AU710404B2

AU710404B2 - Mobile station locating and handover arrangement for a digital cellular telephone

Info

Publication number: AU710404B2
Application number: AU58291/96A
Authority: AU
Inventors: Ying-Chun Lui
Original assignee: HONG KONG TELECOM CSL Ltd
Current assignee: HONG KONG TELECOM CSL Ltd
Priority date: 1995-05-30
Filing date: 1996-05-30
Publication date: 1999-09-16
Anticipated expiration: 2016-05-30
Also published as: CN1191661A; GB9510867D0; WO1996038998A1; EP0829180A1; AU5829196A

Description

WO 96/38998 PCT/GB96/01281 MOBILE STATION LOCATING AND HANDOVER ARRANGEMENT FOR A DIGITAL CELLULAR

TELEPHONE.

The invention relates to digital cellular telephone networks.

Mobile telephone networks are already well established in many geographical regions enabling relatively small and light telephones to be in general use. The smallness and lightness is due in the main part to using relatively low powered transmission between the telephones and fixed communications transmitters and receivers. The communications transmitters and receivers are generally evenly dispersed geographically around the region of the network in an array at base stations and connected to a central network control station which serves to control the communications between all the base stations and the mobile telephones. The central control station allocates frequencies and channels and selects appropriate base stations at the beginning of each telephone call for individual communications. Handover arrangements are provided by the central controller to vary or alter the channels and frequencies to connect the telephones to communicate with different base.stations during each call according to prevailing circumstances. The central controller also monitors and records the calls for user billing.

Normally, the base stations are arranged so as to provide areas of suitable transmission and reception over a number of hexagonal side by side areas or so-called "cells". The WO 96/38998 PCT/GB96/01281 2 carrier frequencies used by one cell is different to any carrier frequencies in an immediately adjacent cell but are the same as the frequencies used in another cell in the network somewhat remote and usually separated by a further two cells of the network. Thus a honeycomb type pattern of communication cells can be conveniently formed using say seven different sets of carrier frequencies. Of course, as is already known, if the mobile telephone moves from one cell to another during a call, the control station is capable and designed to transfer or handover the mobile telephone to communicate at appropriate different frequencies.

At present this is determined simply by signal strength comparisons as reported between the cell with which the telephone is communicating with at any time and adjacent cells. If the signal strength of an adjacent cell increases more than a predetermined amount above the signal strength of the communicating cell, the central controller will handover the mobile telephone to the adjacent cell.

Such arrangements are not sufficiently reliable in practice especially for networks required to operate in difficult terrains adjacent coastal areas of totally open spaces, adjacent mountains and/or in densely populated areas with high rise buildings.

In EP-A-0454638, a locating arrangement is disclosed to determine that a mobile telephone is in a special area where the serving station may have difficulty in serving that special area, within its normal area of transmission, by using signals from other base stations and by centrally storing information concerning the location of such a special area.

It is an object of the invention to provide a network and a mobile telephone that can more satisfactorily operate in difficult terrains.

ooooe According to one aspect of the invention there is n provided a digital cellular telephone network having a central controller and a number of mobile telephones characterised by a mobile telephone locating arrangement wherein a first base station is chosen to •serve over a special geographical area of the network S. and two or more other base stations in the system are selected for use in establishing that a mobile telephone is in the special area, in which the mobile e. telephone is arranged to measure the downlink strength *ooof signals from the first base station and from the two or more selected base stations and to communicate the signal strengths to the first base station for onward transmission to the central controller, in which the central controller compares the difference between the downlink strength of the first base station with the downlink strengths of the two or more selected base stations to identify that the mobile telephone is in the special area.

According to another aspect of the invention there is provided a digital cellular telephone network wherein the mobile telephone is characterised by means for measuring the strength of downlink signals received from the first base station when said mobile telephone is located in the special area over which the first station serves and the downlink strengths of the two or more selected base stations from the special area, and means for transmitting these signal strengths to the first base station.

