HK1034843B - System and method for handing off a cellular call with system and capability change indication - Google Patents

Description

System and method for adding system and service function change indication to cellular call handoff

Technical Field

The present invention relates to systems and methods for handing off cellular calls between base stations, and more particularly to adding system and service function change indications to the handoff.

A cellular mobile telecommunications system includes a mobile station that communicates with any one of a plurality of geographically spaced base stations. Each base station is located within a predetermined "cell". Each base station communicates with a mobile switching center that selectively determines which base station should handle the mobile station's call based on considerations such as signal strength between each available base station and the mobile station.

As a mobile station moves from one cell to another, the signal strength decreases. Therefore, the call must be handed off from one base station to another. This is called "handover". Ideally, the handoff is performed without interfering with the ongoing call.

When a call is handed off, the handoff may be from one cellular system to another. Using different cellular systems may result in different handling of the call. Also, the service functions of different systems are different. For example, some systems may have functions such as voice privacy, message encryption, long message automatic retransmission, or the like. It is helpful for the mobile station to know that a change has occurred in a system or service function. This may be accomplished by a mobile station automatically requesting various service functions by issuing a function update request. Ideally, this should be done before some of the functions that are not possible are activated. However, sending a function request can degrade voice quality at handoff because there is a large amount of data to transmit in the message.

The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.

Summary of the invention

In accordance with the present invention, a system and method for handing off a cellular call includes adding a system and service function change indication to the handoff.

In general terms, there is disclosed herein a handover system for handing over a cellular call with a mobile station from one base station to another base station in a cellular mobile telecommunication system, the cellular mobile telecommunication system comprising: a mobile station in communication with any one of a plurality of geographically spaced base stations; and communicating with a mobile switching center to selectively determine base stations of the base stations that handle a call with a mobile station. The handoff system includes a forward communication channel selectively associated with the one base station for transmitting data from the one base station to the mobile station. The one base station is adapted to receive instructions from the mobile switching center to handoff call processing to another base station, the instructions containing information about the serving function of the other base station. A message generator operatively associated with the one base station transmits a service function change indication to the mobile station over the forward channel and is responsive to a handoff command from the mobile switching center. The service function change indication includes information of a service function of another base station related to the one base station.

It is a feature of the invention that the service function change indication indicates whether said another base station is within the same cellular system as said one base station.

Another feature of the present invention is the indication of a change in service capability indicating whether the other base station has a different service capability than the base station.

It is a further feature of the present invention that the forward channel comprises a forward analog voice channel and the message generator transmits the service function change indication as part of a mobile station control message. The service function change indication comprises two selection bits in the mobile station control message.

It is a further feature of the present invention that the forward channel comprises a forward digital traffic channel and the message generator transmits the service function change indication as part of a handoff message. In one case, the service function change indication is sent as part of a dedicated digital traffic channel handoff of the message.

In accordance with another aspect of the present invention, a method of handing off a cellular call of a mobile station from any one of a plurality of geographically spaced base stations distributed by a cell to another base station in a cellular mobile communication system including the mobile station communicating with any one of the base stations is disclosed. The base stations communicate with a mobile switching center to selectively determine which of the base stations handles a call with a mobile station. The method comprises the following steps: said one base station transmitting data to the mobile station over a forward communication channel selectively in contact with said one base station; the one base station receiving a handover command from a mobile switching center to handover call processing to another base station, the command including information on a service function of the other base station; and the one base station transmitting a message containing a service function change indication containing information of another base station related to the one base station and a service function to the mobile station through a forward channel in response to the handover command from the mobile switching center.

Further features and advantages of the invention will be readily apparent from the description and the accompanying drawings.

Brief description of the drawings

FIG. 1 is a perspective view of a cellular mobile telecommunications system; and

fig. 2 is a flow diagram of a method and system for handing off a cellular call in accordance with the present invention.

Detailed description of the invention

Referring initially to fig. 1, a cellular mobile telecommunications system 10 of conventional construction is illustrated. As is well known, the cellular system 10 is defined by a plurality of cells including representative cell locations A, B, C and D. Each cell a-D contains a respective base station 12, 13, 14 and 15. Each of the base stations 12-15 communicates with a mobile switching center 16 through an inter-cell trunk 17. The mobile station shown at 18 may comprise, for example, a mobile telephone carried by the user in an automobile. It is apparent, however, that the mobile station may be any form of known mobile station capable of communicating in the cellular mobile telecommunications system 10.

