Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W36/00—Hand-off or reselection arrangements
  • H04W36/04—Reselecting a cell layer in multi-layered cells

Definitions

• the present invention relates to a cellular mobile communica ⁇ tion system with radio base stations and mobile stations.
• the system comprises a number of cells which are arranged in two or more different layers or levels and the mobile station connections can be handed over from one cell to another.
• the system comprises means for monitoring and/or measuring at least one signal parameter of at least those cells not being in the uppermost layer and at least one threshold value is given for said signal parameter for at least each of said cells.
• the system furthermore comprises means for controlling said handovers between different cells.
• the invention also relates to a method for controlling handovers in a cellular mobile communication system wherein the cells are arranged in at least two different levels or layers.
• a communication system of this kind comprises a number of base stations which generally are organized into a network.
• Each base station serves a geographical area which is called a cell.
• the geographical area can be said to be given by the radio propagation properties of the base station and of the surrounding radio base stations.
• the system furthermore comprises one or more mobile stations and when a mobile station moves, mobile connections can be handed over from one cell to another which is known as a handover.
• a number of factors play an important role since it is of utmost importance that the most appropriate cell is selected when a handover is carried out, both under normal circumstances as under more or less extraordinary circumstances. It is therefore extremely important that the cells are organized in the most suitable way.
• One known system applies a so called “umbrella” cell treat- ment.
• Different cell layers i.e. preference layers or priority layers, are in this case arranged according to the selection and placement of the radio base stations. Powerful base stations with high antennas then constitute so called umbrella cells whereas low-power base stations e.g. mounted at street level form so called “micro-cells” whereas fur ⁇ thermore so called “pico-cells” can be arranged which then e.g. may be mounted indoors.
• the normal cell selection mechanism, locating is the mechanism responsible for providing the desired behaviour. However, no logic is dedicated to the purpose.
• the intention with the use of so called “umbrella” cells is to provide a safety net to the normal cell network by bridging coverage holes, providing spare capacity at call set up procedures and to have the function of rescue at radio disturbances etc.
• the purpose with micro-cells is to provide the main capacity, particularly in high density traffic areas.
• the system with umbrella cells there ⁇ fore direct traffic to the appropriate cells in the appro- priate layer, in order to assure call continuity and to assure a successful call set up procedure.
• the cell structure with umbrella cells without dedicated logic does not work satisfactorily when base stations belonging to different layers are placed close to each other.
• the cell selection mechanism locating, will then result in a number of unnec ⁇ essary handovers such as from a micro-cell to an umbrella cell even if the micro-cell provides radio conditions which are adequate.
• Fig. 1 which shows two micro-cells under an umbrella cell.
• the micro-cell C A will provide the highest signal strength. Consequently that cell will carry the connection.
• the mobile station MS 2 will be in radio shade from both micro-cell base stations BS 1, BS 2.
• the umbrella cell C j fulfills its purpose and provides coverage where the micro- cells C A , C B fail to do so.
• the micro-cell base station BS 2 is in line of sight and should therefore be able to carry the connection without problems.
• the umbrella cell base station BS u which also is in line of sight has a signal strength which is higher and will therefore take over the connection.
• radio disturbances and call set up congestion connections may unnecessarily be taken over or upheld by the umbrella cell or may be generally upheld or taken over by less appropriate cells. This will lead to a waste of capacity which may lead to loss of connections etc. In other words, the occupation of less appropriate resources will increase.
• an efficient frequency planning and efficient dimensioning of the hard-ware is difficult.
• the number of handovers is unnecessarily high which results in a high load on the switches and thus a non-negligible risk of loosing connections.
• GB-A-2 242 806 describes a cellular system comprising macro-, and micro-cells. Handovers are always effected via the macrocell layer in order to avoid unnecessary handovers from macrocell to microcell and back again. A handover to a lower layers is merely carried out when the link from the equipment to the base station associated with the underlying microcell has a quality which exceeds predefined criteria for a time interval that exceeds a predetermined time interval. Handover from a microcell to another microcell thus never occurs. Thus, also in this case the "wrong" resource will be occupied to a great extent.
