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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 25 — Dec. 10, 2007
  • pp: 16781–16786
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Symmetric reconfigurable capacity assignment in a bidirectional DWDM access network

Beatriz Ortega, José Mora, Gustavo Puerto, and José Capmany  »View Author Affiliations


Optics Express, Vol. 15, Issue 25, pp. 16781-16786 (2007)
http://dx.doi.org/10.1364/OE.15.016781


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Abstract

This paper presents a novel architecture for DWDM bidirectional access networks providing symmetric dynamic capacity allocation for both downlink and uplink signals. A foldback arrayed waveguide grating incorporating an optical switch enables the experimental demonstration of flexible assignment of multiservice capacity. Different analog and digital services, such as CATV, 10 GHz-tone, 155Mb/s PRBS and UMTS signals have been transmitted in order to successfully test the system performance under different scenarios of total capacity distribution from the Central Station to different Base Stations with two reconfigurable extra channels for each down and upstream direction.

© 2007 Optical Society of America

1. Introduction

Passive Optical Networks (PON) topologies have been the basic architectures emerging as low cost solutions for optical communication systems. However, significant fluctuations in the traffic load due to the mobility of the users in the radio cells require dynamic allocated capacity solutions to avoid any waste of capacity and keep reduced costs. In a fiber optical network incorporating feeding multi-sector antennas, each sector or set of sectors needs to be fed by different wavelengths so higher demand requires a higher number of wavelengths.

2. Dynamic Wavelength Router Description

Fig. 1. Symmetric wavelength router for dynamic capacity allocation in downstream and upstream access links.

CAWGRN=[M+N]inputx[M+N]output
(1)

The extra channels carried by the N output fibers are switched in a foldback configuration to AWG input ports and they will emerge from output ports demanding extra capacity.

Fig. 2. Wavelength plan for down and uplink.

Our system has been tested using an UMTS (Universal Mobile Telecommunications System) broadcast signal as the downstream transmission to M=4 BSs using their own wavelengths (λ1, λ2, λ3 and λ4) and two extra optical channels N=2 (λ5 and λ6) are used to assign extra services (in our case, 155 Mb/s PRBS and CATV signals, respectively) or extra capacity to demanding BS. Moreover, in the uplink, own channels in the different BSs (λ19, λ20, λ21 and λ22) are transmitting a digital PRBS 155 Mb/s and extra wavelengths (λ23 and λ24) carry a single tone at 10 GHz and 2.5 GHz, respectively.

In our experimental setup (see also Fig. 3), we employed a 18×18 cyclic AWG with an FSR of 14.37 nm, with a potentiality of including larger M or N channels, and an optical switch which configuration state provides launching λ523) and λ624) into the AWG in order to emerge from output ports corresponding to demanding BSs (1–4). In our demonstrator, the number of input and output ports required in the switch, PSWITCH, are 2 and 5, respectively, which can be easily obtained from the expression:

PSWITCH=[N]INx[M+N1]OUT
(2)

according to cyclical AWG response, which is significantly reduced from other proposals where a large number of equivalent switches 2×2 are required [4

4. T. Koonen, K. Steenbergen, F. Janssen, and J. Wellen, “Flexibility Reconfigurable Fiber-Wireless Network using wavelength routing Techniques: The ACTS Project AC349 PRISMA,” Photon. Netw. Commun. 3, 297–306 (2001). [CrossRef]

].

Fig. 3. Experimental setup.
Fig. 4. Detail of the BSs.

Figure 4 shows the detail of a BS including the AWG to separate downstream channels going to different BSs and, also unmodulated upstream wavelengths. The number of channels in the AWG located in each BS can be obtained as CBSAWG=2CRNAWG, since the free spectral range, FSR, of cyclic AWG response satisfies the following condition:

FSR=(M+N)·ΔλAWGRN
(3)

where ΔλRN AWG is the AWG channel spacing. We employed a 1×40 AWG with a larger number of ports than required for availability reasons. In our experimental case, just six optical filters would be needed. These channels are modulated by an electro-optic modulator (EOM) and finally launched back to the CS across the port 3 of a circulator simplifying maintenance and costs as corresponds to a remote equipment architecture. Upstream channels are demultiplexed in an AWG in the CS, as shown in Fig. 1. Note that all AWG’s are thermally controlled and stabilized to the ITU grid.

