Packet Scheduling for Real-Time Communic

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Packet Scheduling for Real-Time Communication

over LTE SystemsA Comparative Study of Dynamic and Semi-Persistent Scheduling Schemes

Avishek Patra

RWTH Aachen University

Aachen, Germany

[email protected]

Volker Pauli, Lang Yu

Nomor Research GmbH

Munich, Germany

{pauli, yu}@nomor.de

Abstract— With various packet-switched networks coming tothe fore, real-time services like voice and video, transmitted

traditionally using circuit-switched bearers, can have limitedcapacity due to the limited availability of resource-granting

control channels. Such packets are frequent and require more

grants compared to other services like FTP. To compound the

issue, often these packets are large in size compared to available

resources for allocation. To improve the capacity of real-time

communication over LTE (-A), various scheduling methods arebeing studied. However, often the packet sizes are unaccounted

for by these studies. This work deals with the development ofsemi-persistent scheduling (SPS) algorithms based on wide-bandtime-average SINR information for resource allocation to voice

traffic users, with a focus on large packets. A comparative study

between dynamic scheduling (DS) and developed SPS algorithms

is done to determine the suitable scheduling mechanism for voicepackets transmission over LTE (-A) systems in the downlink.

Keywords—semi-persistent scheduling; dynamic scheduling;VoIP; Voice over LTE; radio resource management

I. INTRODUCTION

Over the past decade, there has been a path-breaking shiftin the methods of communication, from just voice–basedcommunication to voice– and data–based communication incellular networks with the global mobile data usage doublingevery year since 2006 [1]. To cater to the ever-increasingdemand for higher data rates, various data-centric standardssuch as Long Term Evolution (LTE) and Long TermEvolution – Advanced (LTE-A) standards from 3GPP havecome up. To meet the required capacity and coverage, variousRadio Resource Management (RRM) algorithms such asHybrid ARQ (HARQ), Link Adaptation (LA), ChannelQuality Indication (CQI) and Packet Scheduling (PS) areimplemented in LTE (-A). One of the main feature of LTE (-A) is that it is supported by packet switched bearers instead ofcircuit switched bearers. Hence, transmission of voice requires

a VoIP-based solution in LTE (-A) that would have at leastsame coverage and capacity as 2G and 3G networks [2].

For data packets in LTE (-A), allocation of resources aredone by schedulers in a dynamic fashion. Based on theimmediate requirement of the user, allocation of resources isdone per Transmission Time Interval (TTI). Physical

Downlink Control Channels (PDCCH) plays the major role insignalling resource allocation. Dynamic allocation works wellfor data packets, which are infrequent and non-periodic.Compared to this, voice and video traffic are bursty in natureand consists of periodically repeating packets and silenceperiods. Problem arises with respect to real-time packetscheduling when multiple users try to access resources in a

single TTI as the allocations of new grants are limited to thenumber of available PDCCHs. To compound the problem,real-time packets are often large and available PhysicalResource Blocks (PRBs) may not be able to accommodatethem. Although to overcome the scheduling problem, differentpersistent, semi-persistent and modified dynamic resourcescheduling algorithms have been studied [3, 4], often thepacket sizes are not taken into consideration.

In this work, we aim to study the capacity achievable byfull dynamic scheduling and semi-persistent scheduling for thedownlink case in multi-user LTE (-A) scenario for voicecommunication. For this, two novel semi-persistent schedulingalgorithms have been proposed, which partition large packetsbefore transmitting them. Based on the partitioning method,these leftovers are transmitted dynamically or semi-persistently. Also, contrary to previous works, which useinstantaneous CQI information, this work is based on time-averaged wideband SINR information.

The work is organised as follows: Section II brieflydiscusses the different scheduling schemes and importance ofpacket size consideration in scheduling. Section III elaboratesthe semi-persistent scheduling algorithms developed. InSection IV, the different simulation specifications andscenarios are explained and the results obtained from thecomparative study between semi-persistent and dynamicscheduling methods are presented. Finally, we conclude thework in Section V.

