Distributed infrared communication system

A two way communication system is described wherein a base unit can transfer information with remotely-located unit such as a portable infrared commuicator. The base unit produces carrier frequency signals to a plurality of distributed transmitter modules which expose different areas to infrared transmissions. A plurality of down-link or base unit associated infrared detection modules are distributed to receive infrared carrier frequency communications from one or more remote or portable units and convert these to electrical signals at the same carrier frequencies. The latter electrical signals are delivered to the base unit through a common cable. This cable also serves to deliver electrical power to the base unit to the latter modules. The electrical carrier frequency signals are combined at the base unit which uses this to provide the base unit with the information detected from the remote unit and produce a signal representative of the detected infrared energy. The latter signal may then be used to control the output power from the remote or portable units.

FIELD OF THE INVENTION 
This invention generally relates to a two-way infrared communication system 
for an enclosed space and more specifically to such a system with a 
plurality of infrared channels for use within a common enclosure such as 
an office, factory, or warehouse, and the like. 
BACKGROUND OF THE INVENTION 
A full two-way infrared telephone system exists as described in my 
copending U.S. patent application Ser. No. 619,803, filed June 12, 1984, 
now U.S. Pat. No. 4757553 entitled COMMUNICATION SYSTEM WITH PORTABLE 
UNIT. This patent application is incorporated herein by reference. The 
infrared telephone system uses a cordless handset that is portable as with 
rf cordless telephones. The infrared telephone system includes a base unit 
having an up-link infrared transmitter to send an audio FM modulated 
infrared carrier to the handset receiver and a base unit infrared receiver 
to detect an audio FM modulated infrared carrier back from a handset 
located infrared transmitter. Battery power of the portable handset is 
conserved by use of a power control circuit whereby the optical power from 
the handset, or down-link infrared transmitter, is kept at a minimum level 
needed to obtain good performance. This is achieved by sensing the level 
of the infrared signals at the infrared receiver in the base-unit and then 
varying the average frequency of the carrier applied in the base unit to 
the up-link infrared transmitter. This carrier frequency shift is sensed 
in the handset and used to vary the optical power from the handset 
completing a control loop. In the process the up-link carrier frequency is 
controlled at the center of the pass band of the handset infrared 
receiver. 
SUMMARY OF THE INVENTION 
In a two way infrared communication system in accordance with the invention 
reliable and full coverage infrared communication can be established over 
variable length optical paths within an enclosed area such as an office, 
factory, storage area, telephone exchange, a vehicle and the like. 
This is achieved in one embodiment in accordance with the invention with a 
base unit that is connected to a plurality of spatially distributed 
infrared transmitter modules and infrared receiver modules so that a 
portable, or down-link, infrared communicator may establish communication 
with the base unit. The optical power signal from the portable unit is 
controlled by detecting the level of the combined base receiver module 
output which is representative of optical power incident upon them. This 
detected level signal is relayed up-link by various means signals and is 
used to establish just enough optical power from the portable communicator 
to establish a satisfactory communication. 
As further described herein, coaxial cables are used to transmit 
communication carrier signals between the modules and the base unit as 
well as provide electrical power to the modules. The modules may be 
connected in parallel and a large number can be used in a large area that 
is serviced by one base unit. 
When optical power from the portable unit is controlled, its battery energy 
is conserved, the signal to noise ratio at the base receiver modules is 
held low at a fixed value and the base frequency of the plurality of the 
up-link transmitters is kept centered on the pass-band of the portable 
unit's receiver. The dynamic range of the optical power incident upon the 
base receiver modules can be kept within acceptable limits so that the 
most proximate up-link receiver module receives neither too little nor too 
much optical power. 
With a communication system in accordance with the invention a plurality of 
such two-way infrared communication systems can be used within a common 
enclosure without causing an overload at any one down-link receiver and 
without interference between the channels. This is achieved in one 
embodiment by placing distributed base infrared receiver modules 
associated with the different channels in proximity to each other, with 
the same optical view The same close placement is made with the up-link 
base transmitter modules of different channels. 
