Note: Descriptions are shown in the official language in which they were submitted.
CA 02254643 1998-11-30
MOBILE CELLULAR TELECOMMUNICATION NETWORR
This invention relates to mobile cellular telecommunication networks.
s There are different systems for allocating channels for uplink
communications. All strive to maintain orthogonality between mobile
terminals in the same cell. Conventionally, the channels allocated to one cell
are not reused in adjacent cells so as to reduce interference.
Against this background, there is provided a mobile cellular
io telecommunication network, comprising a plurality of cells between which
are defined reuse regions each served by a plurality of directional antennas
each at respective base stations located around the reuse region boundary;
means for allocating all uplink channels at all antennas in all reuse regions;
means for determining boundaries of microcells within the reuse regions, a
is plurality, equal to or greater than the plurality of antennas, of
microcells
being served by each antenna in the reuse region; means for allocating to
each microcell a group of uplink channels in an orthogonal reuse pattern
within the reuse region; means for ascribing a position to mobile terminals
within each reuse region; and means for allocating uplink channels for use
2o by a mobile terminal from the group allocated to the microcell which
contains its ascribed position.
Since all channels are reused at all antennas, a11 channels are reusable
several times in all cells, greatly increasing the number of mobile terminals
which may operate without reducing cell size or requiring additional base
2s stations.
CA 02254643 1998-11-30
Preferably, an equal number of channels is allocated to each
microcell.
In order to ascribe positions to the mobile terminals, each mobile
s terminal is preferably adapted to determine power levels of at least the
three
strongest downlink signals from respective base station antennas and to
communicate the power levels and antenna identities to the base station
having the strongest downlink signal; the base station including means for
computing a virtual position of the mobile terminal apparent from the power
io levels. The virtual position may not correspond with the geographic
position of the mobile terminal. Indeed, it will only do so when there is no
shadow fading.
The mobile terminal is preferably adapted to determine the power
levels of pilot signals on the down link.
is The boundaries of the microcells are preferably determined
dynamically such that each microcell within a reuse region carries
approximately the same amount of traffic. To that end, for each antenna a
database is preferably maintained with the ascribed position of all mobile
terminals in the reuse region.
ao One embodiment of the invention will now be described, by way of
example, with reference to the accompanying drawings, in which:
Figure 1 is a schematic plan of part of a cellular mobile
communications network
embodying the invention and showing hexagonal base station cells and
2s reuse regions;
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Figure 2 is a schematic plan of one of the cells of Figure 1 showing
micro cells
therein;
s Figure 3 is a schematic plan similar to that of Figure 2, showing a
different arrangement of microcells;
Figure 4 is a plan similar to that of Figure 3 but showing how the
microcells can be arranged to obtain equal usage in each;
Figure 5 is a schematic plan of a square cell showing microcells
io therein.
Figure 6 is a chart showing a channel allocation pattern which may be
used in the network of Figure 1 or 5;
Figures 7a is a chart showing a channel allocation pattern alternative
to that of Figure 6;
is Figure 7b is a chart showing another alternative channel allocation
pattern;
Figure 8 is a chart showing another alternative channel allocation
pattern; and
Figure 9 is a flow chart showing how channels are allocated in the
ao network of Figure 1 or Figure 4.
Referring to the drawings, base stations 2 each serve a respective
hexagonal base station cell 3 bounded by thin lines in Figure 1. Each base
station comprises three receive stations 4. Each receive station has a 120~
directional antenna 6. The three antennas 6 are directed at respective receive
Zs cells 8 each in an individual l20~ sector within the base station cell 3.
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An area 10 served by three receive stations 4, thus containing three
receive cells 8, constitutes a hexagonal reuse region and is bounded by thick
lines in Figure 1. A reuse region 10 is illustrated, to a larger scale in
Figure
s 2. A reuse region is defined by the fixed directional antenna pattern of the
receive station.
