CHAPTER 1

INTRODUCTION

1.1 OVERVIEW OF OFDM

OFDM based transmitters are being commonly used in wireless communication systems. This

technique allows for the transmission of large amounts of digital data over radio waves. OFDM is

a special case of Frequency Division Multiplexing (FDM) and is a multi-carrier modulation

scheme. In OFDM the data is encoded in parallel over various carriers. The carriers frequencies

differ from each other by integer multiples of the fundamental frequency, making the carriers

orthogonal to each other. This minimizes interference between the different symbols being

transmitted obtained by maintaining the Orthogonality. This immunes from a very important

problem of ISI due to multi-paths, in wireless communications. Recently, research and

development of the OFDM have received considerable attention and OFDM is a wideband

modulation scheme that is specifically able to manage with the problems of the multi-path

reception with sufficient QOS guaranties & high bit rate for 4th generation systems.

1.2 BASIC PRINCIPLE OF OFDMOrthogonal Frequency Division Multiplexing (OFDM) is similar to the Frequency Division

Multiplexing (FDM). OFDM uses the principles of FDM to allow multiple messages to be sent

over a single radio channel which is done in a much more controlled manner, allowing an

improved spectral efficiency.

For example of FDM is the use of different frequencies for each FM (Frequency Modulation)

radio stations. All stations transmit at the same time but do not interfere with each other because

they transmit using different carrier frequencies. Additionally they are bandwidth limited and are

spaced sufficiently far apart in frequency so that their transmitted signals do not overlap in the

frequency domain.

Figure 1.1: FDM with Seven Sub-carriers Using Filters

1

7 Sub-carriers

Frequency

At the receiver, each signal is individually received by using a frequency tune-able band pass

filter to selectively remove all the signals except for the station of interest. This filtered signal can

then be demodulated to recover the original transmitted information.

OFDM is different from FDM in several ways. In conventional broadcasting each radio station

transmits on a different frequency, effectively using FDM to maintain a separation between the

stations. There is no coordination or synchronization between each of these stations. With an

OFDM transmission, the information signals from multiple stations are combined into a single

multiplexed stream of data. This data is then transmitted using an OFDM ensemble that is made

up from a dense packing of many sub-carriers. All the sub-carriers within the OFDM signal are

time and frequency synchronized to each other, allowing the interference between sub-carriers to

be carefully controlled. These multiple sub-carriers overlap in the frequency domain, but do not

cause Inter-Carrier Interference (ICI) due to the orthogonal nature of the modulation .

Fig 1.2: OFDM with Seven Sub-carriers

To transmit FDM signals there is need to have a large frequency guard-band between channels to

prevent interference. The overall spectral efficiency so is very poor. But in OFDM with the dense

orthogonal packing of the each sub carriers eliminates this guard band, and improves the spectral

efficiency.

Fig 1.3: Comparison of the bandwidth utilization for FDM and OFDM

    

2

Conventional Frequency Division Multiplexing (FDM) Multi-carrier modulation technique

Orthogonal Frequency Division Multiplexing (OFDM) multi-carrier modulation technique

Saving of the bandwidth

Frequency

Frequency

Frequency

7 Sub-carriers

1.3 ORTHOGONALITY

   1.3.1 ORTHOGONALITY IN TIME DOMAIN

Signals are said to be orthogonal to each other if they are mutually independent of each other.

This Orthogonality property allows multiple-information signals to be transmitted perfectly

over a widespread channel and detect it without interference. Loss of orthogonality results in

blurring between these information signals and squalor in communications. Many common

multiplexing schemes are naturally orthogonal.

Example: - Time Division Multiplexing (TDM) allows transmission of multiple information

signals over a single channel by assigning unique time slots to each separate information

signal. During each time slot only the signal from a single source is transmitted preventing

any interference between the multiple information sources. Because of this TDM is

orthogonal in nature. In the frequency domain most FDM systems are orthogonal as each of

the separate transmission signals are well spaced out in frequency preventing interference.

Although these methods are orthogonal the term OFDM has been reserved for a special form

of FDM. The sub-carriers in an OFDM signal are spaced as close as is theoretically possible

while maintain Orthogonality between them.

Figure

1.4: Time domain

construction of an OFDM signal.

3

4a

4b

4c

4g

4f

4h

4i4d

4e 4j

Fig.(4a), (4b), (4c) and (4d) show individual sub-carriers with 1, 2, 3, and 4 cycles per symbol

respectively. The phase on all these sub-carriers is zero. Note, that each sub-carrier has an integer

number of cycles per symbol, making them cyclic. Adding a copy of the symbol to the end would

result in a smooth join between symbols. (4f), (4g), (4h) and (4i) show the FFT of the time

waveforms in (4a), (4b), (4c) and (4d) respectively. (4e) and (4j) shows the result for the

summation of the 4 sub-carriers.

In OFDM the available bandwidth is splited into many narrowband channels between 100-8000,

with its own sub-carrier in each case. These sub-carriers are to orthogonal to one another, i.e each

one has an integer number of cycles inverse of a symbol period. So each sub-carrier has a “null”

at the centre frequency of each of the other sub-carriers in the system in its frequency domain, as

shown in figure 5 and 6. This results in no interference between the sub-carriers, allowing them to

be spaced as close as possible. Because of this, there is no great need for users of the channel to

be time-multiplexed, and there is no overhead associated with switching between users.

Fig 1.5: A set of orthogonal signals

Another way of explaining Orthogonality is; if two functions are multiplied and integrated over

symbol period result will be zero but two functions should have integer number of cycles over

symbol period.

