Performance evaluation of WIMAX MAC layer : IEEE- 802-16e

Performance evaluation of WIMAX MAC layer : IEEE- 802-16e
IJECT Vol. 2, Issue 3, Sept. 2011
ISSN : 2230-7109(Online) | ISSN : 2230-9543(Print)
Performance evaluation of WIMAX MAC layer :
IEEE- 802-16e
1
1,2,3
Neha Rathore, 2Dr. Sudhir Kumar, 3Abhishek Choubey
R.K.D.F. Institute of Technology & Science, Bhopal, MP, India
Abstract
The IEEE 802.16 standard which has emerged as a broadband
wireless access technology is capable of delivering very high
data rates. However, providing performance guarantees to
delay sensitive applications like streaming media is still a
challenge. In this paper, the media access control (MAC) layer of
WiMAX has been discussed and simulated; exploiting its flexible
features to dynamically construct the MAC-packet data units
(MPDU). The overhead caused by the MAC-layer of the network
is an important performance indicator, because it significantly
influences the throughput. It is interesting to know how the MACefficiency is dependent on the physical-layer. The MAC efficiency
for point-to-multipoint point topology is simulated based on
IEEE-802.16 standard under the influence of several overhead
parameters: the number of subscriber stations; modulation
techniques; error control coding techniques, and length of MACmessage’s PDU. The performance analysis can be used to the
IEEE 802.16 standard to provide several means to adapt the
MAC- and PHY-layer configuration to the system environment
and user demands. Using the features efficiently, the system
performance can be optimized by maintaining the robustness
and operability of the system.
Keywords
WiMAX-IEEE802.16, Medium Access Control efficiency, and
overhead; physical layer, and MAC-layer.
I. Introduction
New and emerging services such as Video on Demand (VoD),
Internet Protocol Television (IPTV), and triple play bring
multimedia content to end users whenever they want. The
next step is to deliver these contents wherever the users
are. Traditional wire-based access networks can deliver the
contents only to the fixed points. Hence, a new technology that
can deliver the contents to mobile users is needed. Worldwide
Interoperability for Microwave Access (WiMAX) technology is
based on IEEE-802.16–2004 and 802.16e−2005 standards
for fixed and mobile wireless access in metropolitan area
networks (MAN). It can deliver data rates of 70Mbps, cover
ranges in excess of 30km, and it can provide secure delivery
of content and support mobile users at vehicular speeds [1,
2]. WiMAX physical (PHY)-layer uses adaptive modulation
based on Orthogonal Frequency division multiplexing (OFDM)
and orthogonal frequency division multiple access (OFDMA).
Adaptive modulation is used to achieve the highest possible
data rate for a given link quality. Modulation can be adjusted at
very short time intervals, to provide robust transmission links
and high system capacity. The higher modulation constellations
offer a larger throughput per frequency-time slot but not all
users receive adequate signal levels to reliably decode all
modulation types. Users that are close to the base station that
exhibit good propagation and interference characteristics are
assigned with higher modulation constellations to minimize the
use of system resources. While users that are in less favorable
areas use the lower order modulations for communications to
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ensure data is received and decoded correctly at the expense
of additional frequency/time slots for the same amount of
throughput [3]. Assigning modulations based on the link
conditions increases the overall capacity of the system. WiMAX
MAC- layer supports real time poling services (rtPS) that
ensures required bandwidth and minimum latencies for video
services through quality of service (QoS). PHY-layer is resilient
to multipath fading channels. Moreover, it uses forward error
correction (FEC) to increase service quality. Since WiMAX-PHY
supports varying frame sizes and scalable bandwidth, WiMAX
is an ideal choice for new services. WiMAX is considered as an
all IP access network offering transparency for packet based
core networks. Additionally, WiMAX radios are designed not
to add any impairment to the content delivery. Hence, WiMAX
base stations (BSs), subscriber and mobile stations (SSs/
MSs) are ideally suited for the delivery of IP based services;
(triple play) VoIP, IPTV, internet multimedia over wireless MAN.
