Pulse or digital communications – Transmitters – Angle modulation
Reexamination Certificate
2000-02-11
2004-01-20
Chin, Stephen (Department: 2634)
Pulse or digital communications
Transmitters
Angle modulation
C375S261000, C375S269000, C332S103000
Reexamination Certificate
active
06680981
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a &pgr;/4 shift QPSK modulator, as well as a communication device, which is suitable for use of ICs in communications using digital signals.
Conventionally, as a digital signal modulation system, the QPSK (Quadrature Phase Shift Keying) system has been widely used. In this QPSK system, a filter having roll-off characteristics is used as a band-limiting filter so that intersymbol interference is eliminated. This filter having roll-off characteristics is, in many cases, a digital filter because of complex characteristics. However, in this digital filter, since arithmetic processing of signals is executed by multiplications and additions, the arithmetic processing needs to be executed at high speed.
Thus, in order to keep up with this higher-speed arithmetic processing, there has been proposed a technique that the digital filter is replaced with ROM by previously storing processing results in ROM (Read Only Memory) and by feeding input data as ROM addresses (see Japanese Patent Laid-Open Publication SHO 53-24763).
There has also been proposed a symbol tap ROM division method that the digital-filter ROM is divided in every accumulative symbol tap (“&pgr;/4 QPSK Baseband Signal Generator Using Symbol Tap Divided ROM”, Proceedings of the 1992 Spring Conference of IEICE (Institute of Electronics, Information and Communication Engineers)). The digital filter of this &pgr;/4 QPSK baseband signal generator using the symbol tap divided ROM, as shown in
FIG. 6
, comprises: nine unit delay circuits
61
for delaying 2-bit symbol mapping data in symbol cycles; totally nine ROMs
62
to which an address is given by totally 6 bits composed of 2-bit for output of each unit delay circuit
61
and 4-bit for time information; an adder
63
for adding up output data of the ROMs
62
; and a D/A (digital-to-analog) converter
64
for converting output data of the adder
63
into analog form. The ROMs
62
are driven by a clock sixteen times higher than the symbol clock (oversampling). Data lengths of the nine ROMs
62
of the digital filter are 4, 5, 7, 10, 11, 10, 7, 5 and 4 bits, respectively, by simulating dynamic ranges of impulse response of the root Nyquist filter in every symbol interval.
Like this, by determining data lengths corresponding to necessary dynamic ranges in every symbol interval, the total capacity of the ROMs
62
is reduced without lowering the processing precision. Also, by time-dividing I and Q phases of the symbol mapping data, a &pgr;/4 shift QPSK baseband signal generator is implemented with one digital filter.
Further, a &pgr;/4 shift QPSK modulator in which circuit scale and ROM capacity are kept low is disclosed in Japanese Patent Laid-Open Publication HEI 3-235553. There has also been disclosed, in Japanese Patent Laid-Open Publication HEI 7-50693, a technique that in the &pgr;/4 shift QPSK modulator, ROM capacity is reduced by commonizing the phase information I, Q to the ROMs.
FIG. 5
is a main-part block diagram of the ROM capacity reducing technique by using a ROM in common to phase information I, Q, as described in Japanese Patent Laid-Open Publication HEI 7-50693. In
FIG. 5
are shown a mapping circuit
50
, an oversampling counter
51
, an impulse response computing circuit
52
, cumulating circuits
551
,
552
, and D/A converters
571
,
572
. The impulse response computing circuit
52
has therein a ROM
54
in which impulse response data of two kinds of phase information are stored, sign inverting circuits
526
,
526
for performing sign inversion of impulse response data derived from the ROM
54
, and zero output circuits
527
,
527
for replacing outputs of the sign inverting circuits
526
,
526
with zeroes. Since the ROM
54
is provided in common to two systems of phase information (I and Q components) perpendicular to each other, only one ROM
54
will do for phase information (I and Q components) and the storage capacity of the ROM
54
can be reduced.
Further, it is conceivable to make up a &pgr;/4 shift QPSK modulator shown in
FIG. 4
by combining the prior arts of FIG.
5
and
FIG. 6
as described above.
As shown in
FIG. 4
, a signal representing phase information outputted from a mapping circuit
10
is inputted to impulse response computing means
42
. The inputted data is shifted with a shift register
421
by a symbol clock
13
. Then, as shown in a signal arrangement view of
FIG. 2
, signal modulation is done by shifting a reference phase by &pgr;/4 in every symbol cycle. Referring to
FIG. 2
, a signal of a point • is transmitted at an even-numbered timing, and a signal of a point o is transmitted at an odd-numbered timing. That is, a phase state “o” becomes a phase state “•” with a shift of &pgr;/4 at the next symbol timing. Also, after phase information is differentially coded at each • and o, the phase information is divided into vectors of I component and Q component at the individual points • and o, and based on these pieces of information, mapped into magnitude information, sign information and zero replacement information by the mapping circuit
10
. Then, symbol mapping data from the mapping circuit
10
is inputted to the shift register
421
(7 taps), and a total sum of impulse response values corresponding to signals representing the phase information time-delayed by the shift register
421
is computed, by which filter characteristics are fulfilled. Further, outputs from the registers D
1
-D
7
of the shift register
421
are inputted to impulse response storage sections
424
(ROM
1
-ROM
7
), respectively, in which impulse response data have been dividedly stored. In these impulse response storage sections
424
, an impulse response waveform (shown in the schematic view of
FIG. 4
) is divided into 7 symbol intervals, and impulse response data corresponding to a magnitude &agr; and a magnitude &bgr; are oversampled in each symbol interval and stored into the ROM
1
-ROM
7
. Output values from an oversampling counter
11
of
FIG. 4
correspond to sample numbers of
FIG. 3
, and impulse response data corresponding to the sample numbers are stored in the impulse response storage sections
424
.
FIG. 3
shows impulse response data (amplitude values of impulse response waveform) stored in a ROM
2
of the &pgr;/4 shift QPSK modulator shown in FIG.
4
. Referring to
FIG. 3
, according to the sample numbers
1
-
16
and the magnitude information derived from the registers D
1
-D
7
of the shift register
421
, impulse response data is read from the ROM
2
, and the impulse response data read from the ROM
2
is inputted to a numerical value conversion section
426
corresponding to the ROM
2
. Also, sign information and zero-replacement information contained in a signal representing phase information derived from the shift register
421
are time-divided by an IQ time-division clock
14
by a selector
425
and inputted to the numerical value conversion section
426
. This numerical value conversion section
426
executes, as appropriate, sign inversion or zero replacement for each of phase information I and phase information Q with respect to the impulse response data derived from the ROM
2
. Then, outputs from all the numerical value conversion sections
426
are added up by an adder
15
, separated into I component and Q component by latch circuits
161
,
162
, and the separated I component and Q component are converted into analog form by D/A converters
171
,
172
. Thus, modulation signals of I output and Q output are produced, respectively.
In the &pgr;/4 shift QPSK modulator shown in
FIG. 4
, which is based on a system that the principle of convoluting operation is applied to ROM filter implementation, a ROM data map is provided by partitioning a single-pulse root Nyquist filter pass waveform in every symbol interval, and by sampling the partitioned waveforms at an appropriate oversampling frequency, thus giving rise to a need for two kinds of magnitudes of impulse response data for the phase information of symbol intervals. As shown in
Chang Edith
Sharp Kabushiki Kaisha
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