In such case, when a portable unit is close to one of its associated 
infrared down-link base receiver modules, the automatic power control 
causes a cut back of the optical power from the portable unit and avoids 
excessevely high optical power input to the also adjacent up-link infrared 
receiver module of the other channel. 
If, this other channel, for example, happens to be in a high power demand 
mode, because its associated portable unit is at a remote location 
relative to an up-link receiver, the need to provide special skirt filters 
to avoid cross-talk problems and saturation of optical detectors are 
avoided. The combined received desired signal level incident on each 
receiver module group is at the same level as the combined interfering 
signal level As further described herin, the proximity mounting of 
receivers of different channels and the power control feature enables use 
of a common up-link receiver module whose bandwidth is sufficiently wide 
to accomodate more than one channel. In such case the amount of cabling as 
well as the number of different modules can be advantageously reduced. 
Similarly, the colocation of transmitter modules which illuminate a common 
field assures equal desired as well as interfering signal levels at the 
portable unit's receiver. This condition can be conveniently tolerated 
with simple filtering. 
It is, therefore, an object of the invention to provide a two-way infrared 
communication system for a variable optical path within an enclosure and 
which system provides reliable communication with conserved electrical 
power, a low level of complexity and is easily expandable and convenient 
and flexible to install and use. It is another object of the invention to 
provide multiple channel two-way infrared communication systems capable of 
operating within a common enclosure without interference between the 
channels while preserving battery power and the advantages of a single 
channel system. 
These and other objects and advantages of the invention can be understood 
from the folllowing description of several embodiments described with 
reference to the drawings.

DETAILED DESCRIPTION OF THE DRAWINGS 
With reference to FIG. 1 an infrared communication system 20 in accordance 
with the invention is shown inside a room 21 and is formed of a base unit 
22 that is coupled through coaxial cables 24, 26 to respective spatially 
distributed base unit infrared receiver modules 28 and up-link infrared 
transmitter modules 30. The system 20 enables infrared communication with 
a portable unit 32 held by a person as shown. Portable unit 32 is powered 
by a battery pack contained in a transceiver 34 and is formed with at 
least one up-link infrared receiver 36 and a down-link infrared 
transmitter 38. These infrared devices are mounted on a waist belt 40 that 
is conveniently worn as shown. In order to provide maximum spatial 
coverage a second receiver 36 and second transmitter 38 are placed (not 
shown) on the belt, preferably at diametrically opposite belt locations. 
These second down-link infrared devices are not always needed and may be 
deleted particularly when battery power is to be conserved. A microphone 
42 is shown for providing audio signals for transmission via transceiver 
34 and infrared transmitter 38 to any one of the infrared receiver modules 
28 and thence via coaxial cable 24 to base unit 22. In the other 
direction, audio signals can be sent from base unit 22 via coaxial 26 to 
infrared modules 30, all of which simultaneously transmit so that portable 
unit infrared receiver 36 can provide appropriate audio to ear phone 44 
via transceiver 34. The transceiver 34 includes circuitry as illustrated 
in FIG. 7 for the handset portion. This includes the FM receiver dual 
slope gain circuit (optional), filter and earpiece driver networks 
generally indicated at 45 for the receiver section and the amplifier, 
filter, dual slope gain (optional), VCO and infrared diode driver for the 
transmitter section 46. Instead of audio data can be transmitted between 
base unit 22 and portable unit 32. 
In the system 20 many infrared receiver modules 28 and transmitter modules 
30 can be added over very long coaxial cable lengths, which can be of the 
order of a thousand feet long. The modules 28, 30 are all connected in 
parallel and electrical power is supplied through the same coaxial cables 
24, 26 through which the information signals flow. 
