CA1261981A - Wireless telephone system - Google Patents

Abstract

ABSTRACT

A wireless telephone communication system for wireless, voice, data or voice and data terminals comprising apparatus at a central location for receiving one or a plurality of signals for communication with selected ones of wireless communication terminals, apparatus for converting the signals to spread spectrum radio frequency signals, a leaky transmission line located in a communication region, apparatus for applying the spread spectrum radio frequency signals to the transmission line for electromagnetic radiation within the region, at least one wireless communication terminal adapted to receive a predetermined one of the spread spectrum radio frequency signals and for demodulating it into an intelligible signal.

Description

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01 This application is a division of Canadian 02 application Serial No. 499,189 filed January 8, 1986.
03 This invention relates to a telephone 04 communication system utilizing wireless voice, data, 05 or voice and data terminals.
06 It is often difficult to provide telephone 07 service to personnel who are continuously moving, yet 08 must be quickly available at a telephone or data 09 terminal, e.g. warehouse stock takers, personnel responsible for building maintenance or security, etc.
11 Paging recelvers have been used to summon such personnel 12 to call a number via the closest telephone set.
13 Sometimes such personnel are required to enter data into 14 a terminal for storage at a central computer. Yet wired jacks for interfacing a telephone with a PABX or 16 terminal to a computer are often spaced at substantial 17 distances from each other in warehouses, for example.
1~3 Voice communication of such personnel has 19 been partly solved by the use of cordless (wireless) telephones each operating at a different frequency 21 from the other. However such telephones have 22 exhibited substantial problems. There are usually a 23 limited number of channels available, limiting the 24 number of telephone/wireless receiver sets which can be used in a given area. Often interference from 26 adjacent channels or other services is encountered.
27 Transmission or reception nulls are often encountered 28 when a cordless telephone is oriented in a particular 29 direction relative to the main transmitting and receiving station. Such nulls are also caused by 31 shielding by steel and concrete of which the building 32 housing the system is built. The system lacks 33 privacy; since the common radio bands are used, the 34 signals can be readily intercepted.
In addition, the use of wireless presents 36 a range problem. Particularly for large areas, such 37 as when an entire building, a large warehouse, or 38 aircraft hangar, etc., is to be covered by the system, . : :
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'' ~ ~' `'' 01 a high powered central transmitter could be used.
02 However the power of the transmitter cannot be greater 03 than a level prescribed by the regulatory agencies.
04 This places a limitation on the communication range.
05 In the event -that many low power transmitters are used 06 scattered around the building, the cost increases 07 substantially.
08 In the system described in U.S. patent 09 4,462,113 issued July 24, 1984, domestic A.C. power lines are used as an antenna for a low power 11 transmitter. However in this case the null problem 12 still exists, and in addition there is a severe 13 bandwidth limitation to below approximately 150 or 14 200kHz. Further, transmission of signals via the A.C.
power lines is notoriously insecure, since such 16 signals are often carried by the power lines to 17 neighbouring offices and buildings.
18 Due to the inherent lack of security, 19 nulls and power/distance limitations, the use of a mobile telephone handset has been limited to the home 21 environment, or to very special applications. Until 22 the present invention was made it had not yet been 23 found suitable -for commercial applications in which,

2~ for example, an entire multi-story building housing many different companies, some possibly being 26 competitors with each other, could be served mobile 27 telephone and data services reliably from the same 28 PABX. Such prior art systems could not ensure 29 complete security of communications, sufficiently low power radiation outside the confines of the building, 31 and absences of nulls or fade areas within structures.
32 Clearly for the above reasons portable 33 terminals could also not be used for reliable 34 transmission and reception of data to a central computer.
36 The present invention provides a 37 communication system which can be connected to a PABX

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01 switching system, which solves the problems noted 02 above. In addition to the provision of secure mobile 03 (wireless) communications in a low power environment 04 with the substantial elimination of the null or fade 05 areas, it provides multi-channel wideband 06 communications which can reliably carry voice, data 07 and signalling (supervisory) signals.
08 According to the present invention leaky 09 transmission cables are used to radiate and to receive communication signals within the communication 11 region. While at single frequencies such cables 12 exhibit nulls along the cables at regular intervals, 13 the communication signals which are carried and which 14 are received in the present invention are spread spectrum.
16 The use of spread spectrum signals carried 17 by a leaky cable transmission system achieves several 18 highly desirable results. Firstly, nulls are 19 virtually undetectable or are eliminated due to the spreading of the signals over a wide bandwidth.
21 Secondly, since pseudo-noise bandwidth spreading 22 (correlation) code is used for each channel, privacy 23 of communications is virtually assured as well as 24 immunity from interference between channels.
Since a leaky cable is used to distribute 26 the signal in the communication region, t~e 27 transmitter power and resulting effective radiated 28 power can be very low. Furthermore, the power which 29 is used is spread amongst the frequencies of a wide bandwidth, further reducing the power used at any one 31 frequency. Thus the interference that may be caused 32 to other radio signals outside of the building is 33 virtually nil. Also the radiated power within the 34 building can be increased to a substantial degree in comparison with a non-spread spectrum, single or 36 multi-point antenna transmitter to minimize the bit 37 error rate, yet the effective interference with "~ - . , , ~;~6~

