Impact of Extended DRX Cycles on Battery Lifetimes and UE Reachability
VIGNESH NARASARAJU vcna@kth.se
Academic Supervisor: Dr. Cicek Cavdar, KTH
Industrial Supervisor: Dr. Emre A. Yavuz, Ericsson AB Examiner: Prof. Jens Zander, KTH
TRITA-ICT-EX-2016:126
Abstract
Several UE energy consumption optimization techniques have been proposed for Machine Type Communication (MTC) devices. Extended Discontinuous Reception (eDRX) in idle mode is one such technique wherein an UE in idle mode wakes up only during its Paging Occasion (PO) to monitor paging messages from eNodeB (eNB). The PO is located within a Paging frame (PF). The PF is a function of System Frame Number (SFN) cycle of eNB. The paging messages may be sent asynchronously from multiple eNBs to a UE.
Due to asynchronous operation of eNBs, SFN takes on different values at a given point in time and therefore a paging message is transmitted at different points in time from different eNBs. Due to this SFN misalignment between eNBs, an idle mode UE might receive and respond to the same paging message from different eNBs and/or miss a PO and thus the paging message.
Due to this spread in time of SFN and PO, the actual handling of paging message by the UE becomes inefficient leading to increased UE energy consumption and decreased reachability. These issues, resulting from paging handling, will get amplified further if DRX period is extended longer (eDRX).
In this study, we investigate the impact of eDRX cycles and mobility related parameters such as UE speed, cell size and size of SFN misalignment between eNBs on UE energy consumption, use of network resources and UE reachability. Receiving and responding to the same paging message results in increased energy consumption for UE and increased signaling between UE and the network. Missing a PO results in delayed paging reception and hence decreases UE reachability. As the DRX cycle lengths are increased from existing maximum of 2.56 seconds to 10.24 seconds and beyond, we see a reduction in UE energy consumption by more than 90%, but the network signaling and the delay to reach the UE increases linearly as a function of the DRX cycle length. We observe that the number of duplicate paging message receptions/missed POs is minuscule for DRX cycle lengths below 10.24 sec. At DRX cycle length of 10.24 seconds, UEs travelling across 500 m cell radius at speeds of 3, 50, 100 km/h the percentage of duplicate paging receptions are 0.07, 0.11, and 0.15 respectively. This duplicate paging message reception increases the UE energy consumption by 2.31, 6.15 and 12 percent of the total energy units respectively. Similarly, UE misses nearly 0.34, 0.39, and 0.405 percent of the total POs respectively. Depending on the number of consecutive PO misses, the UE reachability decreases. But by reducing the size of SFN misalignment between eNBs, we see that it’s possible to increase the reachability for UEs in eDRX. Further we have proposed solutions based on our analytical study to avoid duplicate paging message reception by UE, increase UE reachability and also reduce UE energy consumption using a windowing technique. We conclude that when a UE is configured with eDRX cycles, the tradeoff between battery lifetimes and UE reachability is based on mobility characteristics and service requirements.
Keywords:
LTE, MTC, Machine-Type-Communication, SFN, DRX, eDRX, Extended DRX, Idle Mode, Paging, M2M, Machine-to-Machine, IoT, Internet-of-Things
K T H R O Y AL I N S T I T U T E O F T E C H N O L O G Y
I N F O R M A T I O N A N D C O M M U N I C A T I O N T E C H N O L O G Y
Acknowledgements
I acknowledge the generous higher education opportunity provided by Sweden and KTH. My learning and time spent at KTH School of ICT have been one of best academic experiences so far.
I extend my sincere thanks to academic supervisor, Dr. Cicek Cavdar (KTH Radio Communications Laboratory, Sweden) and industrial supervisor Dr. Emre A. Yavuz (Ericsson AB, Sweden) for giving me the opportunity to purse this Master Thesis. The technical discussions with Dr. Emre on LTE paging and SFN (System Frame Number) handling were the best. I sincerely thank Dr. Cicek for guidance on energy efficiency in mobile networks and her persistent advice on academic reporting. Together, their continuous support and guidance throughout the thesis work has given me a deep insight in the area of energy efficiency in cellular networks, scientific research and 3GPP standards. Also, special thanks to Prof. Jens Zander for his constructive comments during the problem formulation phase of this work.
Finally, I am ever grateful to my parents and siblings, who encouraged me for pursuing higher education. Despite many hardships, they stood by and encouraged me all the way through. Lastly, but not the least, special thanks to friends and colleagues at Ericsson AB, Kista, Sweden, whose endless support all along the studies and work kept me on my toes to complete this education.
