GB2264210A - Private mobile radio system - Google Patents

Abstract

A plurality of mobile units communicate with at least one base station connectable through a switching station with any other base stations. A controller is arranged to reserve a communication channel at each base station and to command designated mobile units to switch to the reserved channel for open channel communication.

Description

PRIVATE MOBILE RADIO SYSTEM This invention concerns private mobile radio (PMR) systems such as those used by Military and Law Enforcement Agencies. PMR systems are known and generally comprise a base station and a plurality of mobile terminals. Each base station serves a particular area and can communicate with mobile terminals within its area and with other similar base stations serving respective areas through switching centers.

The mobile terminals can communicate with each other via the respective base stations or directly.

There is an increasing reliance on such PMR systems and, with increased use, there is a need more efficiently to utilise the limited radio spectrum. Narrow band channels enable more efficient utilisation but it has been recognised that significant advantages are obtained by "trunking" i.e.

where traffic communication channels are only allocated and used whilst a call is in progress. This is similar to a telephone communication system. DTI document MPT 1318 describes such advantages and indicates that the carrying capacity of a channel within a "trunked" group greater than 10 can approach 0.9E (erlang). The "erlang" is a measure of channel loading (see MPT 1318 pages 16-19).

A trunking technique, as described in MPT 1327 (and associated documents MPT 1343 and MPT 1347), requires the mobile radio terminals to relax to a control channel over which channel call control messages are passed. In this way, a mobile radio terminal may be directed to a particular communication channel when a call is made. Similarly, when a call is terminated, the radio terminal releases the communication channel and relaxes to the control channel.

The same communication channel is then available to other users.

The technique of call control is similar to that of telephoning and inevitably has a delay which may be of the order of 500m secs. This, coupled with a probability of a delay in acquiring a radio channel if all resources are actually in use, is causing concern as, without discipline, speech or word slipping may result. The problem of the 'Don't Shoot' command being 'Shoot' is one aspect of such a problem. Currently, this problem is largely avoided by the communications being undertaken over an 'Open', all available, channel. There is therefore no call set up delay and, as all units are permanently tuned to the channel, there is a comfort to everyone in that all communications are received.

The Shoot/Don't Shoot problem, along with the comfort aspect, is causing significant concern in the organisations defining future radio communication strategies for the Military and Law Enforcement Agencies. It is believed however that, in accordance with the disclosure herein, an open channel capability can be achieved with a trunking system and still maintain the channel efficiencies for one to one communications on an "on demand" basis. Further, in recognition of the requirement to support covert operations and/or any 'low probability of detection' communications, encrypted communications and 'frequency agile' techniques can be incorporated. Such a system is described below.

The communications requirements of a PMR system according to the present invention can be summarised to be: a) One to one private calls.

b) Group, or conference calls.

c) Open channel all available communications.

Requirements (a) and (b) are supported by the MPT 1327 systems described above and are established by similar system call control techniques i.e. Mobile/Radio units are normally tuned to a control channel over which call control is exercised. If it is established that both units to be involved in a one to one call are available, both are instructed to re-tune to a traffic channel. Should both radios be within the coverage of, a single Base Station, then both would utilise a common traffic channel. However, where two base stations are used, different R.F. traffic channels may be used and switch connected by the system infrastructure.

The establishment of a group call is similar to a 'private' call except that a call control message is broadcast to a pre-defined group of mobile radios terminals to re-tune to a particular traffic channel. Again this arrangement could encompass multiple R.F. base stations which would be conferenced to form a wide-area, group connection.

Both 'one to one' and group calls would be set up on demand and suffer the call establishment delay. For general communications and 'ad hoc' non strategic control (e.g.

Traffic Wardens etc) this should not be a problem as system use discipline could be exercised. Under pressure, however, the discipline cannot be guaranteed and the 'Shoot/Don't Shoot' difficulty, gives rise to the requirement for open channel type communications.

In principle, the difficulty of words being missed because of call set up delays can be overcome by introducing a speech delay in the radio equipment. The speech delay would then enable a group call to be established on, for example, receipt of a 'Press to Talk' command whereby to minimise the risk of missed instructions. The use of digitised speech would simplify such realisations as, with 8kb/sec traffic, relatively small digital delay lines (shift registers) could be used. Further, the delay could, by processor control, be dynamically controlled to adapt to various call set up times and after the first speech interchange be effectively switched out by detection of a "non-speech" interval.

