LTE, Telephony and Battery Life

LTE, Telephony and Battery 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 LTE phones in comparison to existing 2G and 3G circuit switched telephony. This white paper aims to address the confusion and uncertainty around the topic and demonstrate the true potential 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-Ericsson makes no representation or warranty of any nature whatsoever (neither expressed nor implied) with respect to the matters addressed in this document, including but not limited to warranties of merchantability or fitness for a particular purpose, interpretability or interoperability or, against infringement of third party intellectual property rights, and in no event shall ST-Ericsson be liable to any party for any direct, indirect, incidental and or consequential damages and or loss whatsoever (including but not limited to monetary losses or loss of data), that might arise from the use of this document or the information in it.

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 Telefonaktiebolaget LM 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 MetroPCS Communications, 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 this 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

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1 About this document

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

1.2 Audience The intended audience for the document is engineers, marketing and planning personnel at 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 new telephony system for LTE known as VoLTE. They main source of the underlying data is a measurement report by Metrico made in MetroPCS’ CDMA+LTE network in late November. It has since then been quoted and analyzed by many media and analysts worldwide.

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 mechanics and also make some projections of what can be expected in well-designed, commercial VoLTE 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 voice call, 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 that VoLTE+LTE consume less power than CDMA+LTE.

LTE networks with a 3GPP-based legacy do not suffer this. Neither do CDMA+LTE solutions 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 the system and the service as energy-efficient as possible.

Much of the media attention has been on first generation VoLTE implementations based on 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 higher and is currently launching LTE solutions which will demonstrate a completely different performance compared to what is currently on the market.

This white paper will offer some simple back-of-an-envelope calculations indicating the kind 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 major factors influencing the battery life of a VoLTE enabled smartphone; software architecture, 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 the IMS protocol stack and the audio processing is distributed across the various processors in the handset. A smartphone typically has a number of processor clusters, the two major ones 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 or smaller ARM cores). Obviously, these two main processor clusters are optimized for completely 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 for decision making and number crunching.

This means that for a task like protocol/audio processing the modem processor is vastly more efficient. For example, comparing the MIPS/milliwatt numbers (the metric telling you how much computing power you get for your electrical power) for a typical dual core application processor and an embedded modem processor you may see ratios between 2x 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 on the modem processor it would perform very poorly.

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


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3.2 Radio hardware Another key factor for VoLTE power consumption is the radio hardware. First of all, the linearity 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 be lowered, which in turn means lower energy efficiency and, consequently, a higher power consumption to reach the same effective antenna radiation.

The difference also includes passives. A 3G radio rarely have more than 3 or 4 bands. An LTE 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 radiated antenna power will be lower for the same battery power spent.

These are factors playing a key role in most first generation LTE devices. For more advanced solutions, such as the Thor™ M7400 modem from ST-Ericsson, these challenges are addressed with new circuits and new hardware architectures, giving no effective penalty on battery power for LTE.

3.3 Transmission protocols Together with protocols for call establishment, codec negotiation etc., the VoLTE standard comes with some new radio protocol enhancements aimed to reduce both spectrum usage 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 control signaling 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 is instead greatly improved service quality at the cell edge. Predictions of up to 4dB have been made, which is dramatic. The effect on battery life in real use is however difficult to predict 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 is a method to switch the smartphone transmitter on and off based on available transmission data. The scheme is useful not only for voice but also for any intermittent traffic, such as web 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 purpose of this paper, we will disregard downlink DRX.

It is a well-established fact that a in a typical phone conversation the two parties speak for around 80% of the time. This means that the smartphone transmitter only needs to be active about 40% of the time. Theoretically the power reduction from using this would be 60% (silence plus when the other party speaks) but due to factors like switching policies and 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 amplifier dominates a smartphone’s power consumption during a voice call. Now, with conservative savings of 20% due to OFDM/multiband optimized circuitry and passives, 15% from SPS and 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 roughly 30%. 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 the market today with battery life just slightly worse than 3G phones, this very conservatively drawn chart indicates a great potential for the more mature products that we will start seeing in 2013.

Comparing with the actual data processing power needs and the actual radio output powers of today’s 3G phones, there is definitely potential for VoLTE phones having significantly 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 are forced to run in the power-hungry application processor (since they are apps) and have no help from advanced protocols; consequently they will be increasingly disadvantaged versus VoLTE. Something they are not today.

Baseband

Baseband

Radio

Radio

Optimized

First generation

Total Power Consumption

LTE, Telephony and Battery Life

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