Method and apparatus for cordless infrared communication

An infrared (IR) communication system is described in which a base unit for a cell can communicate with a plurality of infrared portable devices through distributed infrared receiver/transmitter (RT) modules over a plurality of channels using IR carrier signals. A call processor, referred to as a radio exchange unit, and controlling communication of a cell, communicates with the base unit to place or receive calls with the IR portable devices. The communication occurs in standard communication frames divided into transmission and receiving segments with each segment further divided into slots and with the slots containing digital data, with each communication channel formed by a slot. Each transmission segment to an RT module is immediately followed by a responsive receiving segment. As an IR portable device moves from the vicinity of one RT module to another, the base unit automatically and seamlessly and in a robust manner hands over control to the nearer RT module by monitoring signal strength signals from various RT modules coupled to the base unit. When an IR portable device moves from one cell to another, the call processor hands over control to another base unit using standard protocols. Since an IR portable device may receive IR communication signals from several RT modules, care is taken to avoid signal interference by effectively controlling signal propagation lengths between the base unit and RT modules so that signals arriving from nearby RT modules at a common IR portable device do not have a phase difference more than a predetermined amount. The path lengths can be controlled by selecting cable lengths or by insertion of delays between the base unit and the RT modules to assure that IR signals arrive at portable devices with a minimum amount of interference.

FIELD OF THE INVENTION 
This invention generally relates to a method and apparatus for 
communicating from a base unit to a plurality of portable infrared units 
located randomly throughout a building while using established 
communication protocols. More specifically this invention relates to a 
digital communication method and system for enabling a central control 
connected to telephone lines to communicate with a plurality of portable 
infrared devices, such as handsets and the like, located within an 
enclosed site. 
BACKGROUND OF THE INVENTION 
Several systems for digital communication with portable devices have been 
described. For example in an article entitled Cordless Personal 
Communications by a Dr. W. Tuttlebee and published in the IEEE 
Communications Magazine of December 1992 at pages 42-53, various systems 
for digital cordless telephony are discussed. Much of such activity has 
taken place in Europe where several wireless data standards have emerged 
in recent years, such as the CT2, CT3 and DECT standards. 
The principal function of such standards is to enable digital communication 
from a central control connected to telephone company lines to transfer 
calls to and from portable devices that may be at any location within a 
building. Standard cellular systems cannot adequately serve such function 
because of the long distance range of cellular RF signals and the need to 
accommodate a large number of simultaneous communications within a 
relatively small volume such as a building. 
These wireless standards have been adopted so that both data and speech 
signals can be sent over RF frequencies between a central radio exchange 
and a large number of portable devices. These standards employ a time 
division multiple access/time division duplex/multiple carrier 
(TDMA/TDD/MC) approach. More simply put, digital signals to or from the 
radio exchange unit are sent in time slots. The communication thus occurs 
in frame signals of say twenty milliseconds long, with the time frame 
divided into say ten uplink or transmit slots followed by the same number 
of ten down link or receive slots. Each slot being one millisecond long. 
Each portable unit must respond to a signal addressed to it in one of the 
uplink slots in a corresponding downlink slot in the same frame signal. 
In a radio frequency application of such a cordless digital communication 
system the number of simultaneous communications is limited by the number 
of available slots. If there are say ten slots, then for any one 
particular carrier frequency only ten telephone signals can be carried. In 
order to increase the capacity of the system additional carrier 
frequencies are employed typically about eight. Hence, for each cell, 
formed of a radio exchange unit, a total of eighty active telephone 
communications can be carried out. 
These standard systems are designed to accommodate higher transmission 
requirements to and from any one portable unit by assigning additional 
slots, in which case the number of available slots for other portable 
devices is reduced. Furthermore, the RF communications are difficult to 
limit to specific areas within a particular building so that care must be 
taken that carrier frequencies in one cell do not interfere with those in 
another cell. For example, if such RF system is set up to operate 
communications on adjacent floors of a multistoried building, then a 
similar system on other floors must use sufficiently different carrier 
frequencies to avoid RF interference problems. Since the available RF 
carrier bandwidths tend to be limited, because of FCC or other 
governmental spectrum allocations, a need exists to enable practically 
unlimited digital cordless communications without interference problems. 
Infrared communication systems are well known, see for example the U.S. 
patent to Crimmins U.S. Pat. No. 4,553,267; 4,757,553; 4,977,619; 
5,103,108; 5,319,191 and 5,351,149. In the '619 patent a communication 
system is described wherein a base unit is hard wire connected to a 
plurality of stationary infrared transmitter and receiver (R/T) units 
distributed in an enclosure. An infrared portable unit can communicate 
with anyone of the R/T units to establish a two way communication link 
with the base unit. 
A need exists to accommodate standards for RF or cordless telephone 
communications to infrared communications so that a large number of 
telephone connections can be made at the same time within a cell without 
interference problems in a reliable manner. 
SUMMARY OF THE INVENTION 
With an infrared cordless communication method and system in accordance 
with the invention the channel limitations of radio frequency digital 
cordless systems can be avoided and a high density of portable infrared 
terminals can be accommodated without interference problems while using 
standard communication protocols for RF cordless systems. 
