CA2314964A1 - Paging method for a multi-line terminal in a fixed cellular system - Google Patents

Abstract

A communication system provides telephone service to subscriber units through a private branch exchange (PBX) and a fixed cellular terminal (FCT) that operates within a digital cellular network. The cellular network transmits page information over one or more physical channels that are subdivided into digital control channels (DCCH) that include one or more logical paging channels (PCHs). The FCT includes a number of cellular terminals (CTs) incorporated in multi-line terminals (MLTs). The MLTs connect the subscriber units to the CTs, at least one of which receives the page information on at least one PCH.

Description

PAGING METHOD FOR A MULTI-LINE TERMINAL IN A FIXED CELLULAR SYSTEM
Techriical Field This invention generally relates to the field of communication systems and, more particularly, to a communication system that provides telephone service using a Fixed Cellular Terminal (FCT).
Background In many parts of the world, telephone service is not readily available because of inadequate or nonexistent infrastructure, and people desiring the service have to wait a long time to obtain it. In order to solve this problem, many service providers are installing terminals that are known as Fixed Cellular Terminals (FCT), which combine private branch exchange (PBX) technology and cellular technology, to provide telephone service over a cellular network.
An FCT includes a Multi-Line Terminal (MLT) and a PBX that together provide telephone service to a group of subscribers through a cellular network. In the FCT, a group of subscriber units, such as conventional wired telephones, are multiplexed through the PBX to a number of cellular terminals (CTs) incorporated into the MLT. This arrangement expedites installation of telephone infrastructure in remote locations, where conventional wired telephone service is unavailable. For example, one known FCT provides telephone service to 100 subscribers using an MLT that has 16 CTs. A
control unit in the MLT coordinates the selection of trunks and the communication with the PBX. Under this arrangement, WO 99/31904 PCT/US98/25708 _

2 each MLT services a group of subscriber units that can receive or initiate calls over the cellular network.
When a call is directed to a particular subscriber number of a conventional FCT, the cellular network transmits a page containing the called subscriber number over a control channel. Each one of the CTs, which operates to direct or originate a call to and from any one of the subscriber units, is programmed to respond to pages addressing the group of subscriber numbers assigned to a corresponding MLT.
Conventional FCTs are implemented using two types of cellular networks. The first type of cellular network is an analog cellular network, such as AMPS network, which modulates analog voice and control signals on physical radio frequency (RF) channels that link the CTs and a central controller of the cellular network to each other. Under this implementation, the CTs of a MLT tune to a known RF
control channel for receiving and transmitting analog control information, such as paging information and voice channel information that carry a voice call. Once the control information is received for a called subscriber unit, the MLT selects an idle CT to handle the call.
FCTs are also implemented in a second type of cellular network that uses a hybrid of analog and digital technology. One such hybrid cellular network standard is defined by Telecommunication Industry Associated (TIA) IS-54 standard. This hybrid cellular network uses one or more analog physical channels for communicating control information. Voice and data signals, however, are communicated digitally over logical channels that are formed by subdividing a physical RF channel into a predefined

3 number of time slots. In many digital systems, the logical channels carry digital voice and traffic data using a Time Division Multiple Access (TDMA) technique. Similar to analog cellular networks, the CTs of the FCT operating with the hybrid cellular network tune to a designated physical control channel for receiving analog control information, including the page information.
There also exists a third type of cellular _ __network, which is a . fully_.digitized. This -type -of -cellular network uses logical channels for communicating digital voice or data as well as control information. One such digital cellular standard is defined by TIA's IS-136 standard, which is being adopted rapidly throughout the world. A cellular network implemented using the IS-136 standard transmits control information over digital control channels (DCCHs) that carry page information over logical paging channels (PCHs). Under the IS-136 standard, a Mobile Switching Center (MSC) executes a predefined PCH calculation algorithm for allocating the PCHs. In order to allocate the PCHs, the MSC applies a number of defined parameters, including each mobile station's mobile identification number (MIN, to the PCH calculation algorithm. Because of the arbitrary nature of the MIN pair, however, the PCH
calculation algorithm produces arbitrary results.
Therefore, unlike the first two types of cellular networks that transmit paging information over a known physical control channel, the paging information in an IS-136 system may be scattered on arbitrarily allocated logical PCHs.
Moreover, in addition to being scattered on multiple logical channels of one DCCH physical channel, a cell, depending on the cell configuration of the cellular network, may

