**2. UWB over perfluorinated graded index polymer optical fiber for low-cost in-building networks**

The use of multimode fibers (MMF) in RoF short span networks has attracted much attention in recent years. Nowadays, the majority of building networks are based on MMF. In fact, in-building networks employing MMF topologies for high speed short-range (10 Gb/s; < 300 m) represent about 90% of all in-building networks [7]. Furthermore, it is predicted that the fastest growing part of the optical communications market will be targeting legacy MMF for installed lengths up to 300 meters.

Polymer optical fiber (POF) is an emerging medium for very short reach links. The popularity of polymer optical fiber is due to the advantages brought by its large core diameter and mechanical properties. These include connectorization simplicity due to the large numerical aperture, high tolerance to both misalignments and vibrations, low bending loss that eases installation and simple and low maintenance costs due to its robustness.

2 Will-be-set-by-IN-TECH

(GSM) [3], WiMAX [4] and ultra-wide band (UWB) [5, 6]. Moreover, RoF systems also offer other attractive advantages such as low weight and immunity to electromagnetic interference. However, an optically modulated mm-wave signal can also suffer from several impairments namely nonlinear distortion, power penalty from the electric/optic/electric (E/O/E) conversion process, chromatic dispersion, attenuation from the optical fiber and

Here we study the transmission performance of UWB over two distinctive optical networks. In Section 2, the packet error rate performnace in a low-cost multi-mode fiber (MMF) network composed by a 850 nm vertical-cavity surface emitting LASER (VCSEL) and a PIN photodiode connected by two different polymer optical fibers (POF) is assessed. Then, in Section 3, an analytical and experimental performance evaluation is carried out in a single-mode fiber (SMF) network composed by a reflective electro-absorption modulator (R-EAM) and a PIN

The use of multimode fibers (MMF) in RoF short span networks has attracted much attention in recent years. Nowadays, the majority of building networks are based on MMF. In fact, in-building networks employing MMF topologies for high speed short-range (10 Gb/s; < 300 m) represent about 90% of all in-building networks [7]. Furthermore, it is predicted that the fastest growing part of the optical communications market will be targeting legacy MMF

**2. UWB over perfluorinated graded index polymer optical fiber for**

*BS*

*Control Station*

*Optical Fiber Network*

*BS*

*BS BS*

**Figure 1.** In-building radio-over-fiber concept.

phase noise from LASER sources.

**low-cost in-building networks**

for installed lengths up to 300 meters.

*BS*

photodiode.

Common polymer optical fibers are based on polymethyl methacrylate (PMMA-POF). These fibers exhibit low bandwidth, multimodal dispersion and high attenuation (200 dB/km) hence are not suitable for today's high data rates or RoF systems where signals usually exhibit high bandwidths and high RF frequency carriers. Due to their relatively low bandwidth, a down-conversion of the RF signal to an intermediate frequency would be necessary, which introduces additional complexity and raises the cost of the BSs. Newer perfluorinated graded index polymer optical fiber (PF-GI-POF) from companies such as Sekisui Chemical, Chromis Fiber or Asahi Glass, solve this issue by combining a low attenuation material (about 50 dB/km @ 850 nm) with a graded index profile in their fiber construction. Bandwidth is relatively high for graded-index multimode fibers. Current PF-GI-POFs have bandwidth length products of around 1 GHz·Km, and attenuations as low as 10 dB/Km at 1310 nm [8]. In practical terms, for short links (< 100 m), it is limited by the response of directly modulated laser devices [9].

Comparing to common silica multimode fiber (SI-MMF) with respect to transmission capacity, PF-GI-POF has the potential of high bandwidth and a lower modal dispersion. Moreover, it offers lower material dispersion and higher bandwidth than standard MMF with 40 Gb/s data transmission capability for 100 m links [9].

