GB2273416A - A communications system and method therefor. - Google Patents

Abstract

A wide-area two-way communications system 30 provides two-way communication between a plurality of mobile communications devices 43 - 45, responsive to a first plurality of frequency channels Rx B1-Rx B3 and configured to transmit on a second plurality of frequency channels Tx M1-Tx M3, distributed throughout a plurality of coverage areas 32 - 24, each administered by a base station 37 - 39 that only transmits signals Tx B1-Tx B3, to said plurality of mobile communications devices 43 - 45, on a single frequency channel of said first plurality of frequency channels and that is receptive to transmissions Rx M1-Rx M3 on said second plurality of frequency channels. The method comprises the steps of transmitting a first signal from more than one base station, receiving the first signal at a scanning mobile communications device of the plurality of mobile communications devices and establishing a two-way communication between a particular base station and the scanning mobile communications device by using the single frequency channel and a first frequency channel of said second plurality of frequency channels, wherein the single frequency channel and the first frequency channel have a defined frequency separation. <IMAGE>

Description

A Communications Svstem and Method therefor.

Background to the Invention.

This invention relates, in general, to wide area two-way radio communications systems and is particularly applicable to a wide area two-way radio communications systems having a channel scan and lock methodology.

Summarv of the Prior Art.

There are currently two distinct methodologies for providing a wide area two-way communications system for a mobile transceiver, such as a SABERS portable radio, in an area that is too large for a single fixed transceiver to cover effectively. [SABER is a trade mark of Motorola Inc.] First, simulcast or quasi-synchronous systems, i.e. the broadcast of the same signal from several remote transceivers at the same radio frequency fl, is often used for the broadcast of signals to a mobile transceiver. A second frequency f2 is used by the mobile transceiver to transmit signals to the remote transceivers. The mobile transmissions on frequency f2 are received by the remote transceivers and then processed, by a central controller, in order to present the best received signal thereto.More specifically, the processing is implemented using a receiver voting system whereby the signals received at all the remote transceivers are compared with one another and the one with the highest quality signal selected therefrom. The advantage of a simulcast system is that there is an efficient use of a single radio frequency pair (i.e. a channel comprising frequencies fl & f2) and the simplicity of operation of the system. Unfortunately, there are high costs associated with initially providing a stable, accurately phased audio signal to the remote transceivers. More specifically, land-lines used to interconnect remote transceivers require extensive calibration and adjustment in order to provide an accurate phased audio signal.

Moreover, even the best simulcast systems may have coverage overlap areas where mobile transceivers receive audio signals of indifferent quality.

In the alternative, a scan and lock methodology is used whereby signals are broadcast from a number of transceivers, each located in a remote area and each operating at a different transmission frequency. A receiver of a mobile transceiver scans all the broadcast frequencies of its present area and locks onto any signal of acceptable signal strength. As is the case with simulcast, the mobile transceivers always transmit on one frequency regardless of location, and receiver voting is generally used by a central controller, in an analogous way to simulcast, to select the best signal received by a transceiver at a remote area. The advantages of a scan and lock methodology over simulcast are that scan and lock offers a simplified operation, higher audio quality and lower engineering cost.Moreover, a scan and lock methodology provides a possible migration path to a communications system with digital encryption. Unfortunately, there are several limitations associated with scan and lock. First, a scan and lock methodology is less efficient in terms of radio channel use, i.e. scan and lock requires a greater number of frequencies. Furthermore, mobile transceivers must be equipped with at least some form of scanning circuitry and, preferably, scanning and voting circuitry.

Additionally, the requirement that the remote (base) transceiver transmits at a variable frequency whilst the mobile transceiver transmits at a common defined frequency creates planning problems where channels are customarily assigned in regularly spaced frequency pairs. More specifically, a mobile transceiver entering a first scan and lock area from a second scan and lock area will be unable to operate in the first area because the frequency pairing is different. A further inherent encumbrance of scan and lock is. that scan time of the mobile transceivers may be, in certain instances, of significant duration. Typically, scan times fall within the range of 0.5 to 2 seconds. However, as regular speech occurs at a rate of approximately 3 words per second, scan times may account for a significant or important part of the call.At present, the effects of scanning are reduced by limiting the period of scanning to times when a received signal, transmitted from a remote transceiver, is completely lost for a meaningful time.

