WO1992012580A1 - Switched diversity for optical communications - Google Patents
Switched diversity for optical communications Download PDFInfo
- Publication number
- WO1992012580A1 WO1992012580A1 PCT/US1991/006566 US9106566W WO9212580A1 WO 1992012580 A1 WO1992012580 A1 WO 1992012580A1 US 9106566 W US9106566 W US 9106566W WO 9212580 A1 WO9212580 A1 WO 9212580A1
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- photodetectors
- sensing
- signal
- receiver
- following circuitry
- Prior art date
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Classifications
-
- 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
- H04B10/60—Receivers
Definitions
- the present invention relates to optical communications systems and, more particularly, to a receiver for use in an optical communications system where the receiver must receive incoming light signals from a plurality of differing fields of view and constantly generate an adequate electrical signal therefrom for use by following circuitry comprising, a plurality of photodetectors for providing electrical signal outputs as a result of incident light thereon, the plurality of photodetectors being positioned to receive the incoming light signals from respective ones of the differing fields of view; a plurality of switches connecting respective ones of the photodetectors into the following circuitry; sensing means for sensing a signal quality criteria of respective ones of the photodetectors; and, decision and switching logic means connected to the sensing means and the plurality of switches for connecting and disconnecting respective ones of the photodetectors from the following circuitry to optimize an electrical signal which is a function of incident light output to the following circuitry.
- a major consideration and sometimes problem is the selection of the best signal at the receiving end.
- the main criteria are signal strength and the signal- to-noise ratio.
- sending transmitters at 10 and 12 send radio waves 14 to a receiver at 16.
- multiple antenna may be employed in an effort to obtain the best signals from the transmitters 10 and 12.
- one of the antenna providing the strongest signal with the best signal-to-noise ratio will be employed and the other antenna will be switched off.
- radio frequency as the transmitting medium is replaced by light.
- a modulated light beam 18 is transmitted towards the receiver 20 and strikes a photodetector 22 which provides an electrical signal on the wires 24 therefrom to the connected circuitry 26 as a function of the intensity of the light.
- the photodetector 22 is a photodiode.
- FOV field-of-view
- the use of multiple detectors as implemented in the prior art is not without penalty, however. If one detector is exposed to levels of light sufficient to saturate the device, then the signal-to-noise ratio (SNR) in the receiver can be impaired beyond use. Further, multiple detectors will each produce noise arising from their dark current. This noise is additive in the receiver. In low-level conditions where not all of the detectors are illuminated, the dark current- produced noise from the non-illuminated detectors can also impair the SNR.
- SNR signal-to-noise ratio
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- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Engineering & Computer Science (AREA)
- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- Radio Transmission System (AREA)
- Optical Communication System (AREA)
Abstract
This is a receiver (20') for use in an optical communications system where the receiver must receive incoming light signals from a plurality of differing fields of view (FOV A - FOV E) and constantly generate an adequate electrical signal therefrom for use by following circuitry (26'). There is a plurality of photodetectors (22) for providing electrical signal (24) outputs as a result of incident light thereon, the plurality of photodetectors being positioned to receive the incoming light signals from respective ones of the differing fields of view. A plurality of switches connect respective ones of the photodetectors into the following circuitry. There is sensing circuitry for sensing a signal quality criteria of respective ones of the photodetectors and decision and switching logic connected to the sensing circuitry and the plurality of switches for connecting and disconnecting respective ones of the photodetectors from the following circuitry to optimize an electrical signal which is a function of incident light output to the following circuitry.
Description
SWITCHED DIVERSITY FOR OPTICAL COMMUNICATIONS
Background of the Invention:
The present invention relates to optical communications systems and, more particularly, to a receiver for use in an optical communications system where the receiver must receive incoming light signals from a plurality of differing fields of view and constantly generate an adequate electrical signal therefrom for use by following circuitry comprising, a plurality of photodetectors for providing electrical signal outputs as a result of incident light thereon, the plurality of photodetectors being positioned to receive the incoming light signals from respective ones of the differing fields of view; a plurality of switches connecting respective ones of the photodetectors into the following circuitry; sensing means for sensing a signal quality criteria of respective ones of the photodetectors; and, decision and switching logic means connected to the sensing means and the plurality of switches for connecting and disconnecting respective ones of the photodetectors from the following circuitry to optimize an electrical signal which is a function of incident light output to the following circuitry.
