International Journal of Scientific & Engineering Research Volume 2, Issue 11, November-2011 1

ISSN 2229-5518

Performance Evaluation of Adaptive Modulation Techniques and Offered Load in OFDM-based WiMAX Network by Considering Cyclic Prefix

Pratyush Sharma, Abhishek Sharma, Kailash C.Bandhu

Abstract— WiMax uses orthogonal frequency division multiple access technique in wireless communication. Orthogonal frequency division multiple access (OFDM) is what puts the max in WiMAX, OFDM delivers a wireless signal much farther with less interference.

This technique uses Adaptive modulation coding (AMC) on physical layer of WiMAX. Adaptive modulation technique uses the concept of

cyclic prefix that adds additional bits at the transmitter end. The receiver removes these additional bits.

Cyclic Prefix is used to combat intersymbol inter-ference (ISI) and intercarrier interference (ICI) introduced by the multipath fading channel. This paper investigates the performance of WiMAX network by varying physical layer parameter such as modulation and coding sc heme and cyclic prefix. It also investigates the performance of WiMAX network by increasing traffic (number of downloading nodes) in network with different cyclic prefix.The performance of WiMAX network is measured in terms of throughput and goodput

Index Terms— Downlink (DL), Adaptive Modulation techniques, IEEE-802.16, OFDMA, Cyclic Prefix, Throughput, Goodput

1 INTRODUCTION

——————————  ——————————
iMAX is abbreviation `Worldwide Interoperability for Micro-wave Access', is a new wireless OFDM-based technology that provides high throughput broadband connec-
tion over long distances based on IEEE.802.16 wireless
WiMAX network increasingly more intelligent and agile communication systems, capable of providing spectrally effi- cient and flexible data rate access.
The WiMAX standard supports adaptive modulation, effec-
tively balancing different data rates and link quality.
The modulation method may be adjusted almost instanta- neously for optimum data transfer. WiMAX is able to dynami- cally shift modulations from 64-QAM to QPSK via 16-QAM, displaying its ability to overcome QoS issues with dynamic bandwidth allocation over the distance between the BS and the SS.
As the range increases, modulation step down to lowermodu- lations (in other words, BPSK), but as you are closer you can utilize higher order modulations like QAM for increased throughput.
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 Pratyush Sharma is currently pursuing masters degree program in Informa- tion Technology from MIT,Ujjain, RGPV University, Bhopal(M.P), In- dia.

E-mail:pratyush.sharma@yahoo.co.in

 Abhishek Sharma working as a Reader. in Computer sc. Dept. in

MIT,Ujjain, RGPV University, Bhopal(M.P),India

E-mail: abhi_ujn9@yahoo.co.in

 Kailash C Bandhu working as a Astt.Prof. in Computer sc. Dept. in

AITR,Indore, RGPV University, Bhopal(M.P),India

E-mail: kailash_bandhu@yahoo.co.in
Thus the modulation coding schemes ensure a qualitysignal is delivered over distance by decreasing througput.
An example of utilization of the cited adaptive modultion and coding scheme is illustrated in Fig. 1. It shows that as the range increases, the system steps down to a lower modulation, but as closer to the base station, higher order modulations can be used for increased throughput.
The rest of the paper is structured as follows. The system
model for the investigation is introduced in Section 2.

Fig. 1 Scheme for the utilization of AMC

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ISSN 2229-5518

In Section 3, simulation scenarios are presented and the results are discussed. Finally, we present our conclusions in Section 4.

2 SYSTEM MODEL

This section present the system model used in our investiga- tion. The network setup is shown in Fig. 2.

Fig. 2 The network setup

2.1 Simulation Environment

The investigation was through simulation. The simulation platform is ns-2 and the WiMAX module is from the National Institute of Standards and Technology (NIST) [1]. The simula- tion parameters are summarized in Table I .

2.2 Performance Metrics

We study performance by means of three metrics:
• Throughput that measures the amount of raw bytes sent by a source.
• Goodput that measures bytes that are sent and successfully acknowledged.
• Cyclic Prefix act as a buffer region where delayed informa- tion from the previous symbols can get stored.
In our system we investigated the behavior of adaptive modulation technique of WiMAX network. The adaptive modulation used Binary Phase Shift Keying (BPSK), Quadra- ture Phase Shift Keying (QPSK),16-Quadrature Amplitude Modulation(16-QAM),64-Quadrature Amplitude Modula- tion(64-QAM) for modulating and demodulating the signal. Based on these modulation techniques the throughput and goodput were investigated

