Note: Descriptions are shown in the official language in which they were submitted.
WO 96100484 PCrlSE95100764
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INTR.A-CELL HANDOVER WITH ANTENNA ARRAYS
Field of the Invention
The present invention relates to an infra-cell handover method for use in a
cellular communication system and more particularly to an infra-cell handover
method which utilizes spatial information from an antenna array.
Background of the Disclosure
Current digital cellular systems employ base stations which distinguish
between different mobile stations by using time and frequency orthogonality.
Signals from a mobile station propagate to the base station. The signals are
received in a single antenna or sometimes two antennas to gain diversity
effects.
The receiver processes the signal using the time and frequency orthogonality
to
separate signals from different users. Handover between channels within a base
station, intra~ell handover, is based on quality measurements from the mobile
station and the base station. Infra-cell handover is used, for example, to
avoid
channels with strong interferers. Channel allocation, i.e., the selection of
the
new channel, is in most cases done according to a frequency plan. Adaptive
channel allocation, i.e., assigning a mobile station to a channel based upon
measured quality information, has been proposed for evolved TDMA systems and
is used in the digital cordless DFrCT system.
Spatial filters are lmown in the art and are used to create spatial
selectivity, i.e., to reject interference from certain directions and to
amplify
desired signals from other directions. A spatial filter can be implemented
with,
for example, a passive circuit with a radio frequency, a set of phase shifters
of
analog signals or by signal processing in the baseband. The term spatial
information is used to denote information about how a spatial filter can
process
signals. Spatial information can, for example, consist of the spatial filters
that in
some sense, are optimal to process .signals from mobile stations. The spatial
information can also consist of the direction-of arrival of the power from the
mobile stations.
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It is desirable Lo improve the spectral efficiency of such systems. One
way to improve the spectral efficiency of the system is to use arrays of
antennas,
e.g., to use a number of spatially separated antenna elements. It is then
possible
to distinguish between spatially separated users by using narrow adaptive
antenna
lobes. This can be viewed as a way to utilize orthogonality in spatial
dimension.
Current digital cellular systems employ base stations which use antennas
with wide antenna lobes of approximately 124 or 360 degrees. The base station
receives/transmits signals for all mobile stations within the lobe. It is
hence not
necessary to lrnow the position of the mobile station. It is, on the other
hand, not
possible to suppress mobiles transmitting from other angles. There is hence no
spatial information to take into account and handover has to be made without
spatial information.
A system using an adaptive antenna array can use a narrow antenna lobe
to receiveltransmit the desired signals and to suppress the undesired signals.
This
adds a new dimension to the handover strategies since spatial information can
be
used to fit mobiles into an appropriate channel.
It is necessary to take the spatial disturbance situation into account when
allocating channels in order to increase spe~al efficiency and to avoid an
excessive number of handovers. A mobile should "fit" a channel in the sense
that it is not too disturbed by mobiles already present at the channel in own
or
co-channel cells. At the same time, it is important to chose a channel where
the
new mobile station will not disturb the mobilC station already using the
channel.
It is also important that the transceiver characteristics for mobiles in their
own
cell should sometimes be modified as a new mobile is allocated to the channel.
The new mobile station could otherwise heavily disturb the other mobile
station
and cause lost calls or low quality. There is hence new information that must
be
taken into account in order to perform a correct handova.
In addition, the power levels in the system must also be adjusted so that
the base station power levels (recciveltransmit) are approximately equal for
all
mobile stations. A too large power differ~t will, in practice, destroy the
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spatial orthogouaiity and create problems very similar to the "near-far"
problem
which occurs in CDMA systems.
In systems using an adaptive ant~na array, both transmission and
reception is adaptive for the array base station which implies that the
addition of
a new mobile station can modify both transmit and receive antenna patterns for
surrounding base stations. Another differeace is the fact that multiple users
on a
channel in a base station using an adaptive array are not automatically
orthogonal.
