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Claims  |
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What is claimed is:
1. A wireless system for telephone communication within a building,
comprising:
(a) means at a central location for receiving a plurality of signals for
communication with selected wireless communication terminals,
(b) means at said central location for converting said signals to spread
spectrum radio frequency signals using correlation codes corresponding to
particular idle channels,
(c) a leaky transmission line located in a communication region within the
building,
(d) means for applying the spread spectrum radio frequency signals to said
transmission line for electromagnetic radiation within said region,
(e) at least one mobile wireless communication terminal adapted to receive
a predetermined one of the spread spectrum radio frequency signals within
the building and for demodulating it onto an intelligible signal.
2. A system as defined in claim 1 including means for transmitting a
supervisory signal on a fixed spread spectrum supervisory channel to said
wireless communication terminal, means at said terminal for receiving a
predetermined form of said supervisory signal designative of a particular
correlation code related to a receive channel, and means for correlating
the predetermined one of the spread spectrum signals using the correlation
code to effect said demodulation into said intelligible signal.
3. A system as defined in claim 1 including means at the central location
for receiving a signal at a particular port designative of a particular
junctor or channel of an incoming signal, and means for transmitting said
spread spectrum supervisory signal to said wireless terminal containing
said predetermined form of said supervisory signal designative of said
particular correlation code corresponding to said particular junctor or
channel.
4. A system as defined in claim 1 including means at the central location
for transmitting said spread spectrum communication signals to the
wireless communication terminals using pseudo-random spreading codes which
correspond to predetermined fixed correlation codes associated with
individual ones of the wireless communication terminals.
5. A system as defined in claim 1, in which each wireless cOmmunication
terminals is comprised of:
(i) a telephone set,
(ii) means for PCM encoding signals received from the telephone set,
(iii) means for spread spectrum and RF modulating the PCM encoded signals
using a first pseudo-random correlation code,
(iv) means for applying the RF modulated signals to an antenna through a
first filter for wireless transmission to the leaky transmission line,
(v) means for receiving RF and spread spectrum modulated signals-from the
antenna via a second filter,
(vi) means for RF and spread spectrum demodulating the received signals
using a second pseudo-random correlation code different from the first
correlation code,
(vii) means for PCM decoding the spread spectrum demodulated signals,
(viii) means for applying the PCM decoded signals to the telephOne,
whereby wireless two-way communication via said leaky transmission line on
channels designated by the correlation codes is provided.
6. A system as defined in claim 5 in which the first and second correlation
codes are variable, and means for causing variation thereof whereby
particular designated send and receive spread spectrum channels are
established.
7. A system as defined in claim 6 including a synchronization and control
circuit, a second demodulator having its input connected to the output of
said second filter for receiving a supervisory signal including
synchronization signals, and providing demodulated digital supervisory
signals therefrom to the synchronization and control circuit, the second
demodulator using a fixed predetermined correlation code for demodulation
of the supervisory signals, a second modulator having its input connected
to the synchronization and control circuit for receiving supervisory
signals and spread spectrum modulating said latter signals using a fixed
predetermined pseudo-random code different from that of the correlation
code used in the first modulator, RF modulating these signals and applying
the signals via the second filter to the antenna, and means in the
synchronization and control circuit for receiving supervisory signals from
the telephone, generating supervisory data signals of predefined format
and applying them to the second modulator, and for receiving supervisory
data signals of predefined format from the second demodulator, for
translating said latter supervisory signals therefrom and applying said
translated signals to the telephone.
8. A system as defined in claim 7 including means at the synchronization
and control circuit for deriving designated correlation codes from the
received supervisory signals of predefined format and for enabling the
first modulator and demodulator to use said latter correlation codes in
modulation and demodulation respectively to establish the receive and
transmit channels for said signals.
9. A system as defined in claim 8 including means for generating and
displaying data at said wireless communication terminal, a data interface
circuit connected to said data generating and displaying means, third
means for spread spectrum and RF modulating data signals received via said
interface circuit and applying the RF modulated signals to the first
filter for transmission via the antenna to the leaky transmission line,
third demodulation means having its input connected to the antenna via the
second filter, and its output connected to the data interface circuit for
applying demodulated data signals thereto, and means connecting the third
modulator and demodulator to the synchronization and control circuit for
carrying signals therefrom designative of the pseudo-random correlation
codes to be used by the third modulator and demodulator.