II. BACKGROUND CONCEPTS

A. Voice over LTE

In voice traffic, the voice packets are periodic in naturewith a period of inter-packet arrival interval. Although voicedata rates are low compared to data traffic, being real-time,transmission of voice over LTE (VoLTE) system is highly

978-1-4799-0543-0/13/$31.00 ©2013 IEEE


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sensitive to transmission latency and loss of packets. Thus,even though packet switching may increase resource usageefficiency through multiplexing, it has the limited capability ofguaranteeing the required QoS. E-UTRAN is expected to meetthe capacity limit set by the previous 2G and 3G standards. Inthis context, a satisfied user is defined as the user receiving98% of the packets within the acceptable end-to-end delay –also known as the delay budget [5]. For a VoLTE system, thecapacity is defined as the number of users served in the cell

such that 95% of the users are satisfied [8].To meet these requirements, robust packet scheduling is

required. PDCCH is important in scheduling as it providesuser equipments (UEs) information about the scheduledchannels – primarily, the allocated Physical Resource Blocks(PRBs), and modulation and coding schemes (MCS) – forboth the uplink and downlink. Allocation of resources isnormally done using Dynamic Scheduling (DS) algorithm forthe data packets. However, as voice packet transmission isperiodic with regular voice packets, with the possibility oflarge number of users vying for resource in a TTI, the numberof available PDCCHs for granting new resources can limit thepossible allocation. With only 1-3 symbols of each carrier ineach PRB allocated for control signal, this may not onlyincrease call drop rate but also may fail in providing the

required QoS to the existing users. To overcome thislimitation, various resource scheduling algorithms other thandynamic scheduling are studied.

B. Scheduling Schemes

In this section, various scheduling schemes such asdynamic, persistent, semi-persistent and talk-spurt basedpersistent scheduling are briefly explained.

1) Dynamic Scheduling: In DS, queued packets of the

users are scheduled every TTI by allocating the required PRBs

and the transmission format combination (TFC) to the users

(based on SINR information). As these resource grants are

sent via PDCCHs, VoIP capacity using DS may be limited by

the PDCCH limit. A variation of DS is by using the concept ofpacket bundling, where consecutive packets of the same user

are queued and bundled together before being transmitted.

Although this may increase capacity [6], packet bundling may

increase transmission delay.

2) Persistent Scheduling: Persistent scheduling is the

process of allocation of fixed time and frequency resources to

the user along with fixed TFC for the entire call duration or

duration of burst [4]. This is advantageous in comparison to

DS as the capacity is not limited by the available PDCCH.

However, it is highly inefficient in resource utilization as the

resources are dedicated for long durations even though there is

no transmission of VoIP packets. Also, the capacity is limited

to the bandwidth available as maximum capacity possible C =

(1000 x N)/B, where B – bandwidth/user in KHz and N –Total bandwidth in MHz. Also, lack of link adaptation may

reduce the user experience.

3) Semi-Persistent Scheduling: SPS takes the advantage

of both dynamic and persistent scheduling. In this scheme, the

users are allocated resources for an extended period for

transmission of voice packets. At the end of the burst, the user

resources are deallocated and are allocated to another user.

The TFC may be changed for the duration of the burst based

on the channel state information. Thus, PDCCH are required

only in case of new resource allocation, for changing of the

TFC or transmission power within a burst. For the

retransmission of the packets, they are dynamically scheduled

and this also requires the PDCCH resources.

4)

Talk-Spurts based Scheduling: As silence periodsconsume half the duration of the talk burst or voice call, talk-

spurt based scheduling [4] aims at allocating PRBs every talk-

spurt and deallocating PRBs at the end of the spurt. TFC

remains same for each spurt. The Silence Insertion Descriptors

(SIDs) are transmitted in a dynamic basis, which consumes

PDCCH but as they are less frequent compared to voice

packets, talk-spurt based scheduling perform better than full

dynamic scheduling. The resources for voice packets can

either be allocated for both transmission and retransmission or

only transmission, with retransmission being dynamically

scheduled.

C. Large VoIP Packets and concept of leftovers

For scheduling algorithm, there are three main processes -initial allocation, periodic allocation and retransmission of

VoIP packets and SIDs - that have been considered in most of

the literatures [3-7]. An important issue often ignored is

regarding the size of the VoIP packets compared to the

available PRBs for allocation. Conflict in the schedulers

would arise if the required number of PRBs for a user at a

given TTI is greater than the number of available PRBs. It is

important to consider the large packets and the possibility of

transmission of the leftovers any real-time communication

where the QoS depends on the continuous uninterrupted

transmission and reception. Fig. 1 shows the ratio of VoIP

packets requiring leftover to total VoIP packets in the

downlink scenario to underline the importance of considering

leftover transmission.

