GB2314235A

GB2314235A - Interference Suppression during Mode Switching in a Communication System

Info

Publication number: GB2314235A
Application number: GB9612088A
Authority: GB
Inventors: Kristian G Pedersen; Keld Anderson; Erik Lund; Tommy Hjordt
Original assignee: Motorola AS
Current assignee: Motorola Solutions Danmark AS
Priority date: 1996-06-10
Filing date: 1996-06-10
Publication date: 1997-12-17
Anticipated expiration: 2016-06-10
Also published as: GB2352140A; GB0025418D0; GB9612088D0; GB2314235B; GB2352140B

Abstract

A mobile communication system having a plurality of base stations may be operated in either a simulcast or in site-steering (one base station transmitting/receiving) mode. Switching between the two modes requires keying-up or -down of base station transmitters with attendant generation of interference (spatter). To prevent loss of data due to such interference, one or more dummy frames, which carry no information are transmitted at the moment of switching.

Description

COMMUNICATIONS SYSTEM AND METHOD OF TRANSMITTING SIGNALS Field of the Invention This invention relates to signal transmission and methods of transmitting signals in communication systems. The invention is applicable to, but not limited to, the transmission of signals whilst switching between operational modes in such a communication system.

Background of the Invention Simulcast communication systems are well known and documented in, for example US Patent No. 4,696,051 assigned to Motorola Inc. In a simulcast system, simultaneous transmission is carried out by a number of transmitters on the same frequency in overlapping transmission areas.

The transmissions from the various transmitters must be accurately synchronised, as described in the above patent.

In a site-steering communications system transmission on one transmitter only (monocast) is performed by selecting a particular address of the transmitter site.

A multi-site communications system will typically need to provide the capability of operating in both a simulcast and site-steering operational mode and therefore need to provide the capability of switching between such modes of operation. Transmission sessions in a multi-site digital communications system may include transmission of digital voice and/or data frames and the switching between such modes of operation can cause the voice and/or data frames to be corrupted.

Typically, there are two scenarios in such communications system which may cause voice and/or data frames to be destroyed: 1) Site-steered to Simulcast: When switching from a site-steered session to a simulcast session, the station, e.g. station 1, that was keyed-up during the site-steered session remains keyed, and the other stations required for the simulcast session key-up. During this key-up process, any frames transmitted from station 1 are destroyed by this transient RF interference until all of the stations required for the simulcast session have keyed-up and are ready to participate in the simulcast session.

2) Simulcast to Site-steered: When switching from a simulcast session to a site-steered session all of the stations, except one, e.g. station 1, will de-key. Dung this de-key operation any frames transmitted from station 1 are destroyed by the transient RF interference. Alternatively, all stations may de-key and then station 1 re-key between sessions to avoid lost frames due to the transient RF interference. In doing this, a session change period lasts approx. 500 ms thereby causing the system throughput to decrease.

In multi-site digital communications system operating both simulcast and site-steering operational modes, a problem arises when RF interference, caused by stations keying-up and keying-down when switching between site-steered and simulcast sessions, causes a destruction of the digital frames. This can be a serious problem with data transmissions as error-free transmission is essential and all the frames need to be received in order to process the data correctly. If the data frames are corrupted, a re-transmission is required thereby, reducing the throughput of the system. In voice transmissions, the loss of frames will cause truncation of the speech and hence reduced speech quality.

Thus, there is a need for an improved communication system and signal transmissions therein. In particular, there is a need for an improved method of handling signal transmissions when switching between operational modes, e.g. between simulcast and site-steered operations.

Summarv of the Invention According to a first aspect of the present invention, a method of transmitting signals in a communications system, when switching between a first mode and a second mode of operation, is provided. The communications system includes at least one base station which facilitates communications between a plurality of mobile stations. The method includes the steps of transmitting information signals from at least one first mobile station to at least one second mobile station in the first mode of operation, transmitting at least one dummy signal from the at least one first mobile station to the at least one second mobile station and transmitting information signals from the at least one first mobile station to the at least one second mobile station in the second mode of operation.

According to a second aspect of the present invention, a time domain multi-site radio communication system is provided. The time domain multi-site radio communication system includes a controller for controlling communications within the time domain multi-site radio communication system, the controller having a monitoring function for monitoring at least one operational communication parameter of the time domain mult.-site radio communication system. At least one base station is operably coupled to the controller to facilitate communications to, and/or between, a plurality of communication units, and the controller inserts at least one dummy frame into an information transmission to at least one of the communication units dependent upon the monitored at least one operational communication parameter.

