United States Patent 19
`5,355,242
`Eastmondet al.
`[45] Date of Patent:
`Oct. 11, 1994
`
`US005355242A
`[11] Patent Number:
`
`[54] RECEIVER FOR BINARY CODED
`WIRELESS OPTICAL DATA
`
`[75]
`
`Inventors: Bruce C, Eastmond, Downers Grove;
`Rachid M. Alameh, Schaumburg;
`Kevin W. Jelley, LaGrange Park,all
`of Tl.
`
`[73] Assignee: Motorola, Inc., Schaumburg,Ill.
`[21] Appl. No.: 42,909
`[22] Filed:
`Apr. 5, 1993
`
`[SE] Unt, CaS one eesescesessecsseeseaserssees HO4B 10/06
`[52] U.S. Ch ooeeeeseescesereeeene 359/189; 250/214 A;
`330/59
`{58] Field of Search............... 359/154, 157, 188, 189,
`359/194; 250/214 A; 330/276.59, 308
`
`[56]
`
`References Cited
`.
`U.S. PATENT DOCUMENTS
`
`1/1984 Skerlos et al. 1....esccscsoeees 359/194
`4,426,662
`
`4,679,252 7/1987 Holland .........csssecessseseeesenee 359/194
`5,012,202, 4/1991 Taylor .......cesessessesssseeeseeses 359/194
`
`8/1993 Little, Jr. et al. we 359/194
`5,239,402
`FOREIGN PATENT DOCUMENTS
`
`............ 359/189
`0372742 6/1990 European Pat. Off.
`0100532 4/1990 Japan oo...esesssesseesseeneseeees 359/189
`
`Primary Examiner—Herbert Goldstein
`Assistant Examiner—Kinfe-Michael Negash
`Attorney, Agent, or Firm—Timothy W. Markison;
`Steven G. Parmelee
`
`[57]
`
`ABSTRACT
`
`A wireless binary coded optical data receiver receives
`the binary encoded data via a photodiode. The binary
`encodeddata is then supplied to the primary winding of
`a transformer and thus coupled to the secondary wind-
`ing of the transformer. The secondary winding is cou-
`pled to a transimpedance amplifier wherein the tran-
`simpedance amplifier buffers the received binary en-
`coded data. The buffered binary encoded data is then
`amplitude limited by a limiting circuit.
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`15 Claims, 2 Drawing Sheets
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`U.S. Patent
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`Oct. 11, 1994
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`Sheet 1 of 2
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`5,355,242
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`U.S. Patent
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`Oct. 11, 1994
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`Sheet 2 of 2
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`5,355,242
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`FIGC.2A-2
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`1
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`5,355,242
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`RECEIVER FOR BINARY CODED WIRELESS
`OPTICAL DATA
`
`FIELD OF THE INVENTION
`This invention relates generally to optical data com-
`munication systems and, in particular, to receivers of
`wireless binary coded optical data.
`
`BACKGROUND OF THE INVENTION
`Asis known,data signals may be transmitted by am-
`plitude modulating an optical signal in response to the
`data. The optical signal is typically generated by a semi-
`conductor laser or light-emitting diode, propagates
`through a guiding medium, such as fiber, or through
`space, and is directly detected by a semiconductor pho-
`todiode. The optical signals commonly employed for
`data transmission have a wavelength whichis located in
`the infrared portion of the electromagnetic spectrum.
`An example of an optical fiber data communication
`system is the Fiber Distributed Data Interface (FDDI)
`which transmits a continuous stream of data at 125
`Mbit/sec. Examples of wireless optical data communi-
`cation systemsinclude: the ubiquitous TV/VCR remote
`control, which transmits bursts of pulses at a low effec-
`tive bit rate and typically requires orientation of the
`transmitter;
`the BICC Communications InfraLAN,
`which transmits IEEE 802.5 token-ring standard data
`at a 4 Mbit/second rate and requires orientation of both
`the transmitter and receiver; and the Photonics Corpo-
`ration Infrared Transceiver for Mobile Computing,
`which transmits data packets at a 1 Mbit/sec rate with-
`out requiring orientation. Wireless optical data commu-
`nication systems, unlike their fiber counterparts, must
`contend with the presence of ambient light which,
`when detected, generates a direct current (dc) signal
`which may inhibit reception. In addition, the received
`signal power range of wireless optical data signals is
`significantly greater than the signal power range of
`optical data signals guided through fiber, especially if
`the propagation path of the wireless signals involves
`reflections which diminish the signal amplitude.
