Two-dimensional downlink burst construction in IEEE 802.16 networks

Lai Yuan-Cheng, Chen, Chen Yen-Hung

Description

Several burst construction algorithms for orthogonal frequency division multiple access were proposed. However, these algorithms did not meet the downlink burst characteristics specified in the IEEE 802.16 standard. This article therefore proposes the best corner-oriented algorithm (BCO). BCO not only complies with downlink burst characteristics, but also considers the three issues to obtain high throughput, as follows: BCO maintains all free slots as a continuous area by constructing each burst in the corner of the available bandwidth area for minimizing external fragmentation; BCO shrinks the burst area to minimize internal fragmentation, if the requested bandwidth has been satisfied; and for exploring the continuous subchannels with good channel quality, BCO ensures that the burst adopts an optimal modulation coding scheme by selecting the excellent corner that can generate the maximal throughput. The simulation results indicate that BCO achieves 2-9 times the throughput achieved by the previous algorithms under a heavy load.

Sujets

• Downlink
• IEEE
• WiMAX
• Orthogonal Frecuency Division Multiple Access

Informations

• Publié par: biomed
• Publié le: 01 janvier 2011
• Nombre de lectures: 5
• Langue: English
• Poids de l'ouvrage: 1 Mo

Extrait

Lai and Chen EURASIP Journal on Wireless Communications and Networking 2011, 2011:173
http://jwcn.eurasipjournals.com/content/2011/1/173
RESEARCH Open Access
Two-dimensional downlink burst construction in
IEEE 802.16 networks
*Yuan-Cheng Lai and Yen-Hung Chen
Abstract
Several burst construction algorithms for orthogonal frequency division multiple access were proposed. However,
these algorithms did not meet the downlink burst characteristics specified in the IEEE 802.16 standard. This article
therefore proposes the best corner-oriented algorithm (BCO). BCO not only complies with downlink burst
characteristics, but also considers the three issues to obtain high throughput, as follows: BCO maintains all free
slots as a continuous area by constructing each burst in the corner of the available bandwidth area for minimizing
external fragmentation; BCO shrinks the burst area to minimize internal fragmentation, if the requested bandwidth
has been satisfied; and for exploring the continuous subchannels with good channel quality, BCO ensures that the
burst adopts an optimal modulation coding scheme by selecting the excellent corner that can generate the
maximal throughput. The simulation results indicate that BCO achieves 2-9 times the throughput achieved by the
previous algorithms under a heavy load.
Keywords: burst construction, downlink, IEEE, 802.16, OFDMA
1. Introduction attempted to determine the optimal matches between
Because IEEE 802.16 uses the technique of orthogonal bursts and subchannels [3-8].
frequency division multiple access (OFDMA), the band- The IEEE 802.16 standard defines a number of specifi-
width resources are represented by a two-dimensional cations to alleviate the overhead of management mes-
area of slots, in which the two dimensions are time in sages and to concentrate the transmission power on
units of symbols and frequency in units of subchannels specific subchannels for battery-powered devices, as fol-
[1]. Therefore, the bandwidth allocation in IEEE 802.16 lows: (1) the burst must be a continuous bandwidth
must consider the construction of a two-dimensional area, (2) the shapes of the bursts used in downlink and
bandwidth area, called a burst, assigned to a connection. uplink transmissions should be rectangular and multi-
The subchannel diversity should be considered when rectangular, respectively, and (3) one burst should use
constructing bursts. Subchannel diversity means that a only one MCS based on the worst signal-to-noise ratio
connection uses a different modulation coding scheme (SNR) among the assigned subchannels [1,9].
(MCS) on various subchannels because the connection The previous researches that focused on the maxi-
encounters various channel qualities on various sub- mum matching problem violated the specifications in
channels [2]. Therefore, for each connection, each burst IEEE 802.16 standard, and are thus unpractical. There-
must be constructed in its corresponding best-quality fore, a number of researchers regarded the burst con-
struction problem as a variant of the bin packingsubchannels, i.e., the subchannels on which the connec-
problem. So-In et al. [10] designed the enhanced one-tion receives the optimal channel quality to maximize
bandwidth usage. Several algorithms for the IEEE 802.16 column striping with non-increasing area first mapping
burst construction problem were proposed to obtain the algorithm (eOCSA), which constructs each burst from
higher throughput. A number of researchers regarded bottom right to top left of the available bandwidth area.
this problem as a maximum matching problem and Wang et al. [11] developed the weighted less flexibility
first algorithm (WLFF), which constructs each burst on
athe best edge selected in the free bandwidth area. The
* Correspondence: [e-mail]
best edge is the edge on which a constructed burstDepartment of Information Management, National Taiwan University of