• O-A1-92/02105 discloses a cellular radio system.
• a handover- initiation system comprises means for determining the distance of a mobile station from the base station of a cell and means for measuring the signal strength to determine the path of a mobile station.
• the system furthermore comprises means for storing information-pairs on location and signal character of said location and means to form a current pair comprising these parameters for a mobile station which is moving and means to compare stored parameters with current parameters.
• the wrong resources will be occupied leading to a non-efficient use of the resources and increasing the risk of loosing connections etc.
• a further object of the invention is to provide a system through which it is at the same time assured that traffic is directed to the cells for which the network is dimensioned, that there is free capacity in the cells to provide spare capacity at call set up and to assure that there is free capacity in the cells for them to act as rescue cells at the same time to as to assure call continuity and successful call set up.
• Still another object of the invention is to enable an efficient frequency planning and an efficient dimensioning of the hardware of the system.
• Still another object of the invention is to keep the number of handovers at a low level and to minimize the load on the switches as well as to minimize the risk of loosing connections.
• controlling means which comprises a ranking arrangement which is based on a number of criteria.
• One criterion is based on a comparison of the current moni ⁇ tored value of a signal parameter of the serving cell with the given threshold value for the serving cell and a second criterion is based on a comparison between a comparison of the current monitored value of a signal parameter of a neighbour cell with the given threshold value for that neighbour cell.
• the handovers between cells are governed by the priority ranking arrangement in such a way that a sys ⁇ tematic passing between cells or cell layers or levels is obtained, also for passing up and down between the layers.
• the method comprises the steps of: - introducing a threshold value for the serving cell; introducing a threshold value for at least each cell not being in the uppermost layer; monitoring at least one signal connection parameter for the serving cell; - monitoring at least one signal connection parameter for a number of neighbour cells; comparing the monitored current value of the signal parameter for the serving cell with the threshold value for serving cell; - comparing the monitored value of the signal parameter for the neighbour cells with the threshold value for the respective cell; carrying out the handovers in agreement with the priority ranking arrangement in such a way that a systematic passing between cells or cell layers is obtained, also for passing up and down between cell layers .
• the method can particularly be modified to comprise any embodiment or any combination as further evaluated in relation to the system itself.
• connec ⁇ tions will systematically be directed to a lower layer and at radio disturbances and call set up congestion among others there will be a systematic redirection of connections to cells that confidently can take care of them.
• the threshold can e.g. be for signal strength, path loss or both. Other signal parameters are also possible.
• Which signal parameter (I) is used depends on the general handover strategy that the system applies. In a particular embodiment the so called Mobile Assisted Handover strategy is used (MAHO). Then the mobile station performs signal strength (and/or other) measurements on radio energy transmitted from a number of neighbouring base stations.
• MAHO Mobile Assisted Handover strategy
• the mobile station transmits these measurements to the base station which delivers them to the unit responsible for the decision logic.
• handover strategies can be used such as NCHO (Network Controlled Handover) wherein the mobile is passive, MCHO (Mobile Controlled Handover) wherein the mobile both measures received signal strengths etc and takes decisions as to handover.
• Time Division Multiple Access is used.
• the threshold is used for passing upwards as well as down ⁇ wards, i.e. for passing to cells of a higher level of prefer- ence as well as to cells of a lower level of preference, or having a lower priority.
• the threshold is modified with the hysteresis which is added or subtracted according to the direction of movement.
• one condition for passing to a higher layer is that if the signal strength (in this particular case) monitored from the cell currently serving the connection, i.e.
• the serving cell decreases below the threshold for that particular cell (in an ad ⁇ vantageous embodiment with hysteresis subtracted)
• the system extends the set of neighbour cells which are eligible for handover to cells in a higher hierarchical layer.