3. Experimental Results

3.1 Dynamic capacity allocation

As explained above, the variable capacity assignment to different BSs consists of different routing scenarios for different optical wavelengths in the way that higher demand from a given BS implies that a larger number of wavelengths are being routed to that BS. In our experiment employing 2 extra wavelengths, three different scenarios have been implemented corresponding to 2 extra optical channels in BS1 (scenario 1), 1 extra in BS1 and the other one in BS3 (scenario 2), and 2 extra channels in BS3 (scenario 3). Numbering the optical input ports in the switch (1–2) corresponding to λ5 and λ6 AWG output ports, respectively and output ports (1–5) which will be the foldback AWG input ports, Fig. 5 shows the switch configuration to achieve the described scenarios, since the optical wavelengths in BSs 1 to 4 depend on the input ports in the AWG.

Fig. 5. Optical spectra in BS1, BS3 and BS4 for different switching configurations (left) to provide reconfigurable capacity assignment.

Figure 5 shows the optical spectra in the BSs 1, 3 and 4 for the three different scenarios described above including down and upstream channels with a symmetric channel distribution for both directions as corresponds to the AWG periodical response. Scenario 1 and 3 are complementary in the sense that all variable capacity is located either in BS1 or BS3. However, BS4 keeps unvariant the capacity independently to the changes in the rest of the system and therefore, similar optical spectra have been measured for all scenarios. In all the cases, the crosstalk level is lower than 35 dB when all wavelengths are used due to the cascade of AWGs incorporated in the experimental setup. Insertion losses of the wavelength router are below 3 dB and 10 dB for own and extra downstream channels, respectively, whereas losses for upstream channels are significantly increased up to 6 and 20 dB, respectively, and must be precompensated in the CS.

3.2 Impact on the signals degradation (own channels)

Once the signal routing has been correctly demonstrated, the performance of the router must be tested, firstly, to verify the acceptable performance and secondly, that this is independent of the routing configuration. Figure 6 shows the signal degradation of the BSs own channels for (a) downstream transmission (QPSK signal in UMTS system) and (b) upstream signal (155 Mb/s digital signal) compared to the back to back signal. Degradation of the QPSK signal in all BSs has been measured as being very low, since EVM factor keeps below 1.2%, fully satisfying 3GPP standard requirements and, therefore, shows an acceptable I/Q constellation diagram and spectrum emission mask (insets correspond to measurement in BS1). The differences obtained between the EVMs in each BS are mainly due to the mismatch in optical polarization of each wavelength in the EOM.

The bit error rate (BER) has been measured for the 155 Mb/s PRBS signals upstream traffic coming from the BS1 (λ19) and BS3 (λ21) to the CS. The B2B of each BS is plotted with its corresponding upstream optical signals. We can observe a power penalty lower than 1 dB (BER of 10-12) as shown in (b), corresponding to acceptable eye diagrams (inset corresponds to signal carried by λ19) for both optical carriers. The BER differences between both BSs are due to the different EOMs used in each BS.

Fig. 6. Own channels signal degradation: (a) EVM for QPSK downstream channels in BS 1, 3 and 4. (b) BER for upstream channels launched from BS1 and 3.

In the following, the degradation of the own channels for downstream and upstream signals has been checked to be independent of the capacity distribution scenario, as shown in Fig. 7. EVM factor of QPSK signal carried by λ3 (downlink) has been measured in BS3 under 3 different scenarios and no significant differences have been found, and BER for the digital signal transported by λ21 (uplink) has been measured in the CS with power penalty differences below 0.5 dB at 10-12 bit-error-rate.

Fig. 7. Signal degradation for the down and uplink own channels for different scenarios: (a) λ3 (b) λ21.

3.3 Impact on the signals degradation (extra channels)

In this section, the degradation of the extra channels has been measured and also checked its independency of the routing scenario. Figure 8 shows the degradation of downstream extra channels -digital signal carried by λ5 in a), and analog CATV signal transported by λ6 in b)-measured on the BSs where they have been assigned. For the first one, penalty differences under 0.1 dB are found for a given BER value, and for the analog signal, lower fluctuations on the optical power are found for a fixed SNR level.

