Radiotelephone system in the form of a private branch exchange

A radiotelelphone system in the form of a private branch exchange (12) within and/or outside one or more buildings is equipped with a plurality of mobile subscriber units (28), a plurality of transmitter/receiver units (27) capable of communicating with the subscriber units (28) by radio, and a central unit (26) cooperating with the transmitter/receiver units (27). In order that such a radiotelephone system not only permits mobile telephoning within and/or outside buildings but also can be installed later and adapted to changing requirements in a simple manner, the transmitter/receiver units (27) are arranged and designed to form not only a radio link to at least one of the subscriber units (28) but also, via respective adjacent transmitter/receiver units (27), a radio-link chain (radio bus) to the central unit (26), which has a separate transmitter/receiver unit.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a radiotelephone system, particularly in 
the form of a private branch exchange within and/or outside one or more 
buildings, comprising a plurality of mobile subscriber units, a plurality 
of transmitter/receiver units which can be coupled with the subscriber 
units by radio links, and a central unit co-operating with the 
transmitter/receiver units. 
2. Description of the Prior Art 
In such a radiotelephone system, which is disclosed in EP-A-225 607, the 
individual transmitter/receiver units are connected with the central unit 
of the private branch exchange via connecting lines, so that the 
individual subscriber units are suitable for mobile use but are tied to 
the predetermined coverage of the transmitter/receiver unit. Another 
disadvantage of this prior art radiotelephone system lies in the fact that 
an expensive cable system has to be installed to connect the individual 
transmitter/receiver units with the central unit. This expenditure is 
nearly equal to that required for conventional private branch exchanges 
which are connected with the subscriber units by subscriber lines. 
From Br Telecom Technol J Vol 4 No. Oct. 4, 1986, "Controlled Radio 
Coverage in Buildings", D. A. Palmer et al, a radiotelephone system is 
known in which the transmitter/receiver units are provided, for example, 
on each floor of a building, and in which these transmitter/transceiver 
units are connected with the individual subscriber units via leaky 
feeders. This system suffers basically from the same drawbacks as those 
mentioned for the system referred to above since a fixed wiring system is 
required. 
SUMMARY OF THE INVENTION 
It is the object of the present invention to provide a radiotelephone 
system of the above kind which not only permits mobile telephoning within 
and/or outside buildings but also is easy to install even later and 
readily adaptable to changing requirements. 
This object is attained by a radiotelephone system having a plurality of 
mobile subscriber units, a plurality of transmitter/receiver units which 
can be coupled with the subscriber units by radio links, and a central 
unit cooperating with the transmitter/receiver units, which units are 
arranged and designed to form not only a radio link to at least one of the 
subscriber units, but also, via respective adjacent transmitter/receiver 
units, a radio-link chain to the central unit, which has a separate 
transmitter/receiver unit. 
According to the invention, the permanent connection of the individual 
transmitter/receiver units with the central unit of , e.g., a private 
branch exchange is thus replaced by a radio bus, which eliminates the need 
for expensive wiring systems and permits quick and inexpensive changes in 
the telephone system if changes are made in the layout of the rooms of the 
building. It is also possible to adapt the radiotelephone system to 
changing requirements in a simple manner, e.g., with regard to meetings-at 
which the participants can be provided with cordless telephones as 
subscriber units. In that case, it is also possible to take into account 
changes in the size of the available rooms. 
For further simplification, particularly with respect to equipment design, 
the transmitter/receiver units are of uniform design and are controllable 
from the central unit with respect to carrier frequency and/or channel 
number and/or transmitting power. 
It is advantageous to operate the units or equipment in a time-division 
multiplex mode, wherein the transmitter/receiver unit and the subscriber 
unit operate in a TDMA mode. 
A simple and advantageous solution is obtained if the transmitter/receiver 
units operate with three carrier-frequency pairs, a first of which is used 
for duplex communication with the respective subscriber unit, while the 
two others are used for duplex communication with two adjacent 
transmitter/receiver units, and the subscriber unit operates with a single 
carrier-frequency pair. 
Even greater mobility results if the transmitter/receiver unit is provided 
with a power supply for connection to an element of the electric building 
wiring system, as is the case in a further embodiment of the present 
invention. In that case, it may be advantageous to provide the 
transmitter/receiver units with conventional power plugs and to use 
existing receptable outlets of the electric building wiring system. An 
even simpler and more advantageous solution is obtained if the power 
supply is provided with a base for a conventional lamp socket since 
simultaneously with the electric connection, mechanical support is 
provided for the transmitter/receiver unit. 
