WO2002035861A2

WO2002035861A2 - Call re-origination via a new base station to be placed in soft/softer handoff condition with a serving base station prior to call dropping

Info

Publication number: WO2002035861A2
Application number: PCT/US2001/029780
Authority: WO
Inventors: Gary J. Pregont
Original assignee: Motorola, Inc.
Priority date: 2000-10-24
Filing date: 2001-09-20
Publication date: 2002-05-02
Also published as: AU2002224330A1; WO2002035861A3

Abstract

Once a first downlink traffic channel has degraded, a remote unit (113) will attempt to access a second base station (102) via normal access procedures utilizing an access channel. Once communication has been established with the second base station (102), a second downlink traffic channel is established between the remote unit and the second base station. The remote unit is then placed in soft handoff with the first (101) and the second (102) base stations.

Description

CALL RE-ORIGINATION

Field of the Invention

The present invention relates generally to cellular communication systems and in particular, to call re-origination in such communication systems.

Background of the Invention

As a cellular subscriber (i.e., remote or mobile unit) enters an area of poor communication, there exists situations where the mobile unit can no longer communicate with the base station effectively. During this situation, the mobile unit will usually de-key and try to re-establish a communication link with the base station. If, after a predetermined period of time, (usually after an RF-loss Timer has expired) the communication path cannot be reestablished, the call will drop. The user must then manually redial the person they were talking with in order to be able to continue the conversation. The customer is left with a poor impression of the performance of the communication system. Therefore, a need exists for a method for call re- origination that does not require the user to redial the person they were talking to if the call drops.

Brief Description of the Drawings

FIG. 1 is block diagram of a communication system in accordance with the preferred embodiment of the present invention. FIG. 2 is a flow chart showing the operation of the communication system of FIG. 1 in accordance with the preferred embodiment of the present invention.

FIG. 3 is a block diagram of a communication system illustrating call re-origination in accordance with the preferred embodiment of the present invention.

FIG. 4 is a flow chart showing the operation of the remote unit of FIG. 1 in accordance with the preferred embodiment of the present invention.

Detailed Description of the Drawings

To address the above-mentioned need, a method for call re-origination is provided herein. In accordance with the invention once a first downlink traffic channel has degraded, the remote unit will attempt to access a second base station via normal access procedures utilizing the access channel. Once communication has been established with the second base station, a second downlink traffic channel is established between the remote unit and the second base station. The remote unit is then placed in soft handoff with the first and the second base stations.

In prior-art communication systems, the set up of soft/softer handoff links is accomplished via messaging that takes place over common traffic channels. Unfortunately, once a traffic channel experiences poor channel conditions, it may be impossible for the proper messaging to take place so that an additional soft/softer handoff traffic channel can be established. In the preferred embodiment of the present invention, all messaging for the additional traffic channel takes place by utilizing normal call origination schemes. Since call re-origination takes place by utilizing an access channel, the problem with the prior art is eliminated, and a soft/softer handoff leg can be set up even though poor traffic channel conditions are being experienced by the remote unit. Additionally, because the soft/softer handoff link is set up prior to the first traffic channel being dropped, the user will not be required to redial the person they were talking to if the original traffic channel drops. The present invention encompasses a method for re-originating a call.

The method comprises the steps of communicating with a first base station on a first traffic channel, determining that the first traffic channel has degraded, and prior to dropping the first traffic channel, communicating with a second base station via an access channel in order to establish a second traffic channel. The second channel is established as a result of the step of communicating with the second base station via the access channel. The present invention additionally encompasses a method for call re- origination in a code-division multiple-access (CDMA) communication system. The method comprises the steps of receiving a first downlink traffic channel from a first base station, determining that the first downlink traffic channel has degraded, and initiating an RF-loss timer in response to the determination that the first downlink communication signal has degraded. Call re-origination is performed with a second base station via an access channel to establish a second traffic channel with the second base station based on the call re-origination. Communication via soft/softer handoff takes place with the first and the second base stations utilizing the first and the second traffic channels, respectively.