Vignesh Narasaraju Stockholm, Sweden, 2016
1
2
List of Acronyms
3G 3rd Generation
3GPP Third Generation Partnership Project
4G 4rd Generation
AGC Automatic Gain Control
BCCH Broadcast Control Channel
BCH Broadcast Channel
CCCH Common Control Channel
CMAS Commercial Mobile Alert Service
C-RNTI Cell RNTI
CS Circuit Switched
CSG Closed Subscriber Group
DCCH Dedicated Control Channel DCI Downlink Control Information
DL Down Link
DL-SCH Downlink Shared Channel
DRX Discontinuous Reception
DTCH Dedicated Traffic Channel DTX Discontinuous Transmission
EAB Extended Access Barring
ECM EPS Connection Management
eDRX Extended Discontinuous Reception
EMM EPS Mobility Management
eNB evolved Node B
EPS Evolved Packet System
ETWS Earthquake and Tsunami Warning System
E-UTRAN Evolved Universal Terrestrial Radio Access Network FDD Frequency Division Duplex
GSM Global System for Mobile communications
H2H Human 2 Human
ID Identifier
IMSI International Mobile Subscriber Identity IoE Internet of Everything
IoT Internet of Things
IP Internet Protocol
ISD Inter Site Distance
KHz Kilo Hertz
LCD Liquid Crystal Display
LTE Long Term Evolution
3
M2M Machine 2 Machine
MAC Medium Access Control
MCCH Multicast Control Channel
MCH Multicast channel
MHz Mega Hertz
MME Mobile Management Entity
MP Modification Period
ms Millisecond
MTC Machine Type Communication
MTCH Multicast Traffic Channel
NAS Non-Access Stratum
OFDMA Orthogonal Frequency Division Multiple Access
PA Power Amplifier
PBCH Physical Broadcast Channel PCCH Paging Control Channel
PCH Paging Channel
PDCCH Physical Downlink Control Channel PDCP Packet Data Convergence Protocol PDSCH Physical Downlink Shared Channel
PF Paging Fame
PHY Physical layers
PLL Phase Locked Loop
PMCH Physical Multicast Channel
PO Paging Occasion
PRACH Physical Random Access Channel
P-RNTI Paging RNTI
PS Packet Switched
PTW Paging Transmission Window
PUSCH Physical Uplink Shared Channel
QoS Quality of Service
RACH Random Access Channel
RF Radio Frequency
RLC Radio Link Control
RNTI Radio Network Temporary Identity
RX Reception
RRC Radio Resource Control
S1AP S1 Application Protocol
SAE System Architecture Evolution
SC-FDMA Single-Carrier Frequency Division Multiple Access
SF Sub Frame
4
SFN System Frame Number
S-GW Serving Gateway
SI System Information
S-TMSI SAE Temporary Mobile Station Identifier
TA Tracking Area
TAI Tracking Area Identity
TDD Time Division Duplex
TTI Transmission Timing Interval
TX Transmission
UE User Equipment
UL Up Link
UL-SCH Uplink Shared channel
WCDMA Wideband Code Division Multiple Access
5
6
List of Figures
Figure 1.1: MME sending paging request to eNBs in TA ... 4
Figure 1.2: UE receiving duplicate paging message from eNBs due to SFN misalignment ... 5
Figure 1.3: UE missing its PO from eNB due to SFN misalignment ... 6
Figure 2.1: DTX and DRX opportunity for UE in LTE ... 11
Figure 2.2: LTE type-1 frame structure ... 13
Figure 2.3: LTE DRX cycle ... 14
Figure 2.4: LTE DRX cycle flow chart ... 16
Figure 2.5: LTE DRX cycle transitions ... 17
Figure 2.6: UE RRC_CONNECTED and RRC_IDLE mode transition ... 19
Figure 2.7: UE DRX in connected mode ... 20
Figure 3.1: Cells and TA grouping ... 22
Figure 3.2: SFN cycle ... 23
Figure 3.3: LTE network elements involved in paging procedure ... 23
Figure 3.4: LTE protocol and channels interface ... 25
Figure 3.5: LTE channels involved in paging procedure ... 26
Figure 3.6: Paging procedure sequence diagram ... 27
Figure 4.1: Cell layout and UE distribution ... 31
Figure 4.2: Idle mode UE activity and energy consumption ... 34
Figure 4.3: Illustration of duplicated paging message reception ... 36
Figure 4.4: Illustration of delayed paging reception due to missed PO ... 36
Figure 4.5: Illustration of network delay, UE PO occurrence and SFN misalignment ... 40
Figure 5.1: Number of duplicate paging message receptions at 500 m cell radius ... 43
Figure 5.2: Number of missed POs at 500 m cell radius ... 43
Figure 5.3: Total number of paging messages and the percentage of duplicate paging message reception by UEs at 500 m cell radius ... 44
Figure 5.4: UE total energy consumption and percentage of energy consumption due to duplicate paging message reception at 500 m cell radius ... 45
Figure 5.5: Total number of POs and the percentage of missed POs by UEs at 500 m cell radius ... 47
Figure 5.6: Average delay for the network to reach the UEs at SFN offset of 0 to 1023 between cells and 500 m cell radius ... 48
Figure 5.7: Paging delay due to missed PO at DRX 2.56 sec and 500 m cell radius ... 49
Figure 5.8: Paging delay due to missed POs at DRX 10.24 sec and 500 m cell radius ... 50
Figure 5.9: Paging delay due to missed POs at DRX 163.84 sec and 500 m cell radius ... 50
Figure 5.10: Average energy consumption versus average paging delay due to missed POs by UEs at 100 km/h speed and 500 m cell radius ... 51
Figure 6.1: Illustration of PTW within eDRX cycle ... 53
Figure 6.2: Illustration of UE reachability without PTW technique ... 54
Figure 6.3: Illustration of UE reachability with PTW technique ... 55
7 Figure 6.4: Comparison of network delay to reach the UE using no-PTW and
PTW technique. eDRX=10.24 sec, PTW = 10.24 sec. ... 56
Figure 6.5: Ideal DRX cycle ... 57
Figure 6.6: Non-ideal DRX cycle ... 58
Figure 6.7: Short wake-ups and ON period overlaps ... 59
Figure 6.8: Energy comparison for short wake-up POs ... 60
Figure A.1: Number of duplicate paging message receptions at 1000 m cell radius ... 69
Figure A.2: Number of duplicate paging message receptions at 1500 m cell radius ... 69
Figure A.3: Number of missed POs at 1000 m cell radius ... 70
Figure A.4: Number of missed POs at 1500 m cell radius ... 70
Figure A.5: Messaging exchanges due answering the duplicate paging messages between UE and eNB ... 71
Figure A.6: Number of consecutive PO misses when UE at 3 km/h speed and 500 m cell radius ... 71
