Ultra low power optical feeder for VDSL
Sandra E. M. Dudley, Terence Quinlan, Ian D. Henning, Stuart D. Walker
Electronic Systems Engineering department, University of Essex, Wivenhoe Park, Colchester, Essex, CO4 3SQ, Essex, U.K..
sdudle@essex.ac.uk.
R.P. Davey D.B. Payne and I. Boyd
BT, Adastral Park, Martlesham Heath, Ipswich, IP5 3RE, Suffolk, U.K.
russell.davey@bt.com
Abstract: We report a novel optical fiber system to feed street-based VDSL cabinets. Power
requirement of 165mW is almost 90% lower than conventional systems and can be powered from
the customer end along twisted copper pair.
2005 Optical Society of America
OCIS codes: (060.0060) Fiber optics and optical communications (250.0250) Optoelectronics; (140.0140) lasers;
1. Introduction
Demand for broadband access to the customer’s home has indeed exceeded expectations, driven by applications
such as fast internet access, video on demand etc. However the key factor is the cost to the provider. The use of
wireless access is an attractive option offering potentially low installation costs; but this is unlikely to offer
sufficient capacity to cope with the increasing demand for bandwidth. While optical fiber to the home would
provide the ultimate high bandwidth access solution, copper based transmission over unshielded twisted pair (UTP)
is expected to play a key role for many years [1, 2]. In recognition of this, Asymmetric Digital Subscriber Line
(ADSL) technology has been developed to fully exploit the installed copper pairs from a telephone exchange to a
subscriber’s premises. Using suitable modulation techniques, broadband can be provided at data rates which depend
on the quality of the line and the path length between the subscriber and the exchange. A further extension of this,
Very high data rate Digital Subscriber Line (VDSL) technology, provides higher bandwidths by using copper pairs
electrically driven from fiber-fed distribution points closer to the user. Recognising that copper based access
systems are here to stay for the foreseeable future, broadband providers are currently considering ways to minimize
their overhead expenditure in the cabinet/distribution points to deliver enhanced bandwidth at a much lower cost.
In this paper, the authors present a simple interface with < 200mW power dissipation using a laser
photodiode combination with a hybrid transformer splitter scheme that may be utilized in the cabinet/distribution
point. Traditional VDSL approaches have power consumption per port of ~1-1.5W [3, 4]. Employment of passive
components such as transformers in a magic-tee formation, used to separate downstream and upstream channels for
differential lines, makes this system not only low powered and simple to install, but also modulation agnostic. Thus
it could be utilized in both Carrierless Amplitude Modulation (CAP) and Discrete Multitone (DMT) systems. A
further advantage of this arrangement is that the devices in the cabinet are powered from the customer premises end
(CPE) using 60V-DC. This makes the cabinet/distribution point self-sufficient which aids considerably as providers
are constantly seeking to lower OPEX throughout their networks.
2. Experiment
The experimental arrangement is shown in Fig. 1. The overall operation of the system is as follows. Downstream
electrical signals from a CO (Central Office) modem are used to analogue modulate a 1310nm laser. The optical
signal is transmitted over optical fiber via a low cost 1310nm/1550nm diplexer, to the local distribution point where
another 1310nm/1550nm diplexer is used to select the 1310nm signal for detection by a photodiode. The resulting
electrical signal is delivered using a low power amplifier to the customer premises end (CPE) modem over twisted
pair. In the reverse path the CPE modem remotely modulates a 1550nm laser at the distribution point sending the
optical signal back via the 1310nm/1550nm diplexers to the CO. The hybrid circuits shown in the diagram perform
electrical separation/isolation of upstream and downstream components. In Fig. 1, a Sunset MTT Ethernet tester
connected to a CO ASM-61 CAP VDSL 10Mb/s full duplex modem which is in turn linked to the first hybrid
transformer splitter. The hybrid splitter is a passive element and consists of two 1:1:2 (T2-613-1-kk81 Mini-Circuit)
transformers. The transformer gives ideal impedance matching on all the inputs and output ports [5]. The CO
modem has a 100 impedance (Zin) and the transformer is balanced (Zbal) to match this. The splitter is an updated
embodiment of the classical magic-tee circuit in a 2 to 4 wire repeater set-up. This structure has a higher loss than
the conventional magic-tee at 6dB per output port but is ideal for differential transmission lines and has much
improved isolation properties [6]. Baluns could be an alternative solution at this point, but would be more expensive
and defeat the ideal of a low-cost solution.
18km monomode fiber
1300nm transmit
Eth.
test
CO
Mod
S
1km twisted pair
S
Hybrid
Hybrid
Eth.
