DRX Power Saving LTE

IEEE Communications Magazine • June 200948 0163-6804/09/$25.00 © 2009 IEEE

INTRODUCTION

The evolving fourth-generation (4G) wirelesstechnologies, such as long term evolution (LTE)of Universal Mobile Telecommunications System(UMTS) and WiMAX offer high bandwidth fordata transfer. These high data rates over theaccess part of the network are achieved throughthe use of higher order modulation, such as 64-quadrature amplitude modulation (QAM),advanced coding techniques, convolutional turbocodes combined with advanced antenna tech-niques, such as multiple-input multiple-output(MIMO), space-division multiple access(SDMA), and so on. [1]. The receivers requirecomputationally complex circuitry that drains theuser equipment (UE)’s battery power quickly,thus limiting the use of enriched 4G services.There are various methods, such as discontinu-ous reception (DRX) [2–5] in LTE and idle/sleepmodes in WiMAX, introduced to improve UEbattery lifetime. Furthermore, DRX offers sig-nificant improvement with respect to resourceutilization, particularly for applications charac-terized by extended OFF periods. Based on theapplication type, the DRX parameters are select-ed such that the energy and resource savings aremaximized. However, the cost associated withenabling DRX modes is that there will beextended delay when the UE needs to

transmit/receive data. This may include networkre-entry in some cases. Therefore there is a needto select the DRX parameters prudently to bal-ance the cost associated with the ensuing packetdelay and the power/resource saving.

In DRX mode, the UE powers down most ofits circuitry when there are no packets to betransmitted/received. During this time UE listensto the downlink (DL) occasionally and may notkeep in sync with uplink (UL) transmissiondepending on whether the UE is registered withan evolved node-B (eNB) (radio resource control[RRC] connected) or not (RRC idle state). Fur-thermore, UE has to perform scanning of theneighboring eNB in the event of detecting signalquality degradation with respect to the servingeNB [6, 7]. If the signal quality from one ofneighboring eNBs is better than the serving eNB,UE should come out of DRX mode to performhandover (HO) if the UE is in RRC_CONNECTEDstate or perform a cell reselection if the UE is inRRC_IDLE state. UE may choose to go intoDRX once the handover/cell reselection is suc-cessfully performed. While in the RRC_IDLEstate, UE has to perform tracking area (TA)update whenever a change in TA is detected.

The rest of the article is organized as follows.A detailed description of the UE and networkfunctionalities during different DRX modes isgiven in the next section. Then the DRX modeduring the RRC_CONNECTED state is explainedin detail outlining the advantages with respect tovarious application models. Then the DRXmode during the RRC_IDLE state is described.Examples of network re-entry times are present-ed. Finally, some concluding remarks with point-ers to future evolution are presented.

DRX MODEIn LTE DRX mode can be enabled in bothRRC_IDLE and RRC_CONNECTED states. In theRRC_IDLE state, the UE is registered with theevolved packet system (EPS) mobility manage-ment (EMM) but does not have an active ses-sion. In this state the UE can be paged for DLtraffic. UE can also initiate UL traffic by request-ing RRC connection with the serving eNB.

In LTE DRX mode can also be enabled inRRC_CONNECTED state. In the RRC_CONNECT-ED state DRX mode is enabled during the idleperiods during the packet arrival process. Whenthere are no outstanding/new packets to betransmitted/received, eNB/UE may initiate theDRX mode.

ABSTRACT

Enhanced discontinuous reception mode issupported in long term evolution of 3GPP stan-dards to conserve the mobile terminal’s batterypower. Furthermore, there are additional advan-tages in using DRX, such as over-the-airresource saving on both the uplink and downlinkto increase overall system capacity. One of theenhancements over 3G wireless systems is that inLTE DRX mode can be enabled even when theuser equipment is registered with the evolvednode-B. However, there is a need to optimizethe DRX parameters, so as to maximize powersaving without incurring network re-entry andpacket delay. In particular, care should be exer-cised for real-time services. In this article thepower saving methods in both network attachedand network idle modes as outlined in LTE areexplained. The optimum criteria to select theDRX mode are defined for different applica-tions. Analytical/simulation results are presentedto show the power saving/connection reestablish-ment and packet delay.