It can be appreciated that there is a requirement within the art for a wide area two-way radio communications systems devoid of the inherent deficiencies associated with present wide area two-way radio communications systems. More specifically, there is a requirement for a scanning system that makes more efficient use of the radio spectrum and which permits the use of standard frequency pairs. In addition, it is desirable that a wide area two-way radio communications systems should not be subject to transmission delays which arise during scanning procedures and which result in the periodic loss of signal reception at a mobile transceiver of the communications systems.

Summarv of the Invention.

This invention addresses at least some of the deficiencies described in the prior art above. In accordance with the present invention, there is provided a method for operating a wide-area two-way communications system providing two-way communication between a plurality of mobile communications devices, responsive to a first plurality of frequency channels and configured to transmit on a second plurality of frequency channels, distributed throughout a plurality of coverage areas, each administered by a base station that only transmits signals, to said plurality of mobile communications devices, on a single frequency channel of said first plurality of frequency channels and that is receptive to transmissions on said second plurality of frequency channels.The method comprises the steps of transmitting a first signal from more than one base station, receiving said first signal at a scanning mobile communications device of said plurality of mobile communications devices and establishing a two-way communication between a particular base station and said scanning mobile communications device by using the single frequency channel and a first frequency channel of the second plurality of frequency channels. The single frequency channel and the first frequency channel have a defined frequency separation.

In a preferred embodiment, the step of transmitting a first signal is preceded by the steps of scanning, at each base station, the second plurality of frequency channels, transmitting, on a second frequency channel of the second plurality of frequencies, a second signal from the scanning mobile communications device, and receiving the second signal at a base station. Furthermore, the transmission of the first signal is extended for a period of time.

Additionally, the step of establishing a two-way communication is preceded by the step of determining, at the scanning mobile device, the quality of said received first signal, whereby the step of establishing a two-way communication between the scanning mobile communications device and the particular base station is subject to a quality consideration. In the wide area wide-area two-way communications system of the present invention, the base stations in adjacent coverage areas transmit simultaneously on different frequency channels, and the defined frequency separation is a standard duplex frequency pair.

In a further aspect of the invention there is provided a mobile communications device for a wide-area two-way communications system. The mobile communications device comprises a scanning receiver for selectively receiving a signal at a first frequency, a transmitter for selectively transmitting a signal at a second frequency, detection means for detecting the frequency of the received signal, and a transmitter channel selection means for selecting the frequency of the selectively transmitted signal such that the frequency of the selectively transmitted signal and the detected frequency of the received signal form a frequency pair having a defined frequency separation. The mobile communications device further comprises voting means for differentiating between the qualities of the received signals and for selecting a received signal that satisfies a signal quality consideration.

An exemplary embodiment of the present invention will now be described with reference to the accompanying drawings.

Brief Description of the Drawings.

Fig. 1 shows a prior art scan and lock wide area two-way communication system.

Fig. 2 shows a preferred embodiment of a wide area two-way communication system in accordance with the present invention Fig. 3 shows a central controller of the wide area two-way communication system of Fig. 2.

Fig. 4 shows a base station of the wide area two-way communication system of Fig. 2.

Fig. 5 shows a mobile transceiver of the wide area two-way communication system of Fig. 2.

Detailed Description of a Preferred Embodiment.

In accordance with the preferred embodiment of the present invention, an enhanced scan and lock methodology (Scan and Lock Plus) provides additional scanning by base station-receivers. This additional scanning allows standard Tx-Rx frequency pairs to be maintained whilst retaining fully automatic system operation.

