System which routes radio transmissions to selected repeaters for retransmission

A radio transmission system designed to operate on a single defined frequency channel. The system comprises a central receiver (12), at least one transmitter (14) remote from the receiver and a plurality of repeaters (13) arranged in zones (Z1 to Zn) arround the receiver. Each repeater can receive a signal from a transmitter and re-transmit the message so that it eventually reaches the receiver by way of one or more repeaters. The system includes protection means for protecting the message against loss or corruption, for example by using unique time delays before re-transmission or by addressing a certain sequence of repeaters with alternate paths being provided to guard against the message being lost.

The present invention relates to a radio transmission system in which 
messages sent by a transmitter are picked up by a central receiver via a 
number of repeaters. 
The invention is particularly concerned with low power radio transmissions 
using a single frequency, since low power radio transmitters and their 
associated receivers are licence-exempt, provided that they operate on a 
single defined channel (which may be in the HF, VHF or UHF band), the 
transmitter output power is restricted to a low level, and the radio 
system operates on a single site. 
Such low power, licence-exempt devices are commonly used to send binary 
data messages for various purposes, such as telemetry, telecontrol and 
telecommand applications, for identification tagging and increasingly for 
"in-building" and "on-site" wireless security systems. 
The main disadvantages of these low power licence-exempt radio systems is 
their low operational range due to the low level of permitted transmitter 
output power. Attempts have been made to overcome this disadvantage by 
using radio repeaters to relay messages from outlying transmitters to a 
central receiver, but these have generally not been successful because the 
messages from the repeaters and the transmitters share the same frequency 
channel and may clash. Hence, there is an increased risk of traffic 
contention and subsequent loss or corruption of messages. 
The present invention seeks to overcome this difficulty, whilst retaining 
the advantage of greater operational range resulting from the use of 
repeaters. 
According to the present invention there is provided a radio transmission 
system designed to operate on a single defined frequency channel, the 
system comprising a central receiver, at least on transmitter remote from 
the central receiver and, located between said receiver and transmitter, a 
plurality of repeaters each adapted to receive a message from a 
transmitter or from another repeater and to retransmit said message to the 
receiver either directly or via at least one other repeater, in which the 
system includes protection means associated with the repeaters for 
protecting said message against loss or corruption during transmission of 
said message from said transmitter to said receiver. 
Preferably the protection means comprises means for preventing simultaneous 
transmission of said message by two or more repeaters. 
In one embodiment of the invention each repeater includes delay means for 
imposing a time delay between receipt of said message by said repeater and 
re-transmission of said message by said repeater, the time delay 
associated with each repeater being unique to that repeater. 
Thus, the unique time delay of each repeater prevents a message received by 
more than one repeater from being re-transmitted exactly simultaneously, 
thereby avoiding loss of the message due to traffic contention. 
The unique time delay can be pre-set using data in a further electronic 
memory of the repeater, or alternatively using switches or potentiometers. 
Conveniently, the repeaters are arranged in zones around the central 
receiver, each repeater including the zone address number such that the 
repeaters closest to the central receiver are designated "zone 1", and 
those furthest away from the central receiver have the highest zone 
number. 
With such a system of zoned repeaters, when a transmitter sends a message, 
it is received by one or more repeaters and/or the central receiver. The 
message is stored in the repeater for the unique time delay, and the zone 
number information of that repeater is added to the message, which is then 
re-transmitted. This re-transmitted message (including the zone number 
information) is received by one or more repeaters and/or the central 
receiver, but a repeater with a zone number equal to or greater than that 
in the received message does not process the message further. Any repeater 
with a lower zone number overwrites the zone number information in the 
message with its own zone number and re-transmits the message after its 
own pre-set unique time delay. 
In another embodiment of the invention each repeater has a unique 
identification address, memory means storing a list of identification 
addresses relating to those repeaters within its range and control means 
operative to cause the repeater to re-transmit said message to a first 
identification address on said list and, in the event that said 
re-transmitted message is not received or is incorrectly received by the 
first addressed repeater, to re-transmit said message again, but to a 
second identification address on said list. 
