Cell selection and reselection by mobile station in a multilayer cellular mobile radio network

Cell selection and reselection by mobile station in a cellular mobile radio network with a plurality of layers of cells includes, when the station is in a standby state and after selection of a cell by the station, identification of a layer containing the selected cell, reselection of cells in the layer during a predetermined time-delay, cumulative counting of the number of cells reselected in the layer during that time-delay, and comparison of the cumulative number with two thresholds. The reselection, cumulative counting and comparison steps are repeated if the cumulative number is between the thresholds. Otherwise, a change of layer is effected on the basis of the comparison and the previous steps are repeated, starting from the identification step. The mobile station selects the layer most appropriate to the speed at which it is travelling.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention concerns a method of access cell selection and 
reselection by a mobile station in the standby state in a multilayer 
cellular mobile radio network. 
2. Description of the Prior Art 
In a prior art cellular network, whether of the single-layer or multilayer 
type, a mobile station in the standby state continuously selects and 
reselects the access cell through the intermediary of which the mobile 
station receives or sets up a call. 
In the case of a multilayer cellular radio network comprising a plurality 
of layers (subnetworks) of cells, the sizes of the cells of the respective 
layers are different. Accordingly, a small cell layer is provided for 
low-speed or stationary mobile stations and a large cell layer is provided 
for higher speed mobile stations. 
If a given mobile station can access only one predetermined layer, the cell 
size of that layer is not always appropriate to the speed at which the 
mobile station is moving. This leads to an overloading of large cells 
caused by mobile stations temporarily travelling at low speed, and cutting 
off of calls involving mobile stations temporarily travelling at high 
speed connected to small cells. 
It is therefore preferable for the mobile station to be able to access more 
than one layer of the network; the mobile station user then chooses which 
layer to use. If the choice is left to the user it will not necessarily be 
the most appropriate one, and users tend always to select the same key. 
According to EP-A-0 526 436, the mobile station can change layer during a 
call by intercell handover. The change of layer mid-call is based on an 
estimate of the speed at which the mobile station is travelling, which 
takes several seconds and can lead to cutting off of the call. Further, 
although intercell handovers in the same cell layer are necessary when the 
mobile station is moving, intercell handovers between cells of different 
layers increase operating costs. 
OBJECT OF THE INVENTION 
The main object of this invention is to remedy the aforementioned drawbacks 
by providing a method of access cell selection and reselection by a mobile 
station in the standby state in a multilayer cellular mobile radio network 
that enables a mobile station to select a cell of the most appropriate 
size for the speed at which the mobile station is moving and consequently 
with no risk of a call being cut off. 
SUMMARY OF THE INVENTION 
Accordingly, a method of cell selection and reselection by a mobile station 
in a multilayer cellular mobile radio network in which each layer includes 
a plurality of cells and the cells comprise base stations respectively, in 
which method, when the mobile station is in a standby state and after 
selection of a cell as a selected cell by the mobile station, the 
following steps: 
identifying a layer containing the selected cell in an identified layer, 
reselecting cells included in the identified layer during a predetermined 
time-delay associated with the selected cell in reselected cells, 
cumulatively counting a number of reselected cells which are reselected in 
the identified layer during the predetermined time-delay, 
comparing the number of reselected cells with a first predetermined 
threshold associated with the selected cell and a second predetermined 
threshold associated with the selected cell and lower than the first 
predetermined threshold, 
repeating the reselecting, cumulative counting and comparing steps if the 
number of reselected cells lies between the first predetermined threshold 
and second predetermined threshold, 
changing the identified layer to a first selected layer of the multilayer 
cellular mobile radio network having cells covering the cells of the 
identified layer if the number of reselected cells is above the first 
predetermined threshold and to a second selected layer of the multilayer 
cellular mobile radio network having cells covered by the cells of the 
identified layer if the number of reselected cells is below the second 
predetermined threshold, and 
selecting a cell included in one of the first selected layer and second 
selected layer as another selected cell thereby repeating the previous 
steps from the layer identifying step. 
The mobile station selects the layer in which the cells have the most 
appropriate size to the speed of the mobile station. 
