wdm optical interfaces for future fiber radio systems phần 7 docx

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: Measu

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

0.3 dB 0.6 dB 0.9 dB

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

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

93 % R efl ec

te d

85 % R efl ec

te d

70 % R efl ec

te d

54 % R efl ec

93 % R efl ec

te d

85 % R efl ec

te d

70 % R efl ec

te d

54 % R efl ec

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

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

-19 -18 -17 -16 -15 -14 -13

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

-10.5 -10.2 -9.9 -9.6 -9.3 -9

arr ier

70 % C arri

er

54 % C arr ier

arr ier

70 % C arri

er

54 % C arr ier

Received Optical Power (dBm)

Fig 4.33: Measured BER curves as a function of received optical power for uplink signals

generated by the reuse carriers recovered by the modified WDM optical interface with FBG2

reflectivity of: (i) 54%, (ii) 70%, (iii) 85%, and (iv) 93% respectively

4.8 Modified WDM Optical Interface and Network

Dimensioning

Section 4.6 describes the modified WDM optical interface that enhances the

modulation depths of the downlink signals without employing additional hardware,

and delivers greater reuse optical carrier for uplink communications However, the

incorporation of such modification in the WDM optical interface limits the power

budget of the link, which may restrict the network dimensioning Described in

Section 4.5, fibre-radio network configured in star-tree architecture [36-39], is

expected to contain more than two WDM optical interfaces in cascade in the RNs

Also, the networks configured in ring/bus architecture [40-43], will be having

multiple WDM optical interfaces in cascade, along with a span of fibre within each

pair of cascaded interfaces Therefore, the cascadability of the modified WDM

optical interface in both star-tree and ring/bus architectures are needed to be

explored

The power budget and the power margin of the link incorporating the modified

WDM optical interface can be calculated by:

where PR DL and PM DL are the optical power and the power margin of the desired

downlink signal at DL Drop port of modified WOI, Sensitivity DL is the sensitivity at

the DL Drop port of modified WOI, T LSCO is the optical power from the respective

light-source in the CO, L MOD is the loss in OSSB+C modulator, G BAMP is the gain

from the boost-EDFA in the CO, L SMF is the loss in 10 km SMF, and L DropWOI is the

drop-channel loss in the modified WOI, while the downlink signal traverses from IN

to DL Drop port L DropWOI also includes the reflection of the carrier by the variable

FBG2

The parameters obtained from the experimental results with various reflectivity of

FBG2 are presented in Table 4.6, where L DropWOI-54% , L DropWOI-70% , L DropWOI-85%, and

reflectivity of 54%, 70%, 85% and 93% Sensitivity DL-54% , Sensitivity DL-70%,

while reflectivity of FBG2 are 54%, 70%, 85% and 93% respectively

Table 4.6: Modified WDM Optical Interface parameters used in performance

analysis in networks considerations

By using the Equations (9) and (10) and the values noted in Table 4.6, the optical

power and the power margin at DL Drop port for various reflectivity of FBG2 can be

calculated as:

Star-tree configured fibre-radio networks, described in Section 4.5, are expected

to having multiple WOIs in cascade in the RNs If the power penalty is considered to

add up linearly with increasing number of WOIs in cascade, then the number WOIs

supported by the link (no ‘in between’ fibre) can be calculated by:

where N is the number of WOIs in cascade, PP Through is the power penalty

experienced by the through signals for traversing each stage of WOI, and L ThroughWOI

is the insertion loss experienced by the through channels in a WOI

Section 4.5 has shown that, for each stage of cascade, the through signals

experience a power penalty and an insertion loss of 0.4 dB and 3.2 dB respectively

Therefore, for various reflectivity of FBG2, numbers of WOIs in cascade can be

If the lossy multiport OCs in the WOIs in the experiment are replaced with

standard OCs having typical through channel insertion loss (typical through loss

1dB/WOI), and typical drop channel insertion loss (typical loss 1dB/WOI), the

number of units in cascade will increase to:

Also, if the insertion loss of the OSSB+C generator in CO can reduced to 9 dB, the

number of units in cascade will increase to:

Ring/bus configured fibre-radio networks, described in Section 4.5, will be having

multiple WOIs in cascade, in addition to a span of fibre between each pair of

cascaded WOIs Like before, if the power penalty is considered to add up linearly

with increasing number of WOIs in cascade, then the number WOIs supported by the

link can be calculated by:

where N is the number of WOIs in cascade, PP Through is the power penalty

experienced by the through signals for traversing each stage of WOI, L ThroughWOI is

the insertion loss experienced by the through signals in a WOI, and L FS is the

attenuation loss in the ‘in between’ fibre span The through signals in each stage of

cascade is (shown Section 4.5) experiencing a power penalty and an insertion loss of

0.4 dB and 3.2 dB respectively If the fibre span between the WOIs is considered to

be 1 km with an attenuation of 0.2 dB/km, the number of WOIs supported with

various reflectivity of FBG2 can be calculated as:

If the lossy multiport OCs in the WOIs in the experiment are replaced with

standard optical circulators having typical through channel insertion loss (typical

through loss 1dB/WOI), and typical drop channel insertion loss (typical loss

1dB/WOI), the number of units in cascade will increase to:

Also, if the insertion loss of the OSSB+C generator in CO can reduced to 9 dB, the

number of units in cascade will increase to:

Table 4.7: Cascadability of WOI with different reflectivity FBG2

Thus, the numerical evaluation of the links incorporating modified WDM optical

interfaces thus confirms that the replacement of 50% reflective FBG2 with an FBG

having higher reflectivity will restrict the network dimensioning both for star-tree

and ring/bus configurations, although it improves the overall performances of the

links, both in uplink and downlink directions

4.9 Conclusion

The performance of the proposed WDM optical interface in a single and cascaded

configuration is characterised by both simulations as well as by experiment The

results show that the 37.5 GHz-band 25 GHz-separated WI-DWDM signals can be

routed via the proposed interface without significant performance degradation The

characterisations as well as the modelling results confirm the viability of the

proposed interface in star-tree ring/bus network architectures with observed

negligible power penalty for each stage of cascade The incorporation of the

modification in the proposed interface will enhance the overall performances of the

links, both in uplink and downlink directions, although it is a trade off with the

capacity of network dimensioning

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