Equation below shows a set of orthogonal sinusoids, which represent the sub-carriers for an un-

modulated real OFDM signal.

4

Where fo is the carrier spacing, M is the number of carriers, T is the

symbol period. Since the highest frequency component is Mfo the transmission bandwidth is also

Mfo

1.3.2 FREQUENCY DOMAIN ORTHOGONALITYAnother way to analyze the Orthogonality property of OFDM signals is appear at its spectrum. In

the frequency domain each OFDM sub-carrier has a sinc, sin(x)/x, frequency response, as shown

in Figure 7. This is a result of the symbol time corresponding to the inverse of the carrier spacing.

As the receiver is concerned each OFDM symbol transmitted for a fixed time (TFFT) without any

tapering at the ends of the symbol. This symbol time corresponds to the inverse of the sub-carrier

spacing of 1/TFFT Hz. The rectangular gate function, in the time domain results in a sinc

frequency response in the frequency domain. The sinc shape has a narrow main lobe, with many

side-lobes that decay slowly with the magnitude of the frequency difference away from the

center. Each carrier has a peak at the center frequency and nulls evenly spaced with a frequency

gap equal to the carrier spacing. The orthogonal nature of the transmission is a result of the peak

of each sub-carrier corresponding to the nulls of all other sub-carriers. When this signal is

detected using a Discrete Fourier Transform (DFT) the spectrum is not continuous as shown

below, but has discrete samples. The sampled spectrum is shown as ‘o’s in the figure. If the DFT

is time synchronized, the frequency samples of the DFT correspond to just the peaks of the sub-

carriers, thus the overlapping frequency region between sub-carriers does not affect the receiver.

The measured peaks correspond to the nulls for all other sub-carriers, resulting in orthogonality

between the sub-carriers.

Figure 1.6: (a) Spectrum of each carrier and the discrete frequency samples; each carrier is sin(x)/x, in

shape. (b) Overall combined response of the 5 sub-carriers (thick black line).

5

Optical OFDM Generation & Reception

1.4 OPTICAL OFDM SYSTEM

Orthogonal frequency division multiplexing is a special form of multicarrier modulation where a

single data stream is transmitted over a number of lower rate orthogonal subcarriers. Such a

format has been widely implemented in various digital communication standards like Optical

OFDM system model brief description is provided below OFDM transmits a serial high-speed

data channel by dividing it into blocks of data then using Fourier transform techniques to encode

the data on separate sub carriers in the frequency domain. Our system using OFDM over an

optical channel is shown in Fig.7. Each block of data is presented as N parallel data paths to the

OFDM transmitter. The N paths are modulated onto N equally-spaced sub carriers using

Quadrature-Amplitude Modulation (QAM). It overcomes the complexities and practicalities of

multiple microwave mixers by using an inverse-FFT (IFFT) to generate a dense comb of OFDM

sub-carrier frequencies Each QAM data channel is presented to an input of the IFFT; the IFFT

produces a complex-valued time domain waveform containing a superposition of all of the sub-

carriers. This waveform is modulated onto an RF-carrier, fRF using an I-Q modulator, producing a

real-valued waveform comprising a band of sub-carriers. Next, this band is modulated onto an

optical carrier using a linear optical modulator. The output of the optical modulator is filtered to

remove all frequencies other than the upper side-band (or lower sideband if preferred) and an

attenuated (suppressed) optical carrier.

After propagation through the fiber link, the photodiode produces an electrical waveform. This is

converted to I and Q components by mixing with 0º and 90º phases of a local oscillator fRF. The I

and Q waveforms are then converted to OFDM sub carriers using a FFT, which, if the transmitter

and receiver FFT windows are synchronized in time, acts as a set of closely-spaced narrowband

filters. The periodic boundary conditions of the simulator enforce this synchronization. In a real

system, a cyclic prefix is added to each transmitted block after the IFFT, so that the relative

delays between the received OFDM-sub carriers (due to fiber dispersion) can be accommodated

without destroying the Orthogonality of the OFDM sub-carriers. For a 4000-km link of S-SMF at

1550 nm, the relative delay over the OFDM band is 2560 ps, requiring prefixes that extend the

block by only a few percent. Once in the frequency-domain, each channel is equalized to

compensate for phase and amplitude distortion due to the optical and electrical paths. This is 6

easily achieved by using separate complex multiplication for each channel. The multiplication

coefficients can be determined by training the system with a known data sequence or by

introducing pilot channels to the OFDM band to estimate the dependence of optical phase on

frequency. After equalization, each QAM channel is demodulated to produce N parallel data

channels. These can converted into a single data channel by parallel to serial conversion.[7].

Fig 1.7: Optical OFDM System Block Diagram

1.5 NATURAL PROTECTION AGAINST ISI and ICI

The symbol rate for an OFDM signal is much lower than a single carrier transmission scheme.

E.g. for a single carrier BPSK modulation, the symbol rate corresponds to the bit rate of the

transmission. For OFDM the system bandwidth is broken up into say Oc sub-carriers, the symbol

rate is Oc times lower than the single carrier transmission resulted. This low symbol rate makes

OFDM naturally resistant to effects of Inter-Symbol Interference (ISI) caused by multi-path

propagation.

In order to maintain the Orthogonality in sub carriers an OFDM signal the amplitude and phase of

the sub-carrier must remain constant over the period of the symbol. If they are not constant it

means that the spectral shape of the sub-carriers in frequency domain will not have the correct

sinc shape, and the nulls will not be corresponds to peak of the narrow main lobe, this resulting in

Inter-Carrier Interference.