`Since the release of the IEEE 802.3 1OBASE-T twist-
`ed-pair Ethernet standard, 10 Mbit/sec Ethernet has
`rapidly becomethe de facto standard for local commu-
`nication among PCs (personal computers) and worksta-
`tions located on office desktops. Portable computers are
`now available which include provisions for the Ether-
`net controller function and 10OBASE-T wired network
`connectivity. In addition, operating systems and soft-
`warethat incorporatefile sharing permits portable com-
`puter users to convene and work together on a common
`documentor othertask if a limited-range wireless net-
`work could be readily established to link two or more
`computers together without requiring infrastructure.
`The use of a common format for both wired and wire-
`less data minimizes the hardware and software com-
`plexity of both the portable computer and the wireless
`access ports connected to the wired network.
`While Ethernet offers many advantages, eavesdrop-
`ping is possible if the PC’s Ethernet connection is a
`typical radio frequency (RF) transceiver. A typical RF
`transceiver has a transmission rangeof up to half a mile.
`Thus an eavesdropping receiver could be up to half a
`mile away and still intercept the PC’s transmission.
`Such an interception would be almost impossible to
`detect. Dueto the high transmission rate of Ethernet (10
`Mbit/second), prior art wireless optical data communi-
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`cation systems are not readily usable for Ethernet be-
`cause the transmission rate is much slower.
`As a result, a need exists for a sensitive optical re-
`ceiver for binary coded wireless optical data in a packet
`format, such as Ethernet, which minimizes ambient
`light degradation, has a wide acceptance angle so as to
`minimize the need to point or aim the receiver in a
`particular direction, and has a relatively short transmis-
`sion range.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG.1 illustrates a wireless optical receiver in accor-
`dance with the present invention.
`FIGS. 2A-2B illustrates light collection means for
`use with the wireless optical receiver of FIG. 1.
`
`DESCRIPTION OF A PREFERRED
`EMBODIMENT
`
`FIG.1 illustrates a receiver for high-speed binary
`encoded wireless optical data that comprises optical
`receiving means (100 and 101), first and second trans-
`formers (102 and 103), transimpedance amplifiers (104
`and 105), feedback resistors (106 and 107), and limiting
`means(108). The optical receiving means (100 and 101)
`comprises one or more reversed-biased photodiodes,
`parallel-connected to form arrays. The arrays (100) and
`(101) may be reversed biased by a voltage multiplier
`(109). The first photodiode array (100) is connected to
`the primary winding of the first untuned transformer
`(102) and the second photodiode array (101) is con-
`nected to the primary winding of the second untuned
`transformer (103). An example of a transformer suitable
`for use with high-speed binary encoded data signals
`such as Ethernet is the Coilcraft LAX10T-100SM.
`Whenlight is incident on either array (100) or (101),
`a currentis producedin the primary windingofthefirst
`or second untuned transformer (102) or (103), respec-
`tively, which has a mean value equal to the product of
`the light source irradiance at the surface of the array,
`the photodiode responsivity, the surface area of the
`array andthe optical gain preceding the array. Thelight
`incident on the surface of photodiode arrays may in-
`clude both the desired intensity-modulated infrared data
`signals as well as unmodulated ambient light due to
`daylight, interior lighting, etc. The direct current due to
`unmodulated ambient
`light sources is
`returned to
`ground while the binary encoded data signals are passed
`from the primary to the secondary windingofthefirst
`or second untuned transformers (102) and (103).
`The secondary winding of the first untuned trans-
`former (102) is coupled through capacitor (115)to tran-
`simpedance amplifier (104) having feedback resistor
`(106). In a similar way the secondary winding of the
`second untuned transformer (103) is coupled through
`capacitor (116) to transimpedance amplifier (105) hav-
`ing feedback resistor (107). The presence of feedback
`resistors (106 and 107) ensure that the transimpedance
`amplifiers have the necessary low input impedance,
`approximately 100 ohms, to provide proper impedance
`matching to the first and second untuned transformers
`and minimal frequency response degradation due to the
`photodiode array capacitance. The first and second
`untuned transformers (102) and (103) may be designed
`to have a limited low frequency response. Extended
`low frequency response is not necessary with binary
`encoded data signals such as Manchester-encoded data
`signals since the data signal spectrum has minimal en-
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`ergy at frequencies less than approximately 5% of the
`bit rate. It is advantageous to limit the low frequency
`response of the receiver in order to eliminate interfer-
`ence caused by ambient fluorescent lighting and mini-
`mize the time duration of the dc bias shifts in subsequent
`receiver stages associated with the detection of a data
`packet. The secondary windings offirst and second
`untuned transformers (102) and (103) are reversed in
`phase to provide differential signal
`inputs to tran-
`simpedance amplifiers (104) and (105), respectively.