• the cells in the higher layer have a lower priority than cells in the current layer and in lower layers.
• a condition for passing to a lower layer is that if the signal strength monitored or measured for a neighbouring cell in a lower hierarchal layer increases above the threshold for that cell (in an advantageous embodiment with hysteresis added) that cell will be added to the set of neighbouring cells which are eligible for handover. This cell will have a higher priority than cells in the current layer or in higher layers.
• the interactions with other radio network functions are dealt with.
• Examples of other radio network functions are intra-cell handover, overlaid-underlaid sub-cell handover, extended range, directed retry, assignment to another cell, alarm handover etc.
• Particularly one or more of these or other radio network functions are given different priorities in relation to one another and to normal handover functions. This is particularly relevant when different radio network functions propose different types of actions simultaneously.
• the mobile stations can systematically be directed to the lowest possible layer. This saves the capacity of the higher layers for extraordinary events such as e.g. coverage gaps or call set up congestion etc.
• Fig. 2 very schematically illustrates a mobile communication system
• Fig. 3 illustrates a flow diagram of the main flow in the locating procedure
• Fig. 4a illustrates a table for the ranking order in a two- layer structure wherein the serving cell is in the lowest layer and the signal strength for serving cell is under the threshold
• Fig. 4b illustrates a table for the structure of fig. 4a but wherein the serving cell is in the lowest layer and the signal strength of serving cell is over the threshold
• Fig. 4c illustrates a table for the structure of fig. 4a but wherein the serving cell is in a higher layer (forming an "umbrella- ),
• Fig. 5a illustrates a table for ranking order in a three- layer structure wherein serving cell is in the lowest layer and the signal-strength of the serving cell is under the threshold
• Fig. 5b is a table relating to the same structure as in Fig. 5a but wherein the signal strength of the serving cell is over the threshold
• Fig. 5c is a table relating to the same structure as in Fig. 5a but wherein serving cell is in the second layer and wherein the signal strength of serving cell is under the threshold
• Fig. 5d is a table relating to the same structure as in Fig. 5c but wherein the signal strength of the serving cell is over the threshold
• Fig. 5e is a table relating to the structure of Fig. 5a but wherein serving cell is in the 3rd or uppermost layer,
• Fig. 6 illustrates how categories of handover candidates are ordered in a particular scenario.
• the cellular communication system comprises a cell structure wherein the cells are arranged in at least two layers in a hierarchical manner.
• Fig 2 very schematically illustrates a cellular, mobile communication system, in this case the GSM-system.
• Fig 2 is merely shown for brief explanatory reasons, indicating very schematically a part of a mobile cellular system and it has of course in no way any limiting effect on the invention.
• the Base Station System BSS comprises a number of Base Transceiver Stations BTSs wherein a group of BTSs is controlled by a Base Station Controller BSC and a number of Base Station Controllers BSCs are controlled by a Mobile Switching Centre MSC of the Switching System SS controlling calls to and from a network such as e.g. the Public Switched Telephone Network PSTN, the Public Land Mobile Network PLMN, the Packet Switched Public Data Network PSPDN, the Circuit Switched Public Data Network, the Integrated Services Digital Network ISDN or any other network.
• a network such as e.g. the Public Switched Telephone Network PSTN, the Public Land Mobile Network PLMN, the Packet Switched Public Data Network PSPDN, the Circuit Switched Public Data Network, the Integrated Services Digital Network ISDN or any other network.
• the Switching Center further comprises a Visitor Location Register VLR comprising a data Location Area, a Home Location Register HLR with data on subscribers etc. which however is not relevant for the present invention.
• OMC indicates an Operation and Maintenance Center OMC in a manner known per se. Dashed lines in the figure relate to information trans- mission and full lines relate to call connections and infor ⁇ mation transmission and dashed lines also relates to, in the case of boxes, alternative networks, alternative or optional functions (of the SS etc.).
