GB2304498A - Control Method For Mobile Communications - Google Patents

Abstract

A system (18) is provided which causes mobile units 14 of a multi-cellular communication system 10 to more frequently execute a log-in to re-select a local cell A-L. In this manner, loss of mobile-network contact caused by rapid passage through a small cell F is avoided. This is accomplished by a controller (24) and memory (22) operating a cell base station (18) so as to deliberately introduce errors into the paging signal radiated by the base station (18) when no mobile unit 14 is being called. This fools the mobile units 14 decoding the error containing message into believing that they are approaching a cell boundary and prompts them to execute a re-select step, irrespective of their location in the cell A.

Description

CONTROL MEANS AND METHOD FOR MOBILE COMMUNICATIONS Background of the Invention The present invention pertains to mobile communication systems, especially cellular communication systems.

Cellular communication systems are well known in the art. In general such systems comprise an overlapping array of adjacent communication cells within which a base station transceiver and multiple mobile transceivers operate in such a way that they do not interfere with each other or transceiver units in neighboring cells.

As the traffic requirements of cellular radio systems increase, cell sizes are often reduced. This allows one to increase the density of traffic carrying channels available per unit area. However, users are diverse and some are better served by larger rather than smaller cells.

Additionally, traffic density is not uniform and while some areas need small cells, others do not. Thus, it is extremely difficult to design a simple multi-cellular network that is optimized for all users.

In part to alleviate this problem, multi-cellular networks consisting of a variety of cells of different sizes arranged in layers superpositioned on top of one another have been proposed. In this arrangement a user at a particular location can belong to, for example, a small cell in one layer or a larger cell in another layer, depending upon his communication needs. But neither the single or multi-layer approach is free of problems since, in general, they will still contain some small size cells.

A mobile unit is often able to communicate with several base stations, as for example, those located in neighboring cells. This is particularly true when the mobile unit is located near inter-cellular boundaries. When the mobile unit is waiting to make or receive a call, it is said to be in the "idle" state. Accordingly, a means is provided to select one of the several cell base stations that is within range of the mobile unit at a particular time.

Several things occur during this process: (i) The particular cell base station likely to provide the best communication with the mobile unit is chosen, (ii) the network is informed of the identity of the chosen cell (it needs to know where to send messages intended for that mobile unit), and (iii) the mobile is synchronized to this one cell of the network so that it can decode paging or other messages being transmitted to it.

This process of a mobile logging-on to the network via a particular cell is referred to in the art as "camping-on" or being "camped-on" a particular cell. The selection process must be periodically repeated (i.e., "reselection") to keep the cell assignment current since the mobile units often move from one cell to another. Hand-off needs to occur automatically as the mobile unit approaches a cell boundary.

A problem arises when cells become smaller or layered and the mobile unit moves with significant velocity. The mobile may move beyond the range of its current, camped-on cell into a new cell whose transmissions it is not monitoring before a further re-selection step can take place. When this happens, the mobile unit is temporarily lost to the system. The denser and more complex the network, the more severe the problem of lost units can become. The present invention, among other things, reduces the likelihood of units becoming lost in this way. A very important further advantage is that it does not require physical modification of the mobile units.

Brief Description of the Drawing FIG. 1 is a simplified topographical representation of a multicellular communication system showing a user location and trajectory; FIG. 2 is a simplified block diagram of an electronic system, according to the present invention, for carrying out the process described by the flow chart of FIG. 3; and FIG. 3 is simplified flow chart of a process, according to the present invention, for inducing more frequent re-select steps in mobile units of a cellular communication system.

Description of the Preferred Embodiment The tenns "select", "selection", selected", "register", "registration", "registered", "camping-on" and "camped-on", other than their ordinary differences of usage as verb or noun or the like, are intended to have the same meaning, specifically, the process or result of the process by which a mobile unit enters into communication with a particular cell of a cellular network so that the mobile unit and the network communicate via a particular cell base station. Thus, a mobile unit that has "campedon" a particular cell is "registered" with the network via that cell.The prefix "re-" in connection with these words is intended to means a reexecution of the same or an analogous process or its consequent result, as for example, to "re-select" indicates that the action to "select" has been repeated and that, if successful, the mobile unit is once again "campedon" or "registered" via the same or another cell. The select and re-select process need not be identical since, among other things, the information exchanged can be different.

FIG. 1 shows a simplified topographical representation of an array 10 of multiple overlapping cells A-L (collective identified by reference number 12) of a multi-cellular communication system and representative mobile unit 14. Assume that mobile unit 14 is initially camped-on the cell designated by the letter "A", i.e., "A-cell". Mobile unit 14 receives, for example, paging sub-channel signals from the base station of A-cell which it can decode by virtue of being camped-on A-cell, i.e., the set-up selection process has previously been completed. As long as mobile unit 14 continues to receive and decode such paging subchannel signal from A-cell, it knows that it is in communication with Acell and need not initiate a re-selection process to identify a new cell.