III. SEMI-PERSISTENT SCHEDULING FOR VOLTE

With the basic idea of DS scheme explained, in thissection, we look towards the SPS algorithms for scheduling of

90 100 110 120 130 140 150 160 170 180 190 2000

0.5

1

T o t a l V o I P P a c k e t s a n d

L e f t o v e r V o I P P a c k e t s

Number of Users

VoIP Packets with Leftovers in Downlink

80 100 120 140 160 180 200

5

10

15

L e f

t o v e r P e r c e n t a g e

Total Packets

Packets withLeftovers

Percentage of Packets with Leftovers

10

0

15

19090 130 150 160 170 180 200140120110100

1.0

0.5

0.0

Fig. 1. Ratio of VoIP packets requiring leftover to total VoIP packets


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VoIP packets, designed to take into account the effects oflarge packets. To resolve the problem, two strategies ofallocating PRBs have been adopted with variations based onpartitioning and transmission of leftovers.

A.

Non-Segmentation based Semi-Persistent Scheduling

In Non-Segmentation based Semi-Persistent Scheduling(NS-SPS) scheme, when a packet arrives at the transmissionbuffer initially, users are allocated resources based on

availability and feasibility of PRBs for the initial packet. Thisis referred as Initial Allocation. For this, new TFCs are chosenand hence, PDCCH is required to signal the grant for initialallocation. For further periodic arrival of packets, PRBs areautomatically reserved for the users and allocated periodically(with the repetition interval known as SPS Interval) till the endof the talk burst, with the same TFC as used for previoustransmission being used. This is termed as Periodic Allocation. In case a packet requires less PRBs compared tothe reserved ones, then the extra PRBs are released with thenew allocation being recorded for future reservation. If therequired PRBs are more, then extra PRBs are added to theprevious reservation or a completely new set of PRBs arechosen depending on feasibility. Any change of reservedPRBs, TFC or transmission power requires new grant

allocation through PDCCH. If no packets are received afterthe SPS Interval, then the PRBs are dynamically allocated toother users. In case no further packet is received even after thepre-determined active period of the user, the user is assumedto be dormant, after which the PRBs remain no longerreserved for the user. For further transmission, the user istreated as a ‘new’ user.

For either of the two transmission processes, the packetsmay require more PRBs than the available for the TTI. Asreal-time communication is delay sensitive, queuing of suchpackets until PRBs are available may reduce the QoS. Hence,for such large packets, the available PRBs are allocated for apart of the packet while the leftover is transmitted in thesubsequent TTIs on a dynamic basis in a process known as the

Leftover Allocation. The dynamic transmission of leftoverrequires availability of PDCCH for grant allocation.

For any of the allocation process, in case of packet loss, Retransmission of packets takes place in a dynamic fashion.Thus, the developed NS-SPS algorithm can be separated intofour parts with the following order of priority of allocation: (a)Periodic Allocation, (b) Leftover Allocation, (c) Retransmission, and (d) Initial Allocation. Fig. 2. below showsthe basic block diagram of the NS-SPS algorithm.

B. Segmentation based Semi-Persistent Scheduling

The basic process of Segmentation based Semi-PersistentScheduling (S-SPS) algorithm is same as NS-SPS. Thevariation is only in term of allocation of PRBs to large

packets. When packets are large and require more PRBs thanthe available or feasible, they are divided into multiplesegments. The number of segments is calculated by dividingthe packet size by the available/ feasible PRBs, such that theobtained number is a factor of the SPS Interval. For example,using 20 ms as SPS Interval, the possible numbers of segmentsare 1, 2, 4, 5, 10 and 20. The number of PRBs for these

segments equals the number of available/ feasible PRBs at thepresent TTI (that is, the TTI when the packet is segmented).These segments are transmitted periodically within the SPS Interval of 20 ms before the arrival of the next regular packet.Effectively, by segmentation, the SPS Interval is reduced, suchthat no new grants are required for the allocations of segments2, 3… Thus, for each segment, reservation of PRBs is donefor the duration of SPS Interval and these segments use thesame TFC and transmission power for the SPS Interval. As

this algorithm does not contain a separate Leftover Allocation step, the three allocation steps in order of priority are asfollows: (a) Periodic Allocation, (b) Retransmission, and (c) Initial Allocation. Fig. 3. below shows the basic block diagramof the S-SPS algorithm while Fig. 4. shows the difference inallocation process of large packets in NS-SPS and S-SPS.

