WO1993010602A1

WO1993010602A1 - Method of allocating radio channels

Info

Publication number: WO1993010602A1
Application number: PCT/FI1992/000290
Authority: WO
Inventors: Jussi LÖPPÖNEN; Markku Rautiola
Original assignee: Nokia Telecommunications Oy
Priority date: 1991-11-11
Filing date: 1992-10-28
Publication date: 1993-05-27
Also published as: FI915309A; GB9409316D0; AU2874392A; FI915309A0; FI94202B; GB2277233A; AU663531B2; DE4293920T1; FI94202C; DE4293920C2; GB2277233B

Abstract

The invention pertains to a channel allocation method in a cellular radio system. The mobile radios (MS) have permanently stored in their memories a first group of radio channels scanned in a normal hunt sequence and a second group of radio channels scanned in a comprehensive hunt sequence. According to the invention a) when the control channel of a base station (BS) is changed, a channel permanently belonging to the first group is allocated as a new control channel to the base station whenever such a channel is available, b) if a channel belonging to the second group has to be allocated as a control channel, it is transferred temporarily to the first group and the change of group is announced through control channels to the mobile radios, and c) a channel permanently belonging to the first group is re-established as the control channel of a base station as soon as such a channel is available and the freed channel is returned to the second group.

Description

Method of allocating radio channels

Field of the invention

The invention pertains to a method of allo- eating radio channels in a radio system, comprising mobile radio stations and several fixed radio stations located separate from one another within a geographical area covered by the radio system, traffic channels and one control channel being allocated to each fixed radio station, and each mobile station having, permanently stored in its memory, a first group of channels of the radio system assigned primarily as control channels, the mobile station searching for a control channel in the first group in a normal hunt sequence, and, stored by updating through control channels, a second group of radio channels assigned primarily as traffic channels but also useable as control channels, the mobile station searching for a control channel in the second group in a comprehensive hunt sequence if the normal hunt sequence is unsuccessful.

Background of the invention

In cellular mobile radio systems, communica¬ tion between a fixed radio network and mobile radios takes place on radio channels (pairs of transmitting and receiving frequencies). Radio channels used for transmission of e.g. user speech or data are called traffic channels. In radio systems according to the standard MPT 1343 issued by the British Department of Trade and Industry (DTI) or in other corresponding radio systems a radio channel reserved for signalling between a fixed network and a mobile radio is called a control channel. A control channel also identifies the system and the site of the base station thus functioning as a kind of beacon for mobile radios searching for a suit¬ able control channel.

In two hunt sequences for a control channel defined in the standard MPT 1343, i.e. in an NH sequence (normal hunt sequence) and a CH sequence (comprehensive hunt sequence), a mobile radio scans a number of channels on searching for a suitable control channel (base station) as the connection on the current channel is weakening or lost or a mobile radio is turned on. In the NH sequence a mobile radio scans all the radio channels assigned as control channels. In the CH sequence a mobile radio scans radio channels that are usually traf ic channels but may also be used as control channels. The ΝH channels are permanently programmed in a mobile radio. It is possible for a radio system to add a new radio channel to the list of ΝH channels or to withdraw a radio channel therefrom by means of parti¬ cular signalling messages. A large trunking system may include even over a thousand radio channels, a principally arbitrary group of which may be radio channels of the CH sequence. How¬ ever, a list of ΝH channels to be stored in a mobile radio may usually not include more than 32 channels. It is then important in the system to keep the total number of control channels as close to the number of ΝH chan¬ nels as possible to minimize the time the mobile radios require for hunting control channels.

A known solution is to assign an ΝH channel as a control channel to each base station. However, this is not always possible and, in addition, in the course of time the system will have to change control channels of base stations e.g. due to faults in radio units. A currently free radio channel capable of functioning as a control channel is then assigned as a new control channel. The channel selected is often other than an NH channel. In the course of time, many of the control channels of the system may be CH channels, whereby the hunt sequences for control channels become longer. If an NH control channel is withdrawn from a base station of the system and said channel is not currently used as a control channel in any other part of the system, the system will have to send information on the withdrawal on all control channels. If, on the other hand, a radio channel that is not used anywhere else in the system is assigned to a base station as a new control channel, this should be announced to all the mobile radios. If there are numerous changes of channel and channels are allocated as control channels at random, the above messages load the control channels heavily.

