Method for deciding on a potential load balancing operation
in a wireless network and network element for a wireless
network
Specification
Field of the invention
The present invention refers t o a method for deciding o n a
potential load balancing operation in a wireless network
comprising a first base station having a first cell and a t
1 o s t o i h o d €
first cell and the second cell a t least partially
overlapping each other. Furthermore, the present invention
refers t o a network element o f a wireless network, such a s
a macro base station o r a pico base station, being arranged
for executing such a method.
Background
Hierarchical cellular networks are known in the art.
Hierarchical networks typically have comparatively large
macro cells. So-called pico cells which are smaller than
the macro cells are embedded a t least partially within a
macro cell. A mobile terminal that i s registered with a
macro cell and located within a coverage area o f a pico
cell embedded within the macro cell may perform a handover
from the macro cell t o the pico cell, switching data
traffic originally transferred between the macro base
station and the terminal t o the pico base station.
In many cases, the pico cell improves the overall
performance of the cellular network because the macro base
station may hand over at least some terminals located
within the pico cell to the pico base station so that these
terminals perceive a better quality of service and the
macro base station has more radio resources available to
serve the terminals that remain registered with a macro
base station.
In particular, if the macro base station and the pico base
S . . 3.10 SOU TC S f t S 1 Z 6 O f t € O
cell depends on the transmit power used by the macro base
station and the one used by the pico base station for
transmitting on these radio resources or a portion thereof.
For instance, if the transmit power of the pico base
station can not be increased or if the macro transmit power
is comparatively high then interference between the macro
s t o xi d th pi o stcttxon S s u . t ¾
comparatively low size of the pico cell. A low transmit
power results in a comparatively large pico cell because
the interference is comparatively low.
Summary
The object of the present invention is to provide a method
for deciding on a potential load balancing operation, that
allows for coordinating the operation of the macro base
station and at least one pico base station such that an
overall performance of the macro cell and the pico cell
embedded in that macro cell is improved.
According to an embodiment of the present invention, a
method for deciding on whether to perform a load balancing
operation in a wireless network comprising a first base
station having a first cell and at least one second base
station having a second cell, the first cell and the second
cell at least partially overlapping each other, is
provided. This method comprises evaluating an impact of a
potential load balancing operation to an overall
performance metric, said metric characterizing the
performance of the first cell and the at least one second
cell, and performing the load balancing operation if said
evaluating indicates that the potential load balancing
operation would improve the performance according to the
performance metric. The performance metric may characterize
or depend on an overall throughput of the first cell and
the second cell and/or a fairness of resource assignment to
terminals registered with the first base station or the
second base station. However, in certain embodiments, the
performance metric may depend on different characteristics
of the wireless network.
By evaluating the load balancing operation by means of the
3. ¾ C 1 C
operation the method prognosticates whether the load
balancing operation most probably would improve the
performance according to the performance metric or not.
Thus, inappropriate load balancing operations can be
avoided and the overall performance in terms of throughput,
fairness or the like is improved.
Preferably, the network is a hierarchical cellular network,
the first cell being a macro cell and the second cell being
a pico cell, a coverage area of the pico cell being smaller
tJtl3.Il O G S . O t h 1 C 0 X c G i l
first base station is a macro base station controlling the
macro cell and the second base station is a pico base
station controlling the pico cell. The macro cell and the
pico cell overlap each other at least partially, i.e., the
pico cell may be located completely within the macro cell
or the pico cell may be located at a cell border of the
macro cell so that only a part of the pico cell is located
within the macro cell.
In a preferred embodiment, said evaluating comprises
calculating a current value of the performance metric
related to a current operating station of the first cell
and the at least one second cell and calculating a
predicted value of the performance metric related to an
operating state of the first cell and the at least one
second cell that would appear if the load balancing
operation would be performed. The current value and the
predicted value may be compared wi
may decide depending on this comparison on whether to
perform the load balancing operation.
In another preferred embodiment, the values of performance
metric are determined based on a radio resource management
model and the metric is preferably a minimum terminal
bitrate .
The overall performance metric characterizes the
performance of the first cell including the at least one
second cell that at least partially overlaps that first
cell and therefore relates to multiple cells and the
corresponding base stations. In an embodiment, the method
comprises determining at least one cell specific value of a
cell specific performance metric, said cell specific value
characterizing the performance of the first cell or the
second cell and determining the current value and/or the
predicted value depending on the at least one cell specific
value .
Deciding on the load balancing operation is related to the
first base station and the at least one second base
station. Therefore, it is desirable to coordinate decisions
on whether to perform the load balancing operation among
the first base station and the concerned second base
stations. In a preferred embodiment this coordinating is
carried out by exchanging with network elements, preferably
with the first base station and/or the second base station,
the cell specific values, the values of the overall
performance metric, and/or an indication for indicating
whether said evaluating indicates that the potential load
balancing operation would improve the performance according
to the performance metric .
In an embodiment, performing the load balancing operation
comprises triggering a handover of a terminal from the
first cell to the second cell or from the second cell to
the first cell. According to this embodiment, a potential
handover is evaluated using the performance metric. If this
evaluation shows that a handover would improve the combined
performance of the first cell and the at least one second
is performed. Otherwise, the
handover is postponed or completely cancelled .
Preferably, the method comprises transmitting to a handover
target base station at least one parameter characterizing
radio conditions related to the terminal, preferably a
pathloss between the terminal and at least one base
station .
In an embodiment, performing the load balancing operation
comprises limiting a transmit power of a signal transmitted
by the first base station over a portion of radio resources
of the first cell and the second cell. Limiting the
transmit power typically augments the size of the second
cell so that more terminals residing within the first cell
may register with the second cell. However, increasing the
size of the second cell does not always improve the overall
performance. For example, if there is already a large
number of terminals registered with the second cell and if
there is a comparatively high load in the second cell then
increasing the size of the second cell making even more
terminals register with that second cell will not improve
the overall performance because a large number of terminals
share the second cell whereas the first cell is used only
little. However, if the second cell is almost empty then
increasing the size of the second cell improves the overall
performance because the second cell can reach more
terminals that may leave the first cell and register with
the second cell. When limiting the transmit power depending
on the evaluating of the performance metric allows for
semi -statically increase of the size of the second cell if
appropriate .