This is achieved in accordance with one form of the invention by 
distributing stationary infrared RT (receiver/transmitter) modules 
throughout a building area and connecting these to a base unit that in 
turn is connected to a radio exchange unit for RF cordless systems. The RT 
modules are located to cover a desired area so that portable IR units in 
the building area can communicate through the RT modules with telephone 
lines connected to the radio exchange unit. The signal paths delays 
between the base unit and the RT modules are effectively made 
substantially the same for at least those RT modules that are in each 
other's vicinity. As a result infrared carrier frequencies incident upon 
any one portable unit from several nearby RT modules will not be 
significantly out of phase. 
The signal path delays can be equalized by employing similar cable lengths 
between the base unit and nearby RT modules. In another technique 
described in accordance with the invention signal path delays are 
equalized by introducing appropriate delays of signals sent to and from 
the base unit and the RT modules. Signal path lengths are continually 
monitored and appropriate delays are automatically introduced for each 
signal path. 
Since one or more RT modules may be sending signals to a base unit from the 
same portable unit another aspect of the invention is the selection of the 
best portable signal. For example, when a portable unit responds in a time 
slot, several RT modules may receive the signal and forward it to the base 
unit. As a result the base unit, prior to actually receiving the signal 
from the RT's, makes a selection of the best signal based upon information 
sent to it by the RT modules. This selection is made for each slot 
transmission and enables the best signal to be used for the communication 
even while the portable unit is moving between RT modules in the building 
area. 
With an infrared communication system in accordance with the invention the 
number of infrared portable units that can be connected by a base unit can 
be made quite flexible and much higher than the number that is available 
using conventional RF techniques. This can be done by the use of hubs each 
of which can be connected to a number of infrared RT modules. For example, 
if a base unit has sixteen ports, each of which could be connected to RT 
modules the number of RT modules can be increased by the use of hubs. Each 
hub having, for example, sixteen module connectable ports so that a total 
of 256 RT modules can communicate with a single base unit. As a result one 
base unit can serve a high density of portable users. 
Each base unit can be considered as a separate cell designed to serve a 
particular area and yet be able to establish cordless digital telephone 
communication in a flexible manner. A number of different cells can be 
arranged, as the circumstances may require, with each cell enabling a 
separate telephone communication with a number of different infrared 
portable units. One could thus set up several cells on each floor of a 
large building so that a sufficient number of different simultaneous 
telephone communications can be established even though the cells adjoin 
each other, being only separated by an infrared opaque wall. 
It is, therefore, an object of the invention to provide a cordless infrared 
communication system within a building with which a large and practically 
unlimited number of standard data and voice type telephone connections can 
be made. 
It is a further object of the invention to provide a flexible cordless 
infrared communication system with which a high density of infrared 
portable units can be used. 
These and other objects and advantages of a cordless infrared communication 
system in accordance with the invention can be understood from the 
following detailed description of several embodiments as shown in the 
drawings.

DETAILED DESCRIPTION OF THE DRAWINGS 
With reference to FIGS. 1 through 4 an infrared (IR) communication system 
20 in accordance with the invention is shown connected to telephone lines 
22 through a radio exchange 24. The IR system 20 is made to operate with 
the protocols associated with a standard RF cordless communication system 
known as the CT3 system. However, other systems such as compatible with 
the DECT protocol can be used. 
The use herein of cables crossed by a slash line and a number next to that 
line means the use of a number of paths in that cable equal to the number 
next to the slash. Some cables such as 54 have four twisted pairs 
indicating the use of four transmission paths though eight conductors may 
be involved. In other lines sixteen paths are used with as many 
conductors. 
Also used herein is the practice of identifying items that are alike with 
the same number but with a decimal point and a number on the right of it 
to indicate particular ones of the items. 
The CT3RF cordless standard employs a DECT type digital communication 
wherein a TDMA/TDD/MC system operates just below 2 GHz. The CT3 system 
employs a 16 ms frame cycle 26 formed of a transmit segment 28 using 8 
transmit slots 30 and a receive segment 32 using 8 receive slots 34. Each 
slot 30 or 34 is one millisecond in duration and a transmission within a 
slot includes 480 data bits formed of fields as illustrated at 36. The 
system operates in such a way that when, for example, a transmission from 
radio exchange unit 24 arises by virtue of an incoming telephone call on 
an incoming line 22, a digital signal is placed in one of the outgoing or 
transmit slots 30 and is followed by a response in the same frame in a 
receive slot 34. Note that different frame lengths can be employed in 
different protocols as for example the ten slots used in the DECT system 
as described in the above mentioned IEEE article. 
The infrared system 20 for a cell 21.1 is formed of a base unit 40 which 
communicates with a CT3 or DECT type controller or interface 42 to enable 
digital communication with system 20. System 20 further may be composed of 
a number of hubs 44, which in turn are connected to one or more stationary 
infrared receiver/transmit (RT) modules 46 distributed in a building. The 
RT modules 46 in turn communicate with portable or cordless infrared 
devices 48 such as telephones. The RT modules 46 may be directly connected 
to the base unit 40 such as is shown for modules 46.17-46.24. 
