KR100372052B1 - Method for hand-off hard inter-frequency in mobile communication system - Google Patents

Abstract

The present invention provides a method for hard-to-frequency hard handoff in a mobile communication system that enables hard-to-frequency hard handoff without using pilot beacons by using candidate frequency search provided by the IS-95B standard. As described above, the present invention relates to determining a candidate frequency at a base station controller (CCP) and instructing the mobile station to search for a candidate frequency when it is determined that the frequency is a hard handoff. In addition, the candidate frequency response message is reported to the base station controller (CCP), and the base station controller (CCP) requests periodic or one-time start of candidate frequency search and configures only the PNs having a CF_T_ADD or higher in the candidate frequency search report. Pass it to the controller. In addition, the hard handoff procedure between FAs is determined based on the PN strength of the candidate frequency search set reported by the base station controller, and the base station control processor (Target BCP) corresponding to the new active set using the candidate FA as the target FA. Request a channel to be allocated, allocate a corresponding channel with FA information and sector information in a target BCP, configure an active set with allocated channel information in a base station controller, and instruct hard handoff between FAs. In this case, the mobile station transmits the traffic channel preamble to the target CE through the target FA, thereby performing a hard handoff between frequencies using a candidate frequency search function.

Description

Method for hand-off hard inter-frequency in mobile communication system

The present invention relates to an inter-frequency hard handoff method in a code division multiple access (CDMA) mobile communication system. In particular, the present invention relates to a candidate frequency search provided by the IS-95B standard. The present invention relates to a method for hard-to-frequency hard handoff in a mobile communication system capable of performing a hard-to-frequency hard handoff without a pilot beacon.

In general, in a CDMA mobile communication system conforming to the IS-95A standard, when a mobile station receives a traffic channel and starts a call, the CDMA channel cannot be changed until a new handoff occurs. In addition, other CDMA channels cannot be searched until a new handoff occurs.

The CDMA base station system also unevenly allocates the number of CDMA channels that can be serviced between adjacent base stations. For example, increase the number of CDMA channels in crowded urban centers and relatively reduce the number of CDMA channels in quiet suburbs. Therefore, the problem of handoff between neighboring base stations allocated unevenly to each other has always been a problem.

In a base station with a large number of CDMA channels, a mobile station assigned a CDMA channel that is not served by a base station with a small number of CDMA channels cannot handoff the pilot channel of the corresponding CDMA channel from a base station with a small number of CDMA channels when moving to a base station with fewer CDMA channels. Cannot occur.

1 is a diagram for explaining the above case.

In this case, there is no pilot beacon, and the mobile station moves between adjacent base stations having different numbers of CDMA channels. Where FA stands for Frequency Assignment, which is equivalent to the number of CDMA channels.

When the mobile station moves to base station B while receiving a 2FA allocation from the base station A, which is a congested area, the mobile station does not receive any signal because the base station B does not serve the 2FA, and the call is disconnected.

In order to solve this problem, conventionally, as shown in FIG. 2, a pilot beacon is installed in a base station having a small number of CDMA channels to attract a mobile station so as to smoothly perform an inter-frequency handoff.

This is the case when the mobile station moves between adjacent base stations having a CDMA channel number in the presence of a pilot beacon.

When the mobile station moves to base station B while receiving a 2FA allocation from the base station A, which is a congested area, the mobile station detects a pilot channel transmitted by the base station B because the pilot beacon is installed at the base station B's 2FA. At this time, the mobile station requests a handoff by measuring the pilot strength and transmitting a pilot strength measurement message (PSMM) to the base station controller. Upon receipt of the PSMM, the base station controller requests handoff channel allocation to base station B. At this time, since the base station B has only the pilot beacon installed in the 2FA currently assigned to the mobile station, the base station B allocates a channel of IFA or 0FA and transmits a handoff direction message to the mobile station.

In addition, as shown in FIG. 1, another method for preventing a call disconnection occurring when a mobile station moves between adjacent base stations having a different number of CDMA channels, as shown in FIG. 1, is a round trip between a mobile station and a base station. When the round trip propagation delay increases or the frame error rate increases by using a round trip delay and a frame error rate, the base station itself having a large number of CDMA channels changes the CDMA channel and moves to a neighboring base station.

That is, FIG. 3 illustrates a case in which there is no pilot beacon, the CDMA channel is changed and then handoffed to the neighbor base station.

When the mobile station moves to base station B while receiving a 2FA allocation from the base station A, which is a congested area, the mobile station moves away from the base station A, thereby increasing the round trip propagation delay or the frame error rate. Having determined this situation, the base station controller controls the base station A to allocate a CDMA channel available to the neighboring base station to instruct the mobile station to handoff only the CDMA channel at the base station A.

