Demodulators – Phase shift keying or quadrature amplitude demodulator
Reexamination Certificate
2002-04-15
2004-04-06
Shingleton, Michael B (Department: 2817)
Demodulators
Phase shift keying or quadrature amplitude demodulator
C329S306000, C329S307000, C329S308000, C375S340000, C375S326000, C375S327000, C375S329000, C375S324000
Reexamination Certificate
active
06717462
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to high frequency, high data rate communication systems and, more particularly, to demodulation for band efficient quadrature phase shift keying (QPSK) modulation and quadrature amplitude modulation (QAM) using monolithic microwave integrated circuits (MMIC).
The present invention is applicable to bandwidth efficient modulation communication systems. The invention provides a non-wired approach for high data rate needs, for example, in satellite to satellite communication, satellite to ground communication, terrestrial relay links for line-of-sight, demod/remod systems, and space based demod/remod systems.
The purpose of a demodulator is to perform waveform recovery. The best demodulator will recover the baseband pulse with the best signal to noise ratio (SNR). The error degradation in the signals can be caused by two most prominent sources. One most prominent source is inter-symbol interference (ISI). The filtering used to reject unwanted parts of the signal and noise can cause a non-ideal system transfer causing ISI. The ISI distorts the signal and will produce errors in the received signal. The other most prominent source of degradation is due to noise from electrical sources, atmospheric effects, thermal effects, and inter-modulation products, for example. The demodulator should undistort, or correct distortion of, the pulse to give the best possible received signal.
Conventional modulation systems consist of a modulator operated at an intermediate frequency (IF) and a number of filters, amplifiers and mixers that up convert the modulated signal to the transmit frequency, also called the carrier frequency. The wide band signal on the carrier is transmitted over a communication channel and received by a receiver. At the receiver, the wide band signal on the carrier is down converted to an IF channel and then demodulated. The IF channel may be optimized for the control of noise sources to allow increases in data rates and improve link margins. Down conversion of the wide band signals to optimized IF channels may cause phase errors to be introduced into the data that induce increases in the bit error rate of the communication channel. A significant problem for bandwidth efficient modulation and demodulation is achieving low amplitude and phase error.
An example of QPSK modulation is illustrated by the block diagram of
FIG. 1
, where two-bit data word
102
and carrier
104
are input to phase modulator
100
, which outputs QPSK modulated carrier
106
corresponding to a signal, S, of the form:
S
(
t
)=
A
cos(&ohgr;
s
t
−&thgr;+&psgr;) (1)
where A is the carrier amplitude constant and &psgr; is the phase constant. There are four possible values for two-bit data word
102
. Phase modulator
100
maps each of the four possible values for two-bit data word
102
to a distinct value of the phase angle &thgr;.
The QPSK modulated carrier
106
output from phase modulator
100
may be represented on a phase diagram such as phase diagram
200
seen in FIG.
2
. Phase diagram
200
shows that phase angle &thgr; will take on the form of one of four phases separated by 90 degrees. As shown in
FIG. 2
, each of the four possible values of two-bit data word
102
is represented by a symbol
202
, which is a point, or vector, s
1
, s
2
, s
3
or s
4
, in the phase plane of phase diagram
200
. Two bits of information, or one symbol, is sent every word time corresponding to one of the four vectors, or symbols, in phase diagram
200
.
The symbols of a QPSK signals may also be conceptualized as two pairs of a bi-orthogonal set.