• a threshold is introduced at least for each cell not being in the uppermost layer. This is one feature for the provision of a systematic way of passing between layers.
• the threshold can be for the signal strength or for path loss or for both or for any other parameter depending on the general handover strategy of the system (among others). In the embodiments described in the following merely these cases which relate to a signal strength threshold will be described. This is however by now means limitative and the treatment when e.g. a path loss threshold or another threshold or both is applied, is substantially identical.
• the threshold is used for passing upwards as well as down ⁇ wards in the cell hierarchy.
• the threshold for passing upwards and downwards is generally referred to as a threshold. It may however be modified with a hysteresis which is added or subtracted according to the direction of movement.
• the condition for passing upwards i.e. from a lower layer to a higher layer is that if the signal strength measured from the cell currently serving the connection, i.e. the serving cell, is below the threshold for that cell (particularly with a hysteresis subtracted) the system extends the set of neighbour cells which are eligible for handover to cells in a higher hierarchical layer.
• the cells in the higher layer will have a lower priority than cells in the current layer and in any lower layer.
• the cell structure can be said to be a hierarchical cell structure. Therethrough the locating function is affected and modified.
• the basic ranking is done over the different cell levels and a basic ranking list is formed.
• This list comprises a number of candidates and the list is organized depending on different conditions. The organization may be governed by a table and e.g. supplied as Permanent Exchange Data.
• the candidates in the list are also divided into categories which will be further discussed below.
• Basic ranking means the ranking of cells based on signal strength and/or path loss criteria (or any other appropriate parameter).
• Locating relates to the procedure that, using measurement and parameter data, proposes the most appropriate connection.
• the output from the locating procedure is a list of possible candidates, the candidate list, for handover or assignment.
• Urgency refers to a condition requiring an urgent handover. Such is present if the transmission quality is too low, if Timing Advance (TA) is too large, if the time dispersion is too large or if any other criterion for extraordinary radio events is met depending on which physical measurements are available for the radio connection.
• TA Timing Advance
• a cell may be defined as an umbrella cell.
• An umbrella cell is a cell in a network of large cells encompassing the normal network. However, this invention is based on a concept of hierarchical cells.
• the locating algorithm is here particu- larly defined as a collective term for cell and subcell selection, including all types of intra-cell change of channel.
• the locating function generally describes a func ⁇ tionality that comprises one of many functional components of the locating algorithm.
• a handover generally describes a channel change between cells.
• the measured parameters or quantities Those are quantities which are monitored in the (in a particular embodiment) MS, Mobile Station and in the BTS by a measuring device e.g. as specified in the GSM recommen- dations.
• the quantities measured or monitored in the Mobile Station MS are transferred to the BSC over the air. Quan ⁇ tities measured or monitored in the Base Transceiver Station BTS are transferred to the BSC.
• the measurements that are considered include uplink and downlink signal strength and uplink and downlink signal quality measurements.
• reported quantities refers to quantities which are used in the MS, Mobile Station.
• the reported values are transferred from the MS to the BSC, Base Station Controller over air. Reports considered in the described embodiment include Timing Ad ⁇ vance, TA, coming form the BTS. As in any known system, filtering may be required. It may be necessary to smooth out stochastic variations etc. This will however not be further discussed.
• cell and subcell evaluation may be performed together in a cycle which is completed at least within one SACCH (Slow Associated Control Channel) period, and repeated every SACCH period unless certain mechanisms prevent selection for a time interval.
• SACCH Slow Associated Control Channel
• the cell selection is based on the ranking in the ranking list which it estab- lished.
• the ranking list is preferably established in the beginning of the cycle according to given basic principles.
• the final cell selection is obtained through reorganization of the list according to different principles which depend on the different criteria which will be further discussed later on.
• the locating function continuously monitors and evaluates the radio environment and suggest the most favourable cell (and/or subcell).
• a candidate list is produced.
