Method of assigning frequency using propagation loss

Details Method of assigning frequency using propagation loss Method of assigning frequency using propagation loss

1999-06-03

2004-08-24

Gelin, Jean (Department: 2681)

Multiplex communications

Communication over free space

Having a plurality of contiguous regions served by...

C370S342000, C370S441000, C455S450000

Reexamination Certificate

active

06781974

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of assigning a frequency from a base station to mobile stations in a CDMA (Code Division Multiple Access) communication system.
2. Description of the Related Art
The CDMA communication system is expected to be the next-generation mobile communication system. The CDMA communication system is advantageous in that a plurality of adjacent base stations can share one frequency band.
In a CDMA communication system, different codes are assigned to respective channels, and signals are demodulated on the basis of the fact that there is a difference between autocorrelated values, which are correlated values between the same codes, and cross-correlated values, which are correlated values between different codes. It is therefore necessary that the cross-correlated values between the codes assigned to the respective channels are sufficiently lower than the autocorrelated values. It is most preferable that all the codes which are used be orthogonal to each other (the cross-correlated values be 0). However, since it is difficult for all the codes which are used by base stations that are adjacent to each other to be orthogonal to each other, the cross-correlated values are generally not 0.
If the cross-correlated values are not 0, then there is a possibility that when a mobile station A
1
is present near a base station and a mobile station A
2
is present far from the base station, depending on the ratio of transmission powers of the mobile stations A
1
, A
2
at the time signals from the mobile stations A
1
, A
2
arrive at the antenna of the base station, the correlated value (cross-corrected value) between the signal from the mobile station A
1
and the code assigned to the mobile station A
2
may be greater than the correlated value (autocorrelated value) between the signal from the mobile station A
2
and the code assigned to the mobile station A
2
.
In such a case, because the cross-corrected value is not lower than the autocorrelated value, the signal from the mobile station A
2
cannot properly be demodulated. This could lead to a fatally dangerous situation where if the signal power sent from one mobile station and received by the antenna of a base station is much greater than the signal power sent from another mobile station and received by the antenna of the base station, then the base station is unable to decode signals other than the signal from the mobile station whose reception power is larger at the antenna of the base station. This problem is widely known as a near-far problem. In order to solve the problem, it is necessary to perform a transmission power control process on transmitters for equalizing signal powers from all the transmitters in input signals received by a receiver.
If a mobile communication system is operated by a sole network operator and a handoff is properly carried out as a mobile station moves, then communications are effected via a path whose transmission loss is smallest between the base station and the mobile station. It is thus possible to perform a transmission power control process in order to minimize the transmission power of the mobile station for thereby solving the near-far problem.
Another problem occurs when a plurality of mobile communications are operated in one area by a plurality of network operators. In this situation, if the base stations are located at one site or mutual roaming is carried out between the different network operators, then no difficulty arises because a path whose transmission loss is smallest between base and mobile stations is practically selected. Since, however, such a solution is generally difficult to achieve, it has been customary to solve the above problem by assigning different frequency bands-to the respective network operators, as shown in
FIG. 1
of the accompanying drawings.
FIG. 1
shows the assignment of four frequency channels to each of network operators A, B, C.
If the frequency bands assigned to the respective network operators are successive, then in frequency channels located at a boundary between different frequency bands, the leaking power in an adjacent channel of a transmission signal from a mobile station causes a near-far problem to a frequency channel of another network operator. For example, in frequency channels fa
1
, fa
4
located at a boundary of the frequency band assigned to a network operator A, the leaking power in an adjacent channel of a transmission signal from a mobile station causes a near-far problem to a frequency channel fc
4
of a network operator C and a frequency channel fb
1
of a network operator B.
FIG. 2
of the accompanying drawings shows CDMA communication systems provided by a plurality of network operators and located in one area. As shown in
FIG. 2
, two network operators A, B provide respective CDMA communication systems in one area. The CDMA communication system provided by the network operator A has base stations (not shown) which communicate with mobile stations present in cells A
1
-A
7
. It is assumed that the cells A
1
-A
7
overlap a cell B
1
of a base station of the CDMA communication system provided by the network operator B, and that the frequency channels shown in
FIG. 1
are assigned to each of the network operators A, B.
In
FIG. 2
, &agr; represents the position of the base station of the cell A
1
, &bgr; represents the position of the base station of the cell B
1
, and &ggr; represents the position of the base station of the cell A
4
.
In order to solve a near-far problem in one CDMA communication system, each of the base stations control the transmission powers of mobile stations linked thereto, and the reception powers at the antenna of each of the base stations are controlled at a constant level at all times irrespective of the positions of the mobile stations. Specifically, if the distance between a mobile station and a base station linked thereto is large, then the transmission power of the mobile station is increased, and if the distance between a mobile station and a base station linked thereto is small, then the transmission power of the mobile station is reduced.
In
FIG. 2
, when a mobile station belonging to the CDMA communication system provided by the network operator A moves from the position &agr; to the position &bgr; to the position &ggr;, the transmission power of the mobile station is first progressively increased from a low level to a maximum level at the position &bgr;. Near the position &bgr;, a handoff takes place from the base station of the cell A
1
to the base station of the cell A
4
. As the mobile station approaches the position &ggr;, the transmission power of the mobile station is gradually lowered.
The transmission power of a mobile station in the cell A
1
which uses the frequency channel fa
4
changes as shown in
FIG. 3
a
of the accompanying drawings. The reception power at the base station of the cell A
1
in the position a and the reception power at the base station of the cell A
4
in the position &ggr; change as shown in
FIG. 3
b
of the accompanying drawings.
As shown in
FIG. 3
b
, when the mobile station is located between the position &agr; and the position &bgr;, the reception power at the base station of the cell A
1
is constant, and when the mobile station is located between the position &bgr; and the position &ggr;, the reception power at the base station of the cell A
1
is gradually reduced. The reception power at the base station of the cell A
4
is gradually increased when the mobile station is located between the position &agr; and the position &bgr;, and is constant when the mobile station is located between the position &bgr; and the position &ggr;. When the reception powers are constant, the reception powers have the same level at the base stations of the cells A
1
, A
4
. At this time, the reception power in the frequency channel fb
1
of the base station in the position &bgr; of the cell B
1
of the network operator B is indicated by the solid-line curve in
FIG. 3
c
of the accompanying

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