U.S. patent application number 10/799236 was filed with the patent office on 2004-09-16 for telecommunications system.
This patent application is currently assigned to MICROWAVE PHOTONICS, INC.. Invention is credited to Moodie, David G., Westbrook, Lester D..
Application Number | 20040179852 10/799236 |
Document ID | / |
Family ID | 27237747 |
Filed Date | 2004-09-16 |
United States Patent
Application |
20040179852 |
Kind Code |
A1 |
Westbrook, Lester D. ; et
al. |
September 16, 2004 |
Telecommunications system
Abstract
An optical communications network includes a terminal which can
simultaneously receive and modulate an optical signal. The terminal
includes an optical modulator which is controlled by varying the
bias voltage applied to it.
Inventors: |
Westbrook, Lester D.;
(Southham, GB) ; Moodie, David G.; (Ipswich,
GB) |
Correspondence
Address: |
MORRISON & FOERSTER, LLP
555 WEST FIFTH STREET
SUITE 3500
LOS ANGELES
CA
90013-1024
US
|
Assignee: |
MICROWAVE PHOTONICS, INC.
Los Angeles
CA
|
Family ID: |
27237747 |
Appl. No.: |
10/799236 |
Filed: |
March 11, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10799236 |
Mar 11, 2004 |
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10243838 |
Sep 16, 2002 |
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6731880 |
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10243838 |
Sep 16, 2002 |
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09051147 |
Apr 2, 1998 |
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6525855 |
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09051147 |
Apr 2, 1998 |
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PCT/GB97/01958 |
Jul 21, 1997 |
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Current U.S.
Class: |
398/183 |
Current CPC
Class: |
G02F 1/0157 20210101;
H04B 10/25758 20130101; H04B 10/29 20130101 |
Class at
Publication: |
398/183 |
International
Class: |
H04B 010/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 19, 1996 |
EP |
96305315.2 |
Nov 29, 1996 |
GB |
9624909.9 |
May 20, 1997 |
GB |
9710394.9 |
Claims
1. A method of communicating between a first node and one or more
further nodes in an optical communications system, the method
comprising: i) receiving at the first node, an optical signal
transmitted from a first further node over an optical fibre link;
ii) detecting, at the first node, the optical signal using an
electro-absorption modulator and producing thereby an electrical
signal representing information carried by the optical signal; and
iii) imposing, using said electro-absorption modulator, an
information-carrying modulation onto the received optical signal
and feeding the thus modulated optical signal into an optical fibre
link to transmit it to a second further node; wherein step ii and
step iii are carried out with the electro-absorption modulator
subject to the same DC operating conditions.
2. A method as claimed in claim 1, wherein step ii and step iii are
carried out simultaneously.
3. A method as in claim 1, wherein said first further node and said
second further node are a single node.
4. A method as claimed in claim 1, wherein said electrical signal
is an RF or microwave signal.
5. A method as claimed in claim 4, wherein the electrical signal is
applied to an antenna.
6. A method as claimed in claim 1, wherein in step iii said
information-carrying modulation is an RF or microwave signal.
7. A method as claimed in claim 6, wherein said
information-carrying signal is an RF or microwave signal provided
from an antenna.
8. A method as claimed in claim 7 wherein the electrical signal is
applied to an antenna, the antenna to which the electrical signal
is applied and the antenna which provides said information carrying
signal are constituted by a single antenna.
9. A method as claimed in claim 7, wherein the electrical signal is
applied to a single antenna.
10. A method as claimed in claim 9, wherein the base station, in
use, supports the GSM and DECT systems.
11. An optical communications network includes a remote terminal
characterised in that said terminal comprises an electro-absorption
modulator that i) detects an incoming optical signal from a first
portion of the optical communications network whilst simultaneously
ii) modulating said incoming optical signal, said optical signal
being transmitted to a further portion of the optical
communications network.
12. An optical communications network according to claim 11,
wherein the optical communications network uses a frequency
division multiplexing system.
13. A bi-directional optical-electrical signal transducer
comprising: an electro-absorption modulator having optical signal
input and output ports and an electrical signal input/output port,
whereby: first information-bearing optical signals presented to
said optical signal input port produce corresponding first
information-bearing electrical signals at said electrical signal
input/output port, and second information-bearing electrical
signals presented to said electrical signal input/output port
produce corresponding second information bearing optical signals
modulated onto an optical signal at said optical signal output
port.
14. A bi-directional optical-electrical signal transducer as in
claim 13 wherein said electro-absorption modulator functions
simultaneously and at the same DC operating bias conditions to
produce said output optical and electrical signals.
15. A bi-directional optical-electrical signal transducer as in
claim 13 further comprising at least one RF antenna electrically
connected to said electrical signal input/output port.
Description
[0001] The present invention relates to optical communications
systems, terminals for use in systems, to optical links including
such terminals, to cellular and radio distribution points and base
stations and in particular, but not exclusively, to radio and
microwave systems, including cellular radio systems, including such
links.
[0002] It is known that one of the principal problems that must be
addressed before optical fibre networks can be extended from trunk
networks to the local loop is the cost and complexity of the remote
terminals. These terminals need to be able to both receive and
transmit optical signals. Most solutions to this problem use lasers
as optical transmitters but this necessitates the use of control
circuitry which increases the cost, complexity and electrical power
consumption of the terminals.
[0003] A similar problem, although with a slightly different
application, exists for optical fibre feeds to antennas for
cellular and other radio and microwave systems where the cost and
power consumption of terminal equipment used to supply signals to
remote antennas are important factors. "Antenna remoting" as it is
known is of particular interest for cellular and satellite systems.
The frequencies involved are typically in the hundreds of Mhz to
tens of GHz range. There is also interest in yet higher frquencies,
for example for radar. While, strictly speaking, this range extends
into what is more properly known as the millimetre wave band,
throughout this specification the expression "radio or microwave"
has been used loosely to cover this wider range. Thus, unless the
context clearly requires otherwise, the term should be read in a
broad way to include the millimetre wave band.
[0004] Embodiments of the present invention seek to solve, at least
in part, these problems.
[0005] In the paper by Frigo et al, "A wavelength division
multiplexed passive network with cost-shared components", IEEE
Photonics Technology Letters, 1994, volume 6, pp. 1365-1367, it is
proposed that subscriber terminals in a passive optical network
(PON) each be provided with an optical modulator in place of the
conventionally proposed laser. A single, cost-shared tunable laser
is provided in the central office which feeds the PON and this is
wavelength stepped through the different wavelengths of the various
optical network units (ONUs) which feed the subscribers connected
to the PON. In each subscriber's ONU, the laser light received from
the shared laser is split by a passive tap, with a portion of the
light being detected by a receiver. The remainder is "looped-back"
towards the central office through the modulator. The relevant
subscriber uses the modulator to modulate the time-slice of light
received from the shared laser. The up-and-down-stream signals can
be separated at the central office by time partitioning,
wavelength, sub-carrier frequency, format, modulation depth,
gating, coding, etc. The preferred separation technique involves
the use of radio frequency sub-carrier modulating the downstream
(from the central office) data. As described, half of the packet of
light in each ONU's laser timeslot is modulated at the central
office, the other half is modulated at the ONU to provide the
upstream data link. The same RF sub-carrier frequency is used for
the up and down stream signals. Use of the technique for the
transmission of video on demand signals is described. The nature of
the modulator used in the ONU is not revealed.
[0006] Wood et al describe, in "Bidirectional fibre-optical
transmission using a multiple-quantum-well (MQW)
modulator/detector", Electronics Letters, 1986, volume 22, pp.
528-529, a bidirectional optical fibre transmission system in which
one end of the fibre link has an MQW modulator in place of the more
conventional laser and detector pair. At the other end of the
single-fibre was a laser, and a beam-splitter was used to divert
the return optical signal to an avalanche photodiode receiver. To
send data from the MQW modulator to the laser site, the laser was
operated quasi-CW and the modulator, which included a gold mirror
and was operated in reflection mode, used to intensity modulate the
reflected light. In the opposite direction the laser was directly
modulated and the MQW modulator was used as a photodetector. While
no changes were made to the optical system to achieve reversal of
the direction of information flow, it was necessary both to
re-arrange the electrical drive components and to modify the DC
biases of the laser and of the MQW device. It was thus not possible
to have a fully duplex operation (i.e. simultaneous transmission in
both directions). Half-duplex operation would have required some
electrical switching and bias adjustment function to control the
bias level and to effect the re-arrangement of the circuit in
synchronism with the half duplex rate. In fact, no such electrical
control arrangement is suggested in the paper.