7

1.6 GUARD PERIOD

The effect of Inter Symbol Interference, Inter Carrier Interference and Time offset on an OFDM

signal can be further improved by the adding a guard period at the start of each symbol. This

guard period is a cyclic copy that makes longer the length of the symbol waveform. Each sub-

carrier has an integer number of cycles in a symbol that’s why, placing copies of the symbol end-

to-end results in a continuous signal, with no discontinuities at the joins results in a longer symbol

time.

Figure 1.8: Addition of a guard period to an OFDM signal

The total length of symbol is Ts=TG + TFFT

Total length of the symbol in samples is Ts,

Length of the guard period in samples TG,

TFFT is the size of the IFFT used to generate the OFDM signal.

Guard period provides the

Resistance to effects of (ISI) caused by multi-path propagation.

Protection against TIME OFFSET

Protection against Inter Carrier Interference.

1.7 PERFORMANCE OF OFDM SIGNAL IN FIBERCombined deployment of optical fiber technology and wireless networks has great potential for

increasing the capacity and Quality of Service. By using Radio over Fiber technology, the

capacity of optical networks can be combined with the flexibility and mobility of wireless access

networks without significant cost increment. The Radio over Fiber means to transport information

8

IFFT GuardPeriod IFFT output Guard

PeriodIFFT

TGTFFT

Time

Ts Symbol N

Cyclic copy

over optical fiber by modulating the light with radio signal. OFDM is one of most favored

modulation techniques in WLAN due to its efficient implementation and robustness against

multi-path and narrowband interference. One of the biggest disadvantage of OFDM is its high

peak to average power ratio (PAPR). High PAPR makes it unusable in non-linear systems. The

non linear effects on the transmitted OFDM symbols due to high PAPR are spectral spreading,

inter modulation and harmonic generation. In other words, the non linear distortion causes both in

band and out of band interference to signals. Thus Low PAPR of OFDM signal will improve its

performance in fiber. In fiber, OFDM signal is suitable for long haul transmission systems

because it provides number of advantages (i) increase of the transmission distance (ii)

improvement of spectral efficiency (iii) simplification of dispersion compensation engineering. In

fibers, OFDM signal being extremely tolerant to variation in profiles that may be caused by

fabrication defects.

1.8 DISPERSION TOLERANCE AND CYCLIC PREFIX IN OFDMDispersion from optical transmission medium results in ISI and signal impairment. This may be

overcome with an OFDM signal by adding a cyclic prefix to each symbol .The cyclic prefix is a

small section of the end of each symbol that is added to the beginning of each symbol. The cyclic

prefix contains redundant information. In the receiver, only the centre section of the OFDM

symbol is retained and this is not affected by the chromatic dispersion induced inter symbol

interference.

Figure 1.9: Cyclic prefix is this superfluous bit of signal we add to the front of symbol.

9

We add the prefix after doing the IFFT just once to the signal. After the signal has arrived at the

receiver, first remove this prefix, to get back the perfectly periodic signal so it can be FFT’s to get

back the symbols on each carrier. However the addition of cyclic prefix can perfectly eliminate

inter symbol interference, but increases bandwidth.

1.9 ADVANTAGES OF OOFDM OOFDM offers a significant improvement in spectral efficiency

OOFDM provides increase of transmission distance

OOFDM combat dispersion in optical media. In OOFDM system optical signal to noise

ratio penalty at 10 Gb/s is maintained below 2 db for 3000 Km transmission of standard

SMF without dispersion Compensation.

OOFDM systems mitigate polarization Mode dispersion in optical fibers.

BER better than 1*10-4 is achieved in OOFDM.

1.10 DISADVANTAGES OF OOFDM High PAPR of OFDM makes it unusable in nonlinear systems.

OFDM signal is contaminated by nonlinear distortion of transmitter power amplifier,

because it is a combined amplitude –frequency modulation (it is necessary to maintain

linearity)

At the receiver, it is very difficult to decide the starting time of the FFT symbol

1.11 OFDM FOR OPTICAL COMMUNICATIONSDespite the many advantages of OFDM, and its widespread use in wireless communications,

OFDM has only recently been applied to optical communications. This is partly because of the

recent demand for increased data rates across dispersive optical media and partly because

developments in digital signal processing (DSP) technology make processing at optical data rates

feasible. In typical (nonoptical) OFDM systems, the information is carried on the electrical field

and the signal can have both positive and negative values (bipolar). At the receiver there is a local

oscillator and coherent detection is used. In contrast in a typical intensity-modulated direct

detection optical system, the information is carried on the intensity of the optical signal and

therefore can only be positive (unipolar). There is no laser at the receiver acting as a local

oscillator and direct detection rather than coherent detection is used. A variety of optical OFDM

10

solutions have been proposed for different applications. To understand these different techniques,

it is useful to realize what is fundamental in each domain. For an OFDM system to work

successfully the system must be (approximately) linear between the transmitter IFFT input and

the receiver FFT output. Optical OFDM solutions can be broadly divided into two groups. The

first group comprises techniques for systems where many different optical modes are received,

for example, optical wireless, multimode fiber systems and plastic optical fiber systems. For these

the OFDM signal should be represented by the intensity of the optical signal. The second group

includes techniques for single mode fiber, where only one mode of the signal is received and for

these the OFDM signal should be represented by the optical field.