FIG. 2 illustrates the system 20 with greater detail. At base unit 22 
appropriate FM modulated sinewave carrier signals are generated on a line 
50 by a VCO (voltage controlled oscillator) 52 that is modulated by the 
output of a summing network 54. The latter has an audio or data input 56, 
such as from a telephone line as well as a power level control signal on 
line 58. A sinewave carrier is used to make synchronization of the 
transmitter modules 30 relatively immune to reflections in the coaxial 
cables 26. 
The carrier signal is supplied through a DC blocking capacitor 60 to the 
central conductor 62 of coaxial cable 26 whose shield 64 is connected to 
ground. A dc voltage power source 66 is applied through an rf high 
impedance network such as a coil 68 to center conductor 62 of cable 26 to 
supply electrical power to up-link infrared transmitter modules 30.1 and 
30.2. The coaxial cable 26 is shown connecting the modules 30 in parallel 
and a cable terminator 70 is used to reduce cable end reflections. The 
transmitter modules 30 act as high impedance loads to the carrier sinewave 
signal, yet can draw DC power from the same cable. 
The receiver modules 28 associated with base unit 22 are so designed that 
electrical power is received from the center conductor 72 of coaxial cable 
24. The receiver modules 28, however, act as constant current sources at 
carrier frequencies. Each receiver module 28 responds to incident infrared 
optical power by drawing a corresponding amount of current from an emitter 
80 of a transistor 82. The emitter collector junctions are in series with 
the common central conductor 72. The total carrier current drawn through 
the emitter 80 thus in effect combines, by summing, the various signals 
indicative of the optical power incident on the individual modules 28. 
A tank circuit 84 is used on the collector 86 and the carrier is ac coupled 
to an FM receiver 88. A signal is derived from receiver 88, such as from 
its rf stage, by a level detector 90 whose output is a DC level signal 
used to control the output power of the portable or down-link infrared 
transmitters 38. 
DC power is supplied by power supply 66 through emitter 80 and the center 
conductor 72 to the modules 28. A DC blocked matching terminator 92 is 
used at the end of cable 24 to reduce reflections. 
The portable unit 32 is designed so that it transmits no more than the 
optical power needed for a pre-set signal level at the base receiver 
module inputs. This is achieved by combining the portable receiver module 
outputs 94.1 and 94 2 at an FM receiver 96. The latter has a discriminator 
output 98 that is applied to one input 100 of a pseudo integrator 
differential amplifier 102. The other input 104 of amplifier 102 is 
connected to a DC reference level 106 that is set at the voltage level 108 
(see FIG. 2A) that is generally at the center 110 (FIG. 2A) of the pass 
band B of the discriminator 109 in receiver 96. The output of amplifier 
102 is applied to a driver for the portable unit transmitters 38.1 and 
38.2 to cause a change in infrared output power in the direction required 
to compensate for the change in the level signal on line 58 in base unit 
22 while keeping the carrier frequency centered on the pass band of the 
portable receiver modules. 
With such optical output power control, battery drain can be held to a 
minimum level consistent with good performance. The gain of the loop is 
sufficiently high to restrict carrier excursions to a range equal to about 
ten percent of the receiver bandwidth. Automatic tracking to the remote's 
band center is obtained. 
The audio part of the output from discriminator 109 is applied through 
appropriate networks and amplifier 112 to an earphone 114. 
Audio signals from microphone 42 (see FIG. 1) are applied in FIG. 2 on line 
116 &:o an FM carrier modulator 118 which has its output in turn applied 
to down-link infrared transmitter 38. 
A particularly advantageous feature of the control loop for the infrared 
optical power is that it establishes in effect a generally constant signal 
to noise ratio at the down-link or base unit receivers 28. 
This is desirable for voice recognition systems and other systems which are 
sensitive to noise and require removal of its effect for activities such 
as voice analysis or voice controlled switching. 