Advantageously, the address list would be arranged in order to proximity to 
the central receiver, i.e. the addresses of those repeaters closest to the 
central receiver are at the top of the list. 
Preferably, the control means directs the re-transmitted message first to 
the repeater address at the top of the list, monitors the channel until it 
receives the message repeated by the addressed repeater, and, in the event 
that does not receive the message which should have been repeated by the 
addressed repeater, redirects the message to the second address on the 
list, and so on until the addressing repeater finally receives 
confirmation that its message has been received. 
Clearly, each time a message is repeated it will be received by other 
repeaters as well as the specific repeater to which the message is 
addressed, but only the addressed repeater processes the message any 
further. 
This system of "intelligent" repeaters may be used in conjuction with the 
pre-set time delay and/or interrogation means provided in the first aspect 
of the invention, and is particularly suited to larger sites with a large 
number of repeaters. 
Typically, the repeaters would each be defined as belonging to a particular 
zone, as in the first aspect of the present invention. 
Preferably, the system includes a rechargeable battery back-up facility 
which ensures operation of the repeaters should there be a mains failure. 
Conveniently, if mains fails to a repeater, that repeater automatically 
transmits a message, after its own unique pre-set time delay, to alert the 
central receiver of the mains failure. Thus, the pre-set unique time delay 
also serves to prevent traffic contention in the event that mains failed 
generally to all the repeaters on site.

In an embodiment according to a first aspect of the present invention, a 
site as shown in FIG. 1 has located therein a central receiver 12, 
transmitters 14 and repeaters 13. The site is divided into a number of 
zones, Z1, Z2, Z3 . . . Zn, and the repeaters in each zone are assigned 
zone address numbers related to their zone. The repeaters in zone 1 are 
nearest to the central receiver and those furthest away from the central 
receiver have the highest zone number. 
One type of repeater is illustrated in FIG. 2, and comprises a receiver 
aerial 1, transmitter aerial 5, receiver and demodulator 2, data message 
memory 3, transmitter 4, zone address memory 6, electronic control 
circuitry 7, mains power supply 8 and a rechargeable battery 9. 
The aim of the system is to relay messages which are originated by a 
transmitter 14, via a number of repeaters 13, to the central receiver 12. 
This is achieved as follows. 
Binary data message are originated by transmitters 14 which may be at fixed 
positions on the site or may be mobile units carried by people or live 
stock or attached to objects which are moved around the site. 
FIG. 4 shows a message from transmitter 14, which will consist of n bits of 
binary data 15, including error correction bits if required. The message 
will be received by one or more repeaters 13 and/or the central receiver 
12. When the message is received by a repeater, the message is received 
and demodulated by receiver and demodulator 2, then is stored in 
electronic memory 3 for a pre-set time controlled by electronic circuitry 
7 and unique to that particular repeater. An additional m bits of data 16a 
are added to the message 15 to convey the repeater's zone number 
information. These m bits of zone number information may be stored in the 
zone address memory 6, or alternatively may be set during installation 
using switches, wire links etc, or silimar means. 
The receiver and demodulator 2 sends a "received signal strength 
indication" or RSSI to the control circuitry 7 to allow the repeater to 
ensure that there is no traffic on the channel within its own range before 
re-transmitting the message after the pre-set time delay. If the channel 
is clear, i.e., if the received signal strength indicator does not detect 
traffic on the channel, then the message is re-transmitted by transmitter 
4 through aerial 5. The message 16 which is transmitted (see FIG. 5) is 
exactly the same as the original received message 15 with the zone number 
16a added. 
This repeated message is then received by one or more repeaters 13 and/or 
the central receiver 12. Any repeater with a higher zone number or equal 
zone number to the zone number in the received message does not process 
the message any further. Repeaters with a lower zone number store the 
message in memory and overwrite the zone number with their own lower zone 
number before re-transmitting (if the channel is clear) as described 
above. Thus, messages are passed by repeaters towards the central receiver 
12, where the process stops. 