The present invention additionally encompasses a method for re- originating a call. The method comprises utilizing a first channel to communicate with a first base station, wherein the first channel comprises a first channel type. A determination is made that the first channel has degraded and a second base station is accessed utilizing a second channel, wherein the second channel comprises a second channel type. A third channel is utilized to communicate with the second base station, wherein the third channel comprises the first channel type. Finally communication via the first and the third channels is made with the first and the second base stations, respectively.

Turning now to the drawings, wherein like numerals designate like components, FIG. 1 is a block diagram of communication system 100 in accordance with the preferred embodiment of the present invention. In the preferred embodiment of the present invention, communication system 100 utilizes a Code Division Multiple Access (CDMA) system protocol as described in Cellular System Remote unit-Base Station Compatibility Standard of the Electronic Industry Association/Telecommunications Industry Association Interim Standard 95C (TIA/EIA/IS-95C), which is incorporated by reference herein. (EJLA/TIA can be contacted at 2001 Pennsylvania Ave. N Washington DC 20006). However, in alternate embodiments communication system 100 may utilize other digital cellular communication system protocols such as, but not limited to, the next generation CDMA architecture as described in the UMTS Wideband CDMA SMG2 UMTS Physical Layer Expert Group Tdoc SMG2 UMTS-L1 221/98 (UMTS 221/98), the next generation CDMA architecture as described in the cdma2000 International Telecommunication Union-Radiocommunication (ITU-R) Radio Transmission Technology (RTT) Candidate Submission document, the next generation Global System for Mobile Communications (GSM) protocol, the CDMA system protocol as described in "Personal Station-Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communication Systems" (American National Standards Institute (ANSI) J-STD-008), or the European Telecommunications Standards Institute (ETSI) Wideband CDMA (W-CDMA) protocol.

Communication system 100 includes a number of network elements such as base stations 101-102, remote unit 113, Centralized Base Station Controller (CBSC) 103, and Mobile Switching Center (MSC) 104. In the preferred embodiment of the present invention, all network elements are available from Motorola, Inc. (Motorola Inc. is located at 1301 East Algonquin Road, Schaumburg, EL 60196). It is contemplated that network elements within communication system 100 are configured in well known manners with processors, memories, instruction sets, and the like, which function in any suitable manner to perform the function set forth herein.

As shown, remote unit 113 communicates with base stations 101 and 102 via uplink communication signals 118 and 1 19, respectively. Additionally base stations 101 and 102 communicate with remote unit 1 13 via downlink communication signals 116 and 117, respectfully. In the preferred embodiment of the present invention, base stations 101 and 102 are suitably coupled to CBSC 103, and CBSC 103 is suitably coupled to MSC 104.

In accordance with the preferred embodiment of the present invention, once poor communication with a first base station begins (e.g., base station 101), remote unit 1 13 will establish communication with a second base station (e.g., base station 102) prior to the call being dropped (i.e., prior to the RF-loss timer expiring). In particular the communication with the first base station on a first channel is temporarily abandoned while a remote unit 1 13 uses normal access procedures to set up a new traffic channel with the second base station. In most CDMA communication systems, remote unit 1 13 may be placed into simultaneous communication with more than one base station, which is termed "softer handoff or "soft handoff depending on if the base stations are co- located or not, respectively. When the new traffic channel is assigned, the remote unit is placed in 2-way soft/softer handoff with both the first and the second base stations, utilizing both the first and the second channels for communication.

As discussed above, in prior-art communication systems, the set up of soft/softer handoff links is accomplished via messaging that takes place over common traffic channels. Unfortunately, once a traffic channel experiences poor channel conditions, it may be impossible for the proper messaging to take place so that an additional soft/softer handoff traffic channel can be established. In the preferred embodiment of the present invention, all messaging for the additional traffic channel takes place by utilizing normal call origination schemes. More particularly, a random access channel (RACH) is utilized to set up mobile calls. Since call re-origination takes place by utilizing an access channel, the problem with the prior art is eliminated, and a soft/softer handoff leg can be set up even though poor traffic channel conditions are being experienced by the remote unit. Additionally, because the soft/softer handoff link is set up prior to the first traffic channel being dropped, the user will not be required to redial the person they were talking to if the original traffic channel drops.