The attenuation is not an issue for short silica-based fiber link lengths. But in the case of POF the attenuation can be as high as 20 dB for the PMMA-POF or about 5 dB for the state of the art PF-GI-POF for a 100 m length link. Large-core glass fiber shows lower attenuation than POF, however their core size is restricted to 200 *μ*m due to the inherent inflexibility of glass. In this situation, POF again has advantages concerning easy handling and termination, tolerance to misalignments and high mechanical strength [10]. Furthermore, the typical large core of polymer fiber allows for large tolerance on misalignments that results in the possibility of using cheaper connectors. For comparison, consider the case of the power loss due to lateral (axial) misalignment of connecting two graded index (parabolic case) MMF with different core diameters. Comparing the power loss, assuming uniformly modal power distributions, for a misalignment of 25 *μ*m, yields a loss of 1.76 dB for a 62.5 *μ*m core diameter MMF whereas for the case of POF with a core diameter of 200 *μ*m, the same 25 *μ*m displacement results only in 0.48 dB loss [11]. New PF-GI-POF fibers being developed are able to withstand large temperature variation (−65 C to 125 C) and so may be suitable for applications in harsh critical environments. Their ease of installation, and tolerance to misalignment, vibration and large temperature variation operation makes these fibers suitable for short-range applications in the home environment or in critical applications such as the car and the avionics industry.

Here, we experimentally demonstrate the uplink of a MB-OFDM UWB signal (ECMA-368 standard [12]) over two different PF-GI-POFs from Chromis Fiberoptics using commercial UWB transceivers and cheap commercial off-the-shelf (COTS) components, namely an optical transceiver composed by a low-cost 850 nm VCSEL and a PIN photodiode.

#### **2.1. A low-cost directly modulated RoF system**

VCSELs are characterized by a vertical low divergence, circular beam patterns, low threshold currents (a few mA) and high bandwidths (several GHz). Their vertical wafer growth process enables in-wafer testing, and is well suited for large scale production. Their output light beam pattern enables efficient coupling to large diameter polymer optical fibers. Hence, simple plastic injection molded packages are sufficient as fiber coupling devices. These are reasons that make VCSELs desirable for low cost directly modulated systems in these types of widespread commercial applications.

conversion in the photodiode meaning that reduction on the transmitted optical power has a

Performance Assessment of UWB-Over-Fiber and Applications 341

20 40 60 80 100

Cable length (m)

Figure 3 shows the RF power at the CS as function of the cable length. For this illustrative result, we consider an UWB signal transmission over the setup given by Figure 2 with the following specifications: *PRFMS* = −19.7 dBm, a wireless link of 1 meter (*L* ≈ 40 dB), *RGM* = 0.88, *GTX* = 40 dB and *Zout* ≈ *Zin* = 50Ω. We compare this result with the loss of common silica multimode fiber (SI-MMF) and RF electrical cables (RG-58 and RG-174 considering the same operating band and the same RF gain as applied to the E/O/E converter). Although there is a penalty by using the E/O/E conversion and optical transport, this is nonetheless very small when compared to the loss suffered by the signal when transported with electrical cable at distances of several tens of meters. It can be seen that the electrical to optical and optical to electrical conversion (whose efficiency is given by *RGM*) jointly with the attenuation of POFs is the dominant factor reducing the link power budget of these systems. The relatively high POF attenuation can be partially overcome by post detection amplification, at the expense of some SNR degradation due to amplifier noise. In our experiments, an extra

MB-OFDM UWB radio applications make extensive use of multiple subcarriers and, hence require large dynamic range and highly linear devices. The signal transmission is mainly impaired by the laser nonlinearity, the optical loss due to the fiber, the free space loss and

Figure 4 depicts the RoF conversion board based on low-cost electrical and optical components. An amplifier cascade and a polarizing circuit makes up the laser driver for the E/O conversion circuit and a photodiode with an integrated transimpedance amplifier was

Optical (PF-GI-POF) Optical (SI-MMF)

LNA was not included because the UWB DVK provides enough sensitivity.

RG-58 RG-174

significant impact on the link budget.

−160

**Figure 3.** RF power at the CS as a function of the cable length.

−140 −120

−100

PRF,CS (dBm)

noise added by the system.

used for the O/E conversion circuit.

**2.2. Experimental demonstration of concept**

−80 −60

−40

**Figure 2.** Schematic illustrating the RoF setup used.

A schematic of the MB-OFDM UWB over GI-POF system used is shown in Figure 2, representing a RoF communication uplink between a Mobile Station (MS) and a Control Station (CS), via a Base Station (BS). In order to generate MB-OFDM UWB signals compliant with the ECMA-368 standard, a commercially available UWB transceiver module from WisAir (DVK9110M) was used. The proposed system is based on today's commercially available low cost VCSELs and photodiodes that are not optimized for radio-over-fiber applications.

