Note: Descriptions are shown in the official language in which they were submitted.
= WO 96/06512 2197857 PCT/SE95/00933
CELLULAR MOBILE COMMUNICATION SYSTEM
FIELD OF THE INVENTION
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. In this context
however, 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
WO 96/06512 5 7 PCTlSE95100930
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circumstances. It is therefore extremely important that the
cells are organized in the most suitable way.
STATE OF THE ART
A number of attempts have been done to provide a cellular
mobile communication system with a cell structure which is
appropriate.
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. Thus there are either two or
three different cell layers. In this case 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 (in relation to umbrella cells or normal
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 however, 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
WO 96/06512 PCT/SE95/00933
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3
cell even if the micro-cell provides radio conditions which
are adequate. This is illustrated in Fig. 1 which shows two
micro-cells under an umbrella cell. For the mobile station MS
1 the micro-cell Cõ 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, ES 2. In this case the umbrella cell Cõ
fulfills its purpose and provides coverage where the micro-
cells CA, Ce fail to do so. However, for the mobile station
MS 3, the micro-cell base station BS 2 is in line of sight
and should therefore be able to carry the connection without
problems. Nevertheless, the umbrella cell base station BSu
which also is in line of sight has a signal strength which is
higher and will therefore take over the connection. Also at
e.g. 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.
Moreover an efficient frequency planning and efficient
dimensioning of the hard-ware is--difficult. Furthermore 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.
In another known system different cell layers are given
different priorities and a priority has been given to hand-
overs to the umbrella cell layer which is higher than the
priority of handover due to a number of other radio network
functions. Interactions with a number of radio network func-
tions have been given different priorities. A higher layer
has then been given a higher priority in case of radio
disturbances and call set up congestion etc. Thus the passing
between the layers is triggered by a number of different,
extraordinary events such as e.g. congestion at call set up
j`,p PCT/SE95/00933=
WO 96/06512 7Q~f
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4
and bad quality (high BER, Bit Error Rate). Also in this case
connections are not always directed to the most adequate cell
layer. Moreover coverage holes are not adequately covered.
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.
WO-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. With the use of the location
determinating system it is possible, with the use of therein
stored information and signal character on -the estimated
distance, to determine the location of a mobile station
within a micro-cell. 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. However, also in this case the wrong resources
will be occupied leading to a non-efficient use of the
resources and increasing the risk of loosing connections etc.
WO 96106512 PCT/SE95/00933
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SUMMARY OF THE INVENTION
It is an object of the present invention to provide a
5 cellular mobile communication system as initially referred to
wherein the cells are organized into layers of preference in
such a way that the system resources are used in an optimal
way.
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. It is also an object of the invention to direct
connections to cells which confidently can take care of the
connection and to bridge coverage holes. 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.
These as well as other objects are achieved through a system
of the above mentioned kind comprising 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
WO 96106512 ~. PCTlSE95l00930
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6
neighbour ce11. 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.
It is also an object of the invention to provide a method for
controlling the handover procedure in a cellular mobile
communication system wherein the cells are arranged hier-
archically in at least two different layers or levels so that
the resources of the system are used in the optimal way and
so that the traffic or the connections are directed to the
appropriate cells in agreement with the dimensioning of the
system etc.
These and other objects are achieved through a method wherein
a priority ranking is carried out based on a number of
criteria.
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
~ WO 96/06512 219 PCT/SE95/00933
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7
layers.
The method can particularly be modified to comprise any
embodiment or any combination as further evaluated in
relation to the system itself.
A number of advantageous embodiments are e.g. given by the
features of the appended subclaims.
