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
Network operators aim to provide a broadband access network to facilitate the next generation telecom services, such as cellular communications or wireless local area networks. Fibre access networks are very attractive to accomodate this growth in bandwidth demand, as has been demonstrated in previous papers [1–3], since the optical fiber is employed to transport and distribute multiservice signals showing transparency and flexibility.
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.
Recent technological advances such as wavelength division multiplexing, dynamic wavelength assignment, development of agile optical component modules and survivable networking have extended the capabilities of the basic Broadband Passive Optical Network (B-PON) described in ITU-T standard G.983.1. The use of flexible wavelength routers to assign different number of wavelengths to each base station (BS) depending on the actual demand has been previously proposed but using a large number of components with associated costs . A compact alternative has been recently proposed for the access downlink with a significantly reduced number of components , but the requirement of temporarily higher bandwidths for upstream traffic was still to be discussed and demonstrated.
In this paper we demonstrate symmetric variable capacity allocation from the central station (CS) to different BSs in bidirectional optical networks using a compact low cost routing architecture. The system has been fully tested with two extra optical channels for each up and downstream links by measuring the signal degradation of different services transmitted under different scenarios of capacity distribution throughout the networks. Moreover, the proposed architecture includes the transmitters in the CS and therefore, maintenance and costs are reduced, in addition.
2. Dynamic Wavelength Router Description
The reconfigurable capacity is symmetrically assigned for downstream/upstream links to/from different parts of the access network by means of a flexible wavelength router based on an optically switched foldback cyclic Arrayed Waveguide Grating (AWG). Figure 1 shows a schematic of the experimental demonstrator, where all optical sources are located in the CS and remote node (RN) includes the reconfigurable wavelength router which feeds each BS with a number of wavelengths depending on the actual demand. In the CS, the externally modulated downstream channels and also, the unmodulated optical carriers for the uplink are launched into the network and across it, they reach port 1 of the AWG in the RN. The AWG demultiplexes all wavelengths in the network and routes one own channel per BS. Moreover, the configuration state of the optical switch provides launching extra channels back into the AWG in order to emerge from output ports corresponding to demanding BSs (1–4).
Upstream channels use the next upper spectral period of the AWG respect to the downstream channels (see Fig. 2) and according to the AWG periodical response, a given BS will be reached by its own downstream channel and also the corresponding one in the upstream band. They are modulated in the corresponding BS by the upstream traffic.
The scalability of the proposed router is described in the following discussion. Consider an access network where total capacity must be dynamically allocated to M BS’s, assuming that each BS has one own optical wavelength for assuring a unit capacity channel, the total number of own channels for downlink Nown down and uplink Nown up satisfy M=Nowndown=Nownup. On the other hand, all the BS’s have the possibility of increasing their total bandwidth by a number of extra channels for the downlink and uplink, Nextradown and Nextra up, respectively, satisfying N=Nextra down=Nextra up in a symmetric approach. The number of ports CRN AWG required by the AWG in the RN is given by:
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.
Symmetry in the capacity assignment for both link directions is due to the periodical AWG response since if an extra wavelength is assigned to a given BS for downstream transmission, the corresponding optical channel shifted up by a FSR is available in the same BS to be modulated by the upstream signal.
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 λ5 (λ23) and λ6 (λ24) 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:
according to cyclical AWG response, which is significantly reduced from other proposals where a large number of equivalent switches 2×2 are required .
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:
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.
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.
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.
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.
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.
In this paper we have presented a reconfigurable and flexible optical router to provide dynamic capacity allocation in optical access networks by means of wavelength routing using a reduced number of components. All the optical sources are located in the CS to permit an efficient capacity distribution and system maintenance simplification. By using the cyclical response of the AWG and an optical spatial switch in a foldback configuration, optical carriers can be symmetrically redistributed for both uplink and downlink following the actual demand on variable capacity or services. The wavelength reuse in this system allows to satisfy dynamic changes in the demand keeping the reduced costs. In this sense, the paper also includes some expressions to address the required number of ports and components as a function of the number of channels and BSs in the network.
Our system has been successfully tested for different services: analog signals such as CATV, 2.5 and 10 GHz tones and digital signals such as 155 Mb/s PRBS and UMTS signals. To incorporate variable capacity, two extra optical carriers for each direction link have been used, though this number can be increased according to the network requirements. Own optical channels in all the BSs show low signal degradation and are also independent of the variable capacity scenarios, similarly to what happens to the extra channels QoS. Therefore, the full experimental test performed in this work allows to assure the good performance of the router for any capacity distribution under dynamic changes in the demand.
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
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