C. Additional Points

Some of the additional points are elaborated below:

1) Non–Availability of PDCCH : In case of PDCCH non-availability for Periodic Allocation of resources, as allocationof new grants cannot be done for this TTI, the grant for theprevious transmission is used – even if it is not optimal for thepresent situation. For other allocation processes, no further

allocation is possible for the given TTI.

Fig.2. Block Diagram - Non-Segmentation based Semi-Persistent Scheduling

Fig.3. Block Diagram of the Segmentation based Semi-Persistent Scheduling

PERIODIC ALLOCATION

FOR NS - SPS USERS

LEFTOVER ALLOCATION

FOR NS - SPS USERS

RETRANSMISSION FOR

NS-SPS USERS

INITIAL ALLOCATION

FOR NEW NS-SPS

USERS

TFC SELECTION AND PRB

RESERVATION FROM

PREVIOUS PERIODIC

ALLOCATION

STORE TFC SELECTION

AND PRB RESERVATION

FROM INITIAL

ALLOCATION

DYNAMICALLY SELECT PRBs & TFC

SEMI-PERSISTENT

SCHEDULING



DYNAMIC

SCHEDULING

DYNAMIC

SCHEDULING

SEMI-PERSISTENTSCHEDULING

PERIODIC ALLOCATION OF

SEGMENTS FOR S -SPS

USERS

PERIODIC ALLOCATION OF

PACKETS FOR S- SPSUSERS

RETRANSMISSION FOR

S-SPS USERS

INITIAL ALLOCATION FOR

NEW S-SPS USERS

TFC & PRBs FROM

PREVIOUS PERIODIC

PACKET ALLOCATION

STORE TFC SELECTION

AND PRB RESERVATION

FROM INITIAL PACKET/

SEGMENT ALLOCATION

SEMI-PERSISTENT

SCHEDULING



SEMI-PERSISTENT

SCHEDULING

DYNAMIC

SCHEDULING

SEMI-PERSISTENT

SCHEDULING

TFC & PRBs FROM

PREVIOUS PERIODIC

SEGMENT ALLOCATION


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TTIs 01 02 03 04 05 06 07 08 09 … 16

PRBs

SPS Interval

(a) Large Packets requiring PRBs greater than available/feasible

TTIs 01 02 03 04 05 06 07 08 09 … 16

PRBs

(b) Non-Segmentation based a llocation of Large Packets

TTIs 01 02 03 04 05 06 07 08 09 … 16

PRBs S1 S2 S3 S4

(c) Segmentation based allocation of Large Packets

Fig.4. Difference in allocation process of Large Packets in Non-Segmentationbased and Segmentation based Semi-Persistent Scheduling Algorithm

2) Information Carry-forward: For both the algorithms,information such as the TFC used, reserved PRBs, lastscheduled time, transmission power and number of segments(only in case of S-SPS) are recorded in a database. This

database enables the successful semi-persistent allocation ofresources for further TTIs.

IV. SIMULATION SCENARIO AND RESULTS

All the simulation cases were done in a three tier diamond-pattern macro scenario with 3-sector sites. The deployment ofthe users is on a random basis. The traffic model used for theVoIP capacity analysis is Adaptive Multi-Rate (AMR) audiocodec with bit rate of 12.2 kbps. In this simulation, the activeperiod of user is considered to be 20 ms. If no packet isreceived from user within 20 ms, the user is assumed to bedormant. The simulations were done for the downlink scenarioin a single cell with randomly distributed users. For thesimulation scenarios, the number of users varied from 90 to200, with an increment of 10 users in each scenario. In eachscenario, simulation was done for 15 x 10

5 TTIs or 1500

seconds, with the position of the users being randomlyshuffled every 30 seconds. The measurements are obtained fordelay budgets varying from 40 ms to 100 ms (with a step of 10ms). The main simulation parameters are given in Table Iwhereas the parameters related to VoIP traffic are listed inTable II. The performance of the three algorithms for differentscenarios is shown in Fig. 5. The comparison of performanceof the three different algorithms for a delay budget of 70 msand 90 ms are shown in Fig. 6 and Fig. 7 respectively.

