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Sensitivity Analysis of the Optimal Parameter Settings of an LTE Packet Scheduler

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Sensitivity Analysis of the Optimal Parameter Settings of an LTE Packet Scheduler Sensitivity Analysis of the Optimal Parameter Settings of an LTE Packet Scheduler I. Fernandez Diaz, D.C. Dimitrova K. Spaey R. Litjens University of Twente, The Netherlands IBBT, Belgium and J.L. van den Berg E-mail: [email protected] E-mail: [email protected] TNO ICT, The Netherlands E-mail: fIrene.FernandezDiaz, Remco.Litjens and [email protected] Abstract—Advanced packet scheduling schemes in 3G/3G+ lacking. E.g., for the PF scheduler it is often stated that α close mobile networks provide one or more parameters to optimise the to zero (e.g. α = 0:01) is generally a good choice, see e.g. trade-off between QoS and resource efficiency. In this paper we [2], but mostly without any support of results from extensive study the sensitivity of the optimal parameter setting for packet scheduling in LTE radio networks with respect to various traffic experiments and/or simulations. and environment aspects. For our investigations we consider The aim of the present paper is to get more insight into a reference packet scheduling algorithm containing elements the sensitivity of the optimal parameter setting for packet of proportional fairness and packet urgency to support mixes scheduling in LTE. For that purpose we analyse the optimal of real-time and non-real-time traffic. We present extensive parameter settings of a particular ‘reference’ (downlink) packet simulation results showing the impact of traffic characteristics (like file size distribution, traffic mix) and environment conditions scheduling algorithm under different system and traffic con- (regarding e.g. multipath fading and shadowing) on the optimal ditions. The reference packet scheduler, containing elements parameter setting. Although, in some cases, efficiency gains of of proportional fairness and packet urgency, supports mixes of about twenty percent can be achieved by proper tuning of the real-time and non real-time traffic. Besides the parameter α of scheduling parameters, the overall view from our investigations the ‘PF-part’ of the scheduler it contains a second parameter is that a single, robust setting of the parameters can be deter- mined which provides near optimal trade-offs under almost all (ζ) that can be used to tune the relative importance of the two practically relevant conditions. elements in the scheduling decision. To enable our analysis we have developed a tool for dynamic LTE simulations. I. INTRODUCTION Apart from the evident importance of studying the sensi- One of the key radio resource management mechanisms in tivity of packet schedulers with respect to changes in system 3G+ mobile networks is the packet scheduler, which coor- and traffic conditions, the present study is also motivated by dinates the access to shared channel resources. In OFDMA- our investigations in the field of self-optimisation of radio based LTE systems, for example, this coordination generally resource management in LTE, see [3]. Obviously, before considers two distinct dimensions, viz. the time dimension starting to develop self-optimisation algorithms, its important (allocation of time frames) and the frequency dimension (allo- to get insight into these sensitivity issues and the potential cation of subcarriers). The main challenge in designing packet performance/efficiency gains that may be achieved. schedulers is to optimise resource efficiency (e.g. by exploiting multi-user and frequency diversity), while satisfying the users’ A. Related work Quality of Service (QoS) requirements and achieving some Most of the downlink scheduling schemes for 3G+ wireless degree of spatial fairness. Many packet scheduling schemes for networks adopt the well-known Proportional Fair (PF) algo- mobile access networks have been proposed and implemented rithm. It primarily aims at an appropriate trade-off between of which the so called Proportional Fair (PF) scheduler is throughput efficiency (through diversity gain) and fairness for probably the most well known, see e.g. [1], [2]. It explicitly non-real-time data traffic, see e.g. [1], [4], [5], [6], [7], [8], addresses the trade-off between efficiency, QoS and fairness, [9]. Originally developed for purely time domain scheduling, which can be tuned by a single parameter α, 0 ≤ α ≤ 1. as e.g. in HSDPA, the PF algorithm has been adapted in An important issue is how packet scheduling performance several ways for efficient and fair utilisation of both time and depends on actual system and traffic characteristics regarding frequency domain resources in OFDMA systems like LTE and e.g. shadowing and fast fading, mobility, file size distribution, WiMax, see e.g. [10], [11], [12] and the references therein. traffic mix, etc. A particular relevant question in this light is For scheduling multimedia traffic so-called deadline-based how the ‘optimal’ setting of the scheduling parameters has schemes have been developed, explicitly taking care of packet to be adapted when one or more of these system or traffic delays for real-time services, see e.g Necker [2] and Elsayed