With reference to Fig. 1, there is shown a prior art scan and lock communications system 10. The communications system 10 comprises a number of coverage areas 12-14 that have coverage overlap 15. A fixed base station 17-19 is located at a coverage epicentre of each coverage area. Each base station 17-19 comprises a transceiver for transmitting signals to and receiving signals from mobile units 21-23 in the communications system 10. Furthermore, the base stations 17-19 transmit at frequencies Tx B1, Tx B2 and Tx B3 respectively. Base stations 17-19 are coupled to a central controller 25 that administers control of the communications system 10. Coupling of the base stations 17-19 to the central controller 25 is achieved through, typically, a wire line link.Each mobile unit 21-23 comprises a transceiver that transmits at frequencies Tx M1 and receives at frequencies Rx B1, Rx B2 and Rx B3, corresponding to base station transmission frequencies Tx Bl, Tx B2 and Tx B3 respectively. Each base station 17-19 receives frequency Rx M1 that corresponds to transmission frequency Tx M1 emanating from each mobile unit 21-23. Hence, the communications system 10 appears as a wide area single channel scheme.

When a mobile unit 21-23 initiates a call, the central controller 25, which may include a SPECTRATAC receiver voting unit, manufactured by Motorola Inc., selects the transmitted audio signal received through the base site with the best received signal strength. [SPECTRATAC is a trade mark of Motorola Inc.] To detect a reply, the mobile unit 21-23 scans all transmitted base station frequencies Tx B1, Tx B2 and Tx B3 and either locks onto the first frequency that satisfies a predetermined signal level (scan and lock) or locks onto the frequency with the best signal level (scan, vote and lock). During a call, the mobile units 21-23 are held on channel, i.e.

at the specified received frequency Rx B1, Rx B2, Rx B3 for a pre-set hold time, usually -3 seconds, after a push to talk (PTT) has been released or the received signal has dissipated. This hold time ensures that conversations are rarely interrupted by scanning.

Fig. 2 illustrates a preferred embodiment of a wide area two-way communication system. The communications system 30 comprises a number of coverage areas 32-34 that have coverage overlap 35. A fixed base station 37-39 is located at substantially a coverage epicentre of each coverage area 32-34 and provides radio coverage for the area. The base stations 37-39 each comprise a transceiver 40-42 for transmitting signals to and receiving signals from mobile units 43-45 in the communications system 30.

Furthermore, base stations 37-39 transmit at frequencies Tx B1, Tx B2 and Tx B3 respectively. Base stations 37-39 are coupled to a central controller 46 that administers control of the communications system 30. The central controller has an operator console that allows an operator of the central controller 46 to receive and review information (signals) transmitted from the base stations 37-39 and to input information (signals) to be transmitted to the base stations 37-39. Coupling of the base stations 37-39 to the central controller 46 is achieved through, typically, a wire line or radio link 50-52.

Mobile units 43-45 comprise transceivers 47-49 that each transmit at frequencies Tx M1, Tx M2 and Tx M3, and receive frequencies Rx B1, Rx B2 and Rx B3 that correspond to base station transmission frequencies Tx B1, Tx B2 and Tx B3 respectively. The mobile units are free to roam throughout a coverage area that is serviced by a base station 37-39. Base stations 37-39 each receive frequencies Rx M1, Rx M2 and Rx M3, which corresponds to transmission frequencies Tx M1, Tx B2 and Tx B3 emanating from each mobile unit 43-45.

Fig. 3 illustrates a preferred embodiment of a central controller 46. The central controller 46 incorporates a receive line interface 58 for recovering audio (or other) signals transmitted from base stations 37-39 along the wire line links 50-52. The receive line interface 58 further comprises a mixing circuit or a selection circuit for selecting the highest quality received signal, as will be appreciated by one skilled in the art. It will further be appreciated that the selection of a received signal 51 at the receive line interface 58 may be based on whether the received signal satisfies a predetermined quality or, alternatively, on the received signal strength. Furthermore, the receive line interface 58 detects the presence of a received signal 51.The receive line interface 58 therefore has two signal outputs: a received output signal 60 that is relayed to the operator, via the operator console; and a dedicated switching signal that indicates when a received signal is detected.