10. A system as defined in claim 1, 5 or 9 comprising a plurality of
similar circuits each comprising an interface means at the central
location for connection to an individual junctor of a PABX, for receiving
and transmitting communication signals from said junctor and for receiving
and transmitting supervisory signals from the PABX relating to a telephone
call, a central PCM encoder connected to the interface means for receiving
signals to be transmitted to a wireless communication terminal, a
modulator using a fixed pseudo-random correlation code associated with the
junctor for spread spectrum modulating the encoded signal and generating
an RF modulated signal, means for applying the latter RF modulated signal
through a transmit band limiting filter to the leaky transmission line,
means for receiving an RF modulated signal from the leaky transmission
line through a receive band limiting filter, a central demodulator for
demodulating and despreading a received RF modulated signal using a fixed
pseudo-random correlation code associated with said junctor for
demodulating the latter signal, a PCM decoder for receiving a spread
spectrum demodulated signal and applying it to the interface means for
application to said junctor.
11. A system as defined in claim 1, 5 or 9 comprising a plurality of
similar circuits each comprising an interface means at the central
location for connection to an individual junctor of a PABX, for receiving
and transmitting communication signals from said junctor and for receiving
and transmitting supervisory signals from the PABX relating to a telephone
call, a central PCM encoder connected to the interface means for receiving
signals to be transmitted to a wireless communication terminal, a
modulator using a fixed pseudo-random correlation code associated with the
junctor for spread spectrum modulating the encoded signal and generating
an RF modulated signal, means for applying the latter RF modulated signal
through a transmit band-limiting filter to the leaky transmission line,
means for receiving an RF modulated signal from the leaky transmission
line through a receive band-limiting filter, a central demodulator for
demodulating and despreading a received RF modulated signal using a fixed
pseudo-random correlation code associated with said junctor for
demodulating the latter signal, a PCM decoder for receiving a spread
spectrum demodulated signal and applying it to the interface means for
application to said junctor, a plurality of unidirectional repeaters
connected serially at spaced locations in the leaky transmission line
whereby an input and an output to the line are defined, the output of the
transmit filter being connected to the input of the transmission line and
the input to the receive filter being connected to the output of the
transmission line.
12. A system as defined in claim 1, 5 or 9 comprising a plurality of
similar circuits each comprising an interface means at the central
location for connection to an individual junctor of a PABX, for receiving
and transmitting communication signals from said junctor and for receiving
and transmitting supervisory signals from the PABX relating to a telephone
call, a central PCM encoder connected to the interface means for receiving
signals to be transmitted to a wireless communication terminal, a
modulator using a fixed pseudo-random correlation code associated with the
junctor for spread spectrum modulating the encoded signal and generating
an RF modulated signal, means for applying the latter RF modulated signal
through a transmit band limiting filter to the leaky transmission line,
means for receiving an RF modulated signal from the leaky transmission
line through a receive band limiting filter, a central demodulator for
demodulating and despreading a received RF modulated signal using a fixed
pseudo-random correlation code associated with said junctor for
demodulating the latter signal, a PCM analog decoder for receiving a
spread spectrum demodulated signal and applying it to the interface means
for application to said junctor, a plurality of unidirectional repeaters
connected serially at spaced locations in the leaky transmission line
whereby an input and an output to the line are defined, the transmit
filter being connected to the input of the transmission line and the
receive filter being connected to the output of the transmission line, the
frequency of the R.F. modulated signals being between approximately 150
MHz and 1000 MHz.
