OFDM: Orthogonal Frequency Division Multiplexing

OFDM: Orthogonal Frequency Division Multiplexing

Abstract-in wireless communication system, with the increase of data rate the distortion of the received signal caused by multipath fading channel become a major problem. OFDM (orthogonal frequency division multiple access) technique is a solution of this problem in wireless communication. OFDM provides much more bandwidth efficiency as compared to conventional multicarrier modulation schemes. Simulation results are based on MATLAB completely.

Index Terms- OFDM Interference management..

1. INTRODUCTION

   OFDM is a digital modulation schame in which a wideband signal is split in to a number of narrowband signals.Because the symbol duration of a narrowband signal will be larger than that of a wideband signal, the amount of time dispersion caused by multipah delay spread is reduced.OFDM is a multicarrier modulation scheme in which multiple user symbols are transmitted in parallel using different orthogonal subvarriers. The conventional multicarrier modulation techniques suffer from bandwidth inefficiency due to use of guard interval or spacing between adjacent channels.Our aim is here to achieve high data rate by using limited avalaible frequency bandwidth efficiency [1] [2].

2. REASONS OF ISI IN WIRELESS COMMUNICATION

In wireless communication ISI (Inter Symbol Interference) is the major problem,ISI is caused by high data rate transmission using conventional transmission schemes. Multipath propagation and bandlimited channels are the two factors causing ISI [3].

1. Multipath propagation-when the signal reaches the receiver end propagating through different paths, this is called Multipath Propagation. The time taken by the signal to reach the receiver end from transmitter end is directly proportional to the distance between the transmitter and receiver .Due to different path lengths for multipath propagation the delay time of received signal at the receiver end varies.

2. Band limited Channel-Band limited channel is a channel whose frequency response is zero above and below a certain cut-off frequency. When the signal is passed through such a channel the frequency components above the cut-off frequencies are completely removed therefore the removal of higher frequency components due to band limited channel making the time domain pulse spread. The distortion due to pulse spreading is referred as ISI.

3. COMPARISON OF OFDM

1. OFDM vs. CDMA

   Table 1.Comparison of CDMA and OFDM [1]

   PARAMETERS	OFDM	CDMA
   Near Far Effect	Insensitive	Sensitive
   Synchronization	Yes	No
   Crosscell Interference	Sensitive	Mitigated
   Intercell Interference	NO	Sensitive
   Treatment for ISI	Cyclic Prefix	Rake Receiver
   To achieve high data rate	Hogh order Modulation	Difficult

2. OFDM vs. IDMA

   Table 2 Comparison of OFDM and IDMA [1]

   PARAMETERS	IDMA	OFDM
   Near Far Effect	MUG	Insensitive
   Synchronization	NO	NO
   Crosscell Interference	Mitigated	Sensitive
   Intercell Interference	Suppressed by MUD	NO

   1. OFDM PRINCIPLE

      Figure 1 Working Principle of OFDM System

      As shown in above figure, First the transmitted data is digitally Modulated using modulation schemes. Mostly QAM and PSK digital modulation schemes are used to modulate the transmitted signal. The output of the modulator is converted in to parallel signal. By this techniques signal is transmitted through subcarriers which are orthogonal to each other Due to orthogonality property, sub channels are not overlapped to each other, and ICI (Inter channel Interference) problem is reduced. The output of the serial to parrel converter is then applied to IFFT (Inverse Fast Fourier Transform) .By using IFFT the spectral representation of the data is transferred in to time domain, which is much more computationally efficient. The Cyclic prefix scheme is used at the output of the IFFT [3].

   2. CYCLIC PREFIX

      Cyclic prefix is a process of addition of a guard period to the start of each symbol. This guard period is a cyclic copy that extends the length of the symbol waveform. The addition of cyclic prefix to each symbol solves both ICI and ISI problems. From figure 1,

      Figure 2 Concept of cyclic prefix

      After that, the digital data is transmitted over the channel. At the Receiver side the reverse process is adopted as shown in the figure.2 we can see that the guard period is T1.When the guard period is not added the symbol period of IFFT output is T2 only. But when Guard period is added at the output of IFFT, then the symbol period is T1+T2. Therefore, the symbol period of transmitted data is increased, and ISI is reduced [4] [5].

   3. SIMULATION RESULTS

      BER vs SNR

      0

      10

      -1

      10

      BER

      -2

      10

      -3

      10

      -4

      10

      0 2 4 6 8 10 12 14 16

      SNR (dB)

      Figure 3 BER Performance of OFDM system

      10

      0

      -10 (SNR)-->

      -20

      -30

      -40

      POWER SPECTRAL DENSITY with RCF

      10

      0

      -10 (SNR)-->

      -20

      -30

      -40

      POWER SPECTRAL DENSITY with RCF

      -50

      0 100 200 300 400 500 600 700 800 900

      -50

      0 100 200 300 400 500 600 700 800 900

      frequency,f(hz)-->

      frequency,f(hz)-->

      Figure 4 PSD with RCF

   4. CONCLUSION

In this paper we explained the problem of ISI and ICI in wireless communication and also discuss the Solution for it and this is OFDM system with Cyclic Prefix scheme. Now if we use Raised cosine filter with BPSK technique, the BER can be reduced and ISI is reduced.

REFERENCES

1. Roshani Agrawal, Agya Mishra FUTURE TRENDS OF DIFFERENT MULTIPLE ACCESS GJCAT, Vol. 1 (1),2011,32-38.

2. Rashmi Mishra, Baibaswata Mohapatra, PERFORMANCE EVALUATION OF OFDM SYSTEM,International Journal of Engineering and Advanced Technology ISSN:2249-8958, Volume-1,Issue

   -3, February 2012.

3. Helmut Bolcskei,ETH Zurich MIMO-OFDM WIRELESS SYSTEMS:BASICS, PERSPECTIVES, AND CHALLENGES IEEE WIRELESS COMMUNICATION, August 2006.

4. Pooja YadavBER Analysis with OFDM using PSK/QAM Techniques for wireless communicationIJERT,ISSN:2278-0181 Vol 1 Issue 7,September 2012.

5. Khalis Aslamusing raised cosine filter to reduce inter symbol interference in OFDM with BPSK Technique computer science journal, Volume 1, Issue 2,August2011.