[0007] R B Welstand, et al, describe in "Dual-Function
Electroabsorption Waveguide Modulator/Detector for Optoelectronic
Transceiver Applications", IEEE Photon, Tech. Lett. Vol.8, No.11,
pp 1540-1542, a bulk (non-MQW) electroabsorption modulator device
which is useful both as a modulator and as a detector. The device
is referred to as an optoelectronic transceiver. A suggested
application of the device is in antenna remoting. Again, the
transceiver requires an adjustable dc electrical bias to switch
from modulator to detector operation. It is explained that the
transmit mode and the receive mode can be remotely switched by
control circuitry which can adjust the dc electrical bias with a
switching time limited by the associated electronics. Separate
experiments were performed to determine the optimum performance of
the device as a modulator and as a photodetector. In the modulator
assessment, different bias levels were used depending upon the type
of performance required. Optimum bias levels of 2.0 and 2.93 volts
were found. It was also explained that, in order to maintain high
suboctave and multioctave spurious-free dynamic range modulator
performance over temperature, active modulator bias control was
required. In the detector experiments, the device was biased at 7.0
volts. There is no hint or suggestion that the device could provide
both funtions simultaneously. Thus, again it is clear that full
duplex operation was not possible.
[0008] The present inventors have discovered that it is possible to
provide good performance over both transmission directions in an
optical link in which an optical modulator is used both as a
modulator and as a detector even when the modulator is subject to
the same DC operating conditions in both operational modes. That
is, as distinct from the above-identified teachings, systems
according to the present invention do not need complex electrical
bias control. Indeed, for very many commonly available modulators,
perfectly acceptable system performance can be achieved with no
bias at all, that is with zero bias.
[0009] The fact that a single bias level can be used for
transmission in both directions makes possible full duplex
operation--that is, simultaneous transmission in both directions.
Of course, the invention also has application to systems which are
not, or are not run, full duplex. The advantages of simplicity,
compactness and low-cost of terminal equipment, which all follow
from the invention, are all equally useful in systems run
half-duplex, e.g. ping-pong (time-division multiplex) systems.
Again, for the many installations which can be run without
electrical bias in the remote terminal, the fact that no local
power supply is needed is a tremendous benefit. This is especially
true in the field of antenna remoting.
[0010] Accordingly, in a first aspect the present invention
provides a method of communicating between a first node and one or
more further nodes in an optical communications system, the method
comprising;
[0011] i) receiving at the first node, an optical signal
transmitted from a first further node over an optical fibre
link;
[0012] ii) detecting, at the first node, the optical signal using
an electro-optic device and producing thereby an electrical signal
representing information carried by the optical signal; and
[0013] iii) imposing, using said electro-optic device, an
information-carrying modulation onto the received optical signal
and feeding the thus modulated optical signal into an optical fibre
link to transmit it to a second further node; wherein step ii and
step iii are carried out with the electro-optic device subject to
the same DC operating conditions. Preferably said first further
node and said second further node are a single node. Preferably the
DC bias voltage applied is zero--that is no DC bias at all is
applied. Preferably, the electrical signal is an RF or microwave
signal.
[0014] According to a second aspect of the invention there is
provided an antenna installation for a radio or microwave
transmission system, the installation comprising: antenna means for
transmission and reception of radio-frequency or microwave signals;
optical input and optical output ports; an optical modulator
arranged in the optical path between the optical input port and the
output port and operatively connected to the antenna means and
arranged in use to receive RF or microwave signals therefrom and to
impose a corresponding modulating onto light received via said
input port and which passes to said output port; and optical
detection means arranged in use to receive optical signals via said
input port and operatively connected to the antenna means so that
RF or microwave modulation present in said optical signals is
coupled out to said antenna means from which it is then
radiated.
[0015] Preferably the optical detector means are provided by the
optical modulator.
[0016] Preferably the optical detector means and the optical
modulator are electrically unbiased in operation.
[0017] Additionally it is preferred that the modulator is an
electro-absorption modulator.
[0018] Further preferably the terminal is sited remotely from a
base station or central office from which it receives optical
signals carrying an RF or microwave component.
[0019] According to a third aspect of the invention there is
provided an optical communications network featuring a remote
terminal characterised in that said terminal comprises an
electro-optic device that
[0020] (i) detects an incoming optical signal from a first portion
of the optical communications network whilst simultaneously
[0021] (ii) modulating said incoming optical signal, said optical
signal being transmitted to a further portion of the optical
communications network.
[0022] Preferably the optical signals are analogue signals. In
addition it is preferred that the optical communications network
uses a frequency division multiplexing system.
[0023] According to a fourth aspect of the invention there is
provided a method of operating an optical communications network,
the method being characterised in that an optical signal is
respectively received and modulated simultaneously by controlling a
single electro-optic device.
[0024] The invention will now be described, by way of example only,
with reference to the following drawings in which:
[0025] FIG. 1 is a schematic depiction of an optical communications
network featuring a terminal according to the invention and a
remote antenna;
[0026] FIG. 2 is a schematic depiction of an optical communications
network featuring a terminal according to the invention;
[0027] FIG. 3a is a schematic cross section of an
electro-absorption modulator suitable for use in a terminal
according to the invention;
[0028] FIG. 3b is a schematic depiction of a typical input optical
data signal;
[0029] FIG. 3c is a schematic depiction of the electrical data
signals present at the electrical contacts of a terminal according
to the invention;
[0030] FIG. 3d is a schematic depiction of a typical output optical
data signal;
[0031] FIG. 4 is a graph of fibre to fibre transmission for a
suitable electro-absorption modulator (such as that shown in FIG.
2) alongside the first three derivatives of the transmission
function with respect to bias voltage;
[0032] FIG. 5 is a graph showing the variation of insertion gain
and carrier intermodulation with respect to bias voltage;
[0033] FIG. 6 is a schematic depiction of an optical communications
network featuring a plurality of terminals according to the
invention;
[0034] FIG. 7 is a schematic depiction of an optical communications
network featuring a terminal according to the invention and two
modems;
[0035] FIG. 8 is a schematic depiction of an experimental
transmission system in which an EA modulator is used at the remote
terminal;
[0036] FIG. 9 shows schematically two network configurations using
EA modulators at network nodes;
[0037] FIG. 10 shows the uplink and downlink bit error rates (BER)
versus received optical power for the system of FIG. 8;
[0038] FIG. 11 shows BER against Eb/No for the system of FIG.
8;
[0039] FIG. 12 shows the RF spectra of the downlink commposite
signal, comprising QPSK data and satellite TV signals, for the FIG.
8 system;
[0040] FIG. 13 shows schematically an experimental system in which
EA modulators are used in a single fibre, bi-directional optical
bus dleivering 120 Mbit/s QPSK channels;
[0041] FIG. 14 shows BER against Eb/No for the FIG. 13 system;
[0042] FIG. 15 shows the RF electrical spectra obtained at the
output of EAM 2 of FIG. 13;
[0043] FIG. 16 shows BER against the reverse bias of EAM2;
[0044] FIG. 17 shows schematically a pico cell system according to
the invention;
[0045] FIG. 18 is a map of the site of an outdoor demonstration of
the passive picocell showing downlink power levels against
distance; and
[0046] FIG. 19 shows schematically the central control office and a
picocell of a picocell installation.
[0047] FIG. 1 shows an optical communications network including a
terminal 4 according to the invention. The convention that will be
used in the following analysis is that the network path from the
optical input 2 to the electrical contacts 5 of the terminal will
be termed `downstream` and the network path from the electrical
contacts 5 to the optical output 7 will be termed `upstream`. The
fine lines indicate an optical fibre transmission path and the
arrows on the fine lines indicate the direction of signal
propagation along the optical fibre transmission path. The thicker
lines indicate an electrical transmission path.
