Experimental Distribution of OFDM-UWB Radio Signals along

Directly Modulated Long-reach Pons Indicated for Sparse

Geographical Areas

Tiago M. F. Alves and Adolfo V. T. Cartaxo

Instituto de Telecomunicações, Department of Electrical and Computer Engineering,

Instituto Superior Técnico, Technical University of Lisbon, 1049-001 Lisbon, Portugal

Keywords: Long-Reach Passive Optical Networks, Orthogonal Frequency-Division Multiplexing, Ultra Wideband,

Directly Modulated Lasers, Optical Dispersion Compensation.

Abstract: The distribution of three orthogonal frequency division multiplexing (OFDM) ultra wideband (UWB) bands

along directly modulated long-reach (LR) passive optical networks (PONs) is demonstrated experimentally.

Adequate selection of the UWB signal applied to the directly modulated laser (DML) and fixed in-line

optical dispersion compensation are shown as effective solutions to reach 130 km of standard single-mode

fibre. In addition, error vector magnitude (EVM) levels compliant with the UWB standard are achieved for

LR-PONs with reaches between 75 km and 130 km. It is also shown that, for optical channel spacing as

narrow as 0.2 nm, the proposed system suffers from negligible linear inter-channel crosstalk as the EVM of

the UWB bands transmitted in the central channel of a wavelength division multiplexing (WDM) signal

consisting in three optical channels, is similar to the EVM of the UWB bands transmitted in that channel

when single-channel operation is considered. These results demonstrate that the proposed directly

modulated LR-PON is an adequate solution to deliver the UWB signals to users’ premises located in sparse

take-up geographies.

1 INTRODUCTION

Ultra wideband (UWB) radio communication

systems have been receiving a special attention over

the last years. Such systems can benefit from several

UWB advantages as high data rate broadcasting (480

Mbits/s (Staccato Communications, 2005); (ITU-R,

2004), tolerance to multi-path fading, possibility of

co-existence with other already employed

technologies (IEEE 802.11 and IEEE 802.16),

position monitoring and low power consumption

allowing small size/low cost integration

(Siriwongpairat and Liu, 2008).

UWB is an unlicensed technology that uses radio

modulation techniques with a minimum bandwidth

of 500 MHz or at least 20% greater than the centre

frequency of operation. UWB channels must be

allocated in the band between 3.1 and 10.6 GHz with

a maximum equivalent isotropic radiated power

(EIRP) of -41.3 dBm/MHz (EU, 2007); (FCC,

2002). The UWB radio signals broadcasting is

indicated for small environments like homes and

offices premises. Impulse-radio (IR) and orthogonal

frequency division multiplexing (OFDM) were

proposed as UWB signal modulation formats

(Siriwongpairat and Liu, 2008). The OFDM-UWB

solution has shown enhanced features such as higher

flexibility to provide multiple access inherent to

multi-band techniques, tolerance to multi-path

fading and intersymbol interference (ISI), and

reduced band limitations in the UWB transceiver

due to the 528-MHz-wide channelization of OFDM-

UWB signals rather than 7.5 GHz of bandwidth for

IR-UWB signals. Furthermore, devices for OFDM

modulation and demodulation are already available

due to the use of OFDM in other wireless

applications like IEEE 802.11 and IEEE 802.16.

Hence, this work is focused only on OFDM-UWB

radio signals.

Significant efforts from microelectronic

companies developing UWB terminals and devices

at low cost have been accomplished in order to reach

a large scale penetration of the UWB technology in

the access telecommunication networks (UROOF,

2005). The Wimedia Alliance, a non-profit

412

M. F. Alves T. and V. T. Cartaxo A..

Experimental Distribution of OFDM-UWB Radio Signals along Directly Modulated Long-reach Pons Indicated for Sparse Geographical Areas.