The identification step includes detecting an identifier of the layer 
containing the selected cell, the identifier being sent periodically by 
one of the base stations of the multilayer cellular mobile radio network 
transmitting to the selected cell. 
In response to the identifier, the mobile station detects the predetermined 
time-delay that is sent by one of the base stations of the multilayer 
cellular mobile radio network transmitting to the selected cell. 
In response to the identifier, the mobile station detects the first 
predetermined threshold and second predetermined threshold that are sent 
by one of the base stations of the multilayer cellular mobile radio 
network transmitting to the selected cell. 
The predetermined thresholds and time-delays can differ from one cell to 
another in the same layer and can be modified in the base station. 
The layer identifying step follows any breaking off of communication 
between the mobile station and the multilayer cellular mobile radio 
network. 
Setting up a call between the mobile station and the multilayer cellular 
mobile radio network interrupts a current step between the layer 
identifying step and the cell selecting step.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a cellular mobile radio network comprises I layers 
(also called subnetworks) of cells CC.sub.1 through CC.sub.I, where I is a 
positive integer between 2 and 4, for example. When I=4, the network 
comprises a layer of hypercells for communication with ships and aircraft 
via satellites, a layer of large macrocells for fast vehicles such as 
high-speed trains, a layer of small macrocells (covering a few tens of 
km.sup.2) for automobile vehicles and pedestrians, and a layer of 
microcells (covering a few hundred m.sup.2) essentially for pedestrians. 
To avoid overcomplicating FIG. 1, in that figure I=2. 
The first layer CC.sub.1 comprises first cells or macrocells CEL.sub.1,1 
through CEL.sub.1,J each having an area of about 10 km.sup.2 to about 40 
km.sup.2. The first cells CEL.sub.1,1 through CEL.sub.1,J are the cells of 
the pan-European cellular mobile radio system known as GSM ("Global System 
for Mobile Communication"), for example. The cells are often combined in 
groups of adjacent cells called location areas. Each cell CEL.sub.1,j, 
where j is an integer between 1 and the integer J is associated with a 
fixed base station SB.sub.1,j which controls the radio channels. These 
include one or more signalling channels for exchange of digital data 
between the base station and mobile stations SM.sub.1 through SM.sub.N 
where N is a positive integer and for interface protocols for setting up 
calls between the telephone network and the mobile station. 
The layer of macrocells CC.sub.1 further comprises an infrastructure (not 
shown) including computers, switches and digital data and telephone signal 
transport means to implement functions including switching, location of 
mobile stations in the network, and operation and maintenance. 
The second layer of cells CC.sub.2 of the cellular network comprises second 
cells CEL.sub.2,1 through CEL.sub.2,K, also known as microcells, where K 
is a positive integer. Each has an area of between about a few hundred 
m.sup.2 and about a few km.sup.2. The second layer of cells forms a second 
subnetwork, for example the Digital European Cordless Telecommunications 
(DECT) network. Each second cell CEL.sub.2,k, where k is an integer 
between 1 and K, contains a fixed base station SB.sub.2,k the functions of 
which are similar to those of the base station of a macrocell CEL.sub.1,j. 
The layer CC.sub.2, referred to as the microcell subnetwork, comprises an 
infrastructure of computers, switches and digital data transport means 
similar to that of the macrocell network. 
The layers of cells CC.sub.1 and CC.sub.2 are superposed and serve the same 
geographical area. 
The macrocell CEL.sub.1,j is therefore an umbrella cell having extended 
radio coverage encompassing a plurality of microcells. The macrocells are 
more particularly intended for high-speed mobile stations, while the 
microcells are the preferred means of providing service to low-speed or 
stationary mobile stations. 
A mobile station SM.sub.n, where n is an integer between 1 and N, includes 
a radio transceiver for setting up and receiving outgoing and incoming 
calls via one of the base stations SB.sub.1,1 through SB.sub.1,J or 
SB.sub.2,1 through SB.sub.2,K. When it is switched on, the mobile station 
SM.sub.n selects one of the base stations, exchanges digital data with the 
selected base station and selects a channel associated with the selected 
base station. 