[0048] An analogue optical signal, consisting of a plurality of
frequency division multiplexed (FDM) data streams, is introduced
into the optical fibre 3 at the optical input 2 by the optical
transmitter 1. The terminal 4 includes an electro-absorption
modulator (EAM), a schematic example of which is shown, for
illustrative purposes only, in FIG. 3. The EAM comprises three main
layers of semiconductor material. The outermost semiconductor
layers 12 and 14 enable electrical contacts to be mounted to the
EAM in order that bias voltages and electrical data signals might
be applied to the device. The active central layer 13 is either a
layer of bulk semiconductor material or multi-quantum-well (MQW)
material, which, as is well known, is formed from many interleaved
layers of different semiconductor materials. In order to receive
and modulate optical signals transmitted at a given wavelength it
is necessary to choose suitable semiconductor materials for use in
the central layer of the EAM. A combination of III-V materials are
often chosen, for example gallium arsenide (GaAs), indium phosphide
(InP), indium gallium arsenide phosphide (InGaAsP), etc. as these
have bandgap energies which are appropriate for the wavelengths
used in optical communications systems.
[0049] An example of an EAM with an active central layer formed
from a single semiconductor layer is found in CK Sun et al, "High
Spurious Free Dynamic Range Fiber Link Using a Semiconductor
Electroabsorption Modulator", Electronics Letters, 1995, 31, pp
902-903. In the case cited the central layer is 350 nm of undoped
InGaAsP, which had a bandgap equivalent wavelength of approximately
1.24 .mu.m.
[0050] A suitable MQW structure for use in an EAM is seventeen 9.5
nm wells of InGaAsP (bandgap wavelength .apprxeq.1.55 .mu.m)
separated by 5.5 nm barriers of InGaAsP (bandgap wavelength
.apprxeq.1.10 .mu.m). Further details describing the construction
of such a device can be found in European Patent EP-B-143000.
[0051] Currently, higher saturation thresholds are available with
bulk rather than MQW modulators, but higher bandwidths may be
achieved with MQW rather than bulk devices. While work is underway
in various research groups to increase the saturation threshold
(and the damage threshold) of MQW modulators, these considerations
may affect choice between the use of bulk and MQW modulators in
different applications of this invention.
[0052] It will be understood that subject to the above proviso
either a suitable bulk EAM or a suitable MQW EAM may be used in the
embodiments discussed below. The following discussion relates to
the MQW EAM described above, but the principles of the operation of
the MQW EAM are the same as for a bulk EAM.
[0053] When an optical signal, transmitted at an appropriate
optical wavelength, arrives at the EAM, a proportion of all its
frequency components is absorbed within the active central layer 13
generating an electrical signal at the electrical contacts 5. The
large arrows in FIG. 3a show the transmission of the optical signal
through the active central layer of the EAM. In FIG. 3b optical
signal 24 is an example of an incoming optical signal. It comprises
an optical carrier frequency 24a and identical sidebands 24b, the
sidebands containing the information that is conveyed by the
optical signal. Typically the optical carrier frequency will have a
frequency of hundreds of gigahertz, the sidebands will have a width
of tens or hundreds of megahertz and the centre of the sidebands
will be separated from the carrier frequency by a few
gigahertz.
[0054] The optical signal is partially detected within the EAM and
an electrical data signal 25 with a substantially similar frequency
spectrum to that of one of the sidebands 24b is generated within
the EAM and passes through the electrical contacts 5. Electrical
data signal 26 is applied to the electrical contacts 5 and causes
the modulation of the carrier frequency 24a. FIG. 3c shows
electrical data signals 25 and 26, as present at the electrical
contacts of the terminal.
[0055] The modulation of the carrier frequency 24a by the
electrical data signal 26 gives rise to sidebands 27a. Sidebands
27a have identical frequency spectra, which are both substantially
similar to the frequency spectrum of electrical data signal 26. The
optical signal at the output of the EAM consists of sidebands 27a,
generated by the modulation of the carrier frequency by the EAM,
and sidebands 27b, which are the residual remnant of sidebands 24b.
Sidebands 27a occupy a frequency band which is intermediate to the
frequency of the carrier frequency 24a and the frequency band of
sidebands 24b such that the two sets of sidebands do not overlap
each other. FIG. 3d shows, schematically, the arrangement of output
optical signal 27 comprising sidebands 27a and sidebands 27b.
[0056] With regard to FIG. 1, electrical data signal 25 passes into
the antenna 6 from which a radio signal(which might be a microwave
signal) 28, carrying substantially the same information as
electrical data signal 25, is radiated. The antenna will also
receive radio signals (which again may be microwave) 29, within a
suitable range of frequencies, which will constitute electrical
data signal 26. Electrical data signal 26 will, at the electrical
contacts, modulate the carrier frequency 24a of the received
optical signal.
[0057] A terminal operating in such a manner can be used in a
cellular radio communications network. For example, an optical
signal containing data within the frequency band 925 MHz to 933 MHz
is transmitted by the optical transmitter 1. The signal contains
320 channels, each consuming approximately 25 kHz of bandwidth.
This signal is detected within the EAM and generates an electrical
data signal 25 that is transmitted as a radio signal 28 by the
antenna 6. The data contained within the radio signal is
substantially the same as the data contained within the optical
signal. Cellular telephones receive the radio signal and use
filtering means to extract the 25 kHz channel applicable to the
individual cellular telephone. The cellular telephone transmits a
signal occupying a 25 kHz frequency band, the signal being
contained within the frequency band 880 MHz to 888 MHz. The antenna
6 receives a radio data signal 29 contained within the frequency
band 880 MHz to 888 MHz which is the combination of the
transmission of all the cellular telephones communicating with the
communications network via the said antenna. The antenna converts
said received radio data signal 29 into an electrical data signal
26 which is used to modulate the optical carrier frequency 24a of
the transmitted optical signal to create an optical signal 27 that
contains data in both the 880-888 MHz frequency band and the
925-933 MHz frequency band. This data propagates along the optical
fibre and is received at the optical detector 8.
[0058] It is possible to extract simultaneously an electrical
signal from or input a second electrical signal to the electrical
contacts. The presence of both input and output electrical signals
in the EAM will give rise to signal inter-mixing. This can lead to
distortion of the downstream and upstream optical signals and will
limit the performance of the EAM. FIG. 4 shows the variation in
fibre to fibre transmission with bias voltage 15 in a suitable EAM
with an MQW active central layer which has a composition as
described above. The first derivative of this transmission function
with respect to bias voltage 16 influences the upstream loss, the
second derivative of this transmission function with respect to
bias voltage 17 influences the upstream-downstream mixing and the
third derivative of this transmission function with respect to bias
voltage 18 influences the distortion of the upstream signal. FIG. 5
shows typical RF insertion loss characteristics for the downstream
link 19 and the upstream link 20 as the bias voltage is altered,
using a 4 mW DFB laser with a wavelength of 1560 nm as the
transmitter. The upstream and downstream losses can be made equal
at a reverse bias of -1.4V, not needed obviously, and the insertion
loss of .apprxeq.42 dB for this case is comparable with the
insertion loss of the laser-detector modules used (without a
modulator) of 39 dB. FIG. 5 also shows the ratio of the upstream
carrier to the third order intermodulation 21, which is influenced
by the third derivative of the modulator transmission function 18.
The maxima in 21, observed for bias voltages between -0.9V and
-2.0V broadly correspond to the minima in 18. It can be observed
that for the EAM used in this example the effects of upstream
distortion (influenced by 18) are more significant than the effects
of upstream-downstream mixing (influenced by 17), therefore it is
advantageous in this example to control the modulator bias voltage
in order to minimise distortion of the upstream signal. It will be
understood that if a different EAM were to be used, there would be
a different transmission characteristic with respect to bias
voltage 15 and this will lead to differing derivatives of that
function with respect to bias voltage (16, 17 and 18). The nature
of these derivatives will determine which of them will have the
greatest effect on the performance of the terminal and thus which
derivative should be controlled by varying the bias voltage for
optimal terminal performance where a bias voltage is to be
applied.