In communications systems, a major consideration and sometimes problem is the selection of the best signal at the receiving end. The main criteria are signal strength and the signal- to-noise ratio. In a radio frequency-based system as depicted in Figure 1, for example, sending transmitters at 10 and 12 send radio waves 14 to a receiver at 16. At the receiver, multiple antenna may be employed in an effort to obtain the best signals from the transmitters 10 and 12. Depending on which signal is being received, one of the antenna providing the strongest signal with the best signal-to-noise ratio will be employed and the other antenna will be switched off.
In optical communications systems, radio frequency as the transmitting medium is replaced by light. As depicted in it simplest form in Figure 2, a modulated light beam 18 is transmitted towards
the receiver 20 and strikes a photodetector 22 which provides an electrical signal on the wires 24 therefrom to the connected circuitry 26 as a function of the intensity of the light. Typically, the photodetector 22 is a photodiode. In the design of atmospheric optical communications systems, it is often advantageous to employ multiple detectors. Placement of the detectors at varied offset angles or at strategic locations can dramatically increase the field-of-view (FOV) of the receiver and thereby reduce the effect of alignment errors, for example. Additionally, placement of multiple detectors in the same focal plane will increase the strength of the received signal. The use of multiple detectors as implemented in the prior art is not without penalty, however. If one detector is exposed to levels of light sufficient to saturate the device, then the signal-to-noise ratio (SNR) in the receiver can be impaired beyond use. Further, multiple detectors will each produce noise arising from their dark current. This noise is additive in the receiver. In low-level conditions where not all of the detectors are illuminated, the dark current- produced noise from the non-illuminated detectors can also impair the SNR.
The foregoing problems of the prior art are particularly troublesome if one attempts to communicate with a fixed location receiver by means of a locally-mobile transmitting unit. Such a situation would exist, for example, if one were to optically connect the handset of a telephone system to the base station portion thereof. In such a case, as the user moves about the local area with the transmitting unit, the field of view into which the information- bearing emitted light is transmitted is a constantly changing one. Thus, the receiver needs to possess omni-directional reception where field of view is optimized.
Wherefore, it is an object of this invention to provide an optical communications system employing multiple detectors where the advantages thereof are realized and the problems thereof are virtually eliminated.
It is another object of this invention to provide an optical communications system employing multiple detectors where the available signal is optimized.
It is still another object of this invention to provide an optical communications system employing multiple detectors where the signal-to-noise ratio is optimized.
It is yet another object of this invention to provide an optical communications system employing multiple detectors where the effects of detector saturation are eliminated. It is a further object of this invention to provide omni¬ directional reception to an optical communications system employing multiple detectors where field of view is optimized.
Other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.
Summary:
The foregoing objects have been achieved in the receiver of an optical communications system employing a plurality of photodetectors for providing electrical signal outputs as a result of incident light thereon, by the improvement of the present invention comprising, a plurality of switch means for connecting respective ones of the photodetectors into following circuitry; sensing means for sensing a signal quality criteria of respective ones of the photodetectors; and, decision and switching logic means connected to the sensing means and the plurality of switch means for connecting and disconnecting respective ones of the photodetectors from the following circuitry to optimize an electrical signal which is a function of incident light output to the following circuitry.
According to one aspect of the invention, the decision and switching logic means includes logic for connecting a minimum number of the photodetectors which will provide an adequate signal to the following circuitry. According to another aspect of the invention, the sensing means includes means for sensing saturation of respective ones of
the photodetectors; and, the decision and switching logic means includes logic for disconnecting a photodetector which is saturated. According to still another aspect of the invention, the sensing means includes means for sensing signal strength from connected ones of the photodetectors; and, the decision and switching logic means includes logic for adding an additional photodetector when signal strength is too low to provide an adequate signal to the following circuitry.
According to yet another aspect of the invention, the sensing means includes means for sensing the signal-to-noise ratio from connected ones of the photodetectors; and, the decision and switching logic means includes logic for changing photodetectors when the signal-to-noise ratio is too low to provide an adequate signal to the following circuitry.