Optical OFDM Using Intensity Modulation

The many optical modes that are present at the receiver result in optical wireless systems being

linear in intensity. So, for optical wireless systems and other systems where many modes are

received, the OFDM signal must be represented as intensity. This means that the modulating

signal must be both real and positive, whereas baseband OFDM signals are generally complex

and bipolar. A real baseband signal OFDM signal can be generated by constraining to have

Hermitian symmetry. Two forms of unipolar OFDM have been proposed: dc-biased optical

OFDM (DCO-OFDM) and asymmetrically clipped OFDM (ACO-OFDM) . In dc-biased OFDM,

a DC bias is added to the signal, however because of the large peak-to-average power ratio of

OFDM, even with a large bias some negative peaks of the signal will be clipped and the resulting

distortion limits performance . In ACO-OFDM the bipolar OFDM signal is clipped at the zero

level all negative going signals are removed. If only the odd frequency OFDM subcarriers are non

zero at the IFFT input, all of the clipping noise falls on the even subcarriers, and the data carrying

odd subcarriers are not impaired. It was shown that except for extremely large constellations

ACO-OFDM requires a lower average optical power for a given BER and data rate than DCO-

OFDM. ACO-OFDM has also been shown to be efficient from an information theoretic

perspective. The use of DCO-OFDM has been demonstrated experimentally for optical wireless ,

multimode fiber and plastic optical fiber . A number of simulation studies examine the

performance of DCO-OFDM in more detail, and how adaptive modulation can be used to

improve performance.

11

Optical OFDM Using Linear Field ModulationIn single mode optical fiber systems the best way to achieve linearity between the transmitter

IFFT input and the receiver FFT output is to map each discrete OFDM subcarrier frequency in the

baseband electrical domain to a single discrete frequency in the optical domain. This is achieved

by using linear field modulation, so that there is a linear relationship between the optical field of

the transmitted signal and the OFDM baseband signal. At the receiver the OFDM signal is mixed

with a component at the optical carrier frequency and the signal detected from the carrier signal

mixing products. The component at the optical carrier frequency can either be transmitted with

the OFDM signal as in direct-detection optical OFDM (DD-OOFDM) or coherent detection can

be used where the received signal is mixed with a locally generated carrier signal as in coherent

optical OFDM (CO-OFDM). Both techniques have advantages. DD-OOFDM has a simple

receiver, but some optical frequencies must be unused if unwanted mixing products are not to

cause interference. This is usually achieved by inserting a guard band between the optical carrier

and the OFDM subcarriers. This reduces spectral efficiency. DD-OOFDM also requires more

transmitted optical power, as some power is required for the transmitted carrier. CO-OFDM

requires a laser at the receiver to generate the carrier locally, and is more sensitive to phase noise.

There is currently extensive research into the performance of both systems and on techniques to

mitigate the disadvantages of each. It is useful to understand the problems which arise in single

mode systems if an OFDM subcarrier is mapped to more than one optical frequency. If double-

sideband modulation is used, each OFDM subcarrier is represented by two optical frequencies,

one on either side of the optical carrier, chromatic dispersion (CD) results in two components

with equal amplitude and different phases. Subcarriers for which these two components cancel,

experience deep fades. If intensity modulation is used,but one sideband is suppressed, the

combination of the nonlinear effect of intensity modulation and dispersion in the channel results

in ICI in the received signal.

MIMO-OFDM for Optical CommunicationsIn wireless communications, MIMO OFDM has very quickly moved from theoretical concept to

commercial application. In the literature on RF wireless systems, the term “MIMO” is used to

describe a range of systems with multiple transmit and/or receive antennas. Depending on the

relationship between the signals transmitted from different antennas MIMO schemes can be used

to either increase the overall capacity of the system, or to reduce the probability of outage.

12

Because wireless channels usually introduce significant multipath dispersion, MIMO is often

combined with OFDM. MIMO techniques have also been shown to give significant benefit across

a range of optical systems. In indoor optical wireless, multiple transmitters and or receivers can

be used to increase the probability of line of sight between transmitter and receiver . In this

application MIMO OFDM combines the advantages of MIMO with tolerance to delay spread .

MIMO techniques have also been applied to free space optical systems but none of these have

used OFDM. As signal dispersion is relatively unimportant in these applications, the dispersion

tolerance of OFDM is not a significant advantage, though the power efficiency of ACO-OFDM

has potential benefit. MIMO techniques have also been applied to a range of optical fiber

applications. A number of authors have noted the potential of MIMO techniques in multimode

fiber. Intermodal dispersion is usually considered to be a problem in multimode systems, however

when MIMO techniques are applied, it can be used to increase the information capacity of the

fiber. So far there do not appear to be any papers considering the combination of OFDM with

MIMO in multimode systems, despite the significant potential advantages MIMO, both with and

without OFDM, has been applied very successfully in single-mode fiber applications by

transmitting and receiving signals on both polarizations. In this context, MIMO is also called

polarization multiplexing. MIMO in single-mode fiber systems has very different characteristics

from MIMO in wireless applications and may well give even greater benefits. With polarization

multiplexing, all of the received signal power is divided between the two received polarizations,

whereas in wireless systems, the signals at different receive antennas are at best uncorrelated, and

there is always some probability of outage when no antenna is receiving a good signal. It has been

shown experimentally that by using MIMO/polarization multiplexing very high data rate

transmission can be achieved both in systems using OFDM and systems using single carrier

formats.

13

CHAPTER 2

LITERATURE REVIEW

I have done a comprehensive literature review in the field of optical OFDM whose detail is given

as under:

Eric Lawrey [1997]: [1] has given the effectiveness of Orthogonal Frequency Division

Multiplexing (OFDM) as a modulation technique for wireless radio applications.

