Radio communication system

Details Radio communication system Radio communication system

1999-01-25

2002-06-04

Urban, Edward F. (Department: 2685)

Telecommunications

Radiotelephone system

Zoned or cellular telephone system

C455S452200, C455S562100

Reexamination Certificate

active

06400955

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a sector antenna as well as to antenna control in a radio communication system. More particularly, it is an object of the present invention to reduce the interference in the same frequency by correlating a direction of an antenna to an allocated slot position based on a TDMA system used as a multiple access system.
BACKGROUND OF THE INVENTION
FIG. 14
shows a cellular structure of a radio communication system. Numerals from b
1
to b
12
indicate numbers allocated to base stations, and base stations b
1
, b
5
, b
7
and b
12
indicate that the base stations use radio waves at the same frequency. In cellular radio communications, a method is generally employed in which a geographic frequency is reused by repeatedly using the frequency in radio zones provided at a specified distance apart from each other so that utilization efficiency of a frequency can be improved. The zones (or cells, or base station areas) which use the same frequency are geographically so located that the interference in the same frequency does not cross an allowable level. A relation between the distance R and transmission loss in free space L (R) is expressed by the equation described below (Basics of Mobile Communications: edited by IEICE).
L
(
R
)=10 log(4
&pgr;R
/&lgr;)
2
  (1)
Wherein, assuming that a carrier frequency is 5 GHz, L (R) is as follows:
L
(
R
)=46.4+20 log(
R
)  (2)
Namely, a channel having the radio wave attenuation property as shown in the equation (2) is reused at an allowable interference level. Therefore, when high-speed radio communications are to be realized in cellular radio communications, it is important to consider the problem of sending power as well as the problem of reduction of the interference in the same frequency channel.
Especially, when data transfer and radio packet transfer are to be realized with cellular mobile communications, the interference in the same frequency channel has to be reduced. That is because, when data transfer is to be performed, higher line quality is required as compared to that for speech data. For example, in a PHS system which is actually used, it is required that an error rate to speech data transfer is 1.0E−3 or less, but an error rate lesser than this is required for data transfer. Namely, the fact described above indicates that, when the sending power is constant, a ratio of power of the desired to undesired waves is required to be made larger in the cellular radio communications, which prevents improvement in the utilization efficiency of a frequency. In addition, when high-speed data transfer is to be performed, a problem such as selective fading due to multiple scattered waves noticeably comes up, so that it is difficult to insure desired line quality.
For overcoming those problems, a cellular radio communication system using a directional antenna is known. It is known that, the directional antenna can make a delay spread smaller. Communications between a base station and a terminal station, what is called point to point communications are executed with a beam antenna by narrowing down the directivity of an antenna. Therefore, when a beam is ideally provided between two points, interference to other lines may not exist in theory, so that it is possible to execute multiple communications by dividing space even if the same sign is used at the same frequency as well as at the same time.
FIG. 15
is a view showing directions in a radio communication system having a directional antenna. In
FIG. 15
, the reference numerals b
1
and b
2
indicate base stations which use the same frequency respectively. A sector antenna having horizontal directivity of 60 degree is located in each of the base stations, and the antenna of the base station b
1
covers sector cells (b
1
-a
1
, b
1
-a
2
, b
1
-a
3
, b
1
-a
4
, b
1
-a
5
and b
1
-a
6
), while the base station b
2
covers sector cells (b
2
-a
1
, b
2
-a
2
, b
2
-a
3
, b
2
-a
4
, b
2
-a
5
and b
2
-a
6
). Terminal stations p
1
and p
2
are present in the sector cell b
1
-a
1
, a terminal station p
3
is present in the sector cell b
1
-a
3
, and a terminal station p
4
is present in the sector cell b
2
-a
1
, each of those terminal stations has a sector antenna having horizontal directivity of 60 degree, and communicates with the base station by using each antenna of the sector cells respectively.
FIG.
16
A and
FIG. 16B
show each slot structure of frames in the base stations b
1
and b
2
respectively, in which the reference numerals U
1
to U
6
show slots for the communication line.
FIG. 16A
shows how slots are allocated in the base station b
1
, and shows a situation in which the slot U
1
is a slot allocated thereto for communications with the terminal station p
1
positioned in the sector cell b
1
-a
1
, the slot U
2
is a slot allocated thereto for communications with the terminal station p
2
positioned in the sector cell b
1
-a
1
, and the slot U
3
is a slot allocated thereto for communications with the terminal station p
3
positioned in the sector cell b
1
-a
3
.
FIG. 16B
shows how slots are allocated in the base station b
2
, and shows a situation in which the slot U
2
is a slot allocated thereto for communications with the terminal station p
4
positioned in the sector cell b
2
-a
1
.
If slots in a frame are randomly allocated, as shown in the slot U
2
in FIG.
16
A and
FIG. 16B
, there may occur a case where some of the sector cells oriented in the same direction of b
1
-a
1
and b
2
-a
1
are used at the same timing.
FIG. 15
shows the case where sector cells orient in the same direction at the same timing like the slot U
2
. Each directional area of the terminal station p
4
and the base station b
1
are shown by a dotted line. There are included not only the base station b
2
but also the base station b
1
within the directional area of the terminal station p
4
, and there is included the terminal station p
4
within the directional area of the base station b
1
. Therefore, the terminal station p
4
easily receives radio waves not only from the base station b
2
which is a desired base station but also from the base station b
1
which is an interference station and shows a state in which the interference becomes larger.
As a method of avoiding such a situation, there is known a method of changing the allocated slot when the interference is large. FIG.
18
A and
FIG. 18B
, similarly to FIG.
16
A and
FIG. 16B
, showing a state of how slots are allocated in each of the base stations b
1
and b
2
. It is possible to avoid the antennas from orienting the same direction at the same timing by changing the allocated slot for the terminal station p
4
to the slot U
3
.
FIG. 17
shows directions of the radio communication system in the slot U
3
used by the terminal station p
4
. Each directional area of the terminal station p
4
and the base station b
1
are shown by a dotted line. As the terminal station p
4
is not present within the directional area of the base station b
1
, interference does not occur.
As some other method to avoid such a problem, consideration has been made for channel allocation, and there has been proposed, in Japanese Patent Laid-Open Publication No. HEI 7-193857, a method of computing, when one terminal station generates a request for communications, directions which interference waves from all the channels may come from as well as a direction which a wave from the terminal station comes from, selecting one channel among channels in the order that the difference between the direction from which each interference wave comes and the direction from which the desired wave comes is closer to 180 degree, and allocating any channel first satisfying the conditions for allocation because large interference may occur if sector cells in the same direction give the same channel to terminal stations.
There is also a method of using the directivity within the perpendicular plane of an antenna as a method of suppressing the same frequency interference derived from repetit

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