As has been previously mentioned, repeated messages 16 are delayed before 
being transmitted. The purpose of this delay is to prevent a situation 
where a message received by more than one repeater is re-transmitted 
exactly simultaneously causing the loss of the message due to traffic 
contention. The delay can be set using data in a further electronic 
memory, or alternatively using switches, potentiometers or similar means, 
and each repeater on the site is set with a different delay during 
installation. 
In the case of mains failure, the repeater units continue to operate using 
the rechargeable battery 9. When the mains supply to a repeater fails, the 
repeater automatically transmits a mains fail message 17 (see FIG. 6) 
(after a pre-set delay) to alert the central receiver station of the mains 
failure. The pre-set delay is necessary to prevent traffic contention if 
mains fails generally on site to all the repeaters. To reduce the risk of 
contention further, repeaters operating on battery back-up may be 
programmed not to re-transmit a mains failure message, but simply to send 
an acknowledgement signal to the originating repeater. 
The mains fail message 17 will consist of a repeater identity 47, a mains 
fail message 49 and the zone address 48 as shown in FIG. 6. The mains fail 
message 17 has a similar format to the repeated message 16, but the binary 
data 15 and 16a is replaced by the repeater identity code 47 and the mains 
fail message 49. The repeater identity 47 may be set in an electronic 
memory or using switches. It is also possible that the switches described 
above, which set the pre-set transmit delay, could define the repeater 
identity. 
As an alternative to the repeater of FIG. 2 that shown in FIG. 3 may be 
used; components of FIG. 3 similar to those of FIG. 2 are shown by the 
same reference numeral, with the suffix a. The repeater of FIG. 3 differs 
on that it uses a common transmit and receive aerial 11, and in that the 
control circuitry 7 controls a transmit/receive switch 10. 
The system of "zoned repeaters" and "site zones" as above described 
overcomes the problem of low transmitter operational range, which limits 
site size, and the coverage area can be increased idefinitely by adding 
further zones. The problems of traffic contention are reduced by the 
pre-set time delays in the repeaters which prevents those repeaters which 
received the same message from re-transmitting simultaneously, and also by 
the use of the RSSI output from the receiver 2 which is used to ensure 
that the channel is clear before transmission. The received signal 
strength indicator can be generated using the signal level at the receiver 
intermediate frequency, as is the case for RSSI on commercially available 
receiver integrated circuits, or alternatively the data protocol can be 
designed to enable simple detection of on-channel traffic. For example, if 
data messages are split into groups of bits separated by relatively long 
periods where unmodulated carrier is transmitted, then these long periods 
of "no data" and "no noise" out of the demodulator can be detected using 
appropriate electronic circuitry to indicate traffic on the channel. 
The drawback in this system arises as the number of repeaters on the site 
increases and as the site size increases, since a detailed site survey is 
required in order to define the repeater positions and zone boundaries 
which may cover irregular shapes due to propagation effects, and also the 
requirement on installation to program the repeater zone numbers, 
retransmit delays and identity codes. 
In the system according to a second aspect of the invention, illustrated 
schematically in FIG. 7, individual repeaters 29 (each as illustrated in 
FIG. 8) have the repeater address uniquely set at manufacture and stored 
in repeater address memory 24 before despatch to the site. These 
pre-addressed repeaters are installed on site without regard to this 
address. Each repeater also includes a transmit/receive aerial 18, a 
transmit/receive switch 19, a receiver and demodulator 20, a message 
memory 21, a power supply 22, a battery backup 23, control circuitry 25, a 
destination address memory 26, a zone number memory 27 and a transmitter 
28. 
When all repeaters are positioned, the central receiver/transmitter 34 is 
operated to transmit a set up message 35 (see FIG. 10) which consists of a 
"zone set up message" (i.e. a series of data bits which are defined to 
alert any receiving equipment that a set up message follows), and zone 
number data 37 (the data in this case being 0, since the central 
receiver/transmitter is defined as the sole occupant of zone O). This set 
up messsage is generated by an installation tool 55 (see FIG. 13) and is 
transmitted by transmitter 49 in the central station via switch 48 and 
transmit/receive aerial 47. The installation tool may be a personal 
computer using software which generates standard messages and decodes 
replies, or alternatively can be dedicated equipment or part of the 
central station hardware. The central station also includes decoder 
circuitry 53, a rechargeable battery 54, mains supply 56, a receiver and 
demodulator 51 and control circuitry 52. 