Disclosure of the invention

The object of the invention is to mitigate the above problems.

This is achieved by an allocation method according to the invention, which is characterized in that a) when the control channel of a fixed radio station is changed, a radio channel permanently belong¬ ing to the first group is allocated as a new control channel to the fixed base station whenever such a channel is available, b) if a radio channel belonging to the second group has to be allocated as a control channel to the fixed radio station, said radio channel is transferred temporarily to the first group and the change of group is announced through control channels to the mobile sta¬ tions, c) as soon as a radio channel permanently belonging to . the first group is available, it is assigned as a control channel to the fixed radio sta¬ tion, and d) when a radio channel temporarily transferred to the first group is freed from use as a control channel, said channel is returned to the second group and the change of group is announced through control channels to the mobile stations. The basic idea of the invention is, by means of grouping and reservation of radio channels, to main¬ tain a situation where only NH channels are used as control channels in the system.

The radio channels of the system are divided into groups, the first group including channels scanned in the NH sequence and the second group including channels scanned in the CH sequence. The system favours the channels of the first group in control channel use.

If, for one reason or another, a channel of the second group has to be selected as a control channel, it is temporarily transferred to the first group and the change of group is announced to the mobile radios. The system is always apt to re-establish a channel permanently belonging to the first group as a control channel in order that the size of the group might not grow nor the NH sequence be lengthened.

Owing to the invention the radio system uses almost only NH channels as control channels. Thereby the mobile radios find control channels quickly and it is not necessary to transmit signalling relating to changes in channel groups on the control channels.

However, if a CH channel has to be used as a control channel, it is more probable in the solution of the invention than in the prior art systems that the mobile radios receive information thereof since the number of NH channels is restricted to a few dozen and the likelihood of not exceeding the limit is higher with the method of the invention than with the prior art method.

Brief description of the drawings

In the following the invention will be described in greater detail by means of a preferred embodiment, with reference to the attached drawing wherein

Fig. 1 illustrates a radio system where the invention can be applied, and

Fig. 2 illustrates an interconnection between a mobile exchange and a base station.

Detailed description of the invention

Fig. 1 shows a mobile radio system of a trunk¬ ing type, the geographical area covered by the system being divided into smaller radio areas or cells Cl, C2 and C3 such that adjacent cells advantageously overlap in their peripheral areas. Each cell Cl, C2 and C3 includes at least one fixed multi-channel transceiver apparatus BS1, BS2 and BS3, which in this connection is called a base station. The base stations BS1, BS2 and BS3 are connected to a mobile exchange MX by means of fixed transmission lines, such as cables. A system of this kind is described e.g. in British Patent Applica¬ tion 2,165,127 and its cross references.

For speech or data calls predetermined radio frequencies or radio channels (pairs of transmitting and receiving channels) are assigned to the radio system, the base stations BS establishing radio connections on said radio channels with the mobile radio stations or mobile radios MS moving within the cells. In the preferred embodiment the mobile exchange MX allocates said radio channels to base stations call per call on a so called dynamic allocation scheme such that the same frequencies are not used simultaneously at adjacent base stations, which could interfere with one another. Fig. 2 illustrates an interconnection between a mobile exchange MX and a base station BS. The mobile exchange MX comprises a plurality of base station interface units BSIU, each of them operating as a link between a call control computer CCC and a channel unit ChU of the base station. Each base station BS has several channel units ChU, each ChU comprising a. radio transceiver. The call control computer CCC of the mobile exchange MX allocates free radio channels, channel units ChU and base station interface units BSIU for each call in which one or more mobile radios MS of the exchange MX participates. For each call, the mobile exchange MX may command the channel unit ChU to operate on a desired radio channel to perform the above dynamic allocation. The base station interface unit BSIU has four main functions: to control access to a radio channel associated therewith, to transform and forward messages between radio or channel units ChU and a call control computer CCC, to switch the speech paths of the exchange to a radio channel and to control the operation of the base station- One BSIU controls one channel, which may be either a traffic channel or a control channel. In both cases the BSIU receives messages from the call control computer CCC, controls the 4-wire speech and signalling connection between the exchange and the base station and transmits control data to the call control computer CCC relating to the states of the transmitter, base station and transmission lines.