13. A system as defined in claim 1, 5 or 9 comprising a plurality of
similar circuits each comprising an interface means at the central
location for connection to an individual junctor of a PABX, for receiving
and transmitting communication signals from said junctor and for receiving
and transmitting supervisory signals from the PABX relating to a telephone
call, a central PCM encoder connected to the interface means for receiving
signals to be transmitted to a wireless communication terminal, a
modulator using a fixed pseudo-random correlation code associated with the
junctor for spread spectrum modulating the encoded signal and generating
an RF modulated signal, means for applying the latter RF modulated signal
through a transmit band limiting filter to the leaky transmission line,
means for receiving an RF modulated signal from the leaky transmission
line through a receive band limiting filter, a central demodulator for
demodulating and despreading a received RF modulated signal using a fixed
pseudo-random correlation code associated with said junctor for
demodulating the latter signal, a PCM decoder for receiving a spread
spectrum demodulated signal and applying it to the interface means for
application to said junctor, the transmission line being coaxial cable
having a shield containing gaps sufficient to allow RF energy to radiate
therefrom, a plurality of unidirectional repeaters connected serially at
spaced locations in the leaky transmission line whereby an input and an
output to the line are defined, the output of the transmit filter being
connected to the input of the transmission line and the input of the
receive filter being connected to the output of the transmission line, the
frequency of the R.F. modulated signals being between approximately 150
MHz and 1000 MHz.
14. A system as defined in claim 1 or 5 including a plurality of
unidirectional repeaters connected serially at spaced locations in the
leaky transmission line.
15. A system as defined in claim 1 or 5 in which the transmission line is
comprised of coaxial cable having a shield containing gaps sufficient to
allow RF energy to radiate therefrom, and in which a plurality of
unidirectional repeaters are connected serially at -spaced locations in
the leaky transmission line.
16. A local 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.
17. A telephone system as defined in claim 16 in which the spread spectrum
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.
18. A telephone system as defined in claim 16 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.
19. A telephone system as defined in claim 16, 17 or 18 in which the means
for wireless transmitting and receiving are comprised of antennae.
20. A telephone system as defined in claim 16, 17 or 18 in which the means
for transmitting and receiving associated with the mobile sets are
comprised of antennae, and the means for transmitting and receiving
associated with the central switching system is comprised of one or more
leaky cable transmission lines. |
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Claims |
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Description |
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This invention relates to a telephone communication system utilizing
wireless voice, data, or voice and data terminals.
It is often difficult to provide telephone service to personnel who are
continuously moving, yet must be quickly available at a telephone or data
terminal, e.g. warehouse stock takers, personnel responsible for building
maintenance or security, etc. Paging receivers have been used to summon
such personnel to call a number via the closest telephone set. Sometimes
such personnel are required to enter data into a terminal for storage at a
central computer. Yet wired jacks for interfacing a telephone with a PABX
or terminal to a computer are often spaced at substantial distances from
each other in warehouses, for example.
Voice communication of such personnel has been partly solved by the use of
cordless (wireless) telephones each operating at a different frequency
from the other. However such telephones have exhibited substantial
problems. There are usually a limited number of channels available,
limiting the number of telephone/wireless receiver sets which can be used
in a given area. Often interference from adjacent channels or other
services is encountered. Transmission or reception nulls are often
encountered when a cordless telephone is oriented in a particular
direction relative to the main transmitting and receiving station. Such
nulls are also caused by shielding by steel and concrete of which the
building housing the system is built. The system lacks privacy; since the
common radio bands are used, the signals can be readily intercepted.
In addition, the use of wireless presents a range problem. Particularly for
large areas, such as when an entire building, a large warehouse, or
aircraft hangar, etc., is to be covered by the system, a high powered
central transmitter could be used. However the power of the transmitter
cannot be greater than a level prescribed by the regulatory agencies. This
places a limitation on the communication range. In the event that many low
power transmitters are used scattered around the building, the cost
increases substantially.
In the system described in U.S. Pat. No. 4,462,113 issued Jul. 24, 1984,
domestic A.C. power lines are used as an antenna for a low power
transmitter. However in this case the null problem still exists, and in
addition there is a severe bandwidth limitation to below approximately 150
or 200 kHz. Further, transmission of signals via the A.C. power lines is
notoriously insecure, since such signals are often carried by the power
lines to neighbouring offices and buildings.
Due to the inherent lack of security, nulls and power/distance limitations,
the use of a mobile telephone handset has been limited to the home
environment, or to very special applications. Until the present invention
was made it had not yet been found suitable for commercial applications in
which, for example, an entire multi-story building housing many different
companies, some possibly being competitors with each other, could be
served mobile telephone and data services reliably from the same PABX.