Any repeater 29 within range of the central receiver/transmitter which 
receives this set up message 35 defines itself as an occupant of zone 1 
and this zone number information is stored in memory 27 (see FIG. 8). 
After a pre-set delay (different for each repeater) which may be random or 
may be derived electronically from the unique repeater address, each of 
these zone 1 repeaters transmits a repeater zone set up message which is 
similar to message 35 with the exception that the zone number 37 
information is now set to zone 1. 
Any repeaters within range and which receive this message define themselves 
as zone 2 (unless they have previously been defined as zone 0 or zone 1), 
and this set up procedure continues until all repeaters have defined 
themselves as part of a zone. 
If the operational range of a repeater link is marginal, it is possible 
that messages could be received during set up but could occasionally be 
lost after the system has been commissioned. In order to prevent this, the 
received signal strength indication from the receiver is used to define a 
lower acceptable limit for received signal strength during the set up 
procedure. 
If the signal is not above this limit, the control circuitry 25 of each 
repeater prevents the receiving repeater from defining itself as part of 
the higher zone. Alternatively, if an RSSI is not available from the type 
of receivers being used, then the transmitter power should be reduced to a 
fraction of the normal transmitter power in order to build in a safety 
margin. 
When all the repeaters which receive a zone set up message 35 have defined 
themselves as part of a zone, there is a further time delay after which 
the repeaters transmit their addresses and zone numbers back via other 
repeaters to the central receiver. This information is displayed by a 
display unit 55a or printed by a printer 55b of installation tool 55. At 
this stage, the installation team can note which repeaters have not 
replied, i.e. those which are out of range of all other repeaters, and can 
then site additional repeaters as required. This zone set up procedure is 
repeated until all repeaters are defined as an occupant of a zone. The 
repeater destination addresses are then set up as follows. 
Using the installation tool 55, each repeater is instructed to transmit a 
repeater destination set up request 46 (see FIG. 11) which consists of a 
standard request message 39 and its own address 40 which any receiving 
repeaters recognise as a repeater destination set up request. Any repeater 
which receives this message then transmits a repeater destination set up 
reply 43 (see FIG. 12), consisting of a standard "reply message" 45, its 
address code 41, zone number 42 and the address of the requesting repeater 
40. 
These reply messages are sent afer a delay in order to prevent several 
repeaters replying instantaneously, again the delay can be random or 
derived from the repeater address information. The reply messages 43 
include the requesting repeater's address in order to direct the message 
to the correct repeater. The requesting repeater receives messages from 
any replying repeaters, and compiles a list of neighboring repeaters which 
are in-range. This list is compiled in zone order, those with lowest zone 
number (ie nearest to the central receiver) first. Any replying repeaters 
which have the same zone number are listed in reply order, but this is not 
important to the operation of the system. If desired, the order of 
destination repeaters listed with in the same zone may be periodically 
amended by the repeater control circuitry, based on a statistical 
monitoring of failed transmissions for each destination repeater. These 
destination addresses are stored in memory 26 of the repeater. 
This repeater destination set up procedure is initiated by the installer 
using the installation tool 55 to send a message from the central station 
transmitter which is received by all repeaters (possibly via other 
repeaters) and is recognised as an instruction to initiate the destination 
set up procedure. After a delay, each repeater retransmits the initial 
message without modification, then after a further delay each repeater 
transmits its destination request message 46. 
After all repeaters have transmitted their repeater destination set up 
request messages 46 and stored all repeater destination set up replies 43, 
each repeater will hold in its memory 26 a list of other in-range 
repeaters' addresses and their corresponding zone numbers. At this stage, 
the installer sends a "list addresses" message from the central station 
transmitter using the installation tool which instructs all repeaters 
(which may receive the instruction via other repeaters) to transmit a list 
of the destination addresses in memory 26. Each repeater receives this 
"list addresses" instruction, and after a delay retransmits it on to other 
repeaters, then after a further delay it transmits data containing its own 
address and a list of destination addresses. 