Vector 14 defines a trajectory along which mobile unit 14 moves from one cell to another, first into cell F and then into cells G, K and L.

As mobile unit 14 nears the A-F cell interface, the paging signal from the A-cell base station weakens, the error rate increases and the mobile unit is no longer able to decode the received A-cell paging signal to match the stored pattern provided during the initial cell selection (camp-on) process. The approach of a cell boundary is therefore likely and a reselection process (new camp-on ) is initiated after one or more such failures. In systems conforming to the standard of the Global System for Mobile Communication (commonly referred to as "GSM"), three decode failures occur before a "re-select" is initiated. Thus, mobile unit 14, even in an idle state (i.e., waiting for a call) re-selects a new cell when it can no longer decode the paging signal of its "camp-on" cell. Re-select can be also be triggered by higher signal strength from a base station in a neighboring cell.

If Fcell is small and unit 14 is moving rapidly as it approaches the A-F cell boundary, then the transit time of mobile unit 14 along vector 16 across F-cell can be less than the re-select time. When this occurs, mobile 14 is lost until a re-selection step is completed. Since this might not occur until mobile unit 14 has already passed through F-cell into G-cell or K-cell, communication with mobile unit 14 is not possible in F-cell.

Because of the many millions of cellular phones that are already in service, it is not practical to relieve this problem by modifying the existing phones to have shorter re-select intervals, or requiring fewer decode failures before a re-select is initiated. Thus, there is a great need for a solution which is transparent to the user and the mobile unit and requires no hardware modification of the mobile unit.

As long as mobile unit 14 is receiving a paging or equivalent signal from the camped-on base station, it will not initiate a cell re-select.

By prompting unit 14 to do a re-select more frequently, the problem illustrated by FIG. 1 can be relived or, depending upon the cell size and velocity, eliminated entirely. FIG. 2 shows a simplified block diagram of base station 18 for carrying out process 40 illustrated in FIG. 3, for inducing more frequent re-select steps in cellular subscriber mobile unit 14. Persons of skill in the art will understand that the functions described in connection with FIGS. 2-3 can be carried out at each cell's base station or by a central station to which the individual cell base stations are coupled. The term "base station" as used in conjunction with FIGS. 2-3 is intended to include this embodiment as well.

In the preferred embodiment, base station 18 has electronic system 19 coupled to antenna 20. System 19 comprises memory 22 which communicates via bus 21 with controller 24. Controller 24 communicates via buses 23, 25 with transmitter 26 and receiver 28, respectively. Transmitter 26 sends signals on line 27 to transmit/receive (T/R) separator 30. T/R separator 30 can be a switch or, for duplex operation, a circulator or filter element. This is not critical. T/R separator 30 communicates with antenna 20. Antenna 20 communicates with mobile units 14. Persons of skill in the art will understand based on the explanation herein how to configure memory, controller, transmitter, receiver and T/R separator components to perform the functions described herein.

Memory 22 contains, stored programs 221 which determine the operation of controller 24 and in turn, the operation of transmitter 26 and receiver 28. In step 42 (see FIG. 3), controller 24, operating under the direction of programs 221 uses paging message buffer 222 of memory 22 to store incoming message addresses and/or message addresses from the network to determine whether there are any paging signals required to be sent to a mobile unit within the cell. These are referred to as a "type1" paging messages. An example of a GSM type-1 paging message is a message containing pseudo length, RR management protocol discriminator, skip indicator, message type, page mode, channels, needed, mobile id-1, mobile id-2, etc.Memory portion 223 is desirably provided to store information for use by controller 24 on the types of messages intended to included in the class of "type-l" messages, e.g., templates of the desired or not desired messages.

In step 44 (see FIG. 3), controller 24 examines each paging message in buffer 222 to determined whether it contains an explicit mobile identity address, i.e., showing that there is a message intended to be sent to a particular mobile unit 14. If the answer is "YES", then as indicated by line 45Y an instruction is sent to transmitter 26 to broadcast the paging message, as indicated in block 50 with output 53.

If, however, the answer to step 44 is "NO" then, as indicated by line 45N, controller 24 retrieves from portion 224 of memory 22 an error generating routine which in step 46 adds errors into the routine idlestatus (non-addressed) paging message to be broadcast by base station 18.

Controller 24 further passes this error containing message to transmitter 26 for broadcast from antenna 20 as indicated by block 50 with output 53.

The errors that are introduced may be random, e.g., provided by a random number generator operated by controller 24, or systematic. A separate random number generator (not-shown) can also be included in system 18, but this is not essential, since controller 24 can provide this function in response to stored programs 221.