FIG. 3
shows a common implementation, using that concept, of QPSK modulator
300
employing orthogonal bi-phase shift keying (BPSK) modulators
310
and
320
. The circuit of QPSK modulator
300
shown in
FIG. 3
uses double-balanced mixers for BPSK modulators
310
and
320
. As seen in
FIG. 3
, two-bit data word
302
is extracted from bit sequences
303
and
305
. Bit sequence
303
and carrier
314
are input to BPSK modulator
310
, which outputs BPSK modulated signal
316
. Bit sequence
305
and carrier
324
are input to BPSK modulator
320
, which outputs BPSK modulated signal
326
. BPSK modulated signals
316
and
326
are added by summer
330
and output as QPSK modulated carrier
336
corresponding to a signal, S, of the form:
S
(
t
)=
A
cos(&ohgr;
s
t
−&thgr;+&psgr;) (2)
where A is the carrier amplitude constant and &psgr; is the phase constant. There are four possible values for two-bit data word
302
each of which is mapped to a distinct value of the phase angle &thgr;. Because carriers
314
and
316
differ in phase by 90 degrees, phase angle &thgr; will take on one of four phase values separated by 90 degrees, as shown in
FIG. 2
, with each of the four possible values of two-bit data word
302
represented by a symbol
202
, which is a vector, s
1
, s
2
, s
3
or s
4
, in the phase plane of phase diagram
200
.
FIG. 4
shows how two QPSK modulation systems
410
and
420
may be combined in a QAM modulation system
400
to achieve a
16
QAM signal
436
. A radio frequency (RF) or IF carrier is provided by local oscillator
404
using timing reference
401
, as known in the art. The RF or IF carrier is split into carriers
414
and
424
, and each is fed into QPSK modulation systems
410
and
420
, respectively. Two-bit data word
412
, which includes bits b
0
and b
1
as shown in
FIG. 4
, and carrier
414
are input to QPSK modulation system
410
. QPSK modulation system
410
outputs QPSK modulated carrier
416
corresponding to a signal which may be represented, as described above in connection with
FIG. 2
, by vectors
516
on phase diagram
510
shown in FIG.
5
. Similarly, two-bit data word
422
, which includes bits b
2
and b
3
as shown in
FIG. 4
, and carrier
424
are input to QPSK modulation system
420
. QPSK modulation system
420
outputs a QPSK modulated carrier
426
, which travels through attenuator
427
. Attenuator
427
lowers the amplitude of QPSK modulated carrier
426
. The attenuated QPSK modulated carrier
426
corresponds to a signal which may be represented, as described above in connection with
FIG. 2
, by vectors
526
on phase diagram
520
shown in FIG.
5
.
As seen in
FIG. 4
, the two QPSK modulated carriers
416
and
426
are added by summer
430
and output as QAM modulated carrier
436
corresponding to a signal which may be represented, as described above in connection with
FIG. 2
, by vectors
536
on phase diagram
530
shown in FIG.
5
. The addition of QPSK modulated carriers
416
and
426
is indicated in
FIG. 5
by plus sign
532
and equal sign
534
representing addition of phase diagrams
510
and
520
corresponding to QPSK modulated carriers
416
and
426
, respectively. Because each vector
516
and
526
represents a signal, addition of the phase diagrams is accomplished by adding each possible pair of vectors
516
and
526
to produce a vector or symbol
536
in phase diagram
530
. The configuration formed by symbols
536
is referred to as a 16 QAM constellation. The vectors
516
are also shown in phase diagram
530
to provide a size orientation for the purposes of illustration only, but do not form part of the 16 QAM constellation illustrated in phase diagram
530
. Each symbol
536
represents a pair of two-bit data words
412
and
422
, which may be viewed as a four-bit data word, b
0
, b
1
, b
2
, b
3
. Each four-bit data word has 16 possible values each of which is mapped by QAM modulation system
400
to one distinct symbol
536
of the 16 symbols
536
.
Physical limitations and variances in the circuits used to implement QAM modulation system
400
cause variance, or inexactitude, in the amplitudes and phases of symbols
536
during transmission of the QAM modulated signal. The variances may cause some of the symbols
536
to occasionally be transmitted closer together in phase diagram
530
. In other words, the amplitude and phase of two
Burk Leah N.
Cooper James D.
Kubasek Suzanne E.
Loheit Kurt
Shimokaji I Fritz LLP
Shingleton Michael B
The Boeing Company
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