Phase modulator efficiently utilizing waveform storing section

Details Phase modulator efficiently utilizing waveform storing section Phase modulator efficiently utilizing waveform storing section

1998-06-02

1999-11-23

Lee, Benny

Modulators

Phase shift keying modulator or quadrature amplitude modulator

332104, 375308, H03C 300

Patent

active

059907550

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to a phase modulating apparatus for modulating digital baseband signals with the Quadrature Phase Shift Keying (QPSK).


BACKGROUND OF THE INVENTION

Currently, the QPSK is widely used as a digital modulation method. For example, RCR STD-28 standard for Personal Handyphone System (PHS) stipulates that the .pi./4-shift QPSK should be used as a modulation method. The QPSK including the .pi./4-shift QPSK is a modulation method in which two bits of data are transferred simultaneously by using the inphase (I-phase) component and quadrature-phase (Q-phase) component of a carrier wave.
Given below is the description of the .pi./4-shift QPSK and conventional .pi./4-shift QPSK modulating apparatus 600.
.pi.4-shift QPSK modulating apparatus receives a baseband signal as serial data. The baseband signal is converted through series-to-parallel conversion into a symbol (X.sub.k,Y.sub.k) which is a piece of two-bit parallel data, where "k" represents a natural number, indicating that "(X.sub.k,Y.sub.k)" is the k-th symbol. The symbol (X.sub.k,Y.sub.k) is converted by a differential encoding circuit into a quadrature signal (I.sub.k,Q.sub.k) This conversion is performed based on the following formula: ##EQU1## where .DELTA..phi.(X.sub.k,Y.sub.k) has values shown in the following table:


TABLE 1 ______________________________________ X.sub.k 1 0 0 1 Y.sub.k 1 1 0 0 .DELTA..phi. -34 34 4 -4 ______________________________________
The quadrature signal (I.sub.k,Q.sub.k) obtained in this way is passed through a lowpass filter to have a limited band and to be converted into I-phase and Q-phase components. A modulating circuit provides the inphase and quadrature-phase components for a wireless circuit.
FIG. 1 shows symbols (X.sub.k,Y.sub.k) mapped on a two-dimensional I-Q coordinate. By focusing on the movement of a signal point represented by a quadrature signal (I.sub.k,Q.sub.k), it is found that the signal point becomes any of signal points A-D and any of signal points E-H alternately. As is apparent from FIG. 1, signal points A-D are represented by a two-dimensional coordinate system represented by vector I.alpha. on the I-axis and vector Q.alpha. on the Q-axis as the axes. Also, signal points E-H are represented by a two-dimensional coordinate system represented by vector I.beta. and vector Q.beta. as the axes, which are respectively equivalent to vectors I.alpha. and Q.alpha. rotated by .pi./4. For example, suppose a current baseband signal is a piece of information represented by symbol point F. Then, "0" is given as its I.alpha. coordinate, "0" as its Q.alpha. coordinate, "-1" as its I.beta. coordinate, and "+1" as its Q.beta. coordinate.
Suppose the coordinates of the k-th symbol using the above four vectors are respectively I.sub..alpha., Q, I, and Q.sub.k. Then, the quadrature coordinates of k-th symbol, I.sub.k and Q.sub.k, are represented by Formulae 2 and 3 respectively. ##EQU2##
Accordingly, I-phase signal I(t) and Q-phase signal Q(t) output from the modulating circuit are represented by Formula 4 in which h(t) represents a rectangular waveform response function used in the lowpass filter. ##EQU3##
Formulae 4 and 5 indicate that limiting the bands of quadrature signals I.sub.k and Q.sub.k may be achieved by limiting bands of each term of Formulae 2 and 3.
Here, if it is supposed that in FIG. 1, symbol points A-D are odd and symbol points E-H are even in order, the following formulae are given. ##EQU4## Hence, I(t) and Q(t) are represented by the following formulae. ##EQU5##
As apparent from the above formulae, of symbols I.sub..alpha.k, Q.sub..alpha., I, and Q being four vectors of signals I(t) and Q(t), I and Q.sub..quadrature.k may be represented by using only odd symbol signals, and I.sub..beta. and Q.sub..beta.k by using only even symbol signals.
FIG. 2 is a block diagram showing the construction of the conventional .pi./4-shift QPSK modulating apparatus 600.
.pi.4-shift QPSK modulating apparatus 600 includes input terminal 601, symbol generating cir

REFERENCES:
patent: 4680777 (1987-07-01), Saha
patent: 4910474 (1990-03-01), Tjahjadi et al.
patent: 5077757 (1991-12-01), Cahill
patent: 5175514 (1992-12-01), Iinuma et al.

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