Telecommunications – Transmitter – Power control – power supply – or bias voltage supply
Reexamination Certificate
2001-11-01
2004-08-10
Urban, Edward F. (Department: 2685)
Telecommunications
Transmitter
Power control, power supply, or bias voltage supply
C455S126000, C375S295000, C330S297000
Reexamination Certificate
active
06775524
ABSTRACT:
TECHNICAL FIELD
This invention relates to a signal transmitting apparatus and, more particularly, to a signal transmitting apparatus for controlling the power-supply voltage of a transmission-line driving circuit, which transmits a signal to a transmission line, based upon the output signal amplitude of the transmission-line driving circuit.
BACKGROUND ART
A signal transmitting apparatus according to the prior art fixes the power-supply voltage, which is supplied to a transmission-line driving circuit (driver), at a certain value in conformity with the maximum value of the output signal amplitude. This method makes it possible to transmit a signal with little distortion. On the other hand, the average value of signal amplitude with respect to the value of the supplied power-supply voltage is small. As a consequence, the power of the transmitted signal with respect to the power consumed by the driver circuit is small and a problem that arises is poor power efficiency of signal transmission. In particular, with a multicarrier modulation method such as DMT (Discrete Multitone), described below, the ratio PAR (Peak Average Ratio) of momentary maximum output voltage of a signal to the average output voltage thereof is extremely high and the driver circuit is supplied with a high power-supply voltage owing to the momentary maximum value, which appears only rarely. Power efficiency, therefore, is low.
The ADSL (Asymmetric Digital Subscriber Line) scheme is a typical transmission scheme that uses DMT. In recent years, G.992.1 (G.dmt) and G.992.2 (G.lite) have been adopted as ADSL standards by the ITU. This will be described below taking G992.1 (G.dmt) as an example.
With DMT modulation, as shown in
FIG. 8
, a frequency band of 1.104 MHz is divided into M (M=256) multicarriers #1~#256 at intervals of &Dgr;f (=4.3125 KHz). The S/N ratios that prevail when a transmission is made in accordance with 4-QAM (Quadrature Amplitude Modulation) by each of the carriers #1~#256 are measured in advance and it is decided, depending upon the S/N ratios, with which modulation method among 4-QAM, 16-QAM, 64-QAM, 128-QAM . . . modulation methods data is to be transmitted in each carrier. For example, 4-QAM is assigned to a carrier having a small S/N ratio and 16-QAM, 64-QAM, 128-QAM . . . are assigned successively as the S/N ratio increases. It should be noted that 4-QAM is a modulation scheme in which two bits are transmitted at a time, 16-QAM a modulation scheme in which four bits are transmitted at a time, 64-QAM a modulation scheme in which six bits are transmitted at a time, and 128-QAM a modulation scheme in which seven bits are transmitted at a time.
FIG. 9
is a diagram useful in describing 16-QAM. A serial/parallel converter (S/P converter)
1
stores transmit data, which enters as a bit serial, in a buffer successively four bits at a time and outputs four bits as 2-bit parallel data (a
i
,b
i
), (a
i+1
,b
i+1
). A first binary/quaternary converter
2
converts the parallel data (a
i
,b
i
) to four values (−3, −1, +1, +3), and a second binary/quaternary converter
3
converts the parallel data (a
i+1
,b
i+1
) to four values (−3, −1, +1, +3). A carrier generator
4
generates a cosine wave cos (&ohgr;
c
t) of frequency f
c
(&ohgr;
c
=2&pgr;fc), and a phase shifter
5
shifts the phase of the cosine wave by 90° to output a sine wave sin (&ohgr;
c
t). An AM modulator
6
multiplies the output of the first binary/quaternary converter
2
by the sine wave sin (&ohgr;
c
t), and an AM modulator
7
multiplies the output of the second binary/quaternary converter
3
by the cosine wave cos (&ohgr;
c
t). An adder
8
combines the outputs of the AM modulators
6
and
7
and outputs the combined signal. By executing the operation described above, the 16-QAM modulator outputs signals having the illustrated two-dimensional signal point placement (constellation) in accordance with the combination of parallel data (a
i
,b
i
), (a
i+1
,b
i+1
). For example, if data divided into four bits at a time is 1001, 0011, 1100, 0110, the 16-QAM modulator outputs signals {circle around (1)}→{circle around (2)}→{circle around (3)}→{circle around (4)} in the constellation.