This makes WiMAX a superior choice over conventional cable,
DSL, and satellite solutions. WiMAX access networks will
offer the much desired ubiquity for the contents. Eventually,
WiMAX deployments will deliver IPTV to rural and underserved
regions with high degree of video and audio quality at affordable
prices. WiMAX Support two type of network topologies: point-tomultipoint and mesh. In point-to multipoint, the link connection
is only between BS and SS; a connection, used for the purpose
of transporting MAC management messages, required by the
MAC-layer. The overhead caused by the MAC-layer of the network
is an important performance indicator, because it significantly
influences the throughput [4-6]. This paper aims to discuss
the performance evaluation of WiMAX MAC-layer for point-tomultipoint topology. The influence of several parameters is
examined.
II. Overview of WiMAX
In 1998, IEEE formed a group called 802.16 to develop
standards for what was called a WMAN. The purpose of
developing 802.16 standards is to help industry to provide
compatible and interoperable solutions across multiple
broadband segments and facilitate the commercialization of
WiMAX products. Currently, WiMAX has two main variations:
one is for fixed wireless applications (covered by IEEE-802.162004 standard) and another is for mobile wireless services
(covered by IEEE-802.16e standard). Both of them are evolved
from IEEE-802.16 and IEEE- 802.16a, the earlier versions of
WMAN standards [7, 8]. The 802.16 standards only specify
the PHY-layer and MAC-layer of air interface while the upper
layers are not considered. IEEE-802.16 suite of standards
(IEEE-802.16- 2004/IEEE-802-16e-2005) defines within its
scope four PHY-layers, any of which can be used with the MAC
layer to develop a broadband wireless system. The PHY layers
are defined in IEEE-802.16 are: (i) WMAN-SC: a single carrier
PHY-layer intended for frequencies beyond 11GHz requiring
a LoS condition; (ii) WMAN-SC: a single carrier PHY-layer for
frequencies between 2GHz and 11GHz for point–to–multipoint
operations; (iii)WMAN-OFDM: a 256–point FFT–based OFDM-
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PHY layer for point–to–multipoint operations in NLoS conditions
at frequencies between 2GHz and 11GHz; (iv) WMAN-OFDMA: a
211-point FFT–based OFDMA-PHY layer for point–to–multipoint
operations in NLoS conditions at frequencies between 2GHz
and 11GHz. In the IEEE-802.16e–2005, this layer has been
modified to scalable OFDMA, where the FFT size is variable
and can take any one of the following values: 27, 29, 210, and
211 [6, 7]. The variable FFT size allows for optimum operation/
implementation of the system over a wide range of channel
bandwidths and radio conditions; this PHY-layer has been
accepted by WiMAX for mobile and portable operations and
is also referred to as mobile WiMAX [7]. Provisions have been
made to include advanced antenna systems in the WiMAX
standard to improve throughput and link.
IEEE 802.16 allows for several antennas to be used at the
transmitter and the receiver to create a Multiple-Input MultipleOutput (MIMO) system. Techniques such as space-time coding
can be used to reduce the occurrence of deep fades in the signal
level across the transmission band. An increase in throughput
can be achieved by spatially multiplexing different data streams
on each of the transmit antenna elements at the same time on
the same frequency [8]. The WiMAX MAC layer supports point-tomultipoint (PMP) and mesh topologies, both of which rely upon
a shared access medium. In PMP topology, a WiMAX network
is divided into cells and sectors consisting of one base station
(BS) and many subscriber stations (SS), similar to a cellular
telephone network. This architecture naturally lends itself to
PMP operation in the downlink direction, from BS to SS, where
time division duplex (TDD) or frequency-division duplex (FDD) is
used. In practice, TDD is typically used, where BS dynamically
adjusts the duration of the downlink and uplink portions of the
data frame, depending on the requirements. Uplink access is
usually TDMA, with scheduling fully controlled by the BS. The
MAC layer is connection-oriented and unidirectional. All service
flows are mapped to connections between BS and SS [8].
III. IEEE-802.16 MAC-Layer
The primary task of the WiMAX MAC-layer is to provide an
interface between the higher transport layers and the PHY-layer.
The MAC-layer takes packets from the upper layer- these packets
are called MAC service data units (SDUs) - and organizes them
into MAC protocol data units (PDUs) for transmission over the
air. For received transmissions, the MAC-layer does the reverse.