Figure A.7: Number of consecutive PO misses when UE at 50 km/h speed and 500 m cell radius ... 72
Figure A.8: Number of consecutive PO misses when UE at 100 km/h speed and 500 m cell radius ... 72
Figure A.9: Average delay for the network to reach the UEs at SFN offset of 0 to 511 between cells and 500 m cell radius... 73
Figure A.10: Average delay for the network to reach the UEs at SFN offset of 0 to 255 between cells and 500 m cell radius ... 73
8
List of Tables
Table 2.1: LTE Short-DRX cycle length values ... 18
Table 3.1: Contents in S1AP paging message ... 28
Table 3.2: Contents of RRC paging message ... 29
Table 4.1: Simulation parameters ... 32
Table 4.2: DRX parameters ... 33
Table 4.3: Sub-frame pattern for FDD ... 33
Table 4.4: Sub-frame pattern for TDD ... 33
Table 4.5: UE energy model parameters ... 35
Table 4.6: UE energy consumption parameters ... 37
9
10
Table of Contents
1 Introduction ... 1
1.1 Background and Motivation ... 2
1.2 Thesis Outline ... 3
1.3 Synchronization of SFN and Paging Message Delivery ... 3
1.3.1 Scenario 1: UE Receiving and Responding to the Duplicate Paging Message 5 1.3.2 Scenario 2: UE Missing its PO and Receiving Delayed Paging Message ... 5
1.4 Previous Studies in the Related Field ... 7
1.5 Thesis Objective and Research Questions ... 8
1.5.1 Thesis Objective ... 8
1.5.2 Research Questions ... 8
1.5.3 Methodology ... 9
2 Energy Saving in LTE ... 11
2.1 Energy Saving by DTX and DRX ... 11
2.1.1 Discontinuous Transmission – DTX ...13
2.1.2 Discontinuous Reception– DRX ...14
2.2 DRX Cycle ... 14
2.3 DRX in different UE Modes ... 18
2.3.1 DRX in Connected Mode ...19
2.3.2 DRX in Idle Mode ...20
3 Paging Mechanism in LTE ... 22
3.1 LTE Network Elements in Paging Mechanism ... 23
3.2 LTE Channels Involved in Paging Procedure ... 24
3.3 LTE Idle Mode Paging Procedure ... 26
4 System Setup and Performance Measures ... 31
4.1 Simulation Setup ... 31
4.2 UE Energy Model ... 34
4.3 Performance Metrics ... 36
4.3.1 UE Energy Consumption ...37
4.3.2 Paging Load at the eNB ...38
4.3.3 UE Reachability ...38
5 Simulation Results and Discussions ... 42
5.1 Effect of UE mobility, cell size, SFN misalignment and eDRX cycle on paging message reception ... 42
5.2 Paging Load at eNB and UE Energy Consumption ... 44
5.3 UE Reachability ... 47
6 Analysis of Potential Solutions for UE Energy and Reachability issues52 6.1 Preventing UE from receiving duplicate paging messages ... 52
6.2 Increase UE Reachability ... 52
7 Conclusion and Future Work ... 62
7.1 Conclusion ... 62
7.2 Future Work ... 63
References ... 66
Appendix A ... 69
11
1
1 Introduction
In cellular technologies such as 3G and 4G, energy efficiency has become one of the important goals [1]. This goal is mainly driven by the telecom service providers to reduce carbon emissions and decrease total energy consumption in the network. It is estimated that by using modern mobile cellular technology such as LTE, network operators can reduce the overall energy consumption by 60% by 2020 [2]. Energy efficient techniques are applied to both mobile networks and users respectively. In the mobile network domain, despite increase in network infrastructure and deployment, network operators strive to reduce their annual energy bills by using the network equipment efficiently, adapting to traffic flow in the network.[3]. In the mobile user domain, energy efficiency is focused on reducing both the UE energy consumption and unnecessary use of radio network resources.
Compared to mobile network domain, energy efficiency or reducing energy consumption in mobile user domain/UE is crucial due to the fact that source of energy is limited via size of battery in the UE. Therefore, reducing the UE energy consumption is one of the main challenges being posed to cellular technology standard bodies due to the advent of energy hungry smart communicating devices. The smart UEs are used for both H2H and M2M communication. In H2H communication, the UE is used for communication between human users. In M2M communication [4], UE takes the role of machine (e.g. MTC) that acts as a user and has the ability to communicate with other UEs including human users. Some of the M2M communication services include transportation (e.g. automotive, asset tracking, parking meters), healthcare (e.g. wearable sensors, monitoring devices), and utilities industries (e.g. smart grid for electricity, water and gas). In the smart UE, the main energy consuming parts were analyzed in [5]. For example, wireless modules, LCD, touch screen panel, backlight and maintaining a connection to the network: all these tasks consume significant amount of energy. It is suggested that significant energy savings can be achieved by turning off energy source to unused device components in the UE [5] during no-traffic time instants via DRX mechanism.
Even though one could argue for having an efficient and high energy capacity battery for the UE, the pace of development of smart UE and energy hungry applications has outpaced the slow development of compact sized UE batteries. As a result, the focus has shifted towards saving UE battery energy [6]. Also, since the energy storage is limited by the size of UE battery and available technology to date, efforts are made to reduce the overall UE energy consumption and yet keep the UE operational for longer duration and make it available for use to the user. Energy saving is also necessary for smart UEs in critical communication fields where energy supplies are discontinuous or recharging/changing batteries often is also a challenge [7]. In general, LTE offers energy saving mechanisms via DTX and DRX for network and UE respectively. eDRX functionality is specified by 3GPP via Release-13. In this
2 thesis work, we analyze the impact of eDRX/longer DRX cycles on energy consumption and reachability for idle mode UEs.
