Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-01-16
2002-01-01
Patel, Ajit (Department: 2662)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S342000, C455S522000, C455S069000
Reexamination Certificate
active
06335924
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spread spectrum communication system, and in particular to a spread spectrum cellular system in which a plurality of terminals simultaneously communicate with a base station, and mobile terminals and a transmission power control method applied to the spread spectrum cellular system.
2. Description of the Related Art
FIG. 9
shows an example of a conventional spread spectrum cellular system. A plurality of base stations
100
(
100
-
a,
100
-
b
) connected to a switching unit
10
are distributed to form a plurality of cells
1
(
1
a,
1
b
). In each cell, a plurality of mobile terminals
300
(
300
-
1
,
300
-
2
;
300
-
j,
300
-
k
) communicate with a base station
100
. There has been known a method of using orthogonal codes Wi unique to respective terminals as spreading codes of signals transmitted from each base station
100
to each of terminals included in a cell in such a spread spectrum cellular system.
As represented by codes W
0
, W
1
, W
2
and W
3
shown in
FIG. 10
, for example, orthogonal codes have such a property that the inner product performed on two arbitrary codes included in the codes W
0
, W
1
, W
2
and W
3
over an orthogonal code span becomes “0”.
Therefore, the base station assigns orthogonal codes Wi (i=1, 2, . . . , n) respectively unique in a cell to a plurality of terminals
300
-
1
through
300
-n located in the cell, and spreads a signal or data addressed to one terminal
300
-i by using an orthogonal code Wi unique to that terminal
300
-i. The above described terminal
300
-i de-spreads a signal received from an antenna by using the orthogonal code Wi assigned to itself. By doing so, transmitted signals addressed to other terminals located in the cell which are orthogonal to the transmitted signal addressed to the terminal
300
-i are completely removed in the process of the above described de-spreading process and hence they do not act as interference.
A communication method thus employing spreading with orthogonal codes for communication from each base station to mobile terminals is described in U.S. Pat. No. 5,103,459, for example.
In a spread spectrum cellular system using orthogonal codes, however, signals transmitted from other base stations forming adjacent cells arrive at each terminal besides the signal transmitted from the base station. In this case, signals transmitted from other base stations are not orthogonal to the signal transmitted from the base station in the cell, and hence they cannot be removed in the above described cell by de-spreading process using the unique orthogonal code Wi. That is to say, in receiving operation of each terminal, signals transmitted from base stations of adjacent cells act as an interference cause (noise).
FIG. 11
is a diagram showing the influence of the above described signals transmitted from other base stations and received by each terminal.
Received power of the signal transmitted from the base station is attenuated as the distance from the base station is increased. In a terminal, such as
300
j,
located near the base station and located near the center of the cell, therefore, received power
910
of the signal from the base station in the cell is large whereas received power
911
of the signal coming from other base stations located outside the cell and functioning as interference becomes small. As a result, a high signal-to-noise ratio is obtained. In a terminal, such as
300
k,
located near the boundary of the cell, received power
912
of the signal from the base station located in the cell is weak whereas interference from adjacent cells is received with power
913
larger than that of the above described terminal
300
j.
As a result, the signal-to-noise ratio is degraded.
For the above described reason, it is desired to control transmission power in the cellular system according to the positional relation with respect to a terminal so that a signal to be transmitted from each base station to a terminal may be outputted with small transmission power for the terminal
300
j
located near the center of the cell and with large transmission power for the terminal
300
k
located on the periphery of the cell.
Such a transmission power control method as to change the transmission power according to the terminal position is described in “On the System Design Aspects of Code Division Multiple Access (CDMA) Applied to Digital Cellular and Personal communications Network,” by A. Salmasi and K. S. Gilhousen, IEEE VTS 1991, pp. 57-62, for example.
According to the control method described in the aforementioned paper, each terminal measures the signal-to-noise ratio of a received signal by using a circuit configuration shown in
FIG. 12
, for example, and transmits a power control signal demanding adjustment of transmission power to the base station. By using circuit configurations shown in
FIGS. 13 and 14
, the base station conducts transmission signal power control operation in response to the above described power control signal.
FIG. 12
shows the configuration of a transmitter and receiver circuit of a conventional terminal.
A signal received by an antenna
301
is inputted to a radio frequency circuit
303
via a circulator
302
and converted therein to a base band spread spectrum signal.
The above described base band spread spectrum signal is inputted to a first multiplier
304
, therein multiplied by pseudo-noise PN generated by a pseudo-noise generator
305
, and subjected to a first stage of de-spreading process. The above described pseudo-noise PN has a noise pattern set so that the pseudo-noise PN may become the same as a unique pseudo-noise PN generated by a PN generator
103
of the above described base station when the position of the terminal is registered in the base station.
The signal subjected to the first stage of de-spreading process is inputted to a second multiplier
307
, therein multiplied by an orthogonal code Wi generated by an orthogonal code generator
306
and assigned to the terminal, and subjected to a second stage of de-spreading process.
The signal subjected to the second-stage of de-spreading process is inputted to an accumulator
308
. The signal received during a predetermined time is accumulated by the accumulator
308
. The accumulated signal is decoded by a decoder
309
to form received data.
Conventionally in each terminal, the signal-to-noise ratio of the received signal is measured by utilizing the fact that the variance of probability density distribution relating to the amplitude of the received signal indicates the noise power and its average indicates the amplitude of signal. For the purpose of this measurement of the signal-to-noise ratio, the output of the accumulator
308
is inputted to an absolute value unit
328
and a square unit
325
. The absolute value of the received signal obtained by the absolute value unit
328
and the square value obtained by the square unit
325
are supplied to a signal-to-noise (S/N) ratio measuring unit
329
.
In the signal-to-noise ratio measuring unit
329
, the signal-to-noise ratio is measured by deriving noise power from the difference between the average value of squared value input and the squared value of the average of the absolute value input and deriving signal power from the squared value of the average of the absolute value input. In a comparator
330
, the measured signal-to-noise ratio is compared with a reference signal-to-noise ratio value. From the comparator
330
, a power control signal PC-i for requesting the base station to increase or decrease the transmission power is outputted.
The power control signal PC-i is multiplexed in a multiplexer
317
with a data signal to be transmitted from the terminal and subjected to encoding process for error correction in an encoder
318
. In a multiplier
320
, the encoded signal is multiplied by pseudo-noise generated by a pseudo-noise generator
319
and thereby subjected to spread spectrum modulation. The signal subjected to spread spectrum modul
Doi Nobukazu
Yano Takashi
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Patel Ajit
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