The use of hubs 44 and the distribution of RT modules 46 can be as varied 
as the circumstance require, it being understood that the distribution and 
connections of RT modules in FIG. 1 is for illustration purposes and is 
not intended to be required. 
The base unit 40 is connected to RT modules and to hubs 44 by way of 
twisted pair cables 52. Each cable 52, as shown in FIG. 1a is formed of 
four twisted pairs of conductors 54.1-54.4 to respectively conduct 
distinct signals, namely, the transmit segment Tx, 28 of the frame signal 
26 on pair 54.1, the receive segment Rx, 32, on pair 54.2, a signal to 
noise ratio signal on pair 54.3 and electrical power for the RT modules 46 
on pair 54.4. The arrows are indicative of the direction of signal flow on 
the respective pairs 54. The use of a double headed arrow on signal pair 
54.1 indicates that this pair is used to transfer signals in both 
directions, but at different times as will be further explained. 
With an infrared communication system in accordance with the invention a 
substantial advantage is achieved over a conventional RF type system as 
illustrated in FIG. 3. There, the Telco lines 22 enter a radio exchange 24 
and are passed on by it to RF base cells 60 connected to antennas 62, The 
digital RF signals are sent to portable devices 64 which can roam over a 
large area within or outside a building while maintaining contact for 
communication with the radio exchange 24. 
The effect of the large area coverage of any one RF cell is illustrated in 
FIG. 2 wherein a building 66 of many floors 68 is shown. Any one base cell 
60, such as cell 60.1 tends to range over a volume of space that 
encompasses a number of floors as illustrated with the dashed line 70. As 
a result the number of portable devices that can be distributed or used 
within the cell is limited by the number of available slots in the DECT or 
CT3 type communication system. This then employs multiple carriers to 
increase the available channels, but because of the spectrum allocation 
limitations still may be inadequate for accommodating the required number 
of portable devices 64. 
In contrast, when an infrared communication system 20 in accordance with 
the invention is used, each floor can be provided with one or more hubs 44 
and as a result many RT modules 46 can be made available to accommodate as 
many infrared portable devices 48 as are needed. Signals between a hub 44 
and a portable device 48 do not spill over onto unwanted areas, such as 
separate floors and thus security and interference problems are avoided. 
In IR communication system 20 signals are transferred between the radio 
exchange and the portable units 48 in compliance with the established 
protocol by inserting special signals and using signal lines for 
particular purposes as depicted in the view of FIG. 4a. A signal pattern 
as shown in FIG. 4 has for illustrative purposes transmissions occurring 
during slots 30.1, 30.5 (indicated by check marks) and as a result 
response signals in receive slots 34.1 and 34.5 also evidenced by the 
check marks in these slots. 
Just prior to the sending of a transmission in a slot 30, system 20 inserts 
a delay 71 to assure that the transmissions destined for nearby RT modules 
46 arrive at the same time. The reasons for this can be explained with 
reference to FIG. 6 in which a base unit 40 is shown connected by cable 
52.1 and 52.2 of different lengths to RT modules 46.1 and 46.2 
respectively. As a result the IR carrier energy arriving at a portable 
unit 48.1 may include portions from both nearby RT modules and differ in 
phase, depending upon the different signal path lengths from the base unit 
40. If the signals are about 180 degrees out of phase as shown at 67, the 
net effect at a portable unit 48 is a cancellation of IR carrier signals, 
in effect a null, and thus adversely impacts communication with that 
portable unit. 
Hence, it is desired that the signal path lengths from the base unit to RT 
modules 46 which are near each other be made about the same. This means, 
in accordance with one embodiment of the invention, inserting a delay for 
the signal placed on the cable 52.1 and of sufficient duration to reduce 
the phase difference, delta phi, attributable to cable lengths variations 
to a maximum of about ninety degrees (1/4 wavelength) as illustrated at 
69. 
For simplicity, the inserted delays are selected so that transmissions from 
a base unit during any one slot arrive at all the RT modules 46 at the 
same time. The delays are first automatically determined as shown in FIG. 
4a during those intervals such as 72.2 and 72.6 when slots, such as 30.2 
and 30.6, do not require a transmission. Delays are measured by sending a 
pulse signal 73 from the base unit 40 to each of the RT modules 46 and 
measuring the time for a return signal 74 to arrive from that module. 
Measuring of delays need not be done every time there is no transmission. 
Hence, a counter is employed to allow measuring of delays at some 
increased time interval. 
Another feature of IR system 20 arises from the possibility that more than 
one RT module 46 responds to the return transmission from a portable 
device 48. IR system 20 selects the best RT module signal just prior to 
the occurrence of the return transmission. As shown in FIG. 7 an RT module 
46 includes a photo detector 75 responsive to IR signals from portable 
devices 48. The output of the detector 75, after amplification, is passed 
onto an analog to digital converter 76 to produce a digital Rx receive 
signal for return to the base unit 40. 
A signal strength indication is generated at the output of a comparator 77 
after it has compared the output from the photo detector 75 with a squelch 
level signal from a squelch generator 78. The signal strength indication 
is also converted to digital form with a fast A/D converter 79. This A/D 
converter generates a three bit signal strength signal with each bit 
placed on a separate line 54 for return to the base unit 40. 