Such prior art systems could not ensure complete security of
communications, sufficiently low power radiation outside the confines of
the building, and absences of nulls or fade areas within structures.
Clearly for the above reasons portable terminals could also not be used for
reliable transmission and reception of data to a central computer.
The present invention provides a communication system which can be
connected to a PABX switching system, which solves the problems noted
above. In addition to the provision of secure mobile (wireless)
communications in a low power environment with the substantial elimination
of the null or fade areas, it provides multi-channel wideband
communications which can reliably carry voice, data and signalling
(supervisory) signals.
According to the present invention leaky transmission cables are used to
radiate and to receive communication signals within the communication
region. While at single frequencies such cables exhibit nulls along the
cables at regular intervals, the communication signals which are carried
and which are received in the present invention are spread spectrum.
The use of spread spectrum signals carried by a leaky cable transmission
system achieves several highly desirable results. Firstly, nulls are
virtually undetectable or are eliminated due to the spreading of the
signals over a wide bandwidth. Secondly, since pseudo-noise bandwidth
spreading (correlation) code is used for each channel, privacy of
communications is virtually assured as well as immunity from interference
between channels.
Since a leaky cable is used to distribute the signal in the communication
region, the transmitter power and resulting effective radiated power can
be very low. Furthermore, the power which is used is spread amongst the
frequencies of a wide bandwidth, further reducing the power used at any
one frequency. Thus the interference that may be caused to other radio
signals outside of the building is virtually nil. Also the radiated power
within the building can be increased to a substantial degree in comparison
with a non-spread spectrum, single or multi-point antenna transmitter to
minimize the bit error rate, yet the effective interference with external
or other wireless services remains virtually nil, while the reliability of
communication within the building is increased.
The spread spectrum-leaky cable system according to this invention can
operate side-by-side with other similar systems or with conventional AM or
FM wireless systems with substantially no or minimal interference. The
maximum interference which might be apparent in such other systems would
merely be an increase in the background noise level.
The various handsets or wireless remote terminals to be used in the present
system are preferred to be accessed by address code on a supervisory
channel, and to be controlled to internally select a pseudo-noise
correlation code which matches a predetermined channel transmit
pseudo-noise spreading code. This type of system would benefit by the use
of universal wireless handsets or terminals. However in another type of
system each handset or terminal is channel fixed with a predetermined
pseudo-noise correlation code circuit, and the head end terminal changes
its transmit pseudo-noise spreading code to suit that of the selected
handset.
In the reverse transmission direction which uses a different RF centre
frequency the mobile handset is either fixed with a transmit pseudo-noise
spreading code for encoding the transmitted signal, or it can be caused to
transmit on a channel selected by the head end under control of the head
end via signals on the supervisory channel.
Leaky cable transmission systems have long been used to communicate in
tunnels and mines. One such system is described in U.S. Pat. No. 4,476,574
issued Oct. 9, 1984. A large number of publications is listed in that
patent which will provide background information to the reader on the use
of leaky feeder communication systems both subsurface and above the
surface of the earth. However those systems suffer from one or more of the
problems described above which restrict them from use in a reliable
commercial communication system. A text which describes leaky feeder
systems is LEAKY FEEDERS AND SUBSURFACE RADIO COMMUNICATIONS by P.
Delogne, IEE Electromagnetic Waves Series 14, 1982 Peter Peregrinus Ltd.
Spread spectrum systems have been used in wireless systems in the past, as
well as in certain specialized wired systems. A wireless spread spectrum
system is described in U.S. Pat. 4,455,651 issued June 19, 1984 and in
U.S. Pat. No. 4,086,504 issued Apr. 25, 1978. However in both those cases
the power limitation problem and the null problem described above did not
present problems, since in the first case directional antennas were used,
and in the second case high power could be used and the system was not
used in a communication system of the present kind. Furthermore, the
locations of the various transmitting stations used in the latter patent,
which relates to a seismic exploration system, can all be tested for
proper spectrum prior to use and the transmitters moved in case a null is
encountered. In addition, the locations of the spread spectrum
transceivers are all fixed and preknown, since they are used for
triangulation purposes.