A full list of all repeater addresses and their destination addresses is 
compiled and displayed using the installation tool 55. All repeaters 
should have at least two repeaters in this list. If any do not, then the 
installer can install further repeaters. Thus, the installation procedure 
is considerably simplified since only the transmitter 14 to repeater 29 
propagation needs to be considered in detail. The repeater to repeater 
path can be considered using a approximate "typical" range and sited on 
this basis and any holes in repeater to repeater coverage are 
automatically discovered and made apparent to the installer at either the 
zone set up stage or the destination set up stage. 
Installation may thus be summarized as follows: 
1. The site for the central receiver/transmitter 34 is defined. 
2. Repeaters 29 are sited in order to cover the areas of the site furthest 
from the central station which contain transmitters 14. This stage will 
require consideration of the transmitter to repeater range. 
3. Further repeaters are sited between the central station 34 and the 
previously sited outlying repeaters. These repeaters may be sited using 
approximate repeater to repeater range. 
4. The central station transmits a zone set up message which is relayed on 
by other repeaters until all repeaters are either part of a zone or are 
not set up since they are out of range. 
5. After a further delay all repeaters which have zone numbers transmit 
their address and zone number which is relayed back to the central 
receiver. At this stage any repeaters not set up are noted by the 
installers who can then install fill-in repeaters, to bring them within 
range. 
6. Steps 4 and 5 are reiterated until all repeaters are part of a zone. 
7. The installer sends out a message from the central station instructing 
all repeaters to set up their destination address lists. 
8. After a delay all repeaters request neighboring repeaters to reply with 
their addresses and zone numbers. 
9. The installer sends a message instructing all repeaters to respond with 
a list of their destination addresses. Each repeater should have a list of 
at least 2 destinations. Once this is achieved, the installation procedure 
is complete. 
System operation is as follows: 
When a message 30 is transmitted by a transmitter 14 that message 30 is 
received by one or more repeaters which then format a message 57 as shown 
in FIG. 9. 
The destination address 31 is that which is top of the list of destination 
addresses ie, that which is nearest to the central station. This message 
is then retransmitted and is received by one or more neighboring 
repeaters, but only the repeater with the specified address will process 
the data any further. 
The addressed repeater then places the destination address code from the 
top of its own list in the message which is then retransmitted. This new 
message will be received by neighboring repeaters including the previous 
repeater which monitors the channel until it receives this message which 
acts as confirmation that the message has been received correctly. This 
process is continued until the message is received by the central receiver 
where the repetition stops. Clearly it is desirable that path loss between 
repeaters is equal in both the transmit and the receive direction. It is 
therefore preferred to use a common Tx/Rx aerial on each repeater. 
If a previous repeater fails to receive its message acknowledgement from 
the next repeater in the chain then it selects the second address on its 
list of destination addresses and repeats the message for a second time. 
Thus if a repeater fails to retransmit the message for any reason, for 
example if a receiver has become faulty or if a shield is temporarily 
placed between repeaters in the signal path then the message is 
intelligently re-directed via another repeater. 
Before any repeater transmits a message it monitors the channel for traffic 
(using the RSSI from its receiver for example) to ensure that the channel 
is clear of other transmissions before transmitting data itself, thus 
reducing the possibility of traffic contention. If a message has been 
contended with and has been corrupted then the receiving repeater detects 
this since the error check bits will be incorrect. The receiving repeater 
then transmits a "message corrupted" message to the original repeater, 
which would be monitoring for receipt of an acknowledgement signal in the 
form of a repetition of its own message. Receipt of "message corrupted" 
message rather than the acknowledgement signal initiates a re-transmission 
of the original message. If the mains supply fails to a repeater then the 
repeater transmits a message to the central station to indicate that it is 
operating on battery back-up. 
Thus, the system of intelligent repeaters and site zones provides a system 
which allows site size to be increased with a very low probability of 
traffic contention and a very high probability that messages are received 
by the central station accurately and quickly. The system is designed to 
make the commissioning of repeaters a simple task which can be performed 
by a single installer operating from the central station.