Pilot scan radio channel selection

BACKGROUND OF THE INVENTION present invention relates to a mobile communication system and, more particularly, to a channel control system for a small-zone mobile, communication a cordless Si

The radio radio system, e. g. a , portable telephone system telephone system.

Radio channel control methods known communications art may geneally be classified i. e. , a centralized control type in which

Or the two

11/81 mobile types, station in into a base collectively supervises channels which are assigned to a group of mobile stations associated therewith, and a decentralized control type in which each of mobile stations selects an available channel independently of the others. A personal radio system is one of appications. of the decentralized control type method. A domestic maritime mobile radiotelephone system installed by NTT (Japan) is another application which employs a multi-channel cyclic switching principle, i. e. , assigns no exclusive common control channels and allows pilot signals to be sent on desired channels.

The channel control for the above-mentioned personal radio system is performed using exclusive common control channels. The problem with such a manner of channel control is that where base stations are so distributed as to provide continuity among different zones as viewed in a plane/ such as for portable telephones, cochannel interference occurs in overlapping areas of nearby zones where the desired-to-undesired signal (DU) ratio is comparatively low, The cochannel interference

-making it difficult to set up a call.

[fas customarily been coped with by

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providing each mobile station with a carrier sensing capability, which allows the mobile station to sense presence/absence of a radio wave.

Meanwhile, the radio channel control elaborated for the domestic maritime mobile radiotelephones assigns different channels to different zones because the zones are each comparatively wide. This successfully eliminates the cochannel interference. However, in a system in which the individual zones are very small such as a portable telephone system or a cordless 10 telephone system, it is significantly difficult to set up a channel arrangement which promotes efiidient use of frequencies partly because each zone is not circular but rather linear and partly because base stations cannot be located in adequate places.

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’ . SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a radio channel control method for a mobile communication system which ’efficiently uses frequencies and ' promotes the ease of channel assignment.

It is another object of the present invention to provide a radio channel control method for a mobile communication system which eliminates- cochannel interference to allow a .call to be set up with ease. '

It is another object of the present invention to provide a radio channel control method for a mobile communication system „ which prevents a plurality of base stations to be connectedHo a single mobile station.

vj &gt; · It is another object of the present invention to provide a generally improved radio channel control method for a mobile 30 communication system. ..

A radio channel control method for a mobile communication . system in which a plurality of base stations share a plurality of channels and each selects any of the channels for setting up a &gt; call independently of the other base stations of. the presen/ . 35 ' invent/?n comprises the steps of (a) scanning all the channels at

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ϋ' a time of channel selection, (b) selecting one channel out of the channels which show no reception sensitivity while excluding the channels which show reception sensitivity, (c) transmitting a pilot signal on the selected channel, and (d) when a signal has not been received over a predetermined period of time over the channel^ used to transmit the pilot signal, selecting another channel. · /.,

In accordance with the present invention, a radio channel control method, for a small-zone mobile communication system in which a plurality of base stations share a plurality of channels and each selects any of the channels for setting up a call independently of the others, e. g. a portable telephone system Or a cordless telephone system, is disclosed. Installed in each of all the base stations are a transceiver, free to use all the channels, a device capable of identifying channels on which radio Waves have reached the own Statibn based on reception sensitivity, a device for selecting /a channel, a device for transmitting a pilot signal and othdrs, and a device for setting up a call. The channel for a pilot signal is adequately changed to shorten _va duration of cochannel interference. Connection of multiple basje stations to a single mobile station is eliminated by causing' eaqd base station to add to the pilot signal identification data which is particular to the base station.

The present detailed above and other objects, features and advantages of the invention will become more apparent from the following description taken with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS is a schematic view Of a mobile communication system

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to which the present invention is applicable; ,.

Fig. 2 is a block diagram of a base station included in the system of Fig,, 3 1

Fig. 3 is a block diagram of a mobile station also included in the system of Fig. 1;

Fig. 4 is a flowchart demonstrating a channel selection

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.0 channel system in t channels up a call system or n each of se all the hich radio reception levice for Or setting y changed nection of linated by ntification procedure performed by the system of Fig. 1;

Fig. 5 is a timing chart representative of an exemplary operation in accordance with a radio channel control method of .the present invention; and

Fig. 6 is a schematic view of a system construction in

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j ' 4 $4 =i exemplary method of struction in

DESCRIPTION OF THE PREFERRED EMBODIMENT ' Referring to Fig.

system to which the present invention is applicable is shown.

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As 25, and 32, stations, controller hardware.

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The line control 49 is connected to a telephone circuit.

a timer for 47 and is

The base using their by carrier sensing

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shown, the system includes base stations 21, 22, 23, / \j

26 and 27 which cover radio zones 11, 12, 13, 14, 15,

17, ^respectively. Designated by the reference numerals 33* and 34 are mobile stations, or portable transceivers, shows for simplicity only seven radio zones and four stations which are in communication.

Referring to Fig. 2, each of the mobile stations, automobile telephone or a cordless telephone, ^^ansmitter 41, a receiver 42, a controller 43, an antenna duplexer 45, a handset 46, like an comprises a an antenna 44, and a frequency synthesizer capable of being tuned to any desired channel. As shown in Fig. 3, each of the base stations is provided with essentially the same construction as the mobile stations except that it is connected to a public telephone network and, therefore, provided with a line control 49 in place of the handset 46. The line control 49 is connected to a telephone circuit. In each of the base stations and mobile counting time is usually installed in a implemented either by software or stations and mobile stations perform Receivers 42.

