Note: Descriptions are shown in the official language in which they were submitted.
CA 02299297 2000-02-08
7
WO 99/12284 PCT/SE98/01541
A ~IOD FOR ASSIGNING SPREADING CODES
TECHNICAL FIELD OF THE INVENTION
The present invention relates to methods for assigning spreading
codes to DS-CDMA forward-link connections.
DESCRIPTION OF RELATED ART
CDMA (Code Division Multiple Access) is a well known method for
multiple access in a radio communication system. The CDMA method
uses the spread spectrum technique in which a number of users
simultaneously occupy the same frequency band with their radio
channels.
In a DS-CDMA (Direct Sequence-CDMA) system which is a particular
type of the CDMA techniques, each user is assigned a specific
spreading code by which the user is separated from the other
users of the system. Another name for a spreading code is
spreading sequence or spreading-code sequence.
The transmitted information in the radio signal is coded
(spread) by a specific spreading code in the transmitter. At the
receiving end the coded information is decoded (despread) by
correlating with the same specific spreading code again or by
filtering the received information in a matched filter.
A spreading code of the same length as the symbol interval is
called a short spreading code.
Orthogonal codes are codes that has zero cross correlation for
zero time offset. The use of orthogonal codes will reduce the
intra-cell interference, i.e. interference from other forward-
link signals in the same cell. Normally the intra-cell
CA 02299297 2000-02-08
WO 99/12284 PGTJSE98/01541
2
interference will not be completely eliminated as time
dispersion will partly destroy the orthogonality between signals
coded with orthogonal codes.
In general, a set of orthogonal codes does only contain a finite
number of codes, where the number of codes is always smaller or
the same as the length of the codes.
Consequently, as there are only a finite number of orthogonal
spreading codes available, spreading codes which are almost
orthogonal or non-orthogonal has to be used to increase the
l0 number of simultaneous users or increase the bit-rate of the
system.
On a DS-CDMA forward-link (transmission from a base station to a
radio unit), orthogonal spreading codes are often used to
separate different radio channels.
When a connection through a radio channel uses more than one
spreading code, it is said to use so-called multi-code
transmission.
For variable bit-rate connections, i.e. a connection where the
bit-rate varies during the duration of a call, the number of
spreading codes actually used by each connection will vary in
time and between the different connections. A high bit-rate uses
more spreading codes than a lower bit-rate.
One example of a scheme to assign spreading codes to a
connection with a variable bit-rate in a radio communication
system is the static allocation.
A static allocation means that each connection is, at call set-
up, allocated as many spreading codes as is needed to be able to
transmit at a requested maximum bit-rate.
CA 02299297 2000-02-08
WO 99/11184 PCT/SE98/01541
3
This means that a small number of connections with variable bit-
rate connection might allocate all available spreading codes
even if they do not have to use all of them simultaneous.
Another example of a scheme to assign spreading codes to a
connection is the dynamic allocation.
A dynamic allocation means that all connections share a common
pool of spreading codes that are continuously redistributed by a
base station, according to the instantaneous need of each
connection.
l0 Each time the bit-rate at a connection is to be increased the
base station has to inform the radio unit what new spreading
codes to receive. This will require a significant overhead in
the communication between the base station and radio unit.
The US patent US 5 533 013 describes a method and a system for
assigning complete orthogonal spreading codes and radio channels
in a combined CDMA/TDMA or TDMA/CDMA communication system. Said
method comprises the step of assigning an orthogonal spreading
code selected from a set of complete orthogonal spreading codes.
Said system comprises means for assigning orthogonal spreading
codes selected from at least one code set of complete orthogonal
spreading codes. If more than one set, the code sets of complete
orthogonal spreading codes have been selected so that they are
completely orthogonal in relation to each other.
The US patent US 5 452 328 describes a method for assigning
disjoint sets of binary spreading-code sequences to different
nodes in a multi-node communication network.
Each node in the network is allotted spreading-code sequences
which are selected from a family of "almost orthogonal" binary
sequences. The patent also describes an apparatus and a method
CA 02299297 2000-02-08
WO 99/12284 PCT/SE98l01541
4
for generating said family of sequences by combining a first and
a second multi-stage shift register.
As will be seen herein, each of the methods disclosed in these
patents is of a different construction than the method of the
present invention.
The name radio unit includes all portable and non-portable
equipment intended for radio communication, like mobile phones,
pagers, telex, electronic notebooks and communicators. These
equipment's can be used in any type of radio communication
l0 system, such as cellular networks, satellite or small local
networks.