Figure 9 shows the experimental results corresponding to the degradation of upstream extra channels. In this case, SNR has been measured since a single tone has been transmitted by (a) λ23 (10 GHz-tone) and (b) λ24 (2.5 GHz-tone), and no significant differences have been found between different configurations.

Fig. 8. Impact of different system configurations on the signal degradations for the extra downstream channels: (a) λ5 (b) λ6.
Fig. 9. Impact of different system configurations on the signal degradations for the extra upstream channels: (a) λ23 (b) λ24.

The system was tested under all possible configurations but no significant differences with the presented scenarios were found as expected due to the passive characteristic of the router. According to expressions (1), (2) and (3), the scalability of the system can be achieved.

4. Conclusions

Acknowledgements

The authors wish to acknowledge the national project TEC 2005-08298-C02-01 (ADIRA) funded by the Ministerio de Educación y Ciencia and also the “Ayuda Complementaria para proyectos de I+D+I” ACOMP2007/207 supported by the Generalitat Valenciana.

References and links

1.

G. H. Smith, D. Novak, and C. Lim, “A Millimeter-Wave Full-Duplex Fiber-Radio Star-Tree Architecture Incorporating WDM and SCM,” IEEE Photon. Technol. Lett. 10, 1650–1652 (1998). [CrossRef]

2.

H. Ramanitra, P. Chanclou, Z. Belfqih, M. Moignard, H. Le Bras, and D. Schumacher, “Scalable and Multi-Service Passive Optical Access Infrastructure Using Variable Optical Splitters,” in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, Technical Digest (CD) (Optical Society of America, 2006), paper OFE2. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2006-OFE2

3.

T. Koonen, “Fiber to the Home/Fiber to the Premises:What, Where, and When?,” Proc. of the IEEE 94, 911–934 (2006). [CrossRef]

4.

T. Koonen, K. Steenbergen, F. Janssen, and J. Wellen, “Flexibility Reconfigurable Fiber-Wireless Network using wavelength routing Techniques: The ACTS Project AC349 PRISMA,” Photon. Netw. Commun. 3, 297–306 (2001). [CrossRef]

5.

B. Ortega, J. Mora, G. Puerto, and J. Capmany, “Flexible Capacity Assignment in a Multiwavelength Radio over Fiber Access Network,” in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper JWA88. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2007-JWA88.

OCIS Codes
(060.4230) Fiber optics and optical communications : Multiplexing
(060.4250) Fiber optics and optical communications : Networks
(060.4510) Fiber optics and optical communications : Optical communications

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: July 13, 2007
Revised Manuscript: October 2, 2007
Manuscript Accepted: October 22, 2007
Published: December 3, 2007

Citation
Beatriz Ortega, José Mora, Gustavo Puerto, and José Capmany, "Symmetric reconfigurable capacity assignment in a bidirectional DWDM access network," Opt. Express 15, 16781-16786 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-25-16781


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References

  1. G. H. Smith, D. Novak and C. Lim, "A Millimeter-Wave Full-Duplex Fiber-Radio Star-Tree Architecture Incorporating WDM and SCM," IEEE Photon. Technol. Lett. 10, 1650-1652 (1998). [CrossRef]
  2. H. Ramanitra, P. Chanclou, Z. Belfqih, M. Moignard, H. Le Bras, and D. Schumacher, " Scalable and Multi-Service Passive Optical Access Infrastructure Using Variable Optical Splitters," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, Technical Digest (CD) (Optical Society of America, 2006), paper OFE2. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2006-OFE2
  3. T. Koonen, "Fiber to the Home/Fiber to the Premises:What, Where, and When?," Proc. of the IEEE 94, 911-934 (2006). [CrossRef]
  4. T. Koonen, K. Steenbergen, F. Janssen and J. Wellen, "Flexibility Reconfigurable Fiber-Wireless Network using wavelength routing Techniques: The ACTS Project AC349 PRISMA," Photon. Netw. Commun. 3, 297-306 (2001). [CrossRef]
  5. B. Ortega, J. Mora, G. Puerto, and J. Capmany, " Flexible Capacity Assignment in a Multiwavelength Radio over Fiber Access Network," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper JWA88. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2007-JWA88.

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