1.1 Background and Motivation
Reducing energy consumption for MTC devices, referred as UE in rest of this literature, is extremely important as this helps them to be operational for longer periods (e.g. weeks, months or even years) without any interruption for maintenance. Reducing UE energy consumption also increases system capacity, i.e. by minimizing unnecessary signaling towards the network when the UE is in low energy consumption or sleep mode via DRX. Depending on the configuration from network for different traffic situation and required QoS, the length of DRX can vary from small, long to very long periods respectively. Among many other energy consumption optimization techniques for UE, proposed by 3GPP [8] in LTE, eDRX in idle mode is one such technique where an idle mode UE turns off its receiver circuitry over longer periods of time to save battery energy.
An idle mode UE is reachable to the network/eNB only via paging request within DRX/eDRX cycle. The paging requests are sent by MME to all the eNBs in the UE registered TA. The eNBs further delivers paging messages asynchronously at defined time instants called PO, which repeats every DRX cycle. The PF, which contains the PO, is a function of SFN cycle on the air interface, i.e., the PF is one among the many radio frames which are numbered and delivered in a cyclic manner (SFN cycle) by the eNB. However, SFN cycle in each eNB could start at a different point in time. As a result, the paging message handling by the UE becomes inefficient, leading to increased UE energy consumption and reduced UE reachability, which is explained further in section 1.3. Among all the signaling traffic generated by MME, it is estimated that the signaling due to paging alone contributes to around 29% of the total signaling load in the network [9]. Therefore, one cannot ignore the inefficient handling of paging messages by UE and rest of the network elements, which could further lead to increased UE energy consumption, signaling load and reduced UE reachability under eDRX technique. The inefficient handling of paging messages intensifies as the DRX cycles are extended longer (eDRX). Therefore, it is important to analyze the impact of eDRX cycles on UE energy consumption and reachability due to the paging mechanism.
3 1.2 Thesis Outline
The structure of this thesis report is organized as follows: In section 1.3 we introduce the problem being addressed, as it forms the motivation for our work. Section 1.4 discusses the previous work in the related field and mentions the research gap and section 1.5 lists our objectives and research questions answered in our work and also the methodology followed in this work. Chapter 2 and 3 contains our literature survey highlighting our theoretical understanding of the subject. In chapter 4 we discuss the system simulation model, assumptions, energy model and performance metrics used in our analysis. Chapter 5 contains simulation results, performance analysis, discussions and an analytical study on potential solutions to the problems.
Finally, chapter 6 contains conclusions and scope of improvement for future work.
1.3 Synchronization of SFN and Paging Message Delivery
Our motivation to say that “ eDRX could lead to increased UE energy consumption, signaling load and reduced UE reachability due to inefficient paging handling” is arrived by analyzing the problem causing scenarios explained further via a simple illustration. This is also the basis for our analysis work in this thesis.
Let cells cell-1, cell-2 and cell-3 belong to the same TA and are managed by an MME. Since an idle mode UE is registered and known to the network only at granularity of TA, MME sends paging request to the eNBs in cells cell- 1, cell-2 and cell-3, as shown in Figure 1.1. The UE in idle mode wakes up once every DRX cycle at a specified time instant, PO, to check for a possible paging message. The PO is located within the PF in the SFN cycle of eNB. However, the SFN cycles among eNBs are not synchronized.
For example, at time t = 0 (zero), the SFN may take on different values in each cell. As a result, the paging message at a particular PO for a UE is not sent at same time instant from all the eNBs. But the idle UE wakes up when the time for its PO arrives. If the UE is mobile and wakes up during its PO in another cell, it may miss its PO in the new cell due to SFN misalignment between the eNBs. That is, the paging message from the eNB in the new cell might have been already sent from eNB or is yet to be sent from eNB. Further, due to eDRX, where the cycle lengths can be in the range of minutes, hours or even longer, the occurrence of a PF and hence PO for a UE also increases with time. As a result, the handling of paging message by an idle mode UE that is moving across cells will have negative impact on its energy consumption, reachability and signaling load on the network.
4 MME
Cell-2
Cell-3 Cell-1
@t = 0 SFN = 0
@t = 0 SFN = 512
@t = 0 SFN = 1023
Figure 1.1: MME sending paging request to eNBs in TA
We identify two problems causing scenarios in paging handling due to SFN misalignment and eDRX. The scenarios are described further on:
5 1.3.1 Scenario 1: UE Receiving and Responding to the Duplicate
Paging Message
As shown in Figure 1.2, MME sends paging request for UE to eNBs in TA. An idle mode UE receives its paging message at its PO in cell-1, responds to the paging message and sleeps again. During the sleep period, if the UE is mobile, it ends up in cell-2. When the UE wakes up again, it finds itself in cell- 2. Due to SFN misalignment, the eNB in cell-2 also delivers the same paging message to UE when the UE PO has arrived. UE being unaware of the fact that it has already answered the same paging message in cell-1, it responds to the same paging message in cell-2, which we call as duplicate paging reception.
MME
Cell-2
Cell-3 Cell-1
DRX
Figure 1.2: UE receiving duplicate paging message from eNBs due to SFN misalignment
In this scenario 1, we can observe that:
1. UE answered twice to the same paging message, both in cell-1 and cell-2, and hence UE consumes additional battery energy trying to access the network and process the paging message.
2. UE and eNB consumed additional radio network signaling while answering the same paging message and thus reducing the system capacity for other UEs in the cell that might have to be served.