FIG. 3 illustrates the base unit receiver module 28 and transmitter module 
30 with greater detail. The receiver module 28 includes a plurality of 
infrared sensitive diodes 120 in parallel and in series with a tank 
circuit 122. The drain 124 of an FET 126 acts as a constant current signal 
source to center conductor 72 but at carrier frequencies. Electrical DC 
bias for the photodiodes is supplied through resistor 128. 
The transmitter module 30 has a limiter 130 and a logic amplifier 132 to 
drive infrared output diodes 134. Electrical DC power is supplied through 
a high rf impedance coil 136. 
FIGS. 4 and 5 illustrate various configurations and placements of base unit 
transmitter modules 30 and receiver modules 28 in an enclosed space to 
assure adequate infrared communication with a portable unit 32. 
With reference to FIG. 6 an infrared two-way communication system 150 is 
shown including two separate channels A and B operating at different 
carrier frequencies. Each channel includes a base unit 22 and a plurality 
of infrared receiver modules 28 and infrared transmitter modules 30, 
connected in parallel to coaxial cables 24 and 26 respectively. 
In system 150, however, the base unit receivers 28a and 28b are mounted in 
proximity to each other so as to effectively have the same field of view. 
The same pairing is done with infrared transmitters 30a and 30b. In system 
150 the infrared power control loop is essential. Since each channel 
controls its respective portable unit's output optical power, the received 
infrared signal is constant at a predetermined level. As a result spurious 
responses, interfering crosstalk, and intermodulation effects are reduced 
to a negligible level with only a moderate amount of selectivity required 
to be incorporated that can be implemented with simple circuits. 
Hence, interfering signal levels that could be up to 80db higher than the 
desired signal level, as might occur in certain locations and orientations 
of the portable units, are avoided. Such 80db disparity is well beyond the 
infrared power/current linearity characteristics of silicon photodiodes 
and would introduce intermodulation effects that would not be removable 
with filtering. 
The same problems exist for the portable units 32. Hence, the paired 
location of the base unit transmitter modules at each site forces the 
desired and interfering signals at each portable receiver to be equal and, 
therefore, reduces interference and intermodulations. 
Since the receiver modules from all systems are closely spaced at each 
site, it is possible to use a single receiver module for all the systems 
provided it has a sufficiently broad pass band. By reducing R2 to a value 
of R2/n (See FIG. 3.), where n is the number of systems or channels, the 
bandwidth car: be made sufficiently wide to accomodate signals from all 
the portable transceivers. The resulting effect on sensitivity is as 
follows: 
For systems located in environments where the effect of in-band ambient 
optical noise is substantially greater than the electronic thermal noise 
of R2, then reduction of R2 will have no effect on system sensitivity (in 
effect the signal to noise ratio). 
When the effect of ambient optical noise is less than the electronic noise 
of R2, then reducing R2 to R2/n will degrade the signal voltage to noise 
voltage density ratio (E/(V/&lt;H.sub.z), by a factor 1/&lt;n. To retain the 
same system sensitivity, the photodiode area (or number) must increase by 
a factor of n (the equivalent of n receiver module). 
By using a common receiver module 28, n-1 coaxial cable runs are eliminated 
and a common field of view is assured. In addition n-1 receiver modules 
are eliminated. In the latter case the modules are replaced by a larger 
receiver module, having n times the original receiver module diode area. 
Since transmitter (T) modules 30 from all systems are located at each T 
module site, it is also advantageous to employ a single transmitter module 
for all systems provided intermodulation of transmitter carriers is held 
to acceptable levels. 
The down-link or base unit receiver modules 28 have an appearance as 
illustrated in FIGS. 8-10. A generally open ended channel casing 150 made 
of a reflective metal is closed at one end 152 by a metal cap. A base unit 
infrared receiver circuit is mounted on a circuit board 154, the top of 
which also has a ground plane 156 that is electrically connected to casing 
150. 