In more detail, the base station SB.sub.1,j or SB.sub.2,k broadcasts on an 
associated signalling channel signalling data relating to the location 
area and to the cell CEL.sub.1,j or CEL.sub.2,k covered by the station 
SB.sub.1,j or SB.sub.2,k. In the case of the GSM network, this signalling 
channel is the Broadcast Control CHannel (BCCH) which is time-division 
multiplexed into 235 ms multiframes with three control channels, namely 
the Frequency Correction CHannel (FCCH), the Synchronization CHannel (SCH) 
and the Common Control CHannel (CCCH). The BCCH broadcasts four "System 
Information" messages including network parameters such as the maximal 
power allowed in the cell, the cell access threshold and the identity of 
the location area. 
Each of the four "System Information" messages is broadcast every 
4.times.235=940 ms. One of these four messages contains a data field 
CI.sub.i,j or CI.sub.i,k specific to the invention and shown in FIG. 2, 
where i is 1 or 2 corresponding to layer CC.sub.1 or CC.sub.2. The field 
CI.sub.i,j or CI.sub.i,k is 10 bits long and contains a layer identifier 
IDC.sub.i coded on two bits, a time-delay TEMP.sub.i,j or TEMP.sub.i,k 
coded on five bits and a layer threshold SEUC.sub.i,j or SEUC.sub.i,k 
coded on three bits. As explained below, in the case of a network with at 
least three layers, the data field contains two thresholds and the data 
field is longer than 10 bits. The layer identifier IDC.sub.i identifies up 
to four different layers. The time-delay TEMP.sub.i,j or TEMP.sub.i,k has 
a maximum value of 2.sup.5 =32 seconds (in the case where the step is 1 
s). The layer threshold SEUC.sub.i,j or SEUC.sub.i,k has a maximum value 
2.sup.3 =8 or more generally varies in proportions of 1 to 8. Thus each 
base station periodically transmits information required for layer 
selection. The layer identifier is specific to each layer; the time-delay 
and the layer threshold are fixed for each cell and can differ from one 
cell to another in the same layer. The purpose of the information 
contained in the field CI.sub.i,j or CI.sub.i,k is explained below. 
Referring to FIG. 3, access cell selection and registration of any mobile 
station SM.sub.n are performed in three steps E1 through E3 shown in 
diagrammatic form. The mobile station SM.sub.n is switched on by a user in 
step E1 and goes to the standby state. In step E2 the mobile station 
SM.sub.n listens to the signalling channels associated with the two 
layers. 
In a first embodiment of the invention, the mobile station scans all of the 
frequency band used by the two layers to look for the signalling channels. 
In a second embodiment of the invention the frequencies of the signalling 
channels of the two subnetworks are stored in the mobile station and the 
latter is tuned directly to the signalling channels on a cyclic basis. 
In step E3 the mobile station SM.sub.n selects a signalling channel and 
thus one cell of the network. To do this the mobile station calculates an 
average received signal level from at least five samples per channel 
distributed over a period of 3 to 5 seconds, using the method used in the 
GSM network, for example. When the mobile station finds a signalling 
channel meeting a predetermined selection criterion, it synchronizes to 
that channel and reads the signalling information on it. A cell 
CEL.sub.i,j or CEL.sub.i,k is selected. To simplify the description it is 
now assumed that the cell CEL.sub.i,j is selected. The cell selected 
belongs to one of the two layers of the network, and therefore step E3 
contains by implication the selection of a network layer CC.sub.i. In 
practice the predetermined selection criterion is that the mobile station 
SM.sub.n should select the cell giving the highest power received signal, 
with the facility to correct this choice in accordance with the load of 
the base station SB.sub.i,j of the selected cell CEL.sub.i,j. Different 
layer selection criteria can be used: for example, the user of the mobile 
station SM.sub.n could use a switch to select one of the layers; 
alternatively, when it is switched on, the mobile station SM.sub.n could 
select the layer or the cell that was selected when it was switched off, 
the relevant information being stored for this purpose in the mobile 
station. 
After selecting the cell, the mobile station SM.sub.n stores data such as 
the access channel addresses and the access area number of the cell 
selected. The mobile station is registered in the layer of cells CC.sub.i 
and in this way signals its location to the infrastructures of the 
cellular network via the base station for the cell selected. 