The central controller 46 further comprises a transmit line interface 55 that interfaces the operator, via the operator console, to the base stations 37-39. The transmit line interface 55 controls the communication of signals to be transmitted 57 from the operator console to the mobile units. More specifically, the transmit line interface 55 adds a coded transmit instruction 56 to the signal to be transmitted that, when received and decoded at a base station 37-39, instructs the base station to broadcast the signal to be transmitted. This occurs either when a transmit instruction 56 is received from the operator , via the operator console of the central controller, or when the receive line interface 58 detects the presence of a received signal. In both cases, a hold-on delay 62 extends the duration of the encoded transmit instruction.

Fig. 4 illustrates a preferred embodiment of a base station 37-39. Each base station 37-39 is equipped for duplex operation, i.e.

it can transmit and receive simultaneously. Each base station 37-39 comprises a transceiver 41 having a transmitter 100 and receiver 102 coupled to an antenna 105. The transceiver 41 transmits and receives signals to the remote units 43-45. A key decoder 106 detects a coded transmit instruction from the central controller 46 and directs the transmitter 100 of the transceiver 41 to broadcast to the remote units 43-45. As has already been discussed, the transmitter 100 only transmits on a single dedicated frequency Tx Bl-Tx B3; with different transmit frequencies used in each coverage area 32-34.

Each base station 37-39 further comprises channel control logic 108 that is responsive to the key decoder 106. The channel control logic 108 is further coupled to the receiver 102. The receiver 102 in the base station 37-39 scans all frequencies in the communications system, i.e. frequencies Tx M1, Tx M2 and Tx M3 transmitted by mobile units 43-45. When the receiver 102 detects a transmission on such a frequency, control logic 108 inhibits scanning of the frequencies Tx M1, Tx M2 and Tx M3 by the receiver 102 and locks onto the detected frequency (or channel) until either a signal transmitted thereon is removed or the key decoder 106 detects a transmission instruction from the central controller 46. In response to the detection of the transmission instruction, the channel control logic further forces the receiver 102 to receive a preferred frequency. In this way, the base station 37-39 operates a standard frequency pair whenever a call is in progress.

Fig. 4 illustrates a preferred embodiment of a mobile unit 43-45. Each base station 37-39 is usually equipped for simplex operation, i.e. transmission and reception are not simultaneously possible, although duplex operation is technically feasible. Each mobile unit 43-45 comprises a transceiver 47-49 having a transmitter 120 and receiver 122 coupled to an antenna 125. The transceiver 43-45 transmits and receives signals, through antenna 125, to and from base stations 37-39. Transmitter 120 is responsive to an input signal, such as provided through a transducer 128. The receiver 122 provides an output signal through, for example, an external speaker 130. Control channel logic 126 is coupled to the transmitter 120 and the receiver 122 and provides control thereof.

A transmit instruction, such as provided by a push-to-talk (PTT), is input to the channel control logic and provides the decision to activate the transmitter 120. In the absence of an incoming signal from a base station 37-39, the receiver 122 continuously scans the frequencies of all base stations, i.e. Tx B1, Tx B2 and Tx B3, in the communications system 30. When a signal from a base station is detected, the control channel logic 126 halts scanning and locks onto the frequency of the incoming signal until it disappears.

Unlike the transmitter frequencies Tx B1, Tx B2 and Tx B3 of the base stations, the transmitter frequencies Tx M1, Tx M2 and Tx M3, of the mobile units 47-49 may be tied to their receiver frequencies Rx M1, Rx M2 and Rx M3 in a fixed relationship, such as a standard duplex frequency pair. When the scanning function of the mobile unit has ceased, i.e. when a signal from a base station is detected, the transmitter of the mobile unit 47-49 will operate on whichever transmit frequency is associated with the frequency of the received signal. In the event that the transmitter 120 is operated whilst the mobile unit 43-45 is scanning, the mobile unit will either transmit on a default channel or on the last channel on which scanning ceased, subject to the sophistication of the channel control logic 126.