At the BS, a power amplifier (PA) amplifies the driver signal. The optical signal power (*POPT* in dBm) at the laser output is given by [13, 14]

$$P\_{OPT} = \frac{G\_{TX} + P\_{RF,BS}}{2} + 10\log\_{10}\left(G\_M \sqrt{\frac{1000}{Z\_{\rm in}}}\right) \tag{1}$$

where *GTX* is the PA gain in dB, *PRF*,*BS* = *PRF*,*MS* − *L* is the received electrical power (in dBm) at the BS, *PRF*,*MS* is the transmitter electrical power at the MS, *L* is the wireless link loss, *GM* is the VCSEL modulation gain (or slope sfficiency) in mW/mA and *Zin* (∼50 Ω) is the laser input impedance assumed constant within the band of interest.

The received electrical power at the CS, *PRF*,*CS*, is given by

$$P\_{\rm RF,CS} = 20\log\_{10}{(\rm RG\_M)} + 10\log\_{10}{\left(\frac{Z\_{\rm out}}{Z\_{\rm in}}\right)} - 2OL + G\_{TX} + P\_{\rm RF,MS} - L \tag{2}$$

where *R* is the photodiode responsivity in mA/mW, *Zout* (∼50 Ω) is the photodiode output impedance (also assumed constant within the band of interest) and *OL* is the optical power loss due to both the fiber attenuation and connector loss. Note the factor of 2 multiplying the optical power term which results from the quadratic optical power to electrical power conversion in the photodiode meaning that reduction on the transmitted optical power has a significant impact on the link budget.

**Figure 3.** RF power at the CS as a function of the cable length.

4 Will-be-set-by-IN-TECH

VCSELs are characterized by a vertical low divergence, circular beam patterns, low threshold currents (a few mA) and high bandwidths (several GHz). Their vertical wafer growth process enables in-wafer testing, and is well suited for large scale production. Their output light beam pattern enables efficient coupling to large diameter polymer optical fibers. Hence, simple plastic injection molded packages are sufficient as fiber coupling devices. These are reasons that make VCSELs desirable for low cost directly modulated systems in these types

A schematic of the MB-OFDM UWB over GI-POF system used is shown in Figure 2, representing a RoF communication uplink between a Mobile Station (MS) and a Control Station (CS), via a Base Station (BS). In order to generate MB-OFDM UWB signals compliant with the ECMA-368 standard, a commercially available UWB transceiver module from WisAir (DVK9110M) was used. The proposed system is based on today's commercially available low cost VCSELs and photodiodes that are not optimized for radio-over-fiber applications.

At the BS, a power amplifier (PA) amplifies the driver signal. The optical signal power (*POPT*

<sup>2</sup> <sup>+</sup> 10 log10

where *GTX* is the PA gain in dB, *PRF*,*BS* = *PRF*,*MS* − *L* is the received electrical power (in dBm) at the BS, *PRF*,*MS* is the transmitter electrical power at the MS, *L* is the wireless link loss, *GM* is the VCSEL modulation gain (or slope sfficiency) in mW/mA and *Zin* (∼50 Ω) is the laser

> *Zout Zin*

where *R* is the photodiode responsivity in mA/mW, *Zout* (∼50 Ω) is the photodiode output impedance (also assumed constant within the band of interest) and *OL* is the optical power loss due to both the fiber attenuation and connector loss. Note the factor of 2 multiplying the optical power term which results from the quadratic optical power to electrical power

40 dB Gain

BS

PD 850nm VCSEL

 *GM* 1000 *Zin*

− 2*OL* + *GTX* + *PRF*,*MS* − *L* (2)

(1)

PF-GI-POF

**2.1. A low-cost directly modulated RoF system**

of widespread commercial applications.

 PC1 MASTER

MS

 UWB DVK9110

 UWB DVK9110

*POPT* <sup>=</sup> *GTX* <sup>+</sup> *PRF*,*BS*

input impedance assumed constant within the band of interest.