The mobile station that has changed the CDMA channel from 2FA to 1FA detects a pilot channel emitted by 1FA of base station B and transmits a PSMM to the base station controller while moving toward base station B and requests a handoff. The base station controller receiving the PSMM requests handoff channel assignment to base station B to control the mobile station to handoff to base station B.

However, the conventional inter-frequency handoff method has the disadvantage of requiring additional equipment called a pilot beacon.

In addition, when using a round trip propagation delay or a frame error rate without installing a pilot beacon, a handoff decision has many disadvantages.

That is, even though the frame error rate and the round trip propagation delay may be large, if the handoff decision is made simply by increasing the frame error rate or the round trip propagation delay, an error may occur in the handoff decision.

Accordingly, the present invention has been proposed to solve various problems occurring when handoff between frequencies in the conventional CDMA mobile communication system.

An object of the present invention is to use a candidate frequency search provided by the IS-95B standard to perform a hard hand-to-frequency hard handoff in a mobile communication system without a pilot beacon. To provide an off method.

In the mobile communication system according to the present invention for achieving the above object, the frequency between the hard handoff method,

Candidate frequency searching, supported by the IS-95B standard, allows for more accurate inter-frequency handoffs without additional pilot beacons. Here, the candidate frequency search is a function for the mobile station to temporarily tune to another frequency during a call to search for pilot signal strength and Eb / No (energy-to-noise ratio) detected in the changed frequency band, and then return to the original frequency to maintain the call. to be.

Therefore, if the candidate frequency search function is used, handoff between frequencies can be efficiently performed by using detailed information such as the strength of the pilot and the average reception strength detected in the candidate frequency search measured by the terminal.

FIG. 1 is an explanatory diagram for explaining a call disconnection phenomenon during hard handoff between frequencies when a pilot beacon is not present in a conventional mobile communication system.

2 is an explanatory diagram for explaining a process of performing a hard handoff between frequencies with a pilot beacon in a conventional mobile communication system,

3 is an explanatory diagram for explaining a process of performing a hard handoff between frequencies when there is no pilot beacon in the conventional mobile communication system.

4 is a schematic structural diagram of a mobile communication system to which the present invention is applied;

5 is a flowchart illustrating a process for determining a possibility of hard handoff between FAs in the present invention;

6 is a state transition diagram showing a state transition process for processing a hard handoff between FAs in a base station controller according to the present invention;

7 is a flowchart illustrating a method for hard handoff between frequencies in a mobile communication system according to the present invention.

<Explanation of symbols for the main parts of the drawings>

100 ..... Mobile station

200 ..... Base Station (BTS)

210 ..... Channel Elements (CE)

220 ..... Base station control processor

300 ..... Control station

310 ..... Transcoder and Selector Banks

320 ..... Base station controller

Target CE ..... Target Channel Element

Target BCP ..... Target Base Station Control Processor

Hereinafter, a preferred embodiment of the present invention according to the technical spirit as described above will be described in detail with reference to the accompanying drawings.

4 is a schematic structural diagram of a CDMA mobile communication system to which the present invention is applied.

Here, reference numeral 100 denotes a mobile station (MS) capable of making a call while moving, reference numeral 200 denotes a base station (BTS) that is wirelessly connected to the mobile station 100 to perform channel assignment and wireless communication interface, and reference numeral 300 Represents a control station (BSC) that controls the base station 200 and controls handoff.

The base station 200 includes a channel element (CE) 210 and a base station control processor (BCP) 220 that controls the overall operation of the base station.

The control station 300 also includes a transcoder and selector bank (TSB) 310 for performing the vocoding function, and a base station controller (CCP) 320 for controlling the base station 200.

In the mobile communication system to which the present invention configured as described above is applied, the mobile station 100 transmits the PSMM to the base station 200 for handoff when the pilot strength falls below a threshold value during the call, and the base station 200 receives the PSMM. Transmits the PSMM to the control station 300.

Upon receiving the PSMM, the control station 300 determines whether to handoff and, if determined to be handoff, searches for an adjacent base station and then orders an appropriate base station for handoff.

If a channel is allocated by the discovered base station receiving the handoff command, the inter-frequency handoff is successfully completed.

The present invention is intended to more accurately perform the inter-frequency handoff without additional pilot beacons by using the candidate frequency search function supported by the IS-95B standard in the CDMA mobile communication system.

