25-06-2012, 12:05 PM
Bandwidth Recycling in IEEE 802.16 Networks
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Abstract
IEEE 802.16 standard was designed to support the bandwidth demanding applications with quality of service (QoS).
Bandwidth is reserved for each application to ensure the QoS. For variable bit rate (VBR) applications, however, it is difficult for
the subscriber station (SS) to predict the amount of incoming data. To ensure the QoS guaranteed services, the SS may reserve
more bandwidth than its demand. As a result, the reserved bandwidth may not be fully utilized all the time. In this paper, we propose
a scheme, named Bandwidth Recycling, to recycle the unused bandwidth without changing the existing bandwidth reservation.
INTRODUCTION
The Worldwide Interoperability for Microwave Access
(WiMAX), based on IEEE 802.16 standard standards [1]
[2], is designed to facilitate services with high transmission
rates for data and multimedia applications in
metropolitan areas. The physical (PHY) and medium
access control (MAC) layers of WiMAX have been specified
in the IEEE 802.16 standard. Many advanced communication
technologies such as Orthogonal Frequency-
Division Multiple Access (OFDMA) and multiple-input
and multiple-output (MIMO) are embraced in the standards.
Supported by these modern technologies,WiMAX
is able to provide a large service coverage, high data
rates and QoS guaranteed services. Because of these features,
WiMAX is considered as a promising alternative
for last mile broadband wireless access (BWA).
BACKGROUND INFORMATION
The IEEE 802.16 standard specifies three types of transmission
mediums supported as the physical layer (PHY):
single channel (SC), Orthogonal frequency-division multiplexing
(OFDM) and Orthogonal Frequency-Division
Multiple Access (OFDMA). We assume OFDMA as the
PHY in our analytical model since it is employed to support
mobility in IEEE 802.16e standard and the scheme
working in OFDMA should also work in others. There
are four types of modulations supported by OFDMA:
BPSK, QPSK, 16-QAM and 64-QAM.
This paper is focused on the point-to-multipoint
(PMP) mode in which the SS is not allowed to communicate
with any other SSs but the BS directly. Based on
the transmission direction, the transmissions between BS
and SSs are classified into downlink (DL) and uplink
(UL) transmissions. The former are the transmissions
from the BS to SSs. Conversely, the latter are the transmissions
in the opposite direction.
MOTIVATION AND RELATED WORK
Bandwidth reservation allows IEEE 802.16 networks to
provide QoS guaranteed services. The SS reserves the
required bandwidth before any data transmissions. Due
to the nature of VBR applications, it is very difficult for
the SS to make the optimal bandwidth reservation. It is
possible that the amount of reserved bandwidth is more
than the demand. Therefore, the reserved bandwidth
cannot be fully utilized. Although the reserved bandwidth
can be adjusted via BRs, however, the updated
reserved bandwidth is applied as early as to the next
coming frame and there is no way to utilize the unused
bandwidth in the current frame. In our scheme, the
SS releases its unused bandwidth in the current frame
and another SS pre-assigned by the BS has opportunities
to utilize this unused bandwidth.
Analysis of potential unused bandwidth
Based on the traffic generation rate, the applications
can be classified into two types: constant bit rate (CBR)
and variable bit rate (VBR). Since CBR applications
generate data in a constant rate, SSs rarely adjust the
reserved bandwidth. As long as the reasonable amount
of bandwidth is reserved, it is hard to have unused
bandwidth in this type of applications. Therefore, our
scheme has very limited benefit on CBR traffic. However,
VBR applications generate data in a variable rate. It
is hard for a SS to predict the amount of incoming
data precisely for making the appropriate bandwidth
reservation. Thus, in order to provide QoS guaranteed
services, the SS tends to keep the amount of reserved
bandwidth to serve the possible bursty data arrived in
the future. The reserved bandwidth may not be fully utilized
all the time. Our analysis focuses on investigating
the percentage of potentially unused bandwidth of VBR
traffics.
CONCLUSIONS
Variable bit rate applications generate data in variant
rates. It is very challenging for SSs to predict the amount
of arriving data precisely. Although the existing method
allows the SS to adjust the reserved bandwidth via
bandwidth requests in each frame, it cannot avoid the
risk of failing to satisfy the QoS requirements. Moreover,
the unused bandwidth occurs in the current frame
cannot be utilized by the existing bandwidth adjustment
since the adjusted amount of bandwidth can be applied
as early as in the next coming frame. Our research
does not change the existing bandwidth reservation
to ensure that the same QoS guaranteed services are
provided. We proposed bandwidth recycling to recycle the
unused bandwidth once it occurs.