FIG. 2 is a flow chart showing the operation of the communication system of FIG. 1 in accordance with the preferred embodiment of the present invention. For discussion of FIG. 2, it is assumed that remote unit 1 13 is actively communicating with a base station utilizing a traffic channel (first channel type). The logic flow begins at step 201 where a determination is made if the traffic channel between a first base station (e.g., base station 101) and a remote unit (e.g., remote unit 1 13) has degraded. In the preferred embodiment of the present invention the degradation is determined to have occurred after a predetermined number (e.g., 12) consecutive bad, or poor- quality down-link frames have been received by remote unit 1 13. If, at step 201 it is determined that the traffic channel between base station 101 and remote unit 113 has degraded, the logic flow continues to step 203, otherwise the logic flow returns to step 201.

At step 203 it is determined if communication has not been reestablished between base station 101 and remote unit 113. More particularly, at step 203 it is determined if communication has not been reestablished after a predetermined period of time t_\. If, at step 203 it has been determined that the traffic channel has not been re-established between remote unit 113 and base station 101 after a predetermined period of time, then the logic flow continues to step 205, otherwise the logic flow returns to step 201. At step 205, remote unit 1 13 saves the current call state information, (e.g. existing traffic channel Walsh code assignments, timing advance or frame synchronization, layer 2 message sequence information, . . . etc.) and retunes to the control channel to receive downlink overhead information necessary to originate a call (i.e., establish a traffic channel) at a second base station (e.g., base station 102). In particular, information for authentication, initial power level, and power level step size for access probes is obtained in order to establish a traffic channel with base station 102.

Continuing, at step 206, remote unit 113 utilizes a second channel type (access channel) to establish a traffic channel between remote unit 113 and base station 102. At step 207 a traffic channel is established between base station 102 and remote unit 1 13. Upon establishment of the traffic channel with base station 102, both remote unit 113 and base station 102 put the new base station into the remote unit's active set along with the first base station. In particular, remote unit 113 and all base stations in communication with remote unit 1 13 keep track of an active set for the remote unit. Any one of the base stations in the active set can be selected by the remote unit to transmit for a given frame interval.

From the moment that base station 102 is placed into the active set, normal call processing continues (step 209). As part of normal call control, if the first traffic channel is not capable of re-establishing communications it will be removed from the active set via standard methods. If communication fails with the first traffic channel, communication will continue with all other base stations in the active set (i.e., base station 102). Since call re-origination takes place by utilizing an access channel, the problem with the prior art is eliminated, and a soft/softer handoff leg can be set up even though poor traffic channel conditions are being experienced by the remote unit. Additionally, because the soft/softer handoff link is set up prior to the first traffic channel being dropped, the user will not be required to redial the person they were talking to if the original traffic channel drops. FIG. 3 is a block diagram of a communication system illustrating call re-origination in accordance with the preferred embodiment of the present invention. At step 1 base station 101 and remote unit 113 are actively communicating via uplink and downlink traffic channels. At step 2 remote unit detects that the quality of the downlink traffic channel has degraded. Once the degradation of the downlink traffic channel has been detected, remote unit 113 transmits to a second base station (base station 102) on an uplink access channel so that a call can be originated with base station 102 (step 3). Finally, at step 4 remote unit 1 13 is communicating with both base stations via uplink and downlink traffic channels. If the quality of the first traffic channel does not improve, it will be removed from the active set via normal call procedures.