The spatial orthogonality is something that must be created with adaptive
spatial
filtering and with proper channel allocation.
Summary of the Dixlo~
The present invention discloses an intra~ell handover method which
extends known handover algorithms by utilizing spatial information from an ..
antenna array. In addition, the present invention provides a decentralized
adaptive channel allocation solution where each base station allocates mobile
I5 stations to suitable channels by taking the interference situation into
aunt.
Furthermore, the present invention discloses a method for modifying the away
tranxeiver, i.e., a method for linking a mobile station to an already used
channel, when a new mobile station is introduce.
The present invention has at least three main advantages over the prior art.
ZO First, the capacity of the cellular system is increased since mobile
stations are
distributed in a proper way amongst the channels. Second, network signalling
is
minimized since the inventive solution is decentralized for each base station.
Finally, all transceivers using a channel are modified directly so as to take
the
new disturbance situation into account. As a result, the quality for the old
mobile
25 stations will not degrade as a new mobile station is introduced.
According to one embodiment of the present invention, a decision on
whether an intra~ell handover should be made is based on conventional
information andlor a prediction of increased spatial disturbance. If it is
determined that the power level deviates too much from a nominal value, an
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infra-cell handover is performed, wherein the mobile st~on is handai off to a
channel where it does not disawb other ions too much. However, for all
channels where the guality is above a predetermined level, a relative uplink
s disturbance on the new mobile station is com~od from measurements. Then, a
relative upIink disturbance on the mobile stations that m alrrady p~sant on
the
channel from the new mob~e station are computed using measurements. The
doovalink disaubance from mobile assisted haadafF m~~u~neats is then
calculated to eho~lc the~atistiung dow~nlinlcs. The best channel is they
sc1oc:~d
i o taldag conventional infra-cell handover information into aQOOUat toga with
the
information calculatod above. Finally, all stansoavers using the schannel
and the old channel are modified axarding to the near distiu~n~x sihia~ioa.
Accordingly, in one aspect, the invention provides an infra-cell handover
method
in a cellular communication system having adaptive antenna arrays, the method
IS comprising the steps of measuring spatial information with an antenna
array, determining
whether an infra-cell handover is desirable for a mobile station based upon
the measured
spatial information, and handing over the mobile station from a first channel
to a second
channel when desirable.
In another aspect, the invention provides an infra-cell handover method in a
2o cellular communication system having adaptive antenna arrays, comprising
the steps of
determining whether an infra-cell handover is necessary, measuring power level
of
mobile station which needs to be handed off, assigning the mobile station to a
channel
based upon the measured power level, computing the relative uplink disturbance
on the
mobile station, computing the relative uplink disturbance on other mobiles
from the
25 mobile station, computing the downlink disturbance to check existing
downlinks,
selecting best channel using the computed information, modifying spatial
filters of all
transceivers using the selected channel and the old channel according to new
disturbance
situation.
j~ed De, ~ f.~
3o The prat invention will now be dexn'bod in mono d~ with nfa~eax
to pnefcrred embodizneats of the invention, given only by way of example, and
illustrated in the anymg drawings, in which:
~
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4a
Figure 1 ~llusttaza a cautious handova daxsion according to one
embodima~t of the pirseat invention;
Figure 2 tikusttata channel allocattion aooording to one anbodimatt of the
s present invention; cad
l igiut 3 ~ustrates channel linking xcor~ag Zo one embodiment of the
t invention.
' do ~ of the Did
The presait invention discloses an infra-cell handova method which can
be divided into three main steps: ICHO-docision; channel allocation; and
channel
linking. Briefly, the ICHO-decision step determines wbaha an infra-cell
is handaver is n~sary. When a handover is noassaty, the channel allocation
step
determines as appropriate channel for the mobile station. Af'ia the mobile has
been handed off to the new channel, the traasoeivers of the mobile stations
which
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are presently using the channel are modified in the channel linking step so as
to
take the new interference situation into account. The present invention can be
combined with, for example, a conventional fined frequency plan, a slow
adaptive channel allocation or random frequency hopping. Fach of the
individual
5 steps will be descn'bed in more detail below.