1.3.2 Scenario 2: UE Missing its PO and Receiving Delayed Paging Message
6 As shown in Figure 1.3, MME sends paging request for UE to eNBs in TA. An idle mode UE receives its paging message at its PO in cell-1, responds to the paging message and sleeps again. During this sleep period, MME sends a new paging with new information to the UE. When the UE is awake it finds itself in cell-2. But due to SFN misalignment, the PF and PO carrying the paging message for the UE has not yet arrived or could have already passed in cell-2. As a result, UE goes back to sleep mode to wake up at its next PO. If the idle UE moves into cell-3 during this sleep time, UE may again miss its PO again due to SFN misalignment or receive a paging message at its PO. If the idle UE is moving at a high speed such that it crosses many cells and wakes up once during eDRX cycle in each cell, the possibility of UE missing its PO due to SFN misalignment will be high resulting in more time for the network to reach the UE.
MME
Cell-2
Cell-3
DRX Cell-1
DRX
Figure 1.3: UE missing its PO from eNB due to SFN misalignment
In this scenario 2, we can observe that:
1. UE receives a delayed paging message or might even miss the PO and paging message containing time critical information, such as SI updates and warnings.
2. Missing a paging message causes increased delay for the network to reach the concerned UE.
7 1.4 Previous Studies in the Related Field
Several studies have been carried out w.r.t eDRX cycles, focusing mainly on reducing UE energy consumption or increasing UE energy efficiency. In some studies the authors also mention the effects of eDRX cycles on paging handling. For example, they analyze the effects such as delayed SI acquisition, delayed cell reselection and also UE being paged in a different TA while moving across cells when in idle mode. eDRX cycle is also referred as extended paging cycle in some literature.
A study on extended paging cycles for M2M devices was done in [10].
The authors evaluate the best possible ways to reduce M2M device energy consumption for different traffic scenarios. Extending the paging DRX cycles beyond the current limit of 2.56 sec with shorter RRC inactivity timers reduces UE energy consumption for infrequent M2M traffic but increases the packet buffering delay in the network.
Authors in [11] developed an energy model and analyzed the impact of eDRX cycles on M2M devices that are mobile. Their results suggest that both the energy consumption and device battery lifetime can be improved, whilst the device responsiveness to network reach is compromised.
A study on extended paging cycles for MTC UEs and its effect on unnecessary SI acquisition was done in [12]. In order to eliminate UEs missing the updated SI when eDRX cycles are larger than the MP, the authors proposed a solution with two different MP based SI acquisition via normal MP and extended MP. This two different MP based SI acquisition differentiates between MTC (extended paging cycle and extended MP) and non-MTC (default paging cycle and normal MP) UEs.
In [13] the authors discuss the energy saving due to eDRX cycle and issues related to mobility of MTC UEs in idle mode. The issues discussed are delayed cell reselection due to longer DRX cycle (causing the UE to be paged in the wrong TA), delayed SI change notification if the DRX cycles are longer than the MP and possibility of SFN bit extension for supporting longer DRX cycle lengths. In order to overcome delayed cell reselection and SI change notification issue, the authors propose a solution via early cell-reselection and SI reading just before the UE active time within the eDRX cycle.
But none of the above mentioned studies discuss the effect that SFN misalignment and eDRX cycles have on paging handling for an idle mode UE and its ultimate impact on UE energy consumption and reachability. We try to address this research gap in this thesis.
8 1.5 Thesis Objective and Research Questions
Noting our observations from scenarios 1 and 2, described in section 1.3, we aim to analyze the role of different mobility related parameters in paging handling during eDRX cycle for idle mode UEs. This will pave the way forward to design smart solutions to reduce UE energy consumption, signaling load and increase reachability during eDRX. Our thesis objective and resulting research questions are as follows:
1.5.1 Thesis Objective
Based on the research gap identified in the previous studies in related field and the problem scenarios, we aim to evaluate the impact of eDRX cycles on idle mode UEs with the following objectives:
Evaluate the impact of network deployment parameters on UE energy consumption and reachability for UEs configured with eDRX cycles.
Avoid redundant signaling for call establishment due to receiving duplicate paging messages or delay for the network to reach the UE due to missing POs.
1.5.2 Research Questions
In order to achieve the objectives mentioned above, we address the following research questions:
What is the impact of parameters, such as UE speed, cell size and SFN misalignment between eNBs on UEs configured with eDRX cycles?
How does eDRX configuration impact the trade-off between UE energy consumption and its reachability?
How can we reduce redundant signaling and energy consumption in UEs due to duplicate paging message receptions or delay for network to reach UEs configured with eDRX?
9 1.5.3 Methodology
In this Thesis work, we follow a quantitative analysis method to analyze the effects of different mobility related parameters and duration of SFN misalignment between eNBs on energy consumption and reachability for eDRX configured UEs. In order to create the problem causing scenarios, explained in section 1.3.1 and 1.3.2, and observe the effect of different input parameters, we simulate the scenarios using MATLAB software application.
Through simulation, we have the flexibility to repeat the experiment quickly with different sets of input parameters and collect necessary data for performance analysis. Before performing the simulation work, we do an extensive literature study. Initially, LTE network architecture, different energy efficient mechanism will be studied. Secondly, paging and DRX mechanism will be studied. Finally, we propose solutions for efficient handling of paging messages for eDRX configured UEs, with focus on reducing UE energy consumption and increasing reachability for the network.
10
11
2 Energy Saving in LTE
Before starting our analysis on the problem, we perform an extensive literature study to understand different energy saving options, paging mechanism, and different entities involved in paging mechanism in the LTE network. This literature study will help us to simulate the problem causing scenarios and capture necessary results for our analysis.
