HK1033788B - Method of handing-off a remote unit based on overhead information start times - Google Patents

Description

Method for switching remote units based on additional information start time

Technical Field

The present invention relates generally to a method for handing off a remote unit from a first base station to a second base station in a communication system.

Background

Digital cellular systems are beginning to provide multiple bearer services that simultaneously use a common infrastructure. Mobile stations, commonly referred to as remote units, also support multiple bearer service capabilities. The handover function may be split differently between the mobile station and the cellular structure for different bearer services. As used herein, handover refers to the process of transferring a remote unit from a first base station to a second base station. For example, when the mobile station is operating on a traffic channel allocated by the infrastructure, e.g., for voice or circuit-switched data, the infrastructure will control the handover of the remote unit. This switching may occur based on input from the remote unit or in a form separate from the remote unit. In other cases where the remote unit is using a shared channel, such as for packet-based data, control and access channels, or short message service, the remote unit may autonomously control the handoff.

For an autonomous remote unit handoff, the remote unit needs to obtain information related to the new cell as the remote unit hands off to another RF entity, such as a neighboring cell. Examples of such information include configuration of the cell, configuration of the service at the new cell, or other configuration parameters. In some cases, the remote unit may also need to obtain such information multiple times. This situation occurs when more than one channel is allocated to carry packet data activity in a cell. A user who is to be handed over to that cell will first obtain additional information from the primary channel assigned to packet data and then determine the particular ancillary packet data channel that it must use based on the additional information of the primary channel. The remote unit will then obtain additional information from the attached packet data channel before beginning operation on the attached channel. It is important to reduce the time required to complete the handover, particularly for data services where any data lost or erroneously delivered during the handover must be retransmitted.

Disclosure of Invention

Therefore, there is a need for a method of handover for a remote unit that must obtain additional information from a target cell that reduces the amount of data that is delayed or lost during handover, thereby reducing the time required to perform the handover.

The present invention provides a method of handing off a remote unit from a first base station to a second base station, the method comprising: communicating, by a remote unit, with a first base station; determining a need to handover the remote unit to a second base station; determining an additional information start time, which is a time when the additional information is to be transmitted from the second base station; the remote unit is handed off to the second base station based on the additional information start time, the method further comprising the step of determining a transfer time, the transfer time being the time required for the remote unit to transfer from the first base station to the second base station.

The present invention also provides an apparatus for implementing a handover of a remote unit from a first base station to a second base station in a communication system, the apparatus comprising a logic unit for determining an additional information start time, the additional information start time comprising when additional information is to be transmitted at the second base station, for handing over the remote unit to the second base station based on the additional information start time, and for determining a transfer time, the transfer time being a time required for the remote unit to transfer from the first base station to the second base station.

Brief description of the drawings

Fig. 1 depicts a wireless communication system in accordance with a preferred embodiment of the present invention.

Fig. 2 depicts the operation of the communication system of fig. 1 for additional information start time in accordance with a preferred embodiment of the present invention;

fig. 3 depicts the operation of the communication system of fig. 1 for additional information start time in accordance with another embodiment of the present invention;

fig. 4 is a flow chart illustrating operation of the communication system of fig. 1 in accordance with the preferred embodiment of the present invention.

Description of The Preferred Embodiment

The present invention provides a method of handing off a remote unit from a first base station to a second base station. The remote unit communicates with a first base station. The remote unit establishes communication with the second base station. It is then preferably determined by the remote unit that the remote unit should handoff to the second base station. Then, an additional information start time, i.e., a time when the additional information is to be transmitted from the second base station, is determined. In a preferred embodiment, the remote unit determines when additional information is to be broadcast by the second base station, thereby establishing an additional information start time. In another embodiment, the first base station and the second base station are synchronized to transmit the additional information at approximately the same time. In this manner, the remote unit can determine the additional information start time at the second base station by monitoring the additional information start time at the first base station. The remote unit is then handed off to the second base station based on the additional information start time.

