Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
2000-12-11
2004-11-02
Chin, Stephen (Department: 2634)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
Reexamination Certificate
active
06813317
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention mainly relates to a multicarrier transmitting method and its transmission circuit used for a cellular base station.
2. Related Art of the Invention
Recently, digital mobile communications have made remarkable progress, and the arrangement of the infrastructure including a base station is urgently required. Especially in large cities, a smaller base station is necessary for use in a difficult place for radio communications such as an area encompassed by large buildings, an underground street, etc. Thus, a smaller base station device replacing the conventional large base station device is requested.
The conventional multicarrier transmitter circuit will be described below by referring to FIG.
4
. In
FIG. 4
showing a block diagram of the conventional multicarrier transmitter circuit, an exchange
402
retrieves only a necessary signal from among signals for respective users transmitted by a public telephone network
401
, and the signal is output to each of n channels. The n output signals are handled in an appropriate base band process such as a band limit filtering process by base band process circuits
403
-
1
to
403
-
n
, modulated by modulators
404
-
1
to
404
-
n
, and added up by an addition circuit
405
in an analog system. An output is amplified by a high frequency power amplification circuit
406
, and is transmitted through an antenna
407
.
FIG. 5
shows a conventional common multicarrier transmission signal. FIG.
5
(
a
) shows the frequency spectrum of a common multicarrier transmission signal. FIGS.
5
(
b
),
5
(
c
),
5
(
d
), and
5
(
e
) shows the rotation of the vector of one carrier. FIG.
5
(
f
) shows a case in which four carriers have 90-degree phases each other. FIG.
5
(
g
) shows a case in which vectors of the carriers are composed. FIG.
5
(
h
) shows a change of total power of a multicarrier transmission signal with time.
As shown in FIG.
5
(
a
), each carrier is positioned with its frequency shifted from one another so that the central frequency f
1
, f
2
, . . . , fn cannot be superposed. The vector of one (f
1
) of the carriers is rotating counterclockwise from the stating point shown in FIG.
5
(
b
). After ¼ period, it is rotated to the position shown FIG.
5
(
c
). After ½ period, it is rotated to the position shown in FIG.
5
(
d
). After ¾ period, it reaches the position shown in FIG.
5
(
e
). After a period, it reaches the original point shown in FIG.
5
(
b
). Since the central frequency of the carrier of a multicarrier is shifted little by little, the value obtained by composing each vector changes with time although they have an equal carrier amplitude.
Assuming that there are carriers f
1
to fn having four different frequencies and an equal amplitude, and they are shifted by 90 degrees at a certain moment respectively as shown in FIG.
5
(
f
), f
1
and f
3
have an equal value in the opposite directions, f
2
and fn have an equal value in the opposite directions, and the composite vector reached nearly 0. If the four vectors are arranged in the same direction at a certain moment, the composite vector quadruples. For example, if there are three carriers, that is, f
1
, f
2
, and f
3
, as shown in FIG.
5
(
g
), the composite vector is f which changes with time because respective angular speeds are different a little bit from one another. Therefore, the total power of the transmission signal changes with time, and a peak power at a level considerably higher than that of an average power is occasionally generated as shown in FIG.
5
(
b
).
Furthermore, a present portable telephone in a Code Division Multiple Access (CDMA) system has been developed to replacing the current digital portable telephone by reserving a larger communications capacity. Since the CDMA is described in detail in ‘CDMA System and New Generation Mobile Communications System (edited by Akira Ogawa, in a Trikeps series; Chapter 1 PP12-25; published by Trikeps Ltd.’, the detailed explanation of the system will be omitted here. Since a base station of the digital portable telephone uses a linear modulation system, and a signal is transmitted along a plurality of carriers (multicarrier), a transmission and reception circuit requires strict linearity and a wide dynamic range.
FIG. 6
is a block diagram of the main part of a multicarrier transmission circuit in the conventional CDMA system. In
FIG. 6
, k×n channel signals retrieved from a public network through an exchange (not shown in the attached drawings) are input to channel input terminals
601
-(
1
-
1
) to
601
-(n-k), and multiplied respectively by code multipliers
602
-(
1
-
1
) to
602
-(n-k) using a code selected by a code selection circuit
607
. The resultant k outputs are added up by using digital addition circuits
603
-
1
to
603
-n, and obtains n outputs. Using the resultant n outputs, the n carriers generated by carrier generators
605
-
1
to
605
-
n
are modulated by modulators
604
-
1
-
604
-n. The resultant n outputs are added up in an analog system by an addition circuit
606
, and a multicarrier signal is obtained at an output terminal
608
. The signal is amplified by a high frequency power amplification circuit as shown in
FIG. 4
, and transmitted through an antenna. Especially, a transmission circuit has a circuit for handling high power such as a power amplification circuit, etc., and is designed to cover momentary maximum output (peak) power for average output power to maintain the linearity. Furthermore, since a high transmission rate is required to obtain a larger communications capacity, the bandwidth of a transmission signal ranges from several MHz to tens of GHz. Therefore, it is necessary to use a circuit operable with a signal which is variable by {fraction (1/10)} microsecond.
However, when a ratio of momentary maximum output power to average output power (peak factor) becomes high, the transistor of a power amplification circuit also becomes large, thereby requiring average power obtained by considerably lowering the output level down from a saturated output power. In this manner with the level lowered, the ratio of the DC supply power of a power amplifier to the retrieved transmission power (power conversion efficiency) is deteriorated. Especially, in the CDMA system, the peak factor doubles more than the value in the conventional TDMA system. Furthermore, by multiplexing a code, which is a feature of the CDMA system, the peak factor becomes larger, and the peak factor of about 13 dB can be obtained at the maximum multiplexing operation. If it is furthermore multiplexed through a multicarrier, the peak factor becomes the larger as described above. Therefore, a transmission circuit such as a power multiplication circuit requires strict linearity as compared with the conventional system, and it is necessary to use an element capable of outputting power more than ten times the actual power. As a result, the transmission circuit is larger, and it is difficult to design a small base station.
To reduce a peak factor, a multicarrier transmitter circuit under feedback control as disclosed by the Japanese Patent Laid-Open No.8-274734 and No.8-818249 has been suggested. In these circuits, the signal fluctuation period is higher than tens of microseconds when a narrow band (several kHz to several hundreds of kHz) is transmitted, thereby successfully performing the process through the circuit. However, for a broad band signal of several MHz to tens of MHz, the circuit cannot follow the fluctuation period. Therefore, it is difficult to apply the transmittion the circuit used for a base station.
SUMMARY OF THE INVENTION
To solve the above mentioned problems of the conventional multicarrier transmitter circuit, the present invention aims at providing a multicarrier transmitting method and a multicarrier transmitter circuit for reducing momentary maximum output power to a small value for a broad band signal of several MHz to tens of MHz, and reducing a peak factor of a multicarrier tr
Ishida Kaoru
Kugou Shin'ichi
Ahn Sam K.
Chin Stephen
Smith , Gambrell & Russell, LLP
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