2.1 Energy Saving by DTX and DRX
In LTE, energy saving is possible in both mobile network and the UE.
3GPP has specified DTX and DRX mechanisms for saving both energy and radio resources in radio network and UE respectively. Since LTE is an all packet data network, it offers great potential to save energy by turning off parts of the transmitter and receiver circuitry during no-data or low traffic periods within the data transmission intervals or data frames, as shown in Figure 2.1.
Time domain
Frequency domain
UE1
UE1 UE2
UE2 UE3
UE3
UE1
UE3
Opportunity for DTX-DRX
Figure 2.1: DTX and DRX opportunity for UE in LTE
12 In order to visualize how DTX and DRX fit into empty transmission intervals, it is important to understand the LTE data frame structure in brief.
LTE uses multicarrier technologies such as OFDMA and SC-FDMA for DL and UL transmissions respectively. LTE channel bandwidths are scalable via 1.4, 3, 5, 10, 15 and 20 MHz and can support both FDD and TDD transmission modes [14]. The frame structure for FDD is called Type 1 and for TDD it is called Type 2 [15]. We discuss only Type 1 frame structure here and the same is considered for deriving DRX PF and PO in this thesis work. For Type 2 frame structures, kindly refer [15].
The Type 1 frame structure is divided into hierarchical units as resource elements, slots, sub-frames and radio frames, smallest being the resource element and largest being the radio frame, as shown in Figure 2.2. Each radio frame is 10 ms in time duration and contains ten sub-frames, each 1 ms in time duration. Each sub-frame has two slots, each 0.5 ms time duration. A single UL or DL transmission is done at the granularity of sub-frame (1 ms), referred to as TTI. When there is no transmission, the sub-frame or contiguous of sub-frames offers opportunity for DTX or DRX, as shown in Figure 2.1.
Further down, the sub-frame is divided into smaller units of frequency and time resources in a grid fashion, called as resource elements. In the frequency domain, the frequency band is divided into sub-carriers with sub- carrier spacing of 15 KHz to maintain orthogonality. The total number of sub- carriers depends on the channel bandwidth being used. For example, if the channel bandwidth is 10 MHz, then there are 600 sub-carriers in the frequency domain. In the time domain, the resources are divided into 14 OFDM symbols for normal cyclic prefix and 12 OFDM symbols for extended cyclic prefix per sub-frame. Each OFDM symbol has time spacing of 66.67 µs (1/15 KHz). An UE in the LTE network is scheduled in multiples of resource block pair, contained within one sub-frame. Each resource block pair is comprised of 12 sub-carriers (180 KHz = 12*15 KHz) and 14 OFDM symbols (assuming normal cyclic prefix).
13
Radio frame = 10 ms
Sub-frame = 1 ms
Slot 0 = 0.5 ms Slot 1 = 0.5 ms 14 OFDM symbols
12 Sub-carriers
Resource element
Figure 2.2: LTE type-1 frame structure
2.1.1 Discontinuous Transmission – DTX
It is estimated that in a mobile cellular network: GSM, WCDMA or LTE, around 80% of the energy is consumed at the radio network side, specifically the radio unit at the base station [16]. Within radio unit, it is mainly the PA in the transmitter that consumes most of the energy during operation. In DTX, the radio transmitter is configured to switch off for a defined period of time during low or no traffic to reduce energy consumption.
DTX can be applied in both DL and UL directions. When applied in downlink, it’s called cell-DTX, and when applied in uplink it’s called UE-DTX [17]. In the cell-DTX the PA in the radio unit is put to sleep mode during periods of inactivity within data transmission [16]. In the UE-DTX, the UE transmitter is put to sleep mode, mainly during a speech call as there are instants of inactivity or pauses in a speech session. This helps the UE to save battery energy and increase the energy efficiency. When no data transmission is scheduled, UE-DTX also helps to reduce considerable amount of uplink interference over-the-air and increase uplink capacity in the cell so that more number of UEs can be served [17].
14 2.1.2 Discontinuous Reception– DRX
In DRX, the UE radio receiver is switched off from listening to radio channel for a defined period of time in cycles, called as DRX cycles, during low or no traffic to reduce energy consumption. By switching off its receiver circuitry, UE saves battery energy and increases its operating life time. For example, when DRX is applied to voice calls, it can provide more talk time for the user by keeping the UE operational for longer duration, thereby increasing user satisfaction [18]. DRX also helps to save precious radio resources over- the-air in both DL and UL directions by freeing the channel resource and increase the overall system capacity [19]. However, energy saving via DRX comes at a cost such as extended delay in RX of data and delay in network re- entry for the UE [19]. DRX is configurable and can be turned ON/OFF by eNB via RRC signaling towards the UE. DRX is specified on both MAC [20] and RRC layers in the protocol stack [21].
2.2 DRX Cycle
LTE DRX cycles were specified by 3GPP in [22]. A typical DRX cycle consists of alternating periods of ON and OFF time periods with predefined duty cycles [20], as shown in Figure 2.3. A DRX cycle length in physical terms is basically the number of radio frame/sub-frame time durations that the UE sleeps for.
DRX Cycle TON
UE shall monitor PDCCH
TOFF TInActivity
Figure 2.3: LTE DRX cycle
15 DRX cycle is controlled by a set of timers [22]. These timers govern the ON and OFF time periods of a DRX cycle.
1. TON: Specifies the minimum time duration or the number of sub- frames that the UE is required to monitor the PDCCH every 1 ms to check if there is any resources allocated for transmission in either DL or UL.
2. TInActivity: Specifies the number of consecutive sub-frames that UE shall stay active and monitor PDCCH further, following a successful decoding of a PDCCH and presence of respective UE scheduling information, if any. This timer is started/restarted every time when the UE successfully decodes a PDCCH and if it is scheduled, indicating a new data transmission. This helps the network to keep the UE awake beyond the ON time period, TON, when the data is buffered in the network.