It must be noted that while for DS, the TFC selection isbased on the CQI reporting, in NS-SPS and S-SPS, TFCsdepend on time-averaged wideband SINR measurements. As

can be observed from Fig. 5, the percentage of satisfied usersis directly proportional to the increase in delay budget and isinversely proportional to the decrease in the number of usersin the cell. For DS in Fig. 5 (a), the rate of increase in usersatisfaction percentage with respect to the increase of delaybudget and decrease of user per cell are similar. For the delay

budget of 50 ms, 95% users are satisfied for 110 users per cell.With the increase of delay budget to 70 ms, the capacityincreases to 125. In NS-SPS, as users are semi-persistentlyscheduled, there is no (or rather reduced) dynamic access andhence, PRBs are allocated periodically to all the users that canbe accommodated. Fig. 5 (b) shows that for NS-SPS,percentage of satisfied users in downlink gradually decreaseswith the increase of the number of users in the cell till athreshold value after which the percentage of satisfied users

falls drastically. If the number of users goes beyond themaximum user threshold, no more users can be accommodateddue to unavailability of unreserved PRBs – even if the delaybudget is increased. In the given results, the threshold ofmaximum users is approximately 175 users. The increase inthe percentage of satisfied users with respect to increasingdelay budget is gradual. Although at a lesser rate, it occurs aswith more time, the chances of successful transmission ofleftover packets and retransmissions increases. For the delaybudget of 50 ms, the percentage of satisfied users is 95% for acell capacity of 120. For an increased delay budget of 70 ms,95% users are satisfied for a cell capacity of 160 users per cell.The downlink behavior of the S-SPS algorithm is illustrated inFig. 5 (c). As can be observed from the figure, the variation ofpercentage of satisfied users for S-SPS with respect to increase

of delay budget and with respect to decrease of user per cell issimilar. From the results, it can be seen that for a delay budgetof 50 ms, 95% users are satisfied for a cell capacity of 90users whereas for a delay budget of 70 ms, the capacityincreases to 110 such that 95% users are satisfied.

TABLE I. MAIN SIMULATION PARAMETERS

Parameter Value

Cellular Layout Hexagonal grid with 3-sector sites

Inter-site distance 500 m

Shadow Model 2D uncorrelated grid with bilinearinterpolation

Shadow Standard Deviation 8 dBThermal Noise Density -174 dBm/Hz

Noise Figure at UE 9 dB

System Bandwidth 5 MHzCarrier Frequency 2GHzSub-frame duration 1 ms

Duplexing FDDCarrier per PRB 12

Frequency Reuse 1

DRX EnabledNo. of PDCCH 4 for Uplink; 4 for Downlink

Bundling No

Link Adaptation Fast OLLAMax. eNB Transmission Power 46 dBm

eNB Height 32 m

Max. UE Transmission Power 23 dBmUE Height/ Mobility 1.5 m/ 3 kmph

Downlink Antenna Configuration 2 x Transmitter; 2 x Receiver

Downlink Antenna Gain 14 dBiHARQ Scheme Incremental Redundancy

No. of HARQ Processes 8

Max. HARQ Retransmission 4CQI Delay (only for DS) 4 TTI

CQI Report Rate (only for DS) 20 msCQI Resolution (only for DS) 2 PRBs

CQI Report Rate (only for SPS) Beginning of talk-spurt

CQI Resolution (only for SPS) Wideband CQI

Simulation duration 1500 sec

Leftover Allocation

Segments


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TABLE II. VOIP TRAFFIC PARAMETERS

Parameter Value

Call length 30 sec

Average talk-spurt duration/ Voice activity 3 sec/ 50%AMR Voice Codec Rate (burst rate) 12.2 kbps

SID Rate (during silence periods) 0.24375 kbps

Voice packet inter-arrival time (SPS Interval)/ size 20 ms/ 40 bytes

SID inter-arrival time/ size 160 ms/ 15 bytes

(a) Dynamic scheduling

(b) Non-segmentation based semi-persistent scheduling

(c) Segmentation based semi-persistent scheduling

Fig. 5. Capacity analysis w.r.t. different delay budgets and users per cell fordifferent algorithms (Blue grid represents 95% users level)