All uplink channels used by the network are available for allocation at
all receive stations 4 for their respective receive cells 8. Inside each
receive
cell, uplink channels are allocated according to the position of the mobile
io terminals one of which is illustrated at 12. In the example illustrated in
Figure 2 each receive cell is divided into three microcells 14, 16 and 18
each progressively further from the respective base station. The total set of
uplink channels is divided into three blocks indicated by numerals I, II and
III in Figure 2. As will be seen, all three blocks are allocated at each
receive
i s station 4 and are allocated to the respective microcells 14, 16 and 18 in
an
orthogonal reuse pattern which, if used in all reuse regions, ensures that
adjacent microcells do not share the same blocks of channels either within a
reuse region 10 or between adjacent reuse regions.
Another reuse pattern is illustrated in Figure 3. Here the total set of
Zo channels is divided into channel blocks I to VI. The receive cells are
divided
into microcells according to distance from the receive station and laterally
into left and right sectors thus defining microcells 14L, 16L, 18L, 14R, 16R
and 18R. Again the reuse pattern illustrated ensures orthogonality between
adjacent microcells whether within one reuse region to or between adjacent
2s reuse regions. The antennas 6 cover an entire receive cell. They do not
provide directional reception divided into left and right sectors.
CA 02254643 1998-11-30
The position of a mobile terminal 12 is assessed from the power of
pilot signals transmitted by all base stations on the downlink. The mobile
terminal 12 identifies at least the three pilot signals received with the most
s power and the base station from which they each is transmitted. Among
these the mobile terminal identifies the most powerful pilot signal and its
base station. The mobile terminal 12 transmits the power levels and station
identities to the base station from which the most powerful pilot signal was
received on a call setup channel. The base station then calculates the virtual
io or apparent position of the mobile station by determining the virtual or
apparent distance from each by comparing the power levels of the received
pilot signal. The virtual or apparent position may correspond to the
geographic position, but only in the case where there is no shadow fading.
The base station then determines in which microcell the virtual
is position of the mobile terminal is and allocates a channel from those
available in that microcell.
As shown in Figure 4, the receive cells and the microcells do not have
to be regular or equal. Indeed, it is preferable that the microcells are
dynamically defined expanding and contracting so that within one reuse
2o region, each microcell carries the same amount of traffic. To that end a
database of the virtual positions of mobile terminals is maintained for each
reuse region.
The invention is applicable to cells of any possible shape. An
arrangement of microcells in a rectangular cell is shown in Figure 5.
CA 02254643 1998-11-30
The invention is generally applicable to different types of cellular
mobile telecommunications systems in particular those maintaining an
orthogonal multiple user uplink communication and interfer diversity.
One example is a multicarrier system using a contiguous set of sub-
carriers per user. As shown in Figure 6, sets of subcarriers are hopped
slowly in successive time periods T with orthogonal frequency hopping
patterns between users (U 1 to U4) in the same receive cell 8. The
multicarrier system could apply OFDM modulation (Orthogonal Frequency
io Division Multiplex). The mobile terminals are synchronized such that their
delay difference at the base station .is within the guard time of the OFDM
symbol.
A narrow band TDMA alternative (like GSM) is possible in which
only one sub-carrier is allocated per user. The sub-carriers are frequency
is hopped, the hopping patterns being orthogonal among the users in the same
receive cell. OFDM modulation is not applied. The mobile terminals are
synchronized so that their delay difference at the base station is within the
guard time of the TDMA burst.
In another example, a non-contiguous set of sub-carriers, illustrated in
ao Figure 7a, is allocated per user (U1 and U2 are shown) so that the set of
sub-carriers of users within the same receive cell are disjoint. The set of
sub-carriers is referred to as a sub-carrier code.
The sub-carriers could be slowly frequency hopped as illustrated in
Figure 7b.
2s In the arrangement for a code divisional multiple access spread
spectrum system illustrated in Figure 8, a11 mobile terminals use the same
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frequency band all the time if active. Within a receive cell orthgonality is
provided by orthogonal spreading codes and tight synchronization of the
mobile terminals, or by multi-user detection without either synchronization
or orthogonal spreading codes.