With the cellular system according to the invention 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. Through the
introduction of a threshold for each cell, a systematic way
of passing between layers is obtained. 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. The mobile station transmits
these measurements to the base station which delivers them to
the unit responsible for the decision logic. However, also
other 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.
in a particular embodiment Time Division Multiple Access
(TDMA) is used. With the system according to the invention
the threshold is used for passing upwards as well as down-
wards, i.e. for passing to cells of a higher level of prefer-
WO 96/06512 PCT/SE95/009330
8
ence as well as to cells of a lower level of preference, or
having a lower priority. According to a particular embodiment
the threshold is modified with the hysteresis which is added
or subtracted according to the direction of movement. in the
normal case, e.g. when no extraordinary events or
simultaneous functions are demanded, 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. However,
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.
In advantageous embodiments 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.
Particularly with the invention the mobile stations can
systematically be directed to the lowest possible layer. This
saves the capacity of the higher layers for extraordinary
-- -_ _ - ----- ~
~ WO 96/06512 219785PCT/SE95100933
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9
events such as e.g. coverage gaps or call set up congestion
etc.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will in the following be more fully described
in a non-limiting way under reference to the accompanying
drawings wherein :
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,
WO 96/06512 PCT/SE95/00933
7
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
5 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.
DETAILED DESCRIPTION OF THE INVENTION
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
WO 96/06512 - ' 7 857 PCT/SE95/00933
11
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.).
According to the invention 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. According to advantageous em-
bodiments 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. In a generalized case
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
WO 96/06512 PCT1SE95/00933*
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~. k . tV . . n ..
12
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 corresponding condition for
passing downwards, i.e. from a higher layer to a lower layer,
that is if the signal strength measured from a neighbour cell
in a lower hierarchical layer increases above the threshold
for that cell (particularly with hysteresis added) that cell
will be added to the set of neighbour cells which are
eligible for handover. That cell will have a higher priority
than cells in the current layer or in a higher layer.
Thus 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.
In the following, some concepts are explained. 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
WO 96/06512 21.978 57 ~ PCT/SE95/00933
13
available for the radio connection. As to the umbrella cell
concept, 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.
Generally there will be a great number of different
requirements on the Base Station Controller BSC for the
locating algorithm. 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. In the following a handover generally
describes a channel change between cells. In the following
will be referred to 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. When applying the Mobile Assisted Handover strategy
MAHO, 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. However, the ways in which
measurements are carried out, informed on etc. depend on the
particular handover strategy that is used. In a particular
embodiment, as will be further discussed later on, the
measurements that are considered include uplink and downlink
signal strength and uplink and downlink signal quality
measurements. As already stated, there are of course a
number of other alternatives. In the same embodiment 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,
WO 96/06512 PCT/SE95/009330
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14
filtering may be required. It may be necessary to smooth out
stochastic variations etc. This will however not be further
discussed.
In one embodiment, as in a number of known systems, 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. This however merely constitutes one example among
many others.
As more thoroughly discussed later on, 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).
Based on a number of comparisons of measurements of measured
and reported quantities a candidate list is produced. This
candidate list comprises cells in order of preference. in an
advantageous embodiment 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. Generally the locating comparison and the
preparation of the candidate list according to the given
prerequisites start immediately after the initiation of the
locating algorithm.
WO 96/06512 PCT/SE95100933
219'7857
Once the candidate list is received by the function handling
the candidate list, the candidates are used in attempting to
allocate the channel according to the information in each
5 candidate entry. In the attempts, the candidates are used in
the order they are placed in the list e.g. with one attempt
for each candidate. If channel allocation to a particular
candidate fails (e.g. in the case of a congestion), the
candidate next in the list is to be used for the next attempt
10 and so on until the channel has been allocated successfully
or all candidates have been tried unsuccessfully. This is a
procedure which generally is known per se which of course
could be more or less modified but also completely exchanged
through another procedure without departing from the scope of
15 the invention.
Particularly, the interactions with other radio network
functions such as intra-cell handover, overlaid-underlaid
subcell handover, extended range, directed retry, assignment
to another cell, alarm handover etc. will be discussed later
on. This is particularly relevant when various radio network
functions propose different types of actions at the same
time. 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. Each combination of results from
the evaluations of the radio network control functions is
connected to a defined list of categories describing the
precedence of the candidates in a so called "handover candi-
date list".