The array 120 of silicon photodiodes that are sensitive to infrared are 
surrounded by a plastic ring 158. The diodes face upwardly to receive and 
detect infrared light. The ring 158 is adhesively mounted to board 154 and 
a clear, infrared transparent epoxy material 160 is placed inside the ring 
158. An infrared transparent ball 162, made of an acrylic material, is 
placed in the epoxy, which bonds to it. Since the refractive indices of 
the epoxy 160 and ball 162 are almost the same, the infrared light 
capturing capability of the diodes is enhanced without air/surface 
interfaces. 
The optical structure 164 is covered by an infrared transparent plastic 
plate 166 made of a plastic known as Lexan a polycarbonate plastic , the 
inside of which is provided with a fine grid 168 of conductive material 
that forms a faraday shield. Conducting stand-offs 170 provide electrical 
conduction between the grid 168 and ground plane 156. Hence, the high 
output impedance photodiodes are mounted within a faraday shield to 
protect them from stray electrical fields while allowing infrared light to 
pass through. 
The recessed mounting of the optical structure 164 within casing 159 forms 
a "dead-air" pocket enabling an essentially dust free mounting when 
tilting the receiver module relative to the vertical in a manner as 
illustrated in FIGS. 4 and 5. 
The up-link transmitter modules also are mounted in a casing such as 150. 
However, the complex optical structure 164 is not used and its cover plate 
166 is more deeply recessed. A wide effective field of view is still 
obtained by virtue of the multiple reflections by the inside of casing 
150. 
The portable unit 32 infrared receiver 36 has a construction as shown in 
FIG. 11. A flanged metal cup 180 and 182, externally threaded,enclosed the 
infrared receiver circuit. The circuit includes a plurality of infrared 
sensitive photodiodes 186, as illustrated in the schematic block diagram 
of FIG. 7. A hemispherically shaped lens 188 is placed above the diodes 
186 and directly bonded thereto with an epoxy or another suitable clear 
adhesive that eliminates air interfaces. 
An infrared transparent hemispherical enclosure 190, having a lower metal 
rim 192, is used to cover the lens 186 and circuitry as shown. A faraday 
shield is obtained by placing a wire mesh 194 along the inside of the 
cover 190. The mesh 194 is selected to cause as little light blockage as 
possible while still protecting against electrical interference. A mesh of 
one mil wires at ten mil spacings was found effective and blocked only 20 
percent of the light. The lower rim of the mesh 194 is connected to an 
annular metal tape with a conductive adhesive 198 which makes electrical 
contact with the upper edge of metal ring 182 when rim 192 of cover 190 is 
screwed onto the ring 182. 
An infrared transmitter 38 on portable unit 32 has a similar construction 
as FIG. 11 except it has only photodiodes such as at 200 in FIG. 7 and no 
faraday shield. In one embodiment two infrared transmitters 38 are used, 
one with five infrared generating photo diodes, the other with four. The 
diodes 200 of both transmitters 38 are connected in series by use of 
suitable wires embedded in the belt 40 (See FIG. 1) and emanating from and 
returning to transceiver 34. 
Having thus described several embodiments of the invention, its advantages 
can be appreciated. Variations can be made without departing from the 
scope of the following claims. For example, other techniques can be used 
to establish control over the power of the portable unit's infrared output 
power. For example, inaudible control tones could be used. 
The embodiments described herein illustrate a portable unit 32 that is 
battery powered and moveable depending upon where the person carrying it 
moves. It should be understood, however, that other configuration are 
contemplated by the invention. Hence, the term portable as used herein 
means an infrared transmitter and receiver device which may be powered by 
line power, and may be movable, rotatable or with undefined orientations 
at a fixed location. As such, the optical path loss is variable or 
otherwise undefined so as to be unpredictable. 
The embodiment described depicts base systems with a single run of coaxial 
cable tapped by parallel connected receiver modules or transmitter 
modules. The general method, however, permits use of twisted pair wiring 
(instead of coaxial cable) in some installations. In addition, since the 
carrier frequencies can be quite low (50 KHz to 2 MHz), virtually 
unlimited branching of cables is possible permitting high flexibility of 
configuration.