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

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 wavelength assignment can be done at the CS.

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 solution for a full-duplex RoF transmission.

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 bias.

### **3.1. R-EAM performance analysis**

10 Will-be-set-by-IN-TECH

480 Mbps 200 Mbps 53.3 Mbps

<sup>1</sup> <sup>2</sup> <sup>3</sup> <sup>4</sup> <sup>5</sup> <sup>6</sup> <sup>20</sup>

**Figure 10.** GigaPOF-120LD length vs. wireless link length when transmitting at the maximum allowed

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

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

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,

transmitting at the maximum allowed power for a 0.125 % PER.

Wireless link length (m)

Figure result from the extrapolation used.

POF length (m)

power for a 0.125 % PER.

The experimental setup for the characterization of the 60 GHz R-EAM transceiver (CIP EAM-R-60-C-V-FCA) is shown in Figure 11.

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

**Figure 11.** R-EAM characterization setup.

as a function of reverse bias voltage, for different frequencies and wavelengths. The R-EAM average optical input power, *PO*,*I*, is varied by controlling the CW laser output power. It is apparent that the optimum bias voltage increases with both wavelength and the optical input power. It is also apparent that SE degrades slightly with frequency especially for high *PO*,*I*, and lower wavelengths. Furthermore, it is easily seen that SE at the optimum bias voltage increases with *PO*,*I*. Figure 13 shows the O/E response as a function of reverse bias voltage, for different frequencies and wavelengths. Due to the similarity of results, only two wavelengths are plotted, in order to allow a clear visualization. Similarly to what was observed for the E/O case, the responsivity degrades with frequency, particularly for high values of *PO*,*I*, shorter wavelengths and decreasing reverse bias. Nevertheless, responsivity is shown to be more affected than SE. Furthermore, results also show that responsivity increases monotonically with reverse bias voltage.

<sup>0</sup> <sup>1</sup> <sup>2</sup> <sup>3</sup> <sup>4</sup> <sup>5</sup> −50

λ = 1570 nm

Reverse bias (V)

<sup>0</sup> <sup>1</sup> <sup>2</sup> <sup>3</sup> <sup>4</sup> <sup>5</sup> −50

Reverse bias (V)

(b) *PO*,*<sup>I</sup>* = 0 dBm

**Figure 13.** O/E response vs. reverse bias voltage for different wavelengths and frequencies and optical input powers. Solid, dashed and dotted lines correspond to frequencies of 2.4, 5, 15 GHz, respectively

is assessed in terms of slope efficiency and responsivity for the following cases: bias for maximum SE, bias for maximum *Re* and zero bias. The results of this analysis are plotted

The results in Figure 14 show that the best performance is obtained for a wavelength of 1560 nm, when the EAM is biased for maximum SE. However, when zero biased, the optimum wavelength is reduced to 1530 nm for *P*0,*<sup>I</sup>* = 7 dBm, where a penalty of 13 dB is incurred, compared to the case of maximum SE. Finally, when the EAM is biased for maximum responsivity, the optimum wavelength is 1560 nm and the SE decreases by 15 dB, compared to the zero bias case. It has also been verified experimentally that, as expected [18], the slope efficiency is proportional to the input optical power, as seen in Figure 14 (a) and (b), until it saturates at high optical powers, as shown by Figure 14 (c). Nevertheless, high optical input

Concerning the EAM responsivity, its value is optimum for *PO*,*<sup>I</sup>* = −6 dBm, while a noticeable reduction is observed with increasing *PO*,*I*, especially for zero bias. In both cases of biasing for maximum *Re* and SE, the EAM responsivity improves with increasing wavelength, except

The best responsivity values for both cases of biasing for maximum *Re* and SE are−9.6 dBA/W and −18.6 dBA/W, respectively. For the zero bias case, the responsivity decreases with wavelength for both input optical powers −6 dBm and 0 dBm, reaching a maximum of −31 dBA/W for *PO*,*<sup>I</sup>* = −6 dBm and 1530 nm. However, for *PO*,*<sup>I</sup>* = +7 dBm the responsivity is relatively constant with the wavelength, except for 1570 nm where it degrades by 6 dB. The

for high *PO*,*<sup>I</sup>* = +7 dBm, where its maximum is achieved at *λ* = 1560 nm.

λ = 1530 nm

<sup>0</sup> <sup>1</sup> <sup>2</sup> <sup>3</sup> <sup>4</sup> <sup>5</sup> −50

Reverse bias (V)

(c) *PO*,*<sup>I</sup>* = 7 dBm

−45

−40

−35

−30

−25

−20

−15

−10

−5

Performance Assessment of UWB-Over-Fiber and Applications 349

−45

−40

−35

−30

−25

−20

−15

−10

−5

(a) *PO*,*<sup>I</sup>* = −6 dBm

powers should be used in order to maximize the SE.

in Figure 14 and 15.

−45

−40

−35

REAM Responsivity

−30

−25

 (dBA/W)

−20

−15

−10

−5

**Figure 12.** E/O response vs. reverse bias voltage for different wavelengths and frequencies and optical input powers. Solid, dashed and dotted lines correspond to frequencies of 2.4, 5, 15 GHz, respectively.