If the radio telephone system is used in a private branch exchange 
connected to one or more central-office trunk and/or central-office 
general equipment, the central unit of the private branch exchange is 
provided with a controller for the radio bus, said controller being 
connected to the separate transmitter/receiver unit, it may be 
advantageous to use conventional private branch exchanges or, if a private 
branch exchange with permanently connected subscriber units is already 
present, to convert such a private branch exchange to the radiotelephone 
system according to the invention or to expand it into such a system in a 
suitable manner. 
When the transmitter/receiver units of the radio bus are used in a 
quasi-optical (line-of-sight) path, optimum radio communication quality is 
achieved.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The radiotelephone system shown in FIG. 1 in the form of a private branch 
exchange is connected to a central-office-channel equipment 13 which can 
receive and transmit radio signals, which are transmitted via the public 
radio network, via an antenna 14 in the known manner. This public mobile 
communication network is, for example, the C-net, which is currently used 
in Germany and operates in the 450-MHz range, or the future D-net, which 
will operate in the 900-MHz range. As shown in FIG. 2, the 
central-office-channel equipment 13, which can receive 1 to n radio 
channels in given carrier-frequency ranges, is of conventional design and 
a duplexer 16, a transmitter/modulator unit 17, a receiver/demodulator 
unit 18 connected to the duplexer 16, a frequency-conditioning unit 19, 
and a signal-conditioning unit 20 and a signal-evaluating unit 21 which 
are connected to the transmitter/modulator unit 17 and the 
receiver/demodulator unit 18, respectively, and have their other ends 
coupled to a channel translator 22 which is connected to the private 
branch exchange 12 of the radiotelephone system in accordance with the 
invention. On the connecting line 23, 1 to n PCM central-office channels 
can be transmitted. 
The radiotelephone system, shown and designed here as a private branch 
exchange 12, has a central unit 26 which is connected, or connectable to 
permit two-way communication, via a radio-link chain 27 within and/or 
outside one or more buildings to a subscriber arrangement 28, which is 
present there. The radio-link chain will be referred to as "radio bus". 
The subscriber arrangement 28 consists of a plurality of subscriber units 
31 as shown in FIG. 4, which are designed as conventional cordless 
telephones. These subscriber units 31 are thus mobile and are present at 
different points within the radiotelephone system. The radio bus 27 
consists of a plurality of radio-bus elements 32 of essentially identical 
construction as are shown in FIG. 5. The radio-bus elements 32 are 
distributed within the radiotelephone system or within and/or outside said 
one or more buildings in such a way that essentially a quasi-optical 
(line-of-sight) path exists between adjacent radio-bus elements 32. 
However, depending on the materials used for walls and the like, it is 
frequently not absolutely necessary that there should be a line-of-sight 
path between the individual radio-bus elements 32. For the 
transmitter/receive mode, it is then necessary for the respective 
subscriber unit 31 to be located within the coverage of a radio-bus 
element 32 which enters into two-way radio communication with an adjacent 
radio-bus element 32 which, in turn, enters into two-way radio 
communication with an adjacent radio-bus element 32, etc. 
FIG. 3 shows the central unit 26 of the private branch exchange 12 of the 
radiotelephone system. As can be seen, the private branch exchange 12, 
besides being connected at the terminal A to the central-office-channel 
equipment 13 for radio communication via the public mobile communication 
network, may also be connected to a trunk equipment (not shown) at 
terminal B, so that incoming and outgoing calls can also be transmitted 
via the trunk network. The central unit 26 has a first matching circuit 36 
and an extension controller 37 which are connected both to the line 23 
coming from the central-office-channel equipment (terminal A) and to the 
line 34 coming from the trunk equipment (terminal B). The first matching 
circuit 36 is connected directly and via a switching network 38 to a 
second matching circuit 39, to which extension telephones may be 
permanently connected via lines 41. The extension controller 37, which is 
also connected to the switching network 38, is coupled to a radio-bus 
controller 42. A radio-bus head 33, which is similar in design to a 
radio-bus element 32, is connected both to the second matching circuit 39 
(at a terminal C) and to the radio-bus controller 42 (at a terminal D), 
and has an antenna 43. 
