METHOD FOR MANAGING POWER LEVELS IN A WIRELESS
CELLULAR NETWORK
5 BACKGROUND OF THE INVENTION
The present invention relates to the field of telecommunications and more specifically of
uplink (UL) power level management in a cellular network comprising small cells.
10 The rise of traffic, especially in the urban region, has lead to the development of small cells which
allow to enhance the network capacity. Indeed, as described in Fig.1, the area covered by one macrocell
(a) is covered by many small-cells (b) which contributes to reduce the number of active users per
cell and therefore to increase the bandwidth of a user. Such small cells are generally used in addition
to macro-cells and are mainly used to improve indoor coverage-of-to provide local hot spas
15 In macro-cells, as described in Fig.2, uplink (UL) inter-cell interferences are negligible due to power
control and path loss isolation induced by the distance D between a base station 1 and user
equipments 3 of a neighbouring cell (such as cell Cl with respect to cell C2) and soft handover
control of the user equipments located close to the cell boundaries and consequently in a soft
handover area 5. Thus, power management algorithms used in the macro-cells select the power level
20 of the user equipments 3 in function of the infra-cell interference level.
Unlike macro-cells, small cells such as c3 and c4 represented in Fig.3 do not generally have soft
handover mechanism (in order to reduce the last mile cost) and the reduced distance d between base
stations 1 and neighbouring cells cancels the path loss isolation (of the macro-cells) and produces
non-negligible inter-cell interferences.
Power level management algorithms used in the macro-cells can therefore not be applied efficiently
in small cells. Indeed, an increase of power level of the user equipments 3 of a small cell without
taking into account the user equipments 3 of the neighbouring cells would increase the interferences
5 in both cells, producing an increase of the power level of the user equipments of the neighbouring
cell creating an avalanche effect between the different neighbouring cells which would lead
eventually to an outage of the connections. This problem becomes even more important with
technologies such as enhanced-dedicated channel (E-DCH) as the power level of a user equipment
may reach a high level in order to improve its transmission throughput and therefore produce an
10 uplink overload.
SUMMARY OF THE INVENTION
15
It is therefore an object of the present invention to provide a method allowing the power
management of the user equipments attached to the base stations of small cells.
20 Thus, the present invention refers to a method for managing power levels in the uplink
communications of user equipments attached to the base station of a network small cell wherein
-the-contribution-of-neighbouring-cells user equipments in the interference level produced at said
base station is taken into account in the determination of the attached user equipment power
levels. _
25
According to another aspect of the invention, the maximum power levels of the user
equipments attached to the base station decrease when the contribution of the neighbouring cell
user equipments in the interference level produced at said base station increases.
30 According to an additional aspect of the invention, said management is dynamic in order
to take into account the traffic variations and the interference level variations.
-3-
According to a further aspect of the invention, the maximum power levels of the attached
user equipments vary along the time according to a pseudo-random algorithm.
5 According to another aspect of the invention, the pseudo-random algorithm is configured
in function of the number of neighbouring cells.
According to an additional aspect of the invention, the uplink traffic is transmitted
through an enhanced dedicated channel (E-DCH) technology.
10
According 'to a further aspect of the invention, said method is used in a cell cluster
leading to an interference reduction within said cell cluster.
According to an additional aspect of the invention, the pseudo-random algorithm is
15 configured to reduce the probability that the maximum power levels of the attached user
equipments of two adjacent cells correspond to a high level at the same time.
The invention also refers to a radio resources management algorithm comprising
instructions for managing power levels in the uplink communications of user equipments in
20 network small cells wherein the contribution of neighbouring cells user equipments in the
interference level is taken into account in the determination of the attached user equipment
power levels.
The invention also refers to a small cell base station comprising means for determining
25 power levels in the uplink communications of at least one user equipment attached to said base
station and means for sending power level instructions to said at least one attached user
equipment wherein the contribution of neighbouring cells user equipments in the interference
level produced at said base station is taken into account in the determination of the attached user
equipment power levels.
30
BRIEF DESCRIPTION OF THE DRAWINGS
5
FIG.1 is a diagram of a macro-cell (a) and a small cell (b) coverage;
FIG.2 is a diagram of a macro-cell configuration;
FIG.3 is a diagram of a small cell configuration;
FIG.4 is a diagram of the power distribution in a first configuration of two adjacent small cells
10 according to the present invention;
FIG.5 is a diagram, of the power distribution in a second configuration of two adjacent small cells
according to the present invention;
FIG.6 is a diagram of the evolution along the time of the maximum power level allocated by a base
station to its attached user equipment according to the present invention;
15 FIG.7 is a diagram of the evolution along the time of the maximum power level allocated by two
neighbouring base stations to their attached user equipment along the time according to the present
invention;
20 DETAILED DESCRIPTION OF THE INVENTION
As used herein, the term "ROT" refers to the acronym Rise Over Thermal;
25 As used herein, the term "RRM" refers to the acronym Radio Resource Management;
As used herein, the term "small cell" refers to a cell with a small radius such as, for example,
a femto-cell, a pico-cell or a micro-cell.
