Multiplex communications – Communication over free space – Using trunking
Reexamination Certificate
1999-06-29
2003-04-08
Maung, Nay (Department: 2681)
Multiplex communications
Communication over free space
Using trunking
C370S340000, C370S348000, C455S518000
Reexamination Certificate
active
06545995
ABSTRACT:
This application is based on patent application no. 964,817 filed in Finland on Dec. 2, 1996. The content of the Finnish priority application is incorporated herein by reference.
FIELD OF INVENTION
The invention relates to a method of maintaining a group call in a mobile communication system comprising:
a network infrastructure, and mobile stations that communicate with the network infrastructure over a radio path, whose frame structure comprises time slots,
the method comprising the steps of:
establishing a group call in which a first mobile station is prioritized, the mobile stations that participate in the group call communicating in speech items, and the frame structure comprising time slots that a second mobile station sending a speech item employs in its speech item.
BACKGROUND OF INVENTION
The method of the invention is intended for use particularly in PMR (Private Mobile Radio or Professional Mobile Radio) networks, or trunking networks, which are typically company networks or private mobile radio networks used by authorities, all the channels in these networks being allocated to one or more companies or authority organizations. In addition to subscriber numbers, the subscribers in these networks have been assigned group numbers indicating to which group call group or subscriber group the subscriber belongs. It is thus possible to switch calls directed to the members of a certain group to all the subscribers of this group.
A group call is one of the key functions in a PMR network. A group call is used when several participants are involved, particularly when an entire group must continuously know how things proceed. A group call is a conference call in which all participants can take turns to speak, or transmit speech items, and to listen to each other. In group calls the whole group is called by one group identity code. It is known that many mobile telephone systems, particularly systems used by companies and authorities, apply group calls. As regards the radio path, a group call is typically implemented in simplex form as a point-to-multipoint call, in which speech is transmitted from one talking party to multiple talking parties, and a speech item is allocated to a next talking party according to a predetermined practice. It is naturally also possible to implement a group call as a duplex call.
A call or a group call is here taken to mean complete exchange of information between two or more parties. A call may consist of one or more speech items. In a semi-duplex call, these speech items are sequential ones.
The invention is applicable in mobile communication systems with either digital or analogue radio paths. Analogue mobile communication systems are described, for example, in MPT 1327,
A Signalling Standard for Trunked Private Land Mobile Radio Systems
, January 1988, revised and reprinted November 1991, and MPT 1343
, Performance Specification
, January 1988, revised and reprinted September 1991, both issued by the Radio-communications Agency, published by the British Department of Trade and Industry.
An example of a digital mobile telephone or mobile communication system in which the invention can be applied is the TETRA system (Trans-European Trunked Radio). Its implementation is described in the standard ETS 300 392-2
, Radio Equipment and Systems
(RES);
Trans-European Trunked Radio
(TETRA);
Voice plus Data
(V+D)
Part
2:
Air interface
(AI), Mar. 20, 1996, ETSI, 583 pages. In this system, the allocation of speech items is controlled by a Switching and Management Infrastructure (SWMI) through base stations connected to it.
In FDMA systems (Frequency Division Multiple Access), the facility of sending signalling messages or other data during calls or data calls is called in-band signalling. In this system, part of the transmission capacity of the channel is typically used during the call for signalling that supplants actual speech or circuit-switched data.
WO A1 91/09481 describes traffic between a first and a second communication unit in a quasi-duplex situation, for example on a direct mode channel. The first communication unit transmits a signal with periodically-occurring holes therein, the signal also comprising a speech information portion and information packets that inform of the location of the next hole. The receiving, second communication unit sends an interrupt request signal to the first communication unit in the hole indicated by the information packet. The first communication unit receives the interrupt request signal and processes it, and if the first communication unit determines that the interrupt request signal complies with (desired) predetermined criteria, it sends an acknowledgement to the receiving second communication unit. The acknowledgement informs the second communication unit that the first communication unit will interrupt transmission (for a predetermined period of time) and thereby allow the receiving second communication unit to start a transmission. The second communication unit can thus start the transmission.
U.S. Pat. No. 5,025,442 teaches a quasi-duplex mobile telephone system employing a control time slot during which a receiving communication unit is allowed to interrupt the transmission of a transmitting communication unit by sending the transmitting communication unit an interrupt signal by which the transmitting communication unit is instructed not to transmit.
It is also known that part of the traffic capacity allocated to a user is permanently allocated for signalling. This procedure is called out-band (channel-specific) signalling.
In common mobile telephone networks, such as NMT (Nordic Mobile Telephone) or GSM (Global System for Mobiles) systems, typically all calls have the same priority, i.e. no user is prioritized over any other user.
In private radio networks, i.e. trunking networks, the situation is different. The operation of these PMR networks can be arranged in such a way that users performing certain type—e.g. emergency—servicing or other important duties have a higher priority, i.e. their high priority call is switched first in the event of congestion.
An example for a high priority call is an emergency call, i.e. a call established when at least one subscriber needs immediate help. The subscriber in an emergency can activate an emergency call, or the activation can be performed by some other subscriber station or by a dispatcher controlling the operation. The emergency call can be addressed to a talk group, i.e. a group call, whereby the members of the talk group need not just listen to the conversation but they can also participate in it. In other words, the members send speech items. This is often necessary, for example, when instructions are given to a group member in an emergency. However, the subscriber requiring a communication connection in an emergency should always be allocated a speech item as soon as possible in a group call where several users take turns to speak.
In a mobile telephone system based on a trunking method, the mobile station, however, has to request a speech item from the system, and the system allocates the speech item in order of time or priority. The system is also able to interrupt an ongoing speech item.
A problem in the prior art is that the control channel capacity of a prior art mobile communication system protocol, particularly of the TETRA mobile telephone system, is very limited during speech items in individual calls of a single base station and in group calls at the base station that the subscriber station sending a speech item is attached to. During the speech item, only the 18th frame of the TETRA frame structure has available a Slow Associated Control Channel, i.e. a once-a-second time slot. Particularly in private mobile radio networks which are used by authorities and in which emergency group calls are sent frequently, it is essential that a subscriber or mobile station in an emergency is given a chance to request a speech item sufficiently often during a speech item sent by a subscriber participating in a group call, so that the speech item reque
Kinnunen Kimmo
Schroderus Osmo
Maung Nay
Nokia Corporation
Persino Ray
Squire Sanders & Dempsey L.L.P.
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