FIG. 4 is a flow chart showing the operation of the remote unit of FIG. 1 in accordance with the preferred embodiment of the present invention. The logic flow begins at step 401 where remote unit 1 13 is actively communicating with base station 101 via uplink and downlink traffic channels. At step 403 remote unit 1 13 determines if the downlink traffic channel has degraded. As discussed above, the degradation of the downlink traffic channel is determined by the reception of twelve consecutive bad downlink frames. If at step 403 it is determined that the downlink traffic channel has degraded then the logic flow continues to step 405, otherwise the logic flow returns to step 403. At step 405, remote unit 113 initiates an RF-loss timer and an access timer (t|). Remote unit 113 then determines if the access timer has expired (step 407), and if so the logic flow continues to step 409, otherwise the logic flow returns to step 407. At step 409 remote unit 1 13 saves the current call state information, retunes to the control channel, and receives downlink overhead information necessary to establish a new traffic channel on a second base station. Remote unit 113 then establishes a new traffic channel using a standard access probe sequence (step 41 1). More particularly, remote unit 1 13 transmits an Origination Message as an access channel burst or probe to the base station transceiver. The base station then assigns both an uplink and a downlink traffic channel to remote unit 113. Thus, in accordance with the preferred embodiment of the present invention, prior to dropping the first traffic channel, communication takes place with a second base station via an access channel in order to establish a second traffic channel with the second base station. Upon completion of the traffic channel assignment process, both the subscriber and the base stations will continue with normal call processing. For example, if the first traffic channel does not improve, the first traffic channel will be dropped and communication will continue on the second traffic channel established between remote unit 1 13 and base station 102. If, however, the first traffic channel improves, remote unit 1 13 will continue in soft/softer handoff with both base stations 101 and 102.

While the invention has been particularly shown and described with reference to a particular embodiment, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention. For example, the above described invention has the flexibility to wait until the call drops to start the re- origination process. In doing so, the RF-loss timer is simply set equal to the access timer f . Additionally, situations may exist where it is impossible to establish a second channel. For example, the RF environment may be poor for both the access and the traffic channels. If this situation exists, the RF-loss timer will expire prior to a second channel being established. The traffic channel will then be dropped and the user will be required to manually redial. It is intended that such changes come within the scope of the following claims.

Claims

1. A method for re-originating a call, the method comprising the steps of: communicating with a first base station on a first traffic channel; determining that the first traffic channel has degraded; prior to dropping the first traffic channel, communicating with a second base station via an access channel in order to establish a second traffic channel; and establishing a second channel as a result of the step of communicating with the second base station via the access channel.

2. The method of claim 1 further comprising the step of simultaneously communicating with the first and the second base stations via soft/softer handoff communication.

3. The method of claim 1 wherein the step of communicating with the first base station comprises the step of communicating with the first base station via a downlink traffic channel.

4. The method of claim 1 wherein the step of determining that the first traffic channel has degraded comprises the step of determining if a predetermined number of consecutive bad frames have been received.

5. A method for call re-origination in a code-division multiple-access (CDMA) communication system, the method comprising the steps of: receiving a first downlink traffic channel from a first base station; determining that the first downlink traffic channel has degraded; initiating an RF-loss timer in response to the determination that the first downlink traffic channel has degraded; performing call re-origination with a second base station via an access channel; establishing a second traffic channel with the second base station based on the call re-origination; and communicating via soft/softer handoff with the first and the second base stations utilizing the first and the second traffic channels, respectively.

6. The method of claim 5 wherein the step of determining that the first downlink traffic channel has degraded comprises the step of determining if a predetermined amount of poor-quality frames has been received from the first base station.

7. A method for re-originating a call, the method comprising the steps of: utilizing a first channel to communicate with a first base station, wherein the first channel comprises a first channel type; determining that the first channel has degraded; accessing a second base station utilizing a second channel, wherein the second channel comprises a second channel type; utilizing a third channel to communicate with the second base station, wherein the third channel comprises the first channel type; and simultaneously communicating via the first and the third channels with the first and the second base stations, respectively.