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030 WO 96/38998 PCT/GB96/01281 6 Handover arrangements incorporating a special locating fascility for digital cellular telephone networks and telephones for such networks according to the invention will now be described by way of example with reference to the accompanying diagrammatic drawings in which Figure 1 is an idealised plan view of a cellular network; Figure 2 is another idealised plan view of a cellular network; Figure 3 is a plan view of part of a typical cellular network; Figure 4 is a schematic side view of another cellular network showing a mobile telephone located in a special area; and Figure 5 is an idealised plan view of a cellular network having some cells with super re-use carrier frequencies.

Digital cellular mobile telephone networks are already known and typically each cell is arranged to transmit at three different carrier frequencies providing ei,-ht (time division multiplexed) channels per carrier frequency. Two channels of one of the carrier frequencies is used for control signals and the rest of the one carrier frequency and all the channels of the other two carrier frequencies WO 96/38998 PCT/GB96/01281 7 are used for voice or data transmission, so-called "traffic". The channels are arranged in pairs and one channel of each pair is used for transmission to a mobile telephone (downlink signals) and the other channel of the pair is used for transmission from the mobile telephone (uplink signals).

The mobile telephone is normally connected for communications at first to its nearest cell and allotted a pair of channels by the central network controller for communication of traffic. This is done by exchanges of messages on the control channels in accordance with wellpractised procedures already known. In embodiments of the present invention each mobile telephone is arranged to Periodically respond to signals from other cells and relay to the central controller via the cell radio links the signal strengths of signal received from the other cells (downlink signals) in a manner to be described below. The purpose of receiving and measuring the strength of various downlink signals, including its own downlink signal, is to provide information that the central controller can use to "handover" the particular mobile telephone to another cell. This is to ensure that the quality of the communications remains satisfactory as conditions change which will usually be due to the mobile telephone moving closer to a neighbouring cell or possibly into a poor localised reception area of the cell presently in communication with the mobile telephone.

WO 96/38998 PCT/GB96/01281 8 In Figure i, in the cellular network the control channel frequencies are re-used every seven cells (Cl to C7). The normal traffic channels frequencies are re-used every four cells (V1 to V4). In Figure 1, level of re-use for the traffic channels is significantly higher than the level of re-use of the control channels but it will be noted that the "potential interferers" have control channels using different frequencies. "Potential interferers" is used to identify the cells or base stations in the system using the same set of traffic channel frequencies, so that the signals from potential interferers are potentially strong enough to cause interference within the cell or base station area presently communicating with the mobile telephone.

During the progress of a call, the mobile telephone periodically measures the downlink signal strength of the serving base station and those of potential interferers.

All measurement results are reported to the central controller which calculates the C/I ratio of the in-use radio link according to the following method

L

s downlink signal strength of the serving base station Lis downlink signal strength of the 1st potential interferer Lis 2 downlink signal strength of the 2nd potential interferer WO 96/38998 PCT/GB96/01281 9 S Lism downlink signal strength of the mth potential interferer The C/I ratio of the in-use radio link is calculated as C/I

L

s (Lis 1 Lis 2 Lism) When the C/I ratio falls below a predetermined level, the central controller will handover the mobile telephone to another cell. This type of measurement is sometimes referred to as "direct C/I ratio measurement".

In Figure 2, the level of re-use of the traffic channel frequencies is the same as that of the level of re-use of the control channel frequencies. In the Figure the same traffic channels and the control channel are re-used every seven cells.

When using the network in Figure 2, during the progress of a call the mobile telephone periodically measures the downlink signal strength of the serving base station and those of "reference" base stations. "Reference" base stations is used to identify a set of neighbouring base stations on different control-channel frequencies and are specially selected for estimating the signal level of likely interferers. All these measurement results are reported to the central controller which calculates the WO 96/38998 PCT/GB96/01281 10 approximate C/I ratio of the in-use radio link according to the following method: Ls k downlink signal strength of the kth potential interferer (to be estimated)

L

s downlink signal strength of the serving base station.

Lrk downlink signal strength of the 1st reference base station for the kth potential interferer.

Lrk2 downlink signal strength of the 2nd reference base station for the kth potential interferer.

Lrkx downlink signal strength of the xth reference base station for the kth potential interferer.

The interference level of the kth potential interferer is approximated as Lisk MAX (Lrk I (Lrk2 a 2 (Lrk ax) where al, a2, ax are fixed quotients or scaling factors. The scaling factors are pre-set by prior measurements taken for network and provide necessary adjustment so that the composite signal contour of reference base stations after scaling is approximately equal to that of the kth potential interferer within the service area of the serving base station. This is illustrated in Figure 3.