Looking at the content of the IS-95B, by providing the mobile station with the ability to search other FA, other band class or analog frequency in addition to the current FA, it provides a candidate frequency search function to improve and stabilize hard handoff have.

It also provides the ability to periodically search for frequencies that are currently being served.

To this end, IS-95B adds the following messages and orders.

Forward call channel: Periodic Pilot Measurement Request Order, Candidate Frequency Search Request Message, Candidate Frequency Search Control Message, General Handoff Direction Message.

Reverse call channel: Periodic Pilot Strength Measurement Message, Candidate Frequency Search Response Message, Candidate Frequency Search Report Message.

In the above, the Periodic Pilot Measurement Message is a pilot accumulated for one PN chip period for the total power spectral density in the Ec / Io (receive band) of each pilot of the active set of the service FA currently used by the mobile station. By passing the threshold for energy ratio (dB) and the energy (Ec) of each pilot in the active set of the service FA, the terminal sends a Periodic Pilot Measurement Message when the reception strength falls below this threshold. Indicates a message for transmitting the signal reception environment of the FA to the base station.

Next, the Candidate Frequency Search Request Message is transmitted to the terminal with information necessary to search for candidate frequencies such as a search method, a frequency range to be searched, a threshold of reception power for determining when to search, and a size of a search window. Represents the message used to do this.

Next, the Candidate Frequency Search Control Message indicates a message for the terminal to start searching for a candidate frequency.

In addition, the General Handoff Direction Message indicates a message instructing the UE of various types of handoffs including hard handoff between FAs.

In addition, the Periodic Pilot Strength Measurement Message is a response to the Periodic Pilot Measurement Request Order transmitted by the base station, and searches for the service frequency, and the signal density of the entire band of the service frequency when the frequency falls below the pilot power threshold presented by the Periodic Pilot Measurement Request Order. Density), the pilot receive power of the active set, and so on.

In addition, the Candidate Frequency Search Response Message transmits a time required for searching for a candidate frequency (that is, a time for stopping a communication channel of the current service frequency) to the base station.

In addition, the Candidate Frequency Search Report Message indicates a message used to report an average reception power of a service frequency, an average reception power of a candidate frequency, a pilot reception strength of a candidate frequency search set, and the like as a result of searching for a candidate frequency. This result value is useful for determining handoff at the base station.

The state transition for processing the hard handoff between FAs in the base station controller using the candidate frequency search function of the IS-95B standard as described above is shown in FIG. 6.

As shown in the figure, FAHO_TRAFFIC_STATE (S100) for determining whether a hard handoff between FAs in a call state, FAHO_PPSMM_STATE (S200) for transmitting a PPSMM to a control station, FAHO_CFSRES_STATE (S300) for controlling candidate frequencies, and candidate frequencies FAHO_CFSREP_STATE (S400), which is a discovery and handoff channel assignment state, FAHO_CH_ALLOC_STATE (S500), which is a step of instructing handoff to a mobile station, and FAHO_COMPLETE_STATE (S600), which is a step of completing a handoff.

First, in FAHO_TRAFFIC_STATE (S100), once a call is set up or after a handoff, the terminal communicates with a new active base station, it is possible to determine the possibility of hard handoff between FAs.

Here, the algorithm for determining the possibility of hard handoff between FAs is shown in FIG. 5.

As shown, after the first call is set up in step S10 or after a new handoff has been successfully made, the shape information of the neighbor base station of the current active set is read to check whether the FA of the active set is acceptable.

As a result of the inspection, it is determined in step S20 whether the current FA is acceptable.

As a result of this determination, if the current FA is acceptable, it is determined that there is no possibility of handoff between FAs in step S30, and waits until the next handoff.

On the contrary, if the current FA is not acceptable, it is determined that there is a possibility of handoff between FAs in step S40, and a Periodic Pilot Measurement Request Order is transmitted to the mobile station.

That is, the criterion for handoff possibility is whether the CDMA channel of the service frequency currently being serviced by the terminal is serviceable in neighboring base stations of the base station with which the terminal communicates.

If it is determined that there is a possibility of a hard handoff between FAs, a Periodic Pilot Strength Measurement Request Order is sent to the mobile station so that the mobile station sends a Periodic Pilot Strength Measurement Message when the signal strength of the frequency currently being served falls below a certain value. .

Next, in FAHO_PPSMM_STATE (S200), when a mobile station moves to an adjacent base station that does not currently serve a CDMA channel, the mobile station transmits a Periodic Pilot Strength Measurement Message when the signal strength of the corresponding CDMA channel falls below a threshold given in the Periodic Pilot Strength Measurement Request Order. do.