Spread spectrum systems have also been used in wired systems. For example
in U.S. Pat. 4,438,519 issued Mar. 20, 1984, a spread spectrum system is
used connected to an A.C. power line, which is not a leaky feeder
transmission cable, but constitutes an antenna. In that patent, however,
it is clear that the bandwidth of the power line is very low, and the
transmission region is limited to areas which are not isolated by a
transformer. Hence such systems are not useful for wide band multi-channel
voice and data communication systems.
A spread spectrum signal is applied to telephone lines in the invention
described in U.S. Pat. No. 4,475,208 issued Oct. 2, 1984. In this system
data signals are converted to spread spectrum and are transmitted
simultaneously with voice over already existing telephone lines which are
not leaky feeder transmission cables . The bandwidth of such telephone
lines is so low that the data signals are of very low bit rate. Clearly
the system is not suitable for use in multi-channel wide bandwidth
transmission. In addition, the system is unsuitable for use with wireless
handsets or terminals since the telephone transmission lines cannot carry
radio frequency signals for any significant distance.
A general description of the history and structure of spread spectrum
systems will be found in the publication SPREAD-SPECTRUM COMMUNICATIONS,
edited by Charles E. Cook et al, published by the IEEE Press, Institute of
Electrical and Electronic Engineers, Inc.
Carriage of a plurality of spread spectrum communication signals, each
modulated by a different pseudo-random spreading or correlation code, by a
leaky cable transmission system thus provides a new form of wireless
mobile voice and data communication system previously not thought
possible, that is, a reliable, private and low power communication system
using a large number of noted telephone handsets that can be carried
within a predefined environment which can be used sharing the same
bandwidth as other commercial wireless services without interference
between the two kinds of systems. Such a system for the first time is
viable within a building structure, i.e. steel and concrete, which
previously would not reliably support reliable wireless communication to
the required degree.
Because of the broadband and multi-channel capability of the present
system, the various mobile telephone sets can be provided with a full
range of features normally made available only to wired telephone sets by
the PABX, such as local alphanumeric display, conferencing, abbreviated
dialing, etc., as well as computer access, remote control of various
apparatus such as automatic door locks, etc. Such features are not now
possible with the well known cordless telephones. A 32 channel system of
the type described herein could give typically 200 users at a site mobile
telephone facility depending on traffic. Further, the communication
channel between the remote mobile unit and a PABX which connects to a
larger computer can provide to the remote unit enormous computational
power which would otherwise not be available in a hand held computer due
to its size and cost limitations, and since a large shared data base can
be stored at the central computer.
A preferred embodiment of the invention is a wireless communication system
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.
The invention also facilitates any of the wireless communication terminals
to initiate a communication with the central location, such a
communication being capable of having as its final destination any
telephone connectable to the central location, including others of the
wireless communication terminals.
A better understanding of the invention will be obtained by reference to
the detailed description below of the preferred embodiment, with reference
to the following drawings:
FIG. 1 is a general block diagram of a system according to the present
invention,
FIG. 1A illustrates a code arrangement used in multiplexing the channels of
the spectrum,
FIG. 2 is a block diagram illustrating the mobile handset according to the
preferred embodiment of the invention,
FIG. 3 is a block diagram of the central equipment according to the
preferred embodiment of the invention, and
FIG. 4 is a block diagram of a preferred form of transmit channel used in
the system.
To briefly review the spread spectrum concept, this technique causes the
spectrum of the transmitted signal of each channel to be spread over a
greater amount of bandwidth than would be the case if time or frequency
division multiplexing techniques were used. Indeed, the signals of all of
the channels used are spread over the same band. This is achieved by
multiplying a generated data stream to be transmitted by a sequence with
the correct auto-correlation and cross-correlation properties
(pseudo-random/noise code sequence). The resulting output signal is then a
sequence having a higher data rate than that of the input data stream,
which when used to modulate some form of amplitude, frequency or phase
shift keyed system, causes the spectrum to be spread over a wide
bandwidth.