TOPICS IN RADIO COMMUNICATIONS

Chandra S. Bontu and Ed Illidge, Nortel

DRX Mechanism for Power Saving in LTE

BONTU LAYOUT 5/14/09 10:04 PM Page 48

IEEE Communications Magazine • June 2009 49

The EPS network interfaces are depicted inactive and various DRX enabled modes in Fig.1. LTE-U_u is the new LTE air link interfacebetween the eNB and the UE. S1_c is the con-trol plane reference point between the mobilitymanagement entity (MME) and the eNB. Theserving gateway (SGW) acts as the gateway forthe evolved packet core (EPC). Similarly, thepacket data network gateway (PDNGW) acts asthe gateway to the core network. S1_u is theuser plane reference point between the eNB andSGW. S11 and S5 are the control plane refer-ence point between the MME and SGW, andthe user plane reference point between the SGWand PDNGW, respectively. As shown in Fig. 1,when the UE is in DRX enabled/RRC_CON-NECTED state, the S1, non-access stratum (NAS),and RRC connections are active. Only the dis-continuous data exchange is on the air interface.The rest of the network is unaware of the DRXoperation. When the UE is in DRX enabled/RRC_IDLE state, the S1, NAS, and RRC con-nections are removed. More details on thesemodes are covered in subsequent sections.

As shown in Fig. 2, the UE/eNB starts a timerafter successfully transmitting/receiving a datapacket. When there are no data packets for T1 safter the last transmitted/received packet, UEenters DRX mode. In this mode the UE is stillregistered with eNB (i.e., RRC_CONNECTEDstate). During this state, the UE does not listento the DL all the time, but wakes up only peri-

odically to listen to the DL transmission fromeNB. When UE is not listening to the DL trans-mission, most of its circuitry is turned off. TheUE battery saving depends on the DRX parame-ter settings. DRX parameters in this mode areprovided by the eNB during the radio bearersetup.

When there is no transmission/reception ofpackets for an extended period of time, say T2 s(typically T2 > T1) after the successful transmis-sion/reception of a packet, the eNB may initiateRRC connection release. In this mode eNBremoves the UE context and informs MMEabout the UEs RRC_IDLE state. MME keepsthe UE’s context. Similarly, SGW keeps theUE’s user plane context, such as IP routing etc.During the RRC_IDLE mode, the UE does notkeep the time synchronization with the UL trans-mission. When DRX is enabled, the UE in theidle mode listens to the DL broadcast transmis-sion periodically, thus extending its battery life.

DRX IN RRC_CONNECTED STATEDRX mode can be enabled in RRC_CONNECTEDmode if there is no traffic for longer than a spec-ified timer, T1, which is the DRX inactivitytimer. Optionally on the DL, eNB may sendDRX Command MAC control element to the UEto initiate the DRX mode [8]. During DRX, UElistens to the current subframe and the followingTON – 1 subframes for PDCCH, and then enables

�� Figure 1. Network architecture.

PDNGW

SGW

eNB

LTE-U_u

S5

MME

UE

ACTIVE

S1_u

S1_c

S11

NAS

PDNGW

SGW

eNB

LTE-U_u

S5

MME

UE

DRX in RRC/EMM

connected

S1_u

S1_c

S11

NAS

PDNGW

Active interfaceDRX enabled interfaceLogical interface

SGWUser plane relatedUE context is kept

Control plane relatedUE context is kept

eNB

UE contextis removed

Evolvedpacketcore

Corenetwork

LTE-U_u

MME

UE

DRX in RRC/EMM idle

When UE is not

listening to the DL

transmission, most of

its circuitry is turned

off. The UE battery

saving depends on

the DRX parameter

settings. DRX

parameters in this

mode are provided

by the eNB during

the radio bearer

setup.


the power down mode for the next Tp – TONsubframes. This procedure is repeated cyclically.When multiple data bearers are established,DRX is enabled only when all the data bearersmet their corresponding DRX inactivity timercondition. The shortest DRX cycles among allthe data bearers are followed.

UE resets the DRX mode and returns to theactive mode as soon as a packet arrival is detect-ed. However, as shown in Fig. 2, the UE takes T3s to return to active mode. The delay depends onthe length of the DRX cycle. In the DL the dif-ference between the actual arrival of the packetand the UE listening to the PDCCH results inextra delay of the new transmission. In the ULthe additional delay is a result of the bandwidthgrant from the eNB. DRX cycle has to be opti-mized to reduce T3 on DL.