As shown in FIG. 4a the fast analog to digital conversion of the signal 
strength is sent at 79a to the base unit 40 before it receives the Rx 
receive slot signal 34. A selection of the strongest signal is then made 
at 81a. This is done at 81 shown in FIG. 7, and used at 83 to pass the 
best Rx signal on to the radio exchange unit 24. This selection of the 
strongest signal is also used to choose a best RSSI signal. An RSSI signal 
for each receive slot 34 may be required by the CT3 or DECT protocols and 
represents the strength of the received portable IR signal at an RT module 
46. 
With reference to FIG. 5 one form of a base unit 40 in accordance with the 
invention is shown in further detail. Signals to and from a conventional 
CT3 radio exchange interface 42 occur on lines 90.1-90.5. These signals 
are respectively a serial digital transmission signal T.sub.x, on line 
90.1; a logic control signal T/R on line 90.2; a frame logic signal on 
line 90.3; a received signal R.sub.x on line 90.4; and a signal strength 
indication RSSI signal on line 90.5. 
The transmission signal T.sub.x, destined for all RT modules in a cell 21, 
is applied to an FM modulator 91 wherein a digital voltage controlled 
oscillator, in response to an input from an oscillator 92 at 55 MHz, 
produces zeroes represented by a frequency at 3.429 MHz and ones at a 
frequency of 4.571 MHz. Different frequencies can of course be employed. 
The output 93 from the modulator 91 is applied to an electronically 
controlled switch 94 and then through a multiplexer 95 to a delay network 
96. The delay network 96 delays the transmission signal T.sub.x by an 
amount that is sufficient to assure that transmissions, from at least 
those RT modules 46 which are near each other, occur at essentially the 
same time. For simplicity the delays are selected so that transmissions 
from RT modules 46 during any one slot occur essentially at the same time. 
Hence, the transmission signal T.sub.x, destined for each individual RT 
module 46, is delayed a particular amount, depending upon the length of 
the cable connecting that module to the base unit 40. The delays are 
selected so that they are equivalent to the delay caused by the longest 
cable length involved. 
The delayed T.sub.x signals are then passed through a multiplexer 97 onto 
the transmitter lines 54.1 of the various cables 52 leading to the hubs 44 
and RT modules 46. This process is continued for each of the T.sub.x 
signals in the respective transmission slots 30 of a frame signal 26. 
The lines 54 are twisted pairs and are driven by amplifiers 98a and 
terminated with receivers 98b. These amplifiers and receivers enable a 
tristate condition on the lines 54 so as to preserve power when no 
transmissions are to occur and permit two way signal flow when this is 
needed as for the transmission lines 54.1. The tristate condition is 
regulated by signals generated from a controller 114 as hereafter 
described. 
During the receive cycle each of the RT modules 46 which had passed on a 
T.sub.x signal to the portable devices 48 returns a receive signal R.sub.x 
to the base unit 40 in the slot which corresponds to the transmission slot 
in which the T.sub.x signal being responded to was located. In addition, a 
signal indicative of the signal strength of the infrared signal received 
at the RT modules, from the portable devices 48 sending a response, is 
transmitted to the base unit as an S/N signal. 
Since the receive signals arrive at the base unit 40 at different times, 
because of the delays imparted by connecting cables 52, the receive 
signals R.sub.x from the respective RT modules and hubs are passed through 
the delay network 96 to undergo delays of the same duration as the delays 
imparted to the corresponding transmission signals T.sub.x. The sixteen 
receive signals R.sub.x on cable lines 54.2 are, therefore, coupled to the 
input side of the multiplexer 95 and then passed through the delay network 
96 to essentially arrive simultaneously at the output lines 98 of 
multiplexer 97. 
During the receive segment 32 of a transmission, several receive signals 
R.sub.x from different RT modules 46 are presented and the base unit 40 
includes a network 100 to select that signal representative of the best 
available R.sub.x signal. The R.sub.x signals are applied to a multiplexer 
101 where the best receive R.sub.x signal is selected and placed on line 
90.4 leading to the interface 42. The best R.sub.x signal is selected with 
control signals on lines 98 derived from a digital magnitude comparator 
102. 
The latter comparator 102 compares signals on input lines representative of 
the signal to noise ratios of the infrared inputs to the RT modules as 
previously described with reference to signals 79a and 81a in FIG. 4a. As 
explained, this comparison is done at a time preferably just prior to the 
applicable receive slot. The selection of the best receive signal R.sub.x, 
therefore, occurs during a very brief interval between slots 30 as will be 
further explained. 
The control signals on lines 99 are also applied to a multiplexer 104 whose 
inputs are connected to the respective signal strength lines 54.3 from the 
various hubs 44 and RT modules 46. The best signal strength signal is 
selected for each slot 30 and stored in a register 106 with a clock signal 
presented on the T.sub.x1-16 lines 54.1 from the RT modules 46. The clock 
signal is presented on the output line 103 of a multiplexer 105 whose 
input lines connected to lines 54.1. The value of the signal in the 
register 106 is converted by a digital to analog converter 108 to an 
analog signal and presented on line 90.5 as the RSSI signal associated 
with the slot 34 to which the receive signal R.sub.x relates. 