• This candidate list comprises cells in order of preference.
• the signal quality and timing advance are continuously monitored and evaluated together with signal strength estimates (comprising possible filtering functions).
• the candidate list which is produced will e.g. in a known manner be sent to the call process handler to be used for channel allocation.
• the locating comparison and the preparation of the candidate list according to the given prerequisites start immediately after the initiation of the locating algorithm.
• the cell candidates are according to the invention sorted into categories depending on a number of factors, in one embodiment on three factors, namely the layer, the ranking as compared to the serving cell and a measured parameter such as signal strength (or path loss or similar) compared to the threshold.
• a measured parameter such as signal strength (or path loss or similar) compared to the threshold.
• Fig. 3 illustrates in a general way the main flow of the locating procedure in one particular embodiment. Timing
• Advance TA reports and measurements are filtered in a wherein among other things the Bit Error Rate is checked etc.
• the basic ranking is performed which is more thoroughly discussed later on.
• the hierarchical cell level evalua ⁇ tions are carried out, i.e. the evaluations as to if a cell is over/under a threshold etc.
• the Base Station Controller BSC block is initiated, the channel allocation process is initiated and the table(s) is/are read.
• the initiations are performed at activation of a locating individual or a cell, e.g. at handover at recep ⁇ tion in the Base Station Controller BSC of a complete hand ⁇ over signal from the MS (Mobile Station) in the handover procedure.
• the parameters layer, better/worse, over/under threshold are read.
• the initiation block fur ⁇ thermore comprises the so called CPH (Call Process Handler) process which handles the BSC (Base Station Controller) signalling, data structure, updating and processing the parameters, e.g. signal strengths etc. reports to the active locating instance.
• CPH Common Process Handler
• the measurement reports can e.g. be taken care of directly in the locating procedure itself or be placed in a buffer. However, preferably they are taken care of directly.
• Both urgency conditions and overlaid/underlaid evaluation and intracell evaluation are dealt with in a way similar to known cellular systems comprising umbrella cells.
• the organizing procedure may comprise a grooming procedure whereafter the list is sent in a manner known per se, as well as the ad ⁇ ministration of the allocation reply is dealt with in any appropriate way.
• the organizing procedure may e.g. comprise four procedures wherein cross-reference tables are built for cells in the parameter list, e.g. signal strength etc. to cells in the ranking list, cells in the parameter list to cells in the measurement value list, cells in the ranking list to cells in the parameter list and cells in the ranking list to cells in the measurement value list.
• This procedure is followed by the second procedure wherein each cell in the ranking list is fragmented according to three parameters, namely (as referred to above) 1 - layer, 2 - better or worse than serving cell and
• the ranking list is stepped through in the ranking value order.
• Table entries are found in any appropriate manner which generally is known per se. Particularly relating to one embodiment the known umbrella cell concept can be said to be extended to a hierarchical cell structure. Therethrough it can be used to e.g. cover up holes in coverage from normal cells.
• the transitions between normal cells and "umbrella" cells are controlled by the threshold (the signal strength threshold I tr ) and the modified threshold, i.e. the signal strength threshold modified with hysteresis H tr .
• these parameters are defined for normal cells, i.e.
• over for serv ⁇ ing cells means that the signal strength I 0 I 0 tr -H 0 tr and "under” for serving cells means that the signal strength I 0 ⁇ I tr 0 -H tr 0 whereas for neighbouring cells "over” means that the signal strength I A ⁇ Ii tr + H ⁇ whereas "under” means that the signal strength I x ⁇ Ii tr + H ⁇ .
• the candidate list is organized depending on different conditions. This means that one or more categories which normally might be in the list can be removed from the list, and categories can be added etc. and the categories are arranged in a proper order according to the particular needs and requirements.
• the organisation as such is governed by a table as mentioned above.
• This table form permanent exchange data but it can also be changeable and possible to correct etc.
• the table comprises one part referred to as conditions.