[0059] The advantages of using analogue optical networks for
delivering radio signals from a central location to many remote
antenna sites has long been recognised. In a telecommunications
context, these radio signals may be for fixed radio access,
cordless or mobile networks, but the same principles apply. By
making use of the high bandwidth, low loss characteristics of
optical fibre, all high frequency and signal processing functions
can be performed centrally and the signals can then be transported
over the optical network directly at the carrier frequency. The
remote sites then become very simple, requiring only optoelectronic
conversion, filtering and linear amplification. This shifting of
the complex functionality away from the remote site allows cheap,
reliable, small and lightweight radio access points with low power
consumption to be deployed. Ease of installation coupled with low
maintenance provide compelling arguments for this type of system
architecture. The main telecommunications application area for this
type of system is short range and high capacity wireless systems
where the advantages of small, cheap and low power transceiver
units give the greatest benefit.
[0060] Although the generic radio fibre system architecture
described in the preceding paragraph leads to low power consumption
and low component count in the radio access point, it would be even
more desirable to have zero power consumption. This entirely
passive radio access point would make installation very
straightforward, and give excellent reliability leading to
virtually maintenance-free operation. In this example we show that
an electroabsorption modulator may be used as the single component
in such a radio access point, requiring no amplifiers or power
supply to function effectively as a passive transceiver for
small-sized cells (picocells).
[0061] As described above, the EAM can act as a photodetector for
the downstream path, and as a modulator for the remaining
downstream light for the upstream path. The feasibility of a full
duplex link using a frequency division duplex (FDD) arrangement,
showing that both detection and modulation functions can be enacted
simultaneously, has been described above. In this example we show
that this device will also operate satisfactorily without dc bias
for short range (picocellular) radio systems. In this architecture,
a picocell is envisaged to provide radio coverage for future high
capacity wireless services (e.g. the HIPERLAN standard which will
provide data rates of 10 Mbit/s or more) to a volume no greater
than a single office or room (although as will be seen below,
picocells have application to outdoor and indoor but "non-room"
(e.g. concourse, station, stadium, plaza, etc.) applications where
larger volumes/areas must be servred. At present, these future high
capacity radio systems are still in the research phase. For this
work, therefore, the passive transceiver concept was demonstrated
using a commercial 2.4 GHz spread spectrum radio LAN providing
wireless Ethernet at a raw data rate of 3 Mbps. At this data rate,
mobile video access was feasible and was achieved, and access to
the Internet was obviously no problem.
[0062] The electroabsorption modulator used in this experiment was
designed and fabricated in-house and consisted of a multiple
quantum well absorption layer in a low capacitance buried
heterostructure arrangement, as described by Moodie et al in J
Lightwave Tech, Vol.14, No.9, pp 2035-2043, 1996. The InGaAsP/InP
material system was used for operation around a wavelength of 1550
nm. The chip, with a length of 370 .mu.m, was mounted in a high
frequency package with fibre pigtails, and had a total fibre to
fibre loss of only 5 dB at zero bias. The 3 dBe bandwidth of the
packaged device when operated as a modulator was 14 GHz, although
for this experiment we only require a narrowband response around
2.5 GHz.
[0063] FIG. 8 shows the experimental layout in schematic form. The
radio modems consisted of a BreezeNet (TM) Access Point (AP) at the
central office and a BreezeNet (TM) Station Adaptor (SA) for
connection to the laptop computer located in the picocell. These
modems-operate in the IMS band (2.4-2.48 GHz) and use frequency
hopping spread spectrum to provide good immunity to sources of
interference. The system provides half duplex transmission,
although, as described above, the optical link is not limited to
this arrangement. The AP had twin antennas for spatial diversity,
which could be detached very simply to allow connection to the
optical link.
[0064] For the downstream path, the RF output from the AP was
connected to a commercial analogue laser. The optical signal from
this laser was passed thorugh a polarisation controller (PC) and
across the optical link to the EAM located in the remote picocell.
In this direction, the EAM acts as a photodiode, and the resulting
RF power was then radiated into free-space using an antenna, to the
SA connected to a laptop computer.
[0065] For the upstream path, the RF output from the SA is radiated
into free-space to the antenna connected to the EAM. These RF
signals then modulate the residual optical power that passes
through the EAM, and are then sent back to the central office over
the optical link. The upstream signals are detected using a
commercial photodiode, and are then input to the receiver part of
the AP. The laser and photodiode were designed for analogue
microwave links.
[0066] Results
[0067] The picocell used in this experiment was an office with
dimensions of 6 m.times.3.5 m. The antenna for the passive radio
access point (which was fed with the RF output of the modulator)
consisted of a simple microstrip patch design with a gain of 8 dBi.
This was a bow-tie design with a beam width of around 70 degrees.
The SA was configured to use one antenna for the transmit direction
(2 dBi omnidirectional) and a separate antenna for the receive
direction (8 dBi microstrip patch). This arrangement had the
combined benefits of providing a better power balance for the link
(see below) and also limited the transmit power to below 20 dBm to
ensure compliance with EIRP requirements in Europe for radio LAN
systems.
[0068] Before connecting the radio system, power budget
measurements were performed using a signal source and analyser at a
frequency of 2.5 GHz. Table 1 shows the rf power levels at the
important points throughout this system, when the antennas were
spaced 6 m apart. At this spacing, the free space path loss was 50
dB, which is 6 dB less than calculated when no reflections are
assumed. The optical link RF loss for the downstream path (laser to
electroabsorption modulator) was 40 dB, which was only 5 dB worse
than the loss measured when the photodiode was used in place of the
EAM. The upstream (electroabsorption modulator to photodiode)
optical link RF loss was 35 dB. A balanced optical link loss can be
achieved if the EAM is biased appropriately, but for passive
operation at this wavelength the downstream link loss is higher
than the upstream loss since the EAM is not an efficient
photodetector at zero bias. However, the overall radio-fibre link
(taking the radio link loss into account) was more or less
balanced, as a result of choosing the appropriate antenna gain for
each direction at the SA. In fact, in the total link the downstream
loss was 80 dB, and 75 dB upstream.
1TABLE 1 rf power levels throughout the system (see FIG. 17) Point
A B C D E Downstream power, dBm +17 .fwdarw. -23 .fwdarw. -15
.fwdarw. -65 .fwdarw. -63 Upstream power, dBm -58 -23 -31 +19
+17
[0069] The radio system was connected to the optical link and
connection was made to a local Ethernet network. Full rate
transmission was possible with the passive radio access point
antenna located on a wall at one end of the office, and the SA
antenna free to roam within the picocell. The receiver sensitivity
of the BreezeNet (TM) radio modems was -64 dBm for operation at the
full rate of 3 Mbps, -72 dBm for operation at 2 Mbps and -82 dBm
for the lowest rate of 1 Mbps. From table 1, it can be seen that
the power requirements for 3 Mbps are easily satisfied within the
picocell. Range calculations based on an (upstream-limited)
receiver power of -58 dBm and the receiver sensitivies given above
suggest that (unobstructed) cell sizes of approximately 12 m (3
Mbps), 30 m (2 Mbps) and 95 m (1 Mbps) are achievable with this
system. Assuming that we operate at a data rate of 1 Mbit per
second, the total available margin is 99 dB for each direction
based on the transmit power of +17 dBm and a receiver sensitivity
of -82 dBm. The remaining margin in the picocell is therefore 19 dB
(downlink) and 24 dB (uplink). If we make the simplistic assumption
that the path loss varies as the square of distance (free-space
propagation) then this margin translates into a range of 27 metres
for the downlink and 48 metres for the uplink.
[0070] The radio system chosen for this work to demonstrate the
`passive picocell` concept is well suited for the purpose due to
its half duplex, frequency hopping spread spectrum design, which
means that only one frequency carrier is present at any one time.
Operating the EAM at zero bias results in non-linear operation,
especially in the upstream path, which would introduce
intermodulation distortion in multi-carrier systems such as GSM. If
passive operation were not required, a small bias, around 1V or
less, would be sufficient to move to a linear part of the
modulation characteristic where intermodulation distortion is
acceptable. Since the EAM photocurrent is around 1 mA, the total dc
power requirement is only 1 mW, and can therefore be satisfied by a
small battery or possibly even a solar cell or cells located at or
adjacent the base station or its antenna. Furthermore, it would be
possible to redesign the EAM to operate in a linear regime at zero
applied bias so that passive operation could still be achieved. For
example, MQW modulators having an in-built bias, such as those
described in EP-B-0416879, could be used.