WO 96/38998 PCT/GB96/01281 11 The C/I ratio of the in-use radio link is therefore calculated as C/I L, (Lisl Lis 2 Lism When the C/I ratio falls below a predetermined level, a handover is arranged by the central controller. Such type of criteria is sometimes referred to as based on the "approximated C/I ratio".

Some special circumstances arise for particular locations or terrain as illustrated in Figure 4. Thus, across a planned service area boundary, there can exist a number of neighbouring base stations that exhibit a change in signal Strength relative to the serving cell. A typical situation is depicted in Figure 4 where there is a requirement to establish that the mobile telephone is in a particular or special area.

Base station A is planned to illuminate or serve a part of a road on a mountain road but due to the position of antennas, the base station also covers part of the ground areas. The road is exposed to radio signals from many other base stations so only very few frequencies are clean enough to be assigned to the base station A. It is necessary to prevent traffic from ground level being captured by the base station A otherwise it will run into congestion. To determine where the traffic is coming from, the signals from selected remotely located base WO 96/38998 PCT/GB96/01281 12 stations B and C are used as references. On the mountain, the mobile telephone is in line of sight with base stations B and C so their downlink signal strengths relative to that of the serving base station is high. On ground area, the situation is reversed as signals from base stations B and C are blocked off by buildings.

Thus, in operation the mobile telephone is arranged to monitor the signal strength and pass the information to the central controller which will determine the relative location of the mobile telephone and decide in this embodiment whether a handover is actually necessary or not.

The signal strength information is dealt with as follows:- L

CS

Lcs2 downlink signal station downlink signal base station downlink signal base station strength of the serving base strength of the 1st selected strength of the 2nd selected downlink signal strength of the base station xth selected xth selected The mobile telephone is in the planned service area if and WO 96/38998 PCT/GB96/01281 13 only if: Positive Logic Negative Logic Les1 L, d I Ls L d S or Ls 2 Ls >d2 and Ls 2 d or x L >d x and L s X L dx where d 2 dx are constants, the values of which are determined from the predicted or measured signal strength of the serving base station and those of selected base stations inside and outside the planned service area.

Thus the handover is subject to what can be referred to as "service area control".

A typical logic programme for determining handovers generally is automatically carried out by the central controller as follows:- A radio traffic channel is set up between a mobile telephone and a serving base station.

The mobile telephone periodically measures the downlink signal strength of the serving base station and those of other base stations in accordance with the information received from the network. All measurement results are reported to the central controller via the serving base station. Measurement results include: WO 96/38998 PCT/GB96/01281 14 For the serving base station: Lu uplink signal strength Ld downlink signal strength u uplink quality Qd downlink quality D distance between the mobile and the serving base station For other base stations: Cn1' C n2 C base station identity nL' n2' Lnk downlink signal strength As new measurement results are received, the central controller checks whether handover is necessary by looking at the triggering function: T L,LdQuQd,D ,C C 1 n2' Cnk Ln, Lnk) If T 0, handover is necessary, proceed to step Go back to step otherwise.

As handover is determined to be necessary, from the k non-serving base stations in the measurement reports, the central controller selects a number of base stations as potential handover targets by using the respective qualifying function: Qni L,LdQuQd,D, Cn1,Cn, Cnk, L ,Ln2,...,Lnk Qn, is a function parameterized to determine whether the base station identified by Cni is qualified as a handover target. If Qni 0, the base station is qualified.

WO 96/38998 PCT/GB96/01281 15 The central controller determines the priority of qualified handover targets by using the priority function: Pni LuLdQuQdD, C Cn1Cn2' Cnk Ln 1 Ln, ,Lnk Pn is a function parameterized to determine the relative priority of the handover target identified by Cni The central controller commands the mobile to handover to the target base station having the highest priority.

For handover programmes using the system described with reference to Figure i, the central controller is programmed to operate as follows:- When a radio traffic channel has been set up between a mobile telephone and a serving base station, all potential interferers of the serving base station are picked out from the k non-serving base stations in the signal strength measurement reports.