The identification of the best R.sub.x signal with control lines 99 can be 
used as an indication of the location of the portable which was the source 
of the receive signal. The control signals on lines 99 identify the port 
where the receive signals arrived and are stored in a register 107. The 
port identification signals are clocked out onto the RSSI line 90.5 with 
the clock signals on line 103 from multiplexer 105. 
Control over the operation of the base unit 40 and the functions of the 
above described networks is obtained with a sequencer 110. This may be in 
the form of a micro processor with appropriate programming. However, the 
speed with which the required signals have to occur makes it desirable to 
employ discrete circuits. The sequencer 110 produces appropriate control 
signals with which the various functions of the base unit 40 accomplishes 
its tasks. 
Hence, in response to the T/R and frame signals, on lines 90.2 and 90.3 
respectively, the sequencer 110 produces timing signals such as a sync 
signal on line 112.1, a transmit enable signal on line 112.6, a transmit 
or receive selection signal on lines 112.2 and 112.7 to set up the 
appropriate mode in the multiplexers 95 and 97 respectively and a 
calibration enable signal on lines 112.3. In addition the sequencer 
generates tap register enable signals on lines 112.4 and a best receive 
selection signal on line 112.5. The sequencer includes a calibration 
controller 114 and a 12 MHz clock 115 with which the electrical delays 
produced by the cable lengths may be repetitively measured and then used 
to set the appropriate delays. Selected ones of these signals also control 
the tristate conditions of several amplifiers 98a and receivers 98b 
employed at the base unit 40 to drive the lines 54. 
Before describing the hubs 44 and RT modules 46 in greater detail, the 
operation of the infrared system 20 in accordance with the invention can 
be best understood with reference to FIGS. 4, 5, 8 and 9A-9C. A frame sync 
signal as appears on line 90.3 in FIG. 5 is a square wave 130.1, see FIG. 
9A, having equal transmission and receive segments 132 and 134 
corresponding to the transmission and receive segments 28 and 32 shown in 
FIG. 4. The transition 136 from a transmit segment 132 to a receive 
segment 134 is a timing reference used to initiate certain timing signals 
as shown in FIG. 8. 
The frame sync signal 130 is, therefore, connected in the sequencer 110 to 
a frame pulse generator 137 which causes a resetting of a fourteen bit 
counter 140 driven by 12 MHz clock 115. Certain counts achieved inside 
counter 140 are decoded with a comparator 141 as indicated on lines 142 
with the count number placed adjacent the lines 142. When the register is 
full a pulse is applied to drive a slot clock 146 and its output pulses 
counted in a slot counter 148. This circuitry thus produces timing signals 
on lines 142 to cause certain events to occur during a slot and to enable 
the slots 30 and 32 in a frame 26 to be counted. The numbers placed along 
the lines 142 signify the count in the register 140 that yields an output 
on that line with reference to the frame transition 136 in the frame sync 
signal. 
The T/R logic signal 160 on line 90.2 in FIG. 5 from the radio exchange 
unit 24 signifies when a transmission occurs during a slot 30. In FIG. 9A 
an illustrative example of a T/R signal 160 is shown wherein a 
transmission is to occur in the first slot 30.1 and none in the subsequent 
slots 30.2, 30.3. and 30.4. The occurrence of the various timing signals 
on output lines 142 and obtained from the register 140 are depicted on the 
T/R signal line 160 as shown, with the larger counts being abbreviated as 
illustrated with apostrophes. 
As shown in FIG. 5 data for transmission is sent on line 90.1 commencing 
with the count of 144, see FIGS. 8 and 9A. The transmission begins at a 
time identified also by numeral 162. Transmission ends at the end of 
sending a fixed number of 480 bits, as explained with reference to FIG. 4, 
at a time identified at 164 shown in FIG. 9A. The end of transmission 
occurs just prior to the timing signal on line 142.5 bearing the count 
11904. The time period following the transmission of the last bit until 
the next count of 144 is an interval 166 associated with time between 
sequential transmission slots 30. 
During intervals 166 any signals occurring on the data line 90.1 from the 
radio exchange 24 can be construed as noise and transmissions can and are 
inserted by the base unit 40 to the hubs 44 and RT modules 46 as well as 
received from these devices for the operation of the communication system 
20. 
One calibration mode of operation that is preparatory for the function the 
IR communication system involves an automatic determination of the length 
of delay needed to assure that adjacent or nearby RT modules 46 are 
activated for transmissions at substantially the same time. This employs a 
transmission of a special sync signal 170, see FIG. 9, commencing on a 
line 172, see FIG. 5. The sync signal is passed through the multiplexer 
95, and delay measuring and applying network 96 and sent out on an output 
line 54.1 of a cable 52 to a particular RT module 46. The sync signal is 
recognized by virtue of its unique duration by the RT module 46 to which 
it is sent. 