The principle of operation of scan and lock plus can best be illustrated by considering the example of a call being made from a mobile unit 43-45 and answered by the central controller 46. A mobile unit 43 may initially transmit on any one of it's transmit frequencies; Tx M1, Tx M2 and Tx M3 for the system of Fig. 2. This transmission is received by the transceiver 40-42 of one or more base stations 37-39 which relay it to the central controller 46 through wire line links, for example, for forwarding to the operator.

The central controller 46 automatically issues a transmit instruction to all base stations 37-39 thereby causing them to transmit continuously. The hold-on delay introduced at the central controller 46 ensures that the base stations 37-39 continue to transmit for several seconds after the termination of the incoming transmission.

As soon as the mobile unit 43 ceases transmission, its receiver 122 begins scanning of all frequencies and subsequently lock onto whichever base station transmission is in range. In the case of Fig.

2, the mobile unit 43 would lock onto frequency Tx B1 from base station 37. In an alternative embodiment, the mobile unit 43 may employ a voting methodology in selecting the base station transmission onto which it will lock. A two-way call with the operator (operator console) may now take place.

So long as base station 37 continues to transmit and mobile unit 43 remains in range, all transmissions from the mobile unit 43 will be locked on the correct transmission frequency Tx M1. If the mobile unit 43 moves and subsequently loses contact with base station 37, the mobile unit 43 will once again begin scanning and lock onto a suitable frequency in order to monitor and participate in the call. At the end of the call, all base stations cease transmissions and all mobile units 37-39 begin scanning. It will be appreciated that, in the event of a transmit instruction emanating from an operator at the operator console of the central controller 46, the transmit instruction may be passed through the delay 56 to ensure that all base stations continue to transmit whilst responses to the instruction are awaited from mobile units 37-39.Alternatively, the communications system may have a default setting whereby all base stations transmit continuously throughout the duration of a call.

It will be appreciated by one skilled in the art that it is important to limit the number of frequencies needed for a particular coverage area. It will further be appreciated that a communications system employing channel scanning is more prone to encounter interference than a single channel system. Statistically, if a receiver scans four channels it is four times more likely to encounter interference than if it was fixed on a single channel. It is therefore desirable to prevent scanning being halted by the spurious detection of interference from adjacent areas. The implementation of CTCSS (Continuously Tone Coded Squelch System) circumvents this problem by providing a unique code, usually at a sub-audible frequency, that identifies a particular transmission for a particular group of transceivers.Only if the mobile transceiver detects a tone signal that has been previously affiliated therewith can the mobile transceiver respond to the transmission. Therefore, different areas are protected by different CTCSS tones/codes. Additionally, CTCSS has the further advantage of improving the frequency re-use pattern that is necessary in a system having identical frequency channels in adjacent cells. More specifically, without CTCSS any interference must be kept below the sensitivity of a mobile transceiver. For a system employing frequency modulation in association with CTCSS, interference is effectively rejected provided that the signal strength of a received signal is always greater than an interference signal by an amount at least as great as the capture ratio of the mobile transceivers in the communications system.

Therefore, as will be understood, the use of CTCSS improves the spectral efficiency of the communications system for the available radio spectrum.

As will be appreciated, the foregoing exemplary description of operation is limited to a handling capacity of a single call at a time.

However, since the base stations 37-39 operate on different transmission frequencies, Tx B1, Tx B2 and Tx B3, it is possible to operate them separately. More specifically, a base station 37-39 may be switched out of the wide area system (comprising all coverage areas) and operated independently. For example, an operator, using the operator console of the central controller 46, could inform all mobile units that a certain base station, and hence a certain coverage area, was being removed from wide area operation and switched to 'isolated' use. To achieve isolated use, the receive line interface 58, located at the central controller, for the isolated coverage area would be disabled and require re-routing to a designated frequency channel administered by the central controller 46. In a corresponding manner, transmission from the isolated coverage area to the central controller 46 would also be switch to the dedicated frequency channel. Furthermore, if the wide area system was equipped with CTCSS, the base station that is opting out could be switched to a different CTCSS code, thereby limiting access to the base station. Those mobile units located in the isolated coverage area may then manually select both the appropriate frequency and CTCSS code.

