can be accepted than in an acknowledge setup.However, high values of the residual block errors prevent blocks from beingsent to higher layers and, therefore, could cause lower voice QoEperformance. This is particularly true if triggering the error concealmentmechanism in the VoLTE client is not enough to compensate.

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Figure 6

5.1.1.2 Coverage and Interference

Signal strength RSRP (Reference Symbol Received Power) and signal tointerference ratio SINR/CINR (Signal or Carrier Noise to Interference Ratio)are two of the most important RF parameters for identifying coverage andinterference problems. A coverage-plot based in RSRP distribution on themap gives quick information if the problem is coverage-related. Interferencein LTE typically comes from surrounding cells. Therefore, if a low SINR

value is detected, any of the available neighboring cells should be checkedif they do not exhibit signal strength close to the serving cell. In LTE, it isvery important to have distinct cell borders, since all cells operate on thesame frequency. A demo case for the described scenario is presented inFigure 7. Also, in a TD-LTE network, the timing between cells is verysensitive for good performance and can be measured by drive-testing.

If the coverage is good and no neighbors are detected, it is important toinvestigate interference potentially coming from other sources. Usingscanning receiver equipment enables in-depth analysis of the radioenvironment.

It is also important to analyze the uplink and downlink balance by

comparing the downlink path loss with the UE output power. Additionaldetails on the coverage and interference problems are presented in Ascom’s previously published “Handbook on LTE Optimization.” [21]




Figure 7

5.1.1.3 Handover and SRVCC

As already mentioned, excessive handovers generate delays which directlyimpact VoLTE QoE. Measuring handover interruption time is important, buttricky, due to the fact that many layers are buffered on top of one another;thus, correlating the gap in the physical layer or MAC layer to the speechquality degradation is not as obvious. A better approach is to measurewithin the VoLTE client itself to detect the jitter-buffer level and errorconcealment mechanisms setup. A handover-detected problem should bechecked if it is inter- or intra- sites, since the latter are faster and, therefore,have less impact on QoE. In addition, the inter-RAT handovers could besignificantly affected by the SRVCC function which requires the use, anddefinition, of drive testing measurement events to evaluate the function’s

performance.It is also important to verify the RACH (Radom Access Channel)configuration, which needs to show a reasonably low number of preambles,with an appropriate output power transmitted at each handover, in order toensure low handover interruption time.

A demo case showing the voice quality degradation due to handover (HO)interruption is presented in Figure 8.




Figure 8

5.1.1.4 Cell Load

Cell load can have a significant impact on the performance of the VoLTEservice. Three types of measurements should be monitored: RSRQ(Reference Symbol Received Quality), CFI (Channel Format Indicator) andresource block scheduling rate. RSRQ is a measure of the relationshipbetween signal strength (RSRP) measured on the reference symbols andthe total received signal strength received on all symbols (RSSI). It gives anindication of the load in the cell, but can be difficult to use since smallvariations in RSRQ can be caused by large differences in load. The CFI isthe control format indication and provides the number of symbols used forPDCCH (Physical Dedicated Control Channel), which is the control channelfor the downlink. The number of active users in the cell can be monitored

by checking the distribution of the CFI values (0, 1, 2) (Figure 9);distribution of the CFI toward higher values indicates more users. Last, thescheduling can be checked via the PDSCH (Physical Downlink SharedChannel) and PUSCH (Physical Uplink Shared Channel) resource blocksallocation. These values show how many of the total available resourceblocks are assigned to the UE. Even though the typical usage of thosemeasurements is to troubleshoot high-bandwidth services, they can alsoprovide an indication of how many users there are in the cell.




Figure 9

5.1.2 Beyond the Radio Interface

As discussed in previous sections, the analysis beyond the radio network inthe protocol stack relates to various sources that could impact VoLTE QoE;the device and/or client, the voice codec and VEDs, the core network’s configuration, and the type of call (e.g., VoLTE mobile to/from VoLTE ornon-VoLTE mobile, VoLTE mobile to/from CS domain or to/from mobileOTT). Therefore, it is important not only to perform an end-to-end QoEevaluation, but also to be able to identify the main source of the voiceservice degradation. In the section 5.1.1, radio issues are presented.

Section 3.1 discussed the significant role of codecs, VEDs (e.g., speechsignal levels, echoes) and error concealment schemes (such as timescaling implemented in the device/client) for achieving high VoLTE QoE.These aspects can be captured, and their operability detected, using eitherPOLQA MOS and its additional measurements or additional speech signalevaluation, such asecho detection and measurement (see Figure 10).

Round-trip time and one-way delay can be also estimated with POLQA.

Device-related problems should also consider high CPU load and operatingsystem scheduling.

The core network configuration addresses the QCI and RoCH, as well aspossible subscriber subscription limitations (SIM). The QCI values could beused to explain possible QoE differences between VoLTE and OTT calls,since the latter will always be attributed a higher Quality Class Indicator, asdescribed in section 2.3.

It is helpful to analyze voice quality differences between types of calls,especially in the mobile-to-mobile call scenario when the POLQA MOS

QoE values reflect the impact of the combined uplink and downlink. A low




QoE value on the downlink of one of the devices can be the result of ahandover on the uplink of the other device involved in the call.

A demo screenshot of some of these measurements, collected in drive

testing, is presented in Figure 10.

Figure 10

5.2 Circuit Switched Fallback Testing Scenario Results

As already mentioned in section 2.1, call setup time as well as the impacton data sessions running in parallel are significant for Circuit SwitchedFallback (CSFB) QoE troubleshooting.

