Chapter 20 Power Management for 4G Mobile Broadband Wireless Access Networks

1
Chapter 20Power 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 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.

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 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.

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, its 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 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

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 802.16m
• 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 wont 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 doesnt 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 doesnt 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 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 VTC08-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 VTC11-Fall,
  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/de
  veloper-community/blog/smartphone-traffic-impact-b
  attery-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.