[0071] For the purposes of demonstrating the concept behind this
work, the optical link length was initially only 30 m, although
much greater lengths will be feasible. The major consideration in
this case is optical loss, which affects the upstream path
especially. Every 1 dB of optical loss translates into 4 dB of
upstream electrical loss, which means that the 6 dB of power margin
at 3 Mbps for the 6 m picocell equates to a 1.5 dB margin in
optical loss. Assuming a fibre loss of 0.2 dB/km this gives an
optical link length of 7.5 km. For 1 Mbps operation in the 6 m
picocell, an optical link length of 30 km will be possible. Figure
shows the trade off between optical link length and radio link
length for each system data rate based on this assumption of fibre
loss.
[0072] The ultimate cost of the EAM transceiver will determine
whether this type of system is deployed on a large scale. The
present device is expensive to package, mainly due to the labour
intensive fibre pigtailing process. Integrated mode transformers,
which allow passive positioning of the fibre pigtail, should reduce
packaging cost significantly. Suitable mode transformers are
described in WO95/23445. Further reductions will be possible using
a reflection modulator, which only needs one pigtail attachment.
Particularly interesting reflection modulators are described in
WO91/19219.
[0073] Several options are available to improve the signal-to-noise
ratio at the receiver and therefore increase the radio range. Some
of the potential improvements that can be made are given below
together with an estimate of their general practicality and
effectiveness.
[0074] DOWNLINK
[0075] External Modulator
[0076] Rather than applying the modulation directly to the laser
source, it would be possible to use an external modulator such as a
lithium niobate Mach Zeander modulator. This could give a signal to
noise ratio increase of about 20 dB which would be a range increase
factor of 10--the reason being that lasers capable of being
modulated with RF are typically fairly low power whereas if an
external modulator is used the laser is only required to operate CW
and one could launch, for example, 50 mW at the head end. Instead
of using a lithium niobate modulator one could use another
electro-absorption modulator, but lithium niobate has the advantage
that large bandwidth, low insertion loss devices are relatively
easy to come by. With a high launch power one would need to
consider the problem of saturation in the electro-absorption
modulator in the picocell. One could use either modulators using
bulk absorption layers, such as those available from NEC, or one
could use MQW devices with high saturation powers--for example
those using barriers made of indium aluminium arsenide with wells
of indium gallium arsenside phosphide.
[0077] More Efficient Laser
[0078] As an alternative to the use of an external modulator and
high power source, one could continue to have direct modulation of
the laser but use a more efficient device than that used in our
experiments to date. It should be possible to get a signal to noise
ratio increase of 10 dB, a range increase factor of 3.2.
[0079] Higher RF Transmit Power
[0080] With direct modulation of the laser, this is limited by the
damage threshold of the laser, but even so could give a 3 dB
signal-to-noise ratio increase, or a range increase factor of 1.4.
Clearly, the use of an external modulator would permit a higher RF
power level to be transmitted.
[0081] Optical Amplification
[0082] The limiting factor here is saturation of the
electro-absorption modulator. Also, optical amplification, achieved
through the use of a fibre amplifier or a semi-conductor amplifier,
would add complexity. Nevertheless, if used, one could expect a 6
dB increase in signal-to-noise ratio, which equates to a range
increase factor of 2.
EXAMPLE
[0083] An erbium-doped optical fibre amplifier was used at the head
end to boost the launched optical signal level on the downlink to
15 mW. With this power level an experiment was conducted on outdoor
propagation. The same antennas and remote terminal were used. The
results are shown in FIG. 18. As can be seen, the furthest
separation used was 75 metres, at which the power level at the
remote terminal was -78 dBm.
[0084] Higher Antenna Gain at the Electro-Absorption Modulator
[0085] Here the question is how does one want to trade
directionality for gain. Obviously it is possible to use a highly
directional antenna, for example antennas with gains of 18 dB are
available, but they are typically very bulky and have very narrow
beam widths. In experiments to date we have used an antenna which
is no larger than a packet of cigarettes, which has an 8 dB gain
and a 70.degree. beam width. One could, at the price of extreme
directionality, get a 10 dB increase in signal-to-noise ratio, that
is a range increase factor of 3.2.
[0086] UPLINK
[0087] More Efficient Photo-Detector
[0088] By using a more efficient photo-detector one could obtain a
6 dB increase in signal-to-noise ratio which is a range increase
factor of 2.
[0089] Higher Optical Power
[0090] As indicated above, by using a more efficient laser or an
external modulator or optical amplification one could increase the
optical power on the 30 downlink and hence on the uplink, but this
is limited by the saturation threshold of the electro-absorption
modulator. Nevertheless, one could obtain an effective 6 dB
increase in signal-to-noise ratio, a range increase factor of
2.
[0091] Optical Pre-Amplification
[0092] The limiting factor here would be saturation of the
photodiode. Obviously complexity would be increased but this could
be minimised, for example by using an optical detector of the type
described in our U.S. Pat. No. 5,446,751. Alternatively, an optical
amplifier, either fibre or a semiconductor laser amplifier, could
be included in the uplink path. This could give a 10 dB increase in
signal-to-noise ratio, or a range increase factor of 3.2.
[0093] RF Pre-Amplification
[0094] In fact the RF system in the BreezeNet is highly optimised.
There is thus little scope for improvement. Any improvement would
come at the cost of potentially significant extra complexity. The
limit on maximum allowed radiated power may also be significant
here.
[0095] High Antenna Gain at the Station Adapter
[0096] Again, this is a trade off between directionality and gain,
in addition, the limits on the maximum allowed radiated power are
relevant here. Probably no increase can be obtained.
[0097] It can be seen that although some of these options are
simple to implement, they give substantial improvement to the radio
range of the system. A radio range of between 1 and 200 metres
should be possible therefore without resorting to expensive or
impractical measures.
[0098] Improved performance in the systems depicted in FIGS. 1 and
2 may be achieved if the transmitter 1 comprises two lasers, laser
1 and laser 2, emitting light at different wavelengths.lambda.uand
.lambda.2 respectively, where .lambda.1<.lambda.2 (e.g.
.lambda.1=1.3 .mu.and .lambda.2=1.55.mu.). The idea being that
light from laser 1 is used to carry the downlink signal, while
light from laser 2 is used to carry the uplink signal. Light from
laser 1 is modulated at the transmitter 1 and is detected at the
modulator 4. .lambda.1 can be chosen to be suffuciently short that
only a negligible amount of light at this wavelegth emerges from
the optical output port of the modulator even when the modulator is
unbiased. This leads to an improved downlink RF insertion loss at
low reverse bias voltages (including the unbiased case) over that
shown in FIG. 5. Light from laser 2 is not modulated at the
transmitter 2. It is modulated at the modulator 4 and the uplink
signal is detected at the receiver 8. Wavelegth .lambda.2 can be
chosen so as to maximize both the downlink RF insertion loss and
the uplink carrier to intermodulation ratio.
[0099] Uplink-downlink mixing can arise in the modulator in a full
duplex system owing to the simultaneous presence of electrical
signals correponding to the uplink and downlink signals. as these
signals are frequency duplexed, uplink-downlink mixing can only
arise through non-linearites in the modulator's modulation versus
voltage and detection versus voltage charateristics. At the
modulator, the downlink electrical signal is typically several tens
of dBs weaker than the applied uplink electrical signal.
Consequently, uplink-downlink mixing is a more serious problem in
the downlink than in the uplink. Wavelegth .lambda.1 may be chosen
such that the variation of photodetection responsivity at .lambda.1
with voltage in the modulator is minimal. therefore, the downlink
signas will suffer minimal distortion due to the applied uplink
signal. the two wavelength approach should thus reduce
uplink-downlink mixing in the downlink. While this approach has
been described with reference only to the early Figures of this
application, those skilled in the art will realise that it can be
appiled to most of the embodiments/applications of the present
invention.