The optimum bias points for maximum slope efficiency and responsivity have been extracted from the results of Figure 12 and 13. Considering these values, the R-EAM performance

12 Will-be-set-by-IN-TECH

*PO,R*

as a function of reverse bias voltage, for different frequencies and wavelengths. The R-EAM average optical input power, *PO*,*I*, is varied by controlling the CW laser output power. It is apparent that the optimum bias voltage increases with both wavelength and the optical input power. It is also apparent that SE degrades slightly with frequency especially for high *PO*,*I*, and lower wavelengths. Furthermore, it is easily seen that SE at the optimum bias voltage increases with *PO*,*I*. Figure 13 shows the O/E response as a function of reverse bias voltage, for different frequencies and wavelengths. Due to the similarity of results, only two wavelengths are plotted, in order to allow a clear visualization. Similarly to what was observed for the E/O case, the responsivity degrades with frequency, particularly for high values of *PO*,*I*, shorter wavelengths and decreasing reverse bias. Nevertheless, responsivity is shown to be more affected than SE. Furthermore, results also show that responsivity increases monotonically

<sup>0</sup> <sup>1</sup> <sup>2</sup> <sup>3</sup> <sup>4</sup> <sup>5</sup> −65

Reverse bias (V)

(b) *PO*,*<sup>I</sup>* = 0 dBm

**Figure 12.** E/O response vs. reverse bias voltage for different wavelengths and frequencies and optical input powers. Solid, dashed and dotted lines correspond to frequencies of 2.4, 5, 15 GHz, respectively. The optimum bias points for maximum slope efficiency and responsivity have been extracted from the results of Figure 12 and 13. Considering these values, the R-EAM performance

λ = 1540 nm

λ = 1550 nm

−55

−45

−35

−25

−15

−5

5

15

Optical Circulator

Optical Test Ports

**Figure 11.** R-EAM characterization setup.

with reverse bias voltage.

<sup>0</sup> <sup>1</sup> <sup>2</sup> <sup>3</sup> <sup>4</sup> <sup>5</sup> −65

Reverse bias (V)

(a) *PO*,*<sup>I</sup>* = −6 dBm

λ = 1560 nm λ = 1570 nm

−55

−45

R-EAM Slope Efficiency (dBW/V)

−35

−25

−15

−5

5

15

TX

RX

R-EAM

*P* Port *O,I*

TX/RX

<sup>0</sup> <sup>1</sup> <sup>2</sup> <sup>3</sup> <sup>4</sup> <sup>5</sup> −65

λ = 1530 nm

Reverse bias (V)

(c) *PO*,*<sup>I</sup>* = 7 dBm

−55

−45

−35

−25

−15

−5

5

15

Elecrical Test

**Figure 13.** O/E response vs. reverse bias voltage for different wavelengths and frequencies and optical input powers. Solid, dashed and dotted lines correspond to frequencies of 2.4, 5, 15 GHz, respectively

is assessed in terms of slope efficiency and responsivity for the following cases: bias for maximum SE, bias for maximum *Re* and zero bias. The results of this analysis are plotted in Figure 14 and 15.

The results in Figure 14 show that the best performance is obtained for a wavelength of 1560 nm, when the EAM is biased for maximum SE. However, when zero biased, the optimum wavelength is reduced to 1530 nm for *P*0,*<sup>I</sup>* = 7 dBm, where a penalty of 13 dB is incurred, compared to the case of maximum SE. Finally, when the EAM is biased for maximum responsivity, the optimum wavelength is 1560 nm and the SE decreases by 15 dB, compared to the zero bias case. It has also been verified experimentally that, as expected [18], the slope efficiency is proportional to the input optical power, as seen in Figure 14 (a) and (b), until it saturates at high optical powers, as shown by Figure 14 (c). Nevertheless, high optical input powers should be used in order to maximize the SE.

Concerning the EAM responsivity, its value is optimum for *PO*,*<sup>I</sup>* = −6 dBm, while a noticeable reduction is observed with increasing *PO*,*I*, especially for zero bias. In both cases of biasing for maximum *Re* and SE, the EAM responsivity improves with increasing wavelength, except for high *PO*,*<sup>I</sup>* = +7 dBm, where its maximum is achieved at *λ* = 1560 nm.

The best responsivity values for both cases of biasing for maximum *Re* and SE are−9.6 dBA/W and −18.6 dBA/W, respectively. For the zero bias case, the responsivity decreases with wavelength for both input optical powers −6 dBm and 0 dBm, reaching a maximum of −31 dBA/W for *PO*,*<sup>I</sup>* = −6 dBm and 1530 nm. However, for *PO*,*<sup>I</sup>* = +7 dBm the responsivity is relatively constant with the wavelength, except for 1570 nm where it degrades by 6 dB. The

<sup>0</sup> 0.5 <sup>1</sup> 1.5 <sup>2</sup> 2.5 <sup>3</sup> 3.5 <sup>4</sup> −24

responsivity degradation with increasing *PO*,*I*, translates into a penalty of 11 dB when *PO*,*<sup>I</sup>* is

The R-EAM performance was also assessed in terms of reflected optical power, *PO*,*R*, for different values of input optical power *PO*,*I*, the results of this analysis being shown in Figure 16. The reflected optical power is a relevant parameter with impact in the optical to electrical conversion. Therefore, this analysis considered the wavelengths of 1550 nm and 1560 nm, because these provide the best performance according to the measurements of slope efficiency given in Figure 15. The results given by the current analysis, indicate that the reflected optical power tends to decrease with increasing reverse bias voltage. Furthermore, the optical power reflected at the wavelength of 1550 nm is generally lower than that reflected

In this section, we consider the application of the R-EAM as a base station in a bidirectional transmission system, in a typical RoF network. A diagram of the application scenario is shown