The present invention describes several ways to enhance rapacity. The
first way, that is more conventional, is to use the antenna array to reduce
the
cluster size, i.e., the frequ~cy reuse distance. This implies that there is
only
one mobile station per channel in a cell and that the interference originates
from
users in other cells. The second way is to allow multiple users in a channel
where the antenna array is used to orthogonalize the users. The interference
will
then originate both from co-channel mobile stations inside the cell and from
other
cells. The present invention also covers ICHO in cases where combinations of
the techniques are used.
The ICHO-decision can, for example, be made with known techniques
based upon scalar quality measurements from the mobile station and the base
station. One example of a known technique is described in ETSUGSM
specification 05.08. Additionally, the algorithm can use a cautious strategy
where the spatial information is used to predict a deterioration in
transmission
quality. For example, it is possible to detect that two mobile stations MS 1
and
MS2 are moving close to each other, as illustrated in Figure 1. The mobile
stations can be connected to the same cell or they might be connected to
different
cells. This detection is straightforward given the spatial characteristics,
i.e.,
direction-of arrival, spatial filters,.., of the mobile stations. For example,
the
algorithm can track the angle from where the signal arrives and detect that an
angle difference between to mobile stations is too small. As a result, an ICHO-
decision can be made to move either of the mobile stations MS 1 and MS2 to
another channel. An ICHO decision can also be made in order to free a widelobe
channel from a user that can be processed on a narrowiobe channel. In GSM-
type systems, for example, the BCCH carries must be distributed over the
entire
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cell in a wide antenna lobe and hence the BCCH carrier is a widelobe channel.
A mobile station using the BCCH carrier's traffic slots may then be moved to
an
ordinary carrier where the downlink may be transmitted in a narrow antenna
Lobe, hence the notation narrowlobe channel. The ICHO decision may thus
reduce the interference level in the system. An ICHO decision may thus imply
that the mobile station is linked to a channel together with other mobile
stations.
The power of the mobile stations presently operating on the channel, i.e., old
mobile stations, must not drown the signal from the new mobile stations and it
is
ne~ressary for the mobile station to regulate its uplink power towards a
nominal
value. The base station downlink power is analogously set to a nominal value.
The channel allocation, i.e., the choice of a proper new channel, can be
made as follows. First, the uplink power level of the mobile station is
checked.
If the power level of a mobile station deviates by at least a predetermined
amount
from a nominal value, the mobile station is labeled as 'MS with extreme power
Level" and kept on a widelobe channel for the tune being. The channel
allocation
should also take the stability of the spatial information into account. A
mobile
station having tune variations in the measured spatial information can be kept
in a
wideiobe channel or generally treated with caution in order to reduce
interference
levels.
The uplink characteristics can be measvreci by the base station. For each
channel the disturbance power that will affect the base station receiver for
the
new mobile station caused by the existing disturbances on the channel can be
measured. This disturbance power is then compared to the power and path loss
of the new mobile station to form a signal-interference ratio. This quantity
is
denoted a~ where j is the number of the channel. The mobile stations connected
to other base stations act as uplink disturbances and will thus effect a~.
For each channel, the disturbance power that will affect the base station
receivers for the old mobile stations caused by the disturbance from the new
mobile station is measured. This disturbance power is compared to the power
and path loss of the old mobile stations to form a signal-interference ratio
for
WO 96!00484 PCTlSE95100764
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each old mobile station. This quantity is denoted pw where j is the number of
the channel and i is the number of the mobile.