DOI: 10.5220/0004615104120418

In Proceedings of the 4th International Conference on Data Communication Networking, 10th International Conference on e-Business and 4th

International Conference on Optical Communication Systems (OPTICS-2013), pages 412-418

ISBN: 978-989-8565-72-3

 2013 SCITEPRESS (Science and Technology Publications, Lda.)

organization that defines, certifies and supports

enabling wireless technology for multimedia

applications, defined data rates up to 1 Gbit/s and

the creation of a global UWB radio standard with

guaranteed inter-operability as main targets for the

UWB radio specifications. Hence, UWB technology

presents the advantage of broadcasting higher data

rate between electronic devices than traditional

communications and it is indicated to be used by

high quality multimedia equipment as personal

digital video recorders (DVR), high definition

television (HDTV), laptops and cable/satellite set-

top boxes (ITU-R, 2004); (Wimedia, 2013).

The transmission of UWB radio signals along

optical fibre in short-range environments was

already investigated (Guo et al., 2007). The

application target of such investigation is to cover

buildings/offices with an integrated optical fibre

distribution/wireless broadcasting solution that

allows the end-users to benefit from the high

mobility and high bit-rate over short ranges

capabilities provided by the UWB-based wireless

networks. The transmission of these UWB radio

signals over fibre-to-the-home (FTTH)

infrastructures is a powerful solution to address the

distribution of UWB signals along longer distances.

Moreover, avoiding trans-modulation or frequency

conversion of the UWB radio signals at the end-

users' premises, the subscribers can benefit from low

cost transponders and a deep penetration can be

expected worldwide. The considered FTTH paths

correspond to standard lengths used in passive

optical networks (PONs) to connect distribution

hubs (DH) to UWB end users (up to 60 km).

Recently, the increase of the reach of the FTTH

paths has been a hot research topic supported by the

operators’ point of view (Davey et al., 2009). The

main target of this topic is to reach 100 km between

the central office and the users' premises, and it is

indicated for the metro and access networks

integration envisaged by long-reach (LR) PONs

(Davey et al., 2009). In LR-PONs, the optical line

termination (OLT) at the central office (CO) is

connected to an active remote node (RN) via a fibre

span denominated feeder or trunk line (Davey et al.,

2009). The target reach of this span is around 80 km

and optical amplification is performed at the RN in

order to compensate for the losses introduced along

the optical link. The different optical network units

(ONUs) are then connected to the RN via a

completely PON with reach around 20-30 km. From

the operators’ viewpoint, some of the benefits of the

LR-PONs are (Davey et al., 2009): i) decrease the

number of OLTs deployed and provide a full

integration between the metro and access networks

with the corresponding system cost savings, ii)

sharing the OLT and the feeder fibre by several

users in a sparse take-up geography and iii) decrease

the configuration and management issues of the

network.

External and direct modulation have been

recently proposed and demonstrated as effective

solutions to be employed in LR-PONs supporting

radio-over-fibre signals (Alves et al., 2013). External

modulated LR-PONs have shown better

performance, whereas directly modulated LR-PONs

are viewed as a cost-effective and alternative

solution. However, the maximum reach of directly

modulated LR-PONs is commonly assumed shorter

than when external modulation is employed due to

the combined effect of the chirp introduced by the

directly modulated laser (DML) and the fibre

dispersion.

The study of the directly modulated LR-PON

work proposed in (Alves et al., 2013) for the

provisioning of different wired and wireless OFDM-

based services to the end users showed that the

performance of the bundle of OFDM signals is

impaired mainly by the UWB signals. This is due to

the higher bandwidth of UWB signals and also due

to their higher central frequencies (Alves et al.,

2013).

In this work, we focus our attention only in the

transmission of the UWB signals along directly

modulated LR-PONs. Particularly, we propose and

demonstrate experimentally useful system design

guidelines enabling the distribution of UWB signals

along directly modulated LR-PONs with maximum

reach exceeding 100 km. This directly modulated

extended LR-PON is a powerful solution to

distribute the UWB signals to users’ premises

located at sparse geographical areas in a cost-

effective manner.