The RF downlink (DL) signal, generated by the CS transceiver (TRX) with a power of *Ptx*,*DL*, passes through an electrical circulator and amplifier with gain *GE*,*DL*, and drives an E/O modulator, considered ideal in the present analysis. The optical downlink signal then passes through an optical circulator, and reaches the R-EAM through an optical fiber, with an incident optical power of *PO*,*I*. The RF modulated optical signal is converted to the electrical domain by the R-EAM with a responsivity (*Re*), and then reaches the mobile station through the wireless channel, which induces a signal loss of *L*. Note that the attenuation parameter *L* already includes both BS and MS antenna gains. Conversely, the RF uplink (UL) signal is generated by the mobile station transceiver with a power of *Ptx*,*UL*, and reaches the base station after being attenuated by the wireless link. The weak RF uplink signal can be amplified electrically (*GE*,*UL*) before being converted from the electrical to the optical domain with a conversion

Reverse bias (V)

PO,I = +7 dBm

Performance Assessment of UWB-Over-Fiber and Applications 351

PO,I = 0 dBm

−22 −20 −18 −16 −14 −12 −10 −8 −6 −4

——– λ = 1550 nm — — λ = 1560 nm

at 1560 nm, except for higher reverse bias at low optical input powers.

**Figure 16.** R-EAM reflected optical power, *PO*,*<sup>R</sup>* referred to the input optical power *PO*,*I*.

PO,I = −6 dBm

PO,R

increased from −6 dBm to +7 dBm.

**3.2. System performance assessment**

in Figure 17.

referred to

PO,I (dB)

**Figure 14.** E/O response vs. wavelength for different bias configurations and optical input powers.

**Figure 15.** O/E response vs. wavelength for different bias configurations and optical input powers.

**Figure 16.** R-EAM reflected optical power, *PO*,*<sup>R</sup>* referred to the input optical power *PO*,*I*.

responsivity degradation with increasing *PO*,*I*, translates into a penalty of 11 dB when *PO*,*<sup>I</sup>* is increased from −6 dBm to +7 dBm.

The R-EAM performance was also assessed in terms of reflected optical power, *PO*,*R*, for different values of input optical power *PO*,*I*, the results of this analysis being shown in Figure 16. The reflected optical power is a relevant parameter with impact in the optical to electrical conversion. Therefore, this analysis considered the wavelengths of 1550 nm and 1560 nm, because these provide the best performance according to the measurements of slope efficiency given in Figure 15. The results given by the current analysis, indicate that the reflected optical power tends to decrease with increasing reverse bias voltage. Furthermore, the optical power reflected at the wavelength of 1550 nm is generally lower than that reflected at 1560 nm, except for higher reverse bias at low optical input powers.

#### **3.2. System performance assessment**

14 Will-be-set-by-IN-TECH

Bias for Maximum Slope Efficiency

<sup>1530</sup> <sup>1540</sup> <sup>1550</sup> <sup>1560</sup> <sup>1570</sup> −50

(b) *PO*,*<sup>I</sup>* = 0 dBm

<sup>1530</sup> <sup>1540</sup> <sup>1550</sup> <sup>1560</sup> <sup>1570</sup> −50

(b) *PO*,*<sup>I</sup>* = 0 dBm

**Figure 15.** O/E response vs. wavelength for different bias configurations and optical input powers.

λ (nm)

Bias for Maximum Slope Efficiency

**Figure 14.** E/O response vs. wavelength for different bias configurations and optical input powers.

λ (nm)

<sup>1530</sup> <sup>1540</sup> <sup>1550</sup> <sup>1560</sup> <sup>1570</sup> −50

(c) *PO*,*<sup>I</sup>* = 7 dBm

Bias for Maximum Responsivity

<sup>1530</sup> <sup>1540</sup> <sup>1550</sup> <sup>1560</sup> <sup>1570</sup> −50

(c) *PO*,*<sup>I</sup>* = 7 dBm

λ (nm)

6 dB

−45 −40 −35 −30 −25 −20 −15 −10 −5 0 Bias for Maximum Responsivity

λ (nm)

13 dB

15 dB

−40

−30

−20

−10

0

10

−40

−30

−20

−10

0

10

<sup>1530</sup> <sup>1540</sup> <sup>1550</sup> <sup>1560</sup> <sup>1570</sup> −50

(a) *PO*,*<sup>I</sup>* = −6 dBm

<sup>1530</sup> <sup>1540</sup> <sup>1550</sup> <sup>1560</sup> <sup>1570</sup> −50

(a) *PO*,*<sup>I</sup>* = −6 dBm

Zero Bias

λ (nm)

12.5 dB

9 dB

−9.6 dBA/W

11 dB

−45 −40 −35 −30 −25 −20 −15 −10 −5 0

−45 −40 −35 −30 −25 −20 −15 −10 −5 0

REAM Responsivity

 (dBA/W) Zero Bias

λ (nm)

−40

−30

REAM Slope Efficiency (dBW/V)

−20

−10

0

10

In this section, we consider the application of the R-EAM as a base station in a bidirectional transmission system, in a typical RoF network. A diagram of the application scenario is shown in Figure 17.