SUI~iA,RY
The present invention meets problems related to how a forward-
link connection is assigned specific spreading codes in a DS-
CDMA communication system where only a finite number of
orthogonal spreading codes are available.
One problem occurs when the system is using static allocation
for variable bit-rate connections. The system may run out of
spreading codes even if only a small number of spreading codes
are actually used simultaneous. Each connection has allocated
the amount of spreading codes that is needed for the maximum
bit-rate irrespective of if the maximum..bit-rate is needed for
only a short time.
Another problem occurs when the system is using dynamic
allocation for variable bit-rate connections. A significant
overhead in the communication between the base station and the
radio unit is needed to inform the radio unit what new spreading
codes to receive each time the bit-rate is increased.
CA 02299297 2000-02-08
WO 99/12284 PCT/SE98/01541
S
In light of the foregoing, a primary object of the present
invention is to provide methods and means to assign spreading
codes for radio units in a DS-CDMA communication system with
variable bit-rate connections.
Another object of the present invention is to provide a large
number of available spreading codes which is not limited by the
amount of orthogonal spreading codes available in a DS-CDMA
communication system.
A further object of the present invention is to avoid re
allocation of spreading codes during the call in a DS-CDMA
communication system.
In accordance with a first aspect of the present invention,
spreading codes are assigned to forward-link connections from a
first set of orthogonal spreading codes as long as there are
spreading codes available in the first set. When all spreading
codes in the first set is allocated, a second set of orthogonal
spreading codes which are non-orthogonal to the spreading codes
in the first set is used from which spreading codes are assigned
to the forward-link connections. When applicable, more than two
sets of spreading codes are used.
According to a second aspect of the present invention, spreading
codes assigned to forward-link connection are assigned from two
different code sets. A first group of the spreading codes is
assigned from the first code set and a second group of the
spreading codes is assigned from the second code set. When
applicable, more than two sets of spreading codes are used.
The present invention includes methods for assigning spreading
codes to variable bit-rate forward-link connections. The methods
include the step of assigning spreading codes from a first set
CA 02299297 2000-02-08
WO 99/12284 PGT/SE98/01541
6
of orthogonal spreading codes. The method also includes the step
of assigning spreading codes from a second set of orthogonal
spreading codes which are non-orthogonal to the spreading codes
in the first set.
According to the first aspect of the present invention,
spreading codes are first assigned from the first set of
spreading codes. Spreading codes are then assigned from the
second code set when all spreading codes in the first code set
is allocated. The number of code sets can be extended to more
than two code sets.
According to the second aspect of the invention, the spreading
codes are assigned from two different code sets.
A first group of spreading codes is assigned from the first code
set and a second group of spreading codes is assigned from the
second code set. The first group of the spreading codes
comprises those codes which are most frequently used. The second
group comprises the remaining codes. More than two code sets can
be used.
One advantage with the present invention is that the number of
simultaneous allocated spreading codes is not hard limited by
the size of a code set.
Another advantage is that each connection is allocated a number
of spreading codes at call set-up. No further spreading code re-
allocation is needed.
CA 02299297 2000-02-08
WO 99/12284 PCT/SE98/01541
7
BRIEF DESCRIPTION OF THE DRAWINGS
These above mentioned objects and other features of the present
invention will become more readily apparent upon reference to
the following description when taken in conjunction with the
accompanying drawings.
Figure 1 is an illustration of a base station and four radio
units in a DS-CDMA communication system.
Figure 2 is an illustration of an example of a set of code sets
in accordance with the present invention.
Figure 3 is a first part of a flow chart illustrating a first
embodiment of a method in accordance with the present invention.
Figure 4 is a second part of the flow chart in figure 3.
Figure 5 is an illustration of a set of code sets with assigned
spreading codes in accordance with the first embodiment in
figure 3 and 4.
Figure 6a is a first part of a flow chart illustrating a second
embodiment of a method in accordance with the present invention.
Figure 6b is a second part of the flow chart in figure 6a.
Figure 7 is an illustration of four groups of spreading codes.
Figure 8 is an illustration of a set of code sets with assigned
spreading codes in accordance with the second embodiment in
figure 6a-b.
CA 02299297 2000-02-08
WO 99112284 PCf/SE98/01541
8
DETAILED DESCRIPTION OF EMBODIMENTS
The present invention relates to methods for assigning spreading
codes to forward-link connections in DS-CDMA communication
systems. The forward-link connections are radio connections
where the bit-rate can be varied (variable bit-rate) during the
duration of a call.