Preferably, the portion of the radio resources corresponds
to a time interval, preferably a frame or a subframe of a
framing structure of the wireless network, or a portion of
the frame or the subframe. In particular, the first base
station may limit the transmit power, preferably completely
suppress signal transmissions, within at least one portion
of the frame or the subframe. In this case in a time
synchronized system, the first base station transmits only
in that part of the frame or the subframe that is used for
transmission of reference symbols (e.g. pilots) for
mobility measurements, whereas the remaining parts of the
frame or the subframe are not used by the first base
station at all. This way the suppression by the first base
station also eliminates interference on the portion of the
subframe that is used for the control channel. When
applying the method in the Long Term Evolution (LTE)
system, the first base station may suppress transmission in
all portions of a su frame except these portions of the
subframe that are used for transmitting reference symbols
(pilots) . These subframes are also referred to almost blank
subframes (ABS) .
In an embodiment, the method comprises reverting said
limiting the transmit power. For example, if a large size
of the second cell is not required anymore then the first
base station may stop limiting the transmit power and use
the corresponding portion of the radio resources for
communicating with a terminal registered with the first
xo ,
Preferably, the method comprises evaluating an impact of
reverting limiting the transmit power to the overall
performance metric and reverting said limiting if said
evaluating indicates that said reverting would improve the
performance according to the performance metric.
In order to evaluate the impact of the performance metric,
the method may comprise calculating the current value of
the performance metric relating to the current operating
state, calculating the predicted value of the performance
metric relating to an operating state that most probably
will appear if limiting the transmit power is reverted. For
deciding on reverting limiting the transmit power the
current value of the performance metric may be compared
with the predicted value of the performance metric.
Preferably, the method may comprise determining at least
one cell specific value of a cell specific performance
metric, said cell specific value characterizing the
performance of the first cell or the second cell, and
wherein the predicted value of one cell is derived in
approximation from a load information submitted in relation
to predefined thresholds.
In an embodiment of the present invention, the method
comprises transmitting a handover request message from the
first base station to the second base station and
signalling to the second base station a portion of the
radio resources on which the first base station is willing
to limit the transmit power if the second base station
accepts a handover specified by the handover request. This
allows for combining the two above described load balancing
operations, i.e. triggering a handover and limiting the
transmit power.
In an embodiment, the method comprises transmitting a
limiting request from the second base station to the first
base station for requesting the first base station to limit
the transmit power of the signal transmitted over a portion
of radio resources .Preferably, the method comprises
transmitting a handover request from the first base station
to the second base station and receiving a handover
rejection from the second base station, the handover
rejection indicating whether the second base station cannot
accept the requested handover due to insufficient control
channel radio condition to the terminal. In an embodiment,
the method comprises transmitting a handover request from
the first base station to the second base station and
limiting the transmit power of the signal transmitted by
the first base station over a portion of the radio
resources if a handover rejection related to said handover
request is received from the second base station. So in one
embodiment the method comprises that a handover rejection
indicates that the second base station cannot accept the
requested handover due to insufficient control channel
condition to the terminal. After having received the
handover rejection and after having limited the transmit
power, the first base station may transmit a further
handover request to the second base station. Under normal
circumstance, limiting the transmit power should have
removed the insufficient control channel condition to the
terminal and the second base station should be able to
accept the requested handover.
In an embodiment, the method comprises transmitting a
limiting request from the second base station to the first
base station for requesting the first base station to limit
the transmit power of the signal transmitted over a portion
of the radio resources. In particular when applying the
method in a LTE system, the limiting request may be a
muting request for requesting the first base station to
insert almost blank subframes (ABS) into the framing
structure of LT .
In an embodiment said evaluating is performed on a single
network element, preferably on a base station. For
instance, the performance metric may be calculated by the
first base station only, with the second base station
transmitting values specific to the second cell to the
first base station. In particular, the second base station
may calculate only the current value of the metric specific
to the second cell and transmit this value to the first
base station.
According to an embodiment, a network element of a wireless
network is provided, said network comprising a first base
station having a first cell and at least one second base
station having a second cell, the first cell and the second
cell at least partially overlapping each other, wherein the
network element comprises control means arranged for
evaluating an impact of a potential load balancing
operation to an overall performance metric, said metric
characterizing the performance of the first cell and the at
least one second cell, and performing the load balancing
operation if said evaluating indicates that the potential
load balancing operation would improve the performance
according to the performance metric . The control means maycomprise,
e.g., a processor or micro computer programmed
for executing a method according the present invention,
embodiments of which are described above.
Preferably, the network element is the first base station
or the at least one second base station, the control means
of which being arranged for executing a method a method
according to the present invention, embodiments of which
. -2- -L- " _ ¾ ' C -
Brief description of the figures
Preferred embodiments and further advantages of the present
invention are shown in the f
X c t .
Figure 1 shows a cellular communication network;
figure 2 shows network elements of the cellular network
shown in figure 1 ;
figure 3 shows diagrams of resource allocation in the
network shown in figure 1 ;
figure 4 shows a flow chart of a method for operating a
network element of the network shown in figure 1 ;
and
figure 5 shows a sequence chart of signalling messages
exchanged between a pico base station and a macro
base station of the cellular network shown in
figure 1 .
Description of the embodiments
The description and drawings merely illustrate the
principles of the invention. It will thus be appreciated
that those skilled in the art will be able to devise
various arrangements that, although not explicitly
described or shown herein, embody the principles of the
invention and are included within its spirit and scope.