2 EXPERIMENTAL SETUP

Figure 1 depicts the diagram of the experimental

setup employed to assess the performance of the

OFDM-UWB signals distribution along the directly

modulated LR-PON. Figure 1 depicts a multi-

wavelength setup comprising three optical channels.

However, in this section, we focus the system

description only on the single-channel operation, i.

e., the switch S presented in Figure 1 is in the open

position. Further information about the multi-

wavelength setup is provided in section 3.2.

The OFDM-UWB bands #1, #2 and #3 are

generated and frequency multiplexed through digital

ExperimentalDistributionofOFDM-UWBRadioSignalsalongDirectlyModulatedLong-reachPonsIndicatedforSparse

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413

Figure 1: Schematic diagram of the experimental setup implemented in the laboratory to emulate the distribution of the

UWB signals along a directly modulated LR-PON.

signal processing (DSP) in Matlab. The raw data rate

of each UWB band is 320 Mbaud and the UWB

bands #1, #2 and #3 are centred at the frequencies of

3.4 GHz, 3.9 GHz and 4.4 GHz, respectively, and

each UWB band occupies a bandwidth of 528 MHz.

Hence, after multiplexing, the spectral occupancy of

the three UWB bands is between 3.1 GHz and 4.7

GHz. Quadrature phase-shift keying (QPSK)

mapping and similar power levels between the three

OFDM-UWB bands are considered. Only these three

bands are considered in these experiments because

most of the UWB devices commercially available

nowadays operate only in these UWB bands and

also due to the limited bandwidth of the DML

available in the laboratory. The electrical signal is

generated by an arbitrary waveform generator

operating at 20 Gsamples/s. A radio frequency (RF)

amplifier and a variable electrical attenuator are used

to set the modulation index of the signal applied to

the DML. After amplification, the electrical noise

power is reduced by using a low-pass filter (LPF)

with -3 dB bandwidth of 7.6 GHz. The DML is a

low-cost multi-quantum well DFB laser

characterized by a threshold current of I

=8.1 mA, a

bias current of I

=30 mA, a chirp parameter of 2.6,

nominal wavelength of 1552.85 nm and an intensity

response bandwidth of about 4 GHz (Morgado et al.,

2011).

The OFDM-UWB signal is launched into a 75

km-long feeder standard single-mode fibre (SSMF),

with dispersion parameter of 17 ps/nm/km, with an

average optical power of 0 dBm.

At the remote node (RN), an optical amplifier

compensates for the fibre loss and a dispersion

compensating module (DCM) is used to reduce the

degradation induced by the combined effect of the

DML’s chirp and the fibre dispersion. A noise

loading circuit is used to set the optical signal-to-

noise ratio, in a 0.1 nm bandwidth, to 30 dB. The

amplified spontaneous emission noise power is

reduced by using an optical filter with -3 dB

bandwidth of 16 GHz. The average optical power at

the input of the distribution fibre is also 0 dBm.

Different distribution fibre reaches are

considered along the study in order to measure the

performance of the OFDM-UWB signals

distribution for different LR-PON distances.

Particularly, distribution fibres with reach between

0 km and 60 km are analysed. These distribution

fibre reaches correspond to LR-PON with reaches

between 75 km and 135 km.

At the ONU, the average optical power at the

photodetector input is set to -12.5 dBm. The signal is

photodetected by a 10 GHz PIN including a

transimpedance amplifier stage. After photodetec-

tion, the UWB signal is filtered by a LPF with -3 dB

bandwidth of 10 GHz and sampled by a real-time

oscilloscope operating at 20 Gsamples/s. DSP

algorithms are then applied to the sampled signal

waveform in order to demodulate each OFDM-

UWB band. These algorithms comprise RF carrier

recovery, time synchronization, down-conversion,

ideal filtering, FFT window synchronization,

common phase error compensation and equalization.