[0100] For many situations capacity on the downlink is much more
important than capacity on the uplink, e.g. for remote video access
or other multimedia applications. Conversely, there are other
applications, for example like remote video surveillance, where
only a small downlink capacity is required, but a significant
uplink capacity is important. The precise nature of the application
may therefore determine the choices made from the above list of
options
[0101] Additionally, the terminal of the present invention can be
used in communication networks in a further configuration. FIG. 2
shows the electrical contacts 5 connected to an electrical output
interface 10 and an electrical input interface 11 via bandpass
filters 9a and 9b. The bandpass filters are chosen such that
electrical data signal 25 is present at the electrical output
interface 10 substantially free from inter-mixing with electrical
data signal 26 and that electrical data signal 26 is present at the
electrical output interface 11 substantially free from inter-mixing
with electrical data signal 25. For example, in a cable television
distribution network wherein a plurality of television signals are
transmitted downstream within the frequency range 10 MHz to 600 MHz
and control signals are transmitted upstream within the frequency
range 100-200 kHz, the bandpass filter 9a will reject substantially
all signals below 10 MHz or above 600 MHz whilst allowing
substantially all signals within the range 10 MHz to 600 MHz.
Similarly, the bandpass filter 9b will reject substantially all
signals below 100 kHz or above 200 kHz whilst allowing
substantially all signals within the range 100 kHz to 200 kHz.
Clearly in some situations it may be possible to dispense with the
filter 9a or filter 9b, depending upon the characteristics of the
communications network or equipment connected to the interfaces 10
and 11.
[0102] The electrical interfaces may be connected to a wide range
of suitable equipment, including a pair of antennas, a further
optical transmitter and receiver, an electrical transmitter and
receiver (in order to transmit signals across a LAN in a building
or a number of rooms) or customer premises equipment (CPE) such as
a set-top box for broadcast cable television services or
interactive multimedia services e.g. video on demand. This list is
not exhaustive and should not be interpreted as a limitation to the
scope of the invention. For many of these applications, such as for
example set top boxes or other CPE, there will be a ready power
supply and hence there may be advantage in using a powered rather
than passive terminal.
[0103] It is possible for a plurality of terminals to be connected
in series, as depicted in FIG. 6.
[0104] The above description has been solely concerned with
analogue signals. In order to transmit digital signals it is
necessary to use modems with the optical transmitter and receiver
at the head-end and with the electrical interfaces at the remote
terminal(s). A modem is a modulator-demodulator, which enables
digital signals to be modulated as analogue signals, without
loosing any significant part of the information carried by the
digital signals, transmitted over an analogue communications
network and then demodulated, re-constituting the original digital
signal. FIG. 7 shows an optical communications network which is
configured with two modems in order that digital signals may be
transmitted.
[0105] An electrical digital data stream, from, for example a LAN,
enters modem 22 through modem input 30a and is modulated into an
analogue electrical data stream with no substantial loss of
information. The analogue optical data stream 24a is transmitted
along the optical fibre 3 by the optical transmitter 1, detected
within the EAM generating an analogue electrical data stream 25 at
the electrical contacts 5, which passes through bandpass filter 9a
to the electrical output interface 10 and thence to the modem 23
where the analogue electrical data stream is demodulated in order
to substantially recreate the original digital data stream. The
substantially recreated digital data stream is then output via
modem output 31b to, for example, a second LAN.
[0106] A digital data stream from the second LAN that enters modem
23 through modem input 31a is modulated into an analogue electrical
data stream 26 with no substantial loss of information which then
passes, via the electrical input interface 11 and bandpass filter
9b, to the electrical contacts 5. Incoming optical carrier 24a is
then modulated by the EAM to create analogue optical data stream
27a, which is transmitted along the optical fibre 3 to the optical
receiver 8. The analogue optical data stream is converted into the
electrical domain and fed to modem 22, which demodulates the signal
in order to substantially recreate the digital data stream from the
second LAN, which is then outputted via modem output 30b. Again,
under certain circumstances, one or both of the bandpass filters
9a, 9b may be omitted.
[0107] The frequency at which analogue data signals can be
modulated is limited by the lower value of the laser modulation
bandwidth or the modulation or detection bandwidthof the EAM. In
the example discussed above the modulation bandwidth of the laser
was .apprxeq.6 GHz and the detection and modulation bandwidth of
the EAM was .apprxeq.14 GHz, so in this case the maximum
transmission performance of the network is limited by the
modulation bandwidth of the laser. If modems are used in order to
transmit digital signals then the modulation speed of the modem
will be the limiting factor for the system, unless it is greater
than the lowest modulation speed of either the laser or the
modulator.
[0108] The maximum distance over which optical signals can be
transmitted and detected is limited by the attenuation of the
signals by the optical fibre. For the DFB laser described above,
with an output power of 4 mW, the maximum distance between the
optical transmitter 1 and the optical receiver 8 was found to be
approximately 50 kilometres. Thus if the above laser were to be
used in a simple network, such as those in FIGS. 1 and 2, the
remote terminal could be sited approximately 25 kilometres from the
base station. This maximum transmission distance could be extended
with the use of optical amplifiers, either fibre or semiconductor,
to increase the level of the optical signal. The use of a higher
power source would obviously also increase the maximum transmission
distance and hence range.
[0109] A further example, which makes use of various of the
alternatives set out above, will now be described.
Example 2
[0110] The experimental optical configuration is shown in FIG. 8. A
single, low insertion-loss, EA modulator module is used at the
remote terminal. A practical low cost/low power system would most
likely use EA modulators at each terminal--thereby eliminating
entirely the need for analogue laser transmitters. FIG. 9 shows two
possible network configurations which use only EA modulator
transceivers. In FIG. 9a, several point-to-point links share a
common CW laser optical (high) power source. In each link, a remote
transceiver, EAM#2, is connected to a transmitter, EAM#1, and
receiver, EAM#3. In FIG. 9b, a number of EA transceivers share an
optical bus powered at the ends by two CW lasers (i.e. all
transceivers share the same RF spectrum).
[0111] In our experiment, a 120 MB/s QPSK modem signal,
up-converted to 1.347 GHz, is combined with the down-converted TV
channels from one polarisation of the Astra (TM) group of
satellites. The combined signal is then applied to a commercial
analogue DFB laser module, which emits an optical power of +6 dBm
at .lambda.=1.56 .mu.m. After transmission over 25 km of step-index
fibre, the optical signal is detected in the remote EA modulator
transceiver. A further 120 MB/s modem signal, centred at 140 MHz,
is applied to the modulator via a duplexer/multiplexer and
impressed on the optical signal for the return path to a commercial
photodetector receiver module. The RF multiplexers used to isolate
the two paths were fabricated from complimentary bandpass/bandstop
microstrip filters of the type described by Wenzel (in IEEE Trans.
Microw Theory & Tech., 1968, MTT-16, 147-157) together with 5th
order complimentary lowpass/highpass lumped filters (see Mathei. G.
L, et al, "Microwave filters, impedance matching networks and
coupling structures", McGraw Hill, 1964) giving>80 dB rejection
of the uplink signal in the downlink path.
[0112] The modulator bias was set initially for minimum 3rd order
intermodulation in the uplink (V.sub.b=0.98V). At this bias, the
electrical insertion loss, excluding fibre loss, was
loss.sub.downlink=43 dB and loss.sub.uplink=41 dB and the DC
electrical power consumption was <1 mW. Whereas the downlink RF
insertion loss varies as the square of the one-way optical loss,
the uplink RF loss varies as the one-way optical loss to the fourth
power. Consequently, it is the uplink which defines the maximum
link length/insertion loss.
[0113] Bit error ratio (BER) measurements were performed initially
on the two data streams without the video signals present, using
two methods: In the first method, the optical insertion loss was
varied using an optical attenuator at point A in FIG. 8. Electrical
gain was then adjusted to maintain a constant input signal to the
modem demodulator. In the second method, optical insertion loss was
kept constant and a variable noise source was inserted before the
demodulator to stimulate different bit energy/noise ratio.
[0114] FIG. 10 shows measured BER for both links plotted as
functions of received optical power, with RF input levels of OdBm.
The minimum received power for a BER of 10.sup.-8 in the uplink is
-27 dBm for this modulation depth and it can be seen that the
sensitivities for the two links differ by 7.4 dB, the uplink being
more sensitive (but suffering twice the effects of fibre
attenuation). It may be shown that this difference is equal to the
optical insertion loss of the EA modulator plus half the difference
in the (zero fibre length) RF insertion losses. BER against
E.sub.b/N.sub.o is plotted in FIG. 11, together with the
back-to-back (electrical) measurement for the modem. These curves
clearly demonstrate that there is no power penalty associated with
the use of an analogue EA transceiver for this configuration (the
uplink shows a small improvement).