TABLE I

SIMULATION PARAMETERS

3 SIMULATION RESULTS

This sec- tion, prese nt the simu- lation scena- rios and
dis- cuss the re- sults ob-
tained
. Sev-
eral
scena- rios
are consi- si- dered to high- light the
effects of
of- fered
load and modulation and coding schemes with cyclic prefix

3.1 Scenario 1: Effect of Load

In the first scenario, all SSs download FTP traffic from the server. It has been study the impact of offered load (i.e. num- ber of SSs) and cyclic prefix on aggregate throughputs and goodputs of the system for fixed downlink and uplink ratio (i.e. DL:UL). The value of DL:UL ratio is fixed at 0.5 The re- sults are presented in Fig. 3 & 4.
As can be seen, the throughput and goodput increases with
offered loads are increases and cyclic prefix decreases. cyclic prefix is added to reduce the effect of fading and to give suffi- cient time to the receiver for storage of signal[2-7]. As distance increase fading is more and signal strength is going low. For this higher value of cyclic prefix is consider because large cyc- lic prefix means large time gap between two frames. Large value gives extra time to receive signal from multipath signals.
It is observed that for large value of cyclic prefix throughput
and goodput are decreases.
For cyclic prefix 0.0625ms, maximum throughput is around

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10.58 Mbps for 30 downloading nodes, 8.89 Mbps for cyclic prefix 0.25ms and 5.72 Mbps for cyclic prefix 0.8ms respective- ly.
It is also observed that throughput and goodput increases
with offered loads increase (i.e. number of SSs) for all cyclic prefix values. The system is more utilized with more down- loading SSs (loads). However, the system resources are finite, and when its capacity is reached new connection cannot be admitted

Fig. 3 Aggregate throughput

(All wireless nodes (SSs) are downloading)

Fig. 4 Aggregate goodput

(All wireless nodes (SSs) are downloading)

3.2 Scenario 2: Effect of Modulation and Coding

Scheme

This study considers the same radio conditions and hence the same modulation and coding schemes (MCS) for all SSs. In this section, we change the MCS for all SSs. The offered load is constant with 15 downloading SSs. It has been plot the throughputs and goodput against MCS for fixed DL:UL ratio
0.5 in Fig. 5 and Fig. 6.

Fig. 5 Aggregate throughput for MCS

(For 15 downloading wireless nodes)

It is observed that for higher order modulation coding scheme, the value of throughput and goodput is maximum. Higher order modulations like QAM for increased throughput and goodput.
For cyclic prefix 0.0625ms, maximum throughput is around
9.80Mbps for 64QAM 3/4 modulation coding scheme which is
higher order modulation coding scheme, 8.3Mbps for cyclic
prefix 0.25ms and 5.4Mbps for cyclic prefix 0.8ms respectively.
For cyclic prefix 0.0625ms, maximum, goodput is around
4.82Mbps for 64QAM 3/4 modulation coding scheme,
4.09Mbps for cyclic prefix 0.125ms and 2.68Mbps for cyclic
prefix 0.8ms respectively.
It is also observed that for higher cyclic prefix values and
lower modulation coding scheme, coverage area that would be
covered by the signal is increases but throughput and goodput
are decreases.

Fig. 5 Aggregate goodput for MCS (For 15 downloading wireless nodes)

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4 CONCLUSION

In this paper investigates that the performance of WiMAX network is highly dependent on Cyclic Prefix and Modulation and Coding Scheme.
It is concluded that Cyclic Prefix is key player in WiMAX Network. It is observed that the modulation and coding scheme and Cyclic Prefix affect the performance of WiMAX network. The Cyclic Prefix play an important role to increase and decrease the throughput and goodput.
By increasing the value of Cyclic Prefix the throughput and
goodput of WiMAX network are decreases and gap between
throughput and goodput occurred because of lost of packet in
network.
As the distance increases signal strength decreases and SNR
also decreases, at lower SNR fading is more and signal strength goes low. To overcome this problem by selecting
higher Cyclic Prefix and Lower order modulation and coding scheme but these two parameters is cause of less throughput and goodput. Higher Cyclic Prefix means large time gap be- tween two frames and large Cyclic Prefix give extra time to receive signal from multipath channel.
It is observed that when increase the number of download-
ing wireless nodes the throughput and goodput are also in-
creases for different Cyclic Prefix because of better utilization
of bandwidth, it is also observed that the lower Cyclic Prefix
support higher throughput and goodput.
So the selection of Cyclic Prefix value is based on the cover- age area that would be covered by the signal and keeping
throughput and goodput parameter in consideration.

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