Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2001-02-22
2004-11-23
Cumming, William (Department: 2683)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
Reexamination Certificate
active
06823187
ABSTRACT:
BACKGROUNDS OF THE INVENTION
1. Field of the Invention
The present invention relates to an adjacent carrier frequency interference avoiding method for cellular system and a mobile station. In particular, the present invention relates to an adjacent carrier frequency interference avoiding method for cellular system, by which the occurrence of interference between different sets of signals sent using adjacent carrier frequencies can be avoided, when carrier frequencies used by two separate cellular systems are adjacent to each other on the frequency axis, as well as a mobile station and a base station controller that are used therein
2. Description of the Related Art
A cellular system is generally assigned a specific frequency band, in which multiple different carrier frequencies are set. A mobile station sets a link between itself and a base station that has been set within a service area and communicates with the base station bi-directionally. A transmitter of a base station and that of a mobile station are designed so that, when a base station transmits signals to a mobile station using a certain carrier frequency, or vice versa, power leakage into the frequency band of another carrier frequency that is adjacent on the frequency axis to the carrier frequency used by the transmitter will be minimized. It is impossible, however, for the transmitter to eliminate such power leakage completely, resulting in interference wave power given to signals of an adjacent carrier frequency at a certain ratio.
Each of the mobile stations in one cellular system sets a link between itself and the nearest base station. In uplink, transmitting power from the mobile station is controlled so that the difference among the levels of reception power that the base station receives from multiple mobile stations will not be excessively large. Therefore, the power of interference between adjacent carrier frequencies that is received from signals of an adjacent carrier frequency sent from a mobile station is lower than desired wave power when it reaches the base station.
In downlink, if the level of transmitting power from base station is constant, the interference wave power from signals of a frequency that is adjacent to desired wave signals reaches the mobile station along the same propagation path as desired wave signals. During the travel, the interference wave power attenuates along with desired waves do, and thus the power of interference between adjacent carrier frequencies becomes lower than desired wave power. Therefore, such interference between adjacent carrier frequencies is not a serious problem in one cellular system.
On the other hand, interference between adjacent carrier frequencies may become a serious problem when two cellular systems are in the same service area. More specifically, a serious problem may occur if the first cellular system is assigned a frequency band adjacent to a frequency band of the second cellular system, and the first cellular sets a base station within this service area and communicates with a mobile station of the first cellular system.
This situation will be described using
FIG. 1
as one example. As shown in
FIG. 1
, there are two cellular systems: Cellular Systems A and B. Cellular System A consists of base stations
11
through
13
and a mobile station
21
; Cellular System B consists of base stations
61
through
63
and a mobile station
71
. As shown in
FIG. 2
, Cellular System A is assigned carrier frequencies Fa
1
, Fa
2
, and Fa
3
for uplink, and carrier frequencies Gal, Ga
2
, and Ga
3
for downlink; Cellular System B is assigned carrier frequencies Fb
1
, Fb
2
, and Fb
3
for uplink and carrier frequencies Gb
1
, Gb
2
, and Gb
3
for downlink. Carrier frequencies Fa
3
and Ga
3
of Cellular System A are adjacent to carrier frequencies Fb
1
and Gb
1
of Cellular System B, respectively, on the frequency axis. The mobile station
21
of Cellular System A is communicating using Fb
3
for uplink and Gb
3
for downlink; the mobile station
71
of Cellular System B is communicating using Fb
1
for uplink, and Gb
1
for downlink.
When the mobile station
21
approaches the base station
61
, uplink signals of the mobile station
21
will be controlled by the base station
11
to maintain received power at a desired level. At the same time, power received by the base station
61
will grow sharply, and consequently power leakage into the carrier frequency Fb
1
, which is adjacent to the carrier frequency Fa
3
, will also become extremely high. This interference between the adjacent carrier frequencies will deteriorate the communication quality of uplink from the mobile station
71
to the base station
61
. Similar deterioration in communication quality will take place along the downlink from the mobile station
21
, because of an increase in adjacent carrier frequency interference from the carrier frequency Gb
1
, which is transmitted by the base station
61
, to the carrier frequency Ga
3
.
One method to solve this problem is disclosed in Japanese Patent Laying-Open No. 11-341555. According to this method applied to the examples of
FIGS. 1 and 2
, the mobile station
21
of Cellular System A will measure the received power Qb of the carrier frequency transmitted from the base station
61
of another cellular system, i.e., Cellular System B. Higher received power Qb means that the base station
61
is nearer. Therefore, if the received power Qb is equal to or lower than a pre-determined threshold, any of the carrier frequencies will be used.
If the received power Qb is higher than the predetermined threshold, carrier frequencies Fa
1
or Fa
2
for uplink and carrier frequencies Gal or Ga
2
for downlink will be used since they are not adjacent to the carrier frequencies of Cellular System B. If Fa
3
or Ga
3
, which is adjacent to the carrier frequencies of Cellular System B, is being used, the carrier frequency will be switched to any of the other carrier frequencies.
When the mobile station
21
controls its transmitting power so that the received power of uplink signals at the base station
11
will be constant, received power, transmitting power, and power of adjacent carrier frequency interference will be in the following relationship. The smaller the received power Qa of the carrier frequency transmitted by the base station
11
of Cellular System A is, the higher the transmitting power from the mobile station
21
becomes. This in turn makes the power of interference between the adjacent carrier frequencies affecting the base station
61
greater. Therefore, another possible method is to have the mobile station
21
measure the received power Qa in addition to the received power Qb, and prevent the carrier frequencies Fa
3
or Ga
3
from being used if the difference between received power Qb and Qa is greater than the pre-determined threshold.
These methods can reduce interference between adjacent carrier frequencies, because they use carrier frequencies other than Fa
3
and Ga
3
when the mobile station
21
is likely to cause strong adjacent carrier frequency interference in the uplink from the base station
61
or when the downlink from the mobile station
21
may receive strong adjacent carrier frequency interference from the base station
61
.
According to yet another method disclosed in Japanese Patent Laying-Open No. 11-341555, when the mobile station
21
is using Fa
3
for uplink, it measures the received power Qb of the carrier frequency transmitted by the base station
61
. If the received power Qb is higher than the pre-determined threshold, the mobile station
21
sets the maximum transmitting power for uplink to a value that is lower than the usual maximum value by a difference between the received power Qb and the pre-determined threshold. Another possible method is to have the mobile station
21
additionally measure the received power Qa of the carrier frequency transmitted by the base station
11
, and reduce the setting of maximum transmitting power by a difference between the received power Qb and Qa.
These methods can red
Cumming William
McGinn & Gibb PLLC
NEC Corporation
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