The received electrical power at the CS, *PRF*,*CS*, is given by

*PRF*,*CS* = 20 log10 (*RGM*) + 10 log10

 PC2 SLAVE

**Figure 2.** Schematic illustrating the RoF setup used.

in dBm) at the laser output is given by [13, 14]

CS

Figure 3 shows the RF power at the CS as function of the cable length. For this illustrative result, we consider an UWB signal transmission over the setup given by Figure 2 with the following specifications: *PRFMS* = −19.7 dBm, a wireless link of 1 meter (*L* ≈ 40 dB), *RGM* = 0.88, *GTX* = 40 dB and *Zout* ≈ *Zin* = 50Ω. We compare this result with the loss of common silica multimode fiber (SI-MMF) and RF electrical cables (RG-58 and RG-174 considering the same operating band and the same RF gain as applied to the E/O/E converter). Although there is a penalty by using the E/O/E conversion and optical transport, this is nonetheless very small when compared to the loss suffered by the signal when transported with electrical cable at distances of several tens of meters. It can be seen that the electrical to optical and optical to electrical conversion (whose efficiency is given by *RGM*) jointly with the attenuation of POFs is the dominant factor reducing the link power budget of these systems. The relatively high POF attenuation can be partially overcome by post detection amplification, at the expense of some SNR degradation due to amplifier noise. In our experiments, an extra LNA was not included because the UWB DVK provides enough sensitivity.

MB-OFDM UWB radio applications make extensive use of multiple subcarriers and, hence require large dynamic range and highly linear devices. The signal transmission is mainly impaired by the laser nonlinearity, the optical loss due to the fiber, the free space loss and noise added by the system.

#### **2.2. Experimental demonstration of concept**

Figure 4 depicts the RoF conversion board based on low-cost electrical and optical components. An amplifier cascade and a polarizing circuit makes up the laser driver for the E/O conversion circuit and a photodiode with an integrated transimpedance amplifier was used for the O/E conversion circuit.

(a) Top view. (b) Bottom view.

The total budget of this system prototype does not exceed e50. Large scale production of such a system would undoubtedly have an even lower price, meeting well in the requirements of a

The UWB kit operates is the band group 1 (from 3.168 GHz to 4.752 GHz) and the maximum equivalent isotropic radiated power (EIRP) is −41.3 dBm/MHz using antennas with approximately 4 dBi of gain. This band group consists of three sub-bands, each occupying a bandwidth of 528 MHz and containing 128 subcarriers. Consequently, the power transmitted by the UWB DVK is approximately −19 dBm, disregarding the antenna gain. Although three subbands are available for transmission, the optical transceiver design limitations (commercial VCSEL, photodiode and amplifiers available at the moment) prevented using the entire available bandwidth. Therefore, the time-frequency code was set

Previous experimental demonstrations have shown that a 1 meter wireless link produces similar results to the ones obtained using a 40 dB attenuator, which corresponds to the free-space air attenuation over 1 meter distance, approximately. Therefore, and for simplicity sake, the effect of the wireless link was simulated by the attenuator. In this experimental demonstration we have used two different PF-GI-POFs from Chromis Fiberoptics, namely, the GigaPOF-62LD and the GigaPOF-120LD, with core diameters of 62.5 *μ*m, and 120 *μ*m,

The experimental validation was carried out by transmitting data at bit rates of 53.3 Mbps,

3.2 3.3 3.4 3.5 3.6 3.7 −90

**Figure 6.** Spectrum of the MB-OFDM UWB signal (a) transmitted by the UWB kit and received after a 1

We also used a 64 octet packet length for PER measurements. Figure 6 shows the MB-OFDM UWB signal spectra obtained before the wireless link and after the GigaPOF-120LD. The

meter wireless link and (b) 50 meters and (c) 100 meters of GigaPOF-120LD (RBW = 1 MHz).

Frequency (GHz)

(b)

(a)

Performance Assessment of UWB-Over-Fiber and Applications 343

(c)

widespread commercial application for home or office use.

to TFC 5 (3.168 GHz to 3.696 GHz).

**2.3. Experimental results and discussion**

−80

−70

−60

Power (dBm) −50

−40

−30

respectively.