The RF downlink (DL) signal, generated by the CS transceiver (TRX) with a power of *Ptx*,*DL*, passes through an electrical circulator and amplifier with gain *GE*,*DL*, and drives an E/O modulator, considered ideal in the present analysis. The optical downlink signal then passes through an optical circulator, and reaches the R-EAM through an optical fiber, with an incident optical power of *PO*,*I*. The RF modulated optical signal is converted to the electrical domain by the R-EAM with a responsivity (*Re*), and then reaches the mobile station through the wireless channel, which induces a signal loss of *L*. Note that the attenuation parameter *L* already includes both BS and MS antenna gains. Conversely, the RF uplink (UL) signal is generated by the mobile station transceiver with a power of *Ptx*,*UL*, and reaches the base station after being attenuated by the wireless link. The weak RF uplink signal can be amplified electrically (*GE*,*UL*) before being converted from the electrical to the optical domain with a conversion

#### 16 Will-be-set-by-IN-TECH 352 Ultra Wideband – Current Status and Future Trends Performance Assessment of UWB-Over-Fiber and Applications <sup>17</sup>

**Figure 17.** Considered setup for performance assessment. CW represents a continuous wave light source. Both TRX shown, represent the transceivers at the Control Station and Mobile Station. PD represents the Control Station photodiode. The EDFA depicted, represents an optional Erbium Doped Fiber Amplifier.

efficiency given by the EAM slope efficiency, *sea*, by modulating the reflected optical carrier with power *PO*,*R*. The uplink signal might be further optically (*GO*,*UL*) amplified before reaching the CS transceiver. This optical amplification is only adequate if the optical power is low (typically less than −3 dBm), so that the noise added by the EDFA is still below the noise level at the receiver [22]. In the following analysis, we assume *PO*,*<sup>R</sup>* ∼= *PO*,*D*, since the optical circulator loss and the use of an EDFA are not considered. The metric used to evaluate performance of the system is the signal to noise ratio (SNR).

The SNR of the RF signal arriving at the mobile station can be written as:

$$\text{SNR}\_{rx,MS} = \frac{\langle I\_{rx,DL}^2 \rangle}{\sigma\_{n,MS}^2} = \frac{\langle I\_{tx,DL}^2 \rangle G\_{DL}}{\langle I\_{t,MS}^2 \rangle} \tag{3}$$

modulator, *k* is Boltzmann's constant, *T* = 290 K and *Ta* = 120 K is the antenna temperature. Since the link gain (*GDL*) is low, the noise at the receiver is dominated by the thermal noise of the receiver circuitry itself. Furthermore, since the R-EAM efficiency is higher in the downlink,

<sup>=</sup> �*I*<sup>2</sup>

*rin*� <sup>+</sup> �*I*<sup>2</sup>

�*I*2

*tx*,*UL*�*GUL*

*sn*� = 2*qrd*�*PO*,*D*�Δ*<sup>f</sup>* (11)

2 *ear* 2

*sn*� <sup>+</sup> �*I*<sup>2</sup>

*<sup>e</sup> L* (9)

Performance Assessment of UWB-Over-Fiber and Applications 353

210RIN/10Δ*<sup>f</sup>* (10)

*tx*,*UL*� (13)

*<sup>t</sup>*,*CS*� (8)

*<sup>d</sup>*Δ*<sup>f</sup>* /*REAM* (12)

regardless of the bias conditions, the SNR is considerably higher than in the uplink.

*rx*,*UL*� *σ*2 *n*,*CS*

+(*Ta* + (*Fn*,*UL* − 1)*T*)*kGE*,*ULs*

The noise terms are referred to the photodiode output, and stem from three main components: the relative intensity noise (RIN) from the laser source (proportional to the square of �*PO*,*D*�), the shot noise (proportional to �*PO*,*D*�), and the last term is the thermal noise from both the photodiode load (*R*L), the antenna and the uplink electrical amplifier, where *Fn*,*UL* is the noise factor of the amplifier at the base-station. The thermal noise from the electrical transmitter at the mobile station can be neglected, due to the low total gain of the link (*GUL*). The optical power detected by the photodiode, *PO*,*D*, is expected to have a significant impact on the noise contribution at the receiver. In the presented results, the wireless channel attenuation (*L*) was not considered, the fiber is considered ideal, *GE*,*UL*/*DL* = 1, *Fn*,*UL*/*DL* = 1, *R*<sup>L</sup> = 1000 Ω and

The present analysis considers again the usage of typical UWB transceivers (*Wisair DVK9110*), which operate in band group 1 (from 3.168 GHz to 4.752 GHz) and have a maximum transmission power of approximately *Ptx*,*UL*/Δ*<sup>f</sup>* = −45.3 dBm, when the MS antenna gain

The UWB receiver sensitivity of −70.4 dBm at 480 Mbit/s specified in the standard [12], is not valid for an optical front-end. This value assumes a typical value of noise level in a wireless receiver of −80.5 dBm, which indicates that a receiver should be able to meet the target performance specified in the standard for a SNR of approximately 10 dB. In the present

2 *eaR*<sup>2</sup>

2

(considered to be 4 dB) is lumped into the wireless channel attenuation.

*<sup>d</sup>*�*PO*,*D*�

The SNR of the RF signal arriving at the CS can be written as:

SNR*rx*,*CS* <sup>=</sup> �*I*<sup>2</sup>

*GUL* = *GE*,*ULs*

*<sup>t</sup>*,*CS*� = 4*kT*Δ*<sup>f</sup>* /*R*<sup>L</sup>

where *REAM* represents the impedance of the R-EAM circuitry.

