.pi./4 shifted DQPSK modulator

Details .pi./4 shifted DQPSK modulator .pi./4 shifted DQPSK modulator

1994-12-21

1996-06-18

Chin, Stephen

Pulse or digital communications

Systems using alternating or pulsating current

Angle modulation

375308, 332103, 332104, H04L 2720

Patent

active

055286313

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to a modulator which is provided in a digital radio communications apparatus, such as a digital portable telephone set, digital mobile telephone set and digital cordless telephone set and, in particular, a modulator using, as a modulation system, a .pi./4 shifted, differentially encoded quadrature phase shift keying (hereinafter referred to as a .pi./4 shifted DQPSK) system.


BACKGROUND ART

In recent years, a cellular radio telephone system adopting a digital system has been proposed as one radio communications system. This type of system transmits, in digitized form, communication information, such as a speech message between a base station and a mobile unit in addition to a control signal. This system has several advantages in that it is possible to improve privacy, to secure data/system compatibility, and to effectively utilize the radio frequency.
The digital radio communications apparatus including this type of system uses, for example, a .pi./4 shifted DQPSK system and is so constructed as, for example, set out below. FIG. 1 is a block diagram showing an arrangement of the modulator.
A transmission stream SD is converted by a serial/parallel conversion circuit (S/P) 1 into two data streams X.sub.k and Y.sub.k. Here, the transmission data stream is comprised of an NRZ signal of, for example, 0 to 5 V.
These data streams X.sub.k and Y.sub.k are differentially coded by a differential coding circuit 2 into the following: ##EQU1## where I.sub.k-1, Q.sub.k-1 : the amplitude of the coded data at one previous pulse time; and
The amount of phase variation, .DELTA..phi., is determined by the amplitudes of the input data streams X.sub.k, Y.sub.k as shown in FIG. 3. As the amplitude value of the coded data I.sub.k, Q.sub.k, one is selected out of five values, that is, 0, .+-.1, .+-..sqroot.2.
The coded data I.sub.k, Q.sub.k delivered from the differential coding circuit 2 are input to a mapping circuit 3. At each pulse time of the coded data I.sub.k, Q.sub.k delivered from the differential coding circuit 2, the mapping circuit 3 determines, based on the phase mapping position of the coded data I.sub.k-1, Q.sub.k-1 obtained at one previous pulse time, the phase mapping position of the coded data I.sub.k, Q.sub.k at this current pulse time.
FIG. 4 is a phase space diagram representing the phase mapping positions of the coded data I.sub.k, Q.sub.k delivered from the mapping circuit 3. As evident from this diagram, the mapping position is so determined that, at each pulse time, any of those positions indicated by .quadrature. in FIG. 4 and any of those positions indicated by .smallcircle. in FIG. 4 are so determined as to be alternately selected.
The phase mapping positions of the coded data MI, MQ have eight combinations: (+I, +Q), (0, +Q), (-I, +Q), (-I, 0), (-I, -Q), (0, -Q), (+I, -Q), and (+I, 0). These combinations are represented by (+I, 0, -I) and (+Q, 0, -Q). Here, +I, +Q show the positive positions on the I axis, Q axis, respectively, and -I, -Q show the negative positions on the I axis, Q axis, respectively.
The coded data MI, MQ delivered from the mapping circuit 3 is input to a roll-off filter 4 where they are subjected to low-pass filtering processing. The roll-off filter 4 is used to reduce an influence resulting from the interference between those codes generated on a transmission path. The roll-off filter 4 is comprised of such a transversal type FIR filter as shown, for example, in FIG. 2. Given that, as here, one symbol (i.e., transmission unit: 2 bits in the .pi./4 shifted DQPSK modulation system) is represented by 256 samples, it is necessary that the signal entering the filter 4 corresponds to one obtained by sampling one symbol into 1/256 parts. If the impulse response of the filter is 10 symbols, filtering is carried out such that each sample of input data in (i.e., in 10.times.256 samples)=2,560 samples is multiplied by a corresponding factor.
The frequency response .vertline.H(f).vertline. of the filter is represented by, for example, the foll

REFERENCES:
patent: 5157693 (1992-10-01), Lemersal et al.
patent: 5210775 (1993-05-01), Takahara et al.
patent: 5285479 (1994-02-01), Iwane
patent: 5361047 (1994-11-01), Takahara et al.
patent: 5363410 (1994-11-01), Hayashi et al.
patent: 5369378 (1994-11-01), Kosaka et al.
patent: 5373532 (1994-12-01), Fujita et al.

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