Furthermore, all examples recited herein are principally
intended expressly to be only for pedagogical purposes to
aid the reader in understanding the principles of the
invention and the concepts contributed by the inventors to
furthering the art, and are to be construed as being
without limitation to such specifically recited examples
and conditions. Moreover, all statements herein reciting
principles, aspects, and embodiments of the invention, as
well as specific examples thereof, are intended to
encompass equivalents thereof.
Figure 1 shows a cellular network 11 having multiple macro
cells 13. Each macro cell 13 has a macro base station 15
arranged for controlling the macro cell 13, in particular
terminals 17 located within that macro cell 13 and
registered with the macro base station 15 of that macro
cell 13 . In the shown embodiment a single macro base
station 15 is assigned to three macro cells 13. In another
embodiment, only one macro cell 13 is assigned to a macro
base station 15 .
Furthermore, the cellular network 11 has multiple pico
cells 19, each of them having a pico base station 21. In
the shown exemplary embodiment, each pico base station 21
controls exactly one pico cell 19 and terminals 17
registered with the corresponding pico base station 21. A
maximum transmission power of a radio signal transmitted by
a pico base station 21 is less than a maximum transmission
power of a radio signal sent by the macro base station 15.
Consequently, the size of a pico cell 19, i.e., the
coverage area of a pico cell, is less than the size of a
macro cell 13 . The pico cells 19 are overlapping with at
least one macro cell 13. A pico base station 21 is
preferably located within an area where a density of
terminals 17 is comparatively high. At least a part of the
terminals 17 located within a pico cell 19 may leave the
macro cell 13 and register with the pico base station 21 of
the pico cell 19. In this way, installing pico base
stations 21 in areas with a high density of terminals 17
helps to improve a quality of service and/or a channel
capacity experienced by users of the terminal 17 located in
that area having a high terminal density.
The cellular network 11 may be part of a Long Term
Evolution (LTE) or Long Term Evolution advanced (LTE
advanced) mobile communication system. Both LTE and LTE
advanced are specified by the Third Generation Partnership
project (3GPP) . However, the present invention is not
limited to LTE or LTE advanced. In LTE the base stations
13, 15 are referred to as enhanced nodeB (eNodeB) . The
terminals 17 are often referred to as User Equipment (UE) .
The invention may be applied in connection with different
types of cellular networks or mobile communication systems,
too .
Figure 2 shows network elements of the network 11, such as
the macro base station 15 and the pico base station 21 in
more detail. Each base station 15, 21 has a transceiver 23
coupled with an antenna 25 for transmitting a radio signal
to terminals 17 and for receiving a radio signal sent by
the terminals 17 .
The base stations 15, 21 have interconnection network
interface circuitry 27 connected to interconnection means
for interconnecting the base stations 15, 21 to each other,
such as an interconnection network 29. When using LTE, the
base stations 15, 21 may communicate with each other
according to the so-called X2 interface.
Moreover, the base stations 15, 21 comprise control means
31 such as control circuitry preferably comprising a
processor programmed for executing a method for operating
the base station 15, 21. In particular, the control means
31 may be configured, preferably programmed, for executing
a method for deciding on a potential load balancing
operation in the wireless network 11. An exemplary method
for deciding on the potential load balancing operation is
described below.
When operating the network 11 having at least one pico cell
19 located at least partially inside a coverage of the
macro cell 13 the overall throughputs of all cells 13, 19
and or the quality of service seen by the terminals 17
shall be maximized. To this end, the network 11 may perform
a load balancing operation in order to move load from the
macro cell 13 to a pico cell 19 and vice versa.
If the macro cell 13 and a pico cell 19 use the same radio
resources, in particular if the same radio carrier is used
then time domain inter-cell interference coordination
(ICIC) may be used in order to coordinate interference on
the control channel. If the cells 13, 19 use both multiple
carriers then frequency-domain ICIC may be used in order to
coordinate interference on the control channel.
Because the pico base stations 21 have a comparatively
small form factor and because of regulatory restrictions
the power of a signal emitted by the pico base station 21
is a low compared to the power of a signal emitted by the
macro base station 15. Therefore, a coverage area of a
pico cell 19 is smaller than the coverage area of a macro
cell 13. In case that only a small number of terminals 17
is registered with a pico cell 19 then a possible load
balancing operation may consist in decreasing a maximum
transmit power used by the macro base station 15 for
transmitting a portion of radio transmission resources 32
in order to increase the coverage area of the pico cell 19.
Decreasing of the transmit power used by the macro base
station 15 in that portion of the radio resources 32
reduces interference between the macro base station 15 and
the terminals 17 of the pico base station 21 so that the
pico base station 21 may reach terminals 17 that are
located rather distant from the pico base station 21. Thus
reducing or limiting the maximum transmit power on the
portion of the radio resources 32 by the macro base station
leads to an increased coverage area A2 (see figure 1 ) of
the pico cell 19. The increase of the coverage area of the
pico cell 19 due to reducing the interference by the macro
base station is also referred to as "foot print increase".
Theoretically, it is also possible to augment the coverage
area of the pico cell 19 by increasing the transmit power
used by the pico base station 21. However, in many cases,
the transmit power of the pico base station 21 is limited
by the small form factor of the pico base station 21 or by
regulatory restrictions.
Figure 3 shows a transmit power P of signals emitted by the
macro base station 15 of a macro cell M - and a pico base
station 21 of a pico cell P over a common time axis. The
radio transmission resources 32 comprise a carrier 33 that
is used in both cells M i and P . The network 11 maintains a
framing structure 35 . The framing structure 35 comprises
subsequent radio frames 37. In figure 3 , only one radio
frame 37 is shown. Each radio frame 37 is subdivided into
multiple subframes S ... S , with R indicating the total
number of subframes within a single radio frame 37. A s can
been seen in figure 3 , the base stations 15, 21 of cells
and i are synchronized with respect to each other
concerning the framing structure 35, in particular the
timing of the radio frames 37 and the subframes S ... SR .
A s shown in the diagram in the top of figure 3 , a subframe
S3 with a limited transmission power is inserted into the
sequence of subframes Si, ... S of the macro base station
15 . When using LTE , the macro base station 15 transmits
during this subframe S3 only essential reference symbols.