Description of the Drawings:
Figure 1 is a simplified drawing of a prior art approach to signal diversity as applied to radio frequency environments
Figure 2 is a simplified drawing of a prior art approach to optical communications wherein only one optical detector is employed on the receiving end.
Figure 3 is a simplified drawing of an approach to optical communications according to the present invention wherein multiple optical detector employed on the receiving end in conjunction with diversity selection capability.
Figure 4 is a simplified functional block diagram of an optical communications system according to the present invention.
Figure 5 is a flowchart of possible logic to be implemented in the detector selection logic portion of the system of Figure 4.
Description of the Preferred Embodiment:
The preferred environment for an optical receiver 20' according to the present invention is depicted in Figure 3. There are multiple photodetectors 22 placed in the same focal plane (FOVs WB", MC", and "D") with some photodetectors 22 (e.g. FOVs
"C" and "D") having overlapping FOVs. There are also
photodetectors 22 in alternate focal planes (e.g. FOV "A") and at remote locations (e.g. FOV "E") to provide FOVs not otherwise accessible. In each case, however, the photodetectors 22 are all connected to revised connected circuitry 26' according to the present invention which implements the switched diversity to be described hereinafter.
The concept of switched diversity as implemented by the present invention to achieve its stated objectives is to operate from the least number of photodetectors 22 required for adequate performance. If a single detector is providing adequate performance, then it shall remain the only detector in use. As its performance degrades, then if the degradation is due to device saturation the particular photodetector 22 is switched out an another detector is chosen which will provide an adequate SNR. If the degradation is not due to saturation, but rather to a diminishing SNR, then additional photodetectors 22 may be switched into the circuit.
The reverse-biased photodiodes typically used as the photodetectors 22 in optical communications systems are basically current sources wherein the current produced is directly proportional to the intensity of the incident light. How close the device is to saturation can be measured by sampling the generated photocurrent. In one implementation of the present invention, if the sampled generated photocurrent of a photodetector 22 in use exceeds a pre-established threshold, a switch can be thrown to remove the photodiode from the receive circuitry.
If the photodiode is operated in the non-reverse-biased, or photovoltaic, mode, then the generated photocurrent flows through the diode's internal resistance causing a voltage across the diode. The voltage will increase until the diode is completely forward- biased, which is the point of device saturation. Thus, this photo current-generated voltage can also be sampled and used as a criterion for diversity switching according to the present invention. An additional measure that may be used as a criteria for diversity switching is the SNR. This quantity is often most effectively measured at later stages of the receiver, as opposed to
right at the photodiode. SNR is the most meaningful measure as it directly indicates the operational performance of the receiver. Thus, in an actual implementation of the present invention, use of the SNR attributable to each of the multiple photodetectors is the preferred diversity switching criteria.
Turning now to Figure 4, a receiver 20' according to the present invention is shown in greater detail. As indicated therein, the connected circuitry 26' comprises current/voltage sensing and amplification 28, SNR detection 30, detector selection logic 32, and user circuitry 34. All the possible switching criteria mentioned above are shown for clarity only and those skilled in the art will readily recognize and appreciate that only those criteria of interest in the particular implementation will be employed. Thus, for example, if the SNRs of the photodetectors 22 are employed as the sole switching criteria, the current and voltage sensing portions of block 28 can be eliminated. As a practical matter, however, while the SNRs of the photodetectors 22 may be employed as the sole switching criteria, a current or voltage sensing portion of block 28 should be retained to provide information to the detector selection logic 32 about the status of non-used photodetectors 22 which may be needed for replacement purposes. Thus, the logic can sample the outputs from the unused photodetectors 22 and anticipate which one or ones will provide the best signal if needed. As depicted in the drawing figure, the wires 24 from the photodetectors 22 are each connected to the remaining circuitry through a switch 36 controlled by the detector selection logic 32 via the feedback control connection 38. Thus, the detector selection logic 32 is able to individually control the switches 36 through the feedback control connection 38 and thereby determine which of the photodetectors 22 will be connected to and be employed by the remaining circuitry at any particular time.