Upon recognition of the sync signal 170 the RT module returns a response 
176, see FIG. 9A, to the base unit 40 on the same transmission line 54.1 
on which the sync signal is sent. The response 176 is initiated upon 
detection of the sync signal at the RT module 46. The arrival of the 
response 176 at the base unit 40 is detected by the delay measuring 
network 96. The time of the arrival of the response 176 is indicative of 
the roundtrip travel interval 178 of signals along the cable 52. 
The base unit 40 measures the delay imparted by the length of the cable 54 
connecting the base unit 40 to the RT module 46 by counting the pulses 
from oscillator 92, see FIG. 5, in a shift register 180 associated with 
the particular RT module 46. These pulses are counted starting from the 
time that the sync signal 170 is first sent until the arrival of the 
response 176. The count accumulated in the register 180 is then 
representative of twice the length of the cable 52 between the base unit 
40 and an RT module 46. Since the count represents the roundtrip distance, 
the count is divided by two, obtained with a simple shift of the count in 
the register 180, and then stored in a tap register 182 as equivalent to 
the required one way trip delay. 
The delay count in a tap register 182 is so coupled to an associated shift 
register 180 that the delay count determines where along the shift 
register 180 a transmission of data to is to begin entering the shift 
register 180. In this manner a small delay count, representative of a 
relatively long cable 52, causes data to be entered towards the input end 
of a shift register 180. A large count on the other hand causes data to be 
entered towards the output end of a shift register 180. 
Hence, data destined for a nearby RT module 46 will be delayed longer and 
data for a farther RT module 46 will be delayed less by the shift 
registers 180. In the aggregate, however, taking into account the 
additional delays imparted by the cables 52, data will arrive at RT 
modules 46 at the same time. 
The process for determining the cable delays is initially carried out for 
each of the RT modules 46 as the system is started up. Once the system is 
operational the delay calibration is continued on a repetitive basis 
depending upon the traffic of data along the cables 52, i.e. the 
availability of a transmission slot 30. 
The sequencer 110 provides the appropriate timing signals for the system 
with the calibration controller 114. This regulates, with the T/R line 
90.2 and the frame sync line 90.3 from the radio exchange interface 42, 
and produces signals for enabling transmissions in a slot. It also 
generates the control signals on lines 112 needed to establish a delay 
calibration for a slot if this has not been done within a predetermined 
time or after a certain number of transmissions. This circuitry needed to 
generate signals can be produced with an array logic or such other 
suitable programmed microprocessor. 
During operation of IR communication system 20 signals representative of 
data or voice in formation is sent in slots 30 to all of the RT modules 46 
either directly from the base unit 40 or through a hub 42. The slot signal 
assigned to a particular portable unit 48 is so loaded into a shift 
register 180 associated with a particular RT module 46 as to be delayed in 
time in proportion to the delay previously measured for the associated 
cable 52 and stored in the associated tap register 182. 
Since during transmission each slot signal is sent to all of the RT modules 
46 in a cell, the sequencer 110 enables the loading of each transmission 
slot signal into all of the shift registers 180. Signals are shifted out 
of the registers 180 onto the output lines 54.1 in each cable 52 to arrive 
substantially at the same time at the RT modules 46. 
At the end of the transmission cycle 28, see FIG. 4, those portable units 
48 which had been addressed with a particular slot signal must, if a 
response is to be produced, do so during a receive slot 34 assigned to be 
associated with a particular transmission slot 30. If no response occurs 
then the radio exchange 24 assumes that the portable device is not active. 
The response is generated at a time dependent on the slot of the 
transmission signal that caused the response. This is done in a manner as 
is well known in CT3 or DECT type communication systems. Suffice it to 
note that the return signals, known and described herein as receive 
signals, are preferably placed in the receive slot 34, see FIG. 4, which 
corresponds to the position of the transmission slot 30 in the 
transmission cycle 28. 
In the RF version of a CT3 system there is one receiver for a cell 21. In 
the IR communication system 20 a single cell contains a large number of 
possible IR receivers in the form of RT modules 46. Since several RT 
modules 46 near a portable unit 48 are likely to generate receive signals 
it is desired to provide the radio exchange unit 24 with the best signal 
at one of these RT modules. The best signal is to be selected for each 
receive signal slot 34. The best signal can then also be made available as 
an RSSI (receiver signal strength) signal for transfer to the radio 
exchange unit 24 as representative of the signal strength at the receiver 
from the respective portable unit from where the signal originates. 
In system 20 a best signal selection is done just prior to the start of 
each active receive slot 34. As illustrated with reference to FIG. 9A the 
receive segment 134 of a frame signal 130.2 is shown in synchronized 
relationship with the frame signal 130.1 shown above it for the 
transmission cycle. Several receive slots 34 are illustrated and in 
response to the occurrence of a transmission in transmission slot 30.1 a 
response is to be sent back in slot 34.1. 
Recognition of the beginning of a receive slot commences at the base unit 
40 and with its recognition of the transition 136.2 in the frame sync 
signal 130. When the frame sync signal transition 136.2 occurs, see FIG. 
9A, the control signal generator 114 produces on line 112.1 a sync square 
wave pulse 184 composed of two microsecond segments. This start-receive 
slot sync pulse 184 is sent out to the RT modules at the beginning of each 
receive slot 34 in response to the slot clock signals on line 147, see 
FIG. 8. 