*Ptx*,*UL* = *RMS*�*I*

**3.3. UWB-over-fiber results and discussion**

�*I* 2 *rin*� <sup>=</sup> *<sup>r</sup>*<sup>2</sup>

�*I* 2

�*I* 2

*RMS*/*EO*/*EAM* = 50 Ω.

where,

where,

$$\mathcal{G}\_{DL} = \mathcal{G}\_{\text{E,DL}} \mathsf{s}\_{\text{eo,ideal}}^2 \mathsf{R}\_{\text{e}}^2 \mathsf{L} \tag{4}$$

$$
\langle I\_{t,MS}^2 \rangle = (4T + T\_a)k\Delta\_f / R\_{MS}
$$

$$+(F\_{\text{H},\text{DL}}-1)kT G\_{\text{DL}}\Delta\_f/R\_{\text{EO}}\tag{5}$$

$$P\_{\rm tx,DL} = R\_{\rm CS} \langle I\_{\rm tx,DL}^2 \rangle \tag{6}$$

Therefore, the SNR at the receiver can be written as the following ratio:

$$\text{SNR}\_{\text{rx,MS}} = \frac{(P\_{\text{tx,DL}}/\Delta\_f)\text{G}\_{\text{DL}}/\text{R}\_{\text{EO}}}{(4T + T\_a)k/R\_{MS} + (F\_{\text{n,DL}} - 1)kTG\_{\text{DL}}/R\_{\text{EO}}}\tag{7}$$

where *RMS* represents the impedance of the mobile station circuitry, *REO* the impedance of the E/O converter circuitry, Δ*<sup>f</sup>* the transmission bandwidth, *seo*,ideal the slope efficiency of the ideal E/O modulator, *Fn*,*DL* the noise factor of the electrical amplifier that precedes the modulator, *k* is Boltzmann's constant, *T* = 290 K and *Ta* = 120 K is the antenna temperature. Since the link gain (*GDL*) is low, the noise at the receiver is dominated by the thermal noise of the receiver circuitry itself. Furthermore, since the R-EAM efficiency is higher in the downlink, regardless of the bias conditions, the SNR is considerably higher than in the uplink.

The SNR of the RF signal arriving at the CS can be written as:

$$\text{SNR}\_{\text{rx,CS}} = \frac{\langle I\_{\text{rx,ILL}}^2 \rangle}{\sigma\_{n,\text{CS}}^2} = \frac{\langle I\_{\text{tx,LL}}^2 \rangle \text{G}\_{\text{UL}}}{\langle I\_{\text{rin}}^2 \rangle + \langle I\_{\text{sn}}^2 \rangle + \langle I\_{t,\text{CS}}^2 \rangle} \tag{8}$$

where,

16 Will-be-set-by-IN-TECH

Fiber

*Fn,DL Fn,UL*

*PO,I*

*PO,R*

Circ.

EDFA

*PO,D*

*GO,UL*

**Figure 17.** Considered setup for performance assessment. CW represents a continuous wave light source. Both TRX shown, represent the transceivers at the Control Station and Mobile Station. PD represents the Control Station photodiode. The EDFA depicted, represents an optional Erbium Doped

efficiency given by the EAM slope efficiency, *sea*, by modulating the reflected optical carrier with power *PO*,*R*. The uplink signal might be further optically (*GO*,*UL*) amplified before reaching the CS transceiver. This optical amplification is only adequate if the optical power is low (typically less than −3 dBm), so that the noise added by the EDFA is still below the noise level at the receiver [22]. In the following analysis, we assume *PO*,*<sup>R</sup>* ∼= *PO*,*D*, since the optical circulator loss and the use of an EDFA are not considered. The metric used to evaluate

> *rx*,*DL*� *σ*2 *n*,*MS*

> > 2 *eo*,ideal*R*<sup>2</sup>

*<sup>t</sup>*,*MS*� = (4*T* + *Ta*)*k*Δ*<sup>f</sup>* /*RMS*

2

where *RMS* represents the impedance of the mobile station circuitry, *REO* the impedance of the E/O converter circuitry, Δ*<sup>f</sup>* the transmission bandwidth, *seo*,ideal the slope efficiency of the ideal E/O modulator, *Fn*,*DL* the noise factor of the electrical amplifier that precedes the

SNR*rx*,*MS* <sup>=</sup> (*Ptx*,*DL*/Δ*f*)*GDL*/*REO*

<sup>=</sup> �*I*<sup>2</sup>

(4*T* + *Ta*)*k*/*RMS* + (*Fn*,*DL* − 1)*kTGDL*/*REO*

*tx*,*DL*�*GDL* �*I*2

TRX

*Prx,UL*

Electrical Optical

performance of the system is the signal to noise ratio (SNR).

�*I* 2

The SNR of the RF signal arriving at the mobile station can be written as:

SNR*rx*,*MS* <sup>=</sup> �*I*<sup>2</sup>

*GDL* = *GE*,*DLs*

*Ptx*,*DL* = *RCS*�*I*

Therefore, the SNR at the receiver can be written as the following ratio:

*Ptx,DL*

CW

λ

Fiber Amplifier.

where,

E/O

*seo,*ideal

Central Station

AMP

Circ.

*GE,DL*

PD

*rd*

R-EAM

*sea Re*

> *GE,UL* AMP

Bias

TRX

Base Station

Wireless Link

*Ptx,UL Prx,DL* *L*

*<sup>t</sup>*,*MS*� (3)

(7)

*<sup>e</sup> L* (4)

+(*Fn*,*DL* − 1)*kTGDL*Δ*<sup>f</sup>* /*REO* (5)

*tx*,*DL*� (6)

Mobile Station

$$\mathbf{G}\_{\rm LL} = \mathbf{G}\_{\rm E,LL} \mathbf{s}\_{ea}^{2} \mathbf{R}\_{e}^{2} \mathbf{L} \tag{9}$$

$$
\langle I\_{\rm rim}^2 \rangle = r\_d^2 \langle P\_{\rm O,D} \rangle^2 \mathbf{1}^{\rm RIN/10} \Delta\_f \tag{10}
$$

$$
\langle I\_{\rm sn}^2 \rangle = 2q r\_d \langle P\_{O,D} \rangle \Delta\_f \tag{11}
$$

$$
\langle \mathbf{I}\_{t,CS}^2 \rangle = 4kT\Delta\_f / R\_\mathbf{L}
$$

$$+(T\_a + (F\_{n,UL} - 1)T)kG\_{E,UL}s\_{ea}^2 r\_d^2 \Delta\_f / R\_{EAM} \tag{12}$$

$$P\_{\rm tx,IL} = \mathbb{R}\_{\rm MS} \langle I\_{\rm tx,IL}^2 \rangle \tag{13}$$

where *REAM* represents the impedance of the R-EAM circuitry.