Although the above would minimise the missed message difficulty it would fundamentally still suffer a call set up delay. The open channel facility would avoid all such difficulties and would be provided by pre-allocating a channel (or channels on conference connected base stations) to form an open group. Pre-defined groups of mobile terminals would then automatically be instructed via the call control channel to re-tune to the pre-allocated channel(s). All mobile radio terminals would then be openly conferenced.

Further, by periodically transmitting the re-tune 'to group' instruction on the call control channel new arrivals (i.e. radios that have just been switched on) could be instructed to join the group. The arrangement is not of course restricted to organising a single open group as by pre-allocating several channels a number of independent groups can be established and mobiles/radios having specific ranges of addresses etc directed to the appropriate group call conference. Similarly different open channel groups can be established and cleared as necessary.

The formation of the open channels will be effected, in general, by commanding mobile terminals having particular address ranges. Each mobile terminal however would have a unique identity within the group range. Therefore by employing command signalling over the traffic channel allocated to the group (MPT 1327 signalling enables this) individual mobile terminals can be instructed to re-tune to another channel for a one-to-one call, to be temporarily part of a dynamically established group call or to be a member of another open channel.

By providing the Open Channel operation within a conventional trunked system, both user requirements and radio channel use efficiency can therefore be achieved.

The invention will be described further, by way of example with reference to the accompanying drawings, in which: - Figure 1 is a schematic diagram of a private mobile radio system in accordance with the present invention; and Figure 2 is a diagram indicating the additional features of such a system incorporating "Open Channel" communication.

Referring firstly to Figure 1, a private mobile radio system comprises a first base station 10 arranged for broadcast communication over a first area 12 to a plurality of mobile radio terminals 14 located within such area. An adjacent area 16 is served by a base station 18 and broadcasts to mobile terminals 14 within the adjacent area.

A plurality of such areas adjacent, contiguous or overlapping may be provided and a switching centre 20 serves to connect the various base stations to each other and to other networks.

The system has allocated thereto a plurality of narrow band radio channels, for example, 100 and these, in turn, are allocated to the base stations 10, 18 etc in accordance with the traffic demand in the respective areas 12, 16 etc and in accordance with minimum radio interference in adjacent areas. The allocated channels comprise, for each area, at least one call control channel and one or more communication channels for carrying communication traffic.

Such a system is described fully in document MPT 1327 issued by the Department of Trade and Industry. Its operation is also there described. MPT 1327 is incorporated herein by reference.

A one-to-one call by one mobile radio terminal 14 to another mobile radio terminal 14 whether in the same or a different area is initiated and made in accordance with the protocols described in MPT 1327. The mobile terminal 14 firstly hunts for the strongest control channel system and makes a request on this frequency. When the base station acknowledges, the mobile indicates the address of a terminal to be called. The base station then determines the availability of the called mobile terminal. If the called terminal is available, the base station sends to the calling mobile terminal a go-to-traffic-channel signal. A similar signal is sent to the called mobile terminal by the base station if it is in the same area or via the switching centre and the appropriate base station if it is in a different area. The one-to-one communication is then established.

Similarly, a group or conference call is made from a mobile terminal 14 to a plurality of other mobile terminals in the same or different areas as described in MPT 1327.

The initiation is the same as before. The base station then ascertains the availability of a traffic channel and advises the calling terminal to go to this channel and all the called terminals (either directly or through the switching centre and the respective base stations) to go to the allocated traffic channel (or channels).

In accordance with the present invention the above system is modified to provide "Open Channel" communications when required.

The open channel will be established in similar manner to that of an MPT 1327 group call. In this case, however, a System Manager or Controller 22 provided at a switching centre 20 or at a base station, would define the range of mobile radio terminal 14 addresses to form the group and would define the duration of the group call. The Controller 22 would instigate the call and initiate its clear down.

The system would also be organised periodically to broadcast the 'group call' instruction on the control channel to catch late comers and to allow for personnel shift changes on instances of long duration communication requirements.

Mobility of radios is catered for to ensure use of Base Stations covering specific locations. Under normal MPT 1327 system operation the 'useability' of a particular base station is established by assessment of the control channel.

With open channel operation this, of course, is not practical. Assessment of the received traffic channel radio signal strength is therefore undertaken at the mobile radio terminal 14. If then the level is too low, the terminal or terminals would relax to the control channel and proceed to hunt for another useable one. Upon acquisition and registration with a different base station, the mobile terminal 14 would then be instructed to rejoin the open channel group by the periodic control channel message.

