Ijecet 06 06_003

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International Journal of Electronics and Communication Engineering & Technology
(IJECET)
Volume 6, Issue 6, Jun 2015, pp. 18-28, Article ID: IJECET_06_06_003
Available online at
http://www.iaeme.com/IJECETissues.asp?JTypeIJECET&VType=6&IType=6
ISSN Print: 0976-6464 and ISSN Online: 0976-6472
© IAEME Publication
___________________________________________________________________________
EFFECT OF MULTIPATH CHANNEL ON
OFDM SYMBOL: GUARD INTERVAL
APPROACH
Kavita S. Dhore and Prof. S. G. Hate
Department of Electronic and Telecommunication,
G.H. Raisoni Institute of Engineering and Technology, Pune, India
ABSTRACT
Orthogonal Frequency-Division Multiplexing (OFDM) is multicarrier
frequency-division multiplexing (FDM) scheme, which support to high data
rate. Orthogonal Frequency Division Multiplexing (OFDM) is an attractive
air-interface for next-generation wireless network without complex equalizer.
OFDM is an emerging multi-carrier modulation scheme. It has been adopted
by several wireless standards such as IEEE 802.11a, IEEE 802.16 and
HiperLAN2. OFDM is designed such a way that it sends data over hundreds of
parallel carrier which are overlapped orthogonally.
The paper is aimed at analyzing effect of multipath channel on OFDM
symbol and the role of guard interval. OFDM scheme is suffer by inter-symbol
interference (ISI) problem which is created due to multipath channel. ISI is
distortion in a signal in which one symbol interfere with subsequent symbol,
this will degrade performance of OFDM system. There are several methods
for reducing the effects of ISI. A guard interval insertion (cyclic prefix, cyclic
suffix and Zero Padding) method is effective method to reduce the effect of ISI.
In this paper, we are studied the effect of CP and ZP on Bit Error Rate (BER)
of OFDM technique with AWGN channel and Rayleigh fading channel.
Through simulations, it is analyzed that the both techniques are effective in
mitigating the effects of ISI for high value of the CP and ZP. Different
modulation scheme are used for analyzing performance of OFDM.
Key words: OFDM, CP, ZP, FFT, IFFT and QAM
Cite this Article: Dhore, K. S. and Prof. Hate, S. G. Effect of Multipath
Channel on OFDM Symbol: Guard Interval Approach. International Journal
of Electronics and Communication Engineering & Technology, 6(6), 2015, pp.
18-28.
http://www.iaeme.com/IJECET/issues.asp?JTypeIJECET&VType=6&IType=6
_____________________________________________________________________

Effect of Multipath Channel on OFDM Symbol: Guard Interval Approach
1. INTRODUCTION
Wireless channels are multipath channel which suffer from fading, variation of signal
amplitude over time and frequency. In contrast with the additive noise as the most
common source of signal degradation, fading is another source of signal degradation
that is characterized as a non-additive signal disturbance in the wireless channel.
Fading may either be due to multipath propagation, referred to as multi-path (induced)
fading, or due to shadowing from obstacles that affect the propagation of a radio
wave, referred to as shadow fading [1].
The performance of wireless communication systems is mainly governed by the
wireless channel environment. Optimization of the wireless communication system
has become critical with the rapid growth of mobile communication services and
emerging broadband mobile Internet access services. The combination of multiple-
input multiple-output (MIMO) system with OFDM techniques is regarded as a
promising solution for enhancing the data rates of next-generation wireless
communication systems [12]. Signal from wireless channel generate multiple path and
multiple path affect OFDM symbol, generating ISI. ISI depends on the duration of a
symbol period T. The shorter the symbol period is, the larger the influence of the ISI
As the data rate increases (i.e., decreasing T) increases ISI. Insertion of guard interval
between two successive symbols reduces (ISI) inter symbol interference which is
harmful to improve BER. There are different techniques available to improve BER
but still need some improvement. In single carrier transmission schemes, one
frequency carrier is used to transmit or receive information. Data or information will
be transmitted serially on the channel at low data rate. High data rate is creating a
problem of ISI in single carrier schemes [2]. In order to support the symbol rate Rs,
the minimum required bandwidth is the Nyquist bandwidth (Rs /2 Hz) [4]. It means
that wider bandwidth is required to support a higher data rate in a single-carrier
transmission. When the signal bandwidth becomes larger than the coherence
bandwidth in the wireless channel, the link suffers from multi-path fading, incurring
the inter-symbol interference (ISI) [6, 9]. In general, adaptive equalizers are employed
to deal with the ISI created by the time-varying multi-path fading channel.
Furthermore, the complexity of an equalizer increases with the data rate, modulation
order and the number of multi-paths. In conclusion, a high data rate single-carrier
transmission may not be practically possible due to too much complexity of the
equalizer in the receiver. Simple flat fading channel is used in single carrier
transmission schemes therefore guard interval and guard band are not required. Due to
drawbacks of single carrier system, multicarrier transmission system is used in next
generation communication system but multicarrier system suffer from ISI due to
multipath delay spread [10].
One of the most important properties of OFDM transmission is its robustness
against multipath delay spread. This is achieved by having a long symbol period
which minimizes the inter-symbol interference. The level of robustness can in fact be
increased even more by the addition of a guard period between transmitted symbols.
The guard period allows time for multipath signals from the previous symbol to
decay before the information from the current symbol is gathered. The most effective
guard period to use is a cyclic extension of the symbol. If a mirror in time, of the end
of the symbol waveform is put at the start of the symbol as the guard period, this
effectively extends the length of the symbol, while maintaining the orthogonality of
the waveform. The guard time is chosen to be larger than the expected delay spread,
such that multipath components from one symbol cannot interfere with the next