The IEEE- 802.16-2004 and IEEE 802.16e-2005 MAC design
includes three sublayers which interact with each other through
the service access points (SAPs) as shown in Fig.1. The service
specific convergence sublayer (CS) provides any transformation
or mapping of external network data, received through the CS
service access point (SAP). This includes classifying external
network service data units (SDU) and associating them with
the proper service flow identified by the connection identifier
(CID). A service flow is a unidirectional flow of packets that
is provided with a particular QoS. The MAC common part
sublayer is the core functional layer which provides system
access, bandwidth allocation, connection establishment, and
connection maintenance. It receives data from various CSs
classified to particular connection identifier CIDs. QoS is applied
to transmission and scheduling of data over the PHY layer.
Privacy sublayer provides authentication, secure key exchange
and encryption on the MAC PDUs formed from the MAC SDUs
and passes them over to the physical layer .
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ISSN : 2230-7109(Online) | ISSN : 2230-9543(Print)
IV. Common Part Sublayer Features
We focus on the common part sublayer to explore its rich set
of features. This sublayer controls the on-air timing based on
consecutive frames that are divided into time
slots. The size of these frames and the size of the individual
slots within these frames can be varied on a frame-by- frame
basis. This allows effective allocation of on-air resources [8].
A. Packing and Fragmentation
The common part sublayer is capable of packing more than one
complete or partial MAC SDUs into single MACPDU. In Fig.2, it
can be seen that the payload of the MACPDU can accommodate
more than two complete MACSDUs, but not three. Therefore,
a part of the third MACSDU is packed with the previous two
MAC-SDUs to fill up the remaining payload field preventing
wastage of resources. The payload size is determined by on-air
timing slots and feedback received from subscriber station.
The common part sublayer can also fragment a MAC-SDU into
multiple MAC-PDUs.
B. MAC-PDU Formats
MAC PDUs consist of a fixed-length MACheader, a variable-length payload and an
optional 32-bit cyclic redundancy check (CRC). Since the size
of the payload is variable, this allows the MAC to tunnel various
higher layer traffic types without knowledge of the formats of
those messages. The MAC header might be directly followed
by one or more subheaders. There are several different
subheaders carrying information for various purposes
such as Mesh, automatic repeat request (ARQ), packing or
fragmentation and grant management. Each MAC-PDU consists
of a header. Two MAC-header formats are defined: (i) the
generic MAC PDU generally used for carrying data and MAClayer signaling messages. A generic MAC-PDU starts with a
generic header whose structure is shown in Fig. 4 as followed
by a payload and a CRC. The various information elements in
the header of a generic MAC-PDU are shown in Table 1 and (ii)
the bandwidth request PDU is used by the MS to indicate to the
BS that more bandwidth is required in the UL, due to pending
data transmission. A bandwidth request PDU consists only of
a bandwidth-request header, with no payload or CRC [7].
Fig. 1. IEEE 802.16 protocol layering [4]
C. Quality of Service
One of the key functions of the WiMAX MAC-layer is to ensure
that QoS requirements for MAC-PDUs belonging to different
service flows are met as reliably as possible given the loading
conditions of the system. The WiMAX MAC-layer uses a
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scheduling service to deliver and handle SDUs and MAC PDUs
with different QoS requirements. WiMAX defines five scheduling
services such as: (i) unsolicited grant service (UGS); (ii) realtime Polling Service (rtPS); (iii) Non-real-time Polling Service
(nrtPS); (iv) Best Effort (BE), and (v) Extended real-time Polling
service (ertPS) [4].
D. ARQ Mechanism
The IEEE 802.16 ARQ mechanism is an optional part of the
MAC layer and can be enabled on a per-connection basis
during connection establishment. It is a bitmap-based ARQ
mechanism based on the fragment sequence number of the
fragmentation or packing subheader. The mechanism can
either work as a cumulative, a selective acknowledge or a
combined ARQ mechanism [8].