4 PCT/US98/25708 _ distribute the PCH channels over up to 8 DCCH physical channels.
The arbitrary nature of PCH complicates allocation aver one or more physical channels the implementation of an FCT using a digital control channel. Unlike analog and hybrid cellular networks, which have a fixed control channel to which the CTs can readily tune to for receiving page information, the page information in an existing digital cellular network are carried on PCHs that are-not--fixed. -Since an MLT is responsible for responding to calls directed to a large group of subscriber numbers, e.g., up to 100 subscriber numbers, its associated CTs must calculate a PCH
for each subscriber number based on an assigned MIN pair.
Because of the arbitrarily allocated logical PCHs, theoretically, each mobile station must monitor every PCH on all of the DCCHs. Although the CT has the capability of monitoring every PCH, performing the PCH allocation algorithm for each MIN pair of a large group of subscriber numbers, e.g., 100, within the duration of a logical channel, which under IS-136 standard~is about 640 ms, is beyond the processing power of the existing CTs.
Furthermore, if the allocated PCHs are scattered over two or more physical channels, the existing CTs, without hardware and software modifications, are unable to concurrently monitor PCHs over multiple physical channels.
Because of the wide acceptance of digital cellular , networks, it is desired to implement the FCT under a standard that defines a digital control channel, without substantially modifying an existing infrastructure.
Therefore, there exists a need for a communication system that allows for the operation of an FTC in an existing cellular network that uses digital control channels.
Summary

5 The present invention that addresses this need is exemplified in a communication system that includes a cellular network that transmits page information over one or more physical channels that are subdivided into digital control channels (DCCHs) that include one or more logical paging channels (PCHs). A fixed cellular terminal (FCT) with a number of subscriber units, each having a corresponding subscriber number, and a number of cellular terminals (CTs) communicates with the cellular network over the DCCHs. At least one multi-line terminal (MLT) that is connected to the subscriber units and to the CTs that receives the page information on at least one PCH via at least one of the CTs.
According to some of the more detailed features of the invention the page information directed to all of the subscriber units that are connected to the MLT is transmitted over a common PCH. Preferably this is done by executing a PCH calculation algorithm for allocating the PCH. Under this arrangement, PCH is calculated based on an MLT identification (MLT ID), which is assigned to the MLT.
When transmitting the paging information, the subscriber number of a called subscriber unit is transmitted over an allocated PCH.
According to another embodiment of the invention one CT is reserved for receiving page information over all PCHs. Under this embodiment, other CTs ignore the page information. The reserved CT delivers the paging information

6 to the MLT, which assigns one of the other CTs to respond to a call.
Other features and advantages of the present invention will become apparent from the following description of the preferred embodiment, taken in conjunction with the accompanying drawings.
Detailed Description Referring to FIG. 1, a communication system 10 according to the present invention provides telephone service to subscribers using a digital cellular network 12.
In a preferred embodiment, the cellular network 12 is implemented substantially according to the TIA~s IS-136 standard, which is hereby incorporated by reference.
Therefore, the operation of the cellular network 12 is described to the extent necessary for understanding of the present invention. Although, the present invention is described as embodied using the IS-136 standard, those skilled in the art would appreciate that the present invention could be advantageously used in a wide variety of other digital communication systems, such as those based on PDC or GSM standards.
The communication system 10 provides telephone service to fixed subscriber units 18 that communicate with the cellular network 12 via a fixed cellular terminal (FCT) that connects them to a number of Multi-Line Terminals , (MLTs} 22 via a PBX 24. In a well known manner, the cellular network 12 communicates digital voice, data and control information over physical channels that are subdivided into a number of logical control channels. In addition, the communication system provides cellular service _ WO 99/31904 PCT/US98/25708

