Abstract
Mobile
communication plays vital role in all areas of Engineering. Because of that,
wireless medium is heavily crowded by various users in various applications.
This medium adds much noise to the transmitted signal. Apart from this noise,
it establishes multipath level between the transmitter and receiver. This
multipath Communication will affect the
signal reception at the receiver. Hence, OFDM is selected for Transmission. The
main aim was to assess the overall performance of TCP\IP protocol suite, with
OFDM as a modulation technique in the Physical layer for wireless Networks. The
performance of OFDM in the TCP\IP Physical Layer was assessed by using computer
simulations performed using NETWORK SIMULATOR-2 (NS2), the commonly used
simulator for Networking and wireless studies in which BPSK is the default
modulation technique in the Physical Layer for mobile Communication. In this
simulation, the modulation scheme investigated is QPSK – OFDM with four
sub-carriers, with Guard time of 512 samples with cyclic extension of symbols. Our
results show that having the QPSK-OFDM at the physical layer do improves the
absolute performance of the protocol suite compared with the inbuilt BPSK
modulation scheme of the simulator for various receiver thresholds.
Key words : NS2,
OFDM.
I. INTRODUCTION
The growth of the Internet has spurred interest in TCP/IP. The Internet protocols are often used for local area networking, even when the local network is not connected to the Internet. This model provides a reasonable pictorial representation of the layers in the TCP/IP protocol hierarchy.
The four-layered structure of TCP/IP is seen in the way data is handled as it passes down the protocol stack from the Application Layer to the underlying physical network. Each layer in the stack adds control information to ensure proper delivery. This control information is called a header because it is placed in front of the data to be transmitted.
The Network Access Layer is the lowest layer of the TCP/IP protocol hierarchy. The protocols in this layer provide the means for the system to deliver data to the other devices on a directly attached network.
The layer above the Network Access Layer in the protocol hierarchy is the Internet Layer, is the heart of TCP/IP. IP provides the basic packet delivery service on which TCP/IP networks are built.
The protocol layer just above the Internet Layer is the Transport Layer. The two most important protocols in the Transport Layer are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).
At the top of the TCP/IP protocol architecture is the Application Layer. This layer includes all processes that use the Transport Layer protocols to deliver data.
Why OFDM ?
OFDM is a wideband modulation scheme that is specifically able to cope with the problems of the multipath reception. This is achieved by transmitting many narrowband overlapping digital signals in parallel, inside one wide band. Increasing the number of parallel transmission channels reduces the data rate that each individual carrier must convey, and that lengthens the symbol period. As a result, the delay time of reflected waves is suppressed to within one symbol time. OFDM was found to have total immunity to multipath delay spread provided the reflection time is less than the guard period used in the OFDM signal.
In most studies on mobile communication, simulation models are used for the evaluation of devices and protocols. Typically, such simulations focus on the specific higher layer protocols that are being proposed, and tend to ignore the details of models at other layers, particularly the interactions with physical layer models. In this paper, we present performance of implementing OFDM (one of the technique for wideband wireless communication) in the physical layer of TCP/IP protocol suite for wireless communication.
II. PROBLEM STATEMENT
The paper is aimed at estimating the overall TCP\IP performance of Wireless Networks, with OFDM as the physical layer transmission. Here the performance is measured in terms of Packet error rate at the higher layers rather than Bit Error rate at the physical layer.
We need a simulator which should have the following features.
- It should perform simulations of TCP\IP algorithms. (obviously)
- It should have models for wireless transmissions and battery models.
- It should be scalable enough to modify the simulator according to the requirements.
- It should be efficient for large simulations.
- There should be technical support.
We selected Network Simulator-2 for our simulations, as it has all the above mentioned features.
III. NS2 - WIRELESS SIMULATION ENVIRONMENT
Our simulations are based on the simulation environment described in the ns-2.1b8a network simulator, with extensions from the CMU Monarch project. The Rice Monarch Project has made extensions to the ns-2 network simulator that enable it to simulate mobile nodes communicating by wireless network interfaces, including the ability to simulate multihop wireless networks.
Physical layer parameters like path loss, fading, interference and noise computation are usually not taken into account in wireless simulations in spite of their important effects in simulation results [1]. For example, Rayleigh fading channel and log distance path loss model are used for error model and estimation of received signal respectively. Also, Friis free space propagation model [2] has been employed in this simulation. Further details of this simulation environment are available in [2].
We have modified ns-2 Physical layer modulation (BPSK) to the OFDM and tested for a typical wireless network described in the next section. This is accomplished by implementing OFDM in Matlab6.1 and convert the Matlab code to C code by using the Matlab compiler compatible to the source code of the NS-2
IV. EVALUATION OF OFDM MODEL
Figure 1 shows the network topology used for the following simulations.
Fig-1 Network Configuration
Three mobile
stations are Communicating on a single channel. The
simulation is done for 50 seconds, for a three node wireless network having the
nodes shown as 0,1,2. The nodes start moving from 0.5 sec onwards, away from
each other. From the time instant t = 3.000sec, node-0 starts to send packets
to node-1 and as both the nodes are with in the hearing distance, the
connection is established and we can see the line between node-0 to node-1 in
the figures shown.
As the time
progresses, the two nodes getting away from each other and at time instant
t=19.300 sec onwards, they lost direct-connection. At the same time the link is
established via node-2 which starts forwarding the packets from node-0 to
node-1 and we can see two connections in the figure-1 from t = 19.300 sec
onwards. Still the node was traveling to move away from node-0 and the link is
through node-2 only. Finally from time t = 41.600sec onwards the links got
broken and whatever the packets being sent were lost. At t= 50.000 the
simulation stops.
Over 49912
CBR packets with average size of 179 bytes have been sent in the simulation.
V. SIMULATION RESULTS
Figure 2 shows the comparison of dropped packets for each mode with respect to receiver thresholds ranging form 1.5X10-10 to 4.0X10-10. The dropped packets are the ones which have not properly reached the destination, even though a link is being established between the communicating nodes. OFDM performs better in terms of dropped packets for threshold above 2.0X10-10.
Figure 3 shows the comparison of lost packets for each mode. The lost packets are due to the improper link establishment between the communicating nodes. The number of lost packets here we got is specific for the scenario we employed for the simulation. Here also the OFDM shows improvement for threshold above 2.0X10-10.
fig 2 Comparison
of dropped packets
fig 3
comparison of lost packets
Figure 4 shows the comparison of overall packet error for each mode. Here also QPSK-OFDM shows better performance compared with the BPSK.
According to probability of bit error rate for QPSK and BPSK in [14], QPSK modulation has higher probability of bit error rate compared to BPSK, but the combination of QPSK and OFDM achieves better performance compared to BPSK.
fig 4
comparison of overall packet error
Figure 5 and 6 show the throughput of sending packets for each mode for
the receiver threshold of 3.652X10-10
fig 5 Throughput
of sending packets for OFDM.
Fig 6 Throughput of sending packets for BPSK
VI CONCLUSIONS AND FUTURE WORK
In this paper performance of OFDM wireless standard is investigated by calculating various packet error rates for QPSK-OFDM and BPSK. We have evaluated throughput performance of each mode using a simple topology. Simulations show that OFDM do improves the overall performance.
In future work, we investigate the performance in terms of bandwidth with the constraint of achieving lower packet errors by inserting M-ary ASK schemes in the OFDM model.
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Technical College - Bourgas,