After DSP, the EVM of each received OFDM-UWB

band is evaluated and compared with the EVM limit

OFDM-UWB

generation

Frequency

multiplexing

AWG

LPF

VEA

75 km

SSMF

Remote Node

Noise Loading

Optical Line Termination

Feeder Fibre

SSMF: 0-60 km

Distribution Fibre

DSO

EVM

calculation

LPF

OFDM

demodulator

Optical Network Unit

AWG: arbitrary waveform generator LPF: low-pass filter

DCM: dispersion compensation module OF: optical filter

DML: directly modulated laser OFDM: orthogonal frequency-division multiplexing

DSO: digital storage oscilloscope OSA: optical spectrum analyzer

EA: electrical amplifier PIN: positive-intrinsic-negative

EDFA: erbium doped fiber amplifier SSMF: standard single-mode fibre

EDL: electrical delay line UWB: ultra wideband

EM: external modulator VEA: variable electrical attenuator

EVM: error vector magnitude VOA: variable optical attenuator

OSA

VOA

DML

DCM

EDFA

LPF

VEA

EDL EDL

Laser

OPTICS2013-InternationalConferenceonOpticalCommunicationSystems

414

Figure 2: (a), (b) and (c) PSD of the signal after the DCM inserted at the RN for different modulation indexes. (d), (e) and

(f) EVM of UWB band #1, #2 and #3 as a function of the residual dispersion of the link and for different modulation

indexes. In (d), (e) and (f), the LR-PON reach is 100 km and m=10% (circles), m=12% (squares), m=13% (diamonds),

m=15% (triangles), m=17% (stars).

(-14.5 dB for QPSK) of UWB standard (ECMA Int.,

2007). The EVM limit of UWB standard is defined

at the output of the wireless transmitter, i. e., before

wireless radiation. Therefore, it can be used as

performance threshold of the UWB signals at the

output of optical fibre link.

3 EXPERIMENTAL RESULTS

AND DISCUSSION

3.1 Single-wavelength Operation

In this section, the EVM of each of the three OFDM-

UWB bands simultaneously distributed along the

LR-PON infrastructure described in section 2 is

evaluated experimentally and discussed.

Initially, the optimization of the modulation index of

the OFDM-UWB signal applied to the DML (in

order to optimize the level of nonlinear distortion

induced by the combined effect of the DML and the

PIN) and of the optical dispersion compensation

level of the DCM (to minimize the degradation due

to the joint effect of DML’s chirp and fibre

dispersion) located at the RN is performed. The

optimization outcomes depend on the reach of the

LR-PON considered due to the interaction between

the accumulated dispersion of the link and the chirp

introduced by the DML. Thus, we decided to

perform the optimization for a RN-ONU distance of

25 km (total LR-PON reach of 100 km). The system

optimization is accomplished for this reach as it

represents a compromise between two LR-PON

reaches. First, we want to ensure the distribution of

UWB signals along a minimum LR-PON reach (the

feeder fibre length, 75 km) with acceptable

performance. Second, we want to extend the

maximum LR-PON reach for a distance longer than

100 km while keeping the reception of UWB signals

with acceptable conditions.

The modulation index is defined as





MS b th L

mV I I R









, where

RMS

V is the

root mean square (RMS) voltage of the OFDM-

UWB signal applied to the DML and

is the load

resistance (





Figures 2 (a), (b) and (c) depict the PSD of the

optical signal at the RN for different modulation

indexes levels. Figures 2 (a), (b) and (c) show that

the modulation index increase leads not only to the

power increase of the UWB bands but also to the

power increase of the distortion components

generated due to the nonlinear characteristic of the

DML.