• the conditions an e.g. be according to the following table, hereinafter referred to as Table A: 1 - Assignment Request arrived,
• the table in fig. 4a illustrates the serving cell in layer 1 (normal) wherein the signal strength is under the threshold. 24 different cases are illustrated. In the tables (Fig. 4a- 4c) in referred to whether the serving cell has a signal strength which is over or under the threshold for the serving cell (se also Fig. 5a-5e).
• the table in Fig. 4b illustrates serving cell in layer one (normal) and the signal strength being over the threshold for 24 cases and finally the table in Fig. 4c illustrates the serving cell in layer 2 (which in this case relates to an "umbrella" cell) for 24 different cases. Particularly it is also possible to divide a number of cases into different cases for subcell change conditions which however will not be further discussed here.
• the embodiment described above relates to a case with two cell layers or levels. Of course there can be more layers. In the following another embodiment will be described. In this case there are three different cell layers.
• the hierarchical cells structure according to the invention can be applied to the umbrella cell functionality.
• An um ⁇ brella cell functionality provides a second level in a network comprising large cells which logically (and physi ⁇ cally) are organized above the original cell network and works as a backup network therefor.
• a third level is introduced which logically (and physically) is arranged below the original cell network and comprises small cells.
• this level forms a micro-cell network.
• the first level or the lowest level (level 1) or the bottom level is called the micro-level and the second level or the middle level is called an normal or an original level and the third level or the top level may be called a third or an upper level.
• the intention with allocating mobiles in the hierarchical cell structure is to fill the lowest level first i.e. the mobile station should be served by a cell in the lowest possible level or layer because this level particularly has the highest capacity. This has as a consequence that the mobile station not always will be served by the best cell from e.g. a signal strength or a path loss point of view but by a cell which is good enough and in the lowest possible layer.
• the invention particularly relating to the treatment with a threshold and with a system of tables as e.g. illustrated in Fig. 4, furthermore allows for any other strategy by changing the priorities as e.g. given by table B as referred to in the foregoing.
• system comprising tables as e.g. in Fig. 4, allows for different strategies in different layers. In e.g. this way, traffic can be directed to any layer and not only to the lowest etc.
• the invention likewise relates to networks having more than two or three layers, but since the principle is the same independently of the number of layers only networks comprising a two and a three-layer structure will be more fully described herein.
• a hierarchical cell structure according to the invention can be related both to the umbrella cell concept and to cells generally in different layers or levels.
• I 0 tr the signal strength threshold
• a flag in a manner know per se, indicating that a handover to a higher layer cell is allowed should be set when the signal strength in the serving normal cell falls below I 0 tr -H 0 tr .
• the flag is e.g. be referred to as a Higher Level Change Allowed flag.
• a candidate list containing only better cells according to basic ranking should be sent if the flag is set (and if no other flags are set).
• the mobile station remains in the current cell i.e. in the serving cell. If however, after handover to a higher level cell, the signal strength from a lower layer cell again rises above the threshold, then this cell should not immediately (according to the particular embodiment) become part of the candidate list.
• the signal strength should preferably reach a level somewhat above I tr or threshold I tr + H r or a threshold for downwards transition in order to get a hysteresis effect which will prevent repeated handovers or so called "ping-pong handovers" .
• a flag is set (in this particular embodiment). This flag can bee seen as a Lower Level Change Allowed.
• the candidate list is then sent containing the lower level cell exceeding I tr + H tr .
• I tr and H tr here relate to cell parameters and the serving cell uses its own threshold values when evaluating I tr -H r (I 0 tr - H 0 tr ) (for a possible move to a higher level).
• the serving cell evaluates neighbours it uses the threshold values corresponding to the neighbouring cells ( I ⁇ * + H ⁇ 1" ).
• the cells of that level shall generally be assured traffic in preference to all the other levels. This is achieved if the above reasoning is applied on the bottom level.