01 external or other wirel~ss services remains virtually 02 nil, while the reliability of communication within the 03 builaing is increased.
04 The spread spectrum-leaky cable system 05 according to this invention can operate side-by-side 06 with other similar systems or with conventional AM or 07 FM wireless systems with substantially no or minimal 08 interference. The maximum interference which might be 09 apparent in such other systems would merely be an increase in the background noise level.
11 The various handsets or wireless remote 12 terminals to be used in the present system are 13 preferred to be accessed by address code on a 14 supervisory channel, and to be controlled to internally selec-t a pseudo-noise correlation code 16 which matches a predetermined channel transmit 17 pseudo-noise spreading code. This type of system 18 would benefit by the use of universal wireless 19 handsets or terminals. However in another type of system each handset or terminal is channel fixed with 21 a predetermined pseudo-noise correlation code 22 circuit, and the head end terminal changes its 23 transmit pseudo-noise spreading code to suit that of 24 the selected handset.
In the reverse transmission direction 26 which uses a different RF centre ~requency the mobile 27 handset is either fixed with a transmit pseudo-noise 28 spreading code for encoding the transmitted signal, or 29 it can be caused to transmit on a channel selected by the head end under control of the head end via signals 31 on the supervisory channel.
32 Leaky cable transmission systems have long 33 been used to communicate in tunnels and mines. One 34 such system is described in U.S. Patent 4,476,574 issued October 9, 1984. A large number of 36 publications is listed in that patent which will 37 provide background in~ormation to the reader on the ~.
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~2~L9i!31 01 use of leaky Eeeder communication systems both 02 subsurface and above the surface of the earth.
03 However those systems suffer from one or more of the 04 problems described above which restrict them from use 05 in a reliable co~mercial communication system. A text 06 which describes leaky feeder systems is LEAKY FEEDERS
07 AND S~BSURFACE RADIO COMMU~ICATIONS by P. Delogne, IEE
08 Electromagnetic Waves Series 14, 1982 Peter PeregrinuS
09 Ltd.
Spread spectrum systems have been used in 11 wireless systems in the past, as well as in certain 12 specialized wired systems. A wireless spread spectrum 13 system is described in U.S. patent 4,455,651 issued 14 June 19, 1984 and in U~S. patent 4,086,504 issued April 25, 1978. However in both those cases the power 16 limitation problem and the null problem described 17 above did not present problems, since in the first 18 case directional antennas were used, and in the second 19 case high power could be used and the system was not used in a communication system of -the present kind.
21 Furthermore, the locations of the various transmitting 22 stations used in the latter patent, which relates to a 23 seismic exploration system, can all be tested for 24 proper spectrum prior to use and the transmitters moved in case a null is encountered. In addition, the 26 locations of the spread spectrum transceivers are all 27 fixed and preknown, since they are used for 28 triangulation purposes.
29 Spread spectrum systems have also been used ln wired systems. For example in U.S. patent 31 4,438,519 issued March 20, 1984, a spread spectrum 32 system is used connected to an A.C. power line, which 33 is not a leaky feeder transmission cable, but 34 constitutes an antenna. In that patent, however, it is clear that the bandwidth o~ the power line is very 36 low, and the transmission region is limited to areas 37 which are not isolated by a transformer. ~Ience such .' . `

' ~Z6198~' 01 systems are not useful for wide band multi-channel 02 voice and data communication system~.
03 A spread spec-trum signal is applied to 04 telephone lines in the invention described in U.S.
05 patent 4,475,208 issued October 2, 1984. In this 06 system data signals are converted to spread spectrum 07 and are transmitted simultaneously with voice over 08 already existing telephone lines which are not leaky 09 feeder transmission cables . The bandwidth of such telephone lines is so low that the data signals are oE
11 very low bit rate. Clearly the system is not suitable 12 for use in multi-channel wide bandwidth transmission.
13 In addition, the system is unsuitable for use with 14 wireless handsets or terminals since the telephone transmission lines cannot carry radio frequency 16 signals for any significant distance.
17 A general description of the history and 18 structure of spread spectrum systems will be found in 19 the publication SPREAD-SPECTRUM COMMU~ICATIO~S, edited by Charles ~. Cook et al, published by the IEEE Press, 21 Institute of Electrical and Electronic Engineers, Inc.
22 Carriage of a plurality of spread spectrum 23 communication signals, each modulated by a different 24 pseudo-random spreading or correlation code, by a leaky cable transmission system thus provides a new ~6 form of wireless mobile voice and data communication 27 system previously not thought possible, that is, a 28 reliable, private and low power communication system 29 using a large number oE noted telephone handsets that can be carried within a predefined environment whic7n 31 can be used sharing the same bandwidth as other 32 commercial wireless services without interference 33 between the two kinds of systems. Such a system for 34 the first time is viable within a building structure, i.e. steel and concrete, which previously would not 36 reliably support reliable wireless communication to 37 the required degree.