The present invention may be better understood with reference to fig. 1-4. Fig. 1 depicts a wireless communication system 100 in accordance with a preferred embodiment of the present invention. Communication system 100 is preferably a Personal Digital Cellular (PDC) system, but may alternatively be any other digital cellular communication system, such as Code Division Multiple Access (CDMA), a mobile expert Group (GSM) system, Time Division Multiple Access (TDMA), next generation CDMA, and next generation CDMA protocols including direct sequence spread spectrum or slow frequency hopping spread spectrum systems. Communication system 100 includes a first base station 101, a second base station 102, a remote unit 113, a Central Base Station Controller (CBSC)103, and a Mobile Switching Center (MSC) 104. Communication system 100 may also include additional base stations, but only 2 are shown for simplicity. Remote unit 113, first base station 101 and second base station 102 each include a logic unit capable of processing information, such as determining the start time of additional information and determining whether a handoff of remote unit 113 is required.

In the preferred embodiment of the invention, base stations 101 and 102 are Motorola SC9600 base stations, MSC 104 is a Motorola EMX2500 MSC, and CBSC 103 is a Motorola SG1128BF CBSC component. As shown, remote unit 113 communicates with base stations 101 and 102 via uplink communication signals 119, and base stations 101 and 102 communicate with remote unit 113 via downlink communication signals 116. In the preferred embodiment, base stations 101 and 102 are adapted to be coupled to CBSC 103, and CBSC 103 is adapted to be coupled to MSC 104.

The preferred operation of communication system 100 occurs as follows: remote unit 113 accesses first base station 101. Typically this is done by scanning for the strongest signal and obtaining a synchronization channel or information on the overloaded channel broadcast by the base station 101. During communication with the first base station 101, the remote unit 113 acquires a signal from the second base station 102. The determination to handoff the remote unit 113 to the second base station 102 is made based on a predetermined condition, such as receiving a stronger signal from the second base station 102. Such determination may be implemented in remote unit 113, first base station 101, or second base station 102. Both base stations 101 and 102 transmit additional information. It is desirable to enable remote unit 113 to handoff to second base station 102 in such a manner that reduces the time required for remote unit 113 to obtain additional information from base station 102. When base station 102 has more than one channel allocated to a service used by remote unit 113, remote unit 113 will obtain additional information from the primary channel providing such service in base station 102. Based on the additional information from the primary channel, remote unit 113 will move to the designated auxiliary channel and obtain the additional information from the auxiliary channel.

2 embodiments of the invention are given. In a preferred embodiment as depicted in fig. 2, the base stations 101 and 102 are synchronized to transmit the additional information at approximately the same time. The additional information is thus time aligned and provides additional information start times at base stations 101 and 102. The synchronization of the transmission start times of the additional information is effected via a central clock, for example the global positioning system, or via control communication between the base stations.

In another embodiment as depicted in fig. 3, the remote unit 113 determines when additional information is transmitted from the second base station 102. Base stations 101 and 102 are unsynchronized; however, each base station knows when neighboring stations broadcast their additional information.

As used herein, additional information refers to information related to the communication system. The additional information is broadcast to all remote units in the service area of that system. However, in a communication system the additional information is different in different service areas, because the additional information provides data specific to that service area. Examples of such information are system ID, cell site ID, paging area ID and information about neighboring cells. For packet data transmission, such additional information must include a packet partition ID for registration purposes and information on channels in neighboring cells. This allows the remote unit to reduce the time it takes to acquire a new channel from another cell.

The remote unit 113 remains at the first base station 101 a time before the transmission of the additional information start time in the second base station 102. The remote unit 113 switches in time to receive additional information from the second base station 102. In this manner, the time required for remote unit 113 to obtain additional information is reduced.

The additional information between cell sites and the additional information on the primary and supplemental channels within a cell site are staggered by a predetermined time. The amount of time that the additional information data is interleaved between the primary and secondary channels is equal to the sum of the time that the current additional information is read and the time that the remote unit 113 acquires the secondary channel. Other time intervals may be used by the system depending on the handover method used by that particular system.