200 Mbps and 480 Mbps.

**Figure 4.** RoF conversion board used as E/O/E transceiver.

The amplifiers were chosen among devices with both high IP3 (3rd Order Intercept) and low noise figure. High IP3 is essential for guaranteeing the integrity of multiple carrier ultra wideband signals. The VCSEL (HFD3180-203) and photodiode (HFE4192-581) operate at 850 nm, and have a combined 3 dB bandwidth of about 5 GHz. The laser modulation efficiency, and photodiode (PD) responsivity are 0.07 mW/mA and 12.5 mA/mW, including transimpedance amplifier gain, respectively. It was also experimentally verified that the receiver noise is much larger than the laser RIN, even with short POF lengths.

**Figure 5.** VCSEL transfer function vs. DC bias current.

The VCSEL bias current was also judiciously adjusted. It was found that the current that provides the maximum bandwidth (6 − 7.5 mA, see Figure 5) is not the one corresponding to the optimum operation point. Instead, the bias current was set to the lower level of 4.5 mA, which increases the output optical modulation index (the input RF intensity is fixed), without significantly compromising the bandwidth, and without significantly increasing the laser nonlinear dynamic distortion.

The total budget of this system prototype does not exceed e50. Large scale production of such a system would undoubtedly have an even lower price, meeting well in the requirements of a widespread commercial application for home or office use.

The UWB kit operates is the band group 1 (from 3.168 GHz to 4.752 GHz) and the maximum equivalent isotropic radiated power (EIRP) is −41.3 dBm/MHz using antennas with approximately 4 dBi of gain. This band group consists of three sub-bands, each occupying a bandwidth of 528 MHz and containing 128 subcarriers. Consequently, the power transmitted by the UWB DVK is approximately −19 dBm, disregarding the antenna gain. Although three subbands are available for transmission, the optical transceiver design limitations (commercial VCSEL, photodiode and amplifiers available at the moment) prevented using the entire available bandwidth. Therefore, the time-frequency code was set to TFC 5 (3.168 GHz to 3.696 GHz).

Previous experimental demonstrations have shown that a 1 meter wireless link produces similar results to the ones obtained using a 40 dB attenuator, which corresponds to the free-space air attenuation over 1 meter distance, approximately. Therefore, and for simplicity sake, the effect of the wireless link was simulated by the attenuator. In this experimental demonstration we have used two different PF-GI-POFs from Chromis Fiberoptics, namely, the GigaPOF-62LD and the GigaPOF-120LD, with core diameters of 62.5 *μ*m, and 120 *μ*m, respectively.

#### **2.3. Experimental results and discussion**

6 Will-be-set-by-IN-TECH

(a) Top view. (b) Bottom view.

The amplifiers were chosen among devices with both high IP3 (3rd Order Intercept) and low noise figure. High IP3 is essential for guaranteeing the integrity of multiple carrier ultra wideband signals. The VCSEL (HFD3180-203) and photodiode (HFE4192-581) operate at 850 nm, and have a combined 3 dB bandwidth of about 5 GHz. The laser modulation efficiency, and photodiode (PD) responsivity are 0.07 mW/mA and 12.5 mA/mW, including transimpedance amplifier gain, respectively. It was also experimentally verified that the

1 2 3 4 5 6 7 8 9 10

Frequency (GHz)

The VCSEL bias current was also judiciously adjusted. It was found that the current that provides the maximum bandwidth (6 − 7.5 mA, see Figure 5) is not the one corresponding to the optimum operation point. Instead, the bias current was set to the lower level of 4.5 mA, which increases the output optical modulation index (the input RF intensity is fixed), without significantly compromising the bandwidth, and without significantly increasing the

receiver noise is much larger than the laser RIN, even with short POF lengths.

IBias = 3.0 mA IBias = 4.5 mA IBias = 6.0 mA IBias = 7.5 mA

**Figure 4.** RoF conversion board used as E/O/E transceiver.

−30

**Figure 5.** VCSEL transfer function vs. DC bias current.

laser nonlinear dynamic distortion.

−25

−20

−15

−10

Amplitude (dB)

−5

0

5

The experimental validation was carried out by transmitting data at bit rates of 53.3 Mbps, 200 Mbps and 480 Mbps.