The CCP of the control station receiving the Periodic Pilot Strength Measurement Message sends a Candidate Frequency Search Request to the mobile station to search for the corresponding frequency by selecting a CDMA channel that can be serviced by all neighboring base stations of the active base station of the mobile station. do.

Next, in the FAHO_CFSRES_STATE (S300), the mobile station receiving the candidate frequency search request message calculates the time taken to search for the candidate frequency and sends the candidate frequency search response message to the control station. If the control station determines that the time required for searching is not abundant in the mobile station, the control station sends a candidate frequency search control message to the mobile station to instruct to search for the candidate frequency in earnest.

Next, in the FAHO_CFSREP_STATE (S400), the mobile station searches for the candidate frequency, measures the signal strength of the corresponding pilot, the average received power of the candidate frequency, and the average received power of the service frequency, and transmits the result to the candidate frequency search report message to the control station.

If the control station determines that the difference between the average received power of the service frequency sent by the mobile station and the average received power of the candidate frequency is large enough to cause a handoff, the control station examines the pilot strength of the candidate frequency included in the candidate frequency search report and performs a handoff. It requires a channel allocation for handoff to a base station corresponding to a pilot having enough strength to cause it.

Next, in FAHO_CH_ALLOC_STATE (S500), when the channel assignment is completed, the mobile station commands a handoff using a general handoff direction message.

Then at FAHO_COMPLETE_STATE (S600) the mobile station tunes the receiver and transmitter to the corresponding CDMA channel to maintain the call.

7 is a flowchart illustrating a hard handoff process between FAs of the present invention as described above.

First, in step S701, the base station controller (CCP) determines the possibility of hard handoff between FAs by searching for an ActiveSet at the time of initial call setup or after completing a previous handoff, and performs a periodic pilot search (Periodic Pilot Search). The transcoder and selector bank (TSB) are requested (PPSReq Cs).

In response to the request, the TSB transmits a message (Periodic Pilot Search Request Order (Serving FA)) instructing a periodic pilot search request to the mobile station MS in step S702.

The mobile station (MS) receiving the message transmits the spectral density and the pilot strength of the service frequency measured periodically in step S703 to the TSB (Periodic Pilot Strength Measurement Message).

The TSB transmits the value measured in step S704 to the base station controller (CCP).

The base station controller (CCP) analyzes the received value, and as a result, if it is determined that the AIR environment of the service FA is bad, it selects an appropriate candidate frequency in step S705 and requests the mobile station to search for the candidate frequency (CFSReq). Cs).

Then, in step S706, the TSB transmits a Candidate Frequency Search Request Message to the mobile station.

After calculating the search time of the candidate frequency, the mobile station MS transmits a candidate frequency search response message to the TSB in step S707. In step S708, the TSB is sent to the base station controller (CCP). The search time will be reported (CFSRes Cs).

Next, in step S709, the base station controller (CCP) requests the TSB to start periodic or one-time candidate frequency search (CFSCtrl Cs), and in step S710, the TSB prompts the mobile station to start candidate frequency search (Candidate). Send Frequency Search Control Message).

The received mobile station stores the current configuration, tunes to the candidate frequency, searches all pilots belonging to the candidate frequency search set, and retunes to the service frequency. In step S711, only the PNs of CF_T_ADD or more to the TSB are configured in the Candidate Frequency Search Report and transmitted to the TSB.

In step S712, the TSB forwards the discovery report received from the mobile station to the base station controller (CFSRep Cs).

In step S713, the base station controller determines the hard handoff procedure between FAs using the reported PN strength of the candidate frequency search set, and sets the candidate FA as a target FA to target a base station control processor corresponding to a new active set. BCP) is requested to allocate a channel corresponding to a new active set. At this time, the base station controller backs up information necessary for recovery such as active set information of the service FA (Ho Ch Alloc Req Cb).

The target BCP controls a channel element Target CE to allocate and initialize a corresponding channel with FA information and sector information received from the CCP in step S714 (tc ho assign).

The channel element (Target CE) allocates a channel in step S715, performs initialization, and transfers the initialization to the target base station control processor (cc ho setup).

Then, in step S716, the target base station control processor transmits the allocated channel information to the base station controller (Ho Ch Alloc Rsp Bc).

In step S717, the base station controller configures an active set with the allocated channel information and instructs the TSB to inter-FA hard handoff (GHDM Cs).

In steps S718 and S719, the TSB and the newly allocated channel elements Target CE are time synchronized.