The spreading codes can be assigned at call set-up and e.g. at
handover and during set-up of additional services.
Figure 1 shows a base station B and four radio units U1-U4 in a
DS-CDMA communication system. Each radio unit U1-U4 has a
forward-link connection C1-C4, with a variable bit-rate, between
the base station B and the respective unit U1-U4.
When a first forward-link connection C1 between the base station
B and the radio unit U1 is to be set up a specified number of
spreading codes are assigned to the first forward-link
connection C1 by the base station B or by some other part of the
communication system. The spreading codes assigned to this
forward-link connection C1 can not be assigned to new forward-
link connections C2-C4 within the same cell as long as the first
forward-link connection C1 is up.
The number of spreading codes which are assigned to each of the
connections is determined by the desired bit-rate according to
the following:
A connection with a high bit-rate needs more spreading
codes than a connection with a low bit-rate.
A connection with multi-code transmission uses several
codes in parallel to increase the bit-rate.
CA 02299297 2000-02-08
WO 99/12284 PGTISE98/01541
9
Figure 2 shows an example of a set 20 of code sets S1-Sn
according to the present invention. A first code set S1
comprises a number of orthogonal spreading codes ml.
A second code set S2 also comprises a number of orthogonal
spreading codes m2. These spreading codes are orthogonal in
relation to each other but non-orthogonal in relation to the
spreading codes in the first code set S1.
A third code set S3 also comprises a number of orthogonal
spreading codes m3. These spreading codes are orthogonal in
relation to each other but non-orthogonal in relation to the
spreading codes in the first and second code set S1 and S2.
The number of code sets n with spreading codes can be more than
three.
Figure 3 and 4 show a flow chart of a first embodiment of a
method according to the present invention where a number of
spreading codes corresponding to a requested bit-rate are
assigned to a variable bit-rate forward-link connection C1.
In a step 31a the number of spreading codes needed to transmit
at the requested bit-rate between a base station B and a radio
unit U1 on the forward-link connection C1 is determined.
In a step 31b the number of available (not assigned) spreading
codes in all available code sets is determined. If there are
less spreading codes available than the spreading codes needed
the method ends, otherwise it continues with a step 32.
In step 32 the number of available (not assigned) spreading
codes in a selected first set S1 of orthogonal spreading codes
~is determined. If there are no available spreading codes in the
first set S1 of spreading codes the method continues with a step
35. If there are available spreading codes in the first set S1
of spreading codes the method continues with a step 33.
CA 02299297 2000-02-08
WO 99/12284 PCTISE98/01541
In step 33 a number of spreading codes, not exceeding the number
of spreading codes needed on the forward-link connection C1, are
assigned from the first set S1 of spreading codes to the
forward-link connection C1.
5 In a step 34 the number of spreading codes needed is compared
with the number of spreading codes assigned from the first set
S1 of spreading codes. The method ends if the number of
spreading codes needed are equal to the number of spreading
codes assigned from the first set S1 of spreading codes,
10 otherwise it continues with a step 35.
In step 35 the number of available spreading codes in a selected
second set S2 of orthogonal spreading codes is determined. If
there are no available spreading codes in the second set S2 of
spreading codes the method continues with a step 38. If there
are available spreading codes in the second set S2 of spreading
codes the method continues with a step 36.
In step 36 a number of spreading codes, not exceeding the number
of spreading codes needed on the forward-link connection C1, are
assigned to the forward-link connection C1.
In a step 37 the number of spreading codes needed is compared
with the number of spreading codes assigned from the first and
second set of spreading codes S1, S2 respectively. The method
ends if the number of spreading codes needed are equal to the
number of spreading codes assigned from the first and second set
of spreading codes S1, S2 respectively, otherwise it continues
with step 38.
In step 38, shown in figure 4, the number of code sets is
determined. The method continues with a step 39 if the number of
code sets is three, otherwise it ends.
CA 02299297 2000-02-08
WO 99/11284 PCT/SE98/01541
11
In step 39 the number of available spreading codes in a selected
third set S3 of orthogonal spreading codes is determined. If
there are no available spreading codes in the third set S3 of
spreading codes the method continues with a step 42. If there
are . available spreading codes in the third set S3 of spreading
codes the method continues with a step 40.