Figures 2 (d), (e) and (f) show the EVM of each

OFDM-UWB band as a function of the residual

dispersion (defined as the difference between the

accumulated dispersion of the feeder and

distribution fibres, and the dispersion compensated

by the DCM) of the link. The EVM results are

presented for different modulation indexes. Figures

2 (d), (e) and (f) show that the behaviour of the

EVM when the residual dispersion varies depends

on the UWB band under analysis. Particularly, the

inspection of Figures 2 (d), (e) and (f) show that the

tolerance of the EVM to negative residual dispersion

levels decreases when the number of the UWB band

increases, i. e., when the central frequency of the

UWB band increases. This effect occurs because the

1552.81552.851552.91552.951552.95

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EVM [dB]

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-8

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ersion

s/nm

]

EVM [dB]

UWB #1

UWB #2

UWB #3

m=10%

m=13%

m=17%

(a)

(b)

(c)

(d)

(e)

(f)

Legend:

m=10%(circles)

m=12%(squares)

m=13%(diamonds)

m=15%(triangles)

m=17%(stars)

ExperimentalDistributionofOFDM-UWBRadioSignalsalongDirectlyModulatedLong-reachPonsIndicatedforSparse

GeographicalAreas

415

intensity response of the link is remarkably affected

by the residual dispersion. Further investigation

showed that, if the residual dispersion of the link

changes from -300 ps/nm (DCM adjusted to

compensate for 2000 ps/nm of dispersion) to -

600 ps/nm (DCM adjusted to compensate

2300 ps/nm), an amplitude gain close to 9 dB occurs

at the frequency of 3.4 GHz (central frequency of

UWB band #1). However, this gain is reduced to 5

dB at the frequency of 4.4 GHz (central frequency of

UWB band #3).

On the other hand, Figures 2 (d), (e) and (f) show

also that the performance of the UWB bands is

remarkably dependent on the modulation index. For

low modulation indexes, the EVM improves when

the modulation index increases due to the

improvement of the signal-to-noise ratio. However,

for high modulation indexes, this EVM

improvement is counterbalanced by the degradation

induced by the nonlinear distortion caused by the

DML nonlinear characteristic and the PIN square

law detection.

Figure 3: EVM of UWB band #1, #2 and #3 as a function

of the LR-PON reach. The DCM is compensating for 2200

ps/nm and the modulation index is 13%.

In the following, the analysis of the EVM of the

three UWB bands when the LR-PON reach varies

between 75 km and 135 km is performed. This study

is accomplished with a fixed dispersion

compensation level in the DCM. From the results of

Figures 2 (d), (e) and (f), the DCM is adjusted to

compensate 2200 ps/nm of dispersion. In a 100 km-

long LR-PON, this compensation level leads to a

residual dispersion of -500 ps/nm. The modulation

index of the multiplexed UWB signal applied to the

DML is 13%. The fixed dispersion compensation

level of the DCM and the modulation index value

chosen correspond to the optimum system operating

point shown in Figures 2 (d), (e) and (f).

Figure 3 shows the EVM of UWB band #1, #2

and #3 as a function of the LR-PON reach. LR-PON

reaches between 75 km and 135 km are considered,

which correspond to distribution fibre distances

between 0 km and 60 km. Figure 3 shows that the

EVM of the three UWB bands meets the EVM

requirements of UWB standard (-14.5 dB) for LR-

PON reaches between 75 km and 130 km.

Moreover, Figure 3 shows also that, for almost all

the LR-PON reaches analysed, UWB band #3 is the

UWB band with worst performance. This is

attributed to the limited frequency response of the

electrical devices used in the experiments and also

due to the reduced gain of the intensity response of

the optical link observed for higher frequencies.

