17-12-2012, 06:33 PM
Dynamic admission control and bandwidth reservation for IEEE 802.16e mobile WiMAX networks
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Abstract
The article presents a dynamic connection admission control (CAC) and bandwidth reservation (BR) scheme for
IEEE 802.16e Broadband Wireless Access networks to simultaneously improve the utilization efficiency of network
resources and guarantee QoS for admitted connections. The proposed CAC algorithm dynamically determines the
admission criteria according to network loads and adopts an adaptive QoS strategy to improve the utilization
efficiency of network resources. After new or handoff connections enter the networks based on current admission
criteria, the proposed adaptive BR scheme adjusts the amount of reserved bandwidth for handoffs according to
the arrival distributions of new and handoff connections in order to increase the admission opportunities of new
connections and provide handoff QoS as well. We conduct simulations to compare the performance of our
proposed CAC algorithm and BR scheme with that of other approaches. The results illustrate that our approach
can effectively improve the network efficiency in terms of granting more connections by as large as about 22% in
comparison with other schemes, and can also guarantee adaptive QoS for admitted new and handoff connections
Introduction
Broadband wireless access networks have rapidly been
growing in these years to support the increasing
demands of wireless multimedia services, like streaming
audio/video, Internet Protocol TV, and video conferencing.
Mobile Worldwide Interoperability for Microwave
Access (WiMAX), which has been standardized by IEEE
802.16e [1], is one of the most promising solutions to
provide ubiquitous wireless access with high data rates,
high mobility, and wide coverage. The IEEE 802.16e
Media Access Control (MAC) layer provides differential
Quality of service (QoS) for various classes of scheduling
services, which are Unsolicited Grant Service (UGS),
Extended Real-Time Polling Service (ertPS), Real-Time
Polling Service (rtPS), Non-real-time Polling Service
(nrtPS), and Best Effort (BE). Each scheduling class is
associated with a set of QoS parameters for quantifying
its bandwidth requirement, e.g., maximum/minimum
data rates and maximum delays.
Bandwidth allocation mechanism
The IEEE 802.16e physical layer (PHY) adopts an
Orthogonal Frequency Division Multiple Access
(OFDMA) slot as the minimum possible resource. The
IEEE 802.16e PHY supports Frequency Division Duplex
(FDD) and Time Division Duplex (TDD) for bandwidth
allocation mechanisms. In FDD mode, the uplink (UL)
and downlink (DL) channels are located on split frequencies,
with which a fixed duration frame is used for
both UL and DL transmissions. In TDD mode, the UL
and DL transmissions are arranged at different time periods
using the same frequency. In this article, we focus
on the TDD mode for the IEEE 802.16e bandwidth allocation
mechanism.
In TDD mode, Time Division Multiplexing (TDM) is
used for DL transmissions and Time Division Multiple
Access (TDMA) is used for UL transmissions. As shown
in Figure 2, a TDD frame has a fixed duration and contains
one DL subframe and one UL subframe whose
durations can adapt to the traffic loads of UL and DL
transmissions. The DL subframe consists of a preamble,
Frame Control Header (FCH), and a number of data
bursts.
Packet scheduling mechanism
As shown in Section 2.1, the IEEE 802.16e standard
defines five scheduling classes. However, it does not specify
the scheduling mechanism for the five classes and
the design is left for researchers [25]. The design of a
scheduling mechanism must take into account the specific
QoS constraints of different applications, e.g. the
maximum allowable delay and minimum data rate [3].
A feasible solution is to decide on a service class first
according to the characteristics of each class and next
choose an appropriate user in the selected class [26]. In
the second phase, the packet scheduling of different
users among a given class may consider some performance
metrics such as throughput and fairness, while
the maximum rate scheduling (greedy algorithm) and
Proportional Fairness (PF) scheduling can be applied,
respectively. The maximum rate scheduling is effective
to advance the overall system throughput as it allocates
resources to users with relatively good channel qualities
among them [27]. On the other hand, the PF scheduling
can improve the fairness of channel utilization among
users as it distributes resources among them with consideration
of their previous records of utilization
[28-30].
Estimation of system capacity
The system capacity B may be dynamic as the IEEE
802.16e standards on PHY support multiple transmission
rates by using adaptive modulation and coding
(AMC) schemes. The transmitter will determine one
from various modulation and coding schemes (MCSs)
available according to the channel conditions of packet
delivery to provide reliable link qualities, large network
coverage, and high data rates as possible. The modulation
types supported in the IEEE 802.16e standards
include Binary Phase-Shift Keying (BPSK), Quadrature
Phase-Shift Keying (QPSK), 16-Quadrature Amplitude
Modulation (16-QAM), and 64-QAM. With the Convolutional
Turbo Code (CTC) and different code rates, the
MCSs provided for WiMAX with 5 and 10 MHz channels
are summarized in Table 2[35]. In a DL transmission
for example, each MS informs its current perceived
channel quality to the BS periodically, and then the BS
will choose a specific MCS corresponding to this channel
condition.
Performance evaluations and results
In this section, we conduct simulations of 802.16e transmission
scenarios to demonstrate the effectiveness of
the proposed CAC algorithm and BR scheme. The simulator
is constructed in C and followed the IEEE 802.16e
standard closely [35,38]. The channel spectrum is 10
MHz. The MAC frame duration is 5 ms, which consists
of 1024 OFDM subcarriers (840 data and pilot subcarriers).
One MAC frame includes 48 OFDM symbols,
while the first symbol is used for a preamble. The ratio
of the symbols of the uplink subframe to those of the
downlink subframe is 18:29. In the uplink, three symbols
are used for control signaling, and there are 44 OFDM
symbols used for data transmissions in the uplink and
downlink in total. The simulation set-up considers a 2 ×
2 MIMO mechanism and the AMC schemes.