Pulse or digital communications – Transmitters – Quadrature amplitude modulation
Reexamination Certificate
1999-03-25
2003-01-07
Phu, Phuong (Department: 2631)
Pulse or digital communications
Transmitters
Quadrature amplitude modulation
C375S261000, C332S103000
Reexamination Certificate
active
06504879
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a digital modulation apparatus.
(2) Description of the Related Art
In recent years, with the diversification of communication services and the increase of demands for information communications, high-speed, large capacity and long distance digital transmission has been performed in a basic trunk transmission line using an optical fiber or a CATV transmission line. The improved performance of an optical device and the technological achievement of a high speed for an LSI have greatly reduced costs for an optical transmission device.
Considered as one of radio transmission systems for supporting such an optical transmission foundation is trunk type multiplex radio transmission which uses a microwave band. Radio transmission data provides for an overall framework for frequency assignment, and so on. For achieving a much higher speed for radio digital transmission, a multivalued digital transmission system such as a QAM (Quadrature Amplitude Modulation) or a QPSK (Quadri Phase Shift Keying) has been employed to secure a high bit rate even in a narrow frequency band.
Described below as systems for converting baseband signals into signals of transmission frequencies with reference to
FIGS. 16
to
19
are four kinds including an analog system of one kind and digital systems of three kinds.
Referring to
FIG. 16
, shown are main sections of the transmitter of an analog type radio transmitter-receiver. An analog modulation apparatus
50
shown in
FIG. 16
up-converts a baseband modulating signal into a signal of an RF (Radio Frequency) band and then transmits this signal to a radio channel. The analog modulation apparatus
50
comprises roll-off filters
51
a
and
51
b
for bang-processing baseband signals to reduce intersymbol interference to a minimum, a frequency conversion unit
54
for frequency-converting the outputs of these roll-off filters
51
a
and
51
b
into signals of an RF band and a transmission unit
55
for transmitting the RF signals to the radio channel.
Herein, the frequency conversion unit
54
frequency-converts the outputs of the roll-off filters
51
a
and
51
b
and then up-converts the same into signals of an RF band. The frequency conversion unit
54
is constructed to include a D/A (Digital/Analog) converter
52
a
for converting the output of the roll-off filter
51
a
from digital to analog and thereby obtaining a multivalued baseband signal, a D/A converter
52
b
for D/A-converting the output of the roll-off filter
51
b
, a first frequency conversion unit
53
a
for up-converting the baseband signal outputted from the D/A converter
52
a
into a signal of an RF frequency, a second frequency conversion unit
53
b
for up-converting a baseband signal outputted from the D/A converter
52
b
into a signal of an RF frequency, a 90° phase sifter
53
e
for inputting the output of a carrier generator
53
d
to the first frequency conversion unit
53
a
and an output phase-shifted by 90° to the second frequency conversion unit
53
b
, and a hybrid unit
53
c
for coupling the outputs of these first and second frequency conversion units
53
a
and
53
b.
The transmission unit
55
transmits an RF signal outputted from the hybrid unit
53
c
to the radio channel. The transmission unit
55
is constructed to include a band-pass filter
55
a
for limiting a transmission band and thereby eliminating unnecessary harmonic components and an antenna
55
b
for transmission to the radio channel.
In the analog system configured in the above manner, baseband signals are outputted from the roll-off filters
51
a
and
51
b
, and then the baseband signals are frequency-converted into signals of an RF band. This analog system is disadvantageous for use in that there is an effect of variance in performance between RF elements and a circuit fine adjustment is necessary. However, with the achievement of a high speed for a device and the improvement of a high-level LSI technology, the analog system is replaced by a digital system, and now it is possible to increase the accuracy of and miniaturize a modem.
Referring to
FIG. 17
, shown are main sections of the transmitter of a radio transmitter-receiver of a digital modulation system. A digital modulation apparatus
56
shown in
FIG. 17
performs quadrature amplitude modulation for an inputted data signal and then transmits the signal to a radio channel. The digital modulation apparatus
56
comprises roll-off filters
51
a
and
51
b
, a transmission unit
55
and a quadrature amplitude modulation unit
57
.
Herein, the quadrature amplitude modulation unit
57
performs quadrature amplitude modulation for baseband signals outputted respectively from the roll-off filters
51
a
and
51
b
. The quadrature amplitude modulation unit
57
is constructed to include a first frequency conversion unit
57
a
, a second frequency conversion unit
57
b
, a hybrid unit
57
c
, a carrier generator
57
d
, a counter
57
e
, a cosine information/sine information ROM (Read Only Memory)
57
f
and a D/A converter
57
g.
The counter
57
e
receives a clock of a speed n×f
SYMBOL
(Hz) which is outputted from the carrier generator
57
d
, and then outputs this clock corresponding to phase information. Herein, a code n denotes an integer ≧2 (normally, an integer ≧4), and a code f
SYMBOL
denotes a symbol clock.
The cosine information/sine information ROM
57
f
has an index based on addresses outputted from the counter
57
e
, and outputs amplitude value information regarding digital sine and cosine components. Amplitude value information regarding digital sine and cosine waveforms is outputted according to phase values obtained by subdividing 0 to 2&pgr; at proper intervals.
The first frequency conversion unit
57
a
multiplies an Ich baseband signal outputted from the roll-off filter
51
a
by a digital cosine component outputted from the cosine information/sine information ROM
57
f
. The second frequency conversion unit
57
b
multiplies a Qch baseband signal outputted from the roll-off filter
51
b
by a digital sine component outputted from the cosine information/sine information ROM
57
f
. Then, Ich and Qch modulating signals respectively outputted from the first and second frequency conversion units
57
a
and
57
b
are coupled together in the hybrid unit
57
c
, passed through the D/A converter
57
g
and then outputted from the transmission unit
55
.
The roll-off filters
51
a
and
51
b
and the transmission unit
55
are the same as the functions of the above analog system, and thus explanation thereof will be omitted.
In the digital system configured in the above manner, quadrature amplitude modulation is performed. In other words, in this digital system, a band for a baseband signal is raised by digital processing. Specifically, its band is raised by multiplying the baseband signal by a sampling clock having a speed faster by n times. More specifically, a baseband signal of a symbol clock speed f
SYMBOL
outputted from the roll-off filter
51
a
is multiplied by a digital cosine waveform having a speed faster by n times, and a baseband signal of a symbol clock speed f
SYMBOL
outputted from the roll-off filter
51
b
is multiplied by a digital sine waveform having a speed faster by n times.
A carrier frequency of n×f
SYMBOL
(Hz) is thereby obtained, and the band is directly up-converted to an RF band.
Here, if there is no problem for the use of an n multiple frequency of a symbol clock as a center frequency of a transmitted carrier, it is possible to configure the transmission system such that a baseband signal can be n multiple of a symbol clock synchronized with this baseband signal. But if there is a problem for the use of an n multiple frequency of a symbol clock, it is necessary to convert a signal outputted from the D/A converter
57
g
in the quadrature amplitude modulation unit
57
into yet another frequency.
The need of conversion into yet another frequency arises because in terms of a relationship between an ou
Katten Muchin Zavis & Rosenman
Phu Phuong
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