For each radio bearer, the DRX parametersare defined during the bearer setup procedure.While the UE enters the DRX mode, optionallya short DRX cycle is applied over a predefinedtime before enabling a constant long DRX cycle.This is to reduce the UE wake up time in case ofunexpected data arrival immediately after theDRX cycle is enabled. The provisioning of ashort DRX cycle is mostly dependent on thecharacteristics of the application packet arrival.

The DRX parameters associated with eachdata bearer are as follows [8]:

•DRX inactivity timer (T1) indicating the timein number of consecutive subframes (without thescheduled traffic) to wait before enabling DRX.This timer is reset to zero and enabled immedi-ately after successful reception of PDCCH(resource grant or allocation). When the timerreaches the advertised value for the radio bear-er, the UE initiates the DRX.

•Short DRX Cycle (Tp_S) is the first DRXcycle to be followed after enabling DRX. Proba-ble short DRX cycles are 2n, n = 1,…,9 and5*2n, n = 1,…,6 in terms of subframes or mil-liseconds.1

•DRX Short Cycle Timer (Ns) is expressed innumber of short DRX cycles. This parameterindicates the number of initial DRX cycles tofollow the short DRX cycle before transitioningto the long DRX cycle.

•Long DRX cycle (Tp_L) is the DRX cycle tobe followed after Ns DRX cycles. The definedDRX cycles shall be cyclic with respect to 10,240subframes. For this condition to be valid, theDRX cycle should be of the form 2n or 5*2n forinteger n. The allowed long DRX cycles are 2n,n = 5,…,11 and 5*2n, n = 1, … ,9 in terms ofsubframes or milliseconds.

•ON duration timer (TON) is the number offrames over which the UE shall read the DLcontrol channel every DRX cycle before enter-ing the power saving mode. TON is less than Tp_Land Tp_S. The allowed TON values in number ofsubframes (or milliseconds) are 1, 2, 3, 4, 5, 6, 8,10, 20, 30, 40, 50, 60, 80, 100, and 200.

•DRX offset (TOffset) is used to obtain thestarting subframe number for DRX cycle, Tp, insubframes, with respect to 10,240 subframes ormilliseconds. That is, DRX is enabled startingwith the frame that satisfies the conditio(SFN*10 + n)%Tp = TOffset, where Tp is equalto Tp_S and Tp_L for short DRX cycle and longDRX cycle, respectively. SFN and n representthe radio frame and subframe number, respec-tively (0 ≤ SFN ≤ 1023 and 0 ≤ n ≤ 9). When thecondition outlined by the above equation is met,UE listens to the current subframes and the fol-lowing TON – 1 subframes for PDCCH, and thenenables the power down mode for the next Tp –TON subframes.

•Retransmission timer (TR) indicates the maxi-mum number of subframes the UE should waitbefore turning off the circuits if a retransmissionof data is expected from the eNB. That is, whenretransmissions are expected, TON is extended.

RRC configures the DRX related parametersto optimize the UE power savings and UE wakeup time from the DRX mode. Since variousapplications have varying delay sensitivity, RRCchooses DRX parameters based on the qualityof service for each application.

In the ensuing sections a mathematical for-mulation is derived to give insight into the delayperformance in the DL.

DELAYIf the interpacket arrival times, υ, follow anexponential distribution with a mean of 1/λ ms,

IEEE Communications Magazine • June 200950

�� Figure 2. DRX states in RRC connected/idle modes.

ACTIVEmodeDRX

IDLEmodeDRX

Data packets

RRC_connectedACTIVE

RRC_connectedACTIVE

RRC_connectedDTX/DRX

RRC_connectedACTIVE

RRC_connected

ACTIVE

RRC_connected

ACTIVE

Networkreentry

Paging/ULtransmission

RRC_connectedDTX/DRX

DRX enabled inRRC_connected

mode

eNB initiatesRRC

connectionrelease

T3

T1 T2

T4

1 At the time of writingthis article, the standarddoes not specify these val-ues. These are reasonablevalues suggested by theauthors.

RRC configures

the DRX related

parameters to

optimize the UE

power savings and

UE wake up time

from the DRX mode.