conditions change over time. Somewhat surprisingly, studies and Khattab [13], [14]. In particular, the work by Elsayed and systematically investigating this issue are rare or completely Khattab (in the context of TDMA-based wireless networks) TABLE I describes an interesting and pragmatic scheduling principle, NOTATION denoted the ‘Channel-Aware Earliest Deadline Due’ scheme. It incorporates both channel-awareness and due date aspects (by Ri;c;potential(t) The potential bit rate at which user i can be served on subchannel c at TTI t: These rates extending a PF scheduler with a delay dependent component) are based on a discretisation of the SINR- to allow for a mixture of real-time and non-real-time traffic. to-rate mapping presented in [15]. Our reference scheduler is based on the scheduler proposed Ri(t) The exponentially smoothed average bit rate at which user i has been served, aggre- by Elsayed and Khattab and extends it for deployment in the gated over the subchannels that have been LTE context. assigned to user i, at TTI t The remainder of this paper is organised as follows. First, Ri(t) The bit rate at which user i is served, aggregated over all subchannels assigned to in Section II, we specify the scheduling algorithm that will user i, at TTI t be used as a reference algorithm in our study, and we explain α Exponential smoothing parameter, used for the set-up of the sensitivity analysis of the optimal parameter the smoothing of Ri(t) settings for the reference scheduler. Section III describes the Ti Maximum allowed delay for packet associ- ated with user i (Ti = 1 if user i is a non main modelling assumptions in the LTE simulation tool and real-time user) gives the simulation parameters used in the reference scenario. Wi(t) Delay experienced by HoL packet of user In Section IV we present and discuss the simulation results. i at TTI t, i.e. the present time minus the packet’s arrival time in the buffer. If The conclusions and some topics for further research are Wi(t) > Ti the packet is dropped (real- presented in Section V. time sessions only). ζ Scheduling parameter that affects the rela- tive importance of the ‘channel adaptivity’ II. REFERENCE PACKET SCHEDULER and ‘packet urgency’ components of the priority level function We have developed a reference packet scheduler for use ρservice Service-specific requested bit rate, i.e. the in LTE’s OFDMA downlink, inspired by algorithms encoun- minimum throughput target for non real- tered in the literature, in particular those in [13], [14]. The time sessions or the fixed bit rate for real- time sessions. The purpose of this ‘correc- scheduling principles proposed in these papers have been tion factor’ is to prevent the scheduler from extended from a pure time-domain focus to cover both time- giving undesiredly high preference to real- and frequency-domain scheduling, as is relevant when applied time sessions, which may typically experi- ence lower throughputs due to their limited in the LTE context. The scheduler supports both real-time and source rate, rather than due to any unfairness non real-time services, and in that light contains elements in the scheduling scheme. of proportional fairness (aiming at resource efficiency and fairness) and packet urgency (for adequate support of delay- sensitive services). The reference packet scheduler assigns in where Ri(t) is initialised at the aggregate bit rate at which each TTI (time domain: 1 ms granularity) a priority level for user i can potentially be served at the time of call creation, every subchannel (frequency domain: 180 kHz granularity) to assuming all subchannels are available. The second component the head of line (HoL) packet of every non-empty user buffer, of Pi;c(t) is the packet ‘urgency factor’. The parameter ζ ≥ 0 taking into account the potential bit rate at which the user allows setting the relative importance of the channel adaptivity can be served on the different subchannels (based on channel (i.e. efficiency) and the packet urgency components. quality feedback from the user), as well as the experienced We have developed a heuristic procedure for the assignment and maximum tolerable delay of the packet. of subchannels, based on these priority levels. Due to a lack of The scheduler comprises two steps. In the first step, pri- space we reproduce here only the main principles, i.e. (i) the ority levels are calculated for each combination of user and assignment of a given subchannel to the user with the highest subchannel. In the second step these priority levels are applied priority on that subchannel; and (ii) in order to comply with in the actual assignment of subchannels to users. the uniformity restriction that multiple subchannels assigned At time t, the priority level Pi;c(t) assigned to user i’s HoL to a given user must use the same MCS (modulation and packet associated with subchannel c is calculated according to coding scheme), all subchannels assigned to a given user are the formula jointly considered and a common MCS is selected which ( )( )ζ maximises the aggregate bit rate for that user; subchannels Ri;c;potential(t) Wi(t) that are potentially released in this step can be reassigned to Pi;c(t) = ρservice 1 + − Ri(t) Ti Wi(t) other users.
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