GB2309854A

GB2309854A - Method of Signalling Channel Occupation in a Direct Mode Communications System

Info

Publication number: GB2309854A
Application number: GB9601772A
Authority: GB
Inventors: Stephen Thomas Valentine; Gary Aitkenhead; Andrew John Aftelak; David John Chater-Lea; Eliezer Fogel; Salomon Serfaty
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1996-01-30
Filing date: 1996-01-30
Publication date: 1997-08-06
Anticipated expiration: 2016-01-30
Also published as: DE19701175B4; DE19701175A1; GB2309854B; FR2744318B1; IT1290896B1; ITRM970027A1; FR2744318A1; GB9601772D0

Abstract

A station RX1 communicating on a particular channel with a station TX1 transmits a periodic channel busy signal while it is receiving from TX1 so that a potentially interfering station TX2 is made aware that the channel is in use even if TX2 is out of range of TX1.

Description

METHOD FOR SIGNALLTNG Field of the Invention This invention relates in general to a method for an air interface signalling operation in a communications system, and more particularly to a method for an air interface signalling operation to indicate use of a communications resource.

Background to the Invention In radio systems, operating with no fixed network control e.g.

conventional or direct mode operation, there is a need for radios to communicate without causing interference to on going communications.

Typically, for radios wishing to transmit on a particular communications channel and not interfere with on going communications, the radio first monitors the desired communications channel for activity prior to any transmission and only if the communications channel is deemed idle shall the radio attempt to transmit.

A problem however arises when the radio wishing to begin transmission on a particular communications channel is separated from a transmitter currently transmitting on the communications channel to such an extent that it cannot detect the transmitter's activity. Furthermore, if the radio began to transmit it would jam nearby receivers currently involved in the communication with the transmitter. FIG. 1 shows an example of the problem in the prior art as previously described. A first transmitter 10 is transmitting in direct mode with a first receiver 12 over a first communications channel and sending a desired signal 16. A second transmitter 14 monitors the first communications channel but does not detect the activity of the first transmitter 10 because beyond a limited range and thus, assuming the channel is free, attempts to send a signal 18 that unfortunately interferes with the first receiver's 12 ability to receive the desired signal 16 from the first transmitter 10.

In a TDM system, communication resources are divided into frames 20 that are further sub divided into time slots 22 as shown in FIG. 2.

Eighteen frames comprise a multiframe 24. A particular communication resource may be a time slot on a particular radio frequency or channel.

Communication in direct mode is where receivers and transmitters communicate over a particular communication resource without infrastructure or a base station. The direct mode operation (DMO) air interface and does not address the problem outlined above. A communication on a DMO channel begins with the originating (master) transmitter monitoring the selected DMO channel for activity. Once the radio has established that the channel is free or absent of any activity the master transmitter will begin sending a call set up signalling message in time slots 1 and 3 of the first two frames of a multiframe as shown in FIG. 3.

In the case of a group call, or an individual call with no acknowledgement, the master transmitter will begin transmitting traffic information on time slot 1 of subsequent frames. In the case of an acknowledged individual call the acknowledge signal sent by the called party is detected in time slot 1 of frame 1, then traffic is sent in time slot 1 of frame 2. For systems supporting a pre-emption service (typically public safety users) the master transmitter shall monitor time slot 3 on frames 2, 5, 8, 11, 14, 17 of each multiframe for pre-emption signalling. At the end of each call transaction (i.e. PTT activation to PrT release) the channel is retained by the master transmitter sending a channel reservation signal in time slot 1 and 3 of frames 6, 12, 18, until the end of the channel reservation time (i.e. channel hang time). However, a problem is that the channel reservation signal may not be received by a transmitter desiring to transmit on the same communications resource. Furthermore, if the transmitter were to transmit it may interfere or jam other receivers involved in the master transmitter's communication as described above with reference to FIG. 1. In such instances, direct mode becomes unreliable.

