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DWDM Based System for Optical

Generation and Transmission of Impulse

Radio UWB SignalsAffan Hasan Khan, Omer Khalid, Ateeq Mumtaz, Dr. M. Khawar Islam

Faculty of Telecommunications and Information Engineering

University of Engineering and Technology, Taxila, Pakistan

Abstract—  This paper puts forward a simple technique for

 generation of Impulse Radio Ultra-wideband pulses in optical 

 domain using an optical delay line, optical bias and an optical 

 subtractor. We transmit the data for 32 users across a 1 km

 optical fiber link using Dense Wavelength Division Multiplexing. Moreover, we analyze the relationship between the

input data rate and signal bandwidth and also study the effect of 

 pulse width on the bandwidth of a signal. This paper depicts our

 research and efforts in UWB-over-fiber technology.

 Index Terms —    DWDM,   Impulse Radio, Optical 

 Differentiator, Ultra-wideband (UWB) over fiber 

I INTRODUCTION 

Ultra-wideband is an up-and-coming technology in wireless

communication that has the potential to provide high data

rate broadband wireless access. Though it is an emergingtechnology at present, the concept is not new at all. The

history of using UWB for wireless communication goes

back to early 1900s when Marconi used UWB pulses in his

spark-gap radio transmitter to transmit Morse code

sequences. The prominent qualities of UWB attracting

attention of researchers from around the globe are its low

complexity, low cost, reduced power consumption and

increased data rate. The mere limitation of UWB is its short

range which can be overcome by the use of Optical Fibers

carrying signals from the head-end to user premises. This

technology is commonly referred to as UWB-over-fiber.

The generation of UWB pulses has always been a challengebecause the wide bandwidth requires the signal to be

considerably narrow in the time domain. Moreover, to fully

exploit the advantages of UWB, there is a need for optical

generation of UWB pulses to avoid the use of high cost

electrical components required to produce such narrow

signals and to exceed the speed limitations offered by the

electrical components. Numerous solutions have been

proposed in the past demonstrating all-optical generation of 

UWB pulses. Recent approaches include the use of cross

phase modulation[1], cross-gain modulation in a

semiconductor optical amplifier (SOA) [2], intensity

modulator to generate polarity switchable UWB pulses [3],

LiNbO3  Intensity modulator using the transfer function’s

wavelength dependant characteristics [4]  and the use of  Phase Modulator and AMZI to generate UWB pulses [5].

The problem associated with [1] and [2] is that they require

two laser sources resulting in increased system complexity.

In [3] and [4], the generation is limited to a single UWB

pulse which confines the use of this technique to a limited

number of applications. Also, the requirement of two

wavelengths in [4] renders the system expensive and

complicated. [5] requires two Asymmetric Mach-Zehnder

interferometers (AMZIs) to be cascaded which causes the design to face stability issue.

In this paper we put forward a technique to generate UWB

signals in optical domain and demonstrate a DWDM baseddistribution mechanism. A Mach-Zehnder modulator is

used to intensity modulate electrical Gaussian pulses. An

optical delay line followed by a subtractor is used to

differentiate the optical Gaussian pulse and thus Ultra-

wideband monocycle pulses are generated. Owing to the

use of optical components only, this technique results in an

ultrafast optical pulse source.

II. OPERATING PRINCIPLE

Figure 1 shows the schematics of the proposed design. The

electrical Gaussian pulses are generated from the incoming

data by the help of a Gaussian pulse generator. These

electrical pulses modulate a Mach-Zehnder modulator andoptical Gaussian pulses are produced. The Gaussian pulse

can be expressed as:

√  (

)  (1)

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Fig. 1: Schematics Diagram of the proposed technique

 A. UWB generation principle:

Mathematically, the Ultra-wideband monocycle pulse is

given by the first order differential of a Gaussian pulse. The

width of resultant UWB pulse can be adjusted by

manipulating the input Gaussian pulse. In our design,

differentiation was performed by splitting the optical

Gaussian pulse into two equal components. One component

was provided with an optical bias, which lifts thiscomponent higher than the peak of the other component.

The second component is passed through an optical delay

line, delaying the signal in time by an amount equal to the

one half of the width of Gaussian pulse. Then, optical

subtraction[6][7][8] was used to subtract the later

component from the former. In this way, during the time in

which elevated component goes through its ascending

values, represented by part 1 in figure 2, the delayed

component is zero and the subtraction results in nothing but

the elevated component itself. Next, during part 2 in figure

2 the subtraction results in the values forming part 5 in the

UWB monocycle. And finally, during the part 3 of 

subtraction, the elevated Gaussian pulse has a constant

positive value equal to the value of optical bias provided.

Thus the subtraction of the delayed component from this

constant positive value results in an inverted copy of the

delayed component (part 6) producing UWB pulse in

optical domain.

Figure 2 shows the working of a differentiator. The elevated

and delayed component is shown and formation of UWB

monocycle is depicted.

Fig. 2: Working of a Differentiator

The equation for UWB monocycle can be deduced from

equation (1): 

(2) 

Fig. 3: Gaussian pulse and its frequency spectrum 

Figure 3 shows a Gaussian pulse along with its frequency

spectra. Narrower the pulse width in time domain, broader

is its frequency spectrum.

Fig. 4: Ultra-wideband monocycle pulse with its frequency spectrum

Figure 4 shows an Ultra-wideband monocycle along with

its frequency spectrum. It should be noted that the original

Gaussian pulse used was a baseband signal while the UWB

pulse generated is a band-pass signal with its frequency

spectrum symmetric around a center frequency. Figure 3

and 4 has been plotted using mathematical equations and

will be used later to verify the simulation results.