Therefore , only the parts of the subframe S3 allocated for
the corresponding reference symbols are used by the macro
base station 15, whereas the macro base station 15 does not
transmit at all during the remaining parts of that subframe
S3 . Therefore, the subframe S3 is also referred to as
Almost Blank Subframe (ABS) .
In another embodiment, the transmit power P of the signal
emitted by the macro base station 15 of cell i is limited
to a reduced power level P ed r subframe S3 . In another
embodiment the macro base station 15 of cell does not
transmit at all during the whole subframe S3 .
Because the transmit power of the signal emitted by the
macro base station 21 of cell is considerably reduced
interference with a signal emitted by the pico base station
21 of cell is reduced during subframe S3. Therefore, the
pico base station 21 of cell can reach terminals 17 that
are relatively distant from that pico base station 21. In
other words, the coverage area of the pico cell
increases .
Terminal 17 located in the increased coverage area A can
receive control channel signals emitted by the pico base
station 21 (e.g. the Physical Downlink Control Channel,
PDCCH of LTE) without experience interference from the
macro base station 15 of cell P in subframe S3.
Furthermore, a data channel (e.g. the Physical Downlink
Shared Channel, PDSCH of LTE) of the pico cell P in
subframe S3 does not experience interference from the macro
base station 15 of the cell P if a terminal 17 registered
with the pico base station 21 resides within the increased
coverage area A2 . Preferably, the subframe S3 during which
a transmit power of the macro base station 15 is reduced is
signalled to the terminal 17 in order to avoid problems of
channel estimation, channel state measurements and radio/or
link failure detection that may occur when inserting ABS
into the radio frame 37.
Thus terminals 17 that reside within the increased coverage
area A2 but not within the regularly coverage area are
preferably scheduled in the subframes that are power
restricted by the macro base station 15 because they can
then receive the control channel (PDCCH) from the pico base
station 21.
In the shown embodiment, the pico base station 21 does not
change the transmit power of the signals emitted into the
pico cell The transmit power is always PPi O for all
subframes S ... S .
In the shown embodiment only one subframe S3 with reduced
transmit power is inserted into the radio frame 37.
However, multiple subframes with limited transmit power,
e.g. ABS , may be inserted into a single radio frame 37.
If the coverage area of the pico cell i increases then
more terminals 17 may register with the cell . A s a
consequence, load of the macro cell i is moved to the pico
cell Pi. In this sense, limiting the transmit power during
a subframe Si, ... S (e.g. inserting an ABS into the radio
frame 35) is a load balancing operation.
However, increasing the coverage area of a pico cell 19
does not always improve the performance of the network. 11.
For example, if there are already many terminals 17 using a
single pico cell 19 then adding additional terminals 17 to
this pico cell 19 does not improve the overall performance
pico cell 19 is already heavily loaded. In such
a situation some terminals located in the coverage area of
the pico cell 19 should remain in the macro cell 13 . Thus ,
inserting the subframe S3 with the limited transmit power
is not needed. Moreover, avoiding inserting a subframe S3
with the limited transmit power or removing a previously
inserted subframe with limited transmit power increases the
performance of the network 11 because the macro base
station 15 has more radio transmission resources 32
available for communicating with terminals 17 that are not
registered with the pico base station 19. Therefore, in an
embodiment of the present invention, the decision on
whether to insert a subframe S3 with limited transmit power
is taken semi-statically depending on an operating state of
the network 11.
A second type of load balancing operation consists in
triggering a handover of a communication session of a
terminal 17 from one base station 15, 21 to another base
station 21, 15. Handovers between the macro base station 15
and the pico base station 21 immediately transfers network
load caused by this terminal 17 between the base stations
15, 21.
Figure 4 shows a method for semi-statically deciding on
whether to perform a load balancing operation, e.g.
inserting a subframe with limited transmit power or
triggering a handover between the macro cell 13 and the
pico cell 19, depending on a current operating state of the
network 11, in particular of the macro cell 13 and all pico
cells 19 that are located at least partially within that
macro cell 13. After a start 43 of the method 41, an impact
of a potential load balancing operation on an overall
performance metric characterizing the performance of the
macro cell 13 and all pico cells 19 located at least
partially within that macro cell 13 is evaluated in block
Block 45 comprises a step 47 of calculating a current value
u of the performance metric related to the current
operating state of the macro cell 13 and the pico cells 19.
Furthermore, the block 45 comprises a step 49 of
calculating a predicted value p of the performance metric
related to an hypothetical operating state that will
appear if the load balancing operation is performed. After
steps 47 and 49 a step 51 is executed that compares the
current value u and the predicted value pre of the
performance metric M and determines whether the load
balancing operation would improve the performance
characterized by the performance metric M . Step 51 takes a
decision d on whether the load balancing operation should
be performed.
The method 41 may be executed in a distributed manner. For
example, multiple network elements of the network 11, such
as the macro base station 15 and the pico base station 21
may execute at least some of the steps shown in figure 4 .
In order to coordinate the decision on whether the load
balancing operation should be performed between these
network elements 15, 21, the method 41 may comprise a step
53 that exchanges the values u of the performance
metric M and/or the decision d taken based on these values
Mp e with other network elements 15, 21. Then a branch
55 is executed for definitely decide on whether to perform
the load balancing operation. Step 55 may decide depending
on the decision d and/or a result of the information
exchange 53 . If step 55 decides that the load balancing
operation shall be performed (Y) then the step 57 of the
me
balancing operation. Otherwise (N) step 57 is skipped and
the method 41 is terminated. After step 55 has been
executed the method 41 is terminated.
In an embodiment, branch 55 decides to perform the load
balancing operation if the decision d indicates that the
load balancing operation shall be performed and step 53
shows that the other network elements 15, 21 have taken the
same decision d . In another embodiment the network elements
15, 21 carry out the same method on the same input
parameters independently and come to the same decision. In
further different embodiments the network elements 15, 21
may negotiate whether the load balancing operation shall be
performed in a different way. In yet another embodiment,
step 53 is omitted and branch 55 decides depending on the
decision d only.