Drive test results on these measurements are discussed in this section(Figures 11, 12, 13). It can be seen that the call setup time shows anaverage of about 6 seconds, with deviations of about +/- 2 seconds, whichcan produce an annoying experience for the customer. In addition, the datasessions have been interrupted for about 2.5 seconds on average with

minimums of 1 second and maximums reaching 3.5 seconds. However,these values are not likely to annoy users since data sessions areperformed (and perceived) as background activities. The time needed tomove back to LTE once the CSFB call ends, which consistently showsvalues larger than 25 seconds, can be far more troubling. Although thisdoesn’t have a direct impact on customer experience, it shows thedeficiency of continuous use of increased LTE data capacity.




Figure 11

Figure 12

Figure 13

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6 Conclusions

The evolution from 3G to 4G/LTE networks has accelerated the availabilityof myriad services, resulting in an impending “data tsunami.” Marketanalysts show a 78% wireless data traffic increase by 2016. In theemerging all-IP networks, voice will become more and more “one of themany” data services, but today it still accounts for approximately 70% of thewireless operator’s revenue.

The 3GPP and other wireless industry forums defined solutions proved tobe feasible: Circuit Switch Fallback (CSFB) and Voice over InternetProtocol (VoIP) over IMS (VoLTE). CSFB is seen as an interim andtransitional solution until IMS technology is fully deployed for wirelesscapabilities and can reliably offer complete mobile voice support. However,

the deployment of packet switched (PS) voice (VoIP), and the evolution ofsmartphones and of broadband services, made it possible for the third-party Over the Top (OTT) voice solutions (e.g., Skype, Viber) to be offeredwirelessly over LTE, as well as 3G.

Regardless of the solution, delivering voice over LTE, while ensuring andmaintaining the QoE that customers are accustomed to, raises a series oftechnical challenges. These challenges arise from the complex LTEnetwork and its integration with the legacy 3G network, as well as fromcodecs, VEDs and terminals (phones) designed to support the 4G voiceservice. From the network perspective, the way to ensure the best VoLTEQoE possible requires optimizing the VoLTE protocol stack. Therefore,thorough voice service QoE troubleshooting requires comprehensive

analysis of the lower and upper layers of the protocol stack.

Achieving high VoLTE QoE also depends on a variety of other factors, suchas codecs, VEDs (e.g., speech signal levels, echoes) and errorconcealment schemes (e.g., time scaling), implemented in the device/client.These factors can be captured, and their operability can be detected, in astraightforward manner using QoE metric (POLQA) MOS and its additionalmeasurements, as well as additional speech signal evaluation such asecho detection and measurement.

However, cost-efficient VoLTE QoE troubleshooting needs to rely on athorough understanding of the human factor. Therefore, it is important toperform end-to-end, accurate estimations of the subscriber’s perception ofvoice service quality using standardized QoE metrics. Equally important isdesigning and setting up tests that emulate, as closely as possible, howsubscribers will perceive the voice quality service. This can be ensured onlyby using on-device testing on phones – native VoLTE software clients.

Voice service over LTE is at its dawn. Today, various solutions areavailable (e.g., VoLTE, CSFB, OTT), but new perspectives such as VoIPover HSPA (VoHSPA) are very likely to be standardized and deployed.Therefore, we should expect and anticipate that a variety of testing andevaluation scenarios beyond what the paper discusses will soon also beneeded.



7 References

[1]. Cisco VNI Mobile 2012

[2] 3GPP “One Voice; Voice over IMS profile” – V1.0.0, Nov. 2009[3] IETF, RFC 3261, “Session Initiation Protocol, SIP”

[4] 3GPP TS 23.1xx, 2xx, 3xx series

[5] VoLGA, Forum, www.volga-forum.com

[6] GSMA Forum[7] 3GPP TS 23.272 – Circuit Switched (CS) fallback in Evolved Packet

System (EPS); Stage 2

[8] GSMA IR.92, VoLTE

[9]. GSMA RCS-e

[10] 3GPP TS 23.216 – Single Radio Voice Call Continuity (SRVCC);

Stage 2

[11] 3GPP TS 23.203 – Policy and Charging Control Architecture

[12] ITU-T G.168, “Requirements for network echo cancellers,” 2009.

[13] ITU-T, G.169, “Automatic level control devices,” 1999

[14] Werner Verhelst and Marc Roelands, “ An Overlap-Add TechniqueBased On Waveform Similarity (WSOLA) for High Quality TimeScale Modification of Speech,” Acoustic Speech and SignalProcessing Proceedings, 1993

[15] 3GPP specification 36.306, E-UTRAN-UE Radio AccessCapabilities

[16] ITU-T P.863, “Perceptual Objective Listening Quality Assessment(POLQA) – An advanced objective perceptual method for end-to-end speech quality evaluation of fixed, mobile, and IP-basednetworks and speech codecs covering narrowband, wideband, andsuper-wideband signals,” Jan 2012

[17] I.Cotanis, “POLQA technology,” Ascom white paper, September2011

[18] I.Cotanis, “Understanding the move from PESQ to POLQA,” Ascomwhite paper, November 2011

[19]. S. Chakraborty and others, “IMS Multimedia Telephony overCellular Systems,” Willey & Sons, 2007

[20]. 3GPP TS 36.213, E-UTRAN Physical Layer procedures

[21]. “Handbook on LTE Optimization,” Ascom, 2011 [22]. Andrew Chisholm, “RCS,” Ascom white paper, August 2012

http://www.volga-forum.com/

http://www.volga-forum.com/

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Voice Service on LTE