FIG. 10
is diagram useful in describing the principle of DMT modulation. From bit-serial transmit data, an S/P converter
11
stores a bit sequence that is to be transmitted within a certain period in an internal buffer and subsequently outputs the bit sequence to a carrier mapper
12
. Data transmitted within this fixed period is referred to as a symbol. Since the QAM modulation scheme of each carrier is known, the carrier mapper
12
divides the one symbol's worth of bit sequence b
k
-number of bits at a time in accordance with the QAM modulation scheme of each carrier and inputs the resultant bit sequence to a QAM modulator
13
i
of the particular carrier. As a result, the total number of output bits per symbol is &Sgr;b
k
(k=1 to M). In this case, the carrier mapper
12
performs the bit division of one symbol successively in accordance with the QAM modulation scheme of the carrier, starting from carriers having a low frequency. A frequency multiplexer
14
frequency multiplexes the QAM signals output from the QAM modulators
13
i
of the respective carriers and outputs the multiplexed signal to a transmission line via a transmission-line driver circuit (not shown).
With the method described above, the number of QAM modulators required is equal to the number of carriers. Let X
k1
, X
k2
represent the first and second halves, respectively, of 2·m
k
bits input to a kth QAM modulator (k=1, 2, . . . , M). If the following holds:
X
k
=X
k1
+jX
k2
then the output signal of the frequency multiplexer will be a real-number portion of an inverse Fourier transform of X
k
. Accordingly, transmission based upon DMT modulation is carried out by providing an arithmetic unit, which implements an IFFT (Inverse Fast Fourier Transform), instead of QAM modulators the number of which is the number (M) of carriers.
FIG. 11
is a basic structural diagram of a DMT transmission circuit having an IFFT arithmetic unit. Components identical with those of
FIG. 10
are designated by like reference characters.
When data of a complex frequency region enters via the S/P converter
11
and carrier mapper
12
, an IFFT arithmetic unit
21
converts the frequency signal (frequency-region signal) X
k
(k=1, 2, . . . M) of each carrier to a time-region signal x(m) by an IFFT arithmetic operation. If the time-region signal x(m) is illustrated upon enlarging the time axis, the result will be as shown in
FIG. 12
, by way of example, where m represents time at discrete time intervals &Dgr;t and m per symbol is equal to 1 to M.
A parallel/serial converter (P/S converter)
22
holds M-number of items of time-region data x(
1
) to x(M), which are output from the IFFT arithmetic unit
21
, in an internal buffer and outputs this data in the order x(
1
), x(
2
) . . . x(M). A DA converter
23
converts the time-region data x(
1
), x(
2
) . . . x(M) to an analog signal and outputs the analog signal as well as a signal obtained by reversing the polarity of this analog signal. A band-pass filter (BPF)
24
passes only the necessary band components contained in the signals output from the DA converter and inputs these components to a transmission-line driver circuit (driver)
25
. The driver
25
, which has the structure of a differential amplifier, differentially amplifies the input signals and outputs the results to a transmission line
28
via resistors
26
1
,
26
2
and a transformer
27
.
In the case of an FDM (Frequency Divided Multiplex) scheme in accordance with G992.1 (G.dmt), all 256 carriers are allocated for (1) the upstream direction from the subscriber to the office and (2) the downstream direction from the office to the subscriber; the number of carriers for the latter is 224. Further, symbol frequency fc (&equals
Kobayashi Yuji
Miyoshi Seiji
Takeuchi Yasuaki
Takeyabu Masato
Yoshimi Masahisa
Chow C.
Katten Muchin Zavis & Rosenman
Urban Edward F.
Yoshimi Michiyo
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