Figure 5 is a simplified flowchart of possible logic to be implemented within the detector selection logic 32 to accomplish the objectives stated earlier herein. As indicated by block 5.1, the prime objective of the logic 32 is to employ the least number of photodetectors 22 (preferably one) which will provide an adequate
signal. At decision block 5.2, the logic 32 checks to see if the signa strength is adequate. If it is not, at block 5.3 it attempts to add th signal from another photodetector 22. At decision block 5.4, th logic 32 checks to see if the SNR is too low. If it is, at block 5.5 i attempts to change the photodetector 22 or add anothe photodetector 22, as best calculated to solve the problem under th dynamic conditions encountered. At decision block 5.6, the logic 3 checks to see if there is a saturation condition. If there is, at bloc 5.7 it attempts to change the photodetector 22 to one which is no saturated and can provide an adequate signal.
Wherefore, having thus described the present invention, wha is claimed is:
Claims
1. In the receiver of an optical communications system employing a plurality of photodetectors for providing electrical signal outputs as a result of incident light thereon, the improvement comprising: a) a plurality of switch means for connecting respective ones of the photodetectors into following circuitry; b) sensing means for sensing a signal quality criteria of respective ones of the photodetectors; and, c) decision and switching logic means connected to said sensing means and said plurality of switch means for connecting and disconnecting respective ones of the photodetectors from said following circuitry to optimize an electrical signal which is a function of incident light output to said following circuitry.
2. The improvement to the receiver of an optical communications system of claim 1 wherein: said decision and switching logic means includes logic for connecting a minimum number of the photodetectors which will provide an adequate signal to said following circuitry.
3. The improvement to the receiver of an optical communications system of claim 1 wherein: a) said sensing means includes means for sensing saturation of respective ones of the photodetectors; and, b) said decision and switching logic means includes logic for disconnecting a photodetector which is saturated.
4. The improvement to the receiver of an optical communications system of claim 1 wherein: a) said sensing means includes means for sensing signal strength from connected ones of the photodetectors; and, b) said decision and switching logic means includes logic for adding an additional photodetector when signal strength is too low to provide an adequate signal to said following circuitry.
5. The improvement to the receiver of an optical communications system of claim 1 wherein: a) said sensing means includes means for sensing the signal-to-noise ratio from connected ones of the photodetectors; and, b) said decision and switching logic means includes logic for changing photodetectors when the signal-to-noise ratio is too low to provide an adequate signal to said following circuitry.
6. In a receiver for an optical communications system employing photodetectors for providing electrical signal outputs as a result of incident light thereon, the improved method of construction and operation comprising the steps of: a) prior to operation, al) disposing a plurality of photodetectors in association with the receiver in a pattern most likely to have incident light impinge thereon, and a2) connecting a plurality of switches between respective ones of the photodetectors and following circuitry; and, b) at the time of operation, bl) sensing a signal quality criteria of respective ones of the photodetectors, and t>2) using the signal quality criteria to connect and disconnect respective ones of the photodetectors from the following circuitry to optimize an electrical signal which is a function of incident light output to the following circuitry.
7. The method of claim 6 wherein the steps thereof include the step of: connecting a minimum number of the photodetectors which will provide an adequate signal to the following circuitry.
8. The method of claim 6 wherein the steps thereof include the steps of: a) sensing saturation of respective ones of the photodetectors; and, b) disconnecting a photodetectors which is saturated.
9. The method of claim 6 wherein the steps thereof include the steps of: a) sensing signal strength from connected ones of the photodetectors; and, b) adding an additional photodetector when signal strength is too low to provide an adequate signal to the following circuitry.
10. The method of claim 6 wherein the steps thereof include the steps of: a) sensing the signal-io-noise ratio from connected ones of the photodetectors: and, b) changing photodetectors when the signai-to-noise ratio is too low to provide an adequate signal to the following circuitry.
11. A receiver for use in an optical communications system where the receiver must receive incoming light signals from a plurality of differing fields of view and constantly generate an adequate electrical signal therefrom for use by following circuitry comprising: a) a plurality of photodetectors for providing electrical signal outputs as a result of incident light thereon, said plurality of photodetectors being positioned to receive the incoming light signals from respective ones of the differing fields of view; b) a plurality of switches connecting respective ones of said photodetectors into the following circuitry; c) sensing means for sensing a signal quality criteria of respective ones of said photodetectors; and, d) decision and switching logic means connected to said sensing means and said plurality of switches for connecting and disconnecting respective ones of said photodetectors from the following circuitry to optimize an electrical signal which is a function of incident light output to the following circuitry.