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
WO 96/06512 PCT/SE95/009330
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16
among other things the Bit Error Rate is checked etc. In b
the basic ranking is performed which is more thoroughly
discussed later on. In c the hierarchical cell level evalua-
tions are carried out, i.e. the evaluations as to if a cell
is over/under a threshold etc.
Thereafter follows (d) the evaluations in relation to other
Radio Network Functions, such as e.g. urgency conditions,
overlaid/underlaid evaluation, intra-cell evaluation etc.
These so called flags are introduced into the tables and
constitute examples on conditions which can be controlled or
modified.
Thereafter follows the organizing procedure which depends on
a - d and wherein a candidate list is organized. Thereafter
is proceeded e.g. as in any known system or Locating Flow.
In the particular described embodiment in the initiation pro-
cedure 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. Among others 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. The procedures referred to are referred to
in a simplified manner since most steps in the procedure
correspond to the procedures that are carried out in known
cellular mobile communications systems e.g. with conventional
umbrella cells. E.g. filtering functions are carried out in
a manner known per se etc. The basic ranking procedure will
~
WO 96/06512 2 1 9 PCT/SE95/00933
17
be further discussed below in relation to two examples of
establishing a priority table based on a hierarchical struc-
ture comprising two and three levels respectively.
In the locating procedure 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.
After the intracell evaluation follows an organizing
procedure. This is different from known systems and will be
further discussed and illustrated later on. 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.
In one embodiment 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
3 - if it is over or under its own threshold. Thereafter the
cells are sorted into a three-dimensional datastructure. 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.
WO 96/06512 PCT/SE95/00933
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2197857
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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. According to one em-
bodiment (which may or may not include a hysteresis threshold
or a second threshold), the transitions between normal cells
and "umbrella" cells are controlled by the threshold (the
signal strength threshold I`') and the modified threshold,
i.e. the signal strength threshold modified with hysteresis
H`r. In a particular embodiment these parameters (the first
and the second thresholds) are defined for normal cells, i.e.
not for cells forming so called "umbrellas". in this par-
ticular case this leads to a basic ranking list (among others
referred to in Fig. 3) comprising 7 categories. This is
illustrated in figs. 4a to 4c. For cells in level 1 (the
lowest layer), in this embodiment so called normal cells, the
signal strength is compared with the threshold value. In
relation to the herein described embodiment "over" for serv-
ing cells means that the signal strength Io >_ Io" -Ho`r and
"under" for serving cells means that the signal strength Io
< I io -H ro whereas for neighbouring cells "over" means that
the signal strength I. ?I;" + H," whereas "under" means that
the signal strength IS <Ii`r + H1". 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:
~ WO96/06512 PCTISE95/00933
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1 - Assignment Request arrived,
2 - AW-state (which means Assignment to worse cell),
3 - Excessive Timing Advance (TA) urgency,
4 - Bad Quality urgency,
5 - Overlaid-Underlaid subcell change,
6 - Intra-Cell handover.
(Table A)
Of course other conditions are also possible as well as there
might be fewer or more conditions, depending on the particu-
lar needs and the particular requirements. The conditions can
be true, indicated with a "1", false, indicated as "0" and
finally they may be irrelevant, indicated as "-".
The table furthermore relates to different categories. In the
tables illustrated in figs. 4a - 4c
s - means serving cell,
1 bo - better cell in layer one, SS (Signal Strength)
over threshold,
1 bu - better cell in layer 1, SS under threshold,
1 wo - worse cell in layer 1, SS over threshold,
1 wu - worse cell in layer 1, SS under threshold,
2 b - better cell in layer 2,
2 w - worse cell in layer 2.