The downlink characteristics are not measurable at the base station. The
algorithm should hence use measurements from the mobile station, such as
Mobile Assisted l3andover measurements. The disturbance situation is, however,
reciprocal in the sense that a channel where uplink transmissions do not
interfere
in a base station is a channel where downlink transmission from that base
station
to one mobile station will not disturb the other mobiles significantly. In
other
words, a base station will see the mobile station it is disturbing as a
disturber.
This fact is utilized in the channel allocation and linking to optimize
performance
and quality.
The mobiles will also see other base stations that the first base station
cannot take into account. The signals from these other base stations will, on
the
other hand, usually be weaker than the signal from the first base station.
This
implies that the parameters a~ and ~;~ can be used to pick a suitable channel.
They should also be combined with conventional channel allocafion information
such as mobile station interference measurements. The parameters a~ and ~;~
will
be computed below.
Figure 2 illustrates an example of channel alloation xcording to one
embodiment of the present invention. In Figure 2, a cell border 50 separates
two
cells 52 and 54. Each cell has a base station l~,Sl and BS2, respectively,
which
have antenna arrays. In this example, a first mobile station MS i is serviced
by
the base station 13S 1 and can be allocated to an arbitrary channel C 1. In
addition,
a second mobile station MS2 is servicxd by the base station BS i and can be
allocated to the same channel C 1 since the antenna array ran make the mobile
stations orthogonal in both the uplink and the downlink. As illustrated, a
third
mobile station MS3 has the second mobile station MS2 in the same angle sector
as viewed from the second base station BS2. As a result, the second base
station
BS2 will view the second mobile station MS2 as a distiu~bance in channel C1,
and
will thus allocate the third mobile station MS3 a different channel, such as
C2.
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In this example, a fourth mobile station MS4 will act as an uplink .
disturbance for the first mobile station MS 1 in the upli.ak to the first base
station
BS 1. When the fourth mobile station MS4 performs downiinlc measurements, the
mobile will determine that channel C 1 is disturbed by the first base station
BS 1.
The fourth mobile station reports the measurements to the second base station
BS2 which will assign the fourth mobile station to a channel other than C 1.
However, if the fourth mobile station MS4 is not capable of making sufficient
MAHO measurements, the second base station can allocate the fourth mobile
station to a channel based only on uplink measurements. As a result, the
mobile
station MS4 could be assigned to channel C 1. The first base station BS 1
would
then detect a new disturber for the first mobile station on channel C 1. As a
result, the first mobile station MS 1 should then be reallocated to another
channel
by the first base station BS 1 if the interference caused by the fourth mobile
station is too high.
The present invention can also use a stopping criteria which indicates if a
channel is heavily loaded since a mobile station should not be added if any of
the
users on the channel is too close to a quality limit indicating a possible
handoff.
Channel linking, the linking of the new mobile station to the channel can
be performed as follows. It is necessary to modify the transceivers for the
old
mobiles using the selected channel and the transceiver for the new mobile
station
that is going to be introduced on the selected chancel: The spatial filters
used in
the receivers and transmitters in the array base station are computed with a
steering vector. containing information about the desired signal and using
information about the distwbance situation on the channel. As a new mobile
station enters the channel, the disturbance situation is changed and all of
the
spatial filters must be recalculated to take the new information into account.
The
transceivers for the mobile stations on the old channel should also be
modified as
a disturber is removed.
Figure 3 illustrates an example of channel linlang according to one
embodiment of the present invention. In this example, the mobile station MS 1
is
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using channel C1 when the mobile station MS2 is allocated to channel C1. The
spatial filters in the transceivers for the mobile stations MS 1 and MS2 must
then
be adapted so that they are spatially orthogonal. In other words, the spatial
filter
for mobile station MS 1 should null out the mobile station MS2 and vice versa.
The present invention performs this hulling in one instantaneous step without
performing measurements. When the mobile station MS3 begins using a channel,
the mobile station MS3 will be measured and viewed as a new disturber from the
base station BS 1. However, the hulling of the mobile station MS3 can be
performed based upon spatial interference measurements.