12. The receiver for an optical communications system of 5 claim 11 wherein: said decision and switching logic means includes logic for connecting a minimum number of said photodetectors which will provide an adequate signal to the following circuitry.
o 13. The receiver for an optical communications system of claim 11 wherein: a) said sensing means includes means for sensing saturation of respective ones of said photodetectors; and, b) said decision and switching logic means includes s logic for disconnecting a photodetector which is saturated.
14. The receiver for an optical communications system of claim 11 wherein: a) said sensing means includes means for sensing o signal strength from connected ones of said photodetectors; and, b) said decision and switching logic means includes logic for adding an additional photodetector when signal strength is too low to provide an adequate signal to the following circuitry.
5 15. The receiver for an optical communications system of claim 11 wherein: a) said sensing means includes means for sensing the signal-to-noise ratio from connected ones of said photodetectors; and, b) said decision and switching logic means includes logic for changing photodetectors when the signal-to-noise ratio is too low to provide an adequate signal to the following circuitry.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US63959191A | 1991-01-09 | 1991-01-09 | |
| US639,591 | 1991-01-09 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| WO1992012580A1 true WO1992012580A1 (en) | 1992-07-23 |
Family
ID=24564755
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| PCT/US1991/006566 WO1992012580A1 (en) | 1991-01-09 | 1991-09-11 | Switched diversity for optical communications |
Country Status (2)
| Country | Link |
|---|---|
| AU (1) | AU8734491A (en) |
| WO (1) | WO1992012580A1 (en) |
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2298752A (en) * | 1995-03-06 | 1996-09-11 | Nec Corp | Portable electronic apparatus having an infrared communication function |
| GB2300986A (en) * | 1995-05-15 | 1996-11-20 | Asahi Kogaku Kk | Optical data communication apparatus for printer |
| KR100420232B1 (en) * | 1995-07-05 | 2004-05-31 | 코닌클리케 필립스 일렉트로닉스 엔.브이. | A communication system including a transmitting apparatus and a receiving apparatus |
| WO2018193251A1 (en) * | 2017-04-18 | 2018-10-25 | Purelifi Limited | Mobile device for optical wireless communication |
Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4864651A (en) * | 1985-10-22 | 1989-09-05 | Canon Kabushiki Kaisha | Light communication apparatus with tracking ability |
-
1991
- 1991-09-11 AU AU87344/91A patent/AU8734491A/en not_active Abandoned
- 1991-09-11 WO PCT/US1991/006566 patent/WO1992012580A1/en unknown
Patent Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4864651A (en) * | 1985-10-22 | 1989-09-05 | Canon Kabushiki Kaisha | Light communication apparatus with tracking ability |
Cited By (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2298752A (en) * | 1995-03-06 | 1996-09-11 | Nec Corp | Portable electronic apparatus having an infrared communication function |
| GB2298752B (en) * | 1995-03-06 | 1997-04-09 | Nec Corp | Portable electronic apparatus having an infrared communication function |
| AU695083B2 (en) * | 1995-03-06 | 1998-08-06 | Nec Corporation | Portable electronic apparatus having a plurality of infrared elements |
| GB2300986A (en) * | 1995-05-15 | 1996-11-20 | Asahi Kogaku Kk | Optical data communication apparatus for printer |
| KR100420232B1 (en) * | 1995-07-05 | 2004-05-31 | 코닌클리케 필립스 일렉트로닉스 엔.브이. | A communication system including a transmitting apparatus and a receiving apparatus |
| WO2018193251A1 (en) * | 2017-04-18 | 2018-10-25 | Purelifi Limited | Mobile device for optical wireless communication |
| US10439714B2 (en) | 2017-04-18 | 2019-10-08 | Purelifi Limited | Mobile device for optical wireless communication |
| US10855369B2 (en) | 2017-04-18 | 2020-12-01 | Purelifi Limited | Mobile device for optical wireless communication |
Also Published As
| Publication number | Publication date |
|---|---|
| AU8734491A (en) | 1992-08-17 |
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