7 to subscribers that carry mobile stations 14 while traveling within communication cells 16 of the cellular network 12.
The cellular network 12 is a hierarchal network with multiple levels for managing calls. Using an allocated set of uplink and downlink RF channels, which are sub-divided into logical control channels, mobile stations 14 and the MLTs 22 participate in calls using the allocated logical channels. A Mobile-service Switching Center (MSC) 26 is responsible for routing calls from an originator to a destination. In particular, the MSC 26 is responsible for setup, control and termination of the calls. The MSC 26 also communicates with a Public Switched Telephone Network (PSTN) 28, or other public and private network, which is connected to conventional telephones 30. A subscriber database 32, which is accessible by the MSC 26, maintains records corresponding to each subscriber of the communication system 10. By comparing a called subscriber number, for example, to one originated by a telephone 30 against the subscriber database 32, the MSC 26 determines whether to handle a call or not.
At a lower hierarchal level, the MSC 26 is connected to a group of base station controllers (BSCs) 34, which are primarily responsible for mobility management.
For example, based on reported received signal strength, a BSC 34 determines whether to initiate a hand over, the process by which the calls are maintained without a noticeable break. At a still lower hierarchal level, each one of the BSCs 34 controls a group of base transceiver stations (BTSs) 38. Via antennas 37 and 35, the BTSs 38 primarily provide wireless links for transmission and reception of data bursts over the logical channels. Each BTS 38 includes one or more transceivers (not shown) that use the physical channels to serve a particular common geographical area, such as one or more of the communication ' cells 16.
Referring to FIG. 2, a block diagram of an MLT 22 is shown to include a terminal unit 39 having a number of cellular terminal (CTs) 40 and a control unit 42. In an exemplary embodiment, the MLT 22 includes sixteen CTs 40 for serving 100 subscriber units 18. The control unit 42 includes a processor 44 that controls the overall operation of the MLT 22 and interfaces with the CTs 40 via a CT
interface 46, which in an exemplary embodiment of the invention uses a serial communication protocol for providing communication between the control unit 42 and the CTs 40.
The MLT 22 interfaces with the PBX 24 via a PBX interface 48. The PBX 24 routes calls to and from the subscriber units 18 via the CTs 40 of the MLT 22. The CTs 40 interface with the cellular network 12 in a similar manner as that of the mobile stations 14 of FIG. 1. An RF unit 41, which includes a passive device that combines the outputs of the CTs to a single antenna 35, receives and transmits RF
signals to and from the cellular network according to the IS-136 standard. In this exemplary embodiment, using the sixteen CTs 40, the MLT 22 is responsible for routing calls to and from up to 100 subscriber units 18 through the PBX
24. An operation and maintenance (0&M) controller 50 controls the operation and maintenance of one or more MLTs 22 in a well known manner.
As described above, each physical channel is subdivided into a number logical channels consisting of time slots during which digital voice or data and control information are communicated. Referring to FTG. 3a, a physical channel subdivided into six time slots is shown to form a 40-ms TDMA frame. Digital voice or data are communicated over logical digital traffic channels (DTC), and the control information are transmitted over digital control channels (DCCH). The DCCHs used for transmitting control information to the CTs 40 (and mobile stations 14) are known as Forward DCCH (FDDCH). Conversely, the DCCHs used for receiving control information from the CTs 40 are known as Reverse DCCH (RDDCH). Each FDDCH includes a number of TDMA Superframes. FIG. 3b shows a TDMA Superframe that consists of sixteen 40-ms TDMA frames for a total length of 640 ms. Each FDDCH superframe is comprised of an of an ordered sequence of logical channels, including F-BCCH, E-BCCH, S-BCCH, reserved slots, and SPACH. The F-BCCH
contains data elements that may be relevant to paging, such as page continuance and PCH calculation information. The SPACH time slots serve as the paging slots on the FDCCH.
All mobile station pages are received on these slots. A
predefined minimum and maximum number of these logical channels may be included in each FDDCH superframe. For example, a minimum of 3 and a maximum of 10 F-BCCH slots may be included in a FDCCH superframe. Similarly, each FDCCH
superframe may include a minimum of 2 and a maximum of 28 SPACH time slots.
According to one embodiment of the present invention, a method for paging a subscriber unit causes the MSC to transmit all pages directed to an MLT on a common PCH, which is used by all of the CTs 40 within the MLT for monitoring the pages. More particularly, the paging method of the invention assigns a MLT ID to each one of the MLTs, which are connected to a corresponding group of subscriber units 18 via the PBX 24. In an exemplary embodiment, the MLT IDs are assigned to the MLTs by the O & M controller 50 (shown in FIG. 2). Instead of the MIN/ESN pair, the CTs 40 and the MSC 26 all use a corresponding MLT ID, which is associated with a corresponding group of subscriber numbers, to calculate the common PCH. Based on a called subscriber number, the MSC 26 applies an MLT ID associated with the called subscriber number to the PCH calculation algorithm to 10 calculate a suitable PCH.
In the MLT, the CTs 40 also use their assigned MLT
ID to calculate their allocated PCH. Because the MSC 26 and the CTs 40 of a particular MLT 22 use the same MLT ID, the PCH calculation algorithm produces the same PCH.
Thereafter, the CTs 40 tune to the calculated PCH and monitor for pages directed at the subscriber units 18 that are connected to their corresponding MLTs 22. Once paged by the MSC 26, idle CTs 40 compare the transmitted subscriber number against a list of subscriber numbers assigned to their corresponding MLT 22. If a match is found, the idle CTs 40 direct the subscriber number to the control unit 42.
The control unit 42 routes the subscriber number to the PBX
24, which alerts the called subscriber unit 18 by causing it to ring. If more than one idle CTs 40 respond to the page, using an arbiter block 52 (shown in FIG. 2), the control unit 42 selects one of the idle CTs 40 for handling the , call.
Under an exemplary embodiment of the invention, a single field called "MLT ID" is added to the subscriber database 32. For each FCT subscriber number, the MLT ID
field contains a value corresponding to an MLT ID of an MLT

