Telecommunications – Transmitter and receiver at separate stations – Distortion – noise – or other interference prevention,...
Reexamination Certificate
2001-10-12
2002-08-20
Le, Thanh Cong (Department: 2684)
Telecommunications
Transmitter and receiver at separate stations
Distortion, noise, or other interference prevention,...
C455S522000, C455S069000, C370S318000
Reexamination Certificate
active
06438356
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for minimizing the effect of interference in a radio system which comprises at least one subscriber terminal and a base transceiver station which receives signals from said subscriber terminal, and in which method power control requests are transmitted to said subscriber terminal.
BACKGROUND OF THE INVENTION
In practice, transmitters used in radio networks are non-linear, and a part of a signal ends up outside the frequency band allocated to the transmission. When a signal ends up outside the frequency band allocated to it, it causes interference which is called adjacent channel interference (ACI).
The non-linearity of a transmitter is closely related to the properties of the amplifier of the transmitter. In general, it can be said that linear amplifiers cause minor interference to adjacent frequency bands, but the performance of the amplifiers is relatively low. Instead, non-linear amplifiers cause more interference in relation to the non-linear amplifiers. However, non-linear amplifiers have the advantage that they provide a better performance than the linear amplifiers.
Adjacent channel interference causes problems especially in wide-band systems, such as the WCDMA (wide-band code division multiple access) system. For instance, the WDCMA specification includes a maximum value for interference caused by the adjacent channel.
FIG. 1
shows a curve which describes the increase in noise in a CDMA system in relation to the system load. The load is proportional to the number of subscriber terminals in the system. The curve is obtained by the following formula:
r
=
10
log
(
1
1
-
η
)
,
(
1
)
wherein
r is the amount of noise exceeding thermal noise,
&eegr; indicates the degree of loading of the system.
The load of the CDMA system as a function of the users can be calculated as follows:
η
=
(
n
F
-
1
)
d
G
p
/
(
E
b
/
N
0
)
,
(
2
)
wherein
G
p
is the processing gain,
d indicates the voice activity,
F is the reuse coefficient of the frequency,
n indicates the number of users,
E
b
/N
o
indicates the ratio of the energy of the received bit to the thermal noise density.
The coefficient F obtains the value 1 when this concerns a separate cell. The coefficient F can obtain the value 0.9, for instance, when this concerns a typical micro cell. When this concerns a typical macro cell, the coefficient F obtains the value 0.67, for instance.
When the formulas (1) and (2) are combined, the following formula is obtained for the amount of noise exceeding thermal noise:
r
=
10
log
(
1
1
-
(
(
n
/
F
)
-
1
)
d
G
p
/
(
E
p
/
N
o
)
)
(
3
)
FIG. 2
shows curves
2
a
,
2
b
and
2
c
defined for different cell types, obtained by the formula (3), when typical values are inserted in the variables of the formula. The curve
2
a
shows a load curve of a macro cell, the curve
2
b
shows a load curve of a micro cell, and the curve
2
c
shows a load curve of a separate cell.
FIG. 2
shows that in a micro cell a higher noise is allowed than in a macro cell.
FIGS. 1 and 2
show that when the degree of loading- is between 0.5 and 0.8, the noise increase is in the range of 3 to 6 dB. When the degree of loading is in the above-mentioned range, a small increase in the degree of loading only causes a small increase in the amount of noise. Because the curve rises steeply, it is easy to see that if the degree of loading is higher than the high value of the above-mentioned range, even a small increase in the degree of loading causes a high increase in the amount of noise. The higher the load in the system, the higher the number of subscriber terminals that need to increase their transmission power. However, an increase in transmission power further increases the amount of noise in the network, and the system may become unstable.
In radio networks, an admission control algorithm is used to ensure that for a call there are sufficient resources which allow a sufficiently good signal-to-interference (SIR) ratio and bit rate for the call signal. The admission control algorithm is, for instance, applied when a subscriber terminal begins to establish a connection in a new cell. The admission control algorithm can also be applied during handover.
The admission control algorithm is applied separately in the uplink and downlink directions, especially when the traffic volumes of the different directions differ a great deal from each other. When the algorithm is activated, call set-up can be prevented.
Radio networks also use a load control algorithm which is used to try and maintain the network resources in a predefined range of use. The admission control and load control algorithms contain a parameter indicating noise increase. The parameter and, at the same time, the algorithm can, for instance, be activated when the amount of noise exceeds a predefined noise limit.
Applying the load control algorithm requires constant processing in which interference is monitored. The algorithm is used in defining the load factor. If a predefined load factor value is exceeded, the network reduces the bit rate of the users whose service contract allows the reduction of bit rate. In addition, the network delays transmission of the users who have no requirements with respect to delay. The network can also interrupt low-priority calls.
In some situations, the network can, in principle, drop all its calls. Calls have to be dropped when the noise level remains too high even after all available means have been used to reduce the noise level.
When the network is underloaded, the load control algorithm increases bit rates of those users who are able to process higher bit rates. Increasing or reducing bit rates can be done in order of priority, for instance.
Let us now examine the situation in
FIG. 3
, which clearly shows the problem which arises as a result of using an adjacent channel. In the figure, a subscriber terminal MS
1
is connected to a base transceiver station BTS
1
. The connection uses a frequency F
3
in the uplink direction (MS
1
→BTS
1
). The terminal MS
1
is, however, close to another base transceiver station BTS
2
which receives on a frequency F
4
from a terminal MS
2
of its own.
If the frequencies F
3
and F
4
are adjacent frequency bands in the frequency range, the base transceiver station BTS
2
will experience the transmission of the terminal MS
1
as adjacent channel interference, because the selectivity of the receiver in the base transceiver station BTS
2
is not ideal. Problems would arise even though the receiver did operate selectively, because the signal of the adjacent channel also spreads to the reception band of a selectively operating receiver. The problem situation caused by the interference is especially difficult when BTS
1
and BTS
2
are base transceiver stations of different network operators, for instance, since then the terminal MS
1
cannot make a handover to the base transceiver station BTS
2
.
BRIEF DESCRIPTION OF THE INVENTION
It is thus an object of the invention to provide a method for minimizing the effect of interference and a radio system so as to reduce the above-mentioned problems. This is achieved by a method of the type as claimed in the preamble, which is characterized by separately measuring the transmission frequency of power control requests transmitted to a subscriber terminal, and when the transmission frequency of the power control requests transmitted to the subscriber terminal exceeds a predefined transmission frequency of power control requests, by altering the attenuation of signals arrived at a base transceiver station at least in the case of the signals which are transmitted from the above-mentioned subscriber terminal to the base transceiver station.
The object is also achieved by a method of the type as claimed in the preamble, which is characterized in that for executing the method, there are two method step groups, of which at least one is executed in the method; in the first method step group: the transmission frequency of power control requests transmitted to each subscri
Hämäläinen Seppo
Lilja Harri
Cong Le Thanh
Gantt Alan T.
Nokia Networks Oy
Pillsbury & Winthrop LLP
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