GB2401266A - Optically fed radio station with an electro-absorption modulator with two sets of electrodes - Google Patents

Optically fed radio station with an electro-absorption modulator with two sets of electrodes Download PDF

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Publication number
GB2401266A
GB2401266A GB0309924A GB0309924A GB2401266A GB 2401266 A GB2401266 A GB 2401266A GB 0309924 A GB0309924 A GB 0309924A GB 0309924 A GB0309924 A GB 0309924A GB 2401266 A GB2401266 A GB 2401266A
Authority
GB
United Kingdom
Prior art keywords
electro
absorption modulator
electrode
electrode arrangement
signals
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
GB0309924A
Other versions
GB0309924D0 (en
Inventor
Peter Paul Smyth
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Microwave Photonics Inc
Original Assignee
British Telecommunications PLC
Microwave Photonics Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by British Telecommunications PLC, Microwave Photonics Inc filed Critical British Telecommunications PLC
Priority to GB0309924A priority Critical patent/GB2401266A/en
Publication of GB0309924D0 publication Critical patent/GB0309924D0/en
Priority to PCT/US2004/009733 priority patent/WO2004100228A2/en
Publication of GB2401266A publication Critical patent/GB2401266A/en
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W88/00—Devices specially adapted for wireless communication networks, e.g. terminals, base stations or access point devices
    • H04W88/08—Access point devices
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/25—Arrangements specific to fibre transmission
    • H04B10/2575—Radio-over-fibre, e.g. radio frequency signal modulated onto an optical carrier
    • H04B10/25752—Optical arrangements for wireless networks
    • H04B10/25758—Optical arrangements for wireless networks between a central unit and a single remote unit by means of an optical fibre
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W84/00—Network topologies
    • H04W84/02—Hierarchically pre-organised networks, e.g. paging networks, cellular networks, WLAN [Wireless Local Area Network] or WLL [Wireless Local Loop]
    • H04W84/10—Small scale networks; Flat hierarchical networks
    • H04W84/12—WLAN [Wireless Local Area Networks]
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W88/00—Devices specially adapted for wireless communication networks, e.g. terminals, base stations or access point devices
    • H04W88/08—Access point devices
    • H04W88/085—Access point devices with remote components

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Optical Communication System (AREA)

Abstract

An optically fed point-of-presence apparatus is provided for a wireless network. The point of presence includes an electro-absorption modulator system which is in an optical path along which signals are conveyed respectively from/to a central access site. The point of presence includes a transmit antenna which is part of a transmission path which conveys signals from a first electrode arrangement of the electro-absorption modulator system to the transmit antenna for transmission to wireless terminals in the network. The point of presence also includes a receive antenna which is pan of a reception path which conveys signals received from the wireless terminals at the second antenna to a second electrode arrangement of the electro-absorption modulator system. The electro-absorption modulator system may be a split contact semiconductor modulator or may be formed of two devices, possibly integrated, at least one of which is an electro absorption modulator.