`Thelimiting means (108) comprises a bandpassfilter
`(111), amplifiers (112 and 114), and a low pass filter
`(113). The outputs of transimpedance amplifiers (104)
`and (105) are coupled through a balancedfilter (111) to
`the input of amplifier (112). The balanced filter (111)
`may be a low-passfilter. If the binary encoded data
`signals are Ethernet, a filter such as the Coilcraft
`LAFI0T-5B may be employed. Balanced filter (111)
`may include means for limiting the low frequency re-
`sponse, such as an R-C high-passfilter, if such a means
`is not incorporated in the design of transformers (102)
`and (103) as previously discussed. Amplifier (112) may
`have a bandwidth sufficient to pass the binary encoded
`data signal with acceptably low distortion and may
`have provisions to derive a received signal strength
`indication (RSSI). A suitable amplifier for Ethernet
`data signals is the Motorola MC13155.
`An additional stage of R-C low-pass filtering (113)
`having a bandwidth sufficient to pass the binary en-
`coded data signals with acceptably low distortion may
`be incorporated between the output of amplifier (112)
`andthe inputoflimiting amplifier (114). Limiting ampli-
`fier (114) may have a bandwidth sufficient to symmetzri-
`cally limit the. amplitude of the binary encoded data
`signal and may. have provisions to derive a received
`signal strength indication (RSSI). A suitable amplifier
`for Ethernet data signals is the Motorola MC13155. The
`output of limiting amplifier (114) consists of a binary
`encoded signal output which may be further processed
`by a packet squelch and squelch gate (110).
`FIGS. 2A~2Billustrates two examplesoflight collec-
`tors which provide optical gain for the receiver of FIG.
`1 over an acceptance angle wider than that of conven-
`tional thin convex or fresnel lenses. A hemispherical
`lens (200) may be optically coupled to photodiode ar-
`rays (100) and (101) to provide optical gain having a
`value equal to the square of the index ofrefraction of
`the lens material and an acceptance half-angle ap-
`proaching +90 degrees. A light cone or compound
`parabolic coliector (CPC) (201) may also be optically
`coupled to photodiode arrays (100) and (101) to provide
`substantially uniform optical gain over the angle of
`acceptance. For the CPC, the optical gain may ap-
`proach the limit 1/sin2qc, where qc is the acceptance
`half-angle. The light collectors may advantageously be
`made from material such as Rohm and Haas Plexiglass
`molding resins 58004 or 58015 which blocks visible light
`but transmits infrared light. Note that with any type of
`optical collector mentioned, the transmission range is
`limited by walls, floors, and ceilings of conventional
`design.
`As shown,the present invention provides a means for
`a receiver capable of receiving binary encoded wireless
`optical data. The present invention minimizeslight deg-
`radation, has a wide acceptance angle minimizing the
`need to aim the receiver in a particular direction, and
`has minimal interference with other RF communication
`devices. Since coverage is confined to a room oroffice,
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`eavesdropping and interference from nearby systemsis
`also minimized.
`Weclaim:
`1. A receiver that receives binary coded optical data
`5 packets comprises:
`at least one photodiodethat receives the binary coded
`optical data packets from a transmitting light
`source to produce received binary encoded data;
`at least one untuned transformer having at least a
`primary winding and a
`secondary winding,
`wherein the primary winding is operably coupled
`to the at least one photodiode such that the re-
`ceived binary encoded data is operably coupled
`from the primary winding to the secondary wind-
`ing;
`at least one transimpedance amplifier operably cou-
`pled to the secondary winding, wherein the tran-
`simpedance amplifier buffers the received binary
`encoded data to produce buffered binary encoded
`data; and
`limiting means, operably coupled to the at least one
`transimpedance amplifier, for limiting amplitude of
`the buffered binary encoded data.
`2. The receiver of claim 1 further comprises a second
`photodiode that receives the binary encoded optical
`data packets to produce second received binary en-
`coded data.
`3. The receiver of claim 2 further comprises a second
`untuned transformer having a primary winding and a
`secondary winding, wherein the secondary winding has
`reverse phase as that of the primary winding and
`wherein the primary winding is operably coupledto the
`second photodiode suchthat the second received binary
`encoded data is operably coupled to the secondary
`winding.
`4. The receiver of claim 3 further comprises a second
`transimpedance amplifier that buffers the second re-
`ceived binary encode data from the secondary winding
`of the second untuned transformer to produce buffered
`second received binary encoded data, wherein the buff-
`ered second received binary encoded datais supplied to
`the limiting means.
