wdm optical interfaces for future fiber radio systems phần 7 docx

Chapter 4: Characterisation and Enhancement of Links Performance

Incorporating WDM Optical Interface

FBG2 with reflectivity 54%, 70%, 85% and 93% reduces the CSRs of the downlink

spectra from 12.2 dB to 9.1, 7.1, 5 and 1.7 dB respectively. Therefore, by replacing

the 54% (~ 50%) reflective FBG in the interface with an FBG of 93% reflectivity, a

reduction in CSR by as much as 7.4 dB can be achieved. The 3rd column of the Table

4.5 shows, the sidebands of the downlink signals vary by 1.3 dB; this is due to the

presence of fluctuations in the recovered spectra caused by the imperfect filtering

characteristics of the FBGs used in the experiment.

The optical spectra of the respective reuse carriers while inserted 54%, 70%, 85%

and 93% reflective FBG2 in the interface are recovered via λ-Re-Use port, and

shown in Fig. 4.30.The characteristic parameters of these curves are also illustrated

in Table 4.5. Fig. 4.30 and Table 4.5 show that the insertion of FBG2 with

reflectivity 54%, 70%, 85% and 93% provides optical carriers in the uplink path,

which gradually increases from -7.6 dB to -7.3, -6.7 and -5.8 dB respectively.

Therefore, the replacement of the 54% (~ 50%) reflective FBG in the interface with a

93% reflective FBG enables an increase of uplink reuse carrier by as much as 1.8 dB.

0.3 dB

0.6 dB

0.9 dB

54%

70%

85%

93%

O

ptic

al P

o

w

er (dBm)

-6

-8

-10

Wavelength (nm)

1556.3 1556.4 1556.5

0.3 dB

0.6 dB

0.9 dB

54%

70%

85%

93%

O

ptic

al P

o

w

er (dBm)

-6

-8

-10

Wavelength (nm)

1556.3 1556.4 1556.5

Fig. 4.30: Measured optical spectra of the uplink reuse carriers with various reflectivity of FBG2,

recovered at λ-Re-Use port of the modified WDM optical interface.

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Chapter 4: Characterisation and Enhancement of Links Performance

Incorporating WDM Optical Interface

In compare with the respective downlink carriers at DL Drop port, uplink carriers are

reduced by approximately 1.2 dB. This can be attributed to the insertion loss of the

-5

-6

-7

-8

-9

-19 -18 -17 -16 -15

93%

Reflected

85% Reflected

70% Reflected

54% Reflecte

ol

d

g l 10 (B

E

R)

Received Optical Power (dBm)

-5

-6

-7

-8

-9

-19 -18 -17 -16 -15

93%

Reflected

85% Reflected

70% Reflected

54% Reflecte

ol

d

g l 10 (B

E

R)

Received Optical Power (dBm)

Fig. 4.31: Measured BER curves as a function of received optical power at DL Drop port of

modified WDM optical interface for downlink (λ2, S2) with FBG2 reflectivity of: (i) 54%, (ii)

70%, (iii) 85%, and (iv) 93% respectively.

OC between port 2 to port 3, which has been traversed by the uplink carriers before

being recovered via λ-Re-Use port.

The effects of the reduction in CSR in the downlink direction are quantified by

measuring BER curves for downlink (λ2, S2) at DL Drop port with various

reflectivity of FBG2 mentioned above. The measured BER curves are shown in Fig.

4.31. The curves demonstrate that due to 7.4 dB reduction in CSR (mentioned

above); the overall performance of the recovered downlink (λ2, S2) improves by as

much as 2.9 dB. The changes in sensitivity with respect to the CSRs, as well as the

reduction of CSRs, in the downlink direction of the link are also plotted in Fig. 4.32.

In order to quantify the effects in the uplink direction, the recovered uplink

carriers were reused to generate uplink OSSB+C modulated signals by using another

37.5 GHz mm-wave signal, which was generated by mixing a 37.5 GHz LO signal

166

Chapter 4: Characterisation and Enhancement of Links Performance

Incorporating WDM Optical Interface

with 155 Mb/s BPSK data, the similar way it was generated in the downlink

direction. Each of the uplink signals was then detected to recover data by using the

PD and data recovery circuit used in recovering downlink data. The BER curves for

the recovered uplink data are shown in Fig. 4.33. It shows that 1.8 dB increase in the

uplink reuse carriers by the modified interface improves the performance of the link

in the uplink direction by 1.2 dB. The changes in sensitivity in the uplink direction

with respect to the intensity of the uplink reuse carriers are also plotted in Fig. 4.34.

-19

-18

-17

-16

-15

-14

-13

0 2 4 6 8 10 12

Sensitivit

y (dBm)

CSR and Reduction of CSR (dB)

Sensitivity Vs. Reduction in CSR

Sensitivity Vs. CSR

-19

-18

-17

-16

-15

-14

-13

0 2 4 6 8 10 12

Sensitivit

y (dBm)

CSR and Reduction of CSR (dB)

Sensitivity Vs. Reduction in CSR

Sensitivity Vs. CSR

Fig. 4.32: Changes of sensitivity in the downlink direction of the link : (i) Sensitivity vs. reduction

in CSR, and (ii) Sensitivity vs. CSR respectively.

The experimental results, therefore, clearly indicate that the incorporation of the

variable FBG2 in the WDM optical interface will enhance the modulation depths of

the downlink signals by reducing the CSRs that improves the link performance in the

downlink direction significantly. Also the reduction in CSRs of the downlink signals

allows the interface to maximise the recovery of the uplink reuse carriers that also

exerts notable performance improvement in the uplink direction, while reducing the

difference between the weaker uplink signals and the through downlink signals in the

fibre feeder networks.

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