Performance of Downlink Frequency Domain Packet Scheduling for the UTRAN Long Term Evolution

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The 17th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC’06)

PERFORMANCE OF DOWNLINK FREQUENCY DOMAIN PACKET

SCHEDULING FOR THE UTRAN LONG TERM EVOLUTION

A. Pokhariyal(1)T. E. Kolding(2), P. E. Mogensen(1,2)

1) Aalborg University, CSYS 2) Nokia Networks

Aalborg, Denmark Aalborg, Denmark

ap@kom.aau.dk troels.kolding@nokia.com

ABSTRACT

In this paper we investigate the potential of downlink frequency

domain packet scheduling (FDPS) for the 3GPP UTRAN long-

term evolution. Utilizing frequency-domain channel quality re-

ports, the scheduler ﬂexibly multiplexes users on different por-

tions of the system bandwidth. Compared to frequency-blind,

but time-opportunistic scheduling, FDPS shows gains in both

average system capacity and cell-edge data rates on the order

of 40%. However, the FDPS performance is shown to depend

signiﬁcantly on the frequency-domain scheduling resolution as

well as the accuracy of the channel state reports. Assuming

Typical Urban channel proﬁle, studies show that the schedul-

ing resolution should preferably be as low as 375 kHz to yield

signiﬁcant FDPS gain with two-branch receive diversity and in

20 MHz. Further, to have convincing FDPS gain the std. of

error of radio state reports needs to be kept within 1.5-2 dB.

I. INTRODUCTION

The ambitious design goal of the 3GPP UTRAN long-term evo-

lution (LTE) is to provide spectral efﬁciency gains in the or-

der of 3-4 compared to e.g. Release 6 high-speed downlink

packet access (HSPDA) [1, 4]. For the LTE downlink OFDMA

is considered to be the best access technology for providing

high scalability up to large system bandwidths and to facili-

tate advanced frequency-domain scheduling methods. As LTE

is optimized for packet data transfer only, the packet scheduler

functionality assumes great signiﬁcance. In this paper we study

speciﬁcally the potential of frequency-domain multiplexing of

users according to their instantaneous radio conditions. This is

most often referred to as frequency domain packet scheduling

(FDPS), although time-domain scheduling is inherent as well.

Optimal and sub-optimal sub-carrier based adaptation has

been widely studied in the literature, see e.g. [7, 3, 2] and ref-

erences therein. Most studies are conceptual in nature and dis-

regard practicalities such as limited dynamic range, multi-user

multiplexing aspects, amount of signaling overhead, adaptation

uncertainties, etc. Such factors are, however, highly relevant

in practical system design. Recent studies, e.g. [8, 10, 9],

investigate the potential of frequency domain adaptation un-

der less ideal conditions. In [10] the performance of FDPS

in a 100 MHz OFDMA system is reported. Assuming a ﬁxed

scheduling resolution of 640 kHz and 10 active users, a FDPS

capacity gain over time-domain proportional fair (PF) schedul-

ing of around 60% is reported. Hence, FDPS appears to be

an attractive method although the conditions in [10] are opti-

mistic seen in relation to LTE; e.g. LTE bandwidth is limited

to 20 MHz, ﬂexible data rate adaptation per sub-band is hard

to achieve due to signaling constraints, and the radio quality

estimation errors may exceed the levels assumed in [10].

In this study we include those additional aspects and fo-

cus on the conditions required for high FDPS gain in LTE. In

particular, the impact of scheduling resolution in frequency-

domain, scheduling freedom, radio environment, and chan-

nel quality indication (CQI) estimation error on FDPS perfor-

mance is evaluated. Further, the analysis is expanded to include

both average cell-level throughput and coverage performance.

We initiate the paper by describing the basic FDPS terminol-

ogy and approach. This includes details related to the used

scheduling algorithms and the operating conditions. Next, we

present the simulation methodology before moving on to the

simulation results and conclusions.

CQI

Data &

control info.

Data &

control info.

UE1

UE2

time

freq.

Resource

block (RB)

sub-frame

FDPS

Node-B

Figure 1: Basic FDPS concept and terminology.

II. FREQUENCY DOMAIN PACKET SCHEDULING

The basic FDPS terminology and framework including the

Node-B where the scheduler is based and the user equipment

(UE) is illustrated in Fig. 1. To frequency multiplex users

in OFDMA, the bandwidth (e.g. the number of OFDM sub-

carriers) needs to be divided into separable chunks denoted as

resource blocks (RBs). Chunk based adaptation and user multi-

plexing is preferred for LTE as it reduces the associated signal-

ing overhead. Moreover, coherence bandwidth of the channel

implies that fading is correlated over a group of OFDM sub-

carriers. Two main ways of creating the RBs exist:

•Distributed: The RB is deﬁned by sub-carriers distributed

over the operating bandwidth. No or limited user/selection

diversity exists in frequency domain since all resource

blocks will experience approximately the same average

conditions. Hence, it does not facilitate FDPS but sup-

ports full averaging frequency diversity.

1-4244-0330-8/06/$20.00 c

2006 IEEE

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The17thAnnualIEEEInternationalSymposiumonPersonal,IndoorandMobileRadioCommunications(PIMRC’06)PERFORMANCEOFDOWNLINKFREQUENCYDOMAINPACKETSCHEDULINGFORTHEUTRANLONGTERMEVOLUTIONA.Pokhariyal(1)T.E.Kolding(2),P.E.Mogensen(1,2)1)AalborgUniversity,CSYS2)NokiaNetworksAalborg,DenmarkAalborg,Denmarkap@kom.aau...

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