[0115] Finally, BER measurements were made of the QPSK signals with
the Astra FM TV signals present. The composite RF spectrum applied
to the laser transmitter is shown in FIG. 12. The modem QPSK signal
is evident at 1.347 GHz. Also noticeable are the digital TV
channels in the Astra signal between 1.75 GHz and 2 GHz. The
electroabsorption modulator bias voltage was increased to -3.65V
and the QPSK signal reduced to -5 dBm in order to ensure minimum
interference between the various signals. Even with this reduced RF
power level, a BER<10.sup.-10 was achieved in both links over 25
km, with no significant visible degradation to the TV signals.
Example 3
[0116] In Example 2 we showed how a single electroabsorption
modulator (EAM) can be used in a "star" type of network
architecture as a low cost device to deliver high data rate
channels together with multi-channel FM and digital TV. In this
example, we present the use of EA modulators for single fibre,
bi-directional optical bus topology operation, delivering 120 Mb/s
QPSK channels. The optical bus is powered by two CW lasers, thereby
avoiding the use of expensive high speed lasers. Potentially, the
cost of the CW laser sources would be shared among users located at
the access nodes. Furthermore, since all nodes are connected in
series, the bus has to afford a certain degree of resilience. The
EAM is an ideal candidate for this application: a power failure at
one node (Bbias=0 volt), would leave the EAM to operate in a
virtually transparent optical state, therefore leaving the rest of
the network in tact.
[0117] Experiment: FIG. 13 shows the experimental setup. Two EA
modulators (EAM1 and EAM2) were used as transceivers to share a
common optical bus powered at the ends by two CW optical sources
(DFB 1 and DFB 2). The launched optical powers were+6 dBm
(.lambda.1=1560 nm) and +4 dBm (.lambda.2=1550 nm) respectively at
EAM1 and EAM2 input ports. Polarisation controllers (PC n.sup.o
1,2,3) were tuned so as to ensure TM mode operation of both EA
modulators.
[0118] For the data path, two 120 Mb/s QPSK data channels were
used: the first modem signal was up-converted to a centre frequency
of 1.347 GHz (channel 1), and a further modem signal was centred at
140 MHz (channel 2). EAM1 modulated .lambda.1 with channel 1 (-3
dBm RF drive power) and detected simultaneously channel 2 from EAM2
at .lambda.2. After transmission over 25 km step index fibre,
simultaneous detection of channel at .lambda.1 and modulation of
channel 2 (-3 dBm input RF power) at .lambda.2 is performed by EAM2
via a duplexer/multiplexer. The RF multiplexers are identical to
those described in Example 2, providing>80 dB rejection between
channel 1 and channel 2 at each node.
[0119] To ensure minimum distortion of the received QPSK signals,
the modulator biases were set to Vb1=-2.14V and Vb2=-3.68V
respectively for EAM1 and EAM2. At these biases, the optical
insertion loss is 12 dB and 10 dB for EAM1 and EAM2 respectively.
The electrical insertion loss, excluding fibre loss, was 42 dB for
the link EAM1 to EAM2, and 44 dB in the opposite direction. The DC
electrical power consumption of the EAMs was less than 1 mW. At
each access node, the electrical amplification was adjusted so as
to maintain a constant signal level into the modem demodulator (-35
dBm). Finally, a noise and interference test set was used to
perform BER measurements versus EbiNo (bit energy/noise ratio).
[0120] Results and discussion: FIG. 14 shows the measured BER
curves. The back to back measurement was performed using the output
of the first up-converter at 1.34 GHz back to the down-converter
(dots). Error free transmission was observed on this link. However,
for reverse biases less than -3.68V on EAM2, a power penalty can be
observed. In FIG. 16, we plot the measured BER at EAM2 output
(channel 1) versus Vb2 for a given Eb/No ratio (Eb/No was
maintained at 18 dB). In both curves, there is a sudden break point
which occurs at Vbp=-3.68V. For .vertline.Vb2.vertline.<.v-
ertline.Vbp.vertline., the responsivity gradient of the DC
characteristics is higher than for
.vertline.Vb2>!Vbp.vertline.nd the measured BER is suddenly
significantly degraded. Further evidence of the signal degradation
is shown in FIG. 16 which shows the RF spectra obtained at EAM2
output on channel 1 for two sets of biases.
[0121] For Vb2>-3.64V, distortion can be observed on the
received signal. We believe this distortion is due to the
cumulation of two effects: the presence of the 8th and 9th
harmonics of channel 2 generated by the sudden gradiant
non-linearity, and the presence of a counter propagating optical
signal which result in an increase of EAM2 photocurrent. This was
confirmed as, when either DFB2 or channel 2 were turned off, the
observed RF spectra changed from FIG. 15a to FIG. 15b.
[0122] Finally, to further characterise EA modulators concatenation
in a bus architecture, we changed the setup so that we could
investigate the alteration of a data stream travelling through an
EA modulator. In the new setup, EAM 1 was used to modulate channel
2 only. Both channels were then recovered at EAM2 after
transmission over 25 km of fibre, using the same RF multiplexers,
the same optical laser sources, and keeping the biases constant.
Error free performance was achieved as shown in FIG. 16. However,
there is a 2 dB penalty which is believed to be due to optical
harmonics generated at EAM1 and transported over the fibre.
[0123] Bus optimisation: We expect this penalty to disappear with
further optimisation of the system. For instance, the choice of a
higher carrier frequency for channel 2 would have been more suitabe
for this application (1.1 GHz rather that 140 Mhz). Also, bias
optimisation of Vb1 would have helped reducing this penalty. There
is however a limit on bias tuning which results from the compromise
between good photodetection and high modulation efficiency of the
EA modulator.
[0124] As those skilled in the art will appreciate, the present
invention is of widespread application.
[0125] Code Division Multiplexing (e.g. Code Division Duplexing),
in which a different code is used for each direction of
communication could usefully be used with at least the passive (and
biased) picocell and microcell embodiments of this invention. CDM
would also readily permit multiple base stations or remote nodes to
be served by a single fibre.
[0126] Currently, EA modulators typically have fairly low
saturation thresholds (or low power handling abilities). By using a
(passive) splitter (eg an optical fibre coupler) to split the
output from the central control station, it would be possible to
share a single powerul optical source between several modulators.
Clearly, the moulators 25 could all be located close together,
perhaps to cover a single microcell, or they could be grouped in
clusters or spread out over a wide area. The invention
[0127] Indoor Applications
[0128] Offices
[0129] The wireless office has long been the holy grail for data
communications managers, mainly because the cost and complexity of
re-cabling due to office moves can be very high. Users too
appreciate the benefits of cable-free working--greater mobility and
no unsightly cable spaghetti to worry about. Unfortunately, the low
performance and high cost of present wireless systems have
prevented their widespread use in the office although this is
starting to change as the technology matures. In a few years time
wireless systems based on the HIPERLAN standard (which will use
transmission at 5.2 Ghz) will provide much higher data rates than
present systems and it is expected that costs will be reasonably
low as a result of the large market engendered by the wide
acceptance of this standard. Wireless voice communications in the
office is also very attractive. Cordless PBX systems based on
standards such as DECT (Digitally enhanced cordless
telecommunications) allow a high degree of mobility because of the
built-in roaming capabilities. Cellular radio systems such as
DCS1800 can also be used to provide voice communications within an
office environment at charging rates close to those of a fixed
line.
[0130] A true wireless office will use a combination of cellular
and cordless telephony and wireless LAN systems. Each of these
systems requires its own radio base unit which must be planned
carefully for optimum positions nd power. In contrast, the passive
picocell system should be able to provide all of these services
from a single unpowered radio base unit. Once the fibre
infrastructure is in place, then the passive picocell radio base
units can be connected when and where they are required. This will
give an enormous advantage in terms of flexibility and ease of use.
The future-proofed aspect of these radio base units is also a very
powerful selling point.