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8~' 01 Beca~lse of the broadband and multi-channel 02 capability of the present system, the various mobile 03 telephone sets can be provided with a full xange of 04 features normally made available only to wired telephone 05 sets by the PABX, such as local alphanumeric display, 06 conferencing, abbreviated dialing, etc., as well as 07 computer access, remote control oE various apparatus 08 such as automatic door locks, etc. Such features are 09 not now possible with the well known cordless telephones. A 32 channel system of the type described 11 herein could give typically 200 users at a site mobile 12 telephone facility depending on traffic. Further, the 13 communication channel between the remote mobile unit and 14 a PABX which connects to a larger computer can provide to the remote unit enormous computational power which 16 would otherwise not be available in a hand held computer 17 due to its size and cost limitations, and since a large 18 shared data base can be stored at the central computer.
19 A preferred embodiment of the invention is a telephone system comprising a central switching system 21 having a plurality of line circuits, apparatus for 22 converting signals carried by at least certain ones of 23 the line circuits to spread spectrum RF signals, each 24 signal carried by the certain line circuits having a different spreading code, first apparatus for wireless 26 transmitting the spread spectrum RF signals, and one or 27 a plurality of mobile telephone sets, for receiving, 28 decoding and reproducing the signals carried by the line 29 circuits and for wireless transmitting to the central switching system spread spectrum RF signals originatlng 31 at the mobile sets, each of the latter signals being 32 transmitted using a spreading code different in each 33 mobile set from other mobile sets.
34 The invention also facilitates any of the wireless communication terminals to initiate a 36 communication with the central location, such a 37 communication being capable of having as its final ".~
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01 destination any telephone connectable to the central 02 location, including others of the wireless 03 communication terminals.
04 ~ better understanding oE the invention 05 will be obtained by reference to the detailed 06 description below of the preferred embodiment, with 07 reference to the following drawings:
08 Figure 1 is a general block diagram of a 09 system according to the present invention, Figure lA illustrates a code arrangement 11 used in multiplexing the channels of the spectrum, 12 Figure 2 is a block diagram illustrating 13 the mobile handset according to the preferred 14 embodiment of the invention, Figure 3 is a block diagram of the central 16 equipment according to the preferred embodiment of the 17 invention, and 18 Figure 4 is a block diagram of a preferred 19 form of transmit channel used in the system.
To briefly review the spread spectrum 21 concept, this technique causes the spectrum of the 22 transmitted signal of each channel to be spread over a 23 greater amount of bandwidth than would be the case if 24 time or frequency division multiplexing techniques were used. Indeed, the signals of all of the channels 26 used are spread over the same band. This is achieved 27 by multiplying a generated data stream to be 28 transmitted by a sequence with the correct 29 auto~correlation and cross-correlation properties (pseudo-random/noise code sequence). The resulting 31 output signal is then a sequence having a higher data 32 rate than that of the input data stream, whi~h when 33 used to modulate some foxm of amplitude, frequency or 34 phase shift keyed system, causes the spectrum to be spread over a wide bandwidth. `-36 At the receiver the incoming signal is 37 multiplied by the same pseudo-random/noise sequence : . .
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01 and the spectrum becomes despread to its original 02 bandwidth. It is important to note that any 03 interfering signal is spread at the receiver rather 04 than despread. The signal is then filtered at the 05 receiver to the original bandwidth, leaving the 06 original signal intact but the interEering signal is 07 attenuated and thus its effect is diminished. This 08 occurs whether the interfering signal is a real signal 09 or is a hoLe (i.e. null~ in the spectrum generated by some propagation effect. Thus a spread spectrum 11 system provides not only immunity against interfering 12 signals but also protection against holes in the 13 spectrum.
14 Because a large number of spread spectrum sequences are generally known, it is possible to 16 choose a set for a system which would be very 17 dif~icult to decode by an intruder or eavesdropper.
18 The spread spectrum system therefore contains its own 19 intrinsic security. The interference to which the system is immune may of course be other channels of 21 the system using the same spectrum. Spread spectrum 22 is therefore intrinsically a multiplexing system.
23 Different channels using the same bandwidth can be 24 immune to each other if they use different spreading sequences for modulation and demodulation.
26 For example, for a pseudo random code 27 having length 255 bits, there are probably 28 approximately 40000 codes which are strongly 29 orthogonal, and thus are highly secure. It is preferred in the present system to use a centre 31 frequency of between about 150 mHz to 1000 mHz, 32 although the invention is not limited to this band, 33 each channel being approximately 32k Hz wide, in which 34 the voice signals are digitized in a well known manner. The spread channel is preferred to be 8 mHz 36 wide.
37 Turning now to Figure 1, the basic system 38 _ 9 _ . .
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01 according to the invention is illustrated. Within the 02 confines of a building periphery 1 a leaky cable 2 is 03 laid. The leaky cable can be coaxial cable with holes 04 in its shield, such as described in Canadian Patent 05 1,014,245 issued 3uly 19, 1977, or other types of 06 leaky transmission cables as described in the 07 aforenoted text by P. Delogne. The cable can be 0~ sinuously laid above the false ceiling over the entire 09 width and breadth of the building, can extend down the centre of a narrow building, and can pass from .storey 11 to storey in a multi-storey building as well as extend 12 over the ceiling area of a building. The leaky cable 13 can be located within movable walls or within an 14 electromagnetically transparent floor. Clearly the location of the cable is dependent on the region to be 16 covered and many variations are possible. The 17 important aspect of the placement of the cable is that 18 for a given minimum electromagnetic field strength, 19 the entire working area of the building which defines the communication region should be enveloped by the 21 field strength leaked from the cable which is above 22 the minimum level.
23 It will be seen that since the field 24 strength drops off by between the square and the cube of the distance from the cable, the effective radiated 26 power outside the building periphery will be low or 27 virtually nil.
28 As central equipment, feeding the cable at 29 one end is a transmitter 3, and receiving signals from the other end of the cable is a receiver 4. The input 31 to the transmitter 3 is connected to a PABX S and the 32 output of receiver 4 is connected to the PABX 5. Also 33 connected to the PAB~ are trunks 6 and local telephone 34 sets 7. Of course the PABX can be divided by number code so that groups of local telephone sets 7 can be 36 associated with one business or division while other 37 groups can be associated with another business or .:
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01 division, if desired.
02 One or a plurality o~ remote wireless 03 mobile telephone sets 8 are located within the 04 building. These telephone sets will be referred to 05 herein henceforth as mobile sets and can include data 06 originating and receiving terminals as well as, or in 07 place of voice handsets. Mobile sets 8 preferably are 08 bat-tery operated, can be carried by a user, can be 09 located on a desk, hung on a wall, etc. They are not connected by wire to the telephone system PABX.
11 Depending on the characteristics of the 12 leaky cable 2, its length, etc., repeaters 9 may be 13 required to be connected at regular intervals in 14 series with the leaky cable. I'hese repeaters are preferably wideband, such as CATV television 16 repeaters.
17 The PABX S is primarily a normal PABX
18 which operates to interconnect local telephone sets 19 with each other or with trunks 6. In order to communicate with the mobile sets 8, one o~ the 21 telephone sets 7 dials an appropriate extension number 22 designating a mobile set. The PABX, instead o~
23 connecting the telephone set to one of the other sets 24 7, simply chooses a line terminal 10 which corresponds to the mobile sets 8, connected to a predetermined 26 junctor. The selected line terminal connects to the 27 input of transmitter 3. In one embodiment ringing 28 current is applied to the line terminal in the normal 29 manner. ~he transmitter 3 converts the ringing current to a predetermined digital code, interleaves 31 it with other digital codes on a supervisory channel, 32 converts the resul-ting signal to a spread spectrum 33 supervisory channel signal and applies the resulting 34 signal to the leaky cable 2.
The line terminal however designates which 36 mobile set is to be selected, since it corresponds to 37 the dialed number. Once the line terminal has been 38 ~

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01 selected the transmitter prefixes the digital code 02 with an identifier code which is unique to the mobile 03 set. However it should be noted that rather than 04 using line terminals, junctors of the PABX can be 05 used. Also, instead of applying ringing current to 06 the terminal it can apply a ringing enable signal to a 07 ringing terminal associated with the line terminal.
08 The signal radiates from the leaky cable 09 into the communication region within the building periphery 1, an~ is received by all o~ the sets 8.
11 All of the mobile sets continuously decode the 12 signalling signals, and as soon as the identifier 13 prefix which iden-tifies the designated set has been 14 received, that the identified set converts the remaining code which calls for it to ring. Ringing is 16 effected in the mobile set by keying an internal 17 "warble" or other signal to alert the user in a well 18 known manner.
19 The user upon hearing the ringing signal, switches his mobile set on, the equivalent of going 21 off hook. That mobile set then generates an off-hook 22 supervisory code which is converted to a spread 23 spectrum R.F. signal in the signalling channel, and is 24 transmitted via its own small local antenna to the leaky cahle 2. Preferably the signal is sent within a 26 time slot designated by the synchronization and 27 control signals sent on the aforenoted supervisory 28 channel. Polling and response of the mobile sets in 29 sequence is preferred to be used in the supervisory channel.
31 The spread spectrum off-hook code is 32 received by the leaky cable, and is carried to the 33 input of receiver 4. Receiver 4 demodulates, 34 despreads and decodes this signal and applies the off-hook supervisory signal to the line input port 11 36 of PABX 5 associated with terminal 11 (or to the 37 associated junctor).