`5. In the receiver of claim 1, wherein the limiting
`means further comprises a band pass filter, operably
`connected to a first amplifier, and a low passfilter oper-
`ably connected to said first amplifier, and a second
`amplifier connected to said low passfilter.
`6. In the receiver of claim 1, the at least one photodi-
`ode further comprises a photodiodearray to receive the
`binary encoded optical data packets.
`7. The receiver of claim 1 further comprises a high
`pass filter operably coupled to the secondary winding.
`8. A receiver that receives binary encoded optical
`data packets comprising:
`optical receiving means for receiving the binary en-
`coded optical data packets and for producing re-
`ceived binary encoded data:
`a first untuned transformer having a primary wind-
`ing, a secondary winding, and a predetermined
`frequency response, wherein the primary winding
`is operably coupled to the optical receiving means
`such that the received binary encoded data is cou-
`pled from the primary winding to the secondary
`winding;
`a transimpedance amplifier, operably coupled to the
`secondary winding, wherein the transimpedance
`amplifier buffers the received binary encoded data
`to produce buffered binary encoded data; and
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`limiting means, operably coupled to the transimped-
`ance amplifier, for limiting amplitude of the buff-
`ered binary encoded data.
`9. In the receiver of claim 8, the optical receiving
`means further functions as a light collector with diode
`array, wherein the light collector with diode arrayis
`operably coupled to the first untuned transformer,
`wherein the light collector with diode array receives
`the binary encodedoptical data packets to producefirst
`received binary encoded data.
`10. In the receiver of claim 8, the optical receiving
`means further functions as a photodiode, wherein the
`photodiode is operably coupled to the first untuned
`transformer, wherein the photodiode receives the bi-
`nary encoded optical data packets to producefirst re-
`ceived binary encoded data.
`11. In the receiver of claim 8, the optical receiving
`meansfurther functionsas a first set of photodiodes and
`a second set of photodiodes, wherein the first set of
`photodiodes is operably coupled to the first untuned
`transformer, wherein the first set of photodiodes re-
`ceives the binary encoded optical data packets to pro-
`duce first received binary encoded-data and wherein the
`second set of photodiodes receives the binary encoded
`optical data packets to produce second received binary
`encodeddata.
`12. In tile receiver of claim 8, wherein the limiting
`means further comprises a low pass filter, operably
`connected between first and second amplifiers, and a
`band passfilter operably connected to said first ampli-
`fier.
`13. The receiver of claim 11 further comprises a sec-
`ond untuned transformer having a primary winding and
`a secondary winding, wherein the secondary winding
`has reverse phase as that of the primary winding and
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`6
`wherein the primary winding is operably coupledto the
`second set of photodiodes such that the second received
`binary encoded data is operably coupled to the second-
`ary winding.
`14. The receiver of claim 13 further comprises a sec-
`ond transimpedance amplifier that buffers the second
`received binary encoded data from the secondary wind-
`ing of the second untuned transformer to producebuff-
`ered second received binary encoded data, wherein the
`buffered second received binary encoded data is sup-
`plied to the limiting means.
`15. An improved personal computer that includes a
`binary encoded data detector coupled to a receiverthat
`receives binary coded optical data packets, wherein the
`improvement comprises:
`at least one photodiodethat receives the binary coded
`optical data packets from a transmitting light
`source to produce received binary encoded data;
`at least one untuned transformer having at least a
`primary winding and a
`secondary winding,
`wherein the primary winding is operably coupled
`to the at least one photodiode such that the re-
`ceived binary encoded data is operably coupled
`from the primary winding to the secondary wind-
`ing;
`at least one transimpedance amplifier, operably cou-
`pled to the secondary winding, wherein the tran-
`simpedance amplifier buffers the received binary
`encoded data to produce buffered binary encoded
`data; and
`limiting means, operably coupled to the at least one
`transimpedance amplifier, for limiting amplitude of
`the buffered binary encoded data.
`*
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`UNITED STATES PATENT AND TRADEMARK OFFICE
`CERTIFICATE OF CORRECTION
`
`PATENTNO.
`
`: 5,355,242
`
`: October 11, 1994
`DATED
`INVENTOR(S) : Bruce C. Eastmond, et al.
`It is certified that ervor appears in the above-identified patent and that said Letters Patent is hereby
`corrected as shown below:
`
`In The Claims:
`Column 5 Line 27, "tile" should be --the--.
`
`Signed and Sealed this
`
`Sixth Day of December, 1994
`
`Commissioner of Patents and Trademarks
`Attesting Officer
`
`
`BRUCE LEHMAN
`
`7
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