The method 41 may be executed repeatedly or periodically.
In an embodiment, the method 41 is executed each time a
potential load balancing operation has been determined and
a decision on whether to perform this load balancing
operation is required.
In a preferred embodiment, the metric M comprises the
result of a radio resource management model (RRM model) .
These performance metrics include at least one of: cell
throughput, a minimum throughput of the terminals 17 (e.g.
J _* - - " -
instance the fifth percentile) , a minimum terminal bit rate
in a cell 13, 19, an average or maximum packet delay, or a
fairness metric characterizing an overall fairness of radio
resource allocation to the individual terminals 17.
The RRM model may use one or more of the following input
p . t S Z O f t ΐΏ S 17 X GC with a cell
13, 19, traffic characteristics (e.g. required bit rates)
of the terminals 17, interference experience by the
terminals 17, channels of the serving cell 13, 19, path
losses between a base station 15, 21 and a terminal 17. In
an embodiment, the fact whether a terminal 17 can be
reached by a certain base station 15, 21, in particular
whether a control channel of that base station 15, 21 can
be received by the terminal 17, forms an input parameter of
the RRM model .
The RRM model may use a part of the above parameters only.
For example, a quite simple RRM model may be provided that
models an average terminal throughput.
The load balancing operation 57 may comprise a handover 61
between the macro cell 13 and a pico cell 19. As shown in
the equations below the performance metric is evaluated
before a handover has taken place (symbol "noHO" ) and
evaluated assuming a handover would take place (symbol
"HO") .
PerfPKnoHO)
PerfMKnoHO)
PerfPKHO) =
PerfMl(HO) -
The situation before the handover is a combined resulting
in an the performance indicator Mcur and the operating state
predicted after the handover is combined resulting in a
predicted indicator Mpre . The handover decision is taken
when an overall improvement of the combined performance is
achieved, i.e. the handover is performed if M e > Mcu .
As shown in the above equations, the values Mcu and M e of
the overall performance metric M may be calculated
depending on values PerfPIO, Perf l of cell specific
performance metrics. These values may be calculated by
using a cell specific radio resource model RRMpicoO,
RRMmacroO . In a preferred embodiment, the base stations
15, 21 exchange the cell specific values Perf i (), PerfMlO
and/or the overall values Mc , M e . Step 53 may comprise
exchanging the values Perf I (), PerfMlO of the cell
specific performance metrics as current and predicted
values and/or exchanging values M and M of the overall
performance metric M between the base stations 15, 21. In
an embodiment, the macro base station 15 calculates the
i L C" S -P € 1 ( ) t - - " S J 1 C- ™f C5 - C* t . *
related to the macro cell Ml and/or the pico base station
21 calculates the values PerfMlO of the cell specific
performance metric related to the pico cell PI. In this
case, the macro base station 15 does not need to calculate
the values Perf l and the pico base station 21 does not
need to calculate the values PerfMl .
In an embodiment, the above described predicted evaluation
of the operating state after the handover may facilitated
by sending information about the designated terminal 17
(e.g. its path loss to the serving cell and or interference
cell) to a target base station 15, 21.
The load balancing operation may also comprise limiting
(step 63) the transmit power of a signal send by the macro
base station 15 on a portion of the radio transmission
resources 32. This portion of the transmission resources 32
may correspond to a time interval such as a subframe Si,
... SR . In particular at least one ABS may be inserted into
the framing structure 35 as described above.
In one embodiment, the macro base station 15 offers with a
handover request message sent to a pico base station 21 to
add one or more ABS. The pico base station 21 takes into
account this offered ABS in evaluating the performance
metrics. Due to an improved performance, in particular
control channel performance, in the pico cell 19 due to the
offered ABS it is more likely that the handover will be
performed .
In another embodiment, the macro base station 21 does not
offer with the handover request message to insert an ABS
into the framing structure 45. This could lead to the
performance - as indicated by the estimated value of the
performance metric - decreasing after the potential
handover has been performed such that the handover will not
take place. However, the pico base station 21 may send a
handover rejection message to the macro base station 15,
this message including an indication that the handover
rejection is due to insufficient control channel condition
to the location of the terminal 17. After having received
this handover rejection message the macro base station 21
may change the ABS configuration, in particular the macro
base station 15 may add an ABS in the framing structure 45
and send a new handover request message to the pico base
station 21.
In both above described embodiments as result, a handover
from the macro base station 15 to the pico base station 21
is combined with limiting the transmit power of the signal
send by the macro base station 15 on a portion of the radio
transmission resources 42 .
In an embodiment, the pico base station 21 may send a
qualified muting request 65 to the macro base station 15 as
shown in figure 5 . By sending the muting request 65, the
pico base station 21 requests adding an ABS in the framing
structure 35 of the macro cell 13. The muting request 65
may comprise the current value PerfPl(curABS) being part of
the performance metric M r or the current value c of the
overall metric and the predicted value PerfPKnewABS)
being part of the performance metric Mpre or this metric Mpre
itself. The predicted value PerfPKcurABS) characterizes an
estimated performance the pico base station 21 would have
if the ABS is inserted. The macro base station 15 receives
the muting request 65 or maybe multiple muting requests and
evaluates its own performance without the additional ABS
and the predicted performance when the ABS is added. By
combining the multiple performance metrics into two overall
performance metrics M u and e and comparing them the
decision to add an ABS is taken. To take this decision, the
method 41 may be executed. When the overall performance of
multiple cells (one or more pico cells 19 and one macro
cell 13) is estimated to be improved then the additional
ABS is set, otherwise it is not.