Preferably, the time domain multi-site radio communication system provides for at least two modes of operation and the controller inserts dummy frames into information transmissions when the controller is aware that information may be lost due to interference from other communication units of the system.

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

Brief Description of the Drawings FIG. 1 shows a block diagram of the communication system in accordance with a preferred embodiment of the invention.

FIG. 2 shows a timing diagram of signal transmissions in the communication system of FIG. 1 according to the preferred embodiment of the invention.

FIG. 3 shows a flowchart illustrating switching between operational modes of the communication system of FIG. 1, using the timing format shown in FIG. 2, in accordance with the preferred embodiment of the invention.

Detailed Description of the Drawings Referring first to FIG. 1, a block diagram of a time domain multisite radio communications system, according to a preferred embodiment of the invention, is shown. The time domain multi-site radio communications system utilises time frames for communicating information, e.g. as in a Time Division Multiple Access (TDMA) scheme, and includes a controller 10 for controlling communications within the time domain multi-site radio communication system. The controller 10 includes a monitoring function 11 for monitoring at least one operational communication parameter of the time domain multi-site radio communication system and is operably coupled to a number of base stations, say base stations 12, 14, and 16. The base stations facilitate communications to, and/or between, a plurality of communication units 18, 20, 22 and 24 over a plurality of communication channels 28 to 34 on a number of communication sites.

In operation in the preferred embodiment of the invention, the multisite radio communications system provides both simulcast and monocast communications. In a simulcast operational mode, a simultaneous broadcast transmission is carried out by, say, base stations 12, 14 and 16, on the same frequency, to respective communication units 18, 20, 22 and 24, in overlapping transmission areas. The simultaneous transmissions are controlled by the controller 10. In a site-steered operational mode, transmission from one transmitter only (monocast) is performed, e.g.

communication unit 18 communicates with communication unit 20 via base station 12. When the controller 10 receives via its monitoring function 11 a particular operational communciation parameter, e.g. a request for a new operational mode, for example to switch transmissions from a multicast transmission to a site-steering transmission or from a site-steering transmission to a multi-cast transmission, dummy frames are inserted into the information stream, dependent upon the monitored operational communication parameter, as shown in FIG. 2. The dummy frames are input to the transmitted information stream to avoid desired information being corrupted by transient RF interference from other units keying-up/ down or registering etc due to the commencement of a new, or termination of an old, information session. The use of such dummy frames also allow the transmitter that is to change its mode of operation to remain keyed-up during the change over period.

FIG. 2 shows the preferred embodiment of the invention, with regard to the two scenarios of switching between multi-cast and sitesteered transmissions. However, it is within the contemplation of the invention that any other operational modes of a communication system may employ the same transmission methodology.

Scenario 1: Site-steered to Simulcast: When switching from a transmitting a first session 50, e.g. a site-steered session, to a second session 56, e.g. a simulcast session, dummy frame 1 (DF1) 52 will be transmit as a site-steered frame, at the end of the site-steered session, and dummy frame 2 (DF2) 54 transmitted as a simulcast frame at the beginning of a simulcast session. During the transmission of DF1, the other communication units will start keying-up to be ready to transmit DF2. The keying of the other stations may cause DF1 to be destroyed, which is acceptable as no information is contained within it, while Session 1, DF2 and Session 2 are transmitted without any transient RF interference.

Scenario 2: Simulcast to Site-steered: Alternatively, when switching from a transmitting a first session 50, e.g. a simulcast session, to a second session 56, e.g. a sitesteered session, dummy frame 1 (DF1) 52 will be transmit as a simulcast frame, at the end of the simulcast session, and dummy frame 2 (DF2) 54 transmitted as a site-steered frame at the beginning of a site-steered session). During the transmission of DF2 from communication unit 1 the other communication units will de-key and thereby create transient RF interference and destroy the content of DF2, which is acceptable as no information is contained within it, while Session 1, DF1 and Session 2 will be transmitted without any transient RF interference.

Preferably the two dummy frames are of a duration longer than the key-up and key-down times for the communication units. In the Motorola ASTRO (TM) system key-up/key-down times are approximately 35 ms. The first dummy frame is transmitted as the preceding session, and the second dummy frame is transmitted as the succeeding session. This causes one dummy frame to be destroyed and one dummy frame to survive each session change.