Fig. 2. Generic MAC PDU header [7]
E. MAC Support of PYH-Layer
The system supports a frame-based transmission, in which
the frame can adopt variable lengths. The frame structure of
FDM-PHY-layer operating in time division duplex (TDD) mode
is illustrated in Fig.5. Each frame consists of a downlink (DL)
subframe and an uplink (UL) subframe, with the DL subframe
always preceding the UL subframe. The following frame control
header (FCH) contains the DL frame prefix (DLFP) and occupies
one OFDM symbol. The DLFP specifies the location as well as
the modulation and coding scheme (PHY-mode) of up to four
DL bursts following the FCH. The mandatory modulation used
for the FCH is BPSK with code rate 1/2. The FCH is followed by
one or multiple DL bursts, which are ordered by their PHY-mode.
While the burst with the most robust PHY mode, e.g., BPSK 1/2
is transmitted first, the last burst is modulated and coded using
the highest PHY-mode, i.e., 64-QAM 3/4. Each DL burst is made
up of MAC packet data units scheduled for DL transmission. UL
subframe consists of contention intervals scheduled for initial
ranging and bandwidth request purposes and one or multiple
UL PHY transmission bursts, each transmitted from a different
SS. The initial ranging slots allow a SS to enter the system by
requesting the basic management CIDs, by adjusting its power
level and frequency offsets and by correcting its timing offset.
The bandwidth request slots are used by SSs to transmit the
bandwidth request header [1, 5].
F. MAC Management Messages
In the following the broadcast MAC-management messages are
described. Fig.6 shows the basic MAC management messages
used to specify the internal structure of the MAC frame. The time
references of the messages to the corresponding elements
of the MAC frame are indicated by arrows. The DLFP contains
up to four information elements (IEs). Each IE specifies a DL
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IJECT Vol. 2, Issue 3, Sept. 2011
burst. Thus, the DLFP can specify up to four DL bursts. If the DL
subframe is made up of more than four bursts, an additional
DL-MAP specifies the remaining ones. If there are less than four
bursts present, the DLFP is sufficient and The DLFP IE contains
the length and the PHY mode of the corresponding DL burst. DL
burst 1 contains the broadcast MAC control messages, i.e., DL
and UL channel descriptor (DCD, UCD) as well as the UL- and
DL-MAP. DCD and UCD define the characteristic of the physical
channels. The DL-MAP defines access to the DL channel and
the UL-MAP allocates access to the UL channel. Thus, the whole
MAC frame is specified by the MAC messages included in the
FCH and the DL burst [1, 5].
V. IEEE-802.16 OFDM Physical layer
The role of the PHY-layer is to encode the binary digits that
represent MAC frames into signals and to transmit and receive
these signals across the communication media. The WiMAXPHY-layer is based on OFDM; which is used to enable high-speed
data, video, and multimedia communications and is used by
a variety of commercial broadband systems. The PHY-layer of
WiMAX includes various functional stages: (i) FEC: including;
randomizing, channel encoding, rate matching, interleaving,
and symbol mapping; (ii) OFDM symbol in frequency domain,
and (iii) conversion of the OFDM symbol from the frequency
domain to the time domain [9]. In the following, the basic
modules of IEEE 802.16 transmitter respectively receiver are
outlined [10]. A randomizer adds a pseudo-random binary
sequence to the DL and UL bit stream to avoid long rows of
zeros or ones for better coding performance. The forward error
correction (FEC) scheme consists of the concatenation of a
Reed–Solomon (RS) outer code and a convolutional inner code
(CC). The RS coder corrects burst errors at the byte level. After
the RS encoding process, data bits are further encoded by a
binary CC, which has a native rate of 1/2 and a constraint
length of 7. The CC corrects independent bit errors. Puncturing
is the process of systematically deleting bits from the output
stream of a low-rate encoder in order to reduce the amount
of data to be transmitted, thus forming a high-rate code. The
process of puncturing is used to create the variable coding
rates needed to provide various error protection levels to the
users of the system. The interleaver is defined by a two-step
permutation. First ensures that adjacent coded bits are mapped
onto nonadjacent subcarriers to overcome burst errors. The
second permutation insures that adjacent coded bits are
mapped alternately onto less or more significant bits of the
constellation, thus avoiding long runs of lowly reliable bits [8].