III. SIMULATIONS AND RESULTS

The discussed design was simulated at an input data rate of 

2Gb/s. The optical delay line used for the differentiation of 

Gaussian pulses has an inversely proportional relationship

with input data rate. Table-1 lists the input data rates

corresponding to the optical delay required for proper

differentiation. The value of optical delay can be calculated

from the data rate and width of the Gaussian pulse. For

example, if the input data rate is 2Gb/s, the width of a

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single bit comes out to be 1/(2*109) = 0.5 ns. So, if the

Gaussian pulses are configured to be 0.1 bit wide, the width

of the pulses in nanoseconds comes out to be 0.5*0.1 = 0.05 

ns. Practically, due to limitations of the Gaussian pulse

generator, the actual width is twice this value i.e. 2*0.05 =

0.1 ns. Hence, at a data rate of 2 Gb/s, an optical delay line

of 0.05 ns (one half of Gaussian pulse width) is required for

proper differentiation. From these calculations we canderive an equation to calculate the value of optical delay

line from the data rate and pulse width.

 

UWB pulses generated at 2Gb/s and the corresponding

frequency spectrum is shown in figure 5 and 6 respectively.

The used modulation technique was On-Off keying.

TABLE-1DATA RATES CORRESPONDING TO THE REQUIRED OPTICAL

DELAY

Sr. No. Data Rate(Gb/s)

Optical Delay(ns)

1. 0.5 0.2

2. 1 0.1

3. 2 0.05

4. 4 0.025

Fig. 5: UWB monocycle generated at 2 Gb/s.

Fig. 6: Frequency spectrum of UWB monocycle at 2Gb/s. The spectrum is

centered at 5 Ghz and has a bandwidth of 6 GHz at -10dbm. 

For comparison, the simulation has also been performed at

a data rate of 1Gb/s. With the data rate reduced to one half,

the inverse proportional relationship dictates us to double

the optical delay offered. The required optical delay comes

out to be 0.05*2 = 0.1ns which can be verified from table.

1. Figure 7 and 8 shows the UWB monocycle produced and

the corresponding frequency spectrum respectively, at an

input data rate of 1Gb/s.

Fig. 7: UWB monocycle generated at 1 Gb/s. (Twice the pulse width at 2

Gb/s) 

Fig. 8: Frequency spectrum of UWB monocycle at 1Gb/s. The spectrum iscentered at 2 Ghz and has a bandwidth of 3 GHz at -10dbm.

The width of the UWB monocycle in figure 5 is 0.2 ns

while it is 0.4 ns in figure 7. These widths are twice the

widths of the Gaussian pulses used to generate them. With

the data rate reduced to one half, the width of the pulse in

time domain in doubled. Intuitively, this should have an

effect on the corresponding bandwidths of the pulses as

well. From figure 6 and 8 we can derive conclusions that

there is a direct proportionality between the input data rate

and UWB pulse bandwidth. Doubling the data rate doubles

the bandwidth. Figure 9 shows this relationship in the form

a graph for two different values of Gaussian pulse widthused for UWB monocycle generation.

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Fig. 9: Relationship between input data rate and bandwidth for two

different values of Gaussian pulse width used.

From the graph, it is clear that increasing the width of 

Gaussian pulse used, reduces the bandwidth of output UWB

pulse, while the directly proportional relationship between

input data rate and bandwidth stays the same. This is in

compliance with the fact that, shorter the pulses in time

domain, broader are their frequency spectra.

Figure 10 shows the output of Dense Wavelength Division

Multiplexing. The UWB pulses carrying the data of 32

different users were multiplexed using a frequency spacing

of 100 GHz or a wavelength spacing of 0.8 nm.

Fig. 10: 32 Dense Wavelength Division Multiplexed channels

Finally, figure 11 shows the shape of the signal for one user

after travelling over a 1 km optical fiber link with an

attenuation of 0.2 db/km and dispersion of 16.75 ps/nm/km.

The Signal was received using a PIN photo diode.

Fig. 11: Shape of the UWB monocycle received after passing through 1

Km of optical fiber links

IV. CONCLUSIONS

A simple technique for optical generation of Impulse radio

UWB pulses was described along with the mechanism to

transmit the data of 32 users over an optical fiber link using

Dense Wavelength Division Multiplexing. We found that

the bandwidth available is directly proportional to the input

data rate. Also, we figured out that we can increase thebandwidth by reducing the width of our UWB pulse. The

use of all-optical generator for UWB pulses allows to

exceed the limitations of electrical components and high

data rates can be achieved.

REFERENCES

[1] F. Zeng, Q. Wang and J. Yao, “All-optical UWB impulse generation

based on cross-phase modulation and frequency discrimination”,

ELECTRONICS LETTERS, 18th January 2007, Vol. 43 No. 2

[2] Wang, Q., Zeng, F., Blais, S., and Yao, J., “Optical ultrawidebandmonocycle pulse generation based on cross-gain modulation in a

semiconductor optical amplifier”, Opt. Lett., 2006, 31, pp. 3083 – 3085

[3  Q. Wang and J. Yao, "UWB doublet generation using nonlinearly-biased electro-optic intensity modulator," Electron. Lett. 42, (2006), 1304--

1305

[4] J. Q. Li, S. N. Fu, K. Xu, J. Wu, J. T. Lin, M. Tang, and P. Shum,

"Photonic ultrawideband monocycle pulse generation using a single

electro-optic modulator," Opt. Lett., vol. 33, no. 3, pp. 288-290, Feb.

2008Opt. Lett. 33, 288 (2008).[5] S. Pan and J. P. Yao: “Switchable UWB pulse generation using a phase

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double-grating shearing interferometer,”  Optics Communications

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