As indicated in Figure 2, the call control channel is full-duplex (as far as the base station is concerned) and is transparent to a user of a mobile radio terminal. The transmission by the base stations are in frames each divided into a plurality of slots. The duration of each frame is an integral number of time slots but such number may increase or decrease depending on traffic demand. The frame size accordingly varies. The communication channels (of which only four are shown) are half duplex.

As described above, the system controller 22 initiates the Open Channel by reserving at least one communication channel and instructing the assigned group of mobile terminals 14 via the call control channel to tune to the reserved channel. Thereafter, communication by the controller 22 with a particular mobile terminal within the group may be made by digital instructions on the reserved channel. Such "flash" instructions are unlikely to disturb normal communication on the reserved "open channel" and may be of such brief duration as to pass unnoticed. The controller 22 may wish so to communicate with an individual mobile terminal 14 to instruct the terminal to participate in a one-to-one or a group call on another channel.

The use of a trunked mobile radio communication system including an "open channel" facility by Military and Law Enforcement Groups may require levels of system security not normally required of civil use systems. Problems of speech secrecy, jamming and low probability of communication detection may therefore have to be resolved.

a) Speech Secrecv This can readily be achieved by speech scramblers or encrypters. Such devices have been developed and could be adapted to the trunking environments. Further the secrecy could be switched in or out depending upon requirements.

b) Jamming As discussed above, the MPT 1327 trunking system is organised via a control channel, any jamming of this could therefore effectively disable the system. Such a state could generally be detected by both mobile terminals and fixed infrastructure by virtue of continuous receipt of non MPT messages. The system could then establish a new control channel and rely upon the mobile terminals 'finding' it via normal channel hunt protocols. Elaborate forms of jamming may of course be realised such that MPT messages are generated whilst holding out any access by mobile terminals.

This would have to be avoided by continually changing the control channel frequency and avoiding any that are identified as potentially jammed.

Frequency change of the control channel frequency would result in mobile terminals being 'out of service' whilst they sought the latest control channel. The establishment of calls to the 'hunting' mobile terminals would then not be possible. It would then be beneficial to positively move the mobile radio terminals to 'the next' control channel via a fully encrypted message. Of course the 'fail safe' mechanism would be the mobile hunt routines.

Use of an 'agile' call control channel may cause mobile radio terminals involved in an Open Channel to spend significant time locating its own call control channel.

This however would be minimised by transmitting on the open channel(s), in encrypted form, the identity of the 'in use' call control channel.

Jamming could of course be directed at specific channels and probably the strategically important 'Open Channel'. Making these frequency agile would minimise this and can be achieved by specific 'Move' or 'Go To Channel' commands broadcast in encrypted form over the currently used channels. Mobiles that fail to follow such commands would be organised to hunt for the call control channel and then be redirected to the open channel frequency in use.

c) Low Probability of Detection This characteristic would be realised by the use of both encrypted speech and a totally channel agile system.

This latter aspect probably considering the channels available to a system e.g. 100 as a pool of resources that are dynamically used rather than restricting agility to a set of channels allocated to a base station.

Claims (3)

1. A private mobile radio system comprising at least one base station serving to communicate with a plurality of mobile radio terminals within its broadcast area to provide one-to-one communication or conference communication therebetween and connected to their base stations in respective areas through a switching centre, there being a system controller arranged to reserve, at each base station a communication channel and to command designated mobile radio terminals to switch to the reserved channels to provide open channel communication for the designated mobile radio terminals.

2. In a private mobile radio system permitting one-to-one communication or conference communication between mobile radio terminals via at least one base station and, where there are a plurality of base stations, a switching centre, the improvement comprising a system controller for causing the or each base station to reserve at least one communication channel and for causing a plurality of designated mobile radio terminals to switch to the reserved channel or channels to provide open channel communication therebetween.

3. A method of radio communication between a plurality of mobile radio terminals served by at least one base station and, where there are a plurality of base stations, a switching centre, the or each base station broadcasting over a respective area and communicating with mobile radio terminals within the area via a call control channel and a plurality of communication channels, the method comprising the steps of reserving, in each area, at least one of the communication channels and instructing designated ones of the plurality of mobile radio terminals to switch to the reserved channel of its area to provide open channel communication therebetween.