Hermitian [2001]: [2] ‘Intuitive Guide to Principles of communication’ Introduces OFDM,

OFDM special case of FDM, delay spread and use of cyclic prefix, properties of OFDM,BER

performance.

Su Li [2001]: [3] discussed the advantages of OFDM viz. spectral efficiency; no inter symbol

interference, high data rate, no need of channel equalization etc.

Bryn J. Dixon [2001]: [4] presented the performance of coded OFDM for variety of multimode

fiber profiles, including step index and graded index profiles has been assessed. It has the ability

to perform well in a frequency-selective multipath environment at data rates in excess of 100Mb/s

without equalization. Here the use of OFDM to combat the effects of multimode fiber dispersion

and investigations on the feasibility of its use for a variety of refractive index profiles has been

reported.

Debashis Chanda,Abu Sesay and Bob Davies [2004]: [5] have suggested better ways to overcome

PAPR problem of OFDM signal which will improve its performance in fiber. Low PAPR in

OFDM signal results in low out of band noise generation and robustness against MZ nonlinearity.

The Performance of the clipped OFDM signal was better than unclipped OFDM signal .

O. Gonzalez [2005]: [6] proposed OFDM system for optical wireless transmission, which was

able to withstand multipath dispersion and also more bandwidth efficient than previous systems

The use of channel equalization based on sending initial training sequences to estimate channel

14

response has been demonstrated as a favorable method to combat frequency channel fluctuations,

owing to slowly time varying nature of optical channels. In addition, a simple QAM system has

been described, which was able to outperform system throughput over noisy wireless optical

channels.

N.E Jolley, H.Kee in his paper [2005]: [7] described the generation of the fastest ever DQPSK

encoded OFDM signal at 10 Gb/s. Minimum degradation to the performance was observed due to

differential encoding scheme and use of cyclic prefix in the data. An acceptable error rate of

better than 1 *10 -4 was achieved.

Arthur James Lowery and Jean Armstrong [2005]: [8] presented a method of transmitting OFDM

signals over multimode fibers that increases electrical SNR by 7 db for a given optical power and

new method of transmitting OFDM signals over optical channels by using zero bias.

Ivan B. Djordjevic and Bane Vasic in his paper [2006]: [9] proposed OOFDM provides increase

in transmission distance, improvement of the spectral efficiency to at least 2.9 b/s/Hz and also

provides dispersion compensation. To further improve spectral efficiency to at least 2.9 b/s/Hz the

OOFDM SSB transmission should be combined with sub carrier multiplexing. This paper has

introduced a novel modulation format for long haul transmission systems.16-QAM OOFFDM

can be used in short reach systems and QPSK OFDM in long reach systems can be used.

W.Shieh and C. Athaudage in their paper [2006]: [10] proposed to combat dispersion in optical

media. It was shown that optical –signal to –noise ratio penalty at 10Gb/s is maintained below 2

db for 3000 Km transmission of standard single mode fiber without dispersion compensation and

if the maximum delay spread of multipath fading is smaller than the guard time, the cyclic prefix

can perfectly eliminate the intersymbol interference. The fundamental condition for complete

elimination of ISI in optical medium is that the delay spread due to chromatic dispersion among

the subcarriers should not excced the guard time.

J.Peters and Jean Armstrong [2005]: [11] presented novel method of transmitting OFDM signals

over non linear multimode fibers increases electrical SNR by 7 db for given optical power .

15

Arthur James, Lowery and Jean Armstrong [2006]: [12] show that combination of OFDM and

suppressed –carrier OSSB transmission could be used to dispersion compensate ultra –long haul

optical links at 10 Gb/s over transmission distance of 4000 Km, with a 0.5 db power sensitivity

advantage over NRZ system with perfect extinction ratio and optimized threshold at a BER of 10-

3 . Thus OFDM can compensate for the frequency dependent amplitude and phase characteristics

of a communication channel .

W. Shieh, W. Chen and R.S Tucker [2006]: [13] proposed that PMD in deployed links can be

overcome by CO-OFDM systems at 10 GB/s and beyond .This paper analyses the system for

which chromatic dispersion is near zero, and guard time interval margin is used for PMD

mitigation. In IO-OFDM systems where main optical carrier is sent along with the OFDM sub

carriers, the polarization misalignment between main carrier and OFDM sub carriers will cause

severe fading thus CO-OFDM provides complete PMD mitigation.

Hongchun Bao and William Shieh in his paper [2007]: [14] showed that Q of the WDM channels

at 10 Gb/s is over 13 db for transmission up to 4800 Km of standard SMF without dispersion

compensation. In this paper two things are investigated, Max achievable Q value and optimal

launch power at various transmission distances .A novel technique of partial carrier filling for

improving the nonlinearity performance of transmission is also presented.

W.Shieh ,X.Yi and Y.Tang [2007]: [15] shown that 128 OFDM sub carriers with nominal data

rate of 8 Gb/s have been successfully processed and recovered after 1000 Km transmission

through SSMF fiber without optical dispersion compensation.

Yan Tang ,William Shieh, Xingwen Yi [2007]: [16] proposed the optical I/Q modulator

nonlinearity in CO-OFDM system based on direct up/down conversion has been analyzed .It was

find that in contrast to direct – detected systems, the optical modulator bias point for the coherent

system was ,where Q penalty and excess loss were minimized.

Brendon J.C Schmidt, Arthur James Lowery and Jean Armstrong [2007]: [17] has presented that

optical OFDM using simple direct detection receiver can post – compensate for dispersion in 320

Km of SMF fiber at 20 Gb/s. In this system signal was recovered by mixing between the OFDM

optical sideband and optical carrier, so distortion from phase noise and frequency offset was

completely eliminated.