The following equations show how the values C r and M e of
the overall performance metric M are calculated:
The operating state curABS before the additional ABS is
inserted into the framing structure 35 is evaluated by the
current value M . The operating state newABS predicted
after a potential insertion of an additional ABS is
evaluated by the predicted value pre . The final decision on
whether to insert the ABS is taken if an overall
improvement of a overall performance is expected to be
achieved. As shown in the equation above, values PerfPlO,
Perf l of cell specific performance metrics determined
based on cell specific radio resource models may be
calculated. The values . , M e of the overall performance
metric may be determined depending on the cell specific
values PerfPlO, PerfMlO . In an embodiment the values
PerfPl, Perf l , M u and/or Mp e may be exchanged between the
base stations 15, 21 as described above in connection with
evaluating a possible handover.
The load balancing operation of inserting an ABS (step 63)
may be automatically reverted. To this end, the macro base
station 15 and/or the pico base station 21 may recalculate
the overall performance metric in order to evaluate whether
removing the ABS would increase the performance of the
network 11. Again, the base stations 15, 21 may exchange
the values PerfPl, PerfMl, and/or Mpre as described
above in connection with evaluating a possible reverting of
the ABS setting.
In an embodiment, the decision process of removing the ABS
may be initiated by the macro base station 15 by requesting
load information from at least one pico base station 21.
Preferably, the macro base station 21 indicates to the pico
base station 21 which ABS is intended to be declared to a
normal (non-ABS) subframe. Furthermore, the macro base
station 15 may request the cell specific performance metric
or overall performance metric related to the current
operating state and a predicted performance metric under
the assumption that one ABS is removed from the framing
structure 35.
Moreover, the macro base station 15 calculates the current
and the predicted performance metrics related to the macro
cell 13 and determines an overall performance, i.e., the
values M and M e . The values Mc r and M e may be
determined as descried above, e.g., depending on cell
specific values PerfMl, PerfPl. If comparing the values M u
and M e shows that removing an ABS would improve the
overall performance then the ABS is reverted to a normal
subframe, otherwise not.
In an embodiment, handover decisions and decisions
concerning adding or removing an ABS are taken
independently from each other. In this embodiment, the
macro base station 21 may have determined a fixed set of
subframes during which the macro base station 21 uses a
limited transmit power, e.g. by treating these subframes as
ABS. Thus, a handover of a terminal 17 registered with the
macro base station 15 in a cell border region (region A2
except region A ) is always possible since the terminal 17
can be reached by the pico base station 21 through a
control channel in one of the subframes during which the
transmit power of the macro base station 15 is limited
(e.g. during the ABS) . If the load in one of the pico cells
19 increases further then the pico base station 21 may
request at least one further ABS from the macro base
station 21 by sending a qualified muting request (see
figure 5 ) .
In another embodiment, no fixed set of subframes during
which the transmit power of the macro base station 15 is
limited, such as ABS, is configured. In this case, a
with the
macro base station 15 and located in the cell border region
of a pico cell 19 will fail because the pico base station
21 cannot communicate to this terminal 17 located too far
away from the pico base station 21. In this embodiment, the
handover rejection message send by the pico base station 21
back to the macro base 15 station may comprise the
indication that the handover is rejected because of an
insufficient control channel condition to the terminal due
to a missing resource restriction by ABS in the macro cell
19. After having received this indication, the macro base
station 15 may add at least one ABS into its framing
structure 35 and request the handover again.
Regarding the performance criteria for the handover
decision or the limiting of the transmit power, the minimum
terminal bit rate of a cell may be used. For example, if
the minimum of the two minimum bit rates of the pico cell
19 and the macro cell 13 is estimated to be increased if
the handover is performed then the handover takes place,
otherwise is does not take place.
In an embodiment, the evaluation 45 is performed by a
single network element, e.g., the macro base station 15 or
one of the pico base stations 21. At least one base station
15, 21 may signal a parameter characterizing a load value,
preferably load in the extended coverage region (A 2 except
region Ai) , depending on o of c l l 13 1
controlled by this base station 15, 21 to said single
network element. This parameter can be used to derive the
current and predicted performance value preferably of the
pico cell 19 assuming a potentially changed ABS setting by
the macro base station 15. This parameter may be conveyed
in a muting request. This allows that the receiving side
can use its current and predicted performance metrics to
put into the evaluation and take the decision based on this
information. The load value may also correspond to a number
n of terminals 17 in the extended coverage region (A 2
except region Ai) or the number of terminals ¾ , nm
registered with the cell 13, 19. In an embodiment, the
parameter characterizing the load is quantized in relation
to predefined load thresholds and can have one of a few
discrete values only, such that the parameter can be
represented by a set of 1 bit, 2 bits, 3 bits, 4 bits, or
even more bits. This bitset can easily be integrated into
the muting request.
In general, the radio resource management model (RRM model)
estimates the performance of the network 11, a group of
cells (e.g. a macro cell 13 and the pico cells 19 located
at least partially within that macro cell 13) , or a single
3 1 n p . h n easily
obtained, e.g., by measurement procedures or acquisition of
an operating state of a network element such as the base
stations 15, 21 or the terminal 17.
In the following, two exemplary RRM models are described. A
first RRM model allows for calculating a performance metric
for a handover decision where some subframes in the macro
cell have a limited transmit power (e.g. ABS) .
In a simplified approximation, performance metrics for
handover decision for a Pico cell can be expressed by the
following equation (with round robin assumption) . The Pico
cell throughput (under the assumption that an incoming or
outgoing terminal is served in the subframes with limited
transmit power) can be approximated as follows.
(MSF I10) * N NPMUE+x
RRM picdxHO) = — — Th{SIR(PMUE )) +
-
RRM_Pico (xHO) gives the throughput of the Pico cell. It is
assumed that the available resources in the subframes with
limited transmit power are equally distributed among the
PMUEs which have to be served in the subframes with limited
transmit power because they are located in the overlapping
region (region A2 except region A ) and the available
resources in the normal subframes are equally distributed
among the PNUEs which can be served in the normal subframes
because they are located in the center region of a Pico
cell (region in Figure 1 ) .
Evaluation of performance metrics for handover decision for
a macro cell can be expressed by the following equation.