symbol [1]. This guard interval is usually chosen as 5 times the delay spread. There
are several options for GI. One choice of GI is zero padding. In this scheme no
waveform is transmitted in the GI duration. However, the zero-padded waveform
would destroy the orthogonality of subcarriers and results in inter carrier interference
(ICI). In this paper, we are discussing and analyzing ZP-OFDM and CP-OFDM and
its effects on performance of system. Section II defines the main component of
OFDM, basic of CP and ZP and its analysis. Then, a brief description of the BER
performance of OFDM by using CP and ZP for AWGN Channel and multipath
channel is provided in Section III. Simulation Results are discussed in Section IV with
different value of Cyclic Prefix and Zero padding for AWGN Channel and Multipath
channel. Finally, the concluding remarks in Section V.
2. OFDM
In this technique, Frequency band is divided into number of subcarriers. These
subcarriers are orthogonal to each other. The basic principle of OFDM is to split high-
rate data stream into number of lower rate streams that are transmitted simultaneously
over a number of subcarriers. Because the symbol duration increases for the lower
rate parallel subcarriers, the relative amount of dispersion in time caused by multipath
delay spread is decreased. Inter-symbol interference is eliminated by using guard time
in OFDM symbol. In OFDM system design, a number of parameters are used such as
subcarrier, guard time, symbol duration, subcarrier spacing, modulation type and error
correcting code. The choice of parameters depends on bit rate, bandwidth, and
multipath delay.
3. OFDM TRANS RECEIVER
Figure 1 shows complete OFDM trans-receiver, where the upper path is transmitter
section and the lower path corresponds to receiver section. The three main important
featurest of OFDM system are as follows.
First main component of OFDM is FFT/ IFFT which modulates a block of input
values onto a number of subcarriers. In the receiver, subcarriers are demodulated by
FFT, which perform reverse operation of IFFT. In practice, IFFT can be made by
using FFT. Therefore same hardware will be used for the both which reduces the
complexity of communication system.
Second important feature of OFDM system is coding and interleaving. Some
successive subcarriers in the OFDM system may suffer from deep fading in which the
received SNR is below the required SNR level. In order to deal with the burst symbol
errors, it may be essential to use of FEC (Forward Error Correction) codes. The FEC
codes can make error corrections only as far as the errors are within the error-
Correcting capability, but they may fail with burst symbol errors. Due to this code
average errors convert into random errors, for which interleaving techniques are used.
There are two types of interleaving: block interleaving and convolution interleaving.
Bit-wise, data symbol-wise, or OFDM symbol-wise interleaving can be used for block
interleaving.
The third key principle is the introduction of a cyclic prefix and zero padding as a
Guard Interval to reduce interference between the symbols. Interleaving type and size
must be determined by the type of FEC code, degree of frequency, time fading and
delay due to interleaving.