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 tables of Fig. 4 have been made up according to guide-
lines which have given a certain priority order between the
different radio network functionalities. This priority order
is given by the following table, hereinafter referred to as
Table B:
WO 96/06512 PCT/SE951009339
219.7815:7 ;
1 - Normal Handover
2 - Timing Advance TA urgency
3 - Going to a lower layer
5 4 - Subcell change
5 - Intracell Handover
6 - BQ urgency
7 - Going to a higher layer
(Table B)
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. In the present embodiment
a third level is introduced which logically (and physically)
is arranged below the original cell network and comprises
small cells. In one embodiment this level forms a micro-cell
network. Herein 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
WO 96/06512 PCT/SE95/00933
2197857
21
level and the third level or the top level may be called a
third or an upper level. Generally 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.
Moreover, the 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.
Of course 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. Through the intro-
duction of the signal strength threshold Io" or a threshold
for upwards transition, below which a transition may occur,
transitions to a logically higher level or layer is facil-
itated. 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
WO 96/06512 PCT/SE95l00933
'~~~.=.i , f O. cJ 0
22
Io`r -Ho". 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). If the list is empty
however, i.e. no better cells exist; then 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`' or threshold I" + H" or a threshold for
downwards transition in order to get a hysteresis effect
which will prevent repeated handovers or so called "ping-pong
handovers". When the signal strength from a lower level cell
exceeds I" + H" 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`r + Htr.
I`r and H" here relate to cell parameters and the serving
cell uses its own threshold values when evaluating I"-H" (Io"
- Ha`') (for a possible move to a higher level). When the
serving cell evaluates neighbours it uses the threshold
values corresponding to the neighbouring cells (Iitr + Hitr)
If a third level below the two other levels is introduced,
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. in the
following the bottom level is called level 1. For the provi-
sion of a candidate list 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 Ii`'+H,`r (which refers
to a preferred embodiment) will set a flag indicating Lower
WO 96/06512 219 PCT/SE95/00933
'~~57
23
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. If the Higher Layer Change Allowed flag is set, 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. If however an
urgency flag is set simultaneously to the flag indicating
Higher Layer Change Allowed, than also worse cells may be
appended to the candidates list. 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. However,
in an alternate embodiment the table is not permanent but can
be changed e.g. by command. Of course it is possible to
provide for means e.g. preventing handovers to lower layer
cells under certain conditions, such as in the case of fast
moving mobile stations etc. Moreover it is possible to add
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.
WO 96/06512 2191557 PCTlSE95/00933ig
24
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.
In 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. First
a basic ranking is performed, advantageously among those
cells which fulfil a minimum criterion of exceeding a thres-
hold value of at least one given signal parameter.
Thereafter a hierarchical level evaluation is performed, i.e.
a level-oriented rearrangement is done. This means that a
candidate list is assembled governed by one or more tables.
Examples of such tables are illustrated in Figs. 5a- 5e.
These tables will in the following be explained. As above TA
relates to Timing Advance and AW relates to Assignment to
worse cell. In the tables, 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.
Better and worse is illustrated through b and w respectively
whereas 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.
As in the embodiment relating to a hierarchical 2-layer
structure, the table (Figures 5a - 5e) comprises a number of
WO 96/06512 .2 197857 PCT/SE95/00933
t P:
conditions,
namely:
1 - Assignment request Arrived and AW (Assignment to
5 worse cell) state,
2 - Assignment request Arrived and not AW-state,
3 - Excessive timing Advance TA urgency,
4 - Bad quality urgency,
5 - Overlaid-Underlaid subcell change request,
10 6 - Intra-cell Handover request,
7,8 - Layer, wherein 00 indicates the lowest layer, 01
indicates the second layer, 10 indicates the third
layer and finally 11 indicates the highest layer,
9 Own cell signal strength under the threshold,
15 10 - Not used (but in this case indicated 0).
This table will hereinafter be referred to as Table C.
Table 5a relates to layer 1 under i.e. the own cell (serving
20 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.
25 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. 12 < I2".