**Figure 6.** Spectrum of the MB-OFDM UWB signal (a) transmitted by the UWB kit and received after a 1 meter wireless link and (b) 50 meters and (c) 100 meters of GigaPOF-120LD (RBW = 1 MHz).

We also used a 64 octet packet length for PER measurements. Figure 6 shows the MB-OFDM UWB signal spectra obtained before the wireless link and after the GigaPOF-120LD. The attenuation of these links obtained from equation (2) are 20.6 dB and 25.6 dB, which agrees with the measured values of 21 dB and 26 dB from Figure 6. In addition to the wireless/optical link attenuation, it can be seen that subcarriers suffer slightly different attenuations mainly due to the photodiode and amplifier frequency response. The UWB signal spectrum shows no distortion for the tested fiber lengths, which indicates that the bandwidth-distance product is not the factor limiting the transmission on the fiber.

<sup>0</sup> <sup>50</sup> <sup>100</sup> <sup>150</sup> <sup>200</sup> −25

GigaPOF-62LD GigaPOF-120LD

**Figure 8.** Required signal transmit power after 1 meter wireless link to achieve a PER of 0.125 % as a

POF length (m)

0

(b) 200 Mbps.

**Figure 9.** Required signal transmit power (relative to max.) to achieve a PER of 0.125% as a function of

that the slope of the plots depicted in Figure 8 correspond to the fiber attenuation, which is approximately 50 dB/Km in both cases, indicating that fiber attenuation (and not fiber

Results also show that it is possible to transmit 480 Mbps up to 100 meters of POF when preceded by a 1 meter wireless link as well as 200 Mbps and 53.3 Mbps over 150 meters and 200 meters, respectively. The slight difference found in the back-to-back configuration shows that the large core diameter fiber has a better light coupling efficiency. This difference persists

It was not possible to obtain results for fiber lengths longer than 100 meters, due to the unavailability of suitable GigaPOF-62LD cables at our lab. Nonetheless, by looking at the

POF length (m)

GigaPOF-120LD length and wireless link length for three different transmission rates.

bandwidth) is the dominant transmission penalty (see Figure 6).

2

Wireless link length (m)

4

1

0

(c) 480Mbps.

50

POF length (m)

100 −15 −10 −5 0 5 10

2

Wireless link length (m)

480 Mbps 200 Mbps 53.3 Mbps

Performance Assessment of UWB-Over-Fiber and Applications 345

−20

function of POF length for both GigaPOF-62LD and GigaPOF-120LD.

Required Tx Power (relative to max.) for

0

0

(a) 53.3 Mbps.

100

POF length (m)

in all the POF lengths.

200 −30 −20 −10 0 10 20

Required Tx Power (relative to max.) for PER = 0.125% (dB)

2

Wireless link length (m)

4

−15

−10

−5

PER=0.125%

0

**Figure 7.** PER vs. transmitted power after a 1 meter wireless link for transmission rates of 53.3, 200 and 480 Mbps for both GigaPOF-62LD and GigaPOF-120LD.

Figure 7 depicts the experimental results of PER as a function of transmitted power (relative to the maximum allowed value as defined by the ECMA-368 standard [12]) for different POF lengths preceded by a 1 meter wireless link and considering transmission rates of 53.3 Mbps, 200 Mbps and 480 Mbps. As expected, results show that the PER increases when the POF length is increased. The horizontal dashed line corresponds to a PER of 0.125 % which is the maximum PER allowed for a 64 octet frame body (ECMA-368 [12]).

Figure 8 shows the minimum required signal power (relative to the maximum) corresponding to the maximum allowed PER, as a function of POF length, where a 1 meter wireless channel is included. As expected, the PER increases for larger data rates, and the required power for achieving a valid transmission also increases.