The telecommunications system 10 may process a call with a mobile station 18 by selecting one of the base stations 12-15. As shown, the mobile station 18 is located within cell D. Thus, the ongoing call will be handled by the base station 15. If the mobile station 18 is stationary, the call will likely be handled by the base station 15 until completion. However, if the mobile station is mobile, it may cross into other different cells. For example, as shown in the figure, the mobile station 18 may move in the direction indicated by the dashed line 20 and cross into cell B, then cell a. The typical cellular mobile telecommunications system 10 utilizes handoff to handoff calls from base station 15 to base station 13 and thereafter from base station 13 to base station 12 as the mobile station crosses from cell D to the corresponding cell B and cell a.

The cellular telecommunication system 10 itself does not form part of the present invention. And the present invention relates to adding system and service function change indications to handoffs.

The IS-136.2 standard currently constitutes a compatible standard for cellular mobile telecommunications systems. The standard ensures that the mobile station can obtain service in any cellular system produced in accordance with the standard. The IS-136.2 standard IS incorporated herein by reference.

In the illustration of fig. 1, handoff generally occurs when the mobile station 18 crosses from cell D to cell B. However, the handoff need not occur strictly when the mobile station crosses the boundary. But when the one base station 15 detects a certain deterioration of the communication signal with the mobile station 18. As a result, the one base station 15 issues a handover request to the mobile switching center 16. Without knowing the direction of movement of the mobile station 18, the mobile switching center 16 sends out handoff measurement requests to neighboring base stations, such as base stations 12, 13 and 14. Each of the base stations 12-14 communicates with a mobile station 18. The base stations 12-14 send measurement responses to the mobile switching center 16. The mobile switching center 16 then sends a channel assignment to the base station 15, which channel assignment contains information about another one of the base stations 12-14 to which the call is to be handed off. The base station 15 then sends a handoff command to the mobile station 18 over the forward communication channel. As a result, the mobile station reprograms itself to then communicate with other base stations, such as base station 13.

The present cellular system 10 includes both analog and digital systems. For analog systems, the mobile station 18 communicates with a particular base station, such as base station 15, over an analog voice channel during a call. As is well known, the forward analog channel carries a data stream from the base station 15 to the mobile station 18. The reverse analog voice channel carries a data stream from the mobile station 18 to the base station 15. The mobile station control message is the only message transmitted over the forward voice channel. The mobile station control message includes one or more words that define a communication, as described in IS 136.2, section 3.7.2.1. Mobile station control messages are periodically sent to the mobile station containing information for processing the call, including handoffs.

Digital cellular systems use reverse and forward Digital Traffic Channels (DTCs) to carry user information and signaling between base stations and mobile stations. As described in IS 136.2, the forward digital traffic channel joins two separate control channels, namely a fast secondary control channel (FACCH) and a slow secondary control channel (SACCH).

There are two types of handoff in digital systems. Normal handoff and dedicated handoff. For normal handover, a handover message is sent from the base station to the mobile station over the FACCH channel. For dedicated digital traffic channel handoff, a differential message is sent by the base station to the mobile station to instruct the mobile station to move from one digital traffic channel to another.

When the mobile station IS on a digital traffic channel, it assumes itself to have the service functions of the system to which the DTC belongs, as described in IS-136.2. The basis for these assumptions is in part in the control channels that the mobile station camps on before being assigned to a DTC. However, when a mobile station is handed off to another DTC, it is not always known explicitly whether the new DTC and the old DTC belong to the same system. It is difficult to judge the service function of the new channel. Also, it is desirable for the mobile station to know whether the channel being handed off has the same service capability as the channel it is leaving. This information can be used to make a more efficient search, for example, when a call ends.

In accordance with the present invention, the handoff procedure under the IS-136.2 standard IS modified to enable the mobile station to be informed whether the new channel and the old channel belong to the same system and/or have the same service function.

If the mobile station is unaware of the service functions of the new system, it may attempt to perform certain functions with the result that the system does not respond, which may cause the user interface of the mobile station to malfunction, or cause communication errors between the mobile station and the system, resulting in the call being degraded or dropped. Although the mobile station may query the system service functions after each handoff, the voice quality is degraded at each handoff.

