GB2287378A

GB2287378A - A base station control unit

Info

Publication number: GB2287378A
Application number: GB9404137A
Authority: GB
Inventors: Uwe Haase
Original assignee: Motorola GmbH
Current assignee: Motorola Solutions Germany GmbH
Priority date: 1994-03-04
Filing date: 1994-03-04
Publication date: 1995-09-13
Anticipated expiration: 2014-03-04
Also published as: GB2287378B; GB9404137D0

Abstract

A radio system has a plurality of base stations 10 - 13 with coverage areas 20 - 23 for communicating with mobiles 30. The base stations are sequentially operated, so that only one is operating at a time. If a base station is being used e.g. 10 then adjacent base stations 11 are placed in a "busy" mode. In channel busy mode, the base station transmits a predetermined signal at reduced power indicating that the channel is busy. This signal can be used to prevent mobiles attempting to gain access to a channel in situations where that channel is in use in another coverage area. <IMAGE>

Description

Base Station Control Unit. Base Station Radio Communication Svstem and Method to Control a Base Station Transmitter Field of the Invention This invention relates to a base station control unit in a multiple base station radio communication system, for example in a transmitter steering system and it relates to a multiple base station radio communication system and a method to control a base station transmitter.

Background to the Invention In an one- or two-way radio system where one base station transmitter is not enough to get sufficient coverage, the use of multiple, at least two, base stations sites is necessary. To achieve overall coverage in the system, the distances between the base stations must be so short, that the particular coverage of the single base station transmitters will partially overlap.

Experience shows that even with optimised system design these overlap areas are the largest part of the whole coverage area of the system.

In systems using simulcast technology all base station transmitters are on the air at the same time and therefore costly provisions have to be made to the base station transmitters, such as high stability oscillators, line equalization network and delay lines, to avoid unacceptable distortion caused by interfering of different signals in these overlap areas.

An inexpensive alternative to simulcast is the use of a transmitter steering system, in which the base station transmitters instead of being switched on all at the same time, are operated one after the other, so that there is only one transmitter activated at a time, avoiding distortion problems in the overlap areas. With only one transmitter activated at a time, there is no way for a user in the range of one of the other non-active base station transmitters to know that the channel is busy. A user within the coverage range of one of the other base station transmitters may attempt transmission and interfere or cause interference in the active base station transmitter coverage range.

A two way radio system with automatic transmitter steering is commonly combined with a receiver diversity system to select the best communication path. In such a system a mobile is called from the base station control point selectively and sequentially via all transmitters and sends an acknowledgement after receipt of this call back to the calling party.

Under the process of receiving the acknowledgement, the receiver diversity system selects the best receiving path and the transmitter steering selects the corresponding transmitter that is to be controlled under the subsequent transmissions. The acknow.edgement itself stops further repetitions Ui calls to the mobile. To prevent disturbing of the transmitter steering, communication traffic between mobiles on the same communication channel must be avoided.

In prior art systems a user attempting to transmit within the coverage range of one of the non-active base station transmitters has been denied any response from the central base station controller. This can be perceived by the user as a fault in his mobile radio or in the system.

Summarv of the Invention According to the present invention, a method to control a base station transmitter in a multiple base station radio communication system is provided comprising the steps of: taking a decision whether another base station transmitter of the communication system is in an active mode; and placing the base station transmitter in a channel busy mode if another base station transmitter is in an active mode comprising the step of: commanding the base station transmitter to transmit a predetermined signal indicating that the channel is busy.

The invention has the advantage of providing mobiles with a busy indication possibility in the whole coverage range.

The invention overcomes the problem in prior transmitter steering systems in which there was no radio signal in coverage areas adjacent an active base station transmitter and no busy indication possibility outside the area of the activated transmitter. With the arrangement in accordance with the invention, unsuccessful calls from mobiles to a base control point are reduced, because mobiles that are adjacent the coverage range of the activated transmitter are aware that there is activity nearby on the channel and avoid attempting to access the channel.

