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Chapter 20
Power Management for 4G Mobile
Broadband Wireless Access
Networks
Maruti Gupta, Ali T. Koc, Rath Vannithamby
Intel Labs, Intel Corporation
Introduction (1/3)
• The use of devices such as smart phones, tablets etc. that
offer the ease and convenience of internet applications like
Email 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.
Introduction (2/3)
• LTE and WiMAX offer high-speed data transfer and alwaysconnected 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.
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.
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 802.16e
standards (released in 2008/2006 respectively) timeframe,
application traffic was largely dominated by Web browsing,
Email, 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.
Overview of Power Management (2/3)
A session
Reading Time
First packet
of the session
Reading Time
Packet Activity
Packet Calls
Last packet
of the session
• 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.
Overview of Power Management (3/3)
time
Packets
from File 1
Packets
from File 2
Packets
from File 3
Packet Activity
Packets
from File 4
Packet Calls
Talk
Silence
Packet Activity
Silence
Packets arrive at fixed
intervals during talk period
Eg for AMR codec at 20ms
intervals
time
• 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.
Power Management in LTE
SHORT DRX Cycles
Last Packet
Served
LONG DRX Cycles
...
...
Inactivity Timer
ON
OFF
Packet
ON
OFF
IDLE
OFF Duration is calculated from OFF durations in this interval.
(Excludes Idle)
• 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.
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.
Idle Mode in LTE (2/3)
Paging Group 1
Paging Group 2
Unavailable Interval
Page Listen
Interval
Unavailable Interval
Page Listen
Interval
Page Listen
Interval
Page Listen
Interval
• 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.
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.
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.
DRX Mode in LTE (2/4)
PDCCH
decode
success
1
PDCCH
decode
success
2
3
UE awake
UE sleep
DRX Cycle
Inactivity
DRX Cycle
Inactivity
1 Start Inactivity Timer
2 Reset Inactivity Timer
3 Inactivity Timer expires and DRX starts
• 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.
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.
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.
Power Management in IEEE 802.16e (1/2)
• Two mechanism in IEEE 802.16e – 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
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
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 802.16m
• With subframe level sleep, Sleep can be supported
even for VoIP
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.
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.
Traffic Profile of Diverse Data Apps (1/2)
CDFs of Packet Interarrival Times
1
0.9
0.8
0.7
0.6
CDF
• 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.
0.5
0.4
0.3
0.2
Active User
Only Background Traffic
Active User + Background Traffic
0.1
0
0
20
40
60
80
100
120
Interarrival Time(ms)
140
160
180
200
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
CDFs of Packet Interarrival Times
1
0.9
0.8
0.7
0.6
CDF
•
0.5
0.4
0.3
0.2
Active User
Only Background Traffic
Active User + Background Traffic
0.1
0
0
20
40
60
80
100
120
Interarrival Time(ms)
140
160
180
200
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.
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 LTEAdvanced system in supporting
emerging applications efficiently in
terms of battery power and signaling
overhead.
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.
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.
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].
References
[1] 3GPP TS 36.300, v8.11.0 "Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access (E-UTRAN);
Overall description; Stage 2", Jan 2010.
[2] 3GPP TS 36.321, v8.9.0, “Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification
(Release 8)”, June 2010.
[3] 3GPP TS 36.304, v8.8.0, “Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) procedures in idle mode (Release 8)”,
January 2010.
[4] IEEE Standard for Local and metropolitan area networks Part 16: Air Interface for Broadband Wireless Access Systems, 802.16-2009, May
2009.
[5] IEEE 802.16m_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 2010.
[6] Roony Yongho Kim, Shantidev Mohanty “Advanced Power Management Techniques in Next-Generation Wireless Networks” IEEE
Communication Magazine May 2010.
[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’11Fall, Sept. 2011, pp. 1 –5.
[9] 3GPP TR 36.822 v0.2.0, “Technical Specification Group Radio Access Network; LTE RAN Enhancements for Diverse Data Applications”,
November 2011.
[10] IEEE 802.16m-08/004r5 “IEEE 802.16m Evaluation Methodology Document (EMD)” January 2009
[11] 3GPP TS 36.331 v10.3.0 “Radio Resource Control (RRC); Protocol specification” October 2011.
[12] Chandra S. Bontu, Ed Illidge, “DRX Mechanism for Power Saving in LTE”, IEEE Communications Magazine, vol. 47, no. 6, pp. 48 –55, June
2009.
[13] Per Willars, “Smartphone traffic impact on battery and networks” https://labs.ericsson.com/developer-community/blog/smartphonetraffic-impact-battery-and-networks.
[14] RP-110410 “LTE RAN Enhancements for Diverse Data Applications” March 2011, 3GPP RAN Plenary contribution.
[15] 3GPP TR 36.822 v0.2.0, “Technical Specification Group Radio Access Network; LTE RAN Enhancements for Diverse Data Applications”,
November 2011.