' a Referring to Fig. 4, there il flowchart representative of an algorithm with which each base station may Specifically, each of the base stations 21—27 1 operates As demonstrated by the flowchart in to communicate assuming that a

T select a channel, siwfe^ in Figi order to allow its associated mobile station therewith. In the condition shown in Fig. 1, radio wave emitted in any one of the seven zones can reach all the other zones as well even if the field intensity may be low, all the base stations need to select channels having frequencies other than fi, f₂, fi₃ and f₄. In this illustrative mobile communication system, each base station has the capability for selecting a

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channel independently of the others so that some of the base j*.

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** . ·» «&lt;««» &gt;*&amp; .1(/0 stations may possibly select the same channel. The present invention contemplates to settle a problematic situation which will be given rise to by Such cochannel interference.

Usually, in the decentralized type control, a base station deterinines which ones of the channels that are shared by all the base stations are being used by the nearby base stations using a known carrier sense principle, i. e. by checking whether radio waves are received or not. While carrier sense is usually 10 performed in the vicinity of a base station, a mobile station is used in locations which are rrTOte from a base station. Hence, perfect carrier sensing is ufl^iinable. in accordance with the method of the present invention, when a predetermined period of time expires before a radio wave is received over a channel 15 which is being used as a control channel, it is decided that a signal is under interception due to interference and the control channel is replaced by another. While a conversation is held, it is impossible to see if interference exists in terms of time only and, hence, another suitable means such as sensing a beat 20 signal is required. In accordance with the present invention, therefore, channel switching is not effected while a conversation is under way. ·

Referring to Fig. 5, there is shown a specific example in which base stations #11 and #12 are provided and to which five 25 channels having¹’frequencies fi-f_s are assigned. The reference numeral I designates a duration for which the station #11 uses the channel with the frequency fj for a conversation; 2, 5 and 10 • designate respectively durations for which the station #11 uses ¹ the channels with the frequencies f₂ and f₄ for thetransmission of pilot signals; 3, 6 and 9 designate respectively durations for which the station #12 uses the channels with the frequencies f_s, f&lt; and f₅ for the transmission of pilot signals; and 4 and 8 designate respectively durations for which the Station #12 uses the channels with the frequencies f₃ and f₅ for conversations.

The reference numerals 9 and 8 show a particular situation

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Ο wherein during transmission of a pilot signal over the channel f₅a Signal emitted from a mobile station has arrived on that Further, the reference over which the stations channel (one with the for the transmission of channel f§ to set up a communication, numeral 7 designates a period of time ¢11 and ¢12 have selected the same frequency f₄) simultaneously to use it ‘ pilot signals.

In the above conflicting condition, the method of the present invention causes the station ¢12 to switch the channel from f₄ to f₂ upon elapse of a predetermined period of time T. Therefore, the duration of the conflict on the same channel is short enough for a time diversity effect to be achieved.

Referring to Fig. 6., a system construction in accordance with the present invention is shown. As shown, base stations 61-64 are located in radio zones /54, respectively. The z-’’^: reference numerals 71 and 72 each designate a channel whose frequency is fj and available for the transmission of a pilot signal. Designated by the reference numeral 81 is a mobile station. In the illustrated particular condition, two base , stations 62 and 64 are emitting pilot signals at the same time on the same Channel (with the frequency f j . As the mobile station 81 in the! illustrated location transmits a signal, the two base stations 6 2 and 64 receive it at the same time. However, since each Of $he pilot signals contains an identification data, or number, which is particular to the associated base station, the signal from received by one ofthe station 81, connect themselves concurrently to one mobile station 81.

A practical procedure for Origination of a. call in accordance with the method of the present invention is as follows. A mobile station first tunes itself to a particular control channel which is being activated by a decentralized base station. This particular channel is a one which has a carrier propagating thereover and the mobile station 81 is accompanied. by the signal·/, the own station, i. e. , ¢2 or #4. This allows only base stations 62 and 64 to respond to the mobile that is, prevents the two stations #2 and #4 to

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is known, ' them may receive a response from the mobile of the two approaches is applicable, to the Once the location of the desired mobile the particular basu station may connect the a calling station for a conversation. As , the is modulated by a pilot signal. Then, as the user of the mobile station off-hooks, the mobile station sends a call origination signal to the base station and, after establishment pf a loop with the latter, a dial signal. After the base station has connected the calling mobile station to a called station, the user is allowed to hold a conversation with a person at the called station.

’ Meanwhile, what is necessary for reception of a call is to see where the mobile station is located. In an automobile telephone system or the like,, a particular radio zone in which a mobile station is located is usually determined by either one of two approaches: one in which a telephone exchange memorizes a location of the mobile station! which is filed, and the other in /

which all the base stations call up the mobile station at the same time so station.

present station cabled station to ^{; :} ..2 ' ^; previously described in relation to the origination of a call, mobile Station will receive the call while having tuned itself to a ochannel associated with the particular base station.

In summary, it will be seen that a radio channel control method for a mobile communication system of the present invention achieves various advantages as enumerated below:

h·) The channel for the transmission of a pilot signal is adequately changeable to shorten the duration of cochannel interference;

(2) Simultaneous connection of a plurality of base stations to a single mobile station is. eliminated by causing each base station to add identification data particular thereto to a pilot signal; and (3) The above advantages in combination realize a system which is cost-effective, facilitates channel assignment, and promotes efficient use of frequencies.

Various modifications will become possible for those skilled

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