Since various

applications have

varying delay

sensitivity, RRC

chooses DRX

parameters based on

the quality of service

for each application.



the extra delay, d (in milliseconds), causedbecause of active mode DRX can be computedas d = mod(υ,Tp). The probability distributionof the extra delay, d, can be expressed as follows:

(1)

where ai is the ith order-n root of unity, andpd(k)is the probability that the extra delay isequal to k subframes.

ENERGY SAVINGThe energy UE saves because of the DRXmechanism can be expressed as follows:

It is assumed that there are M frames duringwhich the UE is in the DRX mode and N framesduring which the UE is in the normal operationmode. Furthermore, we assume that the energyspent per frame is Esleep and Eawake, respectively,during the sleep and normal modes. The ratio ofEawake and Esleep is directly related to the numberof circuits powered down during the DRX mode.

The packet delay (95th percentile), as derivedin the previous section, is plotted as a function ofthe percentage UE energy savings in Fig. 3. Herewe assume that the packets are of fixed size, andthe eNB allocates enough resources to transmitthat packet within one subframe. This assump-tion is to make the analysis independent of thequality of service allocated to the user as well asthe type of application. Throughout this article,75 percent energy is assumed to be saved duringthe OFF time. The results show that the packetdelay increases exponentially with the UE energysavings. Various DRX cycles indicated on theplot show that the packet delay increases rapidlywhen the DRX cycle is greater than 80 sub-frames. This result is true for various ON dura-tion timer settings.

RESOURCE UTILIZATIONThe UE in DRX mode is not expected to sendthe channel quality indicator (CQI) and sound-ing reference symbol (SRS) over the UL. Theperiodic CQI and SRS assignments to the UEcan be allocated to other UE. For example, if NUEs are registered with eNB (i.e., RRC con-nection is established), and N1 of those UEsare expected to be actively transmitting packetswith probability > 99 percent at any given time,the UL channel bandwidth is allocated to CQIand SRS based on N1 UE. In the absence ofDRX, the UL bandwidth is allocated based onN users.

DRX FOR DIFFERENT APPLICATIONSVoice over Internet Protocol — For voiceover Internet Protocol (VoIP), since it is a bidi-rectional connection, the DRX is enabled onlywhen the UE need not send or receive the pack-et. If we consider the well-known ON/OFF VoIPmodel, there seems to be no chance of enabling

DRX and thus no power savings. However, LTEallows another way of enabling DRX. VoIP ischaracterized by the periodic arrival of fixedlength packets for the duration of talk spurt.One way of enabling the DRX is to exploit thischaracteristic. Immediately after sending a pack-et, the eNB instructs the UE to go into DRXmode. The DRX cycle should be set such thatthe next packet is scheduled when the UE wakesup to read the PDCCH message on DL.

Assuming that the VoIP packets are arrivingat 20 ms and the power saving (neglecting theretransmissions, etc.) is approximatelyº 60 to 70percent.

Video Streaming — Video streaming is charac-terized by fixed video frame rate (e.g., 10 frames/s) and within the frame there are fixed numberof packets of varying sizes [9]. The interpacketdelay may vary based on the video coder delay.The received packets are buffered and passed onto decoder at the receiver end. Simulations areperformed based on video streaming model pro-posed in [9]. The energy savings are measuredacross multiple video streaming sessions andplotted against 95 percent packet delay. Thelong DRX cycle is fixed at 100 ms, and the shortDRX cycle is varied between 12, 25, and 50 sub-frames. For video traffic the guaranteed packetdata rate affects the packet delay. If the datarate is too low, the time for DRX reduces asshown in Fig. 4a. Figure 4b shows the packetdelay at 95 percent as a function of data rate fordifferent DRX cycle settings. At higher datarates, the short DRX cycles does not affect thepacket delay performance because the DRXopportunity is increased by sending the data toofast. The short DRX cycle can be used efficientlyas a tool to shape the packet delay distribution.An efficient way to enhance the DRX perfor-mance is to increase the DRX cycle exponential-ly from the short DRX cycle to the long DRX

percentage energy savingssleep awake=

+ME NE

M( ++∗

N E).

awake

100

p kT a a e

dp i

kii

Tp

( ) ,=−( )−

=

−

∑1

10

1λ

λ

�� Figure 3. Percentage of energy saving vs. delay for different values of fixedsleep window sizes.