Lis downlink signal strength of the 1st potential interferer of the serving base station.

Lis 2 downlink signal strength of the 2nd potential interferer of the serving base station.

WO 96/38998 PCT/GB96/01281 16 Lism downlink signal strength of the mth potential interferer of the serving base station The C/I ratio for the serving base station can then be calculated: CI Ld is Lis 2 Lis m The triggering function T is defined as: T ci_threshold

CI

where ci_threshold is a predetermined minimum acceptable c/I ratio.

T 0 means that the C/I radio is worse than the minimum acceptable level so handover to another better base station is necessary.

For the potential handover target identified by CN, the associated potential interferers are picked out from the k non-serving base stations in the measurement reports.

Lin downlink signal strength of the 1st potential interferer of base station identified by C.

nj Lj2 downlink signal strength of the 2nd potential interferer of base station identified by Cn..

downlink signal strength of the mth pote......ntial Linim downlink signal strength of the mth potential WO 96/38998 PCT/GB96/01281 17 interferer of base station identified by C The C/I ratio for the base station identified by can then be calculated: CInj Ln Linjl Linj2 Linjm) The associated qualifying function is defined as: Q CInj ci_threshold Q 0 means the C/I radio is better than the minimum acceptable level so the base station is qualified to be a handover target.

In practice, a priority function of the base station identified by Cnj is defined as Pnj CInj The priority of potential handover targets is determined solely on the corresponding C/I ratio. The better the C/I ratio and hence the signal quality, the higher the priority.

It is also possible to determine handover on the basis of approximate C/I ratios as explained earlier with reference to Figure 2. This is carried out by picking out from the k non-serving base stations in the measurement report, all reference base stations for estimating the C/I ratio of the serving base station.

WO 96/38998 PCT/GB96/01281 18 LrS1 downlink signal strength of the 1st reference base station for estimating the C/I ratio of the serving base station Lrs2 downlink signal strength of the 2nd reference base station for estimating the C/I ratio of the serving base station Lrs x downlink signal strength of the xth reference base station for estimating the C/I ratio of the serving base station The C/I ratio for the serving base station is then calculated in the central controller programme.

CIs Ld MAX [(Lr 1 as) (Lrs 2 as2) (Lrsx as,)] The triggering function T for handover is defined as: T Cithreshold CIs where ci_threshold is pre-defined minimum acceptable

C/I

ratio.

The potential handover target is identified by C This is picked out from the associated reference base stations from the k non-serving base stations in the measurement WO 96/38998 PCT/GB96/01281 19 reports.

L

1 rnij Lnj downlink signal strength of the 1st reference base station for estimating the C/I ratio of the base station identified by Cnj.

downlink signal strength of the 2nd reference base station for estimating the C/I ratio of the base station identified by C Lrjx downlink signal strength of the xth reference base station for estimating the C/I ratio of the base station identified by C,.

The C/I ratio for the base station identified by Cnj can then be calculated: CIn Ln/ MAX [(Lrnj anj,), (Lrn 2 anj 2 (Ln *a The associated qualifying function is defined as: Q CI.n ci_threshold Q 0 means that the C/I radio is better than the minimum acceptable level so the base station is qualified to be an handover target.

The priority function of the base station is identified by Cnj and defined as: Pnj CIn nj nj WO 96/38998 PCT/GB96/01281 20 The priority of potential handover targets is determined solely on the corresponding C/I ratio. The better the C/I ratio and hence the signal quality, the higher the priority.

It was earlier mentioned with reference to Figure 4 that certain special arrangement may be required in difficult terrains or locations. Normally locating a mobile telephone in special area is to do with deciding handover occurrences but it may be useful or required for other reasons, such as deciding whether super re-use frequencies, mentioned later, can be used or whether a different C/I ratio should be employed to determine handover decisions. Where handover decisions are made in accordance with location the following programme is provided:- From the k non-serving base stations in the measurement reports, all selected remotely located base stations for estimating whether the mobile is within the planned service area of the in-use base station are picked out.

L =s downlink signal strength of the 1st selected base station for estimating whether the mobile is within the planned service area of the in-use base station.

