1. Chapter 20 Power Management for 4G Mobile Broadband Wireless Access Networks
Maruti Gupta, Ali T. Koc, Rath Vannithamby
Intel Labs, Intel Corporation

2. Introduction (1/3)
The use of devices such as smart phones, tablets etc. that offer the ease and convenience of internet applications like and Web browsing on the go is widespread.
Inevitably user expectations also rise in terms of higher data rates, instant internet connectivity and a much larger variety of applications to play with.
Mobile broadband technologies such as LTE and WiMAX are what make the promise of such expectations real.

3. Introduction (2/3)
LTE and WiMAX offer high-speed data transfer and always-connected capabilities.
The high data rates in these systems are achieved through the use of higher order MCS and MIMO technology.
Higher speed data transmission or reception requires higher power consumption; this in turn drains the battery quickly.
To support battery-operated mobile devices, 4G technology has developed power-saving features that allow mobile device to operate for longer durations without having to recharge.

4. Introduction (3/3)
Power saving is achieved by turning off all or some parts of the device in a controlled manner when it is not actively transmitting or receiving data.
4G technologies define signaling methods that allow the mobile device to switch into
Discontinuous Reception (DRX) during RRC_Connected in LTE and
Sleep mode in WiMAX, and
to Idle mode when inactive both in LTE and WiMAX.

5. Overview of Power Management (1/3)
Power management schemes are designed to adapt to current and expected application traffic workloads in order to obtain the maximum the power savings.
At the time of design of LTE and the initial WiMAX e standards (released in 2008/2006 respectively) timeframe, application traffic was largely dominated by Web browsing, , File transfer, Voice over IP (VoIP) types of applications.
We show below the traffic models of the expected workloads that were used to evaluate LTE schemes to achieve power savings.

6. Overview of Power Management (2/3)
Figure shows a model of HTTP traffic, the protocol used for web browsing. Web browsing applications typically show an ON-OFF behavior which means that the network experiences packet activity for a duration of time known as ON period and then there is no packet activity for OFF period.

7. Overview of Power Management (3/3)
Figures show models of FTP traffic and VoIP traffic
In summary, the power saving mechanisms should be capable of saving power efficiently for any traffic
Furthermore, emerging data traffic patterns are different from the ones shown above.

8. Power Management in LTE
LTE specifications provide two different mechanisms for power management, namely Idle mode and DRX.
UE can enter Idle mode where UE is no longer actively connected to the eNB, though the network is still able to keep track of the UE through a mechanism known as paging.

9. Idle Mode in LTE (1/3)
UE can enter Idle mode where UE is no longer actively connected to the eNB, though the network is still able to keep track of the UE through a mechanism known as paging.
Idle mode allows the UE to remain in very low power mode since the UE needs to perform a very limited set of functions in this mode.
The UE can be paged for DL traffic. For uplink traffic, UE initiates a procedure to re-enter the network by sending a connection request to the serving eNB and re-enters into the RRC_Connected state.

10. Idle Mode in LTE (2/3)
During every paging cycle, the eNB sends out a paging message at a known period of time called as paging occasion.
UE can wake up during the paging occasion and listen to the paging message to check and see if it is being paged.

11. Idle Mode in LTE (3/3)
The paging occasion is kept very short, it’s only a few milliseconds long and it does not require the UE to be connected to the network.
During the Idle mode, the UE alternates between being completely unavailable to the network and being available for short durations during paging occasion.
UE in Idle mode performs 3 major tasks:
Public land mobile network (PLMN) selection
Cell selection and reselection
Location registration
A registration area basically allows the UE to roam freely across all the cells in it without having to perform location registration for each cell.

12. DRX Mode in LTE (1/4)
DRX can be enabled to save power by allowing the UE to power down for pre-determined intervals, as directed by the eNB.
DRX offers significant improvement on resource utilization as well as power saving. However, DRX increases the end to end delay if the parameters are not set correctly.
If the DRX cycle is kept too long there can be some scenarios where we can face with network re-entry.
In DRX, UE consumes minimal power by powering down most of its circuitry.
During DRX UE only listens periodically DL control channels.

