AU701840B2

AU701840B2 - Mobile assisted handover arrangements for digital cellular telephone networks

Info

Publication number: AU701840B2
Application number: AU58290/96A
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: 1999-02-04
Anticipated expiration: 2016-05-30
Also published as: AU5829096A; WO1996038997A1; EP0829179A1; GB9510861D0; CN1191660A; CN1085031C

Description

PCT/GB96/01280 WO 96/38997 Mobile Assisted Handover Arrangements for Digital Cellular Telephone Networks The invention relates to handover arrangements for 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 WO 96/38997 PCT/GB96/01280 2 of hexagonal side by side areas or so-called "cells". The 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.

I

In an article "Handover Technique for a Third Generation Mobile System CHIA Mobile Radio Conference Nice, Fr, 13-15 Nov. 1991 pages 243-249, XP000444200" various general criteria are discussed for handover arrangements. In WO-A-9217953 a handover arrangement is disclosed in which specific C/I ratios are measured from time to time when handover conditions appear imminent.

It is an object of the invention to provide a network and a mobile telephone that can more satisfactorily operated in difficult terrains and also in less difficult terrains provide even better service even ee when the density of use is high.

9**o "According to one aspect of the present invention there :is provided a handover arrangement for a digital cellular telephone network having a central controller and a number of mobile telephones, in which each 20 telephone is arranged to measure and transmit downlink .9 :signal strength of a first base station and downlink signal strength measurements of other base stations on control channels of the network, and in which all these signals are passed to the central controller which determines a C/I ratio and initiates a handover of a mobile telephone from the first base station to another base station whenever this ratio falls below a predetermined value, where C equals the downlink signal strength of the first base station and I is a summation of the downlink signal strengths of the other base stations.

The controller preferably initiates the handover of the mobile telephone from the first base station to another base station only if the C/I ratio for the other base station is above the predetermined value.

The downlink strengths of the base stations are preferably modified before being summated by selected quotients or scaling factors relating to previous calculations or measurements of the network.

According to another aspect of the invention, there is provided a mobile telephone for a digital cellular telephone network having a handover arrangement defined above including means for periodically 15 measuring the strength of downlink signals received from a first base station and the strengths of downlink signals received on the control channels from a number of other base stations, and means for transmitting these measurements to the first base o: 20 station.

The measuring means may be arranged to measure the strength of downlink signals from surrounding base stations operating at the same carrier frequency as the first base station. The measuring means may also be arranged to measure the strength of downlink signals from immediately surrounding base stations operating at different carrier frequencies from the first base station.

Handover arrangements for digital cellular telephone networks and telephone 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 o network; p p o• p p o p o p -6- Figure 5 is an idealised plan view of a cellular network having some cells with super re-use carrier frequencies; and Figure 6 is a glossary for Figures 1 to Digital cellular mobile telephone networks are already known and typically each cell is arranged to transmit at three different carrier frequencies providing eight (time division multiplexed) channels per carrier frequency. Two channels of one of the carrier frequencies is used for AMENDED SHEET WO 96/38997 PCT/GB96/01280 7 control signals and the rest of the one carrier frequency and all the channels of the other two carrier frequencies 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 WO 96/38997 PCT/GB96/01280 8 localised reception area of the cell presently in communication with the mobile telephone.

In Figure 1, 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 ist potential WO 96/38997 PCTIGB96/01280 9interferer Lis2 downlink signal strength of the 2nd potential interferer Lis m 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 (Lisi Ls 52 Li m) 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 WO 96/38997 PCT/GB96/01280 10 specially selected for estimating the signal level of likely interferers. .All these measurement results are reported to the central controller which calculates the approximate C/I ratio of the in-use radio link according to the following method: Lisk

L

Lrkl Lrk 2 downlink signal strength of the kth potential interferer (to be estimated) downlink signal strength of the serving base station.