30
The embodiments of the present invention refer to the adaptation of the maximum power
level of the uplink communications of the user equipments 3 attached to the base station 1 of a
small cell in function of the interferences produced by the user equipments 3 located in the
neighbouring cells of said base station 1.
The power level of the uplink communications of the attached user equipments 3 is determined in
5 function of the signal quality measured by the base station 1 and is upper bounded by a maximum
rise over thermal (ROT) threshold. Thus, if the quality of the uplink communication between a user
equipment 3 and the base station 1 is too low to provide an efficient connection, the uplink
scheduler of the base station 1 sends a request to the user equipment 3 in order to increase its
emission power and therefore improve the quality of the communication. Nevertheless, this power
10 level has to be limited in order to limit interferences and to provide fairness between the different
connected user equipments 3. A maximum power level is therefore determined by a radio resource
management (RRM) algorithm in function of the amount of traffic requested by connected user
equipments 3.
Such distribution of power can be described as a bandwidth allocation and is determined based on
15 received signal strength indication (RSSI) measurements and processed as a rise over thermal (ROT)
factor which is defined as:
ROT=Iro,/No where Itot is the total power received by the base station 1 and N0 is the thermal noise
power.
The total received power Itot can be decomposed as follow:
M
Its=No+ Ei+ ^j
20
with Ed the power received by the i' attached user equipment and Ej the power received by the j'h
user equipment attached to a neighbouring cell.
- As mentioned previously, in the case of macro-cells, Ej is negligible and the power management
algorithm of the uplink scheduler distributes the amount of power between the attached user
25 equipments in function of the total available power and the interference level produced at the base
station 1 respecting a fairness criterion between the different attached user equipments 3.
However, in the case of small cells and notably in the case of networks using enhanced-dedicated
channel (E-DCH), the Ej values are not negligible and contribute to the interference level and
therefore influence the signal quality received at the base station 1. As a consequence, said
contribution implies to modify the management of the power levels by the scheduler and to
introduce an additional power reservation in the uplink (UL) radio resource management (RRMV1)
algorithm corresponding to the contribution of user equipments 3 of the neighbouring cells. It has
to be noted that the reservation may be dependent of the number of neighbouring cells.
5 Such margin is represented in Fig.4 wherein the power distribution is represented for two adjacent
small cells c3 and c4. The power distribution can be divided in several parts:
- a part corresponding to the thermal noise power (N0) 7,
- a part corresponding to the rise over thermal (ROT) of the attached user equipment 9,
- a part corresponding to the rise over thermal (ROT) of the user equipments of the neighbouring
10 cells 11 and,
- a part corresponding to a margin for "convergence" 13 which corresponds to the margin necessary
for a correct functioning in case of new attachment of user equipments to the base station.
Thus, in the present configuration the uplink scheduler of the small cell c3 has to reserve power
resources 11 for the user equipments UE1 and UE2 of the neighbouring cell c4 as
15 they I EJ contribute to the value of cell c3.
In the same way, cell c4 has to reserve UL resources 11 to take into account the user equipment
UE3 of the neighbouring cell c3.
The power distribution remains dynamic within this defined range in order to take into account the
20 variations of attached user equipments and the variations of the number of user
equipments of the neighbouring cells that I EJ contribute to the value and therefore to
I
the-interference level.
Figure 5 represents another configuration of the pair of cells presented in Fig.4 wherein the user
equipment UE3 is no more attached to the base station 1 of cell c3.
25 The overall interference level in cell c4 is therefore reduced and more power can be allocated to the
UE1 and UE2 from the base station 1 without affecting the neighbouring cell c3. As a consequence,
the ROT value of the attached equipments 9 of cell c4 is increased whereas there is no more ROT
part corresponding to the user equipment of the neighbouring cells 11 as there is no more user
equipment attached to cell c3.
30 Concerning cell c3, as there is no more attached user equipments, additional resources can be added
to the resource level reserved to the user equipments UE1 and UE2 of the neighbouring cell c4 11
without any impact on the network performances. The power level distribution of a small cell and
its neighbouring cells is therefore updated each time the number of attached user equipments varies
in order to regulate the interference level and to insure a correct functioning of the network.
Thus, according to the present invention, power resources are also reserved in order to take into
5 account a burst of ROT, that is to say a temporary increase of the interference level, due to user
equipments located in neighbouring cells (using for example a bursty data transfer) and contributing
to the interference level produced at the base station. Based on the computation of the RRM
algorithm, the uplink scheduler defines the power level of the attached user equipments and power
level instructions are sent to said attached user equipments.