test
CPE
Mod
1550nm return
1550nm/1310nm splitters
Fig. 1. Hybrid system set-up
A schematic of the hybrid board is shown in Fig. 2. Both the CO and CPE hybrids are identical in
characteristic. An electrical signal from a CO modem arriving at port S experiences a total loss of 6dB in each
direction as it is divided in two by both transformers. A single ended signal is required to drive the laser, thus only
one arm of port 1 on the hybrid output is used. The other arm is grounded through a 50 resistor, maintaining the
overall balance of the splitter. The resulting CO signal power at port 1 following the transformer is -27dBm. This is
directly applied through a DC-6GHz, (5722 Picosecond Pulse Labs) bias-tee to a Fabry-Perot 1310nm laser
(Cambridge Optical Sciences, Suffolk, UK). Since the input electrical signal is split two ways, the other half
appears at port 2 of the hybrid. This signal is isolated by the upstream detecting photodiode described presently and
so has no adverse effect. The 1310nm optical signal (-6dBm) travels through the wavelength dependant couplers
and 18km of single mode optical fiber down to the hybrid at the cabinet/distribution point
1310nm/1550nm
FP lasers, Port 1
Transformer splitters 1:1:2
Zport S = Zbal = Zport 1 = Zport 2
DC blocking
capacitors
Wall to 60V
DC PSU
Line balance
Zport S = Zbal
Signal in/out
Port S
Transimpedance
Amplifier
Photodiode Port 2
Fig. 2. Hybrid board components
RF blocking
Inductors
CO
Mod
37-72V DC-DC
Converter
3.3V output
Hybrid
DC Blocking
Capacitors
Fig. 3. DC-biasing arrangement from customer premises
The optical signal is collected in the cabinet using a 1310/1550nm 500m active diameter photodiode
(Hamamatsu G8376-05). As bandwidth requirement is low, a large area photodiode can be used which has the
advantage of allowing a higher photocurrent to drive the following circuitry. Output from the photodiode drives a
transimpedance amplifier (Maxim MAX3665) with differential outputs that are directly connected to the transformer
splitter. The electrical signal is connected to port S of the transformer which feeds the copper pair to the CPE
modem. Incoming signals from either modem on the hybrid achieve isolation of up to ~40dB between the ports 1
and 2 with suitable balancing, due to the fact that transformers at those ports are in anti-phase, thus signals are
effectively cancelled on the opposite port from which they enter. The signal from CPE to CO follows a similar path
but uses 1550nm for the upstream optical connection, so creating a complete, isolated upstream/downstream system.
Direct biasing of the unit in the cabinet/distribution point is via a DC supply from the CPE, as shown in Fig. 3. 60V
DC is transmitted along 1km of twisted pair to a 36-72V DC-DC converter (Excalibur PT4662A) to produce a
+3.3V for the cabinet. An inductor and capacitor network is required at both the inputs of the DC supplies and
modems to mutually isolate analog and DC signals respectively.
3. Results
Ethernet testers were used to drive full-Duplex 10Mb/s transmission between the modems. The CO laser was found
to give the greatest modulation depth at a bias current of 24mA. The signal level injected from the CO splitter was -
27dBm. The laser was impedance matched to 50 giving an optimized modulation index (MI) of 0.8%, which was
found necessary to cope with signals having large peak to mean ratios such as CAP and DMT [6]. Figs. 4a and b
show both (a) the CO downstream signal after 18km of monomode fiber at CPE hybrid photodiode with an optical
power of -17dBm on the photodiode, and (b) the CPE upstream signal after 1km UTP and 18km of fiber at CO
hybrid end with an optical power of -16.3dBm. The higher frequency roll-off in the upstream signal is caused by the
frequency response of the twisted pair. CO and CPE signals had Signal to Noise Ratio’s (SNR’s) of approximately
40dB and 30dB after their respective photodiodes, as may be observed in Figs. 4a and b respectively. From Fig. 4b
the downstream signal can be almost observed rising above the noise floor at frequencies below upstream signal.
There is 30dB isolation between downstream and upstream signal showing the appropriate impedance balancing of
the splitter. Optical signals were attenuated and a margin of approximately 7dB was found in both directions before
bit errors of ~10e-6 could be observed, however synchronization with 10Mb/s full duplex transmission between
modems was still maintained as modem error correction came into force. For complete synchronization loss, a drop
of 12dB in optical power was required. The largest system loss occurred over the 1km UTP, but improved SNR and
optical margins could be achieved, using lasers with greater modulation depth and larger area photodiodes in the
hybrid. Devices in the cabinet were biased from the 3.3V DC-DC converter drawing a current of ~50mA and gave
an overall power consumption of ~165mW per board, thus making this scheme one of the lowest powered systems
for long reach VDSL applications yet reported. Over shorter distances, using 100m of twisted pair, photodiode bias
could be removed and short circuit photocurrent was sufficient to make the system function, thus reducing the
cabinet power consumption all the more. This of course did reduce the transmitted distance but is still suitable drop
wire lengths up to 100m.
Fig. 4a. CO downstream signal after CPE hybrid Photodiode
Fig. 4b. CPE Upstream signal after 1km of twisted pair and 18km of fiber
4. Conclusions
The modulation agnostic system described has one of the lowest values of power/port reported for long reach VDSL
applications. It could be used for both for high bandwidth DMT and CAP systems. The system was also
demonstrated over short distances of UTP up to 100m but with the photodiodes zero biased. This restricted the
operational distance, but allowed for even lower power consumption over drop wire operation. The system
described has elegance, simplicity and utilizes passive components such as the hybrid splitters with low power
amplifiers for transmission and receiving. Although used with 10Mb/s modems it is not data rate selective as all
components mentioned are currently used in GHz schemes. With appropriate adjustments, this scheme above could
be easily implemented into a WDM network. The authors are currently looking into CWDM 1310/1550nm and
bidirectional transceivers (BiDi’s) with individual wavelength section for low cost WDM networks.
References
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