In a transmitter steeling system adjacent transmitters are not activated at the same time, so there is only one signal received at a mobile and no interference problem in the overlap areas. This interference problem occurs in the mobiles, when the received signal consists of two or more signals, that are not identical, but arriving with about the same radio signal strength from different base stations transmitting at the same time. If the difference in radio signal strength of these single signals exceeds 6 dB (theoretically), the stronger signal suppresses the weaker signal and the distortion disappears.

For the interference overlap area a value of 15 dB instead of 6 dB is practically used for calculations with respect to irregular signal strength distribution.

According to a further aspect of the invention the base station transmitter which is operated in an active mode transmits at a given power level when operating in the active mode, and the step of placing the base station transmitter in a channel busy mode comprises commanding the base station transmitter to change from the active mode to the channel busy mode and commanding the base station transmitter to transmit the predetermined signal at a power level reduced from the given power level.

The change of the transmission power is preferably a reduction with at least 6 dB and more preferably a reduction with at least 15 dB.

According to a further aspect of the invention, a base station radio communication system is provided comprising a plurality of base stations and a control unit connected to the base stations for control thereof, the control unit comprising: means for selecting at least one base station transmitter, of non-overlapping radio coverage, for active operation and means for commanding each other adjacent base station transmitter to transmit a predetermined signal indicating that the channel is busy.

According to a still further aspect of the invention, a base station control unit is provided for controlling a base station radio communication system comprising a plurality of base station transmitters, the control unit comprising: means for connection to the base station transmitters for control thereof, means for selecting at least one base station transmitter, of nonoverlapping radio coverage, for active operation and means for commanding each adjacent base station transmitter, with areas of coverage overlapping the area(s) of coverage of the at least one base station transmitter, to transmit a predetermined signal indicating that the channel is busy.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings.

Brief Description of the Drawings Fig. 1 shows a diagram of a multiple base station radio communication system in accordance with the present invention.

Fig. 2 shows the system of tig. 1 in a second step of operation.

Fig. 3 shows a chart of signal strength against distance for two adjacent base station transmitters of the system of Fig. 1.

Fig. 4 shows at first arrangement of a console, controller and base station transmitter of the system of Fig. 1; and Fig. 5 shows an arrangement alternative to that of Fig. 4.

Detailed Description of the Preferred Embodiment Referring to Fig. 1, a radio communication system is shown comprising four base stations 10, 11, 12 and 13. These are connected to a controller 14, which may be a Motorola Spectratac (trade mark) controller and a console 15.

The base stations 10 to 13 have transmitters with nominal maximum coverage areas 20 to 23 and within these coverage areas there are a number of mobile units (e.g. unit 30). The mobile units (hereafter "mobiles") are standard units with no special modifications. Each base station also has a receiver (not shown).

In operation, the controller 14, which is for example a Motorola Spectratac (trade mark) controller, controls inbound and outbound communication to the mobiles via the base stations. For inbound communications, the best received signal is selected by the controller 14 at any instant in time. For an outbound communication, for example voice communication from a microphone at the console 15 to a mobile in the field, a call button is pressed at the console 15 and the controller 14 causes each of the base stations 10 to 13 to poll the selected mobile (selected by tone signalling). Upon acknowledgement from this selected mobile, the base station through which that mobile communicates is identified, and the controller 14 sets up a communication from the console 15 to the transmitter of the selected base station.

If there is a collision between two incoming calls, neither mobile receives an acknowledgement and both mobiles have to retrv.

In the status shown in Fig. 1, when a PTT button is pressed at the console 15, the transmitter of base station 10 is in a normal active mode and the transmitter of base station 11 is in a busy mode. In this mode, a communication path has been set up from the transmitter of base station 10 to mobile 30 within the coverage range 20. Controller 14 has identified and set up this communication and has instructed the transmitter of base station 11 to enter the busy mode. In the busy mode, base stations transmits an intermittent busy tone. The tone is preferably intermediate, rather than continuous, to avoid undue'annoyance when received. Mobiles within the coverage ranges of base stations 11 receive the busy tone and enunciate this tone through their audio loudspeakers.This enunciated tone indicates to the respective users that the channel is busy and that they should not attempt to transmit. This situation continues for as long as the transmitter of base station 10 is keyed up.