An extension of scan and lock plus may allow systems with several central controllers to handle separate calls through different base stations provided that the different base-stations are out of range of one another. In order to implement such multiple call handling, some form of receiver voting is required at the central controller to indicate to an operator which specific base stations may receive incoming calls. An outward call (emanating from the operator console) would be broadcast from all available base stations. A receiver voting methodology may then indicate which coverage areas a reply was received from, whereby the operator can employ manual coverage area selection once contact has been made.

All remaining coverage areas are then available for other calls controlled by the central controller. This feature is particularly applicable to communications systems having large coverage areas.

It can be appreciated that an invention so designed and described produces the novel advantage of a wide area two-way radio communications systems that operates standard frequency pairs in every coverage area.

Claims (12)

Claims.

1. A method for operating a wide-area two-way communications system providing two-way communication between a plurality of mobile communications devices, responsive to a first plurality of frequency channels and configured to transmit on a second plurality of frequency channels, distributed throughout a plurality of coverage areas, each administered by a base station that only transmits signals, to said plurality of- mobile communications devices, on a single frequency channel of said first plurality of frequency channels and that is receptive to transmissions on said second plurality of frequency channels, the method comprising the steps of:: a) transmitting a first signal from more than one base station; b) receiving said first signal at a scanning mobile communications device of said plurality of mobile communications devices; c) establishing a two-way communication between a particular base station and said scanning mobile communications device by using said single frequency channel and a first frequency channel of said second plurality of frequency channels, wherein said single frequency channel and said first frequency channel have a defined frequency separation.

2. A method for operating a wide-area two-way communications system in accordance with claim 1, wherein the step of transmitting a first signal is preceded by the steps of: a) scanning, at each base station, said second plurality of frequency channels; b) transmitting, on a second frequency channel of said second plurality of frequencies, a second signal from the scanning mobile communications device; and c) receiving said second signal at a base station;

3. A method for operating a wide-area two-way communications system in accordance with claim 2, wherein the step of transmitting a first signal further comprises the steps of: a) extending transmission thereof for a period of time.

4. A method for operating a wide-area two-way communications system in accordance with claim 1, 2 or 3, wherein base stations in adjacent coverage areas transmit on different frequency channels.

5. A method for operating a wide-area two-way communications system in accordance with any preceding claim, wherein the step of establishing a two-way communication is preceded by the step of: a) determining, at the scanning mobile device, the quality of said received first signal, whereby the step of establishing a twoway communication between said scanning mobile communications device and said particular base station is subject to a quality consideration.

6. A method for operating a wide-area two-way communications system in accordance with any preceding claim, wherein the step of transmitting a first signal from more than one base station is performed simultaneously.

7. A method for operating a wide-area two-way communications system in accordance with any preceding claim, wherein the defined frequency separation is a standard duplex frequency pair.

8. A mobile communications device for a wide-area two-way communications system, comprising: a) a scanning receiver for selectively receiving a signal at a first frequency; b) a transmitter for selectively transmitting a signal at a second frequency; c) detection means for detecting the frequency of the received signal; and d) transmitter channel selection means for selecting the frequency of the selectively transmitted signal such that the frequency of the selectively transmitted signal and the detected frequency of the received signal form a frequency pair having a defined frequency separation.

9. A mobile communications device in accordance with claim 8, wherein the receiver receives a plurality of identical signals, of varying signal quality, at a plurality of frequencies and the mobile communications device further comprises: a) voting means for differentiating between the qualities of the received signals and for selecting a received signal that satisfies a signal quality consideration.

10. A mobile communications device in accordance with claim 8 or 9, wherein the defined frequency separation is a standard duplex frequency pair.

11. A method for operating a wide-area two-way communications system substantially as described herein and with reference to Fig. 2 of the accompanying drawings.

12. A communications device substantially as described herein and with reference to Figs. 2 to 5 of the accompanying drawings.