Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail
Reexamination Certificate
1997-07-02
2001-11-06
Trost, William (Department: 2683)
Telecommunications
Transmitter and receiver at same station
Radiotelephone equipment detail
C455S561000, C455S550100, C455S517000, C455S524000, C455S525000, C455S063300, C455S067700
Reexamination Certificate
active
06314304
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates generally to a mobile communication system using, for example, TDMA (Time-Division Multiple-Access) or FDMA (Frequency-Division Multiple-Access) technique, and more particularly to an improved antenna arrangement for use in cellular radiotelephone communication systems which is designed to reduce the co-channel interference for shortening the distance between radio zones or cells to which the same frequency is assigned for improvement of the spectrum efficiency.
2. Background of Related Art
In conventional cellular radiotelephone communication systems, an omnidirectional antenna or several directional antennas are disposed in a base station to define a circular radio zone or cell. In a TDMA or FDMA system, transmission of a plurality of radio signals at the same frequency will induce the co-channel interference, resulting in a failure in data transmission.
FIG. 21
shows one example of conventional zone layout. Each hexagonal cell has disposed centrally a base station. A number labeled in each cell indicates the frequency. The cells of the same number are assigned the same frequency. In this frequency reuse pattern, reduction in co-channel interference is achieved by increasing the distance between the cells to which the same frequency is assigned. This, however, decreases the spectrum efficiency, thereby resulting in a difficulty in increasing the capacity of telephone subscribers.
In order to avoid the above problem, Japanese Patent Second Publication No. 3-37336 teaches a cellular telecommunication system using the sector layout as shown in FIG.
22
.
A cell is divided into six sectors
1
to
6
surrounding a base station
7
. Six directional antennas each covering a fan-shaped part of the cell are disposed on the base station. An increase in frequency reuse is achieved without lowering the DU (desired signal to undesired signal) ratio by assigning the same frequency group to any two of the antennas oriented in opposite directions. For example, the same frequency group F
1
is assigned to the sectors
1
and
4
. The same frequency group F
2
is assigned to the sectors
2
and
5
. The same frequency group F
3
is assigned to the sectors
3
and
6
. This frequency reuse will cause, for example, the sector
1
to experience the co-channel interference with a radio signal from the sector
4
which uses the same frequency channel and is diametrically opposed to the sector
1
. A radio signal radiated from the sector
1
to sector
4
, however, has a level that is decreased by a front-to-back (F/B) ratio (i.e., ratio of signal strength transmitted in a forward direction to that transmitted in a backward direction). Typical antennas show a F/B ratio of 25 dB or more.
The above prior art communication system having the antennas disposed at the center of the cell, however, encounters the difficulty in improving the spectrum efficiency for the following reasons.
UPLINK SIGNAL
A mobile station lying at an end of the cell is closer to an adjacent cell than any other mobile stations lying inside the end of the cell and has the greatest transmission power. The proximity to an adjacent cell will cause the interference to be maximized. It is thus necessary to design a circuitry taking into account the case where a mobile station lies at the end of the cell.
DOWNLINK SIGNAL
A mobile station lying at an end of the cell is most apart from the base station, and the level of a desired frequency is lowest. Additionally, the mobile station undergoes the greatest interference with a ratio signal radiated from another cell. This, likewise to the above, requires the design of a circuitry taking into account the case where a mobile station lies at the end of the cell.
Specifically, the above problem is inevitably encountered regardless of any cell layouts as long as a base station is centrally located in a cell.
SUMMARY OF THE INVENTION
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide an improved antenna arrangement capable of reducing the co-channel interference, thereby allowing the distance between cells to which the same channel is assigned to be decreased.
According to one aspect of the present invention, there is provided a mobile communication system which comprises a base station and a plurality of antennas connected to the base station. The antennas are located on a periphery of a cell and have directional characteristics oriented toward the center of the cell.
In the preferred mode of the invention, each of the antennas has the directional characteristic oriented to cover 120° area of the cell. The antennas are located at angular intervals of 120°.
The antennas may alternatively have the directional characteristics oriented to cover 120° areas of the cell, respectively, and are located at angular intervals of 60°. An omnidirectional antenna may be located at the center of the cell.
The number of the antennas may be three, one of which is a central antenna having the directional characteristic covering a 60° area of the cell and the others each have the directional characteristic covering a 120° area of the cell and are located on both sides of the central antenna. The central antenna may alternatively have the directional characteristic covering a 120° area of the cell. Each of the antennas on the both sides of the central antenna may alternatively have the directional characteristic covering a 60° area of the cell.
The base station has a diversity circuit which performs a selection diversity operation on radio signals received by the antennas to provide a resultant signal.
The diversity circuit may alternatively perform an equal-gain combining diversity operation on radio signals received by the antennas to provide a resultant signal or a maximal-ratio combining diversity operation on the radio signals received by the antennas to provide a resultant signal.
The base station may have a decision feedback equalizer which removes components inducing co-channel interference from signal received by the antennas. A tap interval of the decision feedback equalizer is set to a fraction of a symbol cycle for minimizing effects caused by a difference in arrival time of the signals at the antennas.
The base station may have a decoder which performs a maximum likelihood sequence estimation to minimize distortion caused by a difference in arrival time of the signals at the antennas.
A mobile station which has a decision feedback equalizer is further provided. A tap interval of the decision feedback equalizer of the mobile station is set to a fraction of a symbol cycle for minimizing effects caused by a difference in arrival time of signals from the antennas at the mobile station.
The mobile station may have a decoder designed to perform a maximum likelihood sequence estimation to minimize distortion caused by a difference in arrival time of the signals from the antennas at the mobile station.
The mobile station has a plurality of antennas and a diversity circuit performing a given diversity operation on the signals received by the antennas. The mobile station has a decision feedback equalizer and a decoder performing a maximum likelihood sequence estimation operation.
The mobile station may have an adaptive array of antennas, a decision feedback equalizer and a decoder performing a maximum likelihood sequence estimation operation.
The base station may include a power monitor and a handover control circuit. The power monitor monitors powers of the signals received by the antennas to determine a position of a mobile station within the cell. The handover control circuit performs handover based on the position of the mobile station determined by the power monitor.
The station may alternatively include a channel estimating circuit and a diversity circuit. The channel estimating circuit estimates a channel state by adjusting an uplink signal frequency and a downlink signal frequency to agree with each other. The divers
Ferguson Keith
Gopstein Israel
Matsushita Electric - Industrial Co., Ltd.
Trost William
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