21-01-2013, 02:59 PM
ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING FOR WIRELESS NETWORKS
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Abstract.
Orthogonal frequency division multiplexing (OFDM) is a special case of
multicarrier transmission, where a single datastream is transmitted over a number of lower rate
subcarriers. In July 1998, the IEEE standardization group decided to select OFDM as the
basis for their new 5-GHz standard, targeting a range of data stream from 6 up to 54 Mbps.
This new standard is the first one to use OFDM in packet-based communications, while the
use of OFDM until now was limited to continuous transmission systems.
In this project, transmitter and receiver were simulated according to the parameters
established by the standard, to evaluate the performance and different possibilities in the
implementation. Also, some considerations about forward error correction coding,
synchronization and channel estimation are given oriented to improve the system performance.
Introduction
OFDM is of great interest by researchers and research laboratories all over the
world. It has already been accepted for the new wireless local area network standards IEEE
802.11a, High Performance LAN type 2 (HIPERLAN/2) and Mobile Multimedia Access
Communication (MMAC) Systems. Also, it is expected to be used for wireless broadband
multimedia communications.
Data rate is really what broadband is about. The new standard specify bit rates of up
to 54 Mbps. Such high rate imposes large bandwidth, thus pushing carriers for values
higher than UHF band. For instance, IEEE802.11a has frequencies allocated in the 5- and
17- GHz bands.
This project is oriented to the application of OFDM to the standard IEEE 802.11a,
following the parameters established for that case.
OFDM can be seen as either a modulation technique or a multiplexing technique.
One of the main reasons to use OFDM is to increase the robustness against frequency
selective fading or narrowband interference. In a single carrier system, a single fade or
interferer can cause the entire link to fail, but in a multicarrier system, only a small
percentage of the subcarriers will be affected. Error correction coding can then be used to
correct for the few erroneous subcarriers. The concept of using parallel data transmission
and frequency division multiplexing was published in the mid-1960s [1, 2].
The Standard IEEE 802.11a
The IEEE 802.11 specification is a wireless LAN (WLAN) standard that defines a set
of requirements for the physical layer (PHY) and a medium access control (MAC) layer.
For high data rates, the standard provides two PHYs - IEEE 802.11b for 2.4-GHz operation
and IEEE 802.11a for 5-GHz operation. The IEEE 802.11a standard is designed to serve
applications that require data rates higher than 11 Mbps in the 5-GHz frequency band.
The wireless medium on which the 802.11 WLANs operate is different from wired
media in many ways. One of those differences is the presence of interference in unlicensed
frequency bands, which can impact communications between WLAN NICs. Interference
on the wireless medium can result in packet loss, which causes the network to suffer in
terms of throughput performance.
Current 2.4-GHz 802.11b radios handle interference well because they support a
feature in the MAC layer known as fragmentation. In fragmentation, data frames are
broken into smaller frames in an attempt to increase the probability of delivering packets
without errors induced by the interferer.
Carrier Sense Multiple Access/Collision Avoidance
Since different physical transmission layers are supported by IEEE 802.11, the
wireless MAC protocol should be transparency to physical layers, which include Direct
Sequence Spread Spectrum (DSSS), Frequency Hopping Spread Spectrum (FHSS)
diffused infrared and recently OFDM. Since spectrum is a scare resource above all
different physical layers, the throughput and packet delay performance is one of the most
critical considerations in the design of a wireless MAC protocol.
The basic protocol level in the 802.11 MAC protocol is the Distributed
Coordination Function (DCF), which supports asynchronous communication between
multiple users [5]. The DCF allows sharing medium between similar and dissimilar
systems through the use of the CSMA/CA and a random back off delay algorithm. The
CSMA/CA is similar to the Carrier Sense Multiple Access with Collision Detection
(CSMA/CD) used in a Ethernet. As the Ethernet, the CSMA/CA uses carrier-sense
mechanism to determine whether other terminals are using the medium. If a channel is
sensed idle, the packet transmission is started immediately in both cases. However, if the
channel is sensed busy, the CSMA/CA and the CSMA/CD operate differently to resolve
the contention. In the case of the CSMA/CD, when a terminal senses a busy channel, it
waits until the channel goes idle and then it transmits a packet with probability one. When
two or more terminals are waiting to transmit, a collision is absolutely occurred because
each terminal will transmit immediately at the end of channel busy period. While a
terminal, operates in the CSMA/CA protocol, senses the busy channel, it waits until the
channel goes idle and waits for delay period, which is called backoff delay. In the
CSMA/CA, the collision probability between multiple terminals under above situation is
reduced since a random backoff arrangement is used to resolve medium contention
conflicts. The Collision Detection (CD) function detects collisions in the CSMA/CD, but
the CD function is not viable in wireless LANs because the dynamic range of signals in the
medium is very large.
Propagation Characteristics of mobile radio channels
In an ideal radio channel, the received signal would consist of only a single direct
path signal, which would be a perfect reconstruction of the transmitted signal. However in
a real channel, the signal is modified during transmission in the channel. The received
signal consists of a combination of attenuated, reflected, refracted, and diffracted replicas
of the transmitted signal. On top of all this, the channel adds noise to the signal and can
cause a shift in the carrier frequency if the transmitter, or receiver is moving (Doppler
effect). Understanding of these effects on the signal is important because the performance
of a radio system is dependent on the radio channel characteristics.