In the meantime, the transmitter of base station 12 enters the active mode and is available to support the same communication, and the transmitter of base station 13 enters the busy mode.

The base station adjacent the active base station 10 (in this case base station 11) is instructed by controller 14 to transmit the busy signal at reduced power. This is represented by coverage area 24. The power from the base station 11 is reduced by at least 6 dB from normal operating power, but more preferably by at least 15 dB. The effect of this reduction in power is that the busy signal transmitted from base station 11 does not interfere with the active signal being transmitted from base station 10, as it would otherwise do in the area of overlap between coverage areas 20 and 21.

If two adjacent base stations are simultaneously placed in the busy mode, it is of no consequence (and typically unavoidable) that there is an area of overlap between their transmission areas. In such an area of overlap a mobile will typically receive a garbled busy tone signal, but the signal will still be an indication that the channel is busy.

When the conversation with the mobile in coverage area 20 is completed, either by releasing of the PTT button at console 15, controller 14 ends the active mode operation of base station 10 by causing base station 10 to cease transmission.

At the same time as controller 14 ends active operation of base station 10, controller 14 causes the base stations 11 to cease transmitting of the busy signal. The procedure when there is further outbound communication is described below.

Upon termination of the active mode of base station 10, another base station can become active, for example as shown in Fig. 2, in which base station 11 is active. The transmitter of base station 11 may enter the active mode under control of controller 14, for example by virtue of a selective call from console 15 to & mobile within the coverage range 21, or by virtue of polling via the controller 14.

As shown in Fig. 2, the example is given where base station 11 becomes active. In this instance, the adjacent base stations 10 and 12 transmit a busy signal at reduced power. Note that the general rule is that any base station transmitters adjacent the active base station reduce their power for transmission of a busy signal to a level that avoids interference in the coverage area of the active base station transmitter. In the meantime, the transmitter of base station 13, which is sufficiently separated in distance from base station 11 so as not to cause interference is in a "don't care" mode, i.e. it can be active or busy (but cannot be idle). In the illustration it is shown as busy.

Referring to Fig. 3, a signal strength/distance graph is shown for the signal strength between two base stations A and B. The solid exponentially decreasing lines show the decay of signal strength for transmission at full power.

It is generally found that it is necessary to provide 15dB of clear signal over and above any adjacent interfering signal in order to achieve "capture", that is to say for the wanted signal to be received free from the interfering signal. In the figure, the dotted lines are represented as being 15dB below full power. Thus, in the central range P (when both transmitters A and B are transmitting at full power) neither signal provides 15dB of signal strength greater than the other signal and there is interference in this range. If the signal transmitted by transmitter A is reduced by 15dB, the curve Q applies for that transmitter and in this situation, the region of interference moves from the central region P to the region R, closer to base station A. If base station A transmits at full power and base station B transmits at 15dB reduced power, curve S applies for the signal strength from the base station B and the region of interference moves to the right as shown by region T.

In summary, reduction of signal strength of one or other base station transmitter causes the area of interference to move closer to the one or closer to the other base station. By ensuring sufficient reduction of signal strength when transmitting a busy signal in a coverage region adjacent an active base station transmitter, the area of interference is moved sufficiently far from the active base station transmitter to provide the active base station transmitter with full coverage within its area of coverage.

By ensuring a sufficient reduction in signal strength when transmitting a busy signal from base station A, when base station B is active and by doing the reverse when base station A is active and base station B is transmitting a busy signal, the situation is achieved where regions R and T do not overlap, thus it is ensured that any mobile in the region between base station A and base station B can be reached with 15dB of clear signal over and above any busy signal from the adjacent base station.

Figs. 4 and 5 shown alternative arrangements for generating the busy signal.