Science and Technology, #43, Sec. 4, Keelung Rd., Taipei 106, Taiwan
© 2011 Lai and Chen; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.Lai and Chen EURASIP Journal on Wireless Communications and Networking 2011, 2011:173 Page 2 of 18
http://jwcn.eurasipjournals.com/content/2011/1/173
generates the minimal variance of the sub-blocks in the 2. Background
free bandwidth area. Thus, constructing the burst on 2.1. IEEE 802.16 network
this best edge provides the most flexibility for the fol- The IEEE 802.16 network consists of a base station (BS)
lowing burst construction. eOCSA and WLFF conform and a number of subscriber stations (SSs). The BS pro-
to the specifications (1) and (2); however, they comple- vides connectivity, radio resource management, and
tely neglect subchannel diversity and the specification control of SS, which supports the connectivity with the
(3). BS.
A number of issues must be addressed to conform to The two layers in the IEEE 802.16 protocol stack are
the specifications and maximize the throughput. First, the physical layer, which transfers raw data, and the
external fragmentation may occur because the burst MAC layer, which supports the physical layer by ensur-
must be a continuous bandwidth area, which means that ing that the radio resources are used efficiently. The
the total available slots are sufficient to satisfy a burst; three duplex modes in the physical layer with OFDMA
however, the lack of contiguity may prevent their use by are Time Division Duplex (TDD), Frequency Division
this burst. Thus, the external fragmentation should be Duplex (FDD), and Half-duplex Frequency Division
avoided. Second, because of the rectangular shape of a (H-FDD). The TDD is the most attractive
downlink burst or improper slot allocation, internal duplex mode because of its flexibility. In addition, the
fragmentation may occur, which results from a burst modulation methods, that is quadrature phase shift key-
with capacity exceeding the requested bandwidth. The ing (QPSK), 16 quadrature amplitude modulation
internal fragmentation must be minimized because the (16QAM), or 64 quadrature amplitude modulation
unused slots internal to a burst are wasted. Third, (64QAM), and the associated coding rate for data trans-
because one burst must use one MCS based on the mission are selected according to the channel quality,
worst SNR among the assigned subchannels, it must be that is, signal-to-noise ratio (SNR).
constructed in its corresponding optimal block, i.e., a An IEEE 802.16 frame for downlink and uplink trans-
block in which a number of continuous subchannels missions is divided into downlink (DL) and uplink (UL)
have good SNRs. subframes in the time domain of the TDD mode (the
Therefore, this article proposes a one downlink burst right part of Figure 1). A burst is an allocated band-
construction algorithm, called the best corner-oriented width assigned to one dedicated connection of one SS
algorithm (BCO), to maximize the throughput. BCO not and is formed by slots. A slot is the minimal bandwidth
only conforms to the constraints in IEEE 802.16 stan- allocation unit, and consists of one subchannel and one
dards, but also considers these issues. To avoid external to three symbols. A subchannel is the smallest allocation
fragmentation, BCO constructs each burst in a corner of unit in the frequency domain, and a symbol is the smal-
the free bandwidth area to ensure that all free slots are lest allocation unit in the time domain. A number of
within a continuous area. A corner is the intersection of other fields in a frame provide specific functions. For
the horizontal edge and left-hand vertical edge of the example, preamble synchronizes each SS, DL/UL-MAP
free bandwidth area. To minimize internal fragmenta- describes the position and measure of each downlink/
tion, BCO shrinks the area of the burst if the requested uplink burst, and frame control header specifies DL sub-
bandwidth is satisfied to enable unused slots internal to frame prefix and the length of DL-MAP message.
this burst to be used by other bursts. BCO evaluates the In the IEEE 802.16, the SS must acquire bandwidth
channel quality in each corner to explore an optimal from the BS before transmitting or receiving data. On
block, and subsequently constructs the optimal burst in downlink, the BS broadcasts to all SSs, and each SS
the corner in which the burst can provide the largest picks up its destined packets. On uplink, SSs must
throughput. inform the BS of the bandwidth they require for data
This article is organized as follows: Section 2 presents transmission by sending a bandwidth request (BWR).
a discussion of the literature on the IEEE 802.16 net- Upon receiving the BWRs, the BS allocates the bursts in
work, the burst construction in downlink transmission, an uplink subframe to each SS, and subsequently broad-
and related studies. In Section 3, the problem statement casts this information through UL-MAP. After receiving
of the downlink burst construction is formally intro- UL-MAP, each SS uses the allocated burst to transmit
duced, and the issues to solve this problem are pre- its data.
sented. Section 4 provides a description of the proposed Figure 1 demonstrates that, for efficient bandwidth
BCO algorithm in detail. In Section 5, the superior per- use, th