Description

2401 266
ACCESS POINT
FIELD OF THE INVENTION
This invention relates to access points in wireless networks, and in particular, but not exclusively, to a distributed access point having optical fibre connections to multiple points-of- presence. The network may Implement a range of protocols such as IEEE 802. 1 la and 802.11b for wireless terminals communicating through each pont- of-presence. Or similarly, cellular protocols found in 2G and 3G or other cellular networks or combinations of these and WEAN protocols. The invention also relates to an electro-absorption modulator for use in a point-of presence device.
BACKGROUND TO THE INVENTION
Wireless LANs (local area networks) are emerging as important infrastructure for a wide range of commercial and domestic premises. They enable mobihty of wireless devices about the premises and are generally more flexible in their utilisation and lower cost than networks with equivalent wired connections. However, a large number of wireless access points may be required properly to serve the coverage volume of a particular premises, and different mobile devices may require service within the volume using different wireless protocols. This increases the number of wireless interface controllers with radio frequency (RF) transceivers that are required by the network, and therefore increases its cost. Similar issues are found in cellular deployments of access points. Distributed access points having a number of relatively simple cells or (points of presence (POPs) for transmission and reception of RF signals are under development. Each access point typically has a central server with a set of transceivers that are typically connected to the POPs by paths including optical fibres, co-axal cables or the like, through a budge or switch. For a use in a cellular network such as GSM or 3G, the "central server" would contain a bank of Base Station transceivers (BTS) and would be similarly connected to the POPs via optical fibres.
A number of papers have proposed POP designs, including both passive and active devices, but few If any have yet achieved a commercially acceptable combination of features. The proposed passive systems can be relatively simple, cheap and have minimal or zero electrical power consumption the radio transmitted power being provided by the optical signal power from the optical fibre. However they also have a transmission range that is limited to up to about 50m radius (e.g. picocell size) at least for omni-directonal transmission. Active systems which typically utlise additional electrical power amphfication have a greater and controllable transmission range but are also more complex and therefore relatively expensive. Most of the currently proposed designs for distributed access points with optically fed POPs involve (electro- absorpton modulators (EAMs) in the POPs and EAMs/Lasers in the central server. These semiconductor devices provide a convenient way of modulating and demodulating optical signals at 1 300nm and 1 500nm for example and find use in a wide range of optical systems. In some proposals for access points each optical path from the central server to a POP is a loop or a reflection arrangement requiring only one fibre and possibly a bi-directonal coupler at the central server avoiding the need for a laser in the POP. Signals for transmission to wireless terminals are demodulated from a carrier on the optical path by an EAM at the POP and signals received at the POP from wireless terminals are modulated onto the carrier by the EAM.
To extend both the transmission and reception ranges of the POP a bidirectional amplifier could be added between the EAM and the antenna or antennas. The frequency response of the amplifier in both directions needs to be sufficiently wide to cover the protocols used in the POP or for future up-grades that might occur. However the amount of electrical amplification that can be added to the POP in order to extend the both the transmission and reception ranges Is ultimately limited by the closed loop path gam between the EAM and the antenna or antennas. In practice it Is has not been possible to achieve sufficient electrical gain in both transmit and receive directions to challenge that of conventional wireless access points.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an Improved POP device for a distributed access point and an improved EAM for use in the POP device or at least to provide an alternative to existing systems.
Accordingly in one aspect the invention may broadly be said to consist in Point-of-presence apparatus for a wireless network comprising: an optical path including an electro-absorption modulator system and input/output ports through which signals are conveyed respectively from/to a central access site a transmission path Including a transmit antenna which conveys signals from a first electrode arrangement of the electro- absorption modulator system to the transmit antenna for transmission to wireless terminals in the network, and a reception path including a receive antenna, which conveys signals received from the wireless terminals at the second antenna to a second electrode arrangement of the electro-absorption modulator system.
In one embodiment the EAM system is a monolithic semiconductor device with separate electrodes connected to the start of the transmission path and to the end of the reception path. In another embodiment the EAM system includes two separate semiconductor devices, one connected to the start of the transmission path and the other to the end of the reception path.
In preferred versions of either of these embodiments, electrical amplification is added to amplify the transmit and / or receive signals between the EAM and the antenna or antennas. Because a high degree of electrical isolation can be achieved by the use of separate electrodes or the use of separate semiconductor devices, significant levels of gain can be provided without risk of electrical oscillation.
In another aspect the invention may be said to consist of a split-contact electro-absorption modulator including an optical waveguide and a first electrode arrangement to enable an electric field to be applied to a first portion of the optical waveguide and a second electrode arrangement to enable an electric field to be applied to a second portion of the optical waveguide.
The invention may also broadly be said to consist in any alternative combination of features that are indicated in this specification. All equivalents of these features are considered to be included whether or not explicitly set out.
LIST OF DRAWINGS
Preferred embodiments of the invention will be described with respect to the accompanying drawings, of which: Figure 1 schematically shows a distributed access point in a wireless network, Figure 2 shows the structure a POP device that may form part of the access pomt, Figure 3 is a plan view of an EAM device that could be used in the POP, and Figure 4 is a cross sectional view of the EAM device.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to these drawings it will be appreciated that the invention can be implemented in a s range of different ways in a range of wireless networks. The embodiments described here are given by way of example only.