In a control channel mode of operation the BSIU may operate as a dedicated control channel, undedicated control channel or time-shared control channel. The control channel is a radio channel which is reserved for signalling between a fixed network and a mobile radio for call set up, registration of mobile radios, transmission of status and data messages and broad- casting of network information to mobile radios. Each mobile radio responds only to a control signal of the single base station to which it is currently registered and signals back to the system (exchange) only through said base station on receiving the control signal. During a call a mobile radio does not normally receive or transmit on a control channel.

In accordance with MPT 1343 a mobile radio leaves a control channel (base station) e.g. when the number of erroneous messages exceeds a preset limit or when a much better adjacent control channel is avail¬ able.

The search for a control channel is carried out primarily in accordance with MPT 1343. The base station BS transmits a SYS code on a control channel, the code enabling the mobile radio MS to identify the system. The SYS code contains an AREA code, which informs the mobile radio MS of the calling area (area of a parti¬ cular mobile exchange MX) and announces whether or not re-registration is necessary. MPT 1343 specifies the five different hunt procedures or sequences below.

Resuming a control channel sequence RCC, where after the call the MS returns to the control channel it was listening to if the system has not commanded it to take up another control channel. Single channel hunt sequence SCH, where the mobile exchange MX may command the MS to take up a particular control channel. This procedure is also used when the MS is switched on and the number of the control channel and base station site used earlier are fetched from the memory of the MS. δ

If the RCC or SCH is unsuccessful, preferential hunt sequence PH is carried out, first scanning the channels that are stored in the data table (ASD table) of the adjacent base stations. For further studying, the base stations with control channels having a field strength stronger than LI are selected. If a control channel is not found in the ASD table (or the table is empty), ^a--~- '^c*^ne control channels of the system are scanned. In this procedure only a control channel that does not require re-registration (has the same AREA code as the old control channel) are accepted.

If the PH is unsuccessful, the MS is switched on or the previous control channel is lost, normal hunt sequence NH is carried out, first scanning the channels in the ASD table. If a control channel is not found in the ASD table, all the control channels in the channel table are scanned in a dedicated system by using threshold levels L2, LI and L0 in the same way as in the PH. In this procedure all AREA codes are accepted. If the ΝH is unsuccessful, comprehensive hunt sequence CH is carried out, scanning all the channels in the channel table, including those assigned as tra¬ ffic channels. The hunt is performed by using threshold levels LI, L2 and L0 in the same way as in the ΝH. Each mobile exchange MX and each mobile radio

MS have in their memory a channel table listing all the radio channels assigned to the system (usually not more than 250). For each channel, the channel table of a mobile radio has a control byte where each bit b0-b7 contains the information whether the definition corresponding thereto is in force. Such definitions are e.g. control channel (bO), free channel (bl), forbidden channel (b6), measured channel (b7), channel included in a comprehensive hunt sequence (b2), simplex channel for direct MS-MS use (b3), simplex channel for a pager (b4) and time-shared control channel (b5). The bit b5 corresponding to a time-shared control channel contains the information that the channel is a time-shared control channel in some part of the system. In some other part the channel may be a dedicated control channel or a traffic channel.

When a mobile radio is switched on, the channel table is read from the non-volatile memory to the RAM. The radio system may change some bits in the channel table stored in the RAM with BCAST signalling messages. The changes are in force only until the power is switched off. The following is a description of some exemplary BCAST messages.

BCAST (sysdef=00000), Announce CC. By the message control channels are added to the channel table.

BCAST (sysdef=00001), Withdraw CC. By the message control channels are withdrawn from the channel table.

BCAST (sysdef=00100), Adjacent site CC. The message announces the control channels of the adjacent base station sites and announces whether the control channels are time-shared or dedicated. The channel number and the corresponding SYS code are stored in the data table (ASD table) of the adjacent base stations. The table is erased as a control channel is accepted. The channels in the ASD table are the first to be scanned when the search for control channels is commence .