In Fig. 4, details of the console 15 the controller 14 and the transmitter of base station 10 are shown. The console 15 has a microphone 40 and a push-to-talk button 41. The controller 14 has a busy signal generator 42, a number of outputs 43, 44 and 45 for connection to different base stations, an input 46 for connection to the console 15, a number of switches 47, 48 and 49 for selectively connecting the outputs 43, 44 and 45 to either the busy signal generator 42 or the input 46 and a power controller 56. The base station 10 has a transmitter 50 having a power control input 51 connected to the controller 14. Also shown in Fig. 4 is a keyboard input 55 to the console 15, having a call button 57.

In operation, a user selects a mobile for communication through the keyboard 55 and presses the call button 57. The controller 14 identifies which base station is required for transmission to that mobile (by identification techniques well known in the art) and connects the input 46 to one of the outputs (in this case output 45) for communication from the microphone 40 through to the selected base station transmitter. The controller 14 causes that base station transmitter to transmit at full power (by means of a power control line not shown).

Simultaneously, controller 14 causes switch 48 to connect outputs 44 to signal generator 42, which generates a busy signal. In this example the base stations connected to outputs 44 and 45 are adjacent each other, with overlapping coverage areas. Where the base station connected to output 43 is not adjacent the base station connected to output 45, the position of switch 47 is "don't care", i.e. busy or active. If the base station connected to output 43 is also adjacent the base station connected to output 45, the switch 47 should be as shown, placing that former base station in a busy mode also.

The busy signal is preferably an intermittent tone signal. Controller 14 controls the power input 51 of base station 10 (being the base station adjacent the active base station connected to output 45) so as to cause amplifier 50 to output the busy signal at 15dB below maximum power.

In controller 14, switches 47, 48 and 49 are represented as hardware switches and power controller 56 is represented separate to these switches, but it should be understood that all the elements 42, 42,47, 48, 49 and 56 can be implemented in a integrated controller.

Referring to Fig. 5, elements already described with reference to Fig. 4 have the same numerals. In Fig. 5, controller 14 has a busy signal command word generator 60 in place of a signal generator 42 and base station 10 has a busy signal generator 62 and switch means 63. In the arrangement of Fig. 5, on pressing of the PTT 41 or during outgoing calls, busy signal command word generator 60 generates a command word which is passed via switch 48 to the base station 10. This command word is fed to signal generator 62 causing that signal generator to generate a busy signal, which is an intermittent busy tone.

Switch means 63 cause this tone to be fed to amplifier 50 for transmission. At the same time base station 10, recognising the command word received from command word generator 60, causes transmitter amplifier 50 to transmit the signal from signal generator 62 at reduced power. The position of switch 47 depends on the circumstances as described above.

In Fig. 5, command word generator 60 and power controller 56 are shown as separate elements, but it will be understood by one skilled in the art that these will generally be implemented in a microcontroller and indeed the same command word or set of command words can be used for instructing busy signal transmission and for general control of base station key-up, power control etc.

A busy indication in the whole coverage area is only possible when all the transmitters of the system are activated at the same time. To achieve this in a transmitter steering system without interference and signal distortion, when transmitting, one or more activated transmitter(s) is/are sending the communication signal with rated output power, whereas (preferably) all non-active transmitters are controlled to transmit too, but sending a busy tone signal with a reduced output power.

The principle of the system wide channel busy indication makes use of the higher carrier detect sensitivity of the mobile receiver, that already allows a busy indication with very low received signal levels, before reliable speech communication is possible. The reduction of the transmitter output power of at least 15 dB (theoretical minimum 6 dB), when sending the busy tone signal, makes sure, that the take over range of the activated or temporary main transmitter is widened so far, that any point in the overlap areas one time in the transmit cycle will be in a distortion free coverage area.

A carrier frequency transmission without modulation would be sufficient for a busy indication on the mobiles, but a modulation with a busy tone is more unambiguous, especially in the distortion free take over areas of the transmitters.

Distortion of the busy tone signal caused by interference in the overlap areas is accepted, because the overlap areas are not part of the temporary main transmitter coverage area.

In any event, to keep the busy tone signal comfortable, it should consist of a short alarm tone of 0.2 seconds, that is transmitted every 1.5 seconds and with the transmitter on the air only during the alarm tone bursts.