4. OFDM TRANSMITTER
OFDM signals are generated digitally and the signal consist of sum of subcarriers that
are modulated using BPSK or QPSK or QAM. The relationship between of the carrier
must be control to maintain the orthogonality of the carriers. After modulating the
input data digitally, the resulting spectrum is converted back to its time domain using
inverse fourier transform (IFFT). The IFFT convert a number of complex data point
of length power into time domain signal of the same number of points.
Insertion of Guard period: Guard time is introduced to each symbol to eliminate
ISI (Inter symbol Interference). To eliminate the ICI (inter carrier interference)
OFDM symbol must be cyclically extended in the guard time. This ensures that
delayed replicas of the OFDM symbols always have an integer number of cycles
within FFT.
5. OFDM RECEIVER
At the receiver side the OFDM signal is demodulated using OFDM demodulator FFT
is used to estimate the amplitude and phase of each subcarrier. Guard period insertion
is removed in receiver side. Performance of the OFDM system will be determined by
the noise seen at the receiver, We get original signal with some distortion at receiver.
Figure 1 OFDM Transceiver
6. ORTHOGONALITYAND OFDM
If the OFDM system had able to use set of subcarrier that were orthogonal to each
other higher level of spectral efficiency could have been achieved. The guard band
that were necessary to allow individual demodulation of subcarrier in FDM system
would no longer be necessary. The orthogonal subcarrier would allow the sub carrier
spectra to overlap thus increasing the spectral efficiency. As long as orthogonality is
maintain it is still possible to recover the individual subcarrier signal despite their

overlapping spectrum. If dot product of two deterministic signal equal to zero this
signal are said to be orthogonal to each other. Orthogonality can also viewed from the
standpoint of stochastic processes. If two random processes are uncorrelated then they
are orthogonal. OFDM is implemented in practice using the discrete fourier
transform. Sinusoid of the DFT form an orthogonal basis set and a signal in the vector
space of the DFT can be represented as linear combination of the orthogonal sinusoids
[3, 5, 7].
The orthogonal and uncorrelated nature of the subcarrier is exploited in OFDM with
powerful result. Since the basis function of the DFT are uncorrelated. The correlation
perform in DFT for a given subcarrier only sees energy for that corresponding
subcarrier. The energy from other subcarrier does not contribute because it is
uncorrelated. This separation of signal energy is the reason that the OFDM subcarrier
spectrum can overlap without causing interference. Orthogonality can break due to
Doppler shift, carrier frequency offset. For avoiding the carrier frequency offset same
carrier frequencies should have to generate in transmitter and receiver.
7. CONTESTING ISI USING GUARD INTERVAL [6]
To optimize the performance of an OFDM link, time and frequency synchronization
between the transmitter and receiver is of absolute importance. This can be achieved
by using known pilot tones embedded in the OFDM , signal or attach fine frequency
timing tracking algorithms within the OFDM signal’s cyclic extension (guard Period/
Period). To prevent ISI, the individual blocks are separated by guard periods wherein
the blocks are periodically extended. In addition, once the incoming signal is split into
the respective transmission sub-carriers, a guard period is added between each symbol
[8]. Each symbol consists of useful symbol duration, sand a guard period, t, in
which, part of the time, and a signal of is cyclically repeated. This is shown in
Figure 2.
As long as the multi path propagation delays do not exceed the duration of the
period, no inter-symbol interference occurs and no channel equalization is required.
For a delay spread that is longer than the effective guard period, the BER (Bit Error
Rate) rises rapidly due to the inter-symbol interference.
The maximum BER that will occur is when the delay spread is very long (greater
than the symbol time) as this will result in strong inter-symbol interference.
Figure 2 Combating ISI Using A Guard Period