13. DRX Mode in LTE (2/4)
DRX is triggered by means of an inactivity timer known as DRX-InactivityTimer, which can range in value from 1ms up to 2.56 sec, though the values in between are not continuous.
Whenever the UE receives any data, the DRX-Inactivity timer is reset.

14. DRX Mode in LTE (3/4)
During DRX ON period, the UE basically monitors the channel for data and control activity and the eNB is able to exchange data with the UE.
During the OFF period, the UE can go into low power mode and the eNB cannot send any data to the UE.
DRX is terminated as soon as the UE either sends UL data or receives DL data.
In LTE, DRX cannot be enabled during an active data exchange without restarting the DRX-Inactivity timer.

15. DRX Mode in LTE (4/4) LTE supports the notion of ShortDRX and LongDRX.
ShortDRX basically allows the UE to have a shorter DRX cycle and it is also limited to a pre-determined number of cycles only.
If no data is exchanged during the ON period of the shortDRX cycles, only then does the UE transition to LongDRX.
LongDRX cycle may be much longer than shortDRX cycle thus allowing the UE to gain greater power savings.
ShortDRX was introduced to reduce delays in case data activities were to occur very soon after initiation of DRX.

16. Power Management in IEEE 802.16e (1/2)
Two mechanism in IEEE e – Idle and Sleep
Idle Mode
Mobile station will be de-register from base station
Mobile station will stay in Idle mode from a few seconds to several minutes
In Idle, MS alternates between periods of Paging Unavailable and Paging Listening Intervals
In order to contact an MS, BS will send a broadcast message to the MS (exit Idle Mode)
A number of BSs are grouped over a contiguous geographical region to make paging group
Paging message is send to all the BSs in the paging group, this will allow the Idle user to move around in a bigger geographical region
It requires network entry to move from Idle mode to Connected mode

17. Power Management in IEEE 802.16e (2/2)
Sleep Mode
For MSs in connected mode, sleep mode conserves while still exchanging data
MS shut itself down for some pre-negotiated interval of time but unlike Idle mode it is still connected to BS
MS can wake up quickly from Sleep mode because it is already connected to network
MS alternates between periods of Sleep Windows and Listen Windows
For each MS, base station needs to keep context about Sleep/Listen Windows which is called Power Saving Class (PSC)
Mobile station saves power during Sleep Windows
MS can support multiple PSCs

18. Power Management in IEEE 802.16m
Sleep Mode enhancements
MS can only support single PSC
Listen window can dynamically be changed
MS can define multiple PSCs and depending on the traffic it switches from one PSC to another.
Subframe level sleep is supported with new frame structure of m
With subframe level sleep, Sleep can be supported even for VoIP

19. Implementation Challenges (1/2)
Main challenge of power saving is to balance the trade-off between user experience and power consumption
Main challenge of Idle mode is to minimize the signaling overhead due to paging/network re-entry and set an optimum paging group size to minimize the location updates
Main challenge of DRX mode is to accommodate latency and throughput requirements of different applications.
A single DRX parameter set won’t be enough for different type of applications. For example VoIP and FTP traffic have different latency requirements.
For low power consumption, it would be nice to have a long DRX cycle. However, long DRX cycle can cause excessive delay and bad user experience.

20. Implementation Challenges (2/2)
Users needs to periodically align their uplink and downlink timing; having a long DRX may cause some synchronization issues.
Power saving mechanisms need to coexist with other MAC operations
Handover
HARQ
Scanning
Multi – RAT (Bluetooth)
Conflicting requirements from each MAC operations result in a complex optimization problem for finding the optimum power saving mechanism.

21. Traffic Profile of Diverse Data Apps (1/2)
Figure shows the CDF of packet inter-arrival times for 3 different cases, namely a user running an active session, a user running background traffic and a user running an active session in addition to background traffic.
Here background traffic refers to the autonomous exchange of user plane data packets between the UE and the network.
There is a substantial difference between packet activity patterns, particularly between a user running an active session vs. a user running only background traffic.

22. Traffic Profile of Diverse Data Apps (2/2)
We observed that it doesn’t make much difference when applications run in background with an active session in place. The active session dominates the CDF.
We can infer from Figure that the amount of background traffic generated by the emerging applications is not insignificant, and furthermore, the behavior of background traffic is different from the active traffic.
If the background traffic is not handled efficiently in the next generation of the mobile broadband, it can drain the battery power and create excessive signaling overhead

23. Signaling Overhead due to Diverse Data Apps (1/2)
Figure shows the ratio of signaling overhead for a user running an active application session
We can observe that the ratio of Data exchanged to the signaling overhead is around 10,000
Active user change states around 5-6 times per minute.