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01 It should be noted that while two 02 unidirectional ports 10 and ll have been indicated, 03 the transmitter and receiver can equally be connected 04 to separate inputs of a hybrid which is connected to a 05 bidirectional line or junctor.
06 The PABX 5, receiving the off hook 07 supervisory signal as if it were from a telephone set, 08 completes the connection between the calling party and 09 mobile set via the transmitter 3 and receiver 4 as i-f 10 it were to be connected to another telephone 7. The 11 transmitter 3 and receiver 4 are associated with fixed 12 voice channel transmit and receive pseudo-random 13 spreading codes. When this occurs control apparatus 14 associated with transmitter 3 and receiver 4 transmits 15 on the supervisory channel a data signal addressed to 16 the now off-hook mobile set 8 which designates the 17 transmit and receive pseudo-random codes for the 18 two-way voice channel to be used for voice 19 communication to match those of the transmitter and 20 receiver. The mobile set adopts the codes and thus 21 can transmit and receive on the designated channel.
22 Voice communication between the telephone set 7 and 23 mobile set 8 now proceeds on the specified channel, 24 while using the dedicated supervisory channel Eor 25 supervisory signals.
26 When either of the sets goes on hook at 27 the conclusion of the communication, the supervisory 2~ signal associated with that function is carried by the 2g supervisory channel as described earlier during the 30 set up of a call. The voice connection is then taken 31 down in a manner analogous to setting up all 32 supervisory and voice or data communication thus can 33 proceed in this manner.

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01 According to the preferred embodiment in 02 the present divisional patent application, any means 03 for wlreless transmitting the spread spectrum RF
04 signals can be used, such as an antenna. In 05 accordance with another embodiment, which is described 06 above, the transmitting and receiving apparatus 07 associated with a central switching system is 08 comprised of one or more leaky cable transmission 09 lines.
Figure lA illustrates the preferred form 11 of channel assignments. Dedicated pseudo-random codes 12 designate the .supervisory channel in each direction, 13 while a plurality of pseudo-random codes (e.g. up to - 13a -, ; .
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01 approximately 40,000) can be used to designate the 02 transmit and receive channels. All channels use 03 essentially the same frequency band, though different 04 bands are used in -the centre to mobile and mobile to 05 centre directions. None will be found to interfere 06 with each other or with other wireless services using 07 the same frequencies, except for random bit errors 08 which will increase with traffic. In a typical system 09 only about 32 two-way channels will be required although there is clearly capacity for many more, 11 given the number of codes available.
12 In a similar manner calls can be made from 13 any mobile set 8. All mobile sets 8 continuously 14 receive and transmit information on the supervisory channel. Thus if a mobile set 8 goes off-hook this 16 information is transmitted by the supervisory channel 17 to receiver 4 where it is demodulated, despread and 18 decoded before passing to PAB~ 5. On receiving the 19 off-hook signal PABX 5 allocates a voice channel as described above, transmits dial tone to the mobile set 21 8, receives signalling information from mobile set 8 22 and sets up the call in the usual manner. The mobile 23 sets 8 are clearly not limited to voice 24 communications; they can be combination voice and data sets, or restricted to being data terminals. In this 26 respect one of the trunks 6 of PABX 5 can be connected 27 to a computer for receiving data from and transmitting 28 data to a mobile data set 8. Alternatively a computer 29 can be connected directly to the main bus of PABX 5 for communication with the mobile sets. In this 31 manner the mobile set 8 can be used as a remote 32 terminal to a central computer. Low speed data 33 communication can be effected with the mobile set 8 34 via the supervisory channel, or high speed via a dedicated data channel, or via a voice channel shared 36 with and carrying data.
37 Block diagrams of the mobile set and - : .

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01 central equipment constitutiny the transmitter and 02 receiver are shown in Figures 2 and 3. The mobile set 03 will be described first, with reference to Figure 2.
04 An analog telephone set 15 to which a 05 handset or handset with display 16 is connected and, 06 if desired, a keypad ]7 has an intexnal hybrid with an 07 output line connected to an analog-to-digital PCM
08 encoder 18. Preferably the encoder is an adaptive 09 differential encoder of toll quality, e.g. it will encode a signal which is output at 32 Xb/sec.
11 The output of encoder 18 i5 connected to 12 the input of modulator 19, which both spread spectrum 13 ana RF modules the incoming signal. The output of 14 modulator 19 is connected to the input of a combiner 21 which is connected through a transmit filter 22 and 16 directional coupler 22A to an antenna 23.
17 The filter preferably is 8 megahertz wide, 18 having a Q between 20 and 100. It can for example be 19 a printed strip line controlled by varacter diodes such as is often found in the tuner of a TV set.
21 The antenna 23 is similarly connected 22 through a directional coupler 23A to the input of the 23 receive filter 24 (which is similar to filter 22) 24 which is connected to the input of a splitter 25. One output of splitter 25 is connected to demodulator 27 26 which both spread spectrum and RF demodulates the 27 signal. The output of demodulator 27 is connected to 28 the input of digital-to-analog PCM decoder 28, the 29 output of which is connected to the input line to tele~hone 15.
31 A data interface circuit 29 is also 32 connected to telephone set 15, which con~ains data 33 encoding and signalling circuitry as well as 34 associated buffers. Considering only the signalling aspect for the moment, off-hook, on-hook, etc. and 36 other signalling signals as are norm lly generated in 37 a telephone set are applied via interface circuit 24 .:

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01 to a synchronization and control circuit 30. The 02 synchroniza-tion and control circuit contains a master 03 clock for the mobile set, and controls the filters 22 04 and 24. The clock signal used in the synchronization 05 and control circuit is obtained from the incoming 06 signal received via antenna 23.
07 A spread spectrum and RF modulator 31 has 08 its input connected to synchronization and control 09 circuit 30, and its output connected to another input of combiner 21. The output of RF modulator 38 is 11 connected to another input of combiner 21.
12 A second output of splitter 25 is 13 connected to an input of demodulator 34, which has its 14 output connected to synchronization and control circuit 30.
16 A voltage controlled oscillator circuit 35 17 is connected to the synchronization and control 18 circuit 30, and has outputs connected to modulators 19 19, 31 and 38 and demodulators 27, 34 and 41.
Synchroniza-tion and control circuit 30 21 also has outputs connected to modulators 19, 31 and 38 22 and demodulators 27, 34 and 41.
23 In operation, according to the preferred 24 embodiment of the invention the pseudo-random codes designating the transmit and receive supervisory 26 channels are Fixed by means of code plugs or other 27 similar code designating means, fixed in modulator 31 28 and demodulator 34. A supervisory signal having a 29 spreading code correlatable by the correlation code in demodulator 34 is received from the leaky cable 31 transmission line 2 (Figure 1) by antenna 23. The 32 signal passes through directional coupler 22A/ is 33 filtered in filter 24, passes through splitter 25 and 34 into demodulators 27 and 34. However since demodulator 27 will not recognize the encoded signal, 36 it outputs only a low level random noise signal.
37 However since spread spectrum modulator 34 does ~, :

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01 recognize the supervisory channel codet it decodes the 02 signal and applies it to synchronization and control 03 circuit 30. Synchronization and control circuit 30 0~ recognizes a data header designating the local mobile 05 set and further recognizes the demodulated code as 06 meaning that ringing should start. It applies a 07 signal to telephone controller 15, which begins 08 ringing. If the local mobile set data header was not 09 recognized, the ringing function would not be enabled.
It should be noted that on the supervisory 11 channel the code which is received can designate any 12 supervisory function or indeed can carry low speed 13 data communication signals. In this respect it is 14 preferred that the signal carried on this channel should contain four 8 bit words in sequence: the 16 first 8 bits designating the station number of the 17 mobile set, and second 8 bits designating what 18 function should be performed, the third 8 bits should 19 contain bits to enable error detection and correction, and the four-th group of 8 bits should contain a 21 synchronization pattern.
22 With the local telephone set going 23 off-hook, telephone controller 15 applies an off-hook 24 signal to synchronization and control circuit 30.
Synchronization and control circuit 30 in turn 26 generates a code sequence containing the local station 27 address identi~ier, a supervisory code designating 28 "off hook", error correction bits and a 29 synchronization pattern and applies it to modulator 31. Modulator 31 modulates the ~ignal using the fixed 31 supervisory channel code and also RF modulates the 32 signal applied to it before applying it to an input of 33 combiner 21. The output signal of combiner 21 is 34 applied through filter 22 and directional coupler 22A
to antenna 23 from which it is transmi-tted to leaky 36 cable transmission line 2.
37 A supervisory signal is also received from :, . . . .
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01 the central equipment in the same manner as noted 02 above which desi~nates which channel or channels the 03 incoming and outgoing analog and/or data signals from 04 and to the mobile set are to be transmitted. This 05 channel designating signal is received by the 06 synchronization and control circuit 30 over the 07 supervisory channel. The synchronization and control 08 circuit 30 upon receiving the channel designating 09 signals applies signals to modulator 19 and demodulator 27 which control the pseudo-random 11 spreading and correlating codes respectively. Once 12 these codes have been established in the modulator and 13 demodulator, subsequent signals will be transmitted on 14 a spread spectrum channel designated by the modulation pseudo-random spreading code and received by the 16 mobile set on a spread spectrum channel designated by 17 the designated correlation.
18 For transmission of analog or analog and 19 data signals from the local handset, such signals pass from handset 16 (and/or keypad 17) into telephone 15 21 in the usual manner, are split into unidirectional 22 signals, e.g. in a hybrid, and the outgoing signals 23 are applied to PCM encoder 18. The resulting digital 24 output signals are applied to spread spectrum modulator 19, are modulated using the channel 26 spreading code established therein as described above, 27 and are applied to RF modulator 20. Under control of 28 the synchronization and control circuit 30 modulator 29 20 modulates the signal, and the resulting RF
modulated spread spectrum signal is applied via 31 multiplexer 21 through filter 22 to antenna 23 ~or 32 transmission to the leaky cable transmission line.
33 A signal received from leaky cable

3~ transmission line 2 by antenna 23 is applied through directional coupler 22A, filter 24, and splitter 25 to 36 demodulators 27 and 3~ which are controlled by 37 synchronization and control circuit 30. The resulting -" ' ' . ': ,:

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01 demodulated and despread signal from demodulator 27 is 02 applied to PCM decoder 28. The resulting analog 03 output signal is applied to the incoming signal line 04 of telephone 15 Erom where it is applied to handset 05 16. However since demodulator 34 will not recognize 06 the pseudo random code used it will not apply any 07 input to synchronization and control circuit 30.
08 In the above manner all of the supervisory 09 functions of the mobile set can be received and transmitted, the analog signal receive and transmit 11 channels established and the analog and low speed data 12 transmitted and received.
13 It is preferred that when no radio 14 frequency signals are being received, decoder 28 should be switched into a silence mode. A silence 16 code genera-tor 36 is connected to the output of 17 demodulator 27. When the silence code generator 36 18 detects silence code at the output of demodulator 27, 19 it applies a signal to decoder 28 which causes it to remain stable in a known state, whereby no analog 21 output signal is generated.
22 Similarly, a silence detector 37 is 23 connected to the outgoing signal line of telephone set 2~ 15 for detecting silence. The output of silence detector 37 is applied to synchronization and control 26 circuit 30,which reduces or shuts off modulator 19 for 27 that interval. This results in a reduced error rate of 28 signals received by the central system and reproduced 29 in the mobile set. The silence code detector 28 and silence detector 37 should be very fas~ acting so as 31 to prevent the clipping of the start of words.
32 Silence detectors have been used in TASI (time 33 assignment speech interpolation) transmission systems.
34 In order to transmit and receive high speed data, data interface circuit 29 interfaces via 36 telephone 15 to the display in the display and handset 37 16 and to the keypad 17, or to an external port (not :