The noise terms are referred to the photodiode output, and stem from three main components: the relative intensity noise (RIN) from the laser source (proportional to the square of �*PO*,*D*�), the shot noise (proportional to �*PO*,*D*�), and the last term is the thermal noise from both the photodiode load (*R*L), the antenna and the uplink electrical amplifier, where *Fn*,*UL* is the noise factor of the amplifier at the base-station. The thermal noise from the electrical transmitter at the mobile station can be neglected, due to the low total gain of the link (*GUL*). The optical power detected by the photodiode, *PO*,*D*, is expected to have a significant impact on the noise contribution at the receiver. In the presented results, the wireless channel attenuation (*L*) was not considered, the fiber is considered ideal, *GE*,*UL*/*DL* = 1, *Fn*,*UL*/*DL* = 1, *R*<sup>L</sup> = 1000 Ω and *RMS*/*EO*/*EAM* = 50 Ω.

#### **3.3. UWB-over-fiber results and discussion**

The present analysis considers again the usage of typical UWB transceivers (*Wisair DVK9110*), which operate in band group 1 (from 3.168 GHz to 4.752 GHz) and have a maximum transmission power of approximately *Ptx*,*UL*/Δ*<sup>f</sup>* = −45.3 dBm, when the MS antenna gain (considered to be 4 dB) is lumped into the wireless channel attenuation.

The UWB receiver sensitivity of −70.4 dBm at 480 Mbit/s specified in the standard [12], is not valid for an optical front-end. This value assumes a typical value of noise level in a wireless receiver of −80.5 dBm, which indicates that a receiver should be able to meet the target performance specified in the standard for a SNR of approximately 10 dB. In the present

**Figure 18.** SNR as a function of reverse bias for three different values of optical input power.

analysis, a minimum SNR of 7.5 dB is considered, which has been measured experimentally for the mentioned commercial UWB transceiver.

−6 −4 −2 <sup>0</sup> <sup>2</sup> <sup>4</sup> <sup>6</sup> <sup>5</sup>

Min. SNR, No Wireless Link

PO,I (dBm)

(a) Zero bias

Equivalent Limit for 0.5m Wireless Link

Equivalent Limit for 1m Wireless Link

Shot Noise Thermal Noise

there is no advantage in analyzing that scenario.

−6 −4 −2 <sup>0</sup> <sup>2</sup> <sup>4</sup> <sup>6</sup> <sup>5</sup>

Limits Shot

Thermal Noise

Noise

PO,I (dBm)

−6 −4 −2 <sup>0</sup> <sup>2</sup> <sup>4</sup> <sup>6</sup> <sup>5</sup>

Shot Noise

PO,I (dBm)

(c) Optimum bias for maximum SNR

––– r<sup>d</sup> = 0.7 A/W —— r<sup>d</sup> = 1 A/W RIN = -160 dBc/Hz RIN = -150 dBc/Hz

RIN Limits

Performance Assessment of UWB-Over-Fiber and Applications 355

Thermal Noise

(b) Optimum bias for maximum SE

From an overview perspective, it can be noted that the SNR performance increases with *PO*,*<sup>I</sup>* for a RIN of −160 dB/Hz, whereas for −150 dB/Hz there is an optimum value of *PO*,*<sup>I</sup>* for maximum SNR. At zero bias, a maximum margin of 9 dB compared to the UWB SNR limit is obtained with a laser RIN of −150 dB/Hz for a *PO*,*<sup>I</sup>* of ∼0 dBm, whereas for a RIN of −160 dB/Hz the performance becomes limited by shot noise, allowing for a SNR margin of ∼ 13.5 dB. However, the results indicate that none of such limits are achieved at zero bias, because of both RIN and shot noise limitations, which means that a totally passive base-station is not practicable, for reasonable wireless link distances. Results for optimum bias for maximum SE indicate that the SNR for a RIN of −150 dB/Hz practically achieves the UWB SNR limit for a wireless link distance of 0.5 meters with a *PO*,*<sup>I</sup>* of ∼6dBm, whereas for a RIN of −160 dB/Hz a 1 meter link is almost achieved. Note that while the former SNR is limited by the RIN term, the latter is limited by the shot noise. At optimum bias for maximum SNR, a margin of 33.5 dB is obtained at the maximum *PO*,*I*, for a RIN of −160 dB/Hz and *Re* = 1 A/W, allowing for an acceptable wireless link distance of approximately 1 meter. For a RIN of −150 dB/Hz, there is enough SNR margin to allow for a wireless link distance between 0.5 and 1 meter. Furthermore, by reducing the UWB throughput, a maximum distance of 2.8 meters would be achievable at 53.3 Mbit/s, at a minimum SNR of 0 dB. Since the R-EAM slope efficiency at optimum bias for maximum responsivity is worse than that at zero bias,

In conclusion, the optimum operation point was found to be the biasing for maximum SNR using an high optical input power and a wavelength of 1560 nm. Although a zero bias configuration is an attractive technique, it is not suitable to provide a reasonable wireless link distance for UWB. Additionally, we also conclude that one of the performance-limiting

**Figure 19.** SNR as a function of optical input power for thre different bias cases.

RIN

SNR (dB)