According to one embodiment of the present invention, a simple channel
allocation method can be used. in this method, the mobile station can be
classified into power classes and used spatial sectors. For example, mobile
stations with approximately equal power levels and well separated spatial
filters
could share the same channel.
According to another embodiment of the preseat invention, a simple
algorithm using fined filters can be used for channel allocation. in this
example,
assume that a number of filters with pencil beams have been precomputed and
stored. For example, filters a(81), a(8~, ...a(8~) can be used with peach
beams in the directions -60 °, -55 °, .. ., 60 ° . The
filter could, for exampk, be a
Hamming window multiplied with the array steering vector for the desired
direction. Each filter is referred to by the direction of its pencil beam, its
direction~f arrival (DOA).
A suitable channel can be found as follows. A DOA is considered as used
by the mobile station if the DOA itself or one of its two closest neighbors
has
been successfully used during the last tea bursts. All channels where the
quality
is good enough are scanned to find a channel where the DOA used by the new
mobile station are neither disturbed by mobile stations in other cells or used
by
mobile stations in own cells. Typically, a DOA is judged as disturbed if the
output power from the filter is above a ftaCtiOQ of the nominal power level.
The
DOA's for the chosen channel are then occupied by the new mobile station and
WO 96/00484 pCTlSE95100764
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link the mobile station to the channel. The spatial transceiver for the new
mobile
station is simply the set of filters it is using. Thus, there is thus no need
to
modify the transceivers of the old mobile station on the channel.
According to one embodiment of the present invention, an advanced
algorithm for channel allocation can be used. The mobile to be reallo~ can
be characterized by a spatial filter matrix ZY, a covariance matrix R and a
power P. The J accessible channels are characterized by their spatial filter
matrices, W;~, i = 1, ..., M~ where M~ is the number of mobiles on channel j,
by
their covariance matrices, R~, and by the power of the mobiles Pw.
The expected power of the disturbance on the new mobile station relative
its useful power is
def trace[WgRjW]
a~ -
The expected disturbance power on the old mobile number i using channel j from
the new mobile is
def trace [Wi j RWi~ j ]
~i.J - pi..7
One choice is then to pick the channel Jo where
x
.To = arg min{ a j + ~ ~i . j}
1=1
Another solution is to measure the maximum distiufiance that will affect
any of the old mobiles and let
WO 96100484 PCTISE95100764
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~1
.To = arg min{ a3 + max~ij)
j i
Conventional scalar information should also be used. For example, a heavily
dis-
turbed channel should not be further loaded wording to the previously
mentioned stopping criteria.
According to one embodiment of the present invention, an advanced
algorithm for channel linking can be used. This example shows how the spatial
filters for the mobiles presently using channel .la and for the new mobile can
be
modified. The spatial filter matrices for the M mobiles using the channel are
denoted W~, ..., W~ and the new mobile is denoted IVM+1. First, the average
cross-correlation for the existing mobiles is computed wherein for notational
simplicity the correlation matrix of channel .lo is denoted Rte.
~xd ~1 ) = Rold Wi 1 = 1, . . . , M
The average cross-correlation for the new mobile, with the associated
correlation
matrix denoted R, is
Rxd ~ M + 1 ) = IdJX+1
The correlation matrix for the channel after the addition of the new mobile
can be
approximated as
Rne~ = Rold + R
It is then possible to compute new spatial filters as
wi - Rne~xd { 1 ) 1 - 1. . . . , M + 1
The foregoing treatment has assumed a unique R~ per channel but it is
straightforward to incorporate a dependence upon sampling phase.
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While a particular embodiment of the present invention has been described
and illustrated, it should be understood that the present invention is not
limited
thereto since modifications may be made by persons skilled in the art: The
present application contemplates any and all modifications that fall within
the
spirit and scope of the underlying invention disclosed and claimed herein.