{NSF / 10 * N R RRM MacroixHO) = — > Th(SIR(MUE ~ NMUE - x h
RRM_Macro (xHO) gives the throughput o f the macro cell
averaged over a radio frame. I t i s assumed that the
available resources in the normal subframes are equally
distributed among the MUEs because they can only b e served
n t h - c s )f s •
The meaning o f the symbols used in the above equations i s
a s follows.
when n o handover i s assumed
when handover i s assumed
0 ; when n o handover i s assumed
= 1 ; when handover from macro t o pico i s assumed
- 1 ; when handover from pico t o macro i s assumed
N B number o f available physical resource blocks
(PRB) for a given frequency band
MSF number o f muted (almost blank) subframes per
radio frame (10 subframes)
NSF number o f normal subf
(10 subframes)
NPMUE number o f Pico UEs served in muted subframes
per radio frame
NPNUE number o f Pico UEs served in normal subframes
per radio frame
NMUE number o f Macro UEs served in the Macro base
station (served in normal subframes)
Th(SIR(PUE ) ) throughput (in bits/s) from one PRB for U E
(spectral efficiency o f U )
The same method with these simplifications can b e applied
t o obtain e.g. the minimum terminal bitrate in a cell and
take this a s RRM _ picdxHO) .
In the following, an example for performance metrics for
additional ABS setting is given when some subframes are
muted in the Macro cell (i.e. some subframes are ABS) . In a
simplified approximation, performance metrics for
additional ABS setting decision for a pico cell can be
expressed by the following equation (with round robin
assumption) .
[(MSF + ) 10] * N RB
P E ,,
RRM pico(xABS) = . Th(SIR(PMUE )) +
NPMUE
—[(NSF - m )—I 10] * N— E
Th(SIR(PNUEh))
NPNUE
RRM_Pico (xABS) gives the throughput of the Pico cell. It
has to be noted that the number NPMUE and NPNUE of UEs
classified in muted or normal subframes can also depend on
the muted subframe setting m .
Evaluation of performance metrics for additional ABS
setting for a Macro cell can be simplified expressed by the
following equation.
RRM )
RRM_Macro (xABS) gives the throughput of the macro cell
averaged over a radio frame .
The additional symbols used in these equations have the
following meaning.
fcurrABS; when ABS settigs not modified
xABS -
[newABS; when ABS settigs modified
0 ; when ABS settigs not modified
m = 1 ; when number of ABS increased by 1
- 1 ; when number of ABS decreased by 1
The same approach with these simplifications can be applied
to come e.g. to the minimum terminal bitrate in the cells
and take this as performance indication to derive the ABS
setting decision from.
To sum up, the embodiments of the present invention allow
for improving the overall performance of a wireless
network, in particular a set of radio cells comprising a
macro cell 13 and at least one pico cell 19 overlapping at
least partially with that macro cell 13. To this end
embodiments of the present invention perform decisions on
restricting resource usage by the macro base station 15, in
- t * -1— J X - t - X " - - __L t * - ¾ _L - X JL
radio transmission resources 32 including inserting ABS in
the framing structure 35 of the macro cell 13. Furthermore,
decisions on reverting these restrictions may be performed.
Moreover, handover decisions concerning handovers of
terminals 17 from the macro base station 15 to a pico base
station 21 may be taken either if the control channel of
the macro base station 15 cannot longer reach the terminal
17 or in order to off load traffic to the pico base station
21. In addition, decision related to handovers of terminal
17 from the pico base station 21 to the macro base station
13 may be performed. These decisions may be taken if the
terminal 17 can no longer be reached by the control channel
transmitted by the pico base station 21. A handover from a
pico base station 21 to the macro base station 15 may also
be determined to be necessary if the traffic in the pico
cell 13 has increased and the traffic should be off loaded
to the macro cell 13. By these decisions, the quality of
service for the terminal 17, e.g. a minimum terminal bit
rate in the system or the overall throughput of both the
macro cell and the at least one pico cell 19, or another
quality criteria, shall be improved.
including any functional blocks labeled as processors or
control means 31', may be provided through the use of
dedicated hardware as well as hardware capable of executing
software in association with appropriate software. When
provided by a processor, the functions may be provided by a
single dedicated processor, by a single shared processor,
or by a plurality of individual processors, some of which
may be shared. Moreover, explicit use of the term
'processor' or 'controller' should not be construed to
f € - u X t O h . . , 1 O Ut X 1
software, and may implicitly include, without limitation,
digital signal processor (DSP) hardware, network processor,
application specific integrated circuit (ASIC) , field
programmable gate array (FPGA) , read only memory (ROM) for
storing software, random access memory (RAM) , and non
volatile storage. Other hardware, conventi onal and/or
custom, may also be included. Similarly, any switches shown
in the figures are conceptual only. Their function may be
carried out through the operation of program logic, through
dedicated logic, through the interaction of program control
and dedicated logic, or even manually, the particular
technique being selectable by the implementer as more
specifically understood from the context.
t should be appreciated by those skilled
any block diagrams herein represent conceptual views of
illustrative circuitry embodying the principles of the
invention. Similarly, it will be appreciated that any flow
charts, flow diagrams, state transition diagrams, pseudo
code, and the like represent various processes which may be
substantially represented in computer readable medium and
so executed by a computer or processor, whether or not such
computer or processor is explicitly shown. A person of
skill in the art would readily recognize that steps of
various above -described methods can be performed by
programmed computers. Herein, some embodiments 3 . .XS O
intended to cover program storage devices, e.g., digital
data storage media, which are machine or computer readable
and encode machine-executable or computer- executable
programs of instructions, wherein said instructions perform
some or all of the steps of said above-described methods.
The program storage devices may be, e.g., digital memories,
magnetic storage media such as a magnetic disks and
magnetic tapes, hard drives, or optically readable digital
data storage media. The embodiments are also intended to
cover computers programmed to perform said steps of the
above-described methods.