A linearly increasing transmitted signal power (in dB) is necessary to compensate for the linearly increasing POF loss with distance, keeping both the receiver SNR and the PER constant, as indicated in Figure 8. This shows that the overall noise level is constant at the output of the receiver board, and that intermodulation products are sufficiently below the receiver noise level (for the chosen link parameters). A further interesting result is

8 Will-be-set-by-IN-TECH

attenuation of these links obtained from equation (2) are 20.6 dB and 25.6 dB, which agrees with the measured values of 21 dB and 26 dB from Figure 6. In addition to the wireless/optical link attenuation, it can be seen that subcarriers suffer slightly different attenuations mainly due to the photodiode and amplifier frequency response. The UWB signal spectrum shows no distortion for the tested fiber lengths, which indicates that the bandwidth-distance product

0 −5 −10 −15 −20 −25

0 −5 −10 −15 −20 −25

B2B 10 m 50 m 100 m 150 m 200 m

PER limit

GigaPOF-62LD GigaPOF-120LD

Tx Power relative to max (dB)

(c) 480Mbps.

10−4

10−3

10−2

10−1

100

Tx Power relative to max (dB)

(b) 200 Mbps.

**Figure 7.** PER vs. transmitted power after a 1 meter wireless link for transmission rates of 53.3, 200 and

Figure 7 depicts the experimental results of PER as a function of transmitted power (relative to the maximum allowed value as defined by the ECMA-368 standard [12]) for different POF lengths preceded by a 1 meter wireless link and considering transmission rates of 53.3 Mbps, 200 Mbps and 480 Mbps. As expected, results show that the PER increases when the POF length is increased. The horizontal dashed line corresponds to a PER of 0.125 % which is the

Figure 8 shows the minimum required signal power (relative to the maximum) corresponding to the maximum allowed PER, as a function of POF length, where a 1 meter wireless channel is included. As expected, the PER increases for larger data rates, and the required power for

A linearly increasing transmitted signal power (in dB) is necessary to compensate for the linearly increasing POF loss with distance, keeping both the receiver SNR and the PER constant, as indicated in Figure 8. This shows that the overall noise level is constant at the output of the receiver board, and that intermodulation products are sufficiently below the receiver noise level (for the chosen link parameters). A further interesting result is

10−4

maximum PER allowed for a 64 octet frame body (ECMA-368 [12]).

10−3

10−2

10−1

100

is not the factor limiting the transmission on the fiber.

0 −5 −10 −15 −20 −25

Tx Power relative to max (dB)

(a) 53.3 Mbps.

480 Mbps for both GigaPOF-62LD and GigaPOF-120LD.

achieving a valid transmission also increases.

10−4

10−3

PER

10−2

10−1

100

**Figure 8.** Required signal transmit power after 1 meter wireless link to achieve a PER of 0.125 % as a function of POF length for both GigaPOF-62LD and GigaPOF-120LD.

**Figure 9.** Required signal transmit power (relative to max.) to achieve a PER of 0.125% as a function of GigaPOF-120LD length and wireless link length for three different transmission rates.

that the slope of the plots depicted in Figure 8 correspond to the fiber attenuation, which is approximately 50 dB/Km in both cases, indicating that fiber attenuation (and not fiber bandwidth) is the dominant transmission penalty (see Figure 6).

Results also show that it is possible to transmit 480 Mbps up to 100 meters of POF when preceded by a 1 meter wireless link as well as 200 Mbps and 53.3 Mbps over 150 meters and 200 meters, respectively. The slight difference found in the back-to-back configuration shows that the large core diameter fiber has a better light coupling efficiency. This difference persists in all the POF lengths.

It was not possible to obtain results for fiber lengths longer than 100 meters, due to the unavailability of suitable GigaPOF-62LD cables at our lab. Nonetheless, by looking at the

with data rates of 200Mbps and 53.3 Mbps. It is also demonstrated that the PG-GI-POF attenuation, and not its bandwidth, is the dominant factor limiting the fiber link length.