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01 shown) which may receive data from a local data 02 collection machine or the like. The high speed data 03 is applied through data interface 29 to modulator 38 04 of construction similar to that of modulator 19. The 05 output of modulator 38 is connected to an input of 06 combiner 21.
07 An output of splltter 25 is connected to 08 an input of demodulator 41, which has its output 09 connected to the incoming data port of data interface 29.
11 For reception and transmission of data, 12 modulator 38 and demodulator 41 operate similarly to 13 modulator 19 and demodulator 27 respectively. The 14 modulator 38 and demodulator 41 are controlled upon receipt of a signal in the synchronization and control 16 channel designating that data is to be received or 17 transmitted, in a manner similar to that described 18 earlier. Channels are designated by the allocation of 19 pseudo-random codes as described earlier. The data can be transmitted using simple packets at 90.2 21 Kilobits per second, ~or example, the packets 22 containing data signals and error correction codes.
23 Turning now to Figure 3, the transmitter 24 and receiver and PABX interface at the central equipment are illustrated. The elements in the 26 transmitter and receiver referred to with respect to 27 ~igure 1 will become evident by the description below 2~ and have not been segregated, for the purpose of 29 clarity of explanation.
A PABX 5 includes a plurality of interface 31 circuits 42, one of which is shown. ~ach inter~ace 32 circuit can be similar to a well known PABX line 33 circuit except that it has an additional communication 34 link with the main bus of the PABX for receiving synchronization pulses for transmission to the mobile 36 se~.
37 ~ When a telephone set 7 wishes to :

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01 communicate with a mobile se~, Eor example, a 02 subscriber will dial digits designative of the 03 particular mobile set to be contacted. The PABX, in 04 the normal manner, can select a line terminal or port 05 which is unique to that mobile set. However use of 06 the system in this manner would require as many line 07 circuits or terminations as there are mobile sets. It 08 is preEerred, instead, to have the PABX select a 09 particular junctor with a PABX interface connected thereto, with the mobile set to be selected designated 11 by address or ground point. Use of the system in this 12 preferred manner will require only as many interface 13 circuits as the traffic requires, clearly a 14 considerably fewer number of interfaces, circuits and channels than the number of mobile sets.
16 With the interface selected and either a 17 line termination enabled or a data code received from 18 the PABX which designates the mobile set to be rung, a l9 sync and control circuit 43 receives both the supervisory signal and designation of the mobile set 21 to be contacted from the PABX via the PABX interface.
22 The sync and control circuit 43 formulates a data 23 packet comprised of the station number, supervisory 24 signal, error correction and synchronization pattern bits and transmits it to a modulator 44. Modulator 44 26 has a dedicated pseudo-random spreading code fixed to 27 the supervisory channel. The modulator 44 spread 28 spectrum and RF modulates the supervisory signal 29 (which in this case contains a supervisory sequence which indicates that a particular mobile set should be 31 rung). The modulated output signal therefrom is 32 applied to multiplexer 46 from which it is passed to 8 33 megahertz filter 47, which is similar to filter 22.
34 The output signal of filter 47 is applied to one end of leaky transmission cable 2. The signal passes 36 along transmission cable 2, radiating as described 37 earlier. The radiated signal is received by mobile ., .

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01 set 8 in the manner described above.
02 A signal received from the mobile set 8 03 passes through 8 megahertz filter 48 (which is similar 04 to filter 47) and splitter 49 and demodulator 51.
05 Since the supervisory signal is on a fixed channel, 06 the pseudo-random correlation code for demodulator 51 07 is fixed, and the received signal is demodulated 08 resulting in a data signal applied to sync and control 09 circuit 43 which constitutes the return supervisory signal (e.g. an off hook indication) from mobile set 11 8. Sync and control circuit 43 applies this signal to 12 PABX interface 42 which applies it in recognizable 13 form to PABX 5.
14 Sync and control circuit 43 also has outputs connected to modulators 44, 53 and 59 and 16 demodulators 51, 56 and 62 for applying a 17 synchroniDation and control signals thereto.
18 Since a particular junctor, and thus PABX
19 interface 42 was selected by the PABX for the forthcoming com~unication, the particular outgoing and 21 incoming channels are thereby designated, and a mark 22 signal related to the particular channels is applied 23 to sync and control circuit 43. Sync and control 24 circuit 43 contains a table of psuedo-random codes corresponding to the selected spread spectrum 26 modulator and demodulator, and another table of mobile 27 set station numbers corresponding to the mark signal 28 (which identifies the called mobile station uniquely-29 A supervisory signal is formulated in sync and control circuit 43, which is sent to the selected mobile set 8 31 identified by station number, advising it ~hat spread 32 spectrum channel to tune itself to.
33 The supervisory signal can send to the 34 mobile set one of two kinds of signals- a signal advising the mobile set to establish its rsceive and 36 transmit spread spectrum modulator and demodulator 37 pseudo-random codes to codes stored in a table in each 38 mobile set, i.e. identifying the codes by number.

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01 Alternatively the sync and control circuit can 02 transmit the actual pseudo-random codes to the 03 selected mobile ~et 8 to enable it to set its 04 modulator and demodulator to the designated spread 05 spectrum channels. In this manner, in a 200 mobile 06 set system, typically 32 junctors and thus 32 P~BX
07 interfaces can be used, rather than 200 interfaces 08 would be required if each line circuit were separately 09 interEaced. However the present invention contemplates the use of individual line circuits for 11 each two-way channel if economics dictate.
12 Since a particular ~unctor, and thus a 13 particular PABX interface circuit 42 has been selected 14 by the PABX in the normal manner to carry the communication, the outgoing and incoming channels are 16 also fixed as noted above. The output signal from the 17 PABX interface circuit is applied to analog digital 18 PCM encoder 52. The resulting encoded output signal 19 is applied to the input of modulator 53 which has a dedicated pseudo-random code related to that 21 particular channel. The RF modulated output signal is 22 applied to an input of combiner 46, from which it 23 passes through filter 47 and is applied to the leaky 24 cable 2. Since the spread spectrum modulation codes in modulator 53 and modulator 44 are different, there 26 will be no interference between the two signals. The 27 signal applied to leaky cable 2 is radiated for 28 reception by mobile sets 8 as described earlier.
29 A received signal from the leaky cable 2 transmitted by mobile set 8 passes through filter 48 31 and combiner 49, is demodulated and despread in 32 demodulator 56, and the resulting signal is passed to 33 analog PCM decoder 57. The resulting output signal is 34 applied to PABX interface 42 for application to the junctor of PABX 5. Since the spread spectrum 36 pseudo-random spreading code at the mobile set for 37 signals transmitted thereat under control of sync and , . ., ... . .