Thus, a system and method are contemplated in accordance with the present invention for directly informing a mobile station of any change in service capabilities of the system and/or new channel during handoff. The system and method adds some of the necessary messages and procedures that enable the cellular system to inform the mobile station during handoff of the change in the system to which the new channel is connected and the change in the service capabilities of the new channel, both relative to the channel currently being used to receive the handoff message.

According to the invention, a 2-bit field called service function change indication is defined to carry change information. The 2-bit field is used to specify the new channel at handoff as follows:

description of the invention	Code
		Same system, same function	00
Same system, different functions	01
		Different systems, same function	10
Different systems, different functions	11

Referring to fig. 2, a flow chart illustrates how a function change indication (CCI) is transmitted from a current base station to a mobile station.

Obviously, the handover takes place in the middle of a call, as indicated by ring 30 in fig. 2. Referring to the example, a call in progress by the mobile station 18 is handled by the base station 15. The decision block 32 periodically determines whether the signal strength from the mobile station 18 is low. If not, call processing continues until completion. If decision block 32 determines that the signal strength is low such that further processing of the call will be degraded, a handoff request is issued to mobile switching center 16 at block 34. The mobile switching center then decides whether or not other base stations should be selected for further processing of the call. Decision block 36 then begins a self-loop until a handoff command is received from mobile switching center 16. Upon receipt of the handoff command, block 38 determines from the information in the handoff command whether the channel to which the call is to be handed off is within the same system and has the same service function. And then generates a CCI message. Block 40 then generates an appropriate call type message based on the type of channel currently processing the call. The handoff message is then sent in block 42. As a result, the mobile station will be switched to the new channel, ending the process with the one base station in block 44.

There are generally four types of handoff that may occur. The handoff may be from one Analog Voice Channel (AVC) to another AVC or to one Digital Traffic Channel (DTC). On the other hand, a handoff may be from one DTC to one AVC or to another DTC. If the call is from one DTC to another DTC, the handoff may be a normal handoff or a dedicated handoff.

As described above, the forward analog voice channel (FVC) transmits mobile station control messages as described in IS-136.2. The mobile station control message contains one or more words. The first word defines the different information units and bit lengths. Mobile station control message Word 1(Word 1) contains several reserved bits. In accordance with the present invention, mobile station control message word 1 is modified such that two of the reserved bits are replaced by CCI and the PSCC data field previously used in word 2 is also replaced by CCI, as follows:

mobile station control message word 1

Information unit		Length (position)
				T1T2＝10		2
SCC		2
				SCC＝11		SCC≠11
Information unit	Length (position)	Information unit	Length (position)
				PSCC	2	PSCC	2
EF(＝0)	1	EF(＝0)	1
				RSVD＝00000000/DVCC	8	RSVD＝0..0	2
LOCAL/MSGTYPE	5	CCI	2
				ORDQ	3	DTX	1
ORDER	5	PVI	1
				P	12	MEM	1
		VMAC	3
						CHAN	11
		P	12

Word 2-digital channel allocation

Information unit	Length (bit)
		T1T2＝01	2
MEM	1
		PM	1
CCI	2
		SBI	2
TA	5
		DMAC	4
CHAN	11
		P	12

The description of the data fields other than CCI IS as described in section 3.7.2.1 of the IS-136.2 standard.

As currently performed, a handoff from one DTC to one AVC or a normal handoff from one DTC to another uses the handoff messages described in section 3.7.3.1.3.2.4 of IS-136.2. There is no space available in this message to incorporate CCI information. In this case, although CCI is possible to replace other bits, CCI information will not be added according to the present invention, and the mobile station will assume that the new channel belongs to the same system and has the same service function as the old channel.

Finally, for dedicated DTC to DTC handoffs, the operation IS performed in accordance with the dedicated digital traffic channel handoff described in section 3.7.3.1.3.2.20 of the IS-136.2 standard, which defines the format of the message. According to the invention, CCI is added to the message and parameter type code as follows:

message format

Information unit	Type (B)	Length (bit)
			Protocol authentication	M	2
Message type	M	8
			Radio frequency channel	M	11
ATS	M	4
			DVCC	M	8
DMAC	M	4
			Time adjustment (Time Align)ment)	M	5
Short pulse indication (short Burst Indicator)	M	2
			PV	M	4
Indication of SOC change	M	1
			BSMC change indication	M	1
DTX control	M	1
			Residual Length (remainingLength)	M	6
Delta time	O	21
			Voice mode (Note 1)	O	16
Data security mode (Note 1)	O	11
			Message encryption mode C (Note 1)	O	19
Target Hyperband (Note 2)	O	12
			Service function Change (CCI) (Note 3)	O	12

Note 1: without this information element, the mobile station will assume that the feature set of the current DTC is reserved for the new DTC. Note 2: without this information element, the mobile station would assume that the new DTC is in the same super band as the current DTC. Note 3: without this information element, the mobile station would assume that the new DTC is on the same system and has the same service capabilities as the current DTC.

