Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
2001-11-07
2003-05-20
Bocure, Tesfaldet (Department: 2631)
Pulse or digital communications
Transmitters
Antinoise or distortion
C332S162000
Reexamination Certificate
active
06567478
ABSTRACT:
TECHNICAL FIELD
This invention relates to a radio apparatus having a function which compensates for non-linear distortion of a transmission power amplifier.
A power amplifier for amplifying a linear modulated signal used in wireless communications is required to have an amplifier with excellent linearity in order to suppress deterioration of transmission characteristics caused by spectrum characteristics and signal distortion. On the other hand, it is required in almost all applications that an amplifier deliver a high power efficiently at all times. In general, linearity and efficiency of an amplifier are characteristics that run counter to each other and a variety of distortion compensation schemes have been proposed in order to reconcile the two.
The field of next-generation mobile telephone systems (IMT-2000, etc.) based upon W-CDMA is one in which the present invention is particularly useful. With W-CDMA, code division multiplexing is used in direct-sequence spread-spectrum modulation and multiple-access for signal modulation. The transmitted signal has a wider band and a higher dynamic range in comparison with the narrow-band modulation and time division multiplexing schemes used heretofore in existing second-generation mobile telephones (PDC), etc. Accordingly, a power amplifier used in a W-CDMA apparatus is required to exhibit better linearity and higher efficiency than in the past.
BACKGROUND ART
FIG. 29
is a block diagram illustrating an example of a radio apparatus according to the prior art. A transmit-signal generator
1
transmits a serial digital data sequence and a serial/parallel (S/P) converter
2
divides the digital data sequence alternately one bit at a time to convert the data to two sequences, namely an in-phase component signal (“I signal”: In-phase component) and a quadrature component signal (“Q signal”: Quadrature component). A DA converter
3
converts the I and Q signals to respective ones of analog baseband signals and inputs these to a quadrature modulator
4
. The latter multiplies the input I and Q signals (the transmit baseband signals) by a reference carrier wave and a signal that has been phase-shifted relative to the reference carrier by 90°, respectively, and sums the results of multiplication to thereby perform quadrature modulation and output the modulated signal. A frequency converter
5
mixes the quadrature-modulated signal and a local oscillation signal to thereby effect a frequency conversion, and a transmission power amplifier
6
power-amplifies the carrier output from the frequency converter
5
. The amplified signal is released into the atmosphere from an antenna
7
.
In a transmitting apparatus of this kind, the input/output characteristic of the transmission power amplifier develops non-linearity, as indicated by the dashed line in FIG.
30
(
a
). Owing to this non-linear characteristic, non-linear distortion occurs and the frequency spectrum in the vicinity of transmission frequency f
0
develops rising side lobes as indicated by the dashed lines in FIG.
30
(
b
). This leads to leakage and interference between neighboring channels. For this reason, various distortion compensating techniques have been proposed, one of which is a predistorter (an distortion compensating device). A predistorter adds a characteristic that is the inverse of the distortion of a transmission power amplifier onto an input signal in advance, whereby the transmission power amplifier outputs the desired distortion-free signal.
FIG. 31
is a block diagram of a radio apparatus having a non-linear distortion compensating function, which uses a digital Cartesian scheme, as a prior-art example of a predistorter. Digital data sent from the transmit-signal generator
1
is converted to two signal sequences, namely an I signal &ngr;
i
and a Q signal &ngr;
q
, in the S/P converter
2
, and these signals enter a predistorter
8
. The predistorter
8
reads distortion compensation values &Dgr;&ngr;
i
(n), &Dgr;&ngr;
q
(n), which correspond to the input baseband signals &ngr;
i
, &ngr;
q
, out of distortion compensation tables
8
a
,
8
b
, adds these compensation values to the signals &ngr;
i
, &ngr;
q
and inputs the results to the DA converter
3
. The latter converts the entered I signal &ngr;
i
and Q signal &ngr;
q
to analog baseband signals and inputs these signals to the quadrature modulator
4
. The latter multiplies the input I and Q signals by a reference carrier wave and a signal that has been phase-shifted relative to the reference carrier by 90°, respectively, and sums the results of multiplication to thereby perform quadrature modulation and output the modulated signal. The frequency converter
5
mixes the quadrature-modulated signal and a local oscillation signal to thereby effect a frequency conversion, and the transmission power amplifier
6
power-amplifies the carrier output from the frequency converter
5
. The amplified signal is released into the atmosphere from the antenna
7
. Part of the transmit signal is input to a frequency converter
10
via a directional coupler
9
, whereby the signal undergoes a frequency conversion and is input to a quadrature detector
11
.
The quadrature detector
11
multiplies the input signal by a reference carrier wave and a signal that has been phase-shifted relative to the reference carrier by 90°, reproduces baseband signals &ngr;′
i
, &ngr;′
q
on the transmitting side and applies these signals to an AD converter
12
. The latter converts the applied I and Q signals to digital data and inputs the digital data to a distortion compensator
8
. At this time a phase shifter
13
applies a phase adjustment in such a manner that the phases of the demodulated baseband signals &ngr;
i
′, &ngr;
q
′ will coincide with the phases of the input signals &ngr;
i
, &ngr;
q
. The AD demodulator
12
applies an AD conversion to the demodulated baseband signals &ngr;
i
′, &ngr;
q
′ obtained by quadrature detection and inputs the resulting signals to the predistorter
8
. The latter compares the demodulated baseband signals &ngr;
i
′, &ngr;
q
′ and the input baseband signals &ngr;
i
, &ngr;
q
, updates the compensation values in the distortion compensation tables
8
a
,
8
b
based upon errors between the signals and stores updated distortion compensation values &Dgr;&ngr;
i
(n+1), &Dgr;&ngr;
q
(n+1) in the memories
8
a
,
8
b
. The operation described above is subsequently repeated.
With the digital Cartesian scheme described above, predistortion is carried out by obtaining distortion of the transmission power amplifier as an error along each axis of a rectangular coordinate system and adding characteristics that are the inverse of these errors to the respective axial components.
FIG. 32
is a prior-art example of distortion compensation based upon a feed-forward (FF) scheme. With the FF scheme, part of a signal that has been amplified by a main amplifier (transmission power amplifier)
6
is branched by a directional coupler
9
, and an arithmetic unit
15
calculates the difference between the branched part of the signal and a signal obtained by subjecting the input signal to a delay adjustment and level adjustment. The difference signal is a non-linear distortion component produced by the main amplifier
6
. The difference signal is amplified by a linear auxiliary amplifier
16
, and a combiner
18
combines, 180° out of phase, the output of the auxiliary amplifier and a signal that is result of delaying the main amplifier output by a delay line
17
. As a result, distortion compensation is achieved by canceling out the distortion components.
Problems of the Prior Art
With the conventional predistorter, the signal that has undergone predistortion is required to have a wide dynamic range in comparison with the dynamic range of the original signal in order to compensate for amplitude distortion of the amplifier (the power transmission amplifier). This means that a higher bit precision is required for the DA converter that subjects the predistortion signal to
Kubo Tokuro
Nagatani Kazuo
Ode Takayoshi
Oishi Yasuyuki
Takano Takeshi
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