In a practical system the length of the guard period can be chosen depending on
the required multipath delay spread immunity required.
Figure 3 illustrates an ISI effect of the multipath channel over two successive
OFD symbols.
Let denote the duration of the effective OFDM symbol without guard
interval.
Since W = 1/ and thus, Δf = W/N = 1/ (N ) and = N = 1/Δf .
By extending the symbol duration by N times (i.e., = N ), the effect of the
multipath fading channel is greatly reduced on the OFDM symbol. However, its effect
still remains as a harmful factor that may break the orthogonally among the
subcarriers in the OFDM scheme. As shown in Figure 3, the first received symbol
(plotted in a solid line) is mixed up with the second received symbol (plotted in a
dotted line), which incurs the ISI. It is obvious that all subcarriers are no longer
orthogonal over the duration of each OFDM symbol. To warrant a performance of
OFDM, there must be some means of dealing with the ISI effect over the multipath
channel. A guard interval between two consecutive OFDM symbols will be essential.
Figure 3 OFDM Symbol without Guard Period
The Guard Period in OFDM System can be inserted in two different ways. One
way is the zero padding (ZP) i.e. pads the guard period with zeros. The other way is
the cyclic extension of the OFDM symbol (for some continuity) by insertion of CP
(cyclic prefix) or CS (cyclic suffix). CP is to extend the OFDM symbol by copying
the last samples of the OFDM symbol into its front.
8. CYCLIC PREFIX
Let denote the length of CP in terms of samples. Then, the extended OFDM
symbols now have the duration of . Figure 4(a) shows two
consecutive OFDM symbols, each of which has the CP of length , while illustrating
the OFDM symbol of length While, Figure 4(b) shows the ISI effects
of a multipath channel on some subcarriers of the OFDM symbol.

(a)
(b)
Figure 4 OFDM symbol with CP
It can be seen from this Figure 5 that if the length of the guard interval (CP) is set
longer than or equal to the maximum delay of a multipath channel, the ISI effect of an
OFDM symbol (plotted in a dotted line) on the next symbol is confined within the
guard interval so that it may not affect the FFT of the next OFDM symbol, taken for
the duration of . This implies that the guard interval longer than the maximum
delay of the multipath channel allows for maintaining the orthogonality among the
subcarriers. As the continuity of each delayed subcarrier has been warranted by the
CP, its orthogonality with all other subcarriers is maintained over .
9. CYCLIC SUFFIX (CS)
Cyclic suffix (CS) is also a cyclic extension of the OFDM system. It is different from
CP only in that CS is the copy of the head part of an effective OFDM symbol, and it is
inserted at the end of the symbol. CS is used to prevent the interference between
upstream and downstream, and is also used as the guard interval for frequency
hopping or RF convergence, and so on. Both CP and CS are used in Zipper-based
VDSL systems in which the Zipper duplexing technique is a form of FDD
(Frequency-Division Duplexing) that allocates different frequency bands (subcarriers)
to downstream or upstream transmission in an OFDM symbol, allowing for
bidirectional signal flow at the same time. Here, the purpose of CP and CS is to
suppress the ISI effect of the multipath channel, while ensuring the orthogonality
between the upstream and Therefore, the length of CP is set to cover the time
dispersion of the channel, while the length of CS is set according to the difference
between the upstream transmit time and downstream receive time. Figure 8 shows the
structure of the OFDM symbol used in Zipper-based VDSL systems, where the length
of the guard interval is the sum of CP length T CP and CS length T CS.
10. OFDM SYMBOL WITH ZERO PADDING
In zero padding guard interval is pad by zeros. This particular approach is adopted by
multiband-OFDM (MB-OFDM) in an Ultra Wide-band (UWB) system [6]. Figure
show OFDM symbols with ZP and the ISI effect of a multipath channel on OFDM
symbols for each subcarrier respectively. Even with the length of ZP longer than the