FIG. 4 shows an algorithm with eight steps E4 through E11. In step E4 the 
mobile station SM.sub.n detects the layer identifier IDC.sub.i of the 
layer containing the cell selected in step E3 or in step E11 (see below), 
the value of the time-delay TEMP.sub.i,j and the value of the threshold 
SEUC.sub.i,j corresponding to the cell previously selected. The mobile 
station stores the latter two values. The mobile station SM.sub.n includes 
a PROM in which the identifiers of the various layers in which the mobile 
station can detect cell identifiers are stored. In this way the mobile 
station "recognizes" the identifier detected by comparing it to the stored 
identifiers. In step E5 a programmable downcounter is triggered in the 
mobile station to count down predetermined clock periods representing a 
time-delay TP which is initialized to the stored time-delay value 
TEMP.sub.i,j. Simultaneously with step E5, a programmable cell counter in 
the mobile station is zeroed so that its count CCS is incremented for each 
new cell selected in the next step E6. 
Step E6 reselects another cell belonging only to the selected layer 
CC.sub.i on the basis of the selection criterion used in step E3. The 
mobile station examines only cells in which the signalling channels have 
an information field CI.sub.i,j containing the identifier IDC.sub.i of the 
previously selected layer CC.sub.i. 
In step E7 the cumulative count CCS of the number of cells reselected in 
the layer CC.sub.i in step E6 is incremented by one unit each time the 
mobile station SM.sub.n selects in step E6 a new cell different from the 
current cell selected during a previous cycle of steps E6 through E8. 
Step E8 checks the time-delay TP. If it has not run out, the mobile station 
SM.sub.n reselects cells in the selected layer (step E6) and the count CCS 
accumulates units as previously (step E7). When the time-delay TP has run 
out (TP=0) the cumulative count of selected cells CCS is compared with the 
threshold SEUC.sub.i,j or SEUC.sub.i,k in step E9. 
The comparison step E9 differs according to the layer. 
For the layer of microcells CC.sub.2, if the cumulative count CCS of cells 
reselected during the time-delay TP is greater than the threshold 
SEUC.sub.2,k associated with the layer of the cell selected before step 
E4, this means that the mobile station has selected too great a number of 
microcells, i.e. it has accelerated, and that it is preferable to change 
layer to select larger cells, in order to avoid too many intercell 
handovers during a call about to be set up. 
On the other hand, for the layer of macrocells CC.sub.1, if the cumulative 
count CCS of cells reselected during the time-delay TP is less than the 
threshold SEUC.sub.1,j associated with the layer of the cell selected 
before step E4, this means that the mobile station SM.sub.n has not moved 
far compared to the size of the macrocells. The layer of microcells is 
then sufficient for this slow-moving or virtually stationary mobile 
station which will require only a few intercell handovers (or none at all) 
during a call about to be set up. It is preferable to change layer to 
select small cells to release the layer of macrocells CC.sub.1 for faster 
mobile stations. FIG. 5 shows the changes of layer as a function of the 
cumulative count CCS of cells selected at the end of each time-delay TP. 
The arrows pointing towards the right indicate an increase in the count 
CCS per unit of time and therefore "acceleration" of the mobile station 
SM.sub.n. The arrows pointing to the left indicate a reduction in the 
count CCS per unit time and thus "deceleration" of the mobile station 
SM.sub.n. 