At the receiver the incoming signal is multiplied by the same
pseudo-random/noise sequence and the spectrum becomes despread to its
original bandwidth. It is important to note that any interfering signal is
spread at the receiver rather than despread. The signal is then filtered
at the receiver to the original bandwidth, leaving the original signal
intact but the interfering signal is attenuated and thus its effect is
diminished. This occurs whether the interfering signal is a real signal or
is a hole (i.e. null) in the spectrum generated by some propagation
effect. Thus a spread spectrum system provides not only immunity against
interfering signals but also protection against holes in the spectrum.
Because a large number of spread spectrum sequences are generally known, it
is possible to choose a set for a system which would be very difficult to
decode by an intruder or eavesdropper. The spread spectrum system
therefore contains its own intrinsic security. The interference to which
the system is immune may of course be other channels of the system using
the same spectrum Spread spectrum is therefore intrinsically a
multiplexing system. Different channels using the same bandwidth can be
immune to each other if they use different spreading sequences for
modulation and demodulation.
For example, for a pseudo random code having length 255 bits, there are
probably approximately 40000 codes which are strongly orthogonal, and thus
are highly secure. It is preferred in the present system to use a centre
frequency of between about 150 mHz to 1000 mHz, although the invention is
not limited to this band, each channel being approximately 32k Hz wide, in
which the voice signals are digitized in a well known manner. The spread
channel is preferred to be 8 mHz wide.
Turning now to FIG. 1, the basic system according to the invention is
illustrated. Within the confines of a building periphery 1 a leaky cable 2
is laid. The leaky cable can be coaxial cable with holes in its shield,
such as described in Canadian Patent 1,014,245 issued Jul. 19, 1977, or
other types of leaky transmission cables as described in the aforenoted
text by P. Delogne. The cable can be sinuously laid above the false
ceiling over the entire width and breadth of the building, can extend down
the centre of a narrow building, and can pass from storey to storey in a
multi-storey building as well as extend over the ceiling area of a
building. The leaky cable can be located within movable walls or within an
electromagnetically transparent floor. Clearly the location of the cable
is dependent on the region to be covered and many variations are possible.
The important aspect of the placement of the cable is that for a given
minimum electromagnetic field strength, the entire working area of the
building which defines the communication region should be enveloped by the
field strength leaked from the cable which is above the minimum level.
It will be seen that since the field strength drops off by between the
square and the cube of the distance from the cable, the effective radiated
power outside the building periphery will be low or virtually nil.
As central equipment, feeding the cable at one end is a transmitter 3, and
receiving signals from the other end of the cable is a receiver 4. The
input to the transmitter 3 is connected to a PABX 5 and the output of
receiver 4 is connected to the PABX 5. Also connected to the PABX are
trunks 6 and local telephone sets 7. Of course the PABX can be divided by
number code so that groups of local telephone sets 7 can be associated
with one business or division while other groups can be associated with
another business or division, if desired.
One or a plurality of remote wireless mobile telephone sets 8 are located
within the building. These telephone sets will be referred to herein
henceforth as mobile sets and can include data originating and receiving
terminals as well as, or in place of voice handsets. Mobile sets 8
preferably are battery operated, can be carried by a user, can be located
on a desk, hung on a wall, etc. They are not connected by wire to the
telephone system PABX.
Depending on the characteristics of the leaky cable 2, its length, etc.,
repeaters 9 may be required to be connected at regular intervals in series
with the leaky cable. These repeaters are preferably wideband, such as
CATV television repeaters.
The PABX 5 is primarily a normal PABX which operates to interconnect local
telephone sets with each other or with trunks 6. In order to communicate
with the mobile sets 8, one of the telephone sets 7 dials an appropriate
extension number designating a mobile set. The PABX, instead of connecting
the telephone set to one of the other sets 7, simply chooses a line
terminal 10 which corresponds to the mobile sets 8, connected to a
predetermined junctor. The selected line terminal connects to the input of
transmitter 3. In one embodiment ringing current is applied to the line
terminal in the normal manner. The transmitter 3 converts the ringing
current to a predetermined digital code, interleaves it with other digital
codes on a supervisory channel, converts the resulting signal to a spread
spectrum supervisory channel signal and applies the resulting signal to
the leaky cable 2.