22 to which the subscriber units 18 are assigned. In other words, the subscriber database 32 associates the FCT
subscriber numbers with an MLT ID. Preferably, the MLT ID
field is added to every subscriber number regardless of whether they are FCT or non-FCT subscriber numbers.
However, it would be appreciated that the MLT ID field could be added only for FCT subscribers. In an exemplary embodiment, the corresponding MLT-ID fields of non-FCT
subscriber numbers may contain no value. When a call is detected, the MSC 26 looks up a received subscriber number in its subscriber database 32. If found, the MSC 26 checks the corresponding MLT_ID field. If this field was loaded with an MLT ID value, the MSC 26 calculates a PCH based on the MLT ID associated with the subscriber number of the called subscriber unit 18. Using the calculated PCH
channel, the MSC 26 transmits a page that includes the subscriber number of the called subscriber unit 18.
Therefore, the MSC 26 only uses the MLT ID for selecting a PCH. Once selected, a corresponding subscriber number is used for paging a called subscriber unit.
In this way, the MSC 26 pages every subscriber unit 18, which is assigned to an MLT, using a single PCH
that is calculated based on the MLT ID. The advantage of this solution over using a specific MIN from the list of 100 already assigned to the MLT is that it is independent of the non-FCT subscribers and does not require the MSC to distinguish between the FCT and non-FCT subscribers. This allows the MSC 26 to page all types of subscribers in the network 12 without requiring significant configuration modification in the existing MSC 26 or the mobile stations 14 .

In another embodiment of the invention, the MLTs operate without being assigned an MLT ID. Rather, the processing of the received pages over PCHs is off loaded to the processor of the control unit 42, which has a higher processing power than that of the CTs 40. While the CTs 40 are capable of monitoring every SPACH slot on the DCCH, they are not capable of processing the pages. The CTs 40 must read every message from every slot and compare it against a large list of stored subscriber numbers, for example 100 subscriber numbers. Since each SPACH could contain 5 pages, under this scenario, a CT 40 must perform up to 500 comparisons for each 20-ms SPACH slot. With maximum of 28 SPACH slots, theoretically, the CT would need to complete up to 14000 comparisons in 560 ms. As described before, the processors of existing CTs do not have such a high processing power. Further more, if an ~~Additional DCCH
Information~~ element is communicated over the F-DCCH, the CTs may be required to monitor for pages on up to 8 different physical DCCHs simultaneously, or multiple logical DCCHs on the same frequency. The current CTs are also incapable of monitoring different physical channels simultaneously.
Therefore, according to this embodiment of the invention, one of the CTs 40 of the MLT 22 is reserved for PCH monitoring. The reserved CT is only responsible for monitoring pages on every PCH in the superframe. The reserved CT does not perform any comparison or other processing on the page information. All other non-reserved CTs in the MLT would ignore any pages encountered in a superframe. The reserved CT passes all page information received on every PCH to the control unit 42 for processing.