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

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

Lkx 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 Lis k MAX (Lk1 (Lrk 2 a 2 (Lrk x a) 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 WO 96/38997 PCTIGB96/01280 11 service area of the serving base station. This is illustrated in Figure 3.

The C/I ratio of the in-use radio link is therefore calculated as C/I Ls (Lisi Lis 2 L.ism 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.

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 WO 96/38997 PCT/GB96/01280 12 from, the signals from selected remotely located base 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

c Lcs downlink signal station downlink signal base station downlink signal base station strength strength strength of the serving base of the ist selected of the 2nd selected downlink signal base station strength of the xth selected WO 96/38997 PCT/GB96/01280 13 The mobile telephone is in the planned service area if and only if: Positive Loqic Negative Logic Lcsl Ls di

LCS

1 L d 1 or Ls 2 L d 2 and Ls 2

L

s d2 S or Lsx -L >d and L L d C^x s x where dl, d 2 d x 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 96/38997 PCT/GB96/01280 14 For the serving base station: L uplink signal strength Ld downlink signal strength Qu uplink quality Qd downlink quality D distance between the mobile and the serving base station For other base stations: Cn 1 i Cn2, Cnk base station identity 0 Ln1' Ln2, L 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,LdQu,QdD, Cn1,Cn2 Cnk, Ln,L Ln 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 ,Ld,Qu d,D, C 1 ,Cn Cn, Ln 1

,L

2 nk Qn is a function parameterized to determine whether the base station identified by Cni is qualified as a handover target. If Qi 0, the base station is qualified.

WO 96/38997 PCT/GB96/01280 15 The central controller determines the priority of qualified handover targets by using the priority function: Pni LLdQuQd,D, Cn1Cn2, Cnk, Ln, L ,Lnk) Pi is a function parameterized to determine the relative priority of the handover target identified by C ni The central controller commands the mobile to handover to the target base station having the highest priority.

For handover programmes in accordance with embodiments of the invention and using the system described with reference to Figure 1, 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.

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

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

WO 96/38997 PCT/GB96/01280 16 Lsm 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: CIs Ld Lis Lis 2 Lis 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 Cn.* Lj2 downlink signal strength of the 2nd potential interferer of base station identified by Cn..

WO 96/38997 PCT/GB96/01280 17 Linj downlink signal strength of the mth potential interferer of base station identified by Cr.

The C/I ratio for the base station identified by Cnj can then be calculated: CInj L LLinj i Linj Li m The associated qualifying function is defined as: Q CInj cithreshold 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 Pni CIn 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 WO 96/38997 PCT/GB96/01280 18 the serving base station.

rs downlink signal strength of the 1st reference base station for estimating the C/I ratio of the serving base station Lrs 2 downlink signal strength of the 2nd reference base station for estimating the C/I ratio of the serving base station

L

rsx °o 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.

CI Ld MAX (Lrs as) (Lrs a 2 (L a,) The triggering function T for handover is defined as: T Ci_threshold CIs where ci_threshold is pre-defined minimum acceptable

C/I

ratio.

The potential handover target is identified by C. This nj WO 96/38997 PCT/GB96/01280 is picked from the reports.

19 out from the associated reference base stations k non-serving base stations in the measurement Lrn downlink signal strength of the 1st reference base station for estimating the C/I ratio of the base station identified by Cj.

Lnj 2 downlink signal strength of the 2nd reference base station for estimating the C/I ratio of the base station identified by Cj.

Lrnjx downlink signal strength of the xth reference base station for estimating the C/I ratio of the base station identified by Cnj The C/I ratio for the base station identified by Cn can then be calculated: CIn Ln MAX [(Lnji anjl) (Lrnj 2 a 2 (Lnjx anj) The associated qualifying function is defined as: Q CInj cithreshold Q 0 means that the C/I ratio is better than the minimum acceptable level so the base station is qualified to be an handover target. In other words, provided the C/I ratio for the potential handover target station is above a predetermined value a handover can be carried out.

