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LTE, Telephony andBattery Life

Björn Ekelund

White Paper

VoLTE Rev A 2012-12-18

© Copyright ST-Ericsson 2012. All rights reserved.

Abstract

In recent media, much attention has been paid to the battery life of VoLTE enabled LTEphones in comparison to existing 2G and 3G circuit switched telephony. This white paperaims to address the confusion and uncertainty around the topic and demonstrate the truepotential of VoLTE.

Keywords

VoLTE, IMS, Telephony, 3G, LTE, Battery life, SPS, DRX



LTE, Telephony and Battery Life White Paper Contents

VoLTE Rev A 2012-12-18 :

Prepared:

Björn Ekelund

Legal information


Disclaimer

The contents of this document are subject to change without prior notice. ST-Ericssonmakes no representation or warranty of any nature whatsoever (neither expressed norimplied) with respect to the matters addressed in this document, including but not limitedto warranties of merchantability or fitness for a particular purpose, interpretability orinteroperability or, against infringement of third party intellectual property rights, and in noevent shall ST-Ericsson be liable to any party for any direct, indirect, incidental and orconsequential damages and or loss whatsoever (including but not limited to monetarylosses or loss of data), that might arise from the use of this document or the information init.

ST-Ericsson and the ST-Ericsson logo are trademarks of the ST-Ericsson group of companies or used under a license from STMicroelectronics NV or TelefonaktiebolagetLM Ericsson.

All other names are the property of their respective owners.

Trademark list

All trademarks and registered trademarks are the property of their respective owners.

MetroPCS MetroPCS is a registered trademark of MetroPCSCommunications, Inc.

Metrico™ Metrico is a registered trademark of Spirent Inc.

ARM™ ARM is a registered trademark of ARM Ltd.

Cortex™ Cortex is a registered trademark of ARM Ltd.



LTE, Telephony and Battery Life White Paper Contents

VoLTE Rev A 2012-12-18 :

Prepared:

Björn Ekelund

Contents

1 About th is document 4 1.1 Purpose 4 1.2 Audience 4 1.3 Revision information 4 1.4 Reference list 4

2 Introduction 5

3 Key Power Consumption Factors 6 3.1 Software architecture 6 3.2 Radio hardware 7 3.3 Transmission protocols 7

4 Summary 9



LTE, Telephony and Battery Life White Paper

VoLTE Rev A 2012-12-18 4 (9)


1 About this document

1.1 Purpose

The purpose of this document is to reduce the confusion in the marketplace aroundexpectations around battery life for VoLTE enabled LTE smartphones.

1.2 Audience

The intended audience for the document is engineers, marketing and planning personnelat device manufacturers and telecom operators.

1.3 Revision information

Table 1 Revision history

Date Rev. Comments

2012-12-17 PA1 Draft

2012-12-19 A First release

1.4 Reference list

[1] 3GPP specifications www.3gpp.org

[2] GSMA recommendations http://www.gsma.com/newsroom/technical-documents




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2 Introduction

Lately there has been much attention on battery life for devices supporting the newtelephony system for LTE known as VoLTE. They main source of the underlying data is ameasurement report by Metrico made in MetroPCS’ CDMA+LTE network in lateNovember. It has since then been quoted and analyzed by many media and analystsworldwide.

For obvious reasons, this white paper does not disclose detailed data about ST-Ericsson’s products and will instead try to shed some light on the underlying mechanicsand also make some projections of what can be expected in well-designed, commercialVoLTE devices compared to what is available in the market in the fall of 2012.

First of all, without VoLTE CDMA/LTE systems suffer a severe penalty in that they use a

concept called Simultaneous Voice and LTE – SVLTE. This means that during a voicecall, both the CDMA radio and the LTE radio are in full operation. The consequence of this is clearly visible in the Metrico data, which also shows the obvious fact thatVoLTE+LTE consume less power than CDMA+LTE.

LTE networks with a 3GPP-based legacy do not suffer this. Neither do CDMA+LTEsolutions based on fall-back schemes (such as the one used in iPhone 5).

When LTE and VoLTE where standardized years ago, great care was put into making thesystem and the service as energy-efficient as possible.

Much of the media attention has been on first generation VoLTE implementations basedon first generation LTE chipsets. Chipsets developed with a focus on time to market and

on plain data services rather than more complex ones.Given its long experience in wireless technology, ST-Ericsson has put its aim much higherand is currently launching LTE solutions which will demonstrate a completely differentperformance compared to what is currently on the market.

This white paper will offer some simple back-of-an-envelope calculations indicating thekind of performance you will soon see in LTE phones in a shop near you.

Apart from the radio usage scheme used (single/double), there are really three majorfactors influencing the battery life of a VoLTE enabled smartphone; softwarearchitecture, radio hardware and transmission protocols.