Thus, it is desired to have a method of improving direct mode reliability by signalling to a transmitter desiring to communicate on a particular resource that the resource is in use.

Summarv of the Invention According to the present invention, a method is provided for an air interface signalling operation in a communications system having a first transmitter and a first receiver communicating in a direct mode on a first communications resource and at least a second receiver capable of monitoring the first communications resource for any signalling activity, the method comprising the steps of periodically sending a broadcast signal by the first receiver while the first receiver is receiving signals from the first transmitter in direct mode communication thereby indicating to the second receiver that the first communications resource is in use.

Brief Description of the Drawing FIG. 1 shows a communication system including a first receiver and a first transmitter communicating in direct mode.

FIG. 2 shows a TDM frame and slot structure.

FIG. 3 shows a TDM call set up signalling structure.

FIG. 4 shows a frame structure according to an embodiment of the present invention.

FIG. 5 shows a frame structure according to a further embodiment of the present invention.

FIG. 6 shows a busy signal level versus a received signal strength level plot.

FIG. 7 shows a two channel direct mode operation.

FIG. 8 shows a flow chart according to an embodiment of the present invention.

FIG. 9 shows a flow chart according to an embodiment of th epresent invention.

Detailed Description ofthe Preferred Embodiment The present invention provides an enhanced air interface signalling and protocol approach which may be used to improve both the reliability and spectrum efficiency (traffic channels per Hz) of the DMO air interface. An overall frame and slot structure was described above with respect to FIG. 2.

The DMO TDM air interface is divided into multiframes 24 with each multiframe 24 consisting of eighteen frames 20 and each frame consisting of four time slots 22. It is assumed that the DMO radios are operating in (single) frequency simplex mode.

The present invention provides an enhanced direct mode operation (DMO). The invention to modifies the protocol previously outlined for DMO.

One embodiment of the invention provides that in time slot 3 of frames 6, 12, 18 of each multiframe all members of a call, except the master transmitter, may transmit a channel broadcast (busy) signal. See FIG. 4. The busy signal may be transmitted either as a CW or reduced symbol rate in order to improve the busy signal recovery range, i.e. lower C/I required for detection.

The modulation and bit rate used in normal operation may also be used for the busy signal transmission.

A second embodiment requires that a half time slot is used for preemption signalling and that the busy signal could be sent in the second half slot of time slot 3 in the selected pre-emption frames, e.g. 5, 11, 17. See FIG.

5.

In order to limit transmissions, preserve battery life or minimise unwanted interference there may also be predetermined conditions for determining whether a busy signal should be transmitted. For example, only receivers which are measuring a received signal strength indicator (RSSI) level in time slot 1 below a predetermined or defined threshold level would transmit the busy signal, e.g. in time slot 3 of frames 6, 12, 18. The actual busy signal power level could also be weighted with respect to actual RSSI level, as shown in FIG. 6, or transmitted at full power to maximise protection range.

One embodiment of the invention is that the busy signal is produced by an symbol sequence which generates an unmodulated carrier at some frequency offset from the nominal carrier frequency e.g. 2.25kHz. This signal has the properties of being recovered using a simple phase locked loop through a reduced bandwidth e.g. 1kHz. This narrow band phase locked loop approach improves the busy signal range over the normal modulated signal in the region of 15- 20dB. The properties of the single-tone time burst also provides a (course) means to determine frame timing of an ongoing transmission thus enabling a new communication which wishes to use the same carrier frequency to align its DMO frame timing appropriately.

A further embodiment of the busy signal may be appropriate in those cases where there is no collision of busy signal transmissions or where the capture effect is operative that the busy signal contain call related information. Thus, the means of transmitting the information on the busy signal could be achieved by introducing an extended training sequence and a SCH"'H channel in the second half of a busy signal time slot. The first half of the time slot could contain an extended symbol sequence which generates an unmodulated carrier at some frequency offset from the nominal carrier frequency. However, it is accepted that in this case the range of such a SCH"'H channel would be much less than the unmodulated carrier and therefore only recoverable by those radios which are within a certain range (i.e., the busy signal tone range).