3. TOFF: Specifies the time period or the number of sub-frames that the UE shall sleep where it is required to turn-off its receiver circuitry.
16 A flow chart illustrating different steps in DRX cycle is as shown in Figure 2.4.
UE sleep
DRX cycle expires ?
Turn ON UE RX
Decode PDCCH
ON duration
expiries ? UE sleep
InActivity timer expires ? Decode PDCCH Start InActivity
timer No
Yes
No Yes
No Yes
No No
Yes
UE scheduled Yes with data ?
UE scheduled with data ?
Figure 2.4: LTE DRX cycle flow chart Source: [23]
17 Depending on how often the UE is allowed to monitor the PDCCH, the DRX cycle lengths are divided into short-DRX and long-DRX cycle. Different DRX cycle transitions are as shown in Figure 2.5.
UE Active
Short DRX Cycle
Long DRX Cycle
UE Idle RRC_CONNECTED
RRC_IDLE
Figure 2.5: LTE DRX cycle transitions
When the UE is in active mode, a short-DRX cycle is applied during short intermittent gaps between data transmission. Following a short DRX cycle, if there are any prolonged intermittent gaps between data transmission, a long-DRX cycle is applied. Also, depending on the configuration, it is possible for the UE to move directly from active mode to long-DRX cycle. The long-DRX cycle is a multiple of short-DRX cycle duration [21]. After the long- DRX cycle, if there is no further data transmission, then eNB commands the UE to idle mode. During either short-DRX or long DRX-cycle or idle mode, if UE receives a data packet then it switches back to active mode.
18 As per the 3GPP specification [21], the values for short-DRX cycles in number of sub-frames (sf) and corresponding time duration are shown in Table 2.1.
DRX Cycle Number of sub-frames DRX cycle length in time [ms]
Short-DRX-Cycle sf2, sf5, sf8, sf10, sf16, sf20, sf32, sf40, sf64, sf80, sf128, sf160, sf256, sf320, sf512, sf640
2, 5, 8, 10, 16, 20, 32, 40, 64, 80, 128, 160, 256, 320, 512, 640
Table 2.1: LTE Short-DRX cycle length values
DRX cycle length values and other configuration parameters are applied by the eNB depending upon the traffic situation, delay sensitiveness of different applications and service subscription by the user, e.g. QoS. Please refer [21] for more details about DRX configuration parameters and their respective values.
2.3 DRX in different UE Modes
In LTE, a UE can be in different modes [19], e.g., DETACHED or ATTACHED (RRC_CONNECTED and RRC_IDLE). Initially when the UE is powered ON, it starts in DETACHED mode where it has no information about the network and also the network is unaware of the UE. For a UE to have network access, it should be first attached to the network by initiating a random access procedure towards an eNB [17]. In simple terms, for any data transmission to happen between UE and the network, there should be a radio connection between UE and eNB. The RRC protocol offers control plane functionality for establishing and tearing down radio connections. Depending on whether there is an RRC connection or not, UE can be in either of two modes, RRC_CONNECTED or RRC_IDLE mode [21] as shown in Figure 2.6.
19 RRC_CONNECTED
RRC_IDLE
eNodeB to UE RRC connection released UE to eNodeB RRC connection established
Figure 2.6: UE RRC_CONNECTED and RRC_IDLE mode transition
In RRC_CONNECTED mode, UE is known to both LTE core and radio network elements respectively. In this mode, the UE has a C-RNTI with a RRC connection towards specific eNB. Therefore UE is able to receive and send unicast data [21]. In RRC_IDLE mode, UE is put to sleep by eNB due to no active data transmission and therefore UE has no radio connection towards specific eNB. But the UE is still registered in the LTE core network element MME. Therefore the UE is visible to the network only at the granularity of TA and not at the eNB level [22] and hence UE can receive a paging message from all eNBs in the TA. But if the UE wishes to initiate an uplink transmission, then UE requests for a radio resource from eNB and if granted it moves to RRC_CONNECTED mode. DRX is applicable in both RRC_CONNECTED and RRC_IDLE mode.
2.3.1 DRX in Connected Mode
In connected mode, DRX is applied when a UE is active and there are inactive or idle periods during the packet arrival process: i.e., when there are no new or outstanding packets at the eNB. In this mode, the short-DRX and long-DRX cycle lengths determine how frequently the UE monitors the PDCCH for any possible downlink transmission or in other words how long the UE sleeps. An illustration of DRX for UE in connected mode is as shown in Figure 2.7.
20
ON duration OFF duration
short DRX cycle short DRX cycle long DRX cycle continuous reception short DRX cycle
scheduled data reception
Figure 2.7: UE DRX in connected mode
In the ON duration the UE monitors the PDCCH to check for any possible scheduled transmission. And in OFF duration, the UE switches most of its receiver circuitry to save its battery energy. When in ON duration, if any transmission is scheduled, UE starts an inactivity-timer and searches for C- RNTI id on the PDCCH in every sub-frame (i.e., every 1 ms). When inactivity- timer expires, the UE may enter the OFF period within short or a long DRX cycle based on the configuration. The transition from a short to a long DRX cycle is controlled by the short DRX cycle timer. Connected mode DRX is defined in [20].