10 As aconsequence, such management of power distribution within a small cell taking into account
the interference level produced by the user equipments of the neighbouring cells allows to introduce
fairness between the different users, to reduce the overall interference level and therefore to improve
the network performances.
Nevertheless, such margin represents an unused amount of power to secure new incoming users
15 which is wasted if not used.
According to another aspect of the invention, such power margin is used temporarily by a base
station according to a pseudo-random algorithm. Extra power 15 is provided sporadically to a base
station 1 with respect to the maximum power 17 computed by the scheduler as described previously.
Such temporal additional power margin allocation is represented in Fig.6. The amount of power and
20 the pseudo-period being adjusted in function of the number of neighbouring cells. Said number of
neighbouring cells can be configured or self-learnt by the base station 1.
Such temporary margin can be compared-to-a-random access channel (RACI-I) and limit the
probability of collision as described in Figure 7 wherein the evolution of the maximum power level
of two neighbouring cells along the time is shown. At time TO, the power level of the first cell 19 is
25 at a high level (which corresponds to the extra temporary margin) while the power level of the
second cell 21 is at a low level. At time T1, the pseudo-period of the RRM algorithm of the second
cell has elapsed and extra margin is granted to the second cell. At time T2, the pseudo-period of the
RRM algorithm of the first cell has elapsed and its power level return to a low level. Then, at time
T3, the power level of the second cell returns to a low level. The different neighbouring cells get
30 sequentially some extra power margin. As represented in Fig.7, the time range during which both
cells are at a high level (between Ti and T2) and which corresponds to a period with a high collision
_g_
probability 23 is very short. The collision probability is therefore limited. Such sequential grant
corresponds to a pseudo-synchronization of the neighbouring cells to share die available extra
power range. Thus, several small cells located in a common area can be "gathered" as a cell cluster
wherein the pseudo-random algorithm distributes in a sequential way an extra power margin to the
5 cells of the cluster. Moreover, the algorithm is configured in order to reduce the probability that the
allocation of an additional power margin occurs at the same time in two adjacent cells leading thus
to a reduction of the uplink interferences probability.
Thus, by taking into account the contribution of user equipments attached to neighbouring cells in
10 the power level determination of its attached user equipments and by reserving a corresponding
power margin, the present invention allows to provide fairness at a scale higher than the cell scale
which is necessary in the case of small cells due to the lack of radio frequency isolation between the
cells and to prevent the risk of damaging the global performances of the network. Furthermore, the
use of a pseudo-synchronization allows to limit the part of wasted power and contributes to the
15 optimization of the network resources.
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CLAIMS
5
1. Method for managing power levels in the uplink communications of user equipments
attached to the base station of a network small cell wherein the contribution of
neighbouring cells user equipments in the interference level produced at said base station
10 is taken into account in the determinati on of the attached user equipment power levels.
2. Method for managing power levels in accordance with claim I wherein the maximum
power levels of the user equipments attached to the base station decrease when the
contribution of the neighbouring cell user equipments in the interference level produced
15 at said base station increases.
3. Method for managing power levels in accordance with claim 1 or 2 wherein said
management is dynamic in order to take into account the traffic variations and the
interference level variations.
20
4. Method for managing power levels in accordance with one of the previous claims
wherein the maximum power levels of the attached user equipments vary along the time
according to a pseudo-random algorithm.
25 5. Method for managing power levels in accordance with claim 4 wherein the pseudorandom
algorithm is configured in function of the number of neighbouring cells.
6. Method for managing power levels in accordance with one of the previous claims
wherein the uplink traffic is transmitted through an enhanced dedicated channel (E-DCH)
30 technology.
-10-
7 Method for managing power levels in accordance with one of the previous claims
taken in combination with claim 4 wherein said method is used in a cell cluster leading to
an interference reduction within said cell cluster.
5
8. Method for managing power levels in accordance with claim 7 wherein the pseudorandom
algorithm is configured to reduce the probability that the maximum power levels
of the attached user equipments of two adjacent cells correspond to a high level at the
same time.
10
9. Radio resources management algorithm comprising instructions for managing power
levels in the uplink communications of user equipments in network small cells wherein
the contribution of neighbouring cells user equipments in the interference level is taken
into account in the determination of the attached user equipment power levels.
15
10. Small cell base station comprising means for determining power levels in the uplink
communications of at least one user equipment attached to said base station and means
for sending power level instructions to said at least one attached user equipment wherein
the contribution of neighbouring cells user equipments in the interference level produced
20 at said base station is taken into account in the determination of the attached user
equipment power levels.