RIN Limits

The results in Figure 18 show the SNR as a function of the reverse bias, for three different values of *PO*,*I*, (a) −6 dBm, (b) 0 dBm and (c) +7 dBm, considering a responsivity of 1 A/W and two different wavelengths, 1550 nm and 1560 nm and RIN values, -150 dB/Hz and -160 dB/Hz. Moreover, SNR results considering only the thermal noise term are also depicted for comparisson purposes. Two additional equivalent SNR limits that consider a wireless link length of 0.5 m and also 1 m are also shown in the results, which account for a signal loss of 28.5 and 34 dB at 4 GHz, respectively, when considering a total gain of the antennas of 10 dBi, among MS and BS. Results show that for low reverse bias the impact of both shot noise and RIN increase with the incident optical power. This result comes in line with the ones of the reflected optical power plotted in Figure 16. Moreover, it is clearly seen that the SNR is optimum for a wavelength of 1560 nm with a maximum SNR 5 dB better than that at 1550 nm, although slightly worse for zero bias.

In Figure 19 the SNR is obtained as a function of *PO*,*I*, for the following cases: (a) zero bias, (b) optimum bias for maximum SE and (c) optimum bias for maximum SNR, considering only the wavelength of 1560 nm. Again, two additional equivalent SNR limits that consider a wireless link length of 0.5 m and also 1 m are also shown in the results. The results considering optimum bias for maximum Slope Efficiency are, at the maximum points, 5 − 6 dB worse than those for maximum SNR, essentially because there is a compromise between signal power, which is affected by the *se*,*a* as a function of bias voltage, and noise power, affected by the *PO*,*<sup>D</sup>* which also depends on the bias voltage.

(a) Zero bias (b) Optimum bias for maximum SE (c) Optimum bias for maximum SNR

**Figure 19.** SNR as a function of optical input power for thre different bias cases.

18 Will-be-set-by-IN-TECH

<sup>0</sup> <sup>1</sup> <sup>2</sup> <sup>3</sup> <sup>4</sup> <sup>0</sup>

<sup>0</sup> <sup>1</sup> <sup>2</sup> <sup>3</sup> <sup>4</sup> <sup>0</sup>

Reverse bias (V)

(c) *PO*,*<sup>I</sup>* = 7 dBm

Reverse bias (V)

(b) *PO*,*<sup>I</sup>* = 0 dBm

analysis, a minimum SNR of 7.5 dB is considered, which has been measured experimentally

The results in Figure 18 show the SNR as a function of the reverse bias, for three different values of *PO*,*I*, (a) −6 dBm, (b) 0 dBm and (c) +7 dBm, considering a responsivity of 1 A/W and two different wavelengths, 1550 nm and 1560 nm and RIN values, -150 dB/Hz and -160 dB/Hz. Moreover, SNR results considering only the thermal noise term are also depicted for comparisson purposes. Two additional equivalent SNR limits that consider a wireless link length of 0.5 m and also 1 m are also shown in the results, which account for a signal loss of 28.5 and 34 dB at 4 GHz, respectively, when considering a total gain of the antennas of 10 dBi, among MS and BS. Results show that for low reverse bias the impact of both shot noise and RIN increase with the incident optical power. This result comes in line with the ones of the reflected optical power plotted in Figure 16. Moreover, it is clearly seen that the SNR is optimum for a wavelength of 1560 nm with a maximum SNR 5 dB better than that at 1550 nm,

In Figure 19 the SNR is obtained as a function of *PO*,*I*, for the following cases: (a) zero bias, (b) optimum bias for maximum SE and (c) optimum bias for maximum SNR, considering only the wavelength of 1560 nm. Again, two additional equivalent SNR limits that consider a wireless link length of 0.5 m and also 1 m are also shown in the results. The results considering optimum bias for maximum Slope Efficiency are, at the maximum points, 5 − 6 dB worse than those for maximum SNR, essentially because there is a compromise between signal power, which is affected by the *se*,*a* as a function of bias voltage, and noise power, affected by the

**Figure 18.** SNR as a function of reverse bias for three different values of optical input power.

<sup>0</sup> <sup>1</sup> <sup>2</sup> <sup>3</sup> <sup>4</sup> <sup>0</sup>

Min. SNR, No Wireless Link

Thermal Noise Only

Thermal + Shot + RIN (-150 dBc/Hz) Thermal + Shot + RIN (-160 dBc/Hz)

Equivalent Limit for 1m Wireless Link

Equivalent Limit for 0.5m Wireless Link

——– λ = 1550 nm — — λ = 1560 nm

Reverse bias (V)

(a) *PO*,*<sup>I</sup>* = −6 dBm

although slightly worse for zero bias.

*PO*,*<sup>D</sup>* which also depends on the bias voltage.

for the mentioned commercial UWB transceiver.

SNR (dB)