LCs 2 downlink signal strength of the 2nd selected base station for estimating whether the mobile WO 96/38998 PCT/GB96/01281

L

cs, 21 is within the planned service area of the in-use base station.

downlink signal strength of the xth selected base station for estimating whether the mobile is within the planned service area of the in-use base station.

The triggering function T 0 or handover function is provided if: Positive Logic LC Ld dsl or Lcs 2 Ld ds 2 1 c s d 2 o or Ls x Ld d csx d sx Negative Logic Ls 1 Ld ds, and Ls 2 Ld ds2 and L Ld d csx dsx T 0 means that the mobile is outside the planned service area of the in-use base station so handover to another base station is necessary.

The potential handover target is identified by the associated reference base stations are chosen from the k non-serving base stations in the measurement reports.

L cnjl downlink signal strength of the 1st selected base station for estimating if the mobile is within the planned service area of the base WO 96/38998 PCTIGB96/01281 Lcnj2 22 station identified by C,. nj" downlink signal strength of base station for estimating within the planned service station identified by Cn..

the 2nd selected if the mobile is area of the base Lcnx cnix downlink signal strength of base station for estimating within the planned service station identified by C.

nj.

the xth selected if the mobile is area of the base The qualifying function Q is greater than 0 if: Positive Logic Lcnjl nj njl or Lcnj2 Ln j dnj2 cnj2 n nj2 or Lcnj Ln j d nx enx ni njx Negative Logic L L d cnjl n j cnj2 ni"n "nj and L c- L d cnjx njni dnjx Q 0 means that the mobile is with the planned service area of the base station identified by CN so it is qualified as an handover target.

For prioritizing functions such factors as signal strength, traffic loading and C/I ratio are applied.

The described handover arrangements can be applied to underlay-overlay cellular networks. Referring to Figure WO 96/38998 PCT/GB96/01281 23 the cellular network has an underlay-overlay configuration where some of the cells have super re-used carrier frequencies. The operating spectrum of the network is divided into two groups, namely a regular-reuse group and a super-reuse group. The regular-reuse frequencies are reused over safe distance such as that in 7-cell reuse pattern and are intended to serve mobile telephones near the cell boundary where the C/I ratio is usually the worst. As the name implies, the super-reuse frequencies are reused very intensively to produce a required increased capacity.

Some of the ordinary base stations are equipped with both types of frequencies. Super-reuse frequencies allocated to a base station are divided into several groups with each group having different sources of interference.

Based on the profile of interference to which each mobile telephone is exposed to, the central controller determines the most appropriate frequency group to be assigned for carrying traffic.

Stand-alone micro cells with antenna height well below the roof level may be equipped solely with super-reuse frequencies. By establishing appropriate handover connections, a micro cell at good location can effectively absorb the traffic of more than one ordinary base stations in its vicinity. With such an arrangement, the surrounding base stations are usually referred to as WO 96/38998 PCT/GB96/01281 24 "parent base stations" and the micro cell is referred to as "child base stations".

The conventional frequency selection mechanism used with underlay-overlay configuration is only based on the signal strength of the serving base station. A strong signal means that the mobile is close to the base station and the system will assign a super-reuse frequency for a call.

Otherwise, a regular-reuse frequency will be used. The arrangement works well over flat, homogeneous terrains but in interference limited environment, signal strength alone is no indication in practice of the susceptibility to interference. In open areas, the signal level is usually high but so is the chance of interference. When the signal is weak, the mobile telephone may be inside buildings and well shielded from interference.

A frequency selection mechanism using embodiments of the invention is therefore more effective. The improved handover arrangements of the present invention may be applied to selection of frequencies. The procedures are as follows: At call set up, the central controller assigns a regular-reuse frequency.

As signal strength measurements become available during the progress of the call, the central WO 96/38998 PCT/GB96/01281 25 controller checks if sufficiently clean super-reuse frequencies are available (including those in associated child base stations) by using qualifying functions based on: direct C/I ratio measurement (Figure 1) approximated C/I ratio (Figure 2) service area control (Figure 4) If clean super-reuse frequencies are found, the central controller prioritizes the usable super-reuse frequencies according to the respective number of idle traffic channels. The more the number of idle traffic channels, the higher the priority.

The central controller hands over the mobile telephone to the super-reuse frequency with the highest priority.