• the bottom level is called level 1.
• a basic ranking is performed among all cells which fulfil a minimum criterion. After that a level oriented rearrangement is done. Any cell in the lower layer with a signal strength exceeding I i tr +H i tr (which refers to a preferred embodiment) will set a flag indicating Lower Layer Change Allowed which will make the cell the top candi ⁇ date in the candidate list. If there are more such cells the basic ranking result will be used at the ranking among them ⁇ selves.
• the assembling of the candidate list proceeds generally as follows: cells better than the serving cell, in the same or in a higher higher layer, may be candidates, cells in the same layer have priority over higher layer cells. Cells among the "better" cells above with a signal strength below the Higher Layer Change Allowed threshold, have a lower priority, (in case there are any at all). In the last category higher level cells have priority over lower level cells. This is due to the fact that going to a lower layer cell with a signal strength below the threshold for Higher Layer Change Allowed would immediately result in a transfer to a higher layer anyway. This saves two unnecessary handovers.
• the assembly to the candidates list is given by a table which implements the in the foregoing considered principle of priority.
• the table can be supplied as a permanent exchange data which allows tuning of the algorithm without changing the actual code.
• the table is not permanent but can be changed e.g. by command.
• means e.g. preventing handovers to lower layer cells under certain conditions, such as in the case of fast moving mobile stations etc.
• further cell types e.g. indoor cells or pico-cells etc. Therethrough it can be necessary to introduce more levels in the cell ranking list.
• the thresholds and hysteresis parameters can be set per BSC or per cell level or per MSC level or per system level. In the case of three cell layers or levels, the basic ranking is made over the three cell levels.
• the candidates in the candidates list are as already discussed above divided into categories and the reported signal strength, the signal strength threshold and hysteresis and the cell level are used to set the categories for the neighbouring cells. Likewise, as already referred to, it does not have to be the signal strength but could also be the path-loss, the signal strength as well as the path-loss or any other convenient parameter.
• Figs. 5a - 5e tables are illustrated from which the candidate lists are formed as already mentioned above in relation to the embodiment comprising two cell layers.
• a hierarchical level evaluation is performed, i.e. a level-oriented rearrangement is done.
• a candidate list is assembled governed by one or more tables.
• TA Timing Advance
• AW relates to Assignment to worse cell.
• the number defines the layer, i.e. in this case three layers, layer 1, layer 2 and layer 3 wherein layer 1 is the lowest layer etc.
• o and u means over and under the threshold respect ⁇ ively, plus or minus the hysteresis as the case may be, e.g. if an hysteresis is applied or not. If a hysteresis is applied, this may be dealt with by the locating function.
• the table ( Figures 5a - 5e) comprises a number of conditions, namely:
• Table 5a relates to layer 1 under i.e. the own cell (serving cell) is in the lower layer (conditions 7 and 8) and the signal strength is under the threshold of that particular cell (condition 9). 32 different cases are indicated in the table.
• Table 5b refers to layer 1 over, i.e. own cell is in the lower layer and the signal strength is over the threshold.
• Table 5c relates to layer 2 under, i.e. the second layer wherein the own cell is in the second (in this case inter- mediate) layer and wherein the signal strength is under the threshold of that particular cell; i.e. I 2 ⁇ I 2 tr .
• Table 5d relates to layer 2 over i.e. the second layer and a signal strength which exceeds the threshold (I 2 > I 2 tr ).
• Table 5e relates to layer 3 over, which indicates layer 3 own cell in upper layer in this case and a signal strength exceeding the threshold. Alternatively the threshold is not checked.
• the interactions with other network functions are given a priority yielding the sorted order in the priority table of the system both if e.g. the tables (such as Fig. 4 and Fig. 5) are based on the priority between the radio network functions or not.
• the intra-cell handover function proposes a change of channel within a cell at the same time as the hierarchical cell structure function proposes a handover to another cell either in a higher layer or in a lower.