In step 40 a number of spreading codes, not exceeding the number
of spreading codes needed on the forward-link connection C1, are
assigned from the third set S3 of spreading codes.
In a step 41 the number of spreading codes needed is compared
with the number of spreading codes assigned from the first,
second and third set of spreading codes S1, S2, S3 respectively.
The method ends if the number of spreading codes needed are
equal to the number of spreading codes assigned from the first,
second and third set of spreading codes S1, S2, S3 respectively,
otherwise it continues with step 42.
In step 42, the number of code sets is determined. If the number
of code sets is three the method ends, otherwise it continues
with more steps similar to the previous steps 39-42 as long as
there are more spreading codes to be assigned and more sets S1-
Sn of codes available.
The method according to figure 3 and 4 is repeated each time a
new forward-link connection is to be set-up.
Figure 5 shows an illustration of a set 50 of code sets S1-Sn
comprising spreading codes according to the first embodiment.
The first forward-link connection C1 between the base station B
and the radio unit U1, see figure 1, has been assigned a first
number of spreading codes x from the first set S1 of spreading
codes. A second forward-link connection C2 has been assigned a
second number of spreading codes y. A first part y-kl of the
CA 02299297 2000-02-08
WO 99/12284 PCC/SE98I01541
12
second number of spreading codes y are taken from the first set
S1 of spreading codes and is assigned to the second forward-link
connection C2. The first part y-kl of the second number of
spreading codes y comprises at least one complete spreading.
code. A second part kl of the second number of spreading codes y
is taken from the second set S2 of spreading codes and is
assigned to the second forward-link connection C2. The second
part kl of the second number of spreading codes y comprises at
least one complete spreading code. The number of non-assigned
spreading codes in the first set S1 of spreading codes were less
than y so more spreading codes where assigned from the second
set S2 of spreading codes.
A third forward-link connection C3 has been assigned a third
number of spreading codes z from the second set S2 of spreading
codes.
A fourth forward-link connection C4 has been assigned a fourth
number of spreading codes w. A first part w-k2 of the fourth
number of spreading codes w is taken from the second set S2 of
spreading codes and is assigned to the fourth forward-link
connection C4. The first part w-kz of the fourth number of
spreading codes w comprises at least one complete spreading
code. A second part k2 of the fourth number of spreading codes w
is taken from the third set S3 of spreading codes and is
assigned to the fourth forward-link connection C4. The second
part kz of the fourth number of spreading codes w comprises at
least one complete spreading code.
Figures 6a-b show a flow chart of a second embodiment of a
method according to the present invention where a number of
spreading codes corresponding to a requested bit-rate are
assigned to a forward-link connection C1 with variable bit-rate.
In a step 61a a total number of spreading codes needed to
transmit at the requested bit-rate between the base station B
CA 02299297 2000-02-08
WO 99/12284 PGT/SE98/01541
13
and a radio unit U1 on the forward-link connection C1 is
determined.
In a step 61b the number of available (not assigned) spreading
codes in all available code sets is determined. If there are
less spreading codes available than the total number of
spreading codes needed the method ends, otherwise it continues
with a step 62.
In step 62 the total number of spreading codes needed is divided
in a first and a second group xl, xZ respectively. The number of
l0 spreading codes needed in the first group xl corresponds to the
number of spreading codes which will be most frequently used on
the forward-link connection C1. The number of spreading codes
needed in the second group x2 corresponds to the number of
spreading codes which will be less frequently used on the
forward-link connection C1. Together the first and second group
xl, xz respectively will include the total number of spreading
codes needed for the forward-link connection C1.
In a step 63 the number of available (not assigned) spreading
codes in a selected first set S1 of orthogonal spreading codes
is determined. If there are no available spreading codes in the
first set S1 of spreading codes the method continues with a step
70, see page 14. If there are available spreading codes in the
first set S1 of spreading codes the method continues with a step
64.
In step 64 a number of spreading codes, not exceeding the number
of spreading codes needed to the first group xl, are assigned to
the first group of spreading codes xl from the first set S1 of
spreading codes.
In a step 65 the number of spreading codes needed in the first
group xl is compared with the number of spreading codes assigned
CA 02299297 2000-02-08
WO 99/12284 PCTISE98/01541
14
from the first set S1 of spreading codes. If the number of
spreading codes needed in the first group xl is equal to the
number of spreading codes assigned from the first set S1 of
spreading codes the method continues with a step 66 to assign
spreading codes to the second group x2, otherwise it continues
with step 70 to assign more spreading codes to the first group
xl.
