IEEE COMMUNICATIONS LETTERS, VOL. 12, NO. 4, APRIL 2008
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Adaptive Delay Threshold-based Priority Queuing Scheme with Opportunistic Packet Scheduling for Integrated Service in Mobile Broadband Wireless Access Systems Chung G. Kang, Member, IEEE, Tae W. Kim, and Jae H. Kim
Abstract— In this letter, we propose a novel method of implementing an opportunistic priority queuing scheme that supports both latency-sensitive real-time (RT) and best-effort non-realtime (NRT) service classes in mobile broadband wireless (MBWA) systems. It employs an adaptive delay threshold as a dynamic reference of prioritizing the urgent RT service users over others, which allows for fully leveraging the multiuser diversity gain of NRT service users under the varying system conditions. It is shown that the overall system capacity can be further increased by the proposed adaptation rule, as compared with the existing opportunistic priority queuing scheme with the fixed delay threshold. Index Terms— Packet scheduling, QoS, priority queuing, integrated service, mobile broadband wireless access.
I. I NTRODUCTION
I
N order to schedule both real-time (RT) and non-real-time (NRT) services in the practical mobile broadband wireless access (MBWA) systems, two different types of scheduling principles can be considered: opportunistic scheduling and priority queuing. Particular examples of opportunistic scheduling for an integrated service include the adaptive EXP/PF algorithm [1][2] and UEPS algorithm [3], in which two different priority metrics are employed, each one specified for an individual service class. In this case, however, the hard QoS constraint of the RT service, e.g., in terms of maximum allowable delay, may not be warranted as the corresponding priority score is dynamically determined upon the varying conditions, e.g., the instantaneous data rate and waiting time. Meanwhile, priority queuing is one particular scheduling scheme that can guarantee a hard QoS constraint as always serving the RT service class users ahead of the NRT service class users. In spite of its QoS guarantee feature and simplicity, the serious disadvantage of a priority queuing scheme is that the multi-user diversity advantage of NRT users cannot be leveraged. This is true because RT users with the worse channel condition must be still served ahead of NRT users with the better channel condition even when some RT users can still wait for a while up to their deadline. To overcome such a Manuscript received November 24, 2007. The associate editor coordinating the review of this letter and approving it for publication was F. Granelli. This letter was presented in part at the IEEE Wireless Communications & Networking Conference, January 2006. This research was supported in part by the University IT Research Center Project, Korea and in part by grant No.R01-2003-000-10155-0(2005) from the Basic Research Program of the Korea Science & Engineering Foundation. C. G. Kang and T. W. Kim are with the Department of Electrical Engineering, Korea University, Seoul, Korea (e-mail:
[email protected]). J. H. Kim is with the Division of Industrial & Information Systems Engineering, Ajou University, Suwon, Korea. Digital Object Identifier 10.1109/LCOMM.2008.071984.
disadvantage of the priority queuing scheme, an opportunistic priority queuing scheme, referred to as the Delay Thresholdbased Priority Queuing (DTPQ) scheme, has been proposed for improving the overall system capacity subject to individual QoS requirements [4]. This scheme takes the relative urgency of the RT service into account only when its head-of-line (HOL) packet delays exceed a given delay threshold. In practice, however, it is not straightforward to configure the optimum delay threshold under varying service scenarios, e.g., depending on the numbers of RT and NRT users in the system, for the given QoS requirements. In this letter, we propose an adaptive version of the DTPQ scheme. Since it allows for varying a delay threshold subject to the relative urgency as the service scenario changes, a multiuser diversity gain of NRT users can be further leveraged. II. A DAPTIVE DTPQ- BASED P RIORITY Q UEUEING S CHEME A. Motivation: Delay Threshold Priority Queuing Scheme For the RT service class, there is a pre-specified packet loss rate requirement, which is governed by the maximum allowable delay, Wmax . A head of line (HOL) packet is dropped if its delay exceeds Wmax . In [4], we have introduced a HOL delay threshold as a design parameter, which is a reference to determine which service class is to be served first. Let the delay threshold be given by kWmax , where k is a control parameter that determines the priority of one service class over the other. Note that the kWmax must be less than Wmax , i.e., 0 ≤ k ≤ 1. In the first step of scheduling, only the HOL packets of RT service class users that exceed this threshold are scheduled. In the second step, all other packets, both HOL packets of remaining RT service class users and those of NRT service class users, are scheduled at the same time for the resource available after the first step. Herein, all users in the same service class are opportunistically served on the basis of their own priority scores. In the current discussion, we consider a well-known proportional fairness (PF) scheduling algorithm [5] for both RT and NRT service classes. The control parameter k must be determined so as to maximize a given objective function subject to the prescribed QoS requirement. The existing DTPQ algorithm in [4] does not address how the delay threshold can be set. In practice, it is not straightforward to determine the optimal value of the delay threshold, simply because it involves too many parameters that are varying from one frame to the other frame, e.g., the number
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IEEE COMMUNICATIONS LETTERS, VOL. 12, NO. 4, APRIL 2008
of RT and NRT service users for the given QoS requirements, and their individual channel conditions. B. Proposed Algorithm: Adaptive DTPQ Scheme In the proposed scheme, the delay threshold is updated in each frame by observing the current data rate ri (t) and the waiting time of the HOL packet Wihol (t). Let Urt and Unrt denote a set of users belonging to the RT and NRT service classes, respectively. In case that more RT service users suffer from a bad channel condition and/or a risk of outage performance, they must be treated more preferentially than the NRT service users, i.e., the delay threshold must be decreased. Towards this end, the number of urgent RT service users may be one of the most critical factors to consider for determining the delay threshold. Herein, the urgent users correspond to the RT service users whose delay has already exceeded the delay threshold in frame t. Since the delay threshold must be determined with respect to the urgency of the RT service users, we introduce an urgency metric, which takes the residual lifetime as well as the instantaneous data rate of the urgent RT service users into account. In particular, an urgent user with the worse channel condition and/or the less residual lifetime must be properly prioritized by reducing the delay threshold. Toward this end, the following urgency metric can be defined so as to represent the overall system urgency:
c(c) =
Yrt (t)
1 Wmax
Wihol (t)
i=1
Yrt (t) 1 ri (t) Yrt (t) i=1
(1)
where Yrt (t) represents the number of RT service users with the pending packets in frame t, i.e., Yrt (t)= hol i∈Urt u(Wi (t)), and u(x) being a unit step function. The urgency metric in (1) plays an essential role of updating the delay threshold in each frame. If it decreases, i.e., c(t)