24. Signaling Overhead due to Diverse Data Apps (2/2)
Figure shows the ratio of signaling overhead for a user running multiple applications running in background.
We can observe that the ratio of Data exchanged to the signaling overhead is around 180.
Basically a lot more signaling is used to send a lot less data.
The initial studies and observations led to focus on application background traffic in order to enhance the LTE-Advanced system in supporting emerging applications efficiently in terms of battery power and signaling overhead.

25. Enhancement for Diverse Data Applications (1/2)
The eDDA work item in 3GPP considers enhancements in the following areas:
Mechanisms to improvements on the system efficiency for background traffic with using existing RRC states.
Mechanisms to reduce UE power consumption for background traffic with using existing RRC states.
DRX enhancements to achieve optimum trade-off between performance and UE power consumption for single or multiple applications running in parallel.

26. Enhancement for Diverse Data Applications (2/2)
DRX enhancements to improve adaptability to time varying traffic profiles.
Improve system resource efficiency for connected mode Ues.
Improve control signal overhead for larger UE population in connected mode.
Improve power consumption and reduce signaling overhead using mechanisms that leverage on the assistance from UE and network.

27. Conclusion
4G mobile broadband systems are very attractive for smart devices that demand always-connected capability. This capability of 4G doesn’t allow the device to be in low power mode as much as it would like to.
This chapter describes the details of the power efficient mechanisms incorporated in 4G standards.
This chapter also points out the inefficiencies in the power efficient mechanisms incorporated in the original 4G standards in supporting emerging diverse data applications such as social networking, IM, etc.
This chapter also addresses the state of the art technologies that are currently being explored in 3GPP standards body in supporting emerging applications under a work item namely “Enhancements for Diverse Data Applications.”
Research outputs from various industries in this area are captured in [15].

28. References
[1] 3GPP TS , v "Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access (E-UTRAN); Overall description; Stage 2", Jan [2] 3GPP TS , v8.9.0, “Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification (Release 8)”, June [3] 3GPP TS , v8.8.0, “Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) procedures in idle mode (Release 8)”, January [4] IEEE Standard for Local and metropolitan area networks Part 16: Air Interface for Broadband Wireless Access Systems, , May [5] IEEE m_D7, “DRAFT Amendment to IEEE Standard for Local and metropolitan area networks Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems, Advanced Air Interface”, July [6] Roony Yongho Kim, Shantidev Mohanty “Advanced Power Management Techniques in Next-Generation Wireless Networks” IEEE Communication Magazine May [7] L. Zhou, H. Xu, H. Tian, Y. Gao, L. Du, and L. Chen, “Performance analysis of power saving mechanism with adjustable DRX cycles in 3GPP LTE,” in Proc. IEEE VTC’08-Fall, Sept. 2008, pp. 1 –5. [8] S. Gao, H. Tian, J. Zhu, and L. Chen, “A more power-efficient adaptive discontinuous reception mechanism in LTE,” in Proc. IEEE VTC’11-Fall, Sept. 2011, pp. 1 –5. [9] 3GPP TR v0.2.0, “Technical Specification Group Radio Access Network; LTE RAN Enhancements for Diverse Data Applications”, November [10] IEEE m-08/004r5 “IEEE m Evaluation Methodology Document (EMD)” January 2009 [11] 3GPP TS v “Radio Resource Control (RRC); Protocol specification” October [12] Chandra S. Bontu, Ed Illidge, “DRX Mechanism for Power Saving in LTE”, IEEE Communications Magazine, vol. 47, no. 6, pp. 48 –55, June [13] Per Willars, “Smartphone traffic impact on battery and networks” [14] RP “LTE RAN Enhancements for Diverse Data Applications” March 2011, 3GPP RAN Plenary contribution. [15] 3GPP TR v0.2.0, “Technical Specification Group Radio Access Network; LTE RAN Enhancements for Diverse Data Applications”, November 2011.