Hence, when an RT module 46 detects the receive slot sync signal 184 the RT 
module 46 samples the received IR signal strength. The sampled value is 
then immediately transmitted to the base unit 40 before beginning a 
transmission of a signal in a receive slot 34. This can be understood with 
reference to the timing diagrams of FIGS. 9B and 9C. 
Detection of the receive slot start sync signal 184 is promptly followed by 
a fast A/D conversion of the IR signal detected by the photo detector 75, 
see FIG. 7, during an interval 166, see FIG. 9A, between slots. The fast 
A/D conversion occurs after the start at 186 of the portable IR carrier 
for a receive slot transmission and allowing at 187 for settling of the 
output 188 from an amplitude detection circuit, not shown, for the IR 
signal. 
Since the interval 166 during which the fast A/D conversion is done is 
quite short the three bit output from the A/D converter 79, see FIG. 7, is 
applied in parallel as shown at 188 in FIGS. 9B and 9C, to the lines 54.1, 
54.2 and 54.3 in the connected cable 52. FIG. 10B shows the circuitry used 
to described functions for an RT module 46. An IR transmitter 200 for 
sending IR signals to portable units 48 and an IR detector 75 for 
detecting responses from portable units are used. 
The RT modules 46 include a programmable array logic () and other 
appropriate circuits 204 (enclosed by the dashed line in FIG. 10B) for 
processing inputs and outputs. The transmission inputs on line 54.1 from 
the base unit 40 or a hub 44 are passed on to a modulator 206 and 
amplifier 208 for activating the IR transmitter 200. The transmitter 200 
may use a suitable number of IR generating diodes 200 in a manner as is 
well known in the art to produce the desired IR signal output to the 
portable units 48. 
An amplifier 210 and a demodulator 212 are used to respond to IR signals 
from the portable units 48 to produce electrical signals for transmission 
to the base unit 40, either directly or through a hub 44. 
P.A.L circuit 204 employs an external clock 214 which drives a counter 216 
to produce clock pulses at three microsecond intervals. A carrier 
detection circuit 218 is used to detect the arrival of a transmission on 
the transmit line 54.1 and apply a signal to that effect on line 220. The 
transmit line 54.1 is also directly applied to circuit 204 to enable it to 
detect appropriate data and logic conditions. A logic network 222 detect 
the presence of a calibration sync pulse 176, see FIG. 9A. The logic 
circuit 222 generates a response signal on output line 224 which is 
returned to the base unit 40 via a multiplexer 226.2 for the previously 
described cable delay calibration. 
The fast three bit A/D converter 79, which is controlled by a signal on 
line 227 from the P.A.L. circuit 204 has outputs 228.1-228.3 respectively 
applied to multiplexers 226.1-226.3. The operations of the multiplexers 
226 are controlled with signals on lines 230.1-3 from circuit 204. The 
various tristate conditions of amplifiers 232.1-4 driving the lines 54.1-3 
in cable 52 are also controlled with signals on lines 234.1-3 from circuit 
204. 
The programming and operation of the P.A.L. circuit 204 can be best 
understood from the self-explanatory state diagram 250 in FIG. 11 in 
conjunction with the counts as illustrated on top of the figure. At 252 
and commencing at start up step 252 the presence of a transmitter carrier 
on a transmission input line 54.1 from the base unit 40 is awaited at 256. 
When a carrier is detected an idle mode is entered at 258. If a sync pulse 
is present as detected at 260 either the occurrence of a calibration mode 
sync occurred or a receiver mode sync pulse has been detected. In the case 
of a calibration sync pulse a return sync pulse is generated at 266 and 
returned to the base unit 40 and the state is returned to step 258. 
In the event a receiver slot sync pulse was detected then control is 
shifted to step 270. A fast abbreviated (three bit) A/D conversion is 
carried out as previously described at 272 followed by a full slower A/D 
conversion at 274 and sending of signals for a receive slot at 276 as 
received from a portable device 48. 
The portable unit 48 shown in FIG. 12 also includes an IR transmitter 320 
and an IR detector 322 respectively connected to a modulator 324 and 
demodulator 326. A logic circuit 328 is used to handle the digital traffic 
and convert the signals to appropriate format for use by the conventional 
handset 330. A key board 332 such as used with conventional handsets is 
available to initiate calls. The logic network 328 provides the functions 
and operations like those in an RF portable unit and need, therefore, not 
be further described. 
FIG. 14 shows a hub 44, which is very similar to a base unit 40. For that 
reason circuits and lines having similar functions have the same numbers 
as described with reference to the base unit 40. A variation from the base 
unit occurs at the input of a hub where the incoming connections are made 
with a cable 52 having the T.sub.x, 54.1, R.sub.x, 54.2, and S/N, 54.3 
lines as previously described. The resulting inputs correspond to the 
lines described with reference to FIG. 5 and have been correspondingly 
numbered 90.1', 90.4' and 90.5'. 
The slot sync 90.2' and frame sync 90.3' are derived from the input line 
90.1' with a sync detector 370. This detector recognizes when a 
calibration sync pulse is being sent and responds with a return signal on 
line 372. 