Energy saving (%)100

0

50

95 P

erce

ntile

pac

ket

dela

y (m

s)

100

150

200

250

300

350

400

450

500

20 30

TD = 512

TD = 320

TD = 256TD = 160

TD = 128TD = 80

TD = 64

TD = 40

TD=32TD = 20

TD = 10

40 50 60 70 80

TON = 1TON = 2TON = 5TON = 10

2 Assuming 1 subframefor ramping up and 1 sub-frame for ramping downthe circuitry.



cycle in multiple steps.

DRX IN IDLE STATEWhen the UE does not have packets to bereceived and/or transmitted for an extendedperiod of time, the eNB may initiate the releaseof UE’s RRC connection and request MME torelease the UE’s S1 connection. Furthermore,eNB removes the UEs context from thedatabase. MME and SGW only remove the eNBspecific part of the UE context. During the idlemode, the UE wakes up periodically to listen tothe DL transmissions, following the DRX cycle.

During the idle mode, the mobility is fullycontrolled by UE, since the network is not awareof the UE existence continuously. UE shouldperform the signal quality measurements withrespect to the serving and neighboring eNBsaccording to measurement thresholds recom-mended by the serving eNB. Based on the signalquality measure, the UE selects a new servingeNB when UE moves away from the currentserving eNB. When the system informationadvertised by the new serving eNB does notinclude its tracking area, UE will perform atracking area update to indicate its presence sothat the network knows where to page the UE incase of DL data transfer.

UE may be paged by the network when thereis data addressed to that particular UE. UEreturns to EMM_ACTIVE /RRC_CONNECTEDmode as soon as packet arrival is detected. How-ever, as shown in Fig. 2, the UE takes T4 s toreenter the network. The delay depends on thepaging DRX cycle, time to acquire UL synchro-nization, and time to set up the RRC connectionwith the eNB. For DL, the difference betweenthe actual arrival of the packet and the UE lis-tening to the PDCCH results in the extra delay

of the new transmission. For UL, the additionaldelay is as a result of the bandwidth grant fromthe eNB. The paging DRX cycle has to be opti-mized to reduce this delay, T4.

PAGING CYCLEeNB advertises the default paging cycle in sys-tem information broadcast. If desired, UE mayrequest a shorter paging DRX cycle during thenetwork attach. UE wakes up to listen toPDCCH periodically during a predefined radioframe followed by OFF time, during which mostof the circuitry is turned off.

UE wakes up when the system frame number(SFN), n, satisfies the condition mod(n,T) = TF.The frame offset, TF, is a parameter thatdepends on the UE’s international mobile sub-scriber identity (IMSI) as follows:

(2)

where j represents the mode to configure theradio frame used for paging, IMSI* representsthe shortened IMSI expressed as mod(IMSI,4096), and T represents the paging DRX cycle inradio frames (10 ms). eNB sends the page mes-sage to the UE during the predefined subframes,ip (within the assigned radio frame), which satis-fies the condition ip = mod(IMSI*, Np), whereNp = 2 or 4 for j = 6 and 7, respectively.

In Mode-0 the paging message is not sched-uled on all the radio frames. Mode-1 allows con-figuration of the paging message on any radioframe. Furthermore, in Mode-1 the paging mes-sage can be distributed across the subframeswithin the radio frame. eNB advertises T and jas part of the system broadcast parameters. Fur-thermore, we explored the possibility of pagemessages repeated over multiple subframes with-in the radio frame to increase the probability ofreception at the UE.

NETWORK REENTRYOn the DL, the UE is paged when the SGWdetects data addressed to the UE. The MMEsends the page command to all the eNBs withinthe tracking area where the UE was last seen.eNBs transmit the paging message over the airto the UE. During the paging, the DRX parame-ters are sent by the MME to eNB. eNB trans-mits the page message over the air. If UE findsits temporary mobile subscriber identity (S-TMSI) in the page message, it responds by initi-ating the random access procedure by sendingthe random access channel (RACH) preamble.If successfully received, eNB responds by send-ing a random access response granting enoughbandwidth to the UE to send the RRC connec-tion request. Once the RRC connection setup issuccessfully completed, UE sends the RRC con-nection setup complete with a transparent pay-load containing the NAS service request. eNBforwards the UE’s NAS request message to theMME. Over-the-air encryption is enabled bysending the security mode command by the eNB.Similarly, the eNB establishes the default data

T

IMSI T j

F

j j

=

= −− −( )

for 2 2 0 45 5

mod , ,

m

* Mode 0

ood , ,*IMSI T j( )

⎧⎨⎪

⎩⎪ = −for 5 7 Mode 1

�� Figure 4. Energy savings as a function of packet data rate.