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3 Key Power Consumption Factors

3.1 Software architecture

The software architecture of the handset ultimately defines how the computing load of theIMS protocol stack and the audio processing is distributed across the various processorsin the handset. A smartphone typically has a number of processor clusters, the two majorones being the Application processor (typically based on one or more ARM Cortex A-class cores) and Modem processor (typically based on or more proprietary solutions orsmaller ARM cores). Obviously, these two main processor clusters are optimized forcompletely different purposes.

The Application processor is optimized to run a high-level operating system such as

Android, whereas the Modem processor typically runs a real-time OS and is optimized fordecision making and number crunching.

This means that for a task like protocol/audio processing the modem processor is vastlymore efficient. For example, comparing the MIPS/milliwatt numbers (the metric telling youhow much computing power you get for your electrical power) for a typical dual coreapplication processor and an embedded modem processor you may see ratios between2x and 4x. Of course these ratios are only valid for very specialized processing tasks. If

we tried to run a high level OS with large memory pools and heavy context switching onthe modem processor it would perform very poorly.

The huge power saving (2-4x) from using the most appropriate processor in the handsetis to some extent offset by the use of the multimedia accelerators in all applicationprocessors. In reality this means that the ratio is closer to 1.5-2x for highly optimizedcode.




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3.2 Radio hardware

Another key factor for VoLTE power consumption is the radio hardware. First of all, thelinearity requirements for an LTE radio are far higher than for a CDMA or WCDMA radio. This means that without changes to the radio circuitry, the operating point has to belowered, which in turn means lower energy efficiency and, consequently, a higher powerconsumption to reach the same effective antenna radiation.

The difference also includes passives. A 3G radio rarely have more than 3 or 4 bands. AnLTE radio can have up to 8 or even 10 bands. Losses related to the large number of switches and filters mean that, again without major architectural changes, the radiatedantenna power will be lower for the same battery power spent.

These are factors playing a key role in most first generation LTE devices. For moreadvanced solutions, such as the Thor™ M7400 modem from ST-Ericsson, thesechallenges are addressed with new circuits and new hardware architectures, giving noeffective penalty on battery power for LTE.

3.3 Transmission protocols

Together with protocols for call establishment, codec negotiation etc., the VoLTE standardcomes with some new radio protocol enhancements aimed to reduce both spectrumusage and battery power. Few if any of them are activated in networks today.

The first one is Semi-persistent Scheduling, SPS. SPS aims to drastically reduce controlsignaling overhead for services that require persistent radio scheduling like VoIP. This of course has a direct effect on battery life. Simulations show between 10 and 20%

depending on coverage conditions and radio architecture.

OFDM Optimized RF

Conventional RF

Power drain for same antenna power




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TTI bundling is another one; however, this has less effect on battery life. Its main effect isinstead greatly improved service quality at the cell edge. Predictions of up to 4dB havebeen made, which is dramatic. The effect on battery life in real use is however difficult topredict and so small that it can be ignored for the sake of this exercise.

The third, and perhaps the most important one, is Discontinuous Reception, DRX. DRX isa method to switch the smartphone transmitter on and off based on available transmissiondata. The scheme is useful not only for voice but also for any intermittent traffic, such asweb browsing or online gaming.

DRX benefits both the uplink (smartphone transmit) and downlink (smartphone receive)but the gains in uplink DRX greatly exceeds those of downlink DRX. So, for the purposeof this paper, we will disregard downlink DRX.

It is a well-established fact that a in a typical phone conversation the two parties speak foraround 80% of the time. This means that the smartphone transmitter only needs to beactive about 40% of the time. Theoretically the power reduction from using this would be60% (silence plus when the other party speaks) but due to factors like switching policiesand protocol overhead the real saving is closer to 30%. Nevertheless, a major saving.




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4 Summary

For “normal” output powers, the radio transmitter circuitry with its power amplifierdominates a smartphone’s power consumption during a voice call. Now, with conservativesavings of 20% due to OFDM/multiband optimized circuitry and passives, 15% from SPSand 30% from DRX, we see a total reduction for the radio circuitry of around 50%.

Assuming a conservative ratio of 1.5 between the power consumption of an “app-like”VoLTE client and a deeply embedded one, provides a baseband power saving of roughly30%. Using realistic numbers from real products these savings give you a chart like this:

Since we see first generation VoLTE enabled phones (not employing SVLTE) in themarket today with battery life just slightly worse than 3G phones, this very conservativelydrawn chart indicates a great potential for the more mature products that we will startseeing in 2013.

Comparing with the actual data processing power needs and the actual radio outputpowers of today’s 3G phones, there is definitely potential for VoLTE phones havingsignificantly longer use time.

On top of this Moore’s law just continues to help and help.

Another final, albeit interesting, observation is that over-the-top services like Skype areforced to run in the power-hungry application processor (since they are apps) and haveno help from advanced protocols; consequently they will be increasingly disadvantagedversus VoLTE. Something they are not today.

Baseband

Baseband

Radio

Radio

Optimized

First generation

Total Power Consumption

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