Advantageously, the keying-up or keying-down of communication units does not impact the continuous transmission of a communication signal whilst switching between operational modes due to the insertion of the dummy frames during this transitional period. In addition to the preferred embodiment of the invention of introducing dummy frames to avoid loss of information from interference when changing modes of operation, it is within the contemplation of the invention that such use of dummy frames is used at any time when it is known beforehand, due to the current status and/or operation of the communication system, that certain information may be lost due to whatever form of radio frequency (RF) interference may be applicable at the time.

Referring now to FIG. 3, a flowchart illustrating a generic approach to switching between operational modes of the communication system of FIG. 1, using the timing format of FIG. 2, is shown, in accordance with the preferred embodiment of the invention.

A communication unit 12 is transmitting in a first, original session as in step 100. If a session (mode of operation) change is required, as in step 102, the controller determines whether and address change is required for the new session, as shown in step 104. If an address change is not required, at least one dummy frame is inserted into the information stream, as shown in step 105, and the new session commences, as in step 110. Should an address change be required in step 104, a dummy frame is created and transmitted at the appropriate time, addressed as the originating session, as shown in step 106. A second dummy frame is then created and transmitted, addressed as the new session, as shown in step 108, and the new session commences as shown in step 110. The communication unit 12 then transmits in the new session as shown in step 114.

It is within the contemplation of the invention that only one dummy frame may be used, thereby avoiding either step 106 or step 108 if the controller wishes, and the controller knows the originating and new session types, for example, changing from a simulcast to a sitesteered session the second dummy frame is addressed and changing from a site-steered session to a simulcast session the first dummy frame is addressed.

Advantageously, the present invention allows communication units to continue their transmissions whilst switching between operational modes, by inserting dummy frames at appropriate times. In this manner, the communication units remain keyed-up during this transitional session phase. There is no need to re-transmit information as only dummy frames are corrupted. The preferred embodiment otlChe invention is primarily applicable to data transmissions, but it is within the contemplation of the invention that voice transmissions would benefit from the proposed arrangement. Thus, an improved method of transmitting signals in a multi-site radio communication system is provided by the present invention.

Claims

1. A method of transmitting signals in a communications system when switching between a first mode and a second mode of operation, wherein the communications system comprises at least one base station facilitating communications between a plurality of mobile staTions, the method comprising the steps of: transmitting information signals from at least one first mobile station via the at least one base station, in the first mode of operation; transmitting at least one dummy signal from the at least one first mobile station; and transmitting information signals from the at least one first mobile station in the second mode of operation.

2. The method of transmitting signals in a communications system in accordance with claim 1, wherein an information signal transmitted in the first mode of operation is to at least one second mobile station and the information signal transmitted in the second mode of operation is to at least one third mobile station.

3. The method of transmitting signals in a communications system in accordance with any one of the preceding claims, wherein the step of transmitting at least one dummy signal from the at least one first mobile station includes the steps of: transmitting at least one first dummy signal from the at least one first mobile station in the first mode of operation; and transmitting at least one second dummy signal from the at least one first mobile station in the second mode of operation.

4. The method of transmitting signals in a communications system in accordance with claim 3, wherein the first mode of operation is a multicast transmission and the second mode of operation is a site steering transmission.

5. The method of transmitting signals in a communications system in accordance with claim 3, wherein the first mode of operation is a site steering transmission and the second mode of operation is a multi-cast transmission.

6. A time domain multi-site radio communication system comprising: a controller for controlling communications within the time domain multi-site radio communication system, the controller having a monitoring function for monitoring at least one operational communication parameter of the time domain multi-site radio communication system; and at least one base station operably coupled to the controller to facilitate communications to, and/or between, a plurality of communication units, wherein the controller inserts at least one dummy frame into an information transmission to at least one of the plurality of communication units dependent upon a monitored at least one operational communication parameter.

7. The time domain multi-site radio communication system according to claim 6, wherein the insertion of at least one dummy frame into an information transmission transmitted to the at least one of the plurality of communication units occurs when the information transmission is susceptible to interference from an operation of other communication units in the time domain multi-site radio communication system.

8. The time domain multi-site radio communication system of claims 6 or 7 wherein the time domain multi-site radio communication system provides at least two modes of operation, wherein a communication link provided by the communication system includes continuous time domain information transmissions in the at least two modes of operation interspersed by dummy frame transmissions.

9. A method of transmitting signals in a communications system substantially as described hereinbefore with respect to FIG. 3 of the drawings.