BPSK, QPSK, 16- QAM and 64-QAM are the modulation schemes
to modulate bits to the complex constellation points. The FEC
options are paired with the modulation schemes to form burst
profiles. OFDM is a multicarrier modulation technique, which
provides high bandwidth efficiency because the carriers are
orthogonal to each other and multiple carriers share the data
among themselves. The main advantage of this transmission
technique is their robustness to channel fading [11]. At the
receiving side, a reverse process (including deinterleaving and
decoding) is executed to obtain the original data bits. As the
deinterleaving process only changes the order of received data,
the error probability is intact. When passing through the CC
decoder and the RS- decoder, some errors may be corrected,
which results in lower error rate.
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ISSN : 2230-7109(Online) | ISSN : 2230-9543(Print)
VII. Conclusions
A detailed introduction to the IEEE 802.16 MAC and PHY-layer
protocol is presented. The MAC-frame structure is illustrated
and the basic control elements are described. The basic PHYlayer chain modules of the IEEE 802.16 transmitter and receiver
are outlined as well. Based on the PHY-layer the MAC-layer
capacity is calculated. The MAC-layer configuration is analyzed
in the context of throughput and overhead. Performance on
the MAC-layer of the IEEE 802.16 WMAN standard will be
simulated.
Fig. 3. WiMAX 802.16e simulator
VI. Simulation
In this work the performance of IEEE 802.16 MAC- frame
structure and WMAN-OFDM PHY-layer characteristics will be
modeled and simulated using MATLAB 2008. The following
parameter will be considered for the simulation study.
A. Simulation Parameters
1. OFDM Parameters
The standard defines two types of parameters, the primitive
parameters, that will be specified by users or system
requirements and the derived parameters, defined in terms
of the primitive ones. OFDM parameters are given in Table.
Table 1 : OFDM Simulation Parametrs
References
[1] R.B. Marks, K. Stanwood, D. Chang, et al., “IEEE
Standard for Local and Metropolitan Area Networks, Part
16: Air Interface for Fixed Broadband Wireless Access
Systems,”October, 2004.
[2] S. Ahson, M. Ilyas, “WiMAX: Standards and Security," CRC
Press, (Taylor and Francis Group), 2008.
[3] F. Ohrtman, “WiMAX Handbook: Building 802.16 Wireless
Networks,” McGraw-Hill, 2008.
[4] J.G. Andrews, A. Ghosh, R. Muhamed, “Fundamentals of
WiMAX: Understanding Broadband Wireless Networking,”
Prentice Hall Communications AND Engineering
Technologies Series, 2007.
[5] A. Roca “Implementation of WiMAX simulator in Simulink”,
Engineering Institute-Vienna, February 2007.
[6] M.A. Mohamed, M.S. Seoud, H.M. Abd-El-Atty, “Performance
Simulation of IEEE 802.16e WiMAX Physical Layer”. IEEEICIME vol.2.p.p 661-667, April, 2010.
[7] WiMAX Forum, “Mobile WiMAX - Part I: A Technical
Overview and Performance Evaluation,” white paper,
August 2006.
[8] M.A. Hasan, “Performance Evaluation of WiMAX/IEEE
802.16 OFDM Physical Layer,” Master of Science in
Technology, Espoo, June 2007.
2. IEEE802.16-OFDM-PHY Layer Parameters
The system supports four modulation schemes and two channel
models: additive white Gaussian noise (AWGN) and Rayleigh
fading. The channel coding part is composed of three steps:
randomization; FEC, and interleaving.
3. IEEE802.16-MAC Layer Parameters:
The model of 16 MAC-frame structures takes all the above
introduced features into account (MAC-header, CRC, broadcast
MAC-management messages. The scheduling is based on a
fair queuing algorithm where all SSs are treated equally; ARQ
also disabled. Besides the main parameter, the number of
Nss, length of data MAC-PDUs, modulation/coding used, and
number of burst profiles are chosen for the evaluated efficiency
of MAC-layer.
The net throughput of MAC-layer is defined by the ratio of the
total number of payload bits, i.e. without all MAC-overhead in
a frame to the frame duration T frame.
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