• a handover to a higher layer has preference over an intra-cell handover (se Table B).
• an intra-cell handover has preference over e.g. a quality alarm handover, but of course e.g. a quality alarm handover may alternatively have preference over an intra-cell handover etc. This depends on the particular system needs and requirements.
• the corresponding cells are categorized according to the combination of three parameters, namely
• a unique sequence of cell categories are assigned to that par ⁇ ticular combination of evaluation results.
• the sequencing of categories can e.g. be done in order to comply with a given priority list, such as e.g. the one given in table B.
• This sequence or candidate list thus represents the list of the handover candidates in the par ⁇ ticular order of preference.
• Fig. 6 further explains how the different categories of cells can be arranged in order of priority in a list, e.g. the cases 6, 7, 8 in Fig. 4a.
• the two categories 2bo and 2bu form one single category 2b.
• o/u (over/under) relates to the signal strength of the neighbour cell Ii being over/under the threshold value for that neighbour (or candidate) cell Ii tr .
• the reason for this splitting up is to avoid unnecessary handover ping-pong effects (first going down a layer and directly thereafter having to go up to a higher layer again).
• the structure comprises two cell layers and wherein the serving cell is in the lower layer.
• the radio network evaluations have proposed three actions simultaneously, namely a bad quality alarm handover, handover to a higher hierarchical layer and an overlaid-underlaid subcell change.
• a handover to a higher hierarchical layer has the highest priority whereas a bad quality alarm handover has the lowest priority.
• the highest priority of all is to remain in the lower layer, i.e. a normal better cell handover initiated by the normal locating.
• a ranking by the locating function is performed wherein cells are denoted better and worse than the serving cell and thereafter the handover candidate list is established.
• the threshold refers to the threshold for that particular cell. According to this list, the highest priority is to stay in the lower layer but only if a better cell appears which is illustrated in the first line. The second priority will be to go up to the upper layer which is illustrated in lines 2 and 3 of fig. 6.
• the advantageous embodiments invention applies to TDMA (Time Division Multiple Access) or FHMA (Frequency Hopping Multiple Access) or CDMA (Code Division Multiple Access).
• the invention is not limited to any particular handover strategy but a number of different strategies can be used such as Mobile Assisted Handover strategy MAHO, Network Controlled Handover NCHO or Mobile Controlled Handover MCHO.
• Cells in a higher layer have priority at coverage holes and at radio disturbances to ensure call continuity.
• At call set ⁇ up congestion cells in a higher layer also have priority in order to ensure successful call set-up procedures.
• the invention can be applied to generally every known standard, such as GSM, PDC, all PCS-standards, IS54, IS90, ADC, (D-)AMPS, DECT etc.

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• Mobile Radio Communication Systems (AREA)

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Cited By (22)

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• 1994
  • 1994-08-18 SE SE9402770A patent/SE505915C2/sv not_active IP Right Cessation
• 1995
  • 1995-08-17 AU AU32690/95A patent/AU696722B2/en not_active Ceased
  • 1995-08-17 WO PCT/SE1995/000933 patent/WO1996006512A1/en not_active Application Discontinuation
  • 1995-08-17 CA CA002197857A patent/CA2197857A1/en not_active Abandoned
  • 1995-08-17 RU RU97104122A patent/RU2143177C1/ru active
  • 1995-08-17 KR KR1019970701066A patent/KR100289374B1/ko not_active IP Right Cessation
  • 1995-08-17 CN CN95195599A patent/CN1080529C/zh not_active Expired - Fee Related
  • 1995-08-17 EP EP95929295A patent/EP0776588A1/en not_active Ceased
  • 1995-08-17 BR BR9508804A patent/BR9508804A/pt not_active Application Discontinuation
  • 1995-08-17 JP JP8507991A patent/JPH10504697A/ja active Pending
• 1997
  • 1997-02-17 FI FI970663A patent/FI970663A/fi unknown

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