User data rate/packet (kb/s)

(a)

10000

10Peow

er s

avin

g (%

)

20

30

40

50

60

200 300 400 500 600 700 800 900 1000

Ns = 1; Ts = 50Ns = 2; Ts = 25Ns = 4; Ts = 12

User data rate/packet (kb/s)

(b)

00

20

95%

pac

ket

dela

y (m

s)

40

60

80

100

100 150 200 250 300

Ns = 1; Ts = 50Ns = 2; Ts = 25Ns = 4; Ts = 12



bearer by initiating RRC connection reconfigu-ration. The detailed call flow is shown in Fig. 5.The MME retransmits the page request a pre-configured number of times if a response is notreceived from the UE. The timer for retransmis-sion should be carefully configured by measuringthe expected delay between the transmission of aPage request message from the MME andthe reception of an NAS request from theUE.

NETWORK RE-ENTRY TIMEAs shown in Fig. 5, the network reentry timeafter the kth successful retransmission of thepage message can be expressed as τk = t0 + t1 +t2 + t3 + τ1 + τ2 + kTrtx, where t0, t1, t2, and t3are defined as shown in Fig. 5, and are assumedto be constant for simplifying the analysis. τ1 andτ2 are delays associated with the paging DRXcycle and RACH process, and assumed to be

variables. k represents the number of retransmis-sions. Trtx is the retransmission timer for thepaging message from the MME.

The probability distribution function (pdf) ofthe network re-attach time, τ, can be written asfollows:

(3)

where τ represents the elapsed time betweenSGW informing the MME about the data arrivaland the resumption of data delivery. The pdf ofτk is defined as convolution of the pdfs of τ1 andτ2. We assume τ1 is a uniformly distributed ran-dom variable with mean T/2. τ2 is evaluatedbased on a RACH procedure as described in [8].Ppage is the probability of UE being paged unsuc-cessfully.

Analytical results on the network reentry

f x P P f xk

k

N

kτ τ( ) ( )_

= −( )=∑1

0page page

PAGE

�� Figure 5. Call flow for idle mode exit for DL data transfer.

Page

res

pons

e ti

me

t0

t1

t2

t3

50 ms

τ1

τ2

τ

UE

10 ms20 ms

RRC connection request

RRC connection setup

RRC connectionreconfiguration

RRC connectionreconfig complete

Security mode command

Security mode complete

RRC connection setupcomplete

Page

Page

S1 Init UE(NAS: service request)

S1 Initial contextsetup request

Initial context setupcomplete

Downlink data

Uplink data

Update bearerrequest

Update bearerresponse

Downlink datanotification

RACH preamble

RACH response

eNodeB MME SGW PDNGW eNB advertises the

default paging cycle

in system informa-

tion broadcast. If

desired, UE may

request a shorter

paging DRX cycle

during the network

attach. UE wakes up

to listen to PDCCH

periodically during a

predefined radio

frame followed by

OFF time, during

which most of the

circuitry is turned off.



times for UE paged to receive DL traffic aredepicted in Fig. 6. These results are generatedassuming that over-the-air paging messages aresuccessfully received by the UE with a probabili-ty of 0.9. The maximum page retransmissions arelimited to 4. The RACH preamble detectionerror rate is assumed be 3 percent, and the max-imum number of preamble retransmissions is setto 5. It is also assumed that UE’s preparationtime to send the RACH preamble is 2 ms. Thetime to receive RACH response from the eNB isset to 15 ms. RACH transmission backoff time isassumed to be 6 ms.

The advantage of sending the page messagemultiple times over the air is also shown. Herethe assumption is that the page messages areindependently decoded. Multiple pages perradio frame improve the reentry time significant-ly.

CONCLUSIONSThere is significant UE power saving and ULresource optimization by implementing DRXmode in both RRC_CONNECTED and RRC_IDLEstates. In particular, for applications character-ized by extended OFF periods, the power sav-ings and resource utilization are maximized.