maximum delay of the multipath channel, a small symbol time offset (STO) causes
the OFDM symbol of an effective duration to have a discontinuity within the FFT
window. Therefore the guard interval part of the next OFDM symbol is copied and
added into the head part of the current symbol to prevent ICI. Since the ZP is filled
with zeros, the actual length of an OFDM symbol containing ZP is shorter than that of
an OFDM symbol containing CP or CS. Accordingly the length of a rectangular
window for transmission is also shorter, so that the corresponding sinc-type spectrum
may be wider. This implies that compared with an OFDM symbol containing CP or
CS, an OFDM symbol containing ZP has PSD (Power Spectral Density) with the
smaller in to band ripple and the larger out-of-band power, allowing more power to be
used for transmission with the peak transmission power fixed[11].
Figure 5 OFDM Symbol with CP & CS
= +
Figure 6 OFDM Symbol with Zero Padding
11. SYSTEM MODEL
Consider Wideband signal which is divided in to N subchannel Consider OFDM
transmission scheme with N subcarriers Insert guard interval of different lengths with
OFDM symbol and analyse result in term of BER performance. we are using AWGN
channel and Rayleigh fading channel for analysis.
Consider X be transmitted complex symbol vector, n is noise vector and H is
channel response then received signal Y will be given as,
12. SIMULATION AND RESULTS
The OFDM trans-receiver was designed and simulated by MATLAB. The BER
performances of the proposed scheme are examined by MATLAB simulation. In the
simulation we consider an OFDM signal with N = 64 subcarriers, 64-Quadrature
Amplitude Modulation (64-QAM) mapping, 16 virtual carrier, 3 symbol per frame.
Additive white Gaussian noise (AWGN) channel and Rayleigh fading channel
Ith OFDM
Symbol
Zero
Padding
(I+1)th OFDM
Symbol
Zero
Padding

response are consider for simulation result. Effect of Variable length of CP and ZP are
analyses on OFDM system with AWGN channel and Rayleigh fading channel.
The Bit error rate performance of OFDM system is depends on many parameters
such as modulation scheme, inter-symbol interference, inter-carrier interference,
channel noise, bit synchronization problems, attenuation and multipath fading. The
BER may be improved by selecting robust modulation scheme, coding schemes,
reducing the effect of ISI by inserting guard interval . In this paper, simulated result
shown in figure that indicated BER performance of an OFDM system with varying
guard interval (CP or ZP) in the WGN and a multipath Rayleigh fading channel. The
effect of inter symbol interference is simulated as the length of CP or ZP varies. The
effect of ISI is negligible in AWGN fading channel because there is no multipath
signal and it is significantly varies in Rayleigh fading channel with varying guard
interval length
Figure 7 BER Performance of OFDM System for AWGN Fading Channel

Figure 8 BER performance of OFDM system for Rayleigh Fading channel
13. CONCLUSION
OFDM is a block modulation scheme where a block of N information symbols is
transmitted in parallel on N sub-carriers. A guard time greater than or equal to
multipath delay spread usually in the form of CP or ZP is essential to avoid inter
symbol interference which will improve performance of the system in term of BER.
BER performance of AWGN channel is consistent with the analytical results
regardless of the length of CP and ZP because there is no multipath delay in the
AWGN channel. Rayleigh fading channel has multipath between transmitter and
receiver. Multipath signal produces the distortion at the receiver side which is due to
inter-symbol interference (ISI). ISI is prevented in OFDM by the insertion of a cyclic
prefix or zero padding between successive OFDM symbols. This cyclic prefix is
discarded at the receiver to to get transmitted signal. From the simulation result it is
clear that effect of ISI on the BER performance becomes significant in the multipath
Rayleigh fading channel as the length of CP & ZP decreases, which eventually leads
to an error. The BER performance in a Rayleigh fading channel is improved with the
guard interval greater than or equal to multipath delay spread. Again GI should be of
such length so that it should not affect the FFT of next symbol in order to preserved
the orthogonality of subcarriers.

14. FUTURE SCOPE
CP or ZP plays an important role in combating multipath effects by reducing ISI and
to maintain orthogonality between subcarriers. But it is also the fact that inserting CP
or ZP has its own cost, a part of signal energy is lost since CP or ZP carries no
information. Length of CP or ZP depends on maximum channel delay. Therefore in
future, it may be possible to proper estimation of channel maximum delays and insert
the CP or ZP according to the estimation to avoid extra energy loss and hence better
performance.
REFERENCES
[1] López-Martínez, F. J., Martos-Naya, E. and Paris, J. F. BER Analysis for MIMO-
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[3] Agrawal, M. and Raut, Y. BER Analysis of MIMO OFDM System for AWGN &
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[4] Bappy, D. M., Dey, A. K. and Saha, S. OFDM System Analysis for reduction of
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[12] Iqbal, Z. and Nooshabadi, S. Senior Member, IEEE, and Heung-No Lee,
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