Figure I schematically shows part of a network serving a wireless terminal WT through a distributed access point. The access point Includes a central server 10 connected to a wired network 11 and to at least one point-of-presence POP, having one or more RF (radio frequency) transmit and receive antennas. Each POP represents a small cell site for radio communication.
The server contains a number of known components that have been summarised as a processing block 12, such as network Interface cards (NICs) or Base station transceiver systems (BTS) that carry out functions such as conversion of signals between protocols and carriers used on the wired and wireless parts of the network. The POP also contains a number of components, including an opto-electrc converter O/E such as an EAM, and perhaps amplifiers or frequency converters summarised as a processing block 13. A path from the server 10 to the POP Is provided by optical fibres 14, formed as a loop in this example, started by a transmitter Tx such as a laser and completed by a receiver Rx such as a photodiode. Alternatively, the two fibres can be replaced with one if the EAM Is Implemented in a reflective mode in which case a optical directional coupler can be used at the central server to separate optical transmit and receive signals (other means of separating the signals could also be used).
Figure 2 shows a POP device in more detail, suitable for use In the access point of Figure 1.
Signals from and to the central site of the access point are carried on an optical path represented by fibres 14, connected to an EAM system 20 by input and output ports. The POP preferably 2s also has separate transmit and receive paths for signals detected on and imparted to the optical path. The transmit path begins at the EAM system and ends in at least one transmit antenna 21.
The receive path beams in at least one receive antenna 22 and ends at the EAM system.
Proposed POPs with conventional EAM devices generally have transmit and receive paths that are at least partly combined and connected to an EAM through a single electrode, leading to a number of practical problems such as a very low hmit to the amount of electrical amphfication which can be provided before the occurrence of oscillation due to feedback between the EAM and the antennas. In this example, the transmit and receive paths are connected to an EAM system using separate electrodes, either on separate EAM devices placed in series in the optical path, or using a monolithic EAM havmg a twin or split electrode as described below. This enables enhanced electrical gains and bias in each of the transmit and receive paths, and thereby extends the range and reduces the cost of a wireless cell that may be served by the POP.
Alternatively, the EAM system could include a semiconductor detector (for example of PIN configuration) ,not being an electro absorption modulator, to detect the optical signals from the central site, with a separate (but preferably integrated with the detector) EAM with which to apply the modulation derived from the incoming, air-borne, radio signals.
A range of optional features are included in the POP of Figure 2, such as the possibility of control by the central site via the optical path, amplification and frequency conversion of incoming and outgoing signals, transmission and reception in multiple frequency bands, and diversity selection. Control by the central site may be enabled by a processor 23, that receives and sends data from the optical path through multiplexer and demultiplexer 24 and 25. In this example, amplification of signals on receipt from the optical path and before transmission to wireless terminals, is carried out by amplifiers Al and A2 respectively. Amplification of signals received from wireless terminals is carried out by amplifier A3. Power detectors 26 and 27 in the transmit and receive path are provided and monitored by the processor for control of the amplifiers A2 and A3. As much as 80dB or more of gam is likely to be achievable in each of the transmit and receive paths when they are separated, before feedback becomes a problem..
Selection for spatial diversity among two or more receive antennas 22 may be enabled by a selector system 28, preferably to connected to the CPU 23. The selector typically contains narrow band filters and power detectors for each of the channels that may be received at the POP.
Figure 2 also indicates frequency conversion in the transmit and receive paths. Signals transmitted over the optical path to and from the POP, modulated on an optical carrier, may be conveyed at modulation frequencies other than the radio frequencies at which they are transmitted and received by the POP. This enables relatively low cost lasers in the central site, and economy in other analogue components for both the central site and POP. In this example, conversion in two frequency ranges Is preferably indicated for each path by filters Fl, F2 and twin mixers 29 with respective local oscillators. The prescribed bands for IEEE 802.1 la and b are at 5.2-5.8GHz and 2.4GHz respectively, and these may be converted to and from IOOMHZ to 800MHZ bands more suitable for modulation of the optical carrier. A range of other protocols may also be available at a POP of the present kind.
Figures 3 and 4 show a simple EAM device in plan and cross-sectional views respectively, suitable for use in the POP device of Figure 2. In this example, the EAM is a semiconductor structure with a waveguide region 40 typically formed as a multi-quantum well, surrounded by cladding formed from n and p doped materials 41 and 42, preferably TnP. The waveguide has refractive index higher than that of the cladding, and is about 500,um m length and about 1-21lm in thickness and In width. Electrodes 43 and 44 are formed on an upper surface parallel with the waveguide, being collinear but preferably separated and isolated by a layer of metallisation 47 provided as a ground plane. A layer of metallisation 48 forms an electrode on the opposite surface. Voltages are applied between electrodes 43 and 48 and between 44 and 48 via the electrodes at contacts 49. An optical carrier propagates on a path including the waveguide 40 and fibres 30. A voltage detected on the contacts at either electrode 43 or 44 may be used to receive signals modulated on the carrier, and similarly a voltage applied to the other electrode may be used to modulate the carrier for transmission.
The twin electrodes 43 and 44 in Figures 3 and 4 provide end points for the transmit and receive paths in the POP of Figure 2. It will be appreciated that a wide range of EAM devices may be manufactured for this purpose, or that two EAM devices may be used in series. In this example, the ground plane is shaped as an "H" on the surface, coplanar with the two electrodes. However, a wide range of different structures are envisaged for different devices.