On super-reuse frequencies, the central controller constantly monitors the in-use radio links to see if it is necessary to return to regular-reuse frequencies. Handover triggering functions based on the following criteria can be used: direct C/I ratio measurement approximated C/I ratio service area control If returning to regular-reuse frequencies is WO 96/38998 PCT/GB96/01281 26 necessary as indicated by the handover triggering function, the central controller evaluates every regular-reuse frequency of the serving base stations and those in its neighbourhood by using qualifying functions and prioritizing functions based on direct C/I ratio measurement approximated C/I ratio service area control The central controller hands over the mobile telephone to the regular frequency with the highest priority.

Go back to step In the described arrangements, downlink signal strengths of control signals are measured and used to determine the handover criteria and in Figure 4 the location of the mobile telephone. It will be appreciated that downlink signal strengths of the traffic channels might also be employed additionally or alternatively.

WO 96/38998 PCT/GB96/01281 Drawing Legend 26a A 1 tier of co-control channel base stations B t tier of co-traffic channel base stations C t tier of co-control channel and co-traffic channel base stations D potential handover target station E reference base station

A

F interfering base station G service area boundary H reference base station

B

I signal level contour of the interfering base station J zone of interference within the service area K scaled signal level contour of reference base stations A and B L unintended (-ve logic)/intended (+ve logic) service area of base station A or (LI -L)>d (LCI-LAl)>da M unintended (+ve logic)/intended (-ve logic) service area of base station A and (L2-L1) (LC2-L 2 >dc N base station

A

0 reference base station

B

P reference base station

C

Q 1 co-channel cell tier for regular-reuse freq.

R It co-channel cell tier for super-reuse freq. in parent base station S t co-channel cell tier for child base station SUBSTITUTE SHEET (RULE 26)

Claims

2. A digital cellular telephone network according to claim 1, wherein the mobile telephone is characterised by means for measuring the strength of downlink signals received from the first base station when said mobile telephone is located in the special area over which the first station serves and the downlink strengths of the two or more selected base stations from the special area, and means for transmitting these signal strengths to the first base station. Kj I
3. A digital cellular telephone network according to claim 1 further characterised by a handover arrangement in which handover is determined on the basis of a C/I ratio being above a predetermined level, in which C is the downlink signal strength of the one or more potential handover target base stations.
4. A digital cellular telephone network according to claim 2 or 3, in which the downlink strengths are measured on control channels of the network.
5. A digital cellular telephone network according to any one of claims 2 to 4, in which the selected base stations for the handover arrangement comprise base stations which are the nearest other base stations in the network having the same carried frequency as the "first base station. *9.e 20 6. A digital cellular telephone network according to any one of claims 3 to 5, in which the selected base stations for the handover arrangement are preselected other base stations geographically associated with a potential handover target base station.
7. A digital cellular telephone network according to any one of claims 2 to 6, in which the downlink strengths of the selected base stations are modified before being summated by selected quotients or scaling factors relating to previous calculations or measurements of the network. WO 96/38998 PCT/GB96/01281 29
8. A mobile telephone for a digital cellular telephone network according to any one of claims 1 to 7, including means for continuously periodically measuring the strength of downlink signals received from a first base station when the mobile telephone is located in a special area over which the first station serves and the downlink strengths of two or more other base stations remotely located from the special area, and means for transmitting these signal strengths via the first base station to the central controller.
9. A mobile telephone according to claim 8, for use in association with handover arrangements, including means for continuously periodically measuring the strength of downlink signals received from one base station and the strengths of downlink signals received from a number of selected other base stations, and means for transmitting these measurements to the one base station, in which the measuring means is arranged to measure the strength of downlink signals from the surrounding base stations operating at the same carrier frequency as the one base station. A mobile telephone locating arrangement for a digital cellular telephone network substantially as herein described with reference to Figure 4 of the accompanying drawings.
11. A mobile telephone for a digital cellular telephone network incorporating a telephone locating arrangement substantially as herein described with reference to Figure 4 of the accompanying drawings. DATED this Twelfth day of July 1999. HONG KONG TELECOM CSL LTD By its Patent Attorneys FISHER ADAMS KELLY .9 S *e A S A a i '2) U, .7<