FIG. 13 shows an alternative IR system 20' in accordance with the 
invention. Instead of an automatic delay generating system, the cables 52 
connecting a base unit 40 to nearby RT modules 46 are made all essentially 
the same in length. This requires that the shorter cables 52 include extra 
lengths that are wound into coils 350. The cable lengths need not be the 
same for those RT modules 46 not sufficiently close or separated by a wall 
and thus not likely to communicate with the same portable device 48 at the 
same time. 
The system 20' may use a simplified base unit 360 as shown in FIG. 15 
wherein like numbers designate similar circuits or networks as previously 
described. Base unit 360 employs a logic network 362 which responds to the 
incoming frame sync signal on line 90.3 to enable an AND gate 364 during 
the transmission segment 28 of the operation. The transmission from the 
base unit 20' is passed directly on to the cables 52 through appropriate 
drivers 98a. 
During the receive segment 32 the R.sub.x signals from the various RT 
modules 46', see FIG. 16, are passed through a best signal selection 
network 366. The circuits and networks described with reference to base 
unit 360 can be implemented by a microprocessor instead of with discrete 
circuitry as shown. The operation of these networks can be best explained 
with reference to the modified RT module 46' as shown in FIG. 16 and 
wherein like numerals designate like components as previously described. 
In FIG. 16 the demodulated IR signal from a portable device is applied as 
an R.sub.x signal to an AND gate 380. A signal representative of the 
received signal strength is applied on line 75a to squelch type networks 
78 and 382. If the IR signal level is very high, thus representing a high 
quality signal, a comparator 384 detects that the signal exceeds an 
adjustable threshold value as set at 386. The output is a high quality 
signal on line 388 which is applied to the signal to noise ratio line 54.3 
from this RT module 46' to the base unit 360. 
As long as the signal level on line 75a is sufficient for passing on to the 
base unit 360, because the signal exceeds an adequate threshold level as 
set at 390, the AND gate 380 is enabled and the digital R.sub.x signal is 
passed on to the R.sub.x data output line 54.2 to the base unit 360. 
Returning to the base unit 360 shown in FIG. 15 and with reference also to 
FIG. 17 the best signal selection process can be explained keeping the 
signals from the RT modules 46' in mind. The signal lines 54 identified in 
FIG. 17 represent the same signals as on lines 54 except that these lines 
are single conductors from the outputs of receivers, not shown, connected 
to lines 54 from the RT modules 46'. A priority network 394 is used to 
first assure that the R.sub.x signal having a high quality level 
associated with it is detected with network 400 and thus first passed on 
to the radio exchange 24 via line 90.4. A second priority network 402 is 
used to pass an R.sub.x signal onto line 90.4 as long as one of the 
signals from the RT modules 46' exceeds the adequate signal threshold 
level set by networks 390 (FIG. 16). 
A final decision network 404 is used to combine the outputs from the 
networks 400 and 402 to present on line 90.4 the R.sub.x signal for the 
radio exchange 24. The best signal selection works by coupling the high 
quality signals on lines 54.3', the S/N signals, from sixteen RT modules 
46' to AND gates 406.1-16. The RT module 46', which could be the one 
connected to port 1 of the base unit 360, has its S/N line 54.3' coupled 
to the input of AND gate 406.1 together with a reference signal on line 
408 representing an inactive signal level. If there is a high quality 
signal level present on line 54.3' leading to AND gate 406.1 then its 
output 412.1 is enabled and in turn enables the AND gate 410.1. This 
allows data from the RT module 46' on line 54.2' to be passed on to the OR 
gate 414 in network 404. 
The occurrence of an active signal level on line 412.1 is coupled through 
an OR gate 416.1 and an inverter 418.1 to disable AND gate 406.2. All 
subsequent portions of circuit 400 are disabled in this manner so that 
only one high quality data signal is passed on to network 404. In a 
similar manner if the only high quality signal occurs on any other line 
54.3' it is passed on to network 404. The occurrence of a high quality 
signal level on any line 54.3' results in the disablement of the outputs 
from the selection network 402 with a signal on line 418 from the last OR 
gate 416.16 in the chain. This disabling signal is applied to an AND gate 
420 in network 404. 
In the event there is no high quality signal level, then a data signal 
having the next acceptable signal level is passed on to network 404 by the 
selection network 402. This process involves the generation of an adequate 
level signal Q on lines 420.1-16. These Q signals are derived from the 
combination of a lack of signals on lines 54.3' and the presence of data 
(R.sub.x) signals on any one of the lines 54.2'. The selection of the best 
R.sub.x signal by circuit 402 employs a similar technique as described for 
circuit 400. 
The first Q signal on line 430 is applied through an inverter to an AND 
gate 432.1 together with the data signals on line 54.2'. If there is an 
adequate signal level then this is passed onto network 404 via OR gate 434 
and thus through AND gate 420 to the output line 90.4 through OR gate 436. 
If the adequate signal level occurs from any other RT module 46', the next 
highest data signal in the chain of priority is passed on. 
Having thus described several embodiments in accordance with the invention 
its advantages can be understood. Variations from the drawings can be made 
without departing from the scope of the invention as defined by the 
following claims.