After this time synchronization, the TSB instructs the mobile station to handoff to the new active set of the target FA in step S720 (General Handoff Direction Message (Target FA_new ActiveSet)).

In step S721, the mobile station delivers the traffic channel preamble to the target CE through the target FA. In step S722, the target CE (Target CE) successfully receives the traffic channel preamble received through the new reverse call channel. Notified to the TSB (vs mob acq ctl msg), and in step S723 the TSB responds to the target CE (tc ack ctl msg).

Next, in step S724, the target channel element Target CE informs that the link is established to the target BCP (cc_call_status (sell link on)).

In addition, in step S725, the mobile station transmits a handoff completion message to the target channel element (Target CE) (Handoff Completion Message), and the received target channel element is successfully performed and handoffed to the TSB in step S726. Signals that the set has been modified (Target FA_new Active Set).

In step S727, the TSB transmits a message requesting resource release to the channel element (Serving CE) of the first service FA (tc release ctrl msg). In step S728, the first service channel element releases resources and Send a response message to the TSB (vs ack ctl msg).

Then, in step S729, the TSB notifies the base station controller (CCP) that the handoff has been successfully performed (HCM Cs), thereby terminating the inter-frequency hard handoff.

According to the above-described "inter-frequency hard handoff method in a mobile communication system", the inter-frequency hard handoff is performed on the basis of information provided by the mobile station, and thus, the advantage of enabling hard-to-frequency hard handoff on an accurate basis There is this.

In addition, since it is possible to determine in which direction the mobile station moves, there is an advantage in that the base station channel can be efficiently used and the call quality can be improved.

In addition, since there is no need for an additional device such as a pilot beacon, there is an advantage that can reduce the overall system design cost.

Claims (4)

Mobile station (MS), currently serving service channel element (Serving CE) and base station control processor (Serving BCP), target service channel element (Target CE) and base station control processor (Target BCP), transcoder and selector bank (TSB), and In the mobile communication system consisting of a base station controller (CCP), the mobile station (MS) is a periodic pilot strength measurement message including a spectral density (Spectral Density) and pilot strength of the service frequency measured periodically from the base station controller ( In the inter-frequency hard handoff method of performing a hard-to-frequency hard handoff according to a predetermined procedure if it is determined in the CCP), and as a result is determined as a hard-off between frequencies, If the base station controller (CCP) determines that an inter-frequency hard handoff, selecting a candidate frequency and instructing the mobile station to search for the candidate frequency; Transmitting a message requesting a candidate frequency search from the TSB to the mobile station; Calculating a search time of a candidate frequency at the mobile station (MS), and reporting a candidate frequency response message to the base station controller (CCP) through the TSB; Requesting the TSB to start a periodic or one-time candidate frequency search at the base station controller (CCP), and transmitting a message to the mobile station to start the candidate frequency search at the TSB which has received the TSB; Configuring only PNs equal to or greater than CF_T_ADD in a candidate frequency search report by the mobile station to the base station controller through the TSB; Based on the PN strength of the candidate frequency search set reported by the base station controller, the inter-FA hard handoff procedure is determined, and the candidate FA is used as a target FA to a base station control processor (Target BCP) corresponding to the new active set. Requesting to allocate a channel; A channel element (Target CE) is controlled to allocate and initialize a corresponding channel with FA information and sector information received from the target BCP, and when the channel allocation is initialized, the allocated channel information is transmitted to the base station controller. Delivering; Configuring an active set with the allocated channel information in the base station controller and instructing the TSB to inter-FA hard handoff; Instructing the mobile station to handoff from the TSB to a new active set of target FAs; Informing the TSB that the mobile station has successfully received the traffic channel preamble received through the new reverse call channel at the target CE when the traffic channel preamble is delivered to the target CE through the target FA; Transmitting, by the mobile station, a handoff complete message to the target channel element (Target CE), and notifying the TSB of the received target channel element to indicate that the handoff was successfully performed and an active set has been modified; The TSB transmits a message requesting resource release to the channel element (Serving CE) of the first service FA and receives a response message to inform the base station controller (CCP) that the handoff was successfully performed. Inter-frequency hard handoff method in a mobile communication system comprising the step of terminating the off. delete The method of claim 1, wherein the base station controller, And requesting the base station control processor (Target BCP) to allocate a channel corresponding to a new active set, and then backing up information necessary for recovery of active set information of a service FA. A method of hard handoff between frequencies in a mobile communication system. The method according to claim 1, wherein the TSB and the newly allocated channel elements (Target CE) further comprise synchronizing time when receiving a hard handoff indication message between FAs. Hard handoff method.