Through prudent selection of various DRXparameters, the packet delays can be reduced.

In the RRC_CONNECTED state, based on theapplication type, the DRX mode parameters areselected such that the additional delay resultingfrom the DRX mode is minimized. For videostreaming application (10 frames/s), enablingDRX in the active mode may save about 40–45percent of UE battery power without significant-ly impacting video quality. Similarly, for VoIPapplications there is a potential saving of about60 percent.

Furthermore, by enabling short DRX cyclefor initial sleep duration will enhance the userexperience by shaping the packet delay distribu-tion as shown in the previous sections.

Also, in the RRC_IDLE state the DRX cyclecan be selected based on the user’s calling pro-file and the UE subscription status. Networkreentry time can be significantly improved bysending multiple copies of the paging messageover the air to the UE.

REFERENCES[1] 3GPP TS 36.300, “E-UTRAN Overall Description — Stage

2,” Rel. 8, v. 8.4.0, Mar. 2008.[2] H. Wu and T. Haustein, “Energy and Spectrum Efficient

Transmission Modes for the 3GPP-LTE UL,” IEEE Symp.

�� Figure 6. Idle mode exit time for different paging cycles.

Time (ms)

One page per radio frame

10000

10-2

10-3

Prob

abili

ty (

reen

try

tim

e >

abs

ciss

a)

10-1

100

2000 3000

T = 320T = 640T = 1280T=2560

Time (ms)

Two pages per radio frame

10000

10-2

10-3

Prob

abili

ty (

reen

try

tim

e >

abs

ciss

a)

10-1

100

2000 3000

Time (ms)

Three pages per radio frame

10000

10-2

10-3

Prob

abili

ty (

reen

try

tim

e >

abs

ciss

a)

10-1

100

2000 3000Time (ms)

Four pages per radio frame

10000

10-2

10-3

Prob

abili

ty (

reen

try

tim

e >

abs

ciss

a)

10-1

100

2000 3000

T = 320T = 640T = 1280T = 2560

T = 320T = 640T = 1280T = 2560

T = 320T = 640T = 1280T = 2560



PIMRC, Sept. 2007, pp. 1–5.[3] J-H. Yeh et al., “Performance Analysis of Energy Con-

sumption in 3GPP Networks,” Wireless Telecommun.Symp., May 2004, pp.67–72.

[4] S-R. Yang et al., “Modeling UMTS Power Saving withBursty Packet Data Traffic,” IEEE Trans. Mobile Comp.,vol. 6, no. 12, Dec. 2007, pp. 1398–1409.

[5] S-R. Yang andY-B. Lin, “Modeling of UMTS Discontinu-ous Reception Mechanism,” IEEE Trans. Wireless Com-mun., vol. 4, no. 1, Jan. 2005, pp. 312–19.

[6] 3GPP TS 36.304, “E-UTRA: User Equipment Proceduresin Idle Mode,” Rel. 8, v. 8.2.0, May 2008.

[7] 3GPP TS 36.331, “E-UTRA; Radio Resource Control (RRC)Protocol Specification,” Rel. 8, v. 8.2.0, May 2008.

[8] 3GPP TS 36.321, “Medium Access Control (MAC) Proto-col Specification,” Rel. 8, v. 8.2.0, May 2008.

[9] 3GPP2 C.R.1002-0, “CDMA2000 Evaluation Methodolo-gy,” Dec. 10, 2004.

BIOGRAPHIESCHANDRA SEKHAR BONTU ([email protected]) has an M.Tech.from the Indian Institute of Technology, Kharagpur, and aPh. D. from Carleton University, Ottawa, Canada, both inelectrical engineering. He joined Nortel in 1996 as part ofWireless Transport Systems. He is currently working as amobility architect in Nortel’s 4G wireless R&D organizationin Ottawa.

ED ILLIDGE ([email protected])graduated from the Uni-versity of Toronto with a Bachelor of Applied Sciencein electrical engineering. He is in the Carrier Networksstandards and architecture team covering wirelessaccess architecture. He has primarily worked in thetelecommunications field since graduation and has 19years’ experience with Nortel. He has held a variety ofroles in Nortel including field support and messagingdesign. He has worked on wireless standards, wirelessarchitecture and wireless design, seeing the architect-


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