The output power reduction when transmitting the busy tone signal causes smaller interference coverage ranges of these transmitters, resulting in a larger distortion free coverage area of the temporary main transmitter. On the other hand the output power reduction is limited so far, that the busy signal coverage range reaches the limit of the coverage range for voice communication.

The "busy mode information" for the transmitters can be sent by the controller to the base stations in a manner that indicates a different "channel", e.g. "channel 2" instead of"channel 1", where "channel 2" is pre-programmed at the base station as representing the same radio frequency but at a reduced output power. The channel indicator is sent to the base station and the base station performs a look-up to obtain the pre-programmed output power and frequency.

Claims

1. A method to control a base station transmitter (e.g. 11) in a multiple base station radio communication system comprising the steps of: taking a decision whether another base station transmitter (10) of the communication system is in an active mode; and placing the base station transmitter (11) in a channel busy mode if another base station transmitter (10) is in an active mode comprising the step of: commanding the base station transmitter (11) to transmit a predetermined signal indicating that the channel is busy.

2. A method according to claim 1, comprising the steps of operating the base station transmitter (11) in an active mode and transmitting at a given power level when operating in the active mode, wherein the step of placing the base station transmitter in a channel busy mode comprises commanding the base station transmitter to change from the active mode to the channel busy mode and commanding the base station transmitter to transmit the predetermined signal at a power level reduced from the given power level.

3. A method to control a base station transmitter according to claim 2 where the change of the transmission power is a reduction with at least 6 dB.

4. A method to control a base station transmitter according to claim 3 where the change of the transmission power is a reduction with at least 15 dB.

5. A method according to claim 1, being a method to control the radio system, wherein the step of taking a decision comprises the step of taking a decision whether any base station transmitter of the communication system is in an active mode; the method further comprising the steps of placing all the other base station transmitters (11, 12, 13j in a busy mode if one base station transmitter (10) is activated; commanding the other base station transmitters to transmit the predetermined signal.

commanding the other base station transmitters to transmit the predetermined signal.

6. A method according to claim 5 wherein the step of placing all the other base station transmitters in a busy mode comprises the steps of: selecting the transmission powers of the base station transmitters; and commanding each base station transmitters to change its transmission power according to the selecting transmission powers, where the selecting of the transmission powers comprises the step of selecting the transmission powers which give the maximum coverage area in the system for the predetermined signal without substantially interfering with signals from at least the base station transmitter in the active mode.

7. A method according to claim 6 where the change of transmission power is dependent on the position of the base station transmitter in the system and on the position of the activated base station transmitter in the system.

8. A method according to any of the preceding claim where the predetermined signal is a carrier frequency signal.

9. A method according to any preceding claim where the predetermined signal is a carrier frequency signal modulated with a tone.

10. A base station radio communication system comprising a plurality of base station transmitters (10, 11, 12, 13) and a control unit (14) connected to the base station transmitters for control thereof, the control unit comprising: means (49) for selecting at least one base station transmitter, of nonoverlapping radio coverage, for active operation and means (42, 47, 48, 56, 60) for commanding each other adjacent base station transmitter to transmit a predetermined signal indicating that the channel is busy.

12. A base station radio communication system according to claim 10 wherein the means for commanding comprises digital control means (6G) for generating digital command, switching means (47, 48) for selectively feeding the digital command to each base station transmitter other than the base station transmitter selected for active operation and busy tone signal generator means (62) in each base station transmitter responsive to receipt of the digital command for generating and transmitting a busy tone signal.

13. A base station control unit (14) for controlling a base station radio communication system comprising a plurality of base station transmitters, the control unit comprising: means (44) for connection to the base station transmitters for control thereof, means (49) for selecting at least one base station transmitter, of nonoverlapping radio coverage, for active operation and means (42, 47, 48, 56, 60) for commanding each adjacent base station transmitter, with areas of coverage overlapping the area(s) of coverage of the at least one base station transmitter, to transmit a predetermined signal indicating that the channel is busy.