IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 55, NO. 3, MAY 2006 839
A Cross-Layer Scheduling Algorithm With QoS
Support in Wireless Networks
Qingwen Liu, Student Member, IEEE, Xin Wang, Member, IEEE, and Georgios B. Giannakis, Fellow, IEEE
Abstract—Scheduling plays an important role in providing
quality of service (QoS) support to multimedia communications
in various kinds of wireless networks, including cellular networks,
mobile ad hoc networks, and wireless sensor networks. The au-
thors propose a scheduling algorithm at the medium access control
(MAC) layer for multiple connections with diverse QoS require-
ments, where each connection employs adaptive modulation and
coding (AMC) scheme at the physical (PHY) layer over wireless
fading channels. Each connection is assigned a priority, which
is updated dynamically based on its channel and service status;
the connection with the highest priority is scheduled each time.
The authors’ scheduler provides diverse QoS guarantees, uses the
wireless bandwidth efficiently, and enjoys flexibility, scalability,
and low implementation complexity. Its performance is evaluated
via simulations.
Index Terms—Adaptive modulation and coding (AMC),
cross-layer design, IEEE 802.16, quality of service (QoS), schedul-
ing algorithm, wireless networks, Worldwide Interoperability for
Microwave Access (WiMAX).
I. INTRODUCTION
T
HE RAPID growth in demand for high-speed and high-
quality multimedia communications is creating opportuni-
ties and challenges for next-generation wired–wireless network
designs. Multimedia communications entail diverse quality of
service (QoS) requirements for different applications including
voice, data and real time, or streaming video/audio. Providing
QoS-guaranteed services is necessary for future wireless net-
works, including cellular networks, mobile ad hoc networks,
and wireless sensor networks, e.g., IEEE 802.16, IEEE 802.11,
and IEEE 802.15 standard wireless networks. Such networks
are envisioned to support multimedia services with different
QoS requirements. However, the aforementioned standards de-
fine only QoS architecture and signaling, but do not specify the
scheduling algorithm that will ultimately provide QoS support.
Scheduling plays an important role in QoS provision. Al-
though many traffic scheduling algorithms are available for
wireline networks [25], they cannot be directly applied to wire-
Manuscript received September 12, 2005; revised December 28, 2005. This
work was prepared through collaborative participation in the Communications
and Networks Consortium sponsored by the U.S. Army Research Laboratory
under the Collaborative Technology Alliance Program, Cooperative Agreement
DAAD19-01-2-0011. The U.S. Government is authorized to reproduce and
distribute reprints for Government purposes notwithstanding any copyright
notation thereon. The material in this paper was presented in part at the 2nd
International Conference on QoS in Heterogeneous Wired/Wireless Networks
(QShine), Orlando, FL, August 22–24, 2005. The review of this paper was
coordinated by Prof. X. Shen.
The authors are with the Department of Electrical and Computer Engineer-
ing, University of Minnesota, Minneapolis, MN 55455 USA (e-mail: qliu@
ece.umn.edu; xinwest@ece.umn.edu; georgios@ece.umn.edu).
Digital Object Identifier 10.1109/TVT.2006.873832
Fig. 1. Network topology.
less networks because of the fundamental differences between
the two [9], [11]. For example, traditional schedulers for wire-
line networks only consider traffic and queuing status; however,
channel capacity in wireless networks is time varying due to
multipath fading and Doppler effects. Even if large bandwidth
is allocated to a certain connection, the prescribed delay or
throughput performance may not be satisfied, and the allocated
bandwidth is wasted when the wireless channel experiences
deep fades. An overview of scheduling techniques for wireless
networking can be found in [9], where a number of desirable
features have been summarized, and many classes of schedulers
have been compared on the basis of these features. To schedule
wireless resources (such as bandwidth and power) efficiently
for diverse QoS guarantees, the interactive queuing behavior
induced by heterogenous traffic as well as the dynamic variation
of wireless channel should be considered in scheduler design.
In this paper, we introduce a priority-based scheduler at the
medium access control (MAC) layer for multiple connections
with diverse QoS requirements, where each connection em-
ploys adaptive modulation and coding (AMC) scheme at the
physical (PHY) layer. We define a priority function (PRF) for
each connection admitted in the system and update it dynami-
cally depending on the wireless channel quality, QoS satisfac-
tion, and service priority across layers. Thus, the connection
with the highest priority is scheduled each time. Our scheduler
provides prescribed QoS guarantees and utilizes the wireless
bandwidth efficiently while enjoying low implementation com-
plexity, flexibility, and scalability.
II. S
YSTEM ARCHITECTURE
A. Network Configuration
Fig. 1 illustrates the wireless network topology under consid-
eration. Multiple subscriber stations (SS) are connected to the
base station (BS) or relay station over wireless channels, where
multiple connections (sessions, flows) can be supported by each
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