Performance Assessment of UWB-Over-Fiber and Applications 347

As stated earlier, phase noise from optical sources is one of the factors impairing RoF system performance. Thus, very stable narrow linewidth optical sources are mandatory but also very expensive [15]. For the downlink signal transmission, an ultra-stable and common optical source can be used since it is located at the CS. However, in the uplink, it is not attractive in terms of complexity, size, power consumption and cost to have an optical source for each BSs. Furthermore, by eliminating the need of an optical source, BSs can be colorless and the

Two main BS schemes are usually used in colorless RoF systems. The first scheme is based on an external modulator, photoreceiver and utilizes optical filtering techniques and wavelength reuse, or a more convenient method in which the optical carrier is remotely provided from the CS [16–18]. Another scheme of source free BSs is based on a single electro-absorption waveguide device in which a single component acts both as a modulator for the uplink and as photoreceiver for the downlink [19, 20]. Therefore, this transceiver device is a very attractive

Although an electro-absorption transceiver (EAT) based RoF system is simple and potentially has low power consumption, alternative solutions based on a dual lightwave approach and passive EATs have also been reported [21]. Using different wavelengths for segregating the uplink from the downlink optical paths makes it possible to minimize the transceiver insertion loss for both uplink and downlink signal transmission. Therefore, optical transceivers are increasingly considered as key components for the implementation of low-cost BSs [21].

The reflective EAM where the rear facet is coated with high reflection layer is an interesting device for operating simultaneously as a modulator and photoreceiver. In the following, the R-EAM performance as a transceiver in RoF systems is discussed. Additionally, an experimental evaluation of the R-EAM in terms of its slope efficiency (SE) and responsivity (*Re*) at different wavelengths, optical powers and bias points is reported. Finally, a case study of UWB signal transmission is reported, where the optimum operation points are discussed for different scenarios: the bias for maximum SE and for maximum *Re* are compared to zero

The experimental setup for the characterization of the 60 GHz R-EAM transceiver (CIP

Both electrical and optical test signals are used in order to obtain the electro-optical (E/O) and optical to electrical (O/E) response, which correspond to the slope efficiency and responsivity, respectively. We assume the decibel units for these variables to be obtained by 20log10(·), as defined in the measurements of the laboratory equipment. Figure 12 shows the EO response

**3. UWB transmission in a radio-over-fiber system based on reflective**

**electro-absorption modulators**

wavelength assignment can be done at the CS.

solution for a full-duplex RoF transmission.

**3.1. R-EAM performance analysis**

EAM-R-60-C-V-FCA) is shown in Figure 11.

bias.

**Figure 10.** GigaPOF-120LD length vs. wireless link length when transmitting at the maximum allowed power for a 0.125 % PER.

plots trend, we can also infer that valid transmissions of 200 Mbps over 150 meters of GigaPOF-62LD and 53.3 Mbps over 200 meters of the same fiber, are likely achievable.

A generalization of the results plotted in Figure 8 is depicted in Figure 9 for the GigaPOF-120LD. In order to obtain these results, the required signal power for the PER of 0.125% was obtained for wireless links up to 6 meters (in one meter spans) in a back-to-back optical configuration. For the 10, 50 and 100 meters of GI-POF cases, results were extrapolated using the start point given by the back-to-back configuration and using the previous derived result that the transmission power increases linearly with the GI-POF length (see Figure 8). Note that the 1 meter wireless link results are the ones plotted in Figure 8. The solid black lines in Figure 9 indicate the intersection of the surface with the 0 dB plane, which represents the maximum transmitted power. Thus, combinations of POF and wireless links with required transmit powers above 0 dB are not allowed by the standard and their representation in the Figure result from the extrapolation used.

With this result it is possible to see that a 1 meter wireless link followed by a range extension of 150 meter of GI-POF requires the same amount of transmitted power as the 6 meter wireless link with a 30 meter GigaPOF-120LD link for a bitrate of 200 Mbps. Similarly, at 480 Mbps, we can see that 100 meters of GigaPOF-120LD preceded by a 1.3 meters wireless link gives the same performance as 3 meter wireless link followed by approximately 35 meters of fiber.

The specific cases given by the black lines in Figure 9, are represented in Figure 10. These results represent the maximum POF length as a function of the wireless link length when transmitting at the maximum allowed power for a 0.125 % PER.

In conclusion, results show that it is possible to transmit 200 Mbps over a 6 meter wireless link followed by 30 meters of GigaPOF-120LD. We have experimentally demonstrated maximum transmission distances of 150 meters and 200 meters, respectively, using GigaPOF-120LD, with data rates of 200Mbps and 53.3 Mbps. It is also demonstrated that the PG-GI-POF attenuation, and not its bandwidth, is the dominant factor limiting the fiber link length.