Claims (13)

1. Point-of-presence apparatus for a wireless network, comprising: an optical path including an electro-absorption modulator system and input/output ports through which signals are conveyed respectively from/to a central access site, a transmission path including a transmit antenna, which conveys signals from a first electrode arrangement of the electro-absorption modulator system to the transmit antenna for transmission to wireless terminals in the network, and a reception path including a receive antenna, which conveys signals received from the wireless terminals at the second antenna to a second electrode arrangement of the electro- absorption modulator system.
2. Apparatus according to claim I wherein the electro-absorption modulator system is a monolithic semiconductor device carrying the first and second electrode arrangements.
3. Apparatus according to claim I wherein the electro-absorption modulator system includes two separate semiconductor devices, one carrying the first electrode arrangement, the other carrying the second electrode arrangement.
4. Apparatus according any one of the preceding claims, wherein the first electrode arrangement is connected to the start of the transmission path and the second electrode arrangement is connected to the end of the reception path.
5. Apparatus according to any one of the preceding claims, further comprising an amplifier In the transmission and an amplifier in the reception path.
6. Apparatus according to any one of the preceding claims, further comprising means to perform frequency up-conversion m the transmission path and means to perform frequency down-conversion m the reception path.
7. Apparatus according to any one of the preceding claims, further comprising a processor configured to control operation of the point-ofpresence according to control signals received from the central site.
8. Apparatus according to any one of the preceding claims, further comprising a second receive antenna in the reception path for diversity selection of signals from the wireless terminals.
9. Apparatus according to any one of the preceding claims, wherein each of the transmission and reception paths convey signals on two or more channels.
10. Apparatus according to any one of the preceding claims, wherein the transmit and receive antennas are multi-band antennas.
11. A split-contact electro-absorption modulator including an optical waveguide and a first electrode arrangement to enable an electric f eld to be apphed to a first portion of the optical waveguide and a second electrode arrangement to enable an electric field to be applied to a second portion of the optical waveguide.
12. An electro-absorption modulator according to claim I I further comprising a ground plane electrode which lies between an electrode of the first arrangement and an electrode of the second arrangement.
13. An electro-absorption modulator according to claim 12 wherein the third electrode has the shape of a capital letter H with the other two electrodes located collinearly m the openings of the third electrode which correspond to the upper and lower portions of the letter H.
GB0309924A 2003-04-30 2003-04-30 Optically fed radio station with an electro-absorption modulator with two sets of electrodes Withdrawn GB2401266A (en)

Priority Applications (2)

Application Number Priority Date Filing Date Title
GB0309924A GB2401266A (en) 2003-04-30 2003-04-30 Optically fed radio station with an electro-absorption modulator with two sets of electrodes
PCT/US2004/009733 WO2004100228A2 (en) 2003-04-30 2004-03-30 Access point

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB0309924A GB2401266A (en) 2003-04-30 2003-04-30 Optically fed radio station with an electro-absorption modulator with two sets of electrodes

Publications (2)

Publication Number Publication Date
GB0309924D0 GB0309924D0 (en) 2003-06-04
GB2401266A true GB2401266A (en) 2004-11-03

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GB (1) GB2401266A (en)
WO (1) WO2004100228A2 (en)

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2306069A (en) * 1995-10-02 1997-04-23 France Telecom An electro-absorbent in-line modulator
WO1998004057A1 (en) * 1996-07-19 1998-01-29 British Telecommunications Public Limited Company Telecommunications system simultaneously receiving and modulating an optical signal
US6486830B1 (en) * 1999-08-26 2002-11-26 Toshiba Tec Kabushiki Kaisha Directional antenna apparatus

Family Cites Families (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6504636B1 (en) * 1998-06-11 2003-01-07 Kabushiki Kaisha Toshiba Optical communication system

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2306069A (en) * 1995-10-02 1997-04-23 France Telecom An electro-absorbent in-line modulator
WO1998004057A1 (en) * 1996-07-19 1998-01-29 British Telecommunications Public Limited Company Telecommunications system simultaneously receiving and modulating an optical signal
US6486830B1 (en) * 1999-08-26 2002-11-26 Toshiba Tec Kabushiki Kaisha Directional antenna apparatus

Also Published As

Publication number Publication date
GB0309924D0 (en) 2003-06-04
WO2004100228A3 (en) 2005-05-26
WO2004100228A2 (en) 2004-11-18

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Owner name: MICROWAVE PHOTONICS INC.

Free format text: FORMER APPLICANT(S): BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY

WAP Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1)