As evident from the Fig. 6, the fitted curve for NS-SPSalgorithm can support a capacity of nearly 160 users.Compared to NS-SPS algorithm, DS algorithm can supportabout 125 users and S-SPS algorithm can support only 110users per cell. Thus, NS-SPS algorithm can sustain a capacity22% and 31% more compared to DS and S-SPS algorithmsrespectively. The capacity gain for NS-SPS over DS occursdue to the semi-persistent scheduling of the users, as they donot have to compete for the available PRBs. This illustrates

the benefit of reservation of packets with periodic arrival.However, this does not ensure that reservation of resourcesalways performs better than dynamic allocation. Even thoughresources are reserved for S-SPS, the performance of S-SPS ispoorer compared to NS-SPS and DS for delay budget of 70ms. This occurs mainly as the segmentation of packets in S-SPS require more reservations within the SPS interval.Compared to NS-SPS, the frequent reservation to some userscauses the number of allowable users for semi-persistentallocation to reduce. Also, due to the segmentation process,the loss of the segments can cause retransmission to start witha delay – in worst case equaling SPS Interval in case of loss ofthe last segment (since the last segment is transmitted after aperiod equaling SPS Interval after the transmission of the firstsegment). This wait period may cause late delivery of the

packets beyond the acceptable delay. However, it must benoted that the rate of decrease in user satisfaction in S-SPS isvery less compared to DS and NS-SPS. As can be seen in Fig.7, with the increase in delay budget, S-SPS out performs bothNS-SPS and DS and support a higher capacity for delaybudget of 80 ms or higher. With a delay budget of 90 ms, acapacity of 200 can be reached using S-SPS whereas thecapacity of DS and NS-SPS with 100 ms delay budget is about155 (22% less) and 165 (17% less) respectively. Therefore, fordownlink at low delay budget, it can be concluded that theperformance of NS-SPS algorithm is better than DS and S-SPS algorithms. However, with better optimization, S-SPS canbe a good candidate for downlink scheduling.

In terms of the efficiency of resource utilization, the resultscan be seen in Fig. 8. The three sets of bar graphs compare thepercentage of resources used with each set showing thepercentage of resources used when the cell contained 90, 140and 190 users respectively. The resources used by NS-SPSand S-SPS are relatively similar and much better compared toDS. The low resource utilization in DS can be attributed to thelack of PDCCH available to allocate new resources to users inqueue every TTI. Compared to DS, in NS-SPS and S-SPS,PDCCH is used only when certain changes are required andnot used for every transmission by the active users in everyTTI. Hence, this may reduce the capacity achievable usingDS. Comparing results for the 140 user scenario, the resourcesused by NS-SPS is about 83% and for S-SPS, about 86%PRBs are allocated. Compared to these, DS is only able toallocate 35% of the resources available. For capacity of 90 and

190 users respectively, NS-SPS uses about 53% and 95%, S-SPS uses 52% and 94%, while DS users only 25% and 38% ofthe available PRBs. Also, it can be intuitively understood thatwith the increase of number of users per cell, the PRBrequirement increases and this can be clearly seen in thevariation shown in the graph. However, it must be noted thatthough NS-SPS and S-SPS definitely has advantage of consu-


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100 120 140 160 180 200

60

70

80

90

100

Number of Users

P e r c e n t a g e o f S a t i s f i e d U s e r s

Percentage of Satisfied Users in Downlink

DS Curve-Fit

DS Markers

S-SPS Marker

S-SPS Curve-Fit NS-SPS Marker

NS-SPS Curve-Fit

Target Level

Fig. 6. Comparative analysis of the performance of dynamic, segmentationbased semi-persistent and non-segmentation based semi-persistent schedulingalgorithms w.r.t. the percentage of satisfied users for delay budget of 70 ms

100 120 140 160 180 20075

80

85

90

95

100

Number of Users

P e r c e n t a g e o f S

a t i s f i e d U s e r s

Percentage of Satisfied Users in Downlink

DS Markers

DS Curve-Fit

NS-SPS Markers

NS-SPS Curve-Fit

S-SPS Markers

S-SPS Curve-Fit

Target Level

Fig. 7. Comparative analysis of the performance of dynamic, segmentationbased semi-persistent and non-segmentation based semi-persistent schedulingalgorithms w.r.t. the percentage of satisfied users for delay budget of 90 ms