The line terminal however designates which mobile set is to be selected,
since it corresponds to the dialed number. Once the line terminal has been
selected the transmitter prefixes the digital code with an identifier code
which is unique to the mobile set. However it should be noted that rather
than using line terminals, junctors of the PABX can be used. Also, instead
of applying ringing current to the terminal it can apply a ringing enable
signal to a ringing terminal associated with the line terminal.
The signal radiates from the leaky cable into the communication region
within the building periphery 1, and is received by all of the sets 8. All
of the mobile sets continuously decode the signalling signals, and as soon
as the identifier prefix which identifies the designated set has been
received, that the identified set converts the remaining code which calls
for it to ring. Ringing is effected in the mobile set by keying an
internal "warble" or other signal to alert the user in a well known
manner.
The user upon hearing the ringing signal, switches his mobile set on, the
equivalent of going off hook. That mobile set then generates an off-hook
supervisory code which is converted to a spread spectrum R.F. signal in
the signalling channel, and is transmitted via its own small local antenna
to the leaky cable 2. Preferably the signal is sent within a time slot
designated by the synchronization and control signals sent on the
aforenoted supervisory channel. Polling and response of the mobile sets in
sequence is preferred to be used in the supervisory channel.
The spread spectrum off-hook code is received by the leaky cable, and is
carried to the input of receiver 4. Receiver 4 demodulates, despreads and
decodes this signal and applies the off-hook supervisory signal to the
line input port 11 of PABX 5 associated with terminal 11 (or to the
associated junctor).
It should be noted that while two unidirectional ports 10 and 11 have been
indicated, the transmitter and receiver can equally be connected to
separate inputs of a hybrid which is connected to a bidirectional line or
junctor.
The PABX 5, receiving the off hook supervisory signal as if it were from a
telephone set, completes the connection between the calling party and
mobile set via the transmitter 3 and receiver 4 as if it were to be
connected to another telephone 7. The transmitter 3 and receiver 4 are
associated with fixed voice channel transmit and receive pseudo-random
spreading codes. When this occurs control apparatus associated with
transmitter 3 and receiver 4 transmits on the supervisory channel a data
signal addressed to the now off-hook mobile set 8 which designates the
transmit and receive pseudo-random codes for the two-way voice channel to
be used for voice communication to match those of the transmitter and
receiver. The mobile set adopts the codes and thus can transmit and
receive on the designated channel. Voice communication between the
telephone set 7 and mobile set 8 now proceeds on the specified channel,
while using the dedicated supervisory channel for supervisory signals.
When either of the sets goes on hook at the conclusion of the
communication, the supervisory signal associated with that function is
carried by the supervisory channel as described earlier during the set up
of a call. The voice connection is then taken down in a manner analogous
to setting up all supervisory and voice or data communication thus can
proceed in this manner.
FIG. 1A illustrates the preferred form of channel assignments. Dedicated
pseudo-random codes designate the supervisory channel in each direction,
while a plurality of pseudo-random codes (e.g. up to approximately 40,000)
can be used to designate the transmit and receive channels. All channels
use essentially the safe frequency band, though different bands are used
in the centre to mobile and mobile to centre directions. None will be
found to interfere with each other or with other wireless services using
the same frequencies, except for random bit errors which will increase
with traffic. In a typical system only about 32 two-way channels will be
required although there is clearly capacity for many more, given the
number of codes available.
In a similar manner calls can be made from any mobile set 8. All mobile
sets 8 continuously receive and transmit information on the supervisory
channel. Thus if a mobile set 8 goes off-hook this information is
transmitted by the supervisory channel to receiver 4 where it is
demodulated, despread and decoded before passing to PABX 5. On receiving
the off-hook signal PABX 5 allocates a voice channel as described above,
transmits dial tone to the mobile set 8, receives signalling information
from mobile set 8 and sets up the call in the usual manner. The mobile
sets 8 are clearly not limited to voice communications; they can be
combination voice and data sets, or restricted to being data terminals. In
this respect one of the trunks 6 of PABX 5 can be connected to a computer
for receiving data from and transmitting data to a mobile data set 8.