The mobile unit cannot ordinarily tell the difference between the deliberately included errors and the errors that occur naturally due to multi-path and signal fading. Thus, the inclusion of errors in the broadcast signal has the same affect on the mobile unit as if it was approaching or exceeding an inter-cellular boundary. The decode errors increase until the error threshold within the mobile unit is exceeded whereupon it executes a re-select. Rather than occurring only near a region of poor signal quality (e, g., near an inter-cellular boundary), the deliberately introduced errors can prompt a re-select to occur no matter where the mobile unit is located in the cell. Thus, re-selects can be made to occur more frequently than they otherwise would.

The above-described process has the effect of fooling the mobile unit into believing that it is approaching a cell boundary before it actually gets there. Thus, more time is available to do a re-select before a rapidly moving mobile unit actually exits the cell. Other aspects of communication with the mobile unit are not affected since the error introducing process only affects the non-addressed paging signal.

Optional step 48 is provided to supply as indicated by arrow 49, a parameter value obtained from memory portion 225 to determine the type and/or number of errors being introduced into the paging message.

Controller 24 can vary the degree of error in response to changing system or cell conditions, e.g., traffic levels, mobile unit locations, mobile unit signal strength, failure of base station equipment and other things.

Varying the error level is desirable, among other reasons, because it allows the sensitivity of the system to be adjusted. For example, for mobile units 14 which are near but not at cell boundaries, relatively small numbers of errors introduced into the paging message can result in decode failures and trigger a reset process. However, mobile units closer to the base station will be unaffected by this small number of included errors because of the higher received signal strength at the mobile unit reduces the number of naturally occurring errors so that the sum of the naturally occurring errors and the introduced errors is still below the reset threshold level of the mobile unit.Another circumstance under which it is desirable to increase the error level is if numerous, short interval, cell-to-cell hand-offs are detected by the network, indicating that rapidly moving mobile units are likely to be present.

Once broadcast of the paging message has occurred, either error free or with varying degrees of error, the process cycles back to step 42, as indicated by line 51. The path described by lines 41, 43 45N and 47 and the steps represented by blocks 42, 44, 46, 48 are not found in prior art systems and are provided by the present invention.

The present invention is not limited to just those modes of operation particularly described above. For example, the errors in the paging signal can be introduced before or after channel coding. Further, the base station can include errors in all messages while controlling the degree or amount of error differently for messages directed to a particular mobile unit. In this example, corruption of the paging signal (i.e., error introduction) is allowed if a particular mobile is being addressed by a paging signal.

It will be apparent to those of skill in the art that the invented system and method can reduce the occurrence of "lost" mobile units which arise due to small cell sizes, multi-layered cells and rapid mobile unit motion. It will be further apparent that this is accomplished without physical modification of the mobile units and is therefore, particularly, effective and practical. It will be still further apparent that the improved performance permitted by the present invention allows denser cellular systems to be created.

Claims (10)

What is claimed is:

1. A method for controlling communications of a multicellular communication network having a cell with a base station therein broadcasting a paging message to mobile units registered within the cell, characterized by, monitoring messages received by the base station from the multi-cellular communication network to determine whether a received message does or does not contain an explicit address for a mobile unit within the cell, and if it does not, then adding errors to the paging message transmitted by the base station to induce some of the mobile units registered within the cell to re-register.

2. The method of claim 1 further comprising varying the number of errors added to the paging message according to operating conditions of the cell.

3. The method of claim 1 further comprising varying the number of errors added to the paging message according to further information received from the network.

4. A means for controlling communications of a multicellular communication network having a cell with a base station therein broadcasting a paging message to mobile units registered within the cell, characterized by, operating a controller and memory within the base station to monitor messages received by the base station from the multi-cellular communication network to determine whether a received message does or does not contain an explicit address for a mobile unit within the cell, and if it does not, then adding errors to the paging message transmitted by the base station to induce some of the mobile units registered within the cell to re-register.

5. The means of claim 4 wherein the controller and memory compare the messages received by the base station from the network with message templates stored in memory to determined whether an explicit address is contained in the messages received by the base station.

6. The means of claim 4 wherein the controller and memory vary the amount of error introduced in the paging message in response to information from the cell or the network.

7. The method of claim 3 wherein the errors are introduced before or after channel coding.

8. The means of claim 6 wherein the amount of error introduced varies depending upon whether a mobile unit is being paged.

9. A method for controlling communications of a multicellular communication network substantially as hereinbefore described with reference to FIGs.2 and 3 of the accompanying drawings.

10. A means for controlling a multi-cellular communication network substantially as herein before described with reference to FIGs.2 and 3 of the accompanying drawings.