Information contained in such a busy signal could include individual identification, group identification, transmit power level, whether the channel is in active mode or reservation mode and current duration of reservation timer (how much longer until channel is dropped). The individual/group identification information could be used to control or suppress new call set-up's to groups or individuals already in a call. The transmit power information could be used by the new transmitter to determine what minimum power level could be transmitted without causing a problem and, if appropriate, this power level could be used in the new call.

Channel timer information could be used to reschedule the call set-up attempt.

An even further embodiment of the present invention includes sharing of a DMO channel using a busy signal. A radio wishing to start a new communication on a direct mode channel may monitor the channel for activity and depending on its geographical location will detect one of the following: - the current transmitter (if close enough), - a busy signal from one (the strongest or closest) or more of the receivers in the communication, - no activity implying the channel is not being used in that area.

The radio then has the option when activity is detected to either find another channel (a free channel) or according to a further embodiment of the present invention may wish to use time slot 2 on the selected channel. In the latter case, the radio would have to align its timing with the current master prior to transmission. See FIG. 7. In the case where a new call setup is to be made on a shared DMO channel the new call may effectively use time slots 2 and 4. A field indicating that the channel is being shared could be included in the call set-up information.

FIG. 8 shows a flow chart of a particular embodiment of the present invention. In step 80, it is determined that a first transmitter is communicating in direct mode on a communications resource with a first receiver. If the first transmitter sends a signal as in step 82, it is determined if the signal is below a predefined threshold as in step 84. For example, a transmitted signal strength of the broadcast signal can be determined from the strength of a first transmitted signal received at the first receiver. It can then be determined if the signal strength is below a predefined level.

If the first transmitter is not sending a signal or the signal is not below the threshold value then the method returns to step 80.

If the signal is below a predefined threshold as determined in step 84, the receiver sends a broadcast or busy signal as in step 86. The broadcast signal may occupy a full time slot or a half time slot.

After a desired time elapses as in step 88, or periodically, the method repeats. Simultaneously, a second receiver desiring to communicate on the communications resource may be monitoring the activity on the communications resource, as in step 90.

Thus, a method for an air interface signalling operation in a communications system having a first transmitter and a first receiver communicating in a direct mode on a first communications resource and at least a second receiver capable of monitoring the first communications resource for any signalling activity is provided. The method comprises the steps of periodically sending a broadcast signal by the first receiver while the first receiver is receiving signals from the first transmitter in direct mode communication. The broadcast signal indicates to the second receiver that the first communications resource is in use.

The method may further allows at least one second receiver to monitor the communications resource and thereby receive the broadcast signal. The broadcast signal may have a range greater than that of the direct mode communication between the first transmitter and the first receiver.

FIG. 9 shows an embodiment where the at least one second receiver monitors at least the first communications resource. For example, in step 91 the second receiver monitors the first communications resource of a first frequency and determines whether there is any channel activity as in step 92. If there is no activity detected, a channel-free counter is incremented as in step 93, if the counter is greater than a maximum number as determined in step 94, then the channel is determined free as in step 95. If the counter is not greater than the maximum number as determined in step 94 then the channel is monitored again in a time delay, step 96, then repeats step 91.

The time delay could be equal to or less than 1/2 time slot duration or any amount of delay to effectively detect any activity on the frequency.

If it is determined in step 92 that there is activity on the channel then a channel-busy counter is incremented as in step 101. If the channel-busy counter is not greater than a maximum value as determined in step 102 then the channel is monitored again in a time delay, step 103, then repeats step 91. The time delay could be three time slot durations in order to look for activity in particular time slot of each frame.

If the channel-busy counter is greater than a maximum value as determined in step 102 then the channel is determined busy as in step 104.