Parameter type code for optional information unit

Type of parameter	Code
		Delta time	0001
Voice mode	0010
		Data privacy mode	0011
Message encryption mode C	0100
		Purpose super band (Target super band)	0101
Service function change	0110

Section 2.7.3.1.3.3 of IS-136.2, which describes the information element, IS modified as follows:

field(s)	Length (position)
		Type of parameter	4
Number of values	6
		Service function change indication	2

The term "forward communication channel" as used herein refers to a forward analog voice channel in analog systems and a forward dedicated traffic channel in digital systems.

Thus, in accordance with the present invention, a system and method are disclosed for adding system and service feature change indications to a handoff when the cellular call is handed off.

Claims (14)

1. A handoff system for handing off a cellular call with a mobile station from any one base station to another base station in a cellular mobile telecommunication system, said cellular mobile telecommunication system comprising: a mobile station capable of communicating with any one of a plurality of geographically spaced, cell-wise distributed base stations; and a base station communicating with a mobile switching center to selectively determine which base station handles a call with the mobile station, the handoff system comprising:

a forward communication channel selectively associated with said one base station for communicating data from said one base station to said mobile station;

a base station adapted to receive instructions from the mobile switching center to handoff processing of a call to another base station, the instructions including information of a service function of the other base station; and

a message generator operatively associated with said one base station for transmitting a service capability change indication to said mobile station over said forward communication channel in response to a handoff command from said mobile switching center, said service capability change indication containing service capability information of another base station associated with said one base station.

2. The handoff system of claim 1 wherein: the service function change indication indicates whether the other base station belongs to the same cellular system as the one base station.

3. The handoff system of claim 1 wherein: the service capability change indication indicates whether the other base station has a different service capability than the one base station.

4. The handoff system of claim 1 wherein: the forward communication channel comprises a forward analog voice channel and the message generator transmits the service function change indication as part of a mobile station control message.

5. The handoff system of claim 4 wherein: the indication of change of function includes two selected bits in the mobile station control message.

6. The handoff system of claim 1 wherein: the forward communication channel comprises a forward digital traffic channel and the message generator transmits the service function change indication as part of a handoff message.

7. The handoff system of claim 1 wherein: the forward communication channel comprises a forward digital traffic channel and the message generator transmits the service function change indication as part of a dedicated digital traffic channel handoff message.

8. A handoff method for handing off a cellular call with a mobile station from any one base station to another base station in a cellular mobile telecommunication system, the cellular mobile telecommunication system comprising: a mobile station capable of communicating with any one of a plurality of geographically spaced, cell-wise distributed base stations; and a base station communicating with a mobile switching center to selectively determine which base station handles a call with the mobile station, the method comprising the steps of:

said one base station transmitting data to said mobile station over a forward communication channel selectively associated therewith;

the one base station receiving a handoff instruction from the mobile switching center to handoff processing of the call to another base station, the instruction including service function information about the other base station; and

said one base station is responsive to a handoff command from said mobile switching center to generate a message including a service function change indication including information about the service function of said another base station associated with said base station for transmission to said mobile station over said forward communication channel.

9. The method of claim 8, wherein: the service function change indication indicates whether the other base station and the one base station belong to the same cellular system.

10. The method of claim 8, wherein: the service capability change indication indicates whether the other base station and the one base station have different service capabilities.

11. The method of claim 8, wherein: the forward communication channel comprises a forward analog voice channel and the generating step transmits the service function change indication as part of mobile station control information.

12. The method of claim 11, wherein: the service function change indication comprises two selected bits in a first word in a mobile station control message.

13. The method of claim 8, wherein: the forward communication channel comprises a forward digital traffic channel and the generating step transmits the service function change indication as part of a handoff message.

14. The method of claim 8, wherein: the forward communication channel comprises a forward digital traffic channel and the generating step transmits the service function change indication as part of a dedicated digital traffic channel message handoff.