Claims
Method (41) for deciding on a potential load balancing
operation 5 7 ) in a wireless network (11) comprising a
first base station (15) having a first cell (13) and
at least one second base station (21) having a second
cell (19) , the first cell (13) and the second cell
(19) at least partially overlapping each other,
wherein the method (41) comprises evaluating (45) an
impact of the potential load balancing operation (57)
to an overall performance metric (M) , said metric (M)
characterizing the performance of the first cell (13)
and the at least one second cell (19) , and performing
the load balancing operation (57) if said evaluating
indicates that the potential load balancing operation
(57) would improve the performance according to the
performance metric (M) , characterized in that the
method (41) comprises transmitting a limiting request
from the second base station (21) to the first base
station (15) for requesting the first base station
(15) to limit (63) the transmit power of the signal
transmitted over a portion (S 3) of radio resources
(32) .
Method of claim 1 wherein the network is a cellular
network (11) , the first cell being a macro cell (13)
and the second cell being a pico cell (19) » a coverage
area of the pico cell (19) being smaller than a
coverage area of the macro cell (13) , and wherein the
fist base station is a macro base station (15)
controlling the macro cell (13) and the second base
station is a pico base station (21) controlling the
pico cell (19) .
3 . Method (41) o f claim 1 o r 2 , wherein said evaluating
(45) comprises calculating (47) a current value (M c )
o f the performance metric (M) related t o a current
operating state o f the first cell (13) and the a t
least one second cell (19) and calculating a predicted
v a Lc (M) related to
a n operating state o f the first cell (13) and the a t
least one second cell (19) that would appear i f the
load balancing operation (57) would b e performed.
. Method according to one o f the preceding claims
wherein the values (M u r M pre ) o f performance metric
(M) are determined based on a radio resource
management model, the performance metric preferably
comprising a minimum terminal bitrate .
5 • s t 1 c CD t o € f t - -* "L s /
wherein the method (41) comprises determining a t least
a cell
specific performance metric, said cell specific value
(PerfMl, Per fPI) characterizing the performance o f the
first cell (13) o r the second cell (19), and
determining the current value (M cur ) and/or the
predicted value (M pre ) depending o n the current value
and predicted values o f the a t least one cell specific
c La .x e t c ( e r f , r f )
6 . Method (41) according t o one o f the precedent claims,
wherein the method (41) comprises exchanging (53) with
network elements (15, 21) , preferably with the first
base station (15) and/or the second base station (21) ,
the cell specific values (PerfMl, PerfPl) a s current
and predicted values, and/or the values (M r , M pre ) o f
the overall performance metric, and/or an indication
(d) o n whether said evaluating (45) indicates that the
potential load balancing operation (57) would improve
the performance according to the performance metric
(M) .
7 . Method (41) according to one of the precedent claims,
wherein performing the load balancing operation
comprises triggering an handover (61) of a terminal
(17) from the first cell (13) to the second cell (19)
or from the second cell (19) to the first cell (13) .
8 . Method (41) according to claim 7 , wherein the method
(41) comprises transmitting to a handover target base
station (15, 21) at least one parameter characterizing
radio conditions related to the terminal (17) ,
preferably a pathloss between the terminal (17) and at
least one base station (15, 21) .
9 . Method (41) according to one of the precedent claims,
wherein performing the load balancing operation (57)
comprises limiting (63) a transmit power of a signal
transmitted by the first base station (15) over a
portion of radio resources (32) of the first cell (13)
and the second cell (19) , said portion preferably
corresponding to a time interval, preferably a frame
or a subframe (S 3 ) of a framing structure (35) of the
wireless network, or a portion of the frame or the
subframe (S 3).
10. Method (41) according to claim 9 , wherein the method
comprises reverting said limiting the transmit power
and evaluating an impact of reverting limiting (63)
the transmit power to the overall performance metric
(M) and reverting said limiting (63) if said
evaluating indicates that said reverting would improve
the performance according to the performance metric.
11. Method (41) according to one of claims 5 to 10,
wherein the method comprises determining at least one
cell specific value (PerfMl, PerfPl) of a cell
specific performance metric, said cell specific value
(PerfMl, PerfPl) characterizing the performance of the
first cell (13) or the second cell (19) , and wherein
the predicted value of one cell is derived in
approximation from a load information submitted in
relation to predefined thresholds .
12. Method (41) according to one of claims 7 to 11,
wherein the load balancing operation comprises
transmitting a handover request message from the first
base station (15) to the second base station (21) and
signalling to the second base station (21) a portion
(S 3) of the radio resources (32) on which the first
base station (15) is willing to limit the transmit
power .
13. Method (41) according to one of claims 7 to 11,
wherein the load balancing operation (57) comprises
transmitting a handover request from the first base
station (15) to the second base station
receiving a handover rejection from the second base
station (21) , the handover rejection indicating
whether the second base station cannot accept the
requested handover due to insufficient control channel
radio condition to the terminal (17) .
14. Network element (15, 21) for a wireless network (11),
said network (11) comprising a first base station (15)
having a first cell (13) and at least one second base
station (21) having a second cell (19) , the first cell
(13) and the second cell (19) at least partiallyoverlapping
each other, wherein the network element
(15, 21) comprises control means (31) arranged for
evaluating (45) an impact of a potential load
balancing operation (57) to an overall performance
metric (M) , said metric (M) characterizing the
performance of the first cell (13) and the at least
one second cell (19) , and performing the load
balancing operation (57) if said evaluating (45)
l d .1 t t p s . 1 oj - s
(57) would improve the performance according to the
performance metric (M) , characterized in that the
control means (31) are arranged for transmitting a
limiting request from the second base station (21) to
the first base station (15) for requesting the first
base station (15) to limit (63) the transmit power of
the signal transmitted over a portion (S 3) of radio
so s (3 *
Network element of claim 14, wherein the network
ion (15) o h .t 1 .S
o c s1 . o o t o 1 means (31)
of which being arranged for executing a method (41)
according to one of claims 1 to 14 ,