[0131] Warehousing and Retailing
[0132] Wireless LAN systems are already finding a niche market for
warehousing applications hwere the absence of trailing cables is
clearly an important consideration. This is therefore an ideal
application for the passive picocell, which is able to offer a
future proofed service that can be integrated with any additional
requirements for voice communications. In the retail sector, the
flexibility offered by wireless systems is beginning to be
appreciated. For example, electronic point of sale equipment can be
moved around easily when the store layout changes. Again, the
passive picocell is in a good position to provide this wireless
connectivity within an integrated system where future proofing is
perhaps the key consideration.
[0133] Airport Concourses
[0134] An airport concourse is a good example of a present day
picocell--a small hotspot of high demand. The full service and
future proofed capabilities of the passive picocell are again
ideally suited to this application. In the near future, airport
authorities are considering the use of radio tagging in an attempt
to `shepherd` travellers through airports and improve security. The
`passive picocell` may also have a role to play here, where this
new radio system can be added to the others delivered by the radio
base units.
[0135] One of the most attractive applications is in cellular
radio, especially for picocellular and microcellular schemes.
Conventional cellular radio base stations include tuned RF power
amplifiers which restrict the wavlengths which can be handled by
each base station. Allocation of new wavelengths to move capacity
requires the replacement/addition of amplifiers etc., which is
obviously relatively slow and expensive to achieve. In addition,
such base stations have fairly significant power consumption, which
means that generally a mains power supply must be provided, with
back-up batteries to cover power outtages. The combined effect of
these limitations is that the minimum volume and weight for such
base stations are relatively high--severely limiting the sites at
which base stations can be placed.
[0136] Although known radio-by-fibre alternatives to this
conventional approach potentially overcome the fixed wavelength
problem and significantly lower the minimum volume and weight
required for a base station, there is still the problem of
providing a power supply and back-up batteries.
[0137] The invention enables base station power consumption to be
reduced considerably and in many cases eliminated completely. Thus,
it becomes possible to reduce still further the minimum size and
volume required for a base station. With the consequent greater
freedom to site base stations, new architectures become possible.
It becomes more interesting to consider capacity on demand
architectures--where capacity is switched, by the central office,
around between passive base stations according to the actual or
expected level of demand.
[0138] Railway stations can be provided with extra capacity during
peak periods, which capacity can then be switched to commercial or
business districts for the main working hours. Sports stadia or
other entertainment facilities can be provided with capacity as and
when needed. The sports stadium is a good example of an outdoor
application requiring high capacity. It is also a good example to
illustrate the concept of dynamic capacity allocation, or capacity
on demand, which is made possible by the passive picocell system. A
typical sports stadium is used for major events for a few percent
of the time at most. Conventional systems would need to provide
fully equipped base stations to cater for peak demand which ties up
valuable resources. The passive picocell system is capable of
switching capacity to where it is needed on a dynamic basis. So
when the football game is on, the passive picocell base units are
connected to sufficient network capacity to cater for the half-time
peak demand, and when the ground stands empty, the same network
capacity can be used for other locations.
[0139] This capacity on demand concept can be used on a larger
scale, for example in and around a major city. During the day, the
capacity could be diverted into the city and in the evening the
same capacity could be sperad across the suburbs--in other words
the capacity can follow the people. This type of scheme can
uniquely meet the demands of high localised capacity at an
economical cost. Even with the distribution range of the various
non-optimised examples, it becomes possible to switch capacity
between quite widely distributed sites. Again, the use of optical
amplification, particularly in the return path from the base
station (to avoid saturation of the modulator) is an easy way to
extend the range of these systems without the need to move away
from the passive base station approach. In these applications the
expensive, bulky and relatively sensitive RF equipment can be sited
at a central office in a benign environment.
[0140] In addition, the invention permits a single base station to
serve several systems (GSM, DECT, etc). Using FDM, it is possible
to use one base station to serve, for example a combination of GSM,
which uses frequencies around 900 mhz, DECT, which operates around
1900 Mhz and current radio LAN systems operating around 2.5 Ghz. If
all three signals are combined, carried optically over fibre to the
base station and radiated from there over free space, only a GSM
handset will respond to the GSM signals since the antenna and input
stages are highly frequency selective and will therefore reject the
DECT and radio LAN signals. Similarly, the relevant receivers for
the radio LAN and for DECT will respond only to their signals. The
cental control station contains the radio modems connected to their
respective networks. These modems provide the radio signals
transmitted to the radio bse units, each radio modem is associated
with a particular system, for example, GSM, DECT, or radio LAN and
their signals are combined for onward transmission. This is shown
schematically in FIG. 19. The central control station on the left
of the Figure shows a variety of radio modems and includes ones for
emerging standars. The picocell, which could be an office or a
street or airport terminal, shows a range of terminals all of which
should be able to operat simultaneously without inter--system
interference.
[0141] Outdoor Applications
[0142] Fixed Radio Access
[0143] Fixed radio access, also known as wireless local loop, is
seen as an economical way of providing service, especially for
other licensed operators trying to take market share from an
incumbent operator. From a BT perspective, this is an attractive
option for joint venture companies. The passive picocell system
could be used in this type of application as long as the range was
not too great.
[0144] An architecture whereby fibre extends to the radio
distribution point (DP) would see radio base units mounted on
street poles with a range of up to 200 m, the range predicted for
the passive picocell with further optimisation. This arrangement
would remove the need for dropwires into customers' premises, and
would also avoid problems caused by using active electronics in
hostile environments. These radio DPs could also act as base
stations for cordless or cellular systems at the same time, a major
bonus that could wing the economics in favour of this
technique.
[0145] Fibre to the home is often referred to as the ultimate
objective for a truly broadband access netowrk, and is likely to be
the access medium of choice in the long term. The passive picocell
has an important role to play in this architecture too. Once the
fibre is installed in the home, the passive picocell base unit
could be used to avoid further cabling. This is clearly a desirable
objective. The base unit could be installed in the roof for example
to give wireless communications throughout the home.
[0146] Microcells
[0147] Microcellular systems are being deployed at an
ever-increasing rate to satisfy the capcity demand on cellular
networks. These systems have a typical range of a few hundred
metres and are used in dense urban environments where the base
stations are below roof height. They radiate low power levels and a
typical coverage area would be a city street. To avoid handover
problems for fast moving mobiles, they will be used in a
hierarchical arrangement with `umbrella` macrocells which are
designed to cover for this eventuality. The prospect of a small,
lightweight, low-cost and unpowered base station is obviously
attractive for this application. This is especially true since the
cost of renting building space for the base site is expensive and
getting more so as building owners begin to realise their value to
cellular operators.
[0148] The outdoor passive picocell example had a range of 75
metres, , even with a non-optimised set-up. With such a range
applications to provide effective radio coverage in radio "black
spots" are perfectly feasible. Passive picocell base stations can
be provided at all the necessary points in a high rise office
district such as are typically found in the financial districts of
major cities) or in road or rail tunnels. This approach seems
particularly suitable as a means of providing localised radio
coverage on the platforms of underground railway stations. Clearly
for office blocks, shopping complexes and the like, the
picocellular approach according to the invention is attractive.
[0149] Temperature Sensitivity
[0150] Present evices are sensitive to temperature. For indoor
applications this is not important, but it is a serious concern for
outdoor applications. Currently preferred solutions involve the
optical source, which can either be broadband (so that changes to
the EAM operating wavelength fall within the source bandwidth) or
made to track the EAM operating wavelength using a control loop
(which would be simple to implement in the looped versions of the
passive picocell.
[0151] In place of the EAM, it would be possible to use an
asymmetric Fabry Perot modulator (for example one made from indium
phosphide or the like). In this case, modulation is achieved by
tuning the modulator (typically by vaAying the refractive index of
the material within the FP cavity) between a resonance peak and
off-peak. Such a device, whose asymmetry is due to its two
reflectors having different reflectivities, would be used in
reflection.
[0152] As indicated at the start of this specification, the
invention in various aspects is applicable to radio and microwave
systems having frequencies in the hundreds of MHz to millimetre
wave bands. For example the invention is applicable to the HIPERLAN
standardwhich will use 5.5 Ghz, as well as to 60 Ghz raio LANs
(e.g. 120 Mbit/s). The invention ca n equally be applied to phased
array and other radars, for example to feed antennas on military
and other aircraft. In addition, as will be clear from the above
description, the invention in some aspects is applicable to much
lower frequencies, e.g. baseband.
* * * * *