As shown in FIG. 4, the cordless subscriber unit 31 is of conventional 
design and has a duplexer 46 which is connected at one end to a 
transmitting/receiving antenna 47 and at the other end both to a 
transmitter/modulator unit 48 and to a receiver/demodulator unit 49, 
between which there is a frequency-conditioning unit 50 connected to a 
console processor 51. The transmitter/modulator unit 48 is connected to a 
microphone 54 via a signal-conditioning unit 52 and a speech encoder 53. 
Correspondingly, the receiver/demodulator unit 49 is connected to an ear 
piece 57 via a signal-evaluating unit 55 and a speech decoder 56. 
As shown in FIG. 5, each radio-bus element 32 has a duplexer 61 which is 
connected at one end to an antenna 62 and at the other end both to a 
transmitter/modulator unit 63 and to a receiver/demodulator unit 64, 
between which there is a frequency-conditioning unit 65 which is connected 
via a control processor 66 to a service-channel demodulator 67. The 
transmitter/modulator unit 63 and the receiver/demodulator unit 64 are 
interconnected directly and via a converter 68. The receiver/demodulator 
unit 64 is also connected to the service-channel demodulator 67. 
Each radio-bus element 32 of the radio bus 27 between the subscriber 
arrangement 28 and the central unit 26 is provided with a power-supply 
unit 69 having a conventional screw base via which the radio-bus element 
32 can be electrically connected with lamp sockets 70 of the electric 
building wiring system. The radio-bus element 32 can thus be inserted into 
any lamp socket 70, so that not only electric connection but also 
mechanical support are provided for the radio-bus element 32. 
The radio-bus head 33, described in connection with the central unit 26, is 
identical in construction to the radio-bus element 32 of FIG. 5 except 
that the power-supply unit 69 is permanently connected with the power 
supply of the central unit 26 instead of being connectable with a socket 
70. In addition, as can be seen in FIG. 3, the control processor 66 (FIG. 
5) is connected at the terminal D to the radio-bus controller 42 (FIG. 3), 
and the converter 68 (FIG. 5) is connected at the terminal C to the second 
matching circuit 39 (FIG. 3). 
Two-way communciation between a subscriber from outside the radiotelephone 
system and a mobile radio subscriber within the radiotelephone system 
takes place as follows. The signal coming from outside is transferred 
through the central-office-channel equipment 13 or trunk equipment into 
the central unit 26 of the private branch exchange 12 or the 
radiotelephone system and is transmitted by radio from the radio-bus head 
33 of the a central unit 26 to the nearest radio-bus element 32, thence to 
the nearest radio-bus element 32, etc., until it reaches that radio-bus 
element 32 within whose coverage the dialled subscriber unit 31 is 
located. The transmission of signals from the respective subscriber unit 
31 to an external subscriber is reverse, i.e., from the subscriber unit 31 
to the nearest radio-bus element 32, thence to the nearest radio-bus 
element 32, etc., up to the central unit 26 and thence to the outside. 
Two-way communication between two subscriber units 31 within the 
radiotelephone system takes place analagously via the respective radio-bus 
elements 32 and the central unit 26 located between said elements. 
If the radiotelephone system 11 is located within or outside a single 
building, it may generally be sufficient to provide one radio-bus head 33 
which is permanently connected with the central unit 26. If two or more 
buildings are connected to the radiotelephone system, or the external 
subscriber station is too far away, it may be advantageous to provide in 
or outside the other building or each of the other buildings an additional 
radio-bus head 33 as a starting point for the radio-link chain in the form 
of the radio-bus 27 and to permanently connect this additional radio-bus 
head 33 to the central unit 26 via a connecting line. 
FIGS. 6 and 7 show the TDMA signal formats for the radio-bus element 32 and 
the subscriber unit 31, respectively, the latter being designed as a 
telephone operating in the time-division multiplex mode. As shown in FIG. 
6, each radio-bus element 32 can process three different groups of 
frequency pairs, one of which serves for transmission to and reception 
from one or more subscriber units 31, while the two others serve for 
transmission to and reception from the two adjacent radio-bus elements 32. 
FIG. 7 shows that the transmission- and reception-frequency ranges of a 
subscriber unit 31 having a single pair of frequencies have 64 channels. 
It is also possible to provide the identical subscriber units 31 with more 
than one carrier frequency (not shown). The same applies analogously to 
the radio-bus elements 32. In these cases, the frequencies, channels, and 
transmitting powers of the radio-bus elements 32 and the subscriber units 
31 are advantageously set from the central unit 26 via a service channel.