BCAST (sysdef=00101), Vote now advice. The message is used to announce the active adjacent control channels in a dedicated system to a mobile radio MS.

A mobile radio MS scans the channel table such that the control channel to be measured next is selected at random. When the MS measures a channel which is not a time-shared control channel (b6=0), it marks the channel with "measured" information (bit b7 = 1), whereby said channel will not be measured again during the same hunt sequence. Time-shared control channels (b6=l) are measured and re-measured since it is not possible to know which one of the base stations BS using the control channel in question is currently transmit¬ ting. The aim is to listen to the control channels of all the base stations.

In the radio system according to the invention only channels which are permanently programmed to mobile radios as NH channels (b0=l) are to be used as control channels.

Therefore the radio channels of the system are divided into groups, the first group consisting of channels scanned in the NH sequence (b0=l) and the second group consisting of channels scanned in the CH sequence (b2=l). The system favours the channels of the f rst group in control channel use such that on changing the control channel of a base station, a radio channel permanently belonging to the first group is allocated as a new control channel to the base station, whenever such a channel is available. If, for one reason or another, a channel of the second group has to be selected as a control channel, it is temporarily trans- ferred to the first group (by setting b2=0, b0=l) and the change of group is announced to the mobile radios by the Announce CC message. To avoid increase in the size of the NH channel group and lengthening of the NH sequence, the system is always apt to re-establish a channel permanently belonging to the first group as a control channel immediately when such a channel is available. Likewise, as soon as a radio channel temporarily transferred to the first group is freed from use as a control channel, the channel is returned to the second group (by setting b0=0 and b2=l) and the change of group is announced through control channels to the mobile radios.

In an embodiment of the invention the radio channels that can be used only as traffic channels are prioritised also in view of use as traffic channels by placing them in a third and fourth group of channels. A channel assigned to the third group is then preferred in selection of a traffic channel. If a free traffic channel is not found in the third group, the channels of the second group are searched. If there are no free channels in the second group, either, the channels of the first group are searched. The last channels to be searched are those of the fourth group. The aim is to keep channels suitable for use as control channels free of use as traffic channels as long as possible.

The Figs, and the description thereof are intended only to illustrate the instant invention. The method according to the invention may vary in its details within the scope of the attached claims.

Claims

1. A method of allocating radio channels in a radio system, comprising mobile radio stations (MSI, MS2, MS3) and several fixed radio stations (BS1, BS2, BS3) located separate from one another within a geo¬ graphical area covered by the radio system, traffic channels and one control channel being allocated to each fixed radio station, and each mobile station (MS) having, permanently stored in its memory, a first group of channels of the radio system assigned primarily as control channels, the mobile station searching for a control channel in the first group in a normal hunt sequence, and, stored by updating through control channels, a second group of radio channels assigned primarily as traffic channels but also useable as control channels, the mobile station searching for a control channel in the second group in a comprehensive hunt sequence if the normal hunt sequence is unsuccess- ful, c h a r a c t e r i s e d in that a) when the control channel of a fixed radio station (BS) is changed, a radio channel permanently belonging to the first group is allocated as a new control channel to the fixed base station (BS) whenever such a channel is available, b) if a radio channel belonging to the second group has to be allocated as a control channel to the fixed radio station (BS), said radio channel is transferred temporarily to the first group and the change of group is announced through control channels to the mobile stations (MS), c) as soon as a radio channel permanently belonging to the first group is available, it is assigned as a control channel to the fixed radio sta¬ tion (BS), and d) when a radio channel temporarily transferred to the first group is freed from use as a control channel, said channel is returned to the second group and the change of group is announced through control channels to the mobile stations (MS).

2. A method according to claim 1, c h a r a c ¬ t e r i s e d in that the radio channels assigned only as traffic channels are placed in the third and fourth groups of radio channels and that channels belonging to the third group are preferred in selection of traffic channels, thereafter scanning the channels belonging to the second group, the first group and finally to the fourth group.