The following is a description of how a remote unit according to the preferred embodiment moves to the primary packet channel at the destination site, reads the additional information and moves further to the secondary packet channel to be used for user data forwarding. As shown in fig. 4, the remote unit determines (401) that it needs to switch. The remote unit selects an appropriate time to move from the first base station to the second base station. Preferably, the remote unit is moved from the current channel of the first base station to the primary packet channel of the second base station. Since the additional information at the first base station and the second base station is staggered a known amount of time, the remote unit initiates (402) a handoff so that the remote unit acquires and arrives at the primary packet channel of the second base station when the additional information at the second base station is about to be transmitted. This is accomplished by determining a transfer time (t) for the remote unit_acq) To accomplish this, the transfer time is the time required for the remote unit to transfer from the first base station to the second base station. Thus, if the additional information on the target channel is at time T₀₂Is available with the remote unit at T₀₂Previous t_acqAnd the second starts the switch. The remote unit 113 reads and processes 403 the additional information. The remote unit then determines (404) whether it should stop on the primary packet channel or whether it should move to the secondary packet channel. If so, the remote unit receives (405) data services on the primary packet channel. However, the remote unit may determine that it needs to acquire an adjunct packet channel to use the packet data service. The main channel additional information start time of the main channel is transmitted at a first time. The auxiliary channel additional information start time of the auxiliary channel is transmitted at the second time. The remote unit obtains (406) an accessory packet channel. The remote unit moves from the primary channel to the auxiliary channel based on the additional information transmitted by the primary channel. The accessory packet channel (407) provides additional information to the remote unit immediately after the remote unit arrives.

From the foregoing examples, 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. The system may require the remote unit to read the additional information from a channel other than the initial packet channel. The remote unit will be directed by this information to a packet channel.

Thus, the present invention provides a method of handing off a remote unit from a first base station to a second base station. By determining the additional information start time at the second base station to handover the remote unit, the remote unit can switch to the second base station in time to receive the start of the next additional information transmission. In this manner, the remote unit is handed off to the two base stations in an efficient manner. The remote unit thus switches in an efficient manner to begin listening for additional information at the second base station without delay.

Further, the present invention keeps the amount of transmitted additional information to a minimum amount on a channel broadcasting the additional information. The present invention also takes into account changes in the network configuration that result in additional information changes being communicated to the remote unit in a timely manner. The present invention may be used for any channel shared by remote units, such as control and access channels.

Although the present invention has been described with reference to a Personal Digital Cellular (PDC) system, the present invention may also be used in any wireless system that performs automatic handover and reads additional information from more than one channel during the handover process. For example, the present invention may be used in Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA) systems, Cellular Digital Packet Data (CDPD) systems, mobile expert Group (GSM) systems, next generation CDMA systems, and next generation CDMA protocols including direct sequence spread spectrum or slow frequency hopping spread spectrum systems.

While this invention has been described in terms of certain examples thereof, it is not intended to be limited to the above description, but only to the extent set forth in the claims that follow.

Claims (8)

1. A method of handing off a remote unit from a first base station to a second base station, the method comprising:

communicating, by a remote unit, with a first base station;

determining a need to handover the remote unit to a second base station;

determining an additional information start time, which is a time when the additional information is to be transmitted from the second base station;

based on the additional information start time, handing off the remote unit to the second base station,

the method further comprises the step of determining a transfer time, the transfer time being the time required for the remote unit to transfer from the first base station to the second base station.

2. The method for handing off a remote unit in accordance with claim 1, wherein the step of handing off the remote unit further comprises handing off the remote unit to the second base station based on the transfer time.

3. The method of handing off a remote unit in accordance with claim 1, wherein the step of determining a need to hand off the remote unit comprises determining the need at the remote unit.

4. The method of handing off a remote unit in accordance with claim 1, wherein the step of determining an additional information start time comprises determining the additional information start time at the remote unit.

5. An apparatus for implementing a handover of a remote unit from a first base station to a second base station in a communication system, the apparatus comprising a logic unit for determining an additional information start time, the additional information start time comprising when additional information is to be transmitted at the second base station, for handing over the remote unit to the second base station based on the additional information start time, and for determining a transfer time, the transfer time being a time required for the transfer of the remote unit from the first base station to the second base station.

6. The apparatus according to claim 5, wherein the remote unit comprises said logic unit.

7. The apparatus according to claim 5, wherein the first base station comprises said logic unit.

8. The apparatus according to claim 5, wherein the second base station comprises said logic unit.