In the case of a network with at least three layers, the comparison step is 
identical to that described for the largest, respectively smallest cell 
layer. For an intermediate layer, the cumulative count CCS is compared to 
two thresholds corresponding to two layers between which the currently 
selected layer lies, one of the two layers having large cells covering 
each of the cells of the current layer and the other of the two layers 
having small cells covered by each of the cells of the current layer. The 
two thresholds define the limits of a range of variation of the cumulative 
count which, if crossed, imposes a change of layer. The information field 
sent by a base station of an intermediate layer contains both thresholds 
and is therefore longer than the information field as previously 
described. For example, FIG. 6 shows changes of layer as a function of the 
cumulative count CCS of cells selected at the end of each time-delay TP 
for a network having four layers CC.sub.1 through CC.sub.4 defined by 
thresholds SEUC.sub.a through SEUC.sub.d assuming that all the threshold 
values in the cells of the same layer are equal. The arrows in FIG. 6 have 
the same meaning as in FIG. 5. It is assumed that the condition SEUC.sub.a 
&lt;SEUC.sub.b &lt;SEUC.sub.c &lt;SEUC.sub.d is met so that changes of layer are 
effected from one layer to another layer which is an "adjacent" layer in 
terms of the size of the cells, for example from layer CC.sub.2 to layer 
CC.sub.3. This regular progress of change of layer is suitable for most 
multilayer networks. Nevertheless, it is possible, at least locally, to 
choose a different order for the threshold values and therefore for the 
changes of layer, for example from layer CC.sub.1 to layer CC.sub.3. 
The time-delay values TEMP.sub.i,j and TEMP.sub.i,k and the threshold 
values SEUC.sub.i,j and SEUC.sub.i,k are a priori different from one layer 
to another, and also from one cell to another in the same layer, because 
not all cells in the same layer cover the same area. These values can 
equally well be defined for a given location area. Furthermore, it is 
possible to modify the above values dynamically in each base station. 
If one of the above conditions is satisfied in step E9, a change of layer 
is effected in step E10. The mobile station SM.sub.n then uses the layer 
CC.sub.m, where m is different from i, which becomes the new selected 
layer. The mobile station SM.sub.n then considers for use only cells in 
the layer CC.sub.m, i.e. signalling channels in which the information 
field contains the layer identifier IDC.sub.m. 
Then, in step E11, the mobile station selects a cell in the new layer 
CC.sub.m most recently selected and is registered in this most recently 
selected layer, in a similar manner to the operations of step E3. 
After step E11, the mobile station reverts to step E4 to memorize data from 
the information field of the cell selected in the most recently selected 
layer CC.sub.m. The detection of the layer identified in step E4 is then 
optional, since the layer identifier has been chosen previously by the 
mobile station in step E10. 
If, in step E9, comparing the cumulative count CCS with one or two 
thresholds does not indicate crossing of either threshold and a change of 
layer, then the algorithm returns to step E5 to zero the count CCS and to 
reinitialize the time-delay TP to the value TEMP.sub.i,j. It then 
reselects cells in the same layer CC.sub.i for a subsequent time-delay TP. 
As a general rule, since the cumulative count CCS for any cell included in 
the layer CC.sub.4 containing the smallest cells is always greater than a 
second threshold equal to zero and the cumulative count CCS for any cell 
included in the layer CC.sub.1 containing the largest cells is always 
below a first threshold equal to a high number, typically 2.sup.3 =8, or 
even infinite, the count CCS is always compared to first and second 
predetermined thresholds depending on the size of the selected cell 
CEL.sub.i,j, CEL.sub.i,k. If the cumulative number CCS is above the first 
threshold in step E9, then the current layer CC.sub.i is changed in step 
E10 for a selected layer CC.sub.m, if any, with cells covering each of the 
cells of the current layer. If the cumulative number CCS is below the 
second threshold in step E9, then the current layer CC.sub.i is changed in 
step E10 for a selected layer CC.sub.m, if any, having cells covered by 
each of the cells of the current layer. 
In the case of an incoming call to be set up by the base station associated 
with the selected cell or in the case of an outgoing call to be set up by 
the mobile station, cell selection and reselection is interrupted and the 
cell used for the call is the last cell selected by the mobile station 
SM.sub.n. The station SM.sub.n does not carry out any intercell handover 
other than between cells of the most recently selected layer during the 
standby state. At the end of the call the mobile station returns to step 
E2 if it is not switched off. Consequently, the cell selection/reselection 
and layer change process of the invention is never executed during a call 
in progress and simultaneously with intercell handover. However, during a 
call, and as shown at the bottom of FIG. 3, the mobile station continues 
to store the information fields CI.sub.i,j, CI.sub.i,k shown in FIG. 2 on 
each intercell handover. The information contained in the last information 
field is then used when the mobile station reverts to the standby state 
for the next cyclic cell selection/reselection procedure.