In step 66, shown in figure 6b, the number of available (not
assigned) spreading codes in a selected second set S2 of
orthogonal spreading codes is determined. If there are no
available spreading codes in the second set S2 of spreading
codes the method continues with a step 69. If there are
available spreading codes in the second set S2 of spreading
codes the method continues with a step 67.
In step 67 a number of spreading codes, not exceeding the number
of spreading codes needed to the second group xz, are assigned
to the second group x2 from the second set S2 of spreading
codes.
In a step 68 the number of spreading codes needed in the second
group x2 is compared with the number of spreading codes assigned
from the second set S2 of spreading codes. The method ends if
the number of spreading codes needed in the second group x2 is
equal to the number of spreading codes assigned from the second
set S2 of spreading codes, otherwise it continues with step 69
to assign more spreading codes to the second group x2.
In step 69, the number of code sets is determined. The method
continues with steps similar to the previous steps 63-69 if the
number of code sets is more than two, otherwise it ends.
CA 02299297 2000-02-08
WO 99/12284 PC'f/SE98/01541
In step 70 the number of available spreading codes in the
selected second set S2 of orthogonal spreading codes is
determined.
If there are no available spreading codes in the second set S2
5 of spreading codes the method continues with a step similar to
step 69 to search for more code sets. If there are available
spreading codes in the second set S2 of spreading codes the
method continues with a step where spreading codes is assigned
to the first group xl from the second code set S2.
l0 The spreading codes to the second group xz is then assigned from
a third set S3 of spreading codes.
The method according to figures 6a-b continue with steps similar
to the previous steps 63-70 as long as there are more spreading
codes to be assigned and more sets of codes S1-Sn available.
15 The method is repeated each time a new forward-link connection
with variable bit-rate is to be set-up.
Figure 7 shows an example of four groups of spreading codes G1,
G2, G3, G4 respectively assigned to four different variable bit-
rate forward-link connections C1-C4. Each group G1, G2, G3, G4
respectively comprises the total number of spreading codes
needed in each forward-link connection Cl-C4. The total number
of spreading codes needed in each forward-link connection C1-C4
is divided in the first and second group of spreading codes xl,
Yl~ zi. wi ~ xz~ Y2~ zz~ wz respectively, where each group xl, yl,
zl, wl , xz, Yz~ zz. wz respectively comprises complete spreading
codes.
Figure 8 shows an illustration of a set 80 of code sets S1-Sn
according to the second embodiment of the method in figure 6.
The first forward-link connection C1 with variable bit-rate
between the base station B and the radio unit U1, see figure 1,
has been assigned spreading codes from the first S1 and second
CA 02299297 2000-02-08
WO 99/12284 PCT/SE98/01541
16
S2 code set. The first group xl of spreading codes has been
assigned from the first code set S1 and the second group x2 of
spreading codes has been assigned from the second code set S2.
The second forward-link connection C2 with variable bit-rate
between the base station B and the radio unit U2, see figure 1,
has been assigned spreading codes from the first S1 and second
S2 code set. The first group yl of spreading codes has been
assigned from the first code set S1 and the second group y2 of
spreading codes has been assigned from the second code set S2.
The third forward-link connection C3 with variable bit-rate
between the base station B and the radio unit U3, see figure 1,
has been assigned spreading codes from the first S1 and second
S2 code set. The first group zl of spreading codes has been
assigned from the first code set S1 and the second group zz of
spreading codes has been assigned from the second code set S2.
The fourth forward-link connection C4 with variable bit-rate
between the base station B and the radio unit U4, see figure 1,
has been assigned spreading codes from the first S1 and third S3
code set. The first group wl of spreading codes has been
assigned from the first code set S1 and the second group wz of
spreading codes has been assigned from the third code set S3.
There were no non-assigned spreading codes in the second code
set S2 left so more spreading codes where assigned from the
third code set S3.
The signals which have been coded by short spreading codes in
the methods according to the present invention can be scrambled.
Signals in a forward-link connection which have been coded by
short spreading codes is scrambled by a common (long) Pseudo-
Noise code (PN-code). The scrambling randomise the interference
between the cells. The scrambling will not affect the
orthogonality between the signals in one cell as all signals
CA 02299297 2000-02-08
WO 99/12284 PCT/SE98101541
17
uses the same PN-code. The neighbouring cells uses different PN-
codes.