If the channel is to be shared, a further embodiment includes a delay to shift the sampling to an adjacent time slot as in step 105. Thus a similar second phase of monitoring may be performed to determine the state of a second channel. For example, in step 201 the second receiver monitors a second communications resource or second channel of the first frequency and determines whether there is any channel activity as in step 202. If there is no activity detected, a channel-free counter is incremented as in step 203, if the counter is greater than a maximum number as determined in step 204, then the channel is determined free as in step 205. If the counter is not greater than the maximum number as determined in step 204 then the channel is monitored again in a time delay, step 206, then repeats step 201.

If it is determined in step 202 that there is activity on the channel then a channel-busy counter is incremented as in step 301. If the channelbusy counter is not greater than a maximum value as determined in step 302 then the channel is monitored again in a time delay, step 303, then repeats step 201. The time delay could be three time slot durations in order to look for activity in particular time slot of each frame.

If the channel-busy counter is greater than a maximum value as determined in step 302 then the channel is determined busy as in step 304.

A busy signal as determined in steps 104 and 304 for the two channels may have respective or different frequency tones to indicate the respective channel is busy. For example, when it is determined in step 104 that a channel 'A' is busy, a particular frequency or tone could be used to indicate that channel 'A' is busy. Such tone may be different from one which indicates as in step 304 that channel 'B' is busy. Such busy signal or broadcast signal could be generated at one offset frequency of broadcast signal, i.e. dependent upon bit sequence of the signal.

Further, a modulating signal may be chosen that produces a broadcast signal which occupies a narrower bandwidth than that produced by a random modulating signal. Thus, it could be detected in a narrow filter.

Also the broadcast signal may contain identification information which can be demodulated by the second receiver. Such information could be used to determine how to share the communications resource between the direct mode communication and a new communication involving the second receiver.

However, in some instances recovery of call related information from the busy signal is not necessary in order to protect the channel, there is therefore no problem when one or more receivers operating below the RSSI threshold transmit the busy signal at the same time.

The invention proposes an enhanced air interface signalling operation for use in systems employing direct mode using TDMA which improves direct mode communications reliability and may allow a single 251 Hz DMO channel to support 2 traffic channels leading to improved frequency efficiency.

Claims

1. A method for an air interface signalling operation in a communications system having a first transmitter and a first receiver communicating in a direct mode on a first communications resource and at least a second receiver capable of monitoring the first communications resource for any signalling activity, the method comprising the steps of: periodically sending a broadcast signal by the first receiver while the first receiver is receiving signals from the first transmitter in direct mode communication thereby indicating to the second receiver that the first communications resource is in use.

2. The method of claim 1 further comprising the steps of: monitoring the communications resource by the second receiver; and receiving the broadcast signal by the second receiver.

3. The method of claims 1 and 2 wherein the broadcast signal has a range greater than that of the direct mode communication between the first transmitter and the first receiver.

4. The method of any of the preceding claims wherein a modulating signal which produces the broadcast signal is chosen such the broadcast signal occupies a narrower bandwidth than that produced by a random modulating signal.

5. The method of claims 1 and 2 wherein the broadcast signal contains identification information which can be demodulated by the second receiver.

6. The method of claims 1 and 2 further comprising the steps of: determining how to share the communications resource between the direct mode communication and a communication involving the second receiver.

7. The method of any of the preceding claims wherein a transmitted signal strength of the broadcast signal can be determined from the strength of a first transmitted signal received at the first receiver.

8. The method of any of the preceding claims further comprising the step of determining that a received signal from the first transmitter falls below a predetermined threshold before the step of periodically sending a broadcast signal.

9. The method of any of the preceding claims wherein the broadcast signal occupies a full time slot or a half time slot.

10. The method of any of the preceding claims wherein the broadcast signal is an unmodulated carrier.

11. A method for an air interface signalling operation in a communications system substantially as herein described with reference to FIG. 2 of the drawing.