Alternatively a computer can be connected directly to the main bus of PABX
5 for communication with the mobile sets. In this manner the mobile set 8
can be used as a remote terminal to a central computer. Low speed data
communication can be effected with the mobile set 8 via the supervisory
channel, or high speed via a dedicated data channel, or via a voice
channel shared with and carrying data.
Block diagrams of the mobile set and central equipment constituting the
transmitter and receiver are shown in FIGS. 2 and 3. The mobile set will
be described first, with reference to FIG. 2.
An analog telephone set 15 to which a handset or handset with display 16 is
connected and, if desired, a keypad 17 has an internal hybrid with an
output line connected to an analog-to-digital PCM encoder 18. Preferably
the encoder is an adaptive differential encoder of toll quality, e.g. it
will encode a signal which is output at 32 kb/sec.
The output of encoder 18 is connected to the input of modulator 19, which
both spread spectrum and RF modules the incoming signal. The output of
modulator 19 is connected to the input of a combiner 21 which is connected
through a transmit filter 22 and directional coupler 22A to an antenna 23.
The filter preferably is 8 megahertz wide, having a Q between 20 and 100.
It can for example be a printed strip line controlled by varacter diodes
such as is often found in the tuner of a TV set.
The antenna 23 is similarly connected through a directional coupler 23A to
the input of the receive filter 24 (which is similar to filter 22) which
is connected to the input of a splitter 25. One output of splitter 25 is
connected to demodulator 27 which both spread spectrum and RF demodulates
the signal. The output of demodulator 27 is connected to the input of
digital-to-analog PCM decoder 28, the output of which is connected to the
input line to telephone 15.
A data interface circuit 29 is also connected to telephone set 15, which
contains data encoding and signalling circuitry as well as associated
buffers. Considering only the signalling aspect for the moment, off-hook,
on-hook, etc. and other signalling signals as are normally generated in a
telephone set are applied via interface circuit 29 to a synchronization
and control circuit 30. The synchronization and control circuit contains a
master clock for the mobile set, and controls the filters 22 and 24. The
clock signal used in the synchronization and control circuit is obtained
from the incoming signal received via antenna 23.
A spread spectrum and RF modulator 31 has its input connected to
synchronization and control circuit 30, and its output connected to
another input of combiner 21. The output of RF modulator 38 is connected
to another input of combiner 21.
A second output of splitter 25 is connected to an input of demodulator 34,
which has its output connected to synchronization and control circuit 30.
A voltage controlled oscillator circuit 35 is connected to the
synchronization and control circuit 30, and has outputs connected to
modulators 19, 31 and 38 and demodulators 27, 34 and 41.
Synchronization and control circuit 30 also has outputs connected to
modulators 19, 31 and 38 and demodulators 27, 34 and 41.
In operation, according to the preferred embodiment of the invention the
pseudo-random codes designating the transmit and receive supervisory
channels are fixed by means of code plugs or other similar code
designating means, fixed in modulator 31 and demodulator 34. A supervisory
signal having a spreading code correlatable by the correlation code in
demodulator 34 is received from the leaky cable transmission line 2 (FIG.
1) by antenna 23. The signal passes through directional coupler 22A, is
filtered in filter 24, passes through splitter 25 and into demodulators 27
and 34. However since demodulator 27 will not recognize the encoded
signal, it outputs only a low level random noise signal. However since
spread spectrum modulator 34 does recognize the supervisory channel code,
it decodes the signal and applies it to synchronization and control
circuit 30. Synchronization and control circuit 30 recognizes a data
header designating the local mobile set and further recognizes the
demodulated code as meaning that ringing should start. It applies a signal
to telephone controller 15, which begins ringing. If the local mobile set
data header was not recognized, the ringing function would not be enabled.
It should be noted that on the supervisory channel the code which is
received can designate any supervisory function or indeed can carry low
speed data communication signals. In this respect it is preferred that the
signal carried on this channel should contain four 8 bit words in
sequence: the first 8 bits designating the station number of the mobile
set, and second 8 bits designating what function should be performed, the
third 8 bits shoul | | |