1 2 30

20

40

60

80

100

P e r c e n t a g e o f P R B s u s e d

Comparision of Resource Utilization

90 Users

140 Users

190 Users

NS-SPS S-SPS DS

Fig. 8. Percentage of resource allocated using different scheduling algorithm

for 90, 140 and 190 users per cell

ming lesser PDCCH, in terms of PDSCH and amount ofresource usage, DS performs better than NS-SPS and S-SPS.

V. CONCLUSION

As LTE (-A) is based on packet switching real-timeservices like voice communication require VoIP basedsolution. Therefore, scheduling assumes importance as, unlikedata communication over a packet switched network, real-time

communication packets arrive more frequently. Therefore itrequires efficient scheduling such that the capacity of thesystem is not limited by the available control channel.Towards this issue, different scheduling mechanisms havebeen proposed in literature. However, many of the proposalsfail to take into account the possibility of large packet sizes forvoice transmission as well as other real-time applications. Thiswork deals with the development of semi-persistentscheduling algorithms and comparing the capacity results with

the dynamic scheduling mechanism. Two variations of semi-persistent algorithms developed are based on the segmentationand non-segmentation of the large packets. While forsegmentation, the leftovers are semi-persistently scheduled,non-segmented leftover packets are dynamically transmitted.Also, unlike previous works, the SPS algorithms are based ontime-averaged wideband SINR information and notinstantaneous CQI information. Simulation of these algorithmsshows that the non-segmentation based semi-persistentalgorithm works best amongst the three in the downlink forlower delay budgets. However, with the increase in delaybudget, segmentation algorithm works better than bothdynamic and non-segmentation based algorithms. Also, thelow resource utilization in dynamic scheduling shows that thecapacity is restricted due to unavailability of PDCCH to grant

new resources. On the other hand, VoIP capacity in SPS isconstrained by the PDSCH availability.

This work has focussed only on voice communication indownlink using AMR 12.2 voice codec. As a future scope forthis work, simulations can be carried out for uplink scenariosas well and also using video transmission packet using thedeveloped scheduling algorithms to determine theperformance for the algorithms with respect to videotransmission.

REFERENCES

[1] "Cisco visual networking index: Global mobile data traffic forecastupdate, 2010-2015," white paper, Cisco Systems Inc., Feb. 2011.

[2] M. Anehill; M. Larsson; G. Strömberg; E. Parsons, "Validating voice

over LTE end-to-end," white paper, Ericsson Review, Jan. 2012.[3] D. Jiang; H. Wang; E. Malkamaki; E. Tuomaala, "Principle and

Performance of Semi-Persistent Scheduling for VoIP in LTE System," International Conference on Wireless Communications, Networking and Mobile Computing (WiCom, ‘07), pp.286-2864, 21-25 Sep. 2007.

[4] J. Puttonen; N. Kolehmainen; T. Henttonen; M. Moisio, "Persistentpacket scheduling performance for Voice-over-IP in evolved UTRANdownlink," IEEE 19th International Symposium on Personal, Indoorand Mobile Radio Communications, (PIMRC’08), pp.1-6, 15-18 Sep.2008.

[5] M. Muhleisen; B. Walke, "Evaluation and improvement of VoIPcapacity for LTE," 18th European Wireless Conference (EW‘12), pp.1-7, 18-20 Apr. 2012.

[6] J. Puttonen; T. Henttonen; N. Kolehmainen; K. Aschan; M. Moisio; P.Kela, "Voice-Over-IP Performance in UTRA Long Term EvolutionDownlink," IEEE 67 th Vehicular Technology Conference (VTC’S08),pp.2502-2506, 11-14 May 2008.

[7] Y. Fan; P. Lunden; M. Kuusela; M. Valkama, "Efficient Semi-PersistentScheduling for VoIP on EUTRA Downlink," IEEE 68 th VehicularTechnology Conference (VTC’F08), pp.1-5, 21-24 Sep. 2008.

[8] "LTE physical layer framework for performance verification," 3GPPR1-070674, St Louis, USA, 12-16 Feb. 2007.

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