Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
1998-01-07
2001-05-29
Vo, Don N. (Department: 2631)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C375S298000, C375S329000, C329S304000, C332S176000
Reexamination Certificate
active
06240142
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved quadrature modulator and demodulator.
II. Description of the Related Art
In many modern communication systems, digital transmission is utilized because of improved efficiency and the ability to detect and correct transmission errors. Exemplary digital transmission formats include binary phase shift keying (BPSK), quaternary phase shift keying (QPSK), offset quaternary phase shift keying (OQPSK), m-ary phase shift keying (m-PSK), and quadrature amplitude modulation (QAM). Exemplary communication systems which utilize digital transmission include code division multiple access (CDMA) communication systems and high definition television (HDTV) systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, both assigned to the assignee of the present invention and incorporated by reference herein. An exemplary HDTV system is disclosed in U.S. Pat. No. 5,452,104, U.S. Pat. No. 5,107,345, and U.S. Pat. No. 5,021,891, all three entitled “ADAPTIVE BLOCK SIZE IMAGE COMPRESSION METHOD AND SYSTEM”, and U.S. Pat. No. 5,576,767, entitled “INTERFRAME VIDEO ENCODING AND DECODING SYSTEM”, all four patents are assigned to the assignee of the present invention and incorporated by reference herein.
In the CDMA system, a base station communicates with one or more remote stations. The base station is typically located at a fixed location. Thus, power consumption is less important consideration in the design of the base station. The remote stations are typically consumer units which are produced in high quantity. Thus, cost and reliability are important design considerations because of the number of units produced. Furthermore, in some applications such as a CDMA mobile communication system, power consumption is critical because of the portable nature of the remote station. Tradeoffs between performance, cost, and power consumption are usually made in the design of the remote stations.
In digital transmission, the digitized data is used to modulated a carrier sinusoid using one of the formats listed above. The modulated waveform is further processed (e.g. filtered, amplified, and upconverted) and transmitted to the destination device. At the destination device, the transmitted RF signal is received and demodulated by a receiver.
A block diagram of an exemplary transmitter
100
of the prior art which is used for quadrature modulation of QPSK, OQPSK, and QAM signals is illustrated in FIG.
1
A. Transmitter
100
can be used at the base station or the remote station. Within quadrature modulator
110
a
of transmitter
100
, the inphase (I) and quadrature (Q) signals are provided to mixers
112
a
and
112
b
which modulate the signals with the inphase and quadrature intermediate frequency (IF) sinusoids, respectively. Quadrature splitter
114
receives the IF sinusoid (IF LO) and provides the inphase and quadrature IF sinusoids which are approximately equal in amplitude and 90 degrees out of phase with respect to each other. The modulated I and Q signals from mixers
112
a
and
112
b
are provided to summer
116
and combined. In many applications, the signal from summer
116
is provided to mixer
118
which upconverts the signal to the desired frequency with the radio frequency (RF) sinusoid (RF LO). Although not shown in
FIG. 1A
for simplicity, filtering and/or amplification can be interposed between successive stages of summers and mixers.
The modulated signal from mixer
118
is provided to filter
130
which filters out undesirable images and spurious signals. The filtered signal is provided to amplifier (AMP)
132
which amplifies the signal to produce the required signal amplitude. The amplified signal is routed through duplexer
134
and transmitted from antenna
136
to the destination device.
A block diagram of an exemplary direct quadrature modulator
110
b
is shown in FIG.
1
B. Within direct quadrature modulator
110
b,
the I and Q signals are provided to mixers
152
a
and
152
b
which modulate the signals with the inphase and quadrature RF sinusoids, respectively. Quadrature splitter
154
receives the direct RF sinusoid (direct RF LO) and provides the inphase (I LO) and quadrature (Q LO) sinusoids which are approximately equal in amplitude and 90 degrees out of phase with respect to each other. The modulated I and Q signals from mixers
152
a
and
152
b
are provided to summer
156
and combined to provide the modulated signal.
Quadrature modulator
110
a
performs modulation using a two steps process whereby quadrature modulation is performed at an IF frequency and upconverted to the desired RF frequency. Quadrature modulator
110
a
offers several advantages. First, quadrature splitter
114
can be more easily designed and manufactured to meet the required specification at the lower IF frequency. Second, the two sinusoids design (IF LO and RF LO) offers flexibility in the frequency plan and simplification of the filtering.
Direct quadrature modulator
110
b
performs the same functions as quadrature modulator
110
a
. However, direct quadrature modulator
110
b
performs modulation directly at the desired RF frequency using a single step process, thereby eliminating the upconversion step. The simplicity in the design of modulator
110
b
is offset by the performance requirements of quadrature splitter
154
. In particular, it is much more difficult to design and manufacture quadrature splitter
154
having the required amplitude balance and quadrature phase at the higher RF frequency.
A method for generating inphase and quadrature sinusoids at RF frequency having the required performance is disclosed in U.S. Pat. No. 5,412,351, entitled “QUADRATURE LOCAL OSCILLATOR NETWORK”, and incorporated by reference herein. A block diagram of quadrature local oscillator network
170
as disclosed in U.S. Pat. No. 5,412,351 is shown in FIG.
1
C. Within quadrature local oscillator network
170
, the IF sinusoid is provided to quadrature splitter
172
which provides the inphase and quadrature IF sinusoids. The inphase IF sinusoid is provided to mixers
176
a
and
176
d
and the quadrature IF sinusoid is provided to mixers
176
b
and
176
c
. Similarly, the RF sinusoid is provided to quadrature splitter
174
which provides the inphase and quadrature RF sinusoids. The inphase RF sinusoid is provided to mixers
176
b
and
176
d
and the quadrature RF sinusoid is provided to mixers
176
a
and
176
c
. Mixers
176
a
and
176
b
mix the two input signals and provide the upconverted signals to summer
178
a
which combines the signals to provide the inphase direct sinusoid (I LO). Similarly, mixers
176
c
and
176
d
mix the two input signals and provide the upconverted signals to summer
178
b
which combines the signals to provide the quadrature direct sinusoid (Q LO). The inphase and quadrature direct sinusoids can be provided to mixers
152
a
and
152
b
, respectively, as shown in FIG.
1
B.
Ideally, the inphase and quadrature sinusoids from a phase splitter are equal in amplitude and 90 degrees out of phase with respect to each other. At the RF frequency, this is difficult to achieve. For ideal quadrature splitters
172
and
174
(with no amplitude imbalance and no phase error), the inphase (I LO) and quadrature (Q LO) sinusoids are exactly equal in amplitude and 90 degree out of phase with respect to each other. Each sinusoid comprises a single tone at the difference frequency (f
RF
−f
IF
) and no other mixing terms. The I LO and Q LO can be expressed as:
I
—
LO
(
t
)=cos(&ohgr;
RF
−&ohgr;
IF
)
t
Q
—
LO
(
t
)=sin(&ohgr;
RF
−&ohgr;
IF
)
t
(1)
Although quadrature local oscillator network
Aparin Vladimir
Kaufman Ralph E.
Qualcomm Incorporated
Vo Don N.
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