16

W.Shieh in his paper [2007]: [18] showed the PMD benefit to fiber nonlinearity reduction in CO

- OFDM systems was predicted. In CO-OFDM systems not only does PMD not cause any

impairment, but it also provides a benefit of polarization-dependent –loss induced fading and

consequently improves system margin.

Xingwen Yi, William Shieh [2007]: [19] in his paper presented phase estimation and

compensation methods for a transmission for a CO-OFDM transmission experiment with a

nominal data rate of 8 GB/s over 1000-Km SSMF without optical dispersion compensation. The

OSNR penalty was found to be about 1 db at BER of 10 -3.

Arthur James Lowery [2007]: [20] showed that nonlinear power limit of optical links using

optical OFDM for dispersion compensation can be significantly improved using optimum

combination of nonlinearity pre-compensation and post compensation. An optimum combination

of pre and post compensation allows a 2 db increase in launch power for standard SMF over 2000

KM or 5 db increase in signal quality for given launch power.

Xingwen Yi, William Shieh, Member, IEEE, and Yiran Ma [2008]: [21] showed the phase noise

effects on High Spectral Efficiency. This paper includes the three major advantages for coherent

optical orthogonal frequency-division multiplexing (CO-OFDM) transmission using digital signal

processing. First, coherent detection is realized by digital phase estimation without the need for

optical phase-locked loop. Second, OFDM modulation and demodulation are realized by the well-

established computation-efficient fast Fourier transform (FFT) and inverse FFT. Third, adaptive

data rates can be supported as different Quadrature amplitude modulation (QAM) constellations

are software-defined, without any hardware change in transmitter and receiver.

Don F. Hewitt and Nishaanthan Nadarajah [2009]: [22] gave the comparison of Double and single

side band direct base band transmission. The paper investigates the complex interactions between

sidebands for baseband DSB coherent optical OFDM systems and proposes the use of SSB

coherent OFDM to avoid sideband interaction in dispersive fiber .

Liang Du, Arthur Lowery [2009]: [23] studies the Improvement in Nonlinear Pre-compensation

in Direct-Detection Optical OFDM Communications Systems. It studies the Carrier boosting of

the receiver enabled direct detection optical OFDM (DDO-OFDM) to outperform coherent

17

OOFDM in the nonlinear limit. Boosting also improves the effectiveness of nonlinearity pre-

compensation substantially.

Y. Tang, X. Yi, W. Shieh and R. Evans [2009]: [24] studies the Optimum Design for Coherent

Optical OFDM Transmitter. Optimum design for coherent optical OFDM transmitter has been

analyzed. In contract to the direct-detection system, the optimal modulator bias point for the

coherent system is π where the nonlinearity and excessive loss are minimized

Wei-Ren Peng, Jason (Jyehong) Chen, and Sien Chi [2010]: [25] studied the On the Phase Noise Impact in Direct-Detection Optical OFDM Transmission. In this letter, they characterized the impact of laser phase noise (PN) in direct-detection optical orthogonal frequency-division multiplexing (OFDM) and emphasize its several differences from those in coherent optical OFDM. They also analyzed the system performance in the presence of PN for various Quadrature-amplitude-modulation formats and provide the bit-error-rate estimation method which can yield reliable results when the PN -induced optical

signal-to-noise ratio penalty is lower than 2 dB.

Alan Barbieri, Giulio Colavolpe, Tommaso Foggi, Enrico Forestier and Giancarlo Prati [2010]: [26] studied the OFDM versus Single-Carrier Transmission for 100 Gbps Optical Communication. They analyzed the orthogonal frequency division multiplexing (OFDM) technique in long-haul next generation optical communication links and compare it with the well-established single-carrier (SC) data transmission using high-level modulation formats and coherent detection. The analysis of the two alternative solutions is carried out in the 100 Gbps scenario, which is commonly considered to be the next upgrade of existing optical links, with special emphasis on quaternary phase-shift keying (QPSK) modulations.

Nir Shaffy and Dan Sadot IEEE member [2010]: [27] studied the Direct Modulation and

Coherent Detection Optical OFDM. It studies about the Optical orthogonal frequency division

multiplexing (O-OFDM) with a novel direct modulation and coherent detection system at 34.3

Gb/s and 16 QAM is proposed. Electronic pre-compensation of laser frequency response is

applied at the transmitter.

18

Kai ming Feng and Wei Ren Peng IEEE [2010]: [28] studied the Enhancing Spectral Efficiency

and Receiving Sensitivity in a Direct-Detected OFDM System. In this paper, they reviewed the

latest developments in improving the spectral efficiency and receiving sensitivity in a direct-

detected OFDM transmission system. An RF-tone assisted virtual single sideband (VSSB) OFDM

modulation format is proposed to enhance the spectral efficiency in both electrical and optical

domains for direct detected OFDM applications.

Satana Suppitux, Soonthorn Tangkachavana, Thipvadee Vinichhayakul, and Prapun Suksompong

[2010]: [29] studied the enhancing PAPR reduction for Tone Reservation Algorithms by deep

clipping. Large Peak-to-Average Power Ratio (PAPR) has been a crucial problem in OFDM

system. Tone reservation (TR) is a well known technique to reduce PAPR. Its advantages include

no side information and no distortion on the data-bearing carriers.