2.3.2 DRX in Idle Mode
In idle mode, DRX is applied when there is no scheduled transmission to the UE. As there is no data transmission between eNB and UE, the UE wakes up to search for P-RNTI id in PDCCH only at pre-defined instants of time, PO, to check for any possible incoming scheduled transmissions on the PCH via paging message [19] [17]. When in idle mode, the UE is configured to monitor the PDCCH for P-RNTI only at pre-defined instants of time as per DRX cycle length. In idle mode, DRX cycle is referred to as paging DRX cycle due to the fact that UE is paged for incoming notifications. The default paging DRX cycle lengths are 0.32, 0.64, 1.28 and 2.56 seconds respectively [21]. Idle mode DRX is defined in [24].
21
22
3 Paging Mechanism in LTE
Paging procedure is initiated in the core network to inform UEs in both connected and idle modes about an incoming session or event (e.g. inform UE to reattach in case of network failure, to start a CS fallback procedure for mobile terminating call, incoming PS call or message, trigger UE to acquire latest SI, deliver ETWS alerts etc.) [22]. For an idle mode UE, the core network element MME initiates the paging procedure by sending paging message to every eNB in the TA, as shown in Figure 1.1. Also, note that the each eNB can contain cells belonging to different tracking areas, but each cell can belong only to one TA, as shown in Figure 3.1 [22].
Cell-2 (TAI-2)
Cell-3 (TAI-1) Cell-1
(TAI-1)
Cell-4 (TAI-2)
Cell-5 (TAI-2)
Cell-6 (TAI-3) Cell-7
(TAI-3) Cell-8
(TAI-1)
Cell-9 (TAI-3)
MME
Figure 3.1: Cells and TA grouping
Paging message is delivered by eNB to UE at predefined instants of time in a cyclic manner, called as paging DRX cycle. The UE wakes up at these defined time instants to monitor and receive a paging message, if any. The time instants are called PF and PO. PF is equal to a radio frame and PO is equal to a sub-frame within the radio frame. The radio frames are numbered in sequential order from 0 (zero) to 1023 and are called SFN (10 bits). The Figure 3.2 shows a layout of single SFN cycle.
23
SFN cycle length
SFN-0 SFN-1 SFN-1023
0 1 2 3 4 5 6 7 8 9
Sub-Frame
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
Radio Frame
Figure 3.2: SFN cycle
3.1 LTE Network Elements in Paging Mechanism
A simplified LTE network model with different network elements mainly used in the paging procedure is illustrated in the Figure 3.3.
eNB
MME / S-GW MME / S-GW
eNB
eNB
E-UTRAN EPC
Figure 3.3: LTE network elements involved in paging procedure Source: [22]
24 The LTE network elements involved in paging functionality are S-GW, MME and eNB. The interconnection between these network elements is as shown in the Figure 3.3. S-GW is responsible for terminating the interfaces towards E-UTRAN and inter-eNB handovers. MME controls and manages the mobility of the UEs in the E-UTRAN. eNB provides radio interface and connection between UE and the core network. eNB main responsibilities are radio resource control, management, measurements, scheduling and data transmission.
The paging functionality is split across the network elements S-GW, MME and eNB [22]:
S-GW:
Maintains and monitors the UE context information during UE idle mode.
Buffers idle mode downlink sessions and generates paging requests for the UE.
Initiates network triggered service request procedures.
MME:
Manages TA list for UEs in idle mode.
Manages the reachability of idle mode UEs via initiation and management of paging procedures.
Manages re-transmissions of paging messages due to failure or paging response timeouts.
eNB:
Schedules and transmits paging messages originating from MME.
3.2 LTE Channels Involved in Paging Procedure
Further down the network elements, several LTE channels are involved in the paging procedure. LTE channel structure is hierarchical with different channel types: Logical, Transport and Physical. Each channel in the lower layer offers different services to upper layers via SAP between the layers, as shown in Figure 3.4 [17].
25
IP packet
PHY MAC RLC PDCP
eNB RRC
IP packet
PHY MAC RLC PDCP
Transport channels
Logical channels
Radio bearers
UE
Physical channels RRC
Figure 3.4: LTE protocol and channels interface
Logical Channels:
The logical channels are used by the MAC sub layer to provide data transfer services to RLC layer. The services in these channels are characterized by what type of information is transferred [22].
Transport Channels:
The transport channels are used by the physical layer to offer information transfer services to MAC and upper layers. The services in these channels are characterized by how and with what characteristics the data is transferred over the air interface [22].
Physical channels:
The physical channels carry the actual information to the higher layers further on. The information in the physical channels is divided into time-frequency grid entities called as resource elements and blocks respectively [17].
26 The channels involved in the paging procedure are highlighted in Figure 3.5. Please refer [22] for more details about different LTE channels.
PCCH BCCH CCCH DCCH DTCH MCCH MTCH
PCH BCH UL-SCH DL-SCH MCH RACH
PUSCH PDSCH PMCH PRACH
PDSCH PBCH
Logical channels
Transport channels
Physical channels Physical Layer
MAC Layer
PDCCH Control DCI
Information
Figure 3.5: LTE channels involved in paging procedure
PCCH is a downlink control channel that supports transfer of paging information to all the registered UEs in the eNB area and also notifies the changes in SI.
PCH is related to PCCH. PCH is used for broadcasting the information over the eNB coverage area and supports UE DRX.
PDCCH carries the transport format and any resource allocation of the PCH.
PDSCH is used for delivering the actual user information and is used by the PCH.
3.3 LTE Idle Mode Paging Procedure
In idle mode, MME fully manages the UE mobility and the paging procedure [21]. MME paging procedure is defined in 3GPP via [25], [24], [22].
In idle mode, UE satisfies the following conditions.
UE does not have any NAS signaling connection with the core network.
No RRC connection between eNB and UE.
UE location is known to only to MME at TA granularity and not at the eNB level.
UE is registered with MME and has an EMM context in the MME.