3.2 Multi-wavelength Operation

Our investigation of the single-channel LR-PON

was also extended to a wavelength division

multiplexing (WDM) LR-PON. This extension

allows assessing the impact of the inter-channel

crosstalk induced by adjacent optical channels on a

WDM LR-PON. To perform this activity, two

optical channels adjacent to the one used in the

single-channel system were introduced into the

system, as shown in Figure 1. However, these two

additional optical channels were deployed using

external modulation instead of direct modulation due

to the lack of DMLs in our laboratory with adequate

features to generate a dense WDM signal. Figure 4

shows the PSD of the WDM signal after the DCM

inserted at the RN. Figure 4 shows that the spectra of

the optical signal generated by the directly or

externally modulated solutions are similar and,

consequently, crosstalk features similar to the ones

obtained if only directly modulated channels were

employed are expected.

The two adjacent channels are generated through

a distributed feedback laser (DFB) and an external

cavity laser (ECL), and using 10 GHz Mach Zehnder

modulators. The nominal wavelengths of the

adjacent optical channels are set to 1552.65 nm and

75 85 95 105 115 125 135

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EVM [dB]

75 85 95 105 115 125 135

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-16

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Link length [km]

EVM [dB]

75 85 95 105 115 125 135

-18

-16

-14

-12

Link length [km]

EVM [dB]

UWB #1

UWB #2

UWB #3

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1553.05 nm and, therefore, the channel spacing is

0.2 nm. The average optical power of the WDM

signal launched into the feeder and distribution

fibres is kept below 0 dBm to avoid degradation due

to the nonlinear fibre transmission effects. The

impact of the inter-channel linear crosstalk was

performed for LR-PONs with reach of 75 km, 100

km and 130 km. As our focus is in directly

modulated LR-PONs, we only evaluated the

performance of the UWB bands transmitted in

central optical channel.

Figure 4: PSD of the WDM signal after the DCM inserted

at the RN.

Table 1 presents the EVM results of the central

channel of the WDM signal. Table 2 presents the

EVM results of the UWB bands in single-channel

operation, corresponding to the results of Figures

2(d), (e) and (f), as a reference. The comparison

between the EVM results of Table 1 and Table 2

shows an EVM variation lower than 0.3 dB between

single and WDM operation. This variation is mainly

due to the fluctuation of the EVM measurements

resulting from e. g. the noise introduced along the

system. Those reduced EVM variations indicate that,

for the channel spacing and the optical power levels

considered in this work, the inter-channel linear and

nonlinear crosstalks are not a relevant concern.

Table 1: EVM of the UWB signals transmitted by the

central channel of a WDM signal comprising three optical

channels.

EVM [dB] UWB #1 UWB #2 UWB #3

75 km -14.8 -15.0 -15.1

100 km -15.1 -15.4 -15.3

130 km -15.6 -15.6 -15.1

Table 2: EVM of the UWB signals transmitted in single-

channel operation.

EVM [dB] UWB #1 UWB #2 UWB #3

75 km -14.5 -15.1 -15.0

100 km -15.0 -15.5 -15.3

130 km -15.6 -15.8 -15.2

4 CONCLUSIONS

The distribution of OFDM-UWB radio signals along

LR-PONs employing directly modulated lasers has

been demonstrated experimentally as an effective

solution to serve users’ premises located at 130 km

away from the central office. Directly modulated

LR-PONs has been also demonstrated as an effective

solution to be deployed in sparse geographical areas,

as EVM levels compliant with UWB standard are

achieved for LR-PONs with reach between 75 km

and 130 km, i. e., for a maximum distribution fibre

reach of 55 km. In addition, the performance of the

directly-modulated optical channel has shown an

EVM variation lower than 0.3 dB when two adjacent

channels (with a spacing of 0.2 nm) were introduced

to assess the impact of the WDM operation.

This successful demonstration has been achieved

by using fixed optical inline dispersion

compensation at the RN and through adequate

selection of the level of the UWB signal applied to

the DML.

ACKNOWLEDGEMENTS

This work was supported by Fundação para a

Ciência e a Tecnologia from Portugal under the

TURBO-PTDC/EEA-TEL/104358/2008 project.

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