> From an overview perspective, it can be noted that the SNR performance increases with *PO*,*<sup>I</sup>* for a RIN of −160 dB/Hz, whereas for −150 dB/Hz there is an optimum value of *PO*,*<sup>I</sup>* for maximum SNR. At zero bias, a maximum margin of 9 dB compared to the UWB SNR limit is obtained with a laser RIN of −150 dB/Hz for a *PO*,*<sup>I</sup>* of ∼0 dBm, whereas for a RIN of −160 dB/Hz the performance becomes limited by shot noise, allowing for a SNR margin of ∼ 13.5 dB. However, the results indicate that none of such limits are achieved at zero bias, because of both RIN and shot noise limitations, which means that a totally passive base-station is not practicable, for reasonable wireless link distances. Results for optimum bias for maximum SE indicate that the SNR for a RIN of −150 dB/Hz practically achieves the UWB SNR limit for a wireless link distance of 0.5 meters with a *PO*,*<sup>I</sup>* of ∼6dBm, whereas for a RIN of −160 dB/Hz a 1 meter link is almost achieved. Note that while the former SNR is limited by the RIN term, the latter is limited by the shot noise. At optimum bias for maximum SNR, a margin of 33.5 dB is obtained at the maximum *PO*,*I*, for a RIN of −160 dB/Hz and *Re* = 1 A/W, allowing for an acceptable wireless link distance of approximately 1 meter. For a RIN of −150 dB/Hz, there is enough SNR margin to allow for a wireless link distance between 0.5 and 1 meter. Furthermore, by reducing the UWB throughput, a maximum distance of 2.8 meters would be achievable at 53.3 Mbit/s, at a minimum SNR of 0 dB. Since the R-EAM slope efficiency at optimum bias for maximum responsivity is worse than that at zero bias, there is no advantage in analyzing that scenario.

> In conclusion, the optimum operation point was found to be the biasing for maximum SNR using an high optical input power and a wavelength of 1560 nm. Although a zero bias configuration is an attractive technique, it is not suitable to provide a reasonable wireless link distance for UWB. Additionally, we also conclude that one of the performance-limiting

factors comes from the laser RIN that imposes a limit on the achievable SNR, especially for a zero biased modulator. A laser RIN of −160 dB/Hz would be required in order to avoid the RIN limitation. In this case the performance becomes limited by shot noise. Although a zero bias configuration is an attractive technique, it is not suitable to provide a reasonable wireless link distance for UWB signals.

**5. References**

2002.

*Electronics Letters*, 40(21):1353–1354, 2004.

*Technology*, 26(15):2594–2603, 2008.

*2007*, pages 1–3, 2007.

1360–1370. IEEE, 2004.

*Int. Conf*, volume 1, pages 41–44, 2002.

*Lightwave Technology*, 28(4):390–405, 2010.

*Technology Letters, IEEE*, 18(19):2056–2058, 2006.

*Communications*, 282(24):4706–4715, 2009.

[1] Federal Communications Commission. First report and order, revision of part 15 of commission's rule regarding ultra-wideband transmission system. FCC 02-48, 22:2,

Performance Assessment of UWB-Over-Fiber and Applications 357

[2] JE Mitchell. Performance of OFDM at 5.8 GHz using radio over fibre link. *IEEE*

[3] Pak Kay Tang, Ling Chuen Ong, A. Alphones, B. Luo, and M. Fujise. PER and EVM measurements of a radio-over-fiber network for cellular and WLAN system

[4] N.J. Gomes, M. Morant, A. Alphones, B. Cabon, J.E. Mitchell, C. Lethien, M. Csörnyei, A. Stöhr, and S. Iezekiel. Radio-over-fiber transport for the support of wireless

[6] M. Sauer, A. Kobyakov, and J. George. Radio over fiber for picocellular network

[7] C. Lethien, C. Loyez, J.P. Vilcot, L. Clavier, M. Bocquet, and P.A. Rolland. Indoor coverage improvement of mb-ofdm uwb signals with radio over pof system. *Optics*

[8] O. Ziemann, H. Poisel, and J. Vinogradov. Potential of high speed, short distance optical data communication on large diameter optical fibers. In *Proc. 1st Electronics*

[9] A. Polley, R. J. Gandhi, and S. E. Ralph. 40gbps links using plastic optical fiber. In *Proc. Conf. Optical Fiber Communication and the National Fiber Optic Engineers Conf. OFC/NFOEC*

[10] T. Ishigure, Y. Aruga, and Y. Koike. High-bandwidth pvdf-clad gi pof with ultra-low

[11] T. Kibler and E. Zeeb. Optical data links for automotive applications. In *Electronic Components and Technology Conference, 2004. Proceedings. 54th*, volume 2, pages

[13] X. N. Fernando and A. Anpalagan. On the design of optical fiber based wireless access systems. In *Proc. IEEE Int Communications Conf*, volume 6, pages 3550–3555, 2004. [14] T. Marozsak and E. Udvary. Vertical cavity surface emitting lasers in radio over fiber applications. In *Proc. MIKON-2002 Microwaves, Radar and Wireless Communications 14th*

[15] C. Lim, A. Nirmalathas, M. Bakaul, P. Gamage, K.L. Lee, Y. Yang, D. Novak, and R. Waterhouse. Fiber-wireless networks and subsystem technologies. *IEEE Journal of*

[16] Z. Jia, J. Yu, and G.K. Chang. A full-duplex radio-over-fiber system based on optical carrier suppression and reuse. *IEEE Photonics Technology Letters*, 18(16):1726–1728, 2006. [17] L. Chen, H. Wen, and S. Wen. A radio-over-fiber system with a novel scheme for millimeter-wave generation and wavelength reuse for up-link connection. *Photonics*

applications. *IEEE Journal of Lightwave Technology*, 22(11):2370 – 2376, 2004.

broadband services [invited]. *Journal of Optical Networking*, 8(2):156–178, 2009. [5] M. Jazayerifar, B. Cabon, and J.A. Salehi. Transmission of multi-band OFDM and impulse radio ultra-wideband signals over single mode fiber. *IEEE Journal of Lightwave*

architectures. *IEEE Journal of Lightwave Technology*, 25(11):3301–3320, 2007.

*Systemintegration Technology Conf*, volume 1, pages 409–414, 2006.

bending loss. *Journal of Lightwave Technology*, 25(1):335–345, 2007.

[12] S. ECMA. Ecma-368: High rate ultra wideband phy and mac standard, 2005.