Gong Lin，Yang Shu-hui, Chen Yinchao IEEE [2010]: [30] studied Research on the reduction of

PAPR for OFDM signals by companding and clipping method. In this paper, they proposed a

union algorithm of Companding and clipping method to reduce Peak-to-average Ratio (PAPR) in

Orthogonal Frequency Division Multiplexing (OFDM) system.

Jean Armstrong IEEE [2010]: [31] studied the New OFDM PAPR reduction technique. This

paper describes a new peak-to-average power ratio (PAPR) reduction scheme for orthogonal

frequency division multiplexing (OFDM. A time domain version of the OFDM signal is

generated using an oversized inverse discrete Fourier transform (DFT).

Neil Carson and T. Aaron Gulliver [2010]: [32] studied the PAPR Reduction of OFDM Using

Selected Mapping, Modified RA Codes and Clipping. The demand for high speed wireless

communications is constantly increasing. Orthogonal Frequency Division Multiplexing (OFDM)

is a modulation technique that can be used to address this demand as it performs well in fading

channels. However, OFDM has one significant drawback, the potential for a high Peak-to-

Average Power Ratio (PAPR). This paper presents a novel approach to reducing the PAPR using

a modified Repeat- Accumulate (RA) code and signal clipping.

19

Eprahim B. Al-Safadi and Tareq Y. Al-Naffouri [2010]: [33] studied the Reducing of the

complexity of Tone-Reservation based PAPR reduction schemes by compressive sensing. In this

paper, they described a novel design of a Peak-to-Average-Power-Ratio (PAPR) reducing system,

which exploits the relative temporal sparsity of Orthogonal Frequency Division Multiplexed

(OFDM) signals to detect the positions and amplitudes of clipped peaks, by partial observation of

their frequency content at the receiver.

Zhiyuan Huang, Juhao Li, Su Zhang, Fan Zhang, Zhangyuan Chen [2010]: [34] studied the

Investigations of SPM Suppression by PAPR Reduction in Coherent Optical OFDM Systems.

They investigated SPM suppression for coherent optical OFDM systems utilizing three PAPR

reduction methods including the clipping, the selective mapping and the partial transmit

sequence.

Hamidreza Bakhshi, Mohammadamin Shirvani [2010]: [35] studied the Peak-to-Average Power

Ratio Reduction by Combining Selective Mapping and Golay Complementary Sequences. They

studied a new method for reduction of Peak-to-Average Power Ratio in OFDM systems by

combining selective mapping and Golay complementary sequences.

Yafei Hou, Tomohiro Hase [2010]: [36] studied a new way of PAPR reduction for OFDM

system. This paper proposes a new peak-to-average power ratio (PAPR) reduction technique for

OFDM systems. The proposed system selects the specific duration of each time-domain OFDM

symbol which can achieve a promising PAPR, to transmit equal bits of data.

Carole Devlin, Anding Zhu, and Thomas J. Brazil [2010]: [37] studied a PAPR Reduction

Technique for OFDM Signals Using Unused Tones with Phase Information. A major drawback of

Orthogonal Frequency Division Multiplexing (OFDM) is the high Peak-to-Average Power Ratio

(PAPR) of the transmit signal which can significantly impact power efficiency and performance.

In this paper they presented a PAPR reduction technique which exploits the phase of the signal in

the frequency domain.

Y. Z. Jiao, X. J. Liu, and X. A. Wang [2010]: [38] studied a novel Tone Reservation Scheme with

Fast convergence for PAPR reduction in OFDM systems. OFDM is facing great opportunities and

challenges in current broadband communication era. These opportunities and challenges derive

from the native advantages and disadvantages of OFDM technology respectively. Too high PAPR 20

is one of the main problems that prevent OFDM from being used more generally in broadband

systems. Many approaches such as clipping and filtering, coding, SLM, PTS, and tone reservation

have been studied to reduce the peak magnitude of OFDM symbols. In these approaches, tone

reservation is considered as one of the most promising methods because of no additional

distortion, no side information, and low implementation cost. In this paper, a novel tone

reservation scheme is presented. It’s essential idea is that a subcarrier selected from all reserved

subcarriers for PAPR reduction should have a phase close to one of φ, π/2+φ, π+φ and -π/2+φ, at

the peak location in time domain, where φ is the phase of the peak sample.

21

CHAPTER 3

PROBLEM FORMULATION

3.1 OBJECTIVE OF STUDY

To Design and Simulate Optical OFDM system using OPTSIM.

To carry out the PAPR performance analysis of above data format on the basis of

performance matrices using Instantaneous Power.

To improve the PAPR in optical transmission link by employing different techniques

using MATLAB.

3.2 PROBLEM FORMULATION

Since Optical OFDM system involves analogue modulation, and detection of light, it is

fundamentally an analogue transmission system. Therefore, signal impairments such as noise and

distortion, which are important in analogue communication systems, are important in these

systems as well. These impairments tend to limit the Noise Figure (NF) and Dynamic Range of

these links.

The noise sources in analogue optical fiber links include the laser’s Relative Intensity Noise

(RIN), the laser’s phase noise; the photodiode’s shot noise, the amplifier’s thermal noise. In

Single Mode Fiber (SMF) based OOFDM, system, chromatic dispersion may limit the fiber link

lengths and may also cause phase de-correlation leading to increased RF carrier phase noise. In

Multi-Mode Fiber based OOFDM systems, modal dispersion severely limits the available link

bandwidth and distance. It must be stated that although the OOFDM transmission system itself is

analogue, the radio system being distributed need not be analogue as well, but it may be digital

(e.g. WLAN, UMTS), using comprehensive multi-level signal modulation formats such as

xQAM, or Orthogonal Frequency Division Multiplexing (OFDM).