WO 96/38997 PCT/GB96/01280 20 The priority function of the target base station is identified by Cnj and 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 was earlier mentioned with reference to Figure 4 that certain special arrangement may be required in difficult terrains or locations. 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.

Lo downlink signal strength of the ist 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 is within the planned service area of the in-use base station.

WO 96/38997 PCT/GB96/01280 21 L L

O

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 1 0 Lcs 1 Ld ds1 or Lc 2 L ds2 or Lsx Ld dsx Negative Logic L s L dS and Ls 2 Ld ds2 and Lsx Ld ds x 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 C the associated reference base stations are chosen from the k non-serving base stations in the measurement reports.

Lnj downlink signal strength of the 1st selected base station for estimating if the mobile is within the planned service area of the base station identified by C..

Ssignal strenj Lnj2 downlink signal strength of the 2nd selected WO 96/38997 PCT/GB961280 22 cnjx base station for estimating within the planned service station identified by C nj downlink signal strength of base station for estimating within the planned service station identified by Cn.

n if the mobile is area of the base the xth selected if the mobile is area of the base The qualifying function Q is greater than 0 if: Positive Logic Lcnjl Lj dnji or Lnj L dj2 or Lnjx L.j dn anix 03nix Negative Logic L nj- Lj d and L2 L d cnj2 nj nj and L L d.

cnjx nj nix Q 0 means that the mobile is with the planned service area of the base station identified by C. 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 5, the cellular network has an underlay-overlay configuration where some of the cells have super re-used WO 96/38997 PCT/GB96/01280 23 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 "parent base stations" and the micro cell is referred to as "child base stations".

WO 96/38997 PCT/GB96/01280 24 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 controller checks if sufficiently clean super-reuse frequencies are available (including those in associated child base stations) by using qualifying WO 96/38997 PCT/GB96/01280 25 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 necessary as indicated by the handover triggering function, the central controller evaluates every regular-reuse frequency of the serving base stations WO 96/38997 PCT/GB96/01280 26 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.

It will be appreciated that in embodiments of the invention the handover criteria is determined on the basis of C/I ratios and when this ratio falls below a predetermined value for a serving base station, or otherwise, C/I ratios of potential handover target base stations are monitored to identify and choose an appropriate of those stations for handover.

WO 96/38997 PCT/GB96/01280 26a Drawing Legend A 1 tier of co-control channel base stations B 1 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 (LcI-Lil)>d( (L*Cl-.1l) >dC M unintended (+ve logic)/intended (-ve logic) service area of base station A and (L 32

-L

1 >d 1 I Lc _1 12 >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 I co-channel cell tier for super-reuse freq. in parent base station S I t co-channel cell tier for child base station SUBSTITUTE SHEET (RULE 26)

Claims

2. A handover arrangement according to claim i, 20 characterised in that the controller initiates the :handover of the mobile telephone from the first base station to another base station only if the C/I ratio for the other base station is above the predetermined value.
3. A handover arrangement according to claim 1 or 2, characterised in that the downlink strengths of the base stations are modified before being summated by selected quotients or scaling factors relating to previous calculations or measurements of the network. 28
4. A mobile telephone for a digital cellular telephone network having a handover arrangement according to claim 1 to 2, characterised by means for periodically measuring the strength of downlink signals received from a first base station and the strengths of downlink signals received on the control channels from a number of other base stations, and means for transmitting these measurements to the first base station. A mobile telephone according to claim 4, characterised in that the measuring means is arranged to measure the strength of downlink signals from surrounding base stations operating at the same 15 carrier frequency as the first base station. i
6. A mobile telephone according to claim 4, characterised in that the measuring means is arranged to measure the strength of downlink signals from 20 immediately surrounding base stations operating at different carrier frequencies from the first base station.
7. A handover arrangement for a digital cellular telephone network as hereinbefore described with reference to the accompanying drawing figures. DATED this Ninth day of December 1998. HONG KONG TELECOM CSL LTD By its Patent Attorneys FISHER ADAMS KELLY