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01 control circuit 43, were desiyna~ed by the selection 02 of a particular junctor and PABX interface circuit 42 03 by normal operation of the PABX, which is identical to 04 that pseudo-random code in spread spectrum demodulator 05 56, the signal received from mobile set 8 is properly 06 decoded in demodulator 56, but is rejected by 07 demodulator 51.
08 The PABX interface can also interface to 09 high speed data junctors, or to a data bus in the PABX
which designates by code which mobile set is to be 11 communicated with. Either by junctor se]ection 12 as described above or by decoded selection from the 13 PABX data bus, the high speed data signal is applied 14 to outgoing data interface circuit 58. The output signal is applied to modulator 59, in a manner 16 analogous to that described earlier. The RF modulated 17 signal is applied to an input of combiner 46, passes 18 through filter 47 and is applied to the leaky cable 2.
19 Received high speed data signals from mobile set 8 are received by leaky cable 2 and pass 21 through filter 48, splitter 49 and are applied to 22 demodulator 62. The resulting data output signal 23 after spread spectrum and RF demodulation is applied 24 to data interface circuit 63, from which the data signal is applied to the junctor or data bus of the 26 PABX through PABX interface circuit 42. The data 27 channel selection at the mobile set 8 is established 28 as the incoming and outgoing data channels in a manner 29 analogous to that described above for the ougoing and incoming analog channel.
31 It is preferred that a silence detector 32 and a silence code detector should be used in each of 33 the incoming and outgoing analog channels of the 34 central equipment (not shown) which are similar to those described with reference to Figure 2 and are 36 similarly connected. The silence and silence code 37 detectors should be very fast acting. Since the error ~' , - .- .: :

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01 rates of the signals which are received are dependent 02 on the total number of channels sharing the same 03 bandwidth, it is highly desirable -to switch the RF or 04 IF signals of~ or to extremely low level during silent 05 periods. Fox a given error rate, the use of silence 06 detectors will increase the number oE channels which 07 can share the same bandwidth at the same time.
08 Synchroni~ation will not be lost since the supervisory 09 channel will alwaysbe operating and carries s~nchronization signals. Thus the receivers at the 11 mobile sets and at the central equipment can always 12 regain synchronization if it is lost.
13 When communication has been set up the 14 mobile sets thus each will have two receive addresses, one which is a polling address which is used on the 16 synchronization channel and the other which is the 17 pseudo-random code, i.e. the correlation code that it 18 is instructed to use via the supervisory channel. It 19 will use two transmit addresses, one which designates it and allows it to be recognized in the superVisorY
21 receive channel at the central equipment and one a 22 pseudo-random code which matches the analog or digital 23 receive channel code at the central equipment. Thus 24 the instruction to use a particular correlation code is similar to the designation to the mobile set to use 26 a particular junctor, and is directly analogous to the 27 junctor selected at the PABX. It can additionally 28 have separate high speed data channel receive and 29 transmit spreading and correlation code addresses.
The pulse code modulation scheme which is 31 used is preferred to be adaptive differential PCM, a 32 full description of which can be obtained in the draft 33 recommendation G721 of CCITT. According to this 34 specification PCM is transcoded from 64 Kilobits per second to 32 Rilobits per second. It is also 36 preferred that the pseudo noise code sequence used in 37 the spread spectrum modulator should be 255 bits, .

01 although it is expected that other sequence lengths 02 could be used. It is also preferred that the RF
03 modulation should be phase shift keyed, and can be 04 minimum shift keyed, bandwidth phase shift keyed, QPSK
05 or staggered phase shift keyed. It is also preferred 06 that the data channels should transmit at 90.2 07 kilobi-ts per second.
08 It should be noted that only one sync and 09 supervisory channel modulator and demodulator 44 and 51 need be used for the entire system while the PABX
11 interface and decoders, modulators and demodulators 12 are duplicated for each channel. Of course apparatus 13 used for the data, or Eor the analog channels need not 14 be used if one or the other kind of communication is not to be provided for a particular junctor or for 16 communication to the mobile sets in general.
17 Referring now to Figure 4, the modulation 18 portion of the transmitter is shown. An incoming PCM
19 or data signal from encoder 52, for example, is carried on line 64 to a data circuit 65, in which the 21 incoming signal is synchronized and speed adjusted.
22 The outgoing signal from data circuit 65 is applied to 23 an exclusive OR gate 66. A sequence generator 67 24 generates a pseudo-random code which is specific to the channel to be transmitted and applies its output 26 to another input of exclusive OR gate 66. One 27 complete pseudo-random code, of preferred length 255 28 bits, is Exclusively ORed with each data bit. The 29 resulting P~ sequence of exclusive OR gate 66 is applied to an input of data modulator 68. The data 31 circuit 65, sequence generator 67 and exclusive OR
32 gate 66 provides the spread spectrum modulation.
33 An intermediate frequency (IF) oscillator 34 69 generates a signal which is applied to data modulator 68, where it modulates the signal, resulting 36 in a IF signal on line 70. The IF signal is applied 37 to a summer 71, along with the IF signals of other ~6~8,~, 01 data modulators, illustrated by line 71A. The output 02 signal o~ summer 71 is applied to a mixer 72, to which 03 is applied an RF carrier signal generated in a carrier 04 generator 73. The carrier signal is mixed with the 05 sum IF signal and the resulting RF modulated output 06 signal of mixer 72 is applied to an 8 megahert~ filter 07 74. The output ~ilter of signal 74 is applied to 08 leaky transmission line 2.
09 It is preferred that the data modulator should modulate the IF signal with the output of 11 E~clusive OR gate ~6 using phase shift modulation.
12 It will be noted that in the circuit of 13 Figure 4 the modulated signals have been summed prior 14 to RF modulation in mixer 72. The summer 71 is of course equivalent to combiner 46. The IF signals can 16 be summed prior to RF modulation as shown in Figure 4, 17 or the RF signals can be summed following RF
18 modulation as shown in Figure 3.
19 The receive channel is similar to Figure 4 in that the mixer outputs to a splitter the IF signal 21 in a well known manner and the resulting signal is 22 applied to a data demodulator. The demodulator 23 multiplies the incoming signal by an IF signal 24 modulated by the same pseudo-random code used in the transmitter. The output of the demodulator is then 26 low~pass filtered to recover the data.
27 While the circuit of Figure 4 can be used 28 in the transmit and receive channels of the central 29 equipment shown in Figure 3, the major different between that circuit and the circuit used in mobile 31 set 8 is that the sequence generator can generate a 32 selected code sequence in the latter. As noted 33 earlier the sequence is established either by a look 3~ up table in the mobile set which is designated by the supervisory signal received from the central equipment 36 or b~ reception of the actual sequence to be used. Of 37 course the receive channel is directly analogous to ..
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~1 the transmit ehannel.
02 Returning to the supervisory channel, 03 during idle intervals i-t is preferred that the cen-tral 04 equipment should transmit a 32 bit supervisory polling 05 message to each mobile set in sequence and to wait for 06 a response in the following 32 bit message. In case 07 erroneous messages are received, it is preferred that 08 a reques-t for a repetition should be transmitted in 09 the next supervisory sequence designated for the particular mobile set or the particular ~unctor 11 channel time period. In the analog and data channels 12 either repe-tition or error correction, or ignoring 13 packets which have been designated as erroneous can be 14 implemented. Since the supervisory channel operates by polling, transmission collisions are avoided.
16 Each of the elements described above can 17 be implemented in dedicated logic to provide the 18 func-tions described, or can be grouped and implemented 19 in microprocessor-memory combinations operated using firmware written using the algorithms described 21 herein.
22 Since radiation from a leaky transmission 23 cable is used, extremely low powers can be used, e.g.
24 ten milliwatts per channel. Clearly the power used in the mobile sets~ typically operated by battery, is 26 greatly economized.
27 Since spread spectrum is used in 28 combination with the leaky cables, nulls which are 29 usually encountered using leaky feeder systems, and signal dropout regions often encountered using fixed 31 antenna radiators are substantially avoided. Since 32 there is a fast drop off of signal level with distance 33 from the leaky cable radiator, the judicious placement 34 of leaky cable in the ceiling or other peripheral region of the building will establish detectable power 36 levels throughout the building, but virtually 37 undetectable RF signals outside of the environs of the 38 - 2~ --~Z6~