AM\AM distortions are a major focus of research with OFDM systems. Because of the large

number of carriers used in an OFDM system especially for UWB, the dynamic range for the

output of the RF signal can be quite large. Thus, researcher have been facing with the problem of

minimizing the amount of harmonic distortions caused by driving the modulator into saturation

yet maintaining an efficient operating point .The non-linearity causes two effects on the detected

samples:

22

• Constellation warping of amplitude and phase distortions.

• Nonlinear distortion, which causes a cluster of received values around each constellation point

rather than a single point.

3.3 High Peak to Average Power Reduction Because of the difficulty with filtering out near-in inter-modulation (IM) products, IM is the most

difficult to deal with. Equally important situation is when the harmonic distortions are caused by

the input signal driving the amplifier into its saturation region. In the saturation region, an

increase in input drive level does not result in an increase in output power level. The definition for

the beginning of the saturation region is specified relative to the 1 dB compression point. Shown in

Figure the 1 dB compression point is labeled “P1dB” and is defined as the point at which a 1 dB

increase in input power results in 1 dB decrease in the linear gain of the analog device. There for, the

dynamic range of analog device, which also corresponds to the linear region of operation for an

analog device, is defined between the noise-limited region and the saturation region.

Fig 3.10 Represents Power Transfer Function

23

3.4 High Peak to Average Power of OFDM Large peak-to-average ratio (PAPR) distorts the OFDM signal if the transmitter contains

nonlinear components such as laser sources. The nonlinear effects on the transmitted OFDM

symbols are spectral spreading, inter-modulation, and changing the signal constellation. In other

words, the nonlinear distortion causes both in-band and out-of-band interference to signals. The

in-band interference increases the BER of the received signal through warping of the signal

constellation and inter-modulation while the out-of-band interference causes adjacent channel

interference through spectral spreading. The latter is what prevents the usage of OFDM in many

systems even if the in-band interference is tolerable. Therefore the laser source requires a back

off, which is approximately equal to the PAPR for distortion less transmission. So, reducing the

PAPR is of practical interest. The OFDM baseband signal for N subcarriers is:

1

where the an and bn are the in-phase and quadrature modulating symbols.

If each carrier has amplitude A, the maximum PAPR will be: (NA)^2 / [N*(A2/2)] = 2N When

the number of subcarriers N is small, a PAPR of 2N has reasonable chances of occurring.

However, if N is large enough so that the central limit theorem applies, the amplitude distribution

of the OFDM signal is better approximated by a Rayleigh distribution since a PAPR of 2N has

exceedingly small probability of occurring. The cumulative distribution function for the peak

power per OFDM symbol is shown in equation 2

2

Where z is the complex envelope power of x (t) and 2δz represents envelope power to average

symbol power ratio.

24

3.5 PAPR Reduction Methods The high PAPR of OFDM means that if the signal is not to be distorted, many of components in the transmitter and receiver must have a wide dynamic range. In particular the output amplifier of the transmitter must be very linear over a wide range of signal levels. In wireless systems the expense and power consumption of these amplifiers is often an important design constraint. Inter modulation resulting from any nonlinearity results in two major impairments: out-of-band (OOB) power and in-band distortion. In wireless communications OOB power is usually the more important, because of the near-far problem; interference from the OOB power of a close transmitter may swamp reception from a distant transmitter. For this reason the specifications on OOB power in wireless are very stringent. OOB power caused by transmitter nonlinearities may be much less of a problem in optical applications of OFDM. As we will show, in-band distortion is a relatively small effect and becomes important only for large signal constellations.

3.6 CHOICE OF KEY ELEMENTS FOR OPTICAL OFDM SYSTEM

(1) Useful symbol duration

The useful symbol duration T affects the carrier spacing and coding latency. To maintain

the data throughput, a longer useful symbol duration results in increase of the number of

carriers and the size of FFT (assume the constellation Fixed) In Practice, carrier offset and

phase stability may affect how close two carriers can be placed. Its application is for the

mobile reception, the carrier spacing must be large enough to make Doppler shift

negligible. Generally the useful symbol duration should be chosen so that the channel is

stable for the duration of a symbol

(2) Number of carriers

The number of sub-carriers can be determined based on channel bandwidth, data

throughput and useful symbol duration. The carriers are spaced by the reciprocal of the

useful symbol duration. The number of carriers corresponds to the number of complex

points being preceded in FFT. For HDTV applications, the number of sub-carriers are in

25

the range of several thousands, so as to accommodate the data rate and guard interval

requirement.

(3) Modulation scheme

The modulation scheme in an OFDM system can be selected based on the requirement of

power spectrum efficiency and the desired BER performance. The type of modulation can

be specified by the complex number a+jb. The selection of the modulation scheme

applying to each sub-channel depends solely on the compromise between the data rate

requirement and transmission robustness and another advantage of OFDM is that different

modulation schemes can be used on different sub-channels.

(4) Guard interval

The trade-off of guard interval is to set it large enough to avoid inter symbol interference

depending upon the memory channel and transmitter position spacing in a single

frequency network. On the other hand, we want it to be as small as possible as it carries no

information and can be seen as a spoil of bandwidth. In wireless system; a guard interval

of 25% of symbol period is often met.

26

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[1] Eric Lawrey, thesis on ‘The suitability of OFDM as a modulation technique for wireless

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[4] Bryn J. Dixon, ‘OFDM in wirelesss communication system with multimode fiber

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27

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[19] Xingwen Yi,William Shieh ‘ Phase Estimation for coherent Optical OFDM’ ,IEEE

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29