Under this embodiment, the reserved CT acts as a data gathering unit in the MLT 22. Upon receiving the page information from the reserved CT, the control unit 42 compares the MIN transmitted in the page information with a list of corresponding subscriber numbers contained in the control unit 42. If a match is found, the control units 42 selects an idle CT as the trunk to handle the received page and sends the page to the selected CT 40. The selected CT
40 would then respond to the page as if it had received it from the MSC 26 under a normal cellular environment.
Preferably, if a reserved CT became inoperable, the control unit 42 reserves a different CT for monitoring the PCH. If a cell uses multiple DCCHs, up to 8 CTs 40 may be designated as reserved CTs. Alternatively, multiple receivers may be incorporated into the RF unit 41 (shown in FIG. 2) for monitoring multiple DCCHs.
From the foregoing description, it will be appreciated that the two embodiments of the invention allow for implementation of an FCT that uses an existing digital cellular network for providing telephone service for subscriber units via a PBX. By using separate MLT IDs in the PCH calculation algorithms of the CTs and the MSC, one embodiment of the invention forces paging of the subscriber units on a common PCH. This way, the overhead associated with handling arbitrary PCH allocation is avoided. In the other embodiment, the present invention reduces the burden of monitoring all PCHs by CTs by designating a reserved CT, which monitors and reports page information received over all of the PCH to the control unit. This embodiment of the invention, therefore, off-loads the processing requirement of the monitoring pages transmitted on arbitrary channels from the CTs to the more powerful processor of the control unit. Accordingly, both embodiments of the invention allow for implementation of an FCT that operates with a cellular network that uses a digital control channel.
Although the invention has been described in detail with reference only to a preferred embodiment, those skilled in the art will appreciate that various modifications can be made without departing from the invention. Accordingly, the invention is defined only by the following claims which are intended to embrace all equivalents thereof.

Claims (14)

WHAT IS CLAIMED IS:

1. A communication system comprising:
a cellular network that transmits page information over one or more physical channels that are subdivided into digital control channels (DCCH) that include one or more logical paging channels (PCHs);
a fixed cellular terminal (FCT) that includes:
a number of subscriber units each having a corresponding subscriber number;
a number of cellular terminals (CTs);
at least one multi-line terminal (MLT) that is connected to the subscriber units and to the CTs, wherein at least one of the CTs receives the page information on at least one PCH.

2. The method of claim 1, wherein the page information directed to all of the subscriber units that are connected to the MLT are transmitted over a common PCH.

3. The method of claim 1 further including the step of executing a PCH calculation algorithm for allocating the at least one PCH, wherein the at least one PCH is calculated based on an MLT identification (MLT ID), which is assigned to the at least one MLT.

4. The method of claim 2 further including the step of transmitting the subscriber number of a called subscriber unit over an allocated PCH.

5. The method of claim 1 further including the step of reserving a CT for receiving page information over all PCHs, wherein other CTs ignore the page information.

6. The method of claim 5, wherein the MLT assigns one of the other CTs to respond to a call.

7. A method for calling subscriber units having.
corresponding subscriber numbers through an MLT connected to at least one of CT, comprising:
assigning an MLT ID to the MLT;
receiving page information by the at least one CT on a PCH that is allocated based on the MLT ID.

8. The method of claim 7 further including the step of selecting the at least one of CT for processing a call.

9. The method of claim 7 further including the step of allocating the PCH by executing a PCH calculation algorithm using the MLT ID.

10. The method of claim 7 further including the step of transmitting page information that include the subscriber number of a called subscriber unit.

11. A method for paging a number of subscriber units having corresponding subscriber numbers through an MLT, comprising:
determining an MLT ID associated with a subscriber number;

allocating a PCH for transmitting page information based on the MLT ID; and allocating a PCH for receiving the page information based on the MLT ID.

12. A communication system providing communication capability to a number subscriber units having corresponding subscriber numbers, comprising:
a number of MLTs having corresponding unique MLT IDs, wherein each MLT ID is associated with a group of subscriber numbers;
an MSC that allocates PCHs for paging the subscriber units based on the MLT IDs; and CTs that receive the page information over the allocated PCHs.

13. An MLT, comprising:
a plurality of CTs, wherein at least one of the CTs is reserved for receiving page information on a plurality of PCHs;
a control unit that receives the page information from the reserved CT and assigns one of the other CTs to handle a call directed at a subscriber unit.

14. A method for paging subscriber units having corresponding subscriber numbers, comprising:
assigning a MLT ID to at least one MLT;
associating MLT IDs with the subscriber numbers;
receiving a call to a called subscriber unit;

determining an MLT ID associated with the subscriber number of the called subscriber unit;
allocating a PCH based on the MLT ID;
transmitting page information directed at the called subscriber unit; and receiving the page information on a channel calculated based on the MLT ID.