01 building. Thus the system is highly localized, 02 minimizing any interference with any other kinds of 03 systems.
04 Further, because the system is spread 05 spectrum, it is inherently private, which is highly 06 unusual in a wireless telephone system. It is 07 economical of spectrum space, since substantially the 08 same bandwidth is used for all channels. With the 09 very low level of power which is used, and each channel being spread over a wide bandwidth, the actual 11 transmitted signal appears to be little more than very 12 low level noise to conventional wireless systems. Yet 13 because there are such a great number of pseudo random 14 codes which can be used, the possibility of interference between channels, or of intercep-tion 16 outside of the present system is rendered almost nil.
17 The system can be used for conventional 18 analog voice communication, as noted earlier, or in 19 addition or in alternative the mobile set can be a hand held computer terminal. However since each 21 mobile set can transmit on either a designated or 22 centrally controlled secure channel, the mobile set 23 can also be used for remote control of apparatus such 24 as automatic doors, various building services, etc.
with high security. For example it can control 26 robots, domestic appliances, etc. The mobile set is 27 thus a highly versatile unit used in conjunction with 28 the system described above.
29 It should also be noted that while the modulators and demodulators at the central equipment 31 have fi~ed pseudo random codes and those at the mobile 32 sets have codes which are variable, in an alternative 33 system the codes at the mobile sets can be fixed, and 34 the codes at the central equipment can be varied to select a channel corresponding to -the designated 36 mobile set. However in this case the number of 37 variable pseudo-random codes which are used will ,: ~

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01 correspond to t~ice the number of mobile sets (two 02 one-way channels to each mobile set), plus two for 03 supervisory while in the case in which the mobile sets 04 change their correlation codes, the number of 05 correlation codes used will correspond to twice the 06 number of junctors or toial channels expected to be 07 used for communication (plus two for supervisory), a 08 far fewer number.
09 Further, the central apparatus described herein could usefully be employed to operate with one 11 or a group of distributed antennae, rather than, or in 12 addition to, the leaky cable. Such a structure would 13 find great utility in buildings or outdoor areas in 14 which it is not feasible to wire telephone system connected by wires, or to deploy a leaky cable.
16 A person understanding this invention may 17 now conceive of various alternative structures using 18 the principles described herein. All are considered 19 to be within the scope of t~e invention as defined in the claims appended hereto.

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Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A telephone system comprising:
(a) a central switching system having a plurality of line circuits, (b) means for converting signals carried by at least certain ones of said line circuits to spread spectrum RF signals, each signal carried by said certain line circuits having a different spreading code, (c) first means for wireless transmitting said spread spectrum RF signals, (d) one or a plurality of mobile telephone sets, for receiving, decoding and reproducing said signals carried by the line circuits and for wireless transmitting to said central switching system spread spectrum RF signals originating at said mobile sets, each said latter signal being transmitted using a spreading code different in each mobile set from other mobile sets.

2. A telephone system as defined in claim 1 in which the spread sprectrum RF signals transmitted by said first means are in a first frequency band having one centre frequency, and in which the spread spectrum RF signals transmitted by the mobile sets are in a second frequency band having a different centre frequency.

3. A telephone system as defined in claim 1 or 2 further including a supervisory channel associated with the central switching system for providing spread spectrum RF signals carrying digital control data receivable by the mobile sets for controlling the transmit and receive spreading and despreading codes of the mobile sets which correspond to the receive and transmit despreading and spreading codes associated with respective ones of said line circuit.

4. A telephone system as defined in claim 1 or 2 in which the means for wireless transmitting and receiving are comprised of antennae.

5. A telephone system as defined in claim 1 or 2 in which the means for transmitting and receiving associated with the mobile sets are comprised of antannae, and the means for transmitting and receiving associated with the central switching system is comprised of one or more leaky cables transmission line.