Table 5d relates to layer 2 over i.e. the second layer and a
signal strength which exceeds the threshold (IZ > IZ`r)
Finally Table 5e relates to layer 3 over, which indicates
f *
W096706512 PCT1SE95/00933~
219785'~
26
layer 3 own cell in upper layer in this case and a signal
strength exceeding the threshold. Alternatively the threshold
is not checked.
In the following two particular examples will be given on the
interactions of the hierarchical cell structure with other
network functions. Particularly 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.
In a first example the interaction with the intra-cell
handover function will be described for a particular embodi-
ment. Two or more radio network functions may make their
propositions at the same time. In this case 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. In this particular embodiment a handover to a
higher layer has preference over an intra-cell handover (se
Table B). However, 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.
When neighbour cells measurements have been received from the
mobile, the corresponding cells are categorized according to
the combination of three parameters, namely
1- the hierarchical layer they belong to,
2 - their ranking as to if they are better or worse than the
serving cell according to normal locating criteria and
finally
3 - if the measured signal strength is above or below the
WO 96/06512 21 978 57 PCT/SE95/00933
27
threshold for that cell. In this case, if three hierarchical
layers are implemented, twelve categories are obtained.
Generally all possible situations and combinations of situ-
ations which are relevant to the various radio network con-
trolled function evaluations are analyzed. Based thereon 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, as already explained above 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. In
Fig. 4a the two categories 2bo and 2bu form one single
category 2b. For 2w the corresponding throwing together has
been done for 2wo and 2wu. Furthermore, in Fig. 6 o/u
(over/under) relates to the signal strength of the neighbour
cell Ii being 6ver/under the threshold value for that
neighbour (or candidate) cell Ii". 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).
In. the following an example will be considered wherein the
structure comprises two cell layers and wherein the serving
cell is in the lower layer. It is assumed that besides the
normal locating 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. In this case 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
WO 96/06512 PCT/SE95/00933i
2197~57 ~I
28
locating. With this combination of evaluation results, the
candidate list of handover candidates as illustrated in fig.
6 is obtained. First 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. As referred to above, above/below 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. If going to a better cell will result in immediately going
up to an upper layer or an "umbrella" cell which is the case
if the signal strength of that cell is below the threshold,
the first handover will not take place and instead there will
be a handover directly to the upper cell or the "umbrella"
which is illustrated on lines 1 and 4. The third priority
will be to perform a subcell change which is illustrated on
line 5. Finally, the lowest priority is to go to a cell which
is ranked as worse than the cell currently serving the
connection. Also in this case a cell in the lower layer has
preference over a cell of the upper layer but not if a second
handover to the upper layer would ensue which can be seen
in lines 6 to 8 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). Moreover,
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.
With the invention it is possible to apply a systematical
approach upon designing hierarchical cell structures. Of
course an arbitrary number of layers can be designed and the
WO 96/06512 2197" 7 PCTlSE95/00933
29
detailed conditions for switching between layers can be
controlled in terms of criteria for changing between layers
as well as a detailed interplay with other radio network
controlled functions.
Due among others to the fact that passing between layers is
generally predictible, a systematical cell planning and
dimensioning is possible. Furthermore it is possible to
direct mobile stations to the lowest layer in a systematical
way which, as already stated above, saves the capacity of the
higher layers for e.g. coverage gaps or call setup congestion
etc. Alternatively it is possible to direct mobile stations
to any layer of preference.
With the invention it is among others thus intended to be
able to direct the traffic to the cells for which the network
or the Hierarchical Cellular System HCS actually is dimen-
sioned and to ensure that there is free capacity in those
cells which shall provide spare capacity at call set-up and
which have the function of rescue cells.
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 is of course not limited to the shown embodi-
ments but can be varied in a number of ways within the scope
of the claims.
Particularly the invention can be applied to generally every
known standard, such as GSM, PDC, all PCS-standards, IS54,
IS90, ADC, (D-)AMPS, DECT etc. ,. .. l. .
