Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
2000-12-21
2004-11-30
Liu, Shuwang (Department: 2634)
Pulse or digital communications
Receivers
Angle modulation
C375S261000, C329S304000
Reexamination Certificate
active
06826238
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a receiver for receiving signals modulated by quadrature amplitude modulation (QAM) and, more particularly, to a QAM receiver and a carrier recovery method for receiving QAM signals and recovering the frequency offset and the phase jitter of carrier waves.
2. Description of the Related Art
The transmission methods of digital TV's are largely classified into two methods: the one is the vestigial side band (VSB) method using a single carrier and the other is the coded orthogonal frequency division (OFDM) method using multiple carriers.
The OFDM method using multiple carriers readily recovers the signals damaged by multi-path channels and, unlike the VSB method using a single carrier, supports a single frequency network.
OFDM data are mapped by the QAM method prior to being transmitted, which transmission method is widely used for cable TV's in U.S.A.
A QAM receiver receives radio frequency (RF) signals, mapped by the QAM method, via a tuner to perform a data recovery. Meanwhile, the tuner or the RF generator incurs frequency offset of several hundreds of KHz and phase jitter, which have to be minimized in order to achieve an accurate data recovery. The acquisition/tracking procedure for minimizing frequency offset and phase jitter is called “carrier recovery”.
FIG. 1
is a schematic block diagram showing the structure of a carrier recovery device in the conventional QAM receiver, in which the carrier recovery device includes a polar decision-oriented phase error detector
101
, a loop filter
101
d
, and a numerical control oscillator (NCO)
101
e
. The polar decision-oriented phase error detector
101
includes a mixer
101
a
, a decider
101
b
, and a polar decision-oriented phase error generator
10
c.
Referring to
FIG. 1
, the mixer
101
a
of the polar decision-oriented phase error detector
101
demodulates a passband digital signal having frequency offset and phase jitter, generated by a preprocessing unit
100
, with sine/cosine waves generated by the numerical control oscillator
101
e
to produce a baseband digital signal (R
I
, R
Q
) with the frequency offset and phase jitter recovered.
The decider
101
b
generates a deciding signal character (D
I
, D
Q
) conformable to the individual signal level of the baseband digital signal (R
I
, R
Q
) demodulated by the mixer
101
a.
For example, when the baseband digital signal (R
I
, R
Q
) falls in the deciding region of the first quadrant in the QAM character diagram of
FIG. 2
, the decider
101
b
generates a deciding signal character (D
I
, D
Q
) judging that the baseband digital signal (R
I
, R
Q
) is present in the first quadrant.
The polar decision-oriented phase error generator
101
c
detects a phase error by using the baseband digital signal (R
I
, R
Q
) demodulated by the mixer
101
a
and the deciding signal character (D
I
, D
Q
) generated from the decider
101
b.
Namely, the polar decision-oriented phase error generator
101
c
calculates the difference between the phase &thgr; of the demodulated baseband digital signal (R
I
, R
Q
) and the phase &phgr; of the deciding signal character (D
I
, D
Q
), and detects the polarity of the phase difference. The characteristic function e(&phgr;) of the polar decision-oriented phase error generator
101
c
can be expressed by the following equation 1. Diagrams (a) and (b) of
FIG. 2
represent the geometrical characteristics of e(&phgr;).
Diagram (a) of
FIG. 2
shows that the result of e(&phgr;) has a positive value, i.e., sgn(&thgr;−&phgr;)>0 because the phase &thgr; of the demodulated signal character is greater than the phase &phgr; of the deciding signal character. Contrarily, diagram (b) of
FIG. 2
shows that the result of e(&phgr;) has a negative value, i.e., sgn(&thgr;−&phgr;)<0 because the phase &thgr; of the demodulated signal character is less than the phase &phgr; of the deciding signal character.
e
(&phgr;)=
sgn
(&thgr;−&phgr;)=
sgn
(
R
Q
*D
I
−R
I
*D
Q
) [Equation 1]
In the equation 1, the sng(#) operator represents an operator for detecting the polarity of #; R
I
and R
Q
the in-phase and quadrature components of the demodulated signal character, respectively; &thgr; the phase of the demodulated signal character; D
I
and D
Q
the in-phase and quadrature components of the deciding signal character, respectively; &phgr; the phase of the deciding signal character.
FIG. 3
is a detailed block diagram showing the hardware configuration of the above mechanism, i.e., polar decision-oriented phase error generator
101
c.
In the polar decision-oriented phase error generator
101
c
, multipliers
301
and
302
and subtracter
303
determine the phase error between the deciding signal character (D
I
, D
Q
) generated by the decider
101
b
and the demodulated signal character (R
I
, R
Q
) generated by the mixer
101
a
, according to the equation
1
. Polarity detector
304
detects the polarity from the phase error determined by the substracter
303
. Accordingly, the polar phase error e(&phgr;) thus detected has a value of +1, 0 or −1. The output of the polar decision-oriented phase error generator
101
c
is fed into the loop filter
101
d.
The loop filter
101
d
, which uses a general primary baseband loop filter, cumulates the phase error e(&phgr;) detected by the polar decision-oriented phase error generator
101
c
to generate an intermediate frequency &ohgr;
c
as the sum of the frequency offset &Dgr;&ohgr; and the phase jitter &Dgr;&thgr;.
The numerical control oscillator
101
e
generates sine and cosine waves of which the center frequency is the intermediate frequency &ohgr;
c
generated by the loop filter
101
d
. The sine and cosine waves are output to the mixer
101
a.
However, the conventional carrier recovery device applies the polar phase error e(&phgr;) to all characters regardless of the magnitude of the deciding signal character, i.e., D
I
2
+D
Q
2
, so that the phase jitter of the demodulated signal character increases with an increase in the magnitude of the deciding signal character D
I
2
+D
Q
2
, as shown in FIG.
9
. That is, the demodulated signal character has a phase jitter that increases with an increased distance from the origin. This results in a deterioration of the signal-to-noise ratio (SNR) performance of the receiver, i.e., a deterioration of the acquisition/tracking performance even at a relatively small input SNR.
Such a narrow acquisition/tracking range not only causes a need of using a high-quality tuner of excellent mechanism, thus raising the expense of the tuner, but also leads to a deterioration of the BER performance of the receiver due to the great residual phase jitter.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to solve the problems with the prior art and to provide a QAM receiver and a carrier recovery method in which a weighted value variable depending on the magnitude of the deciding signal character is applied to the polar phase error so as to stably detect the accurate phase error.
To achieve the above object of the present invention, there is provided a QAM receiver including: a signal generator for multiplying the passband digital signal by a sine/cosine wave into a demodulated baseband digital signal, and generating a deciding signal character conformable to the individual signal level of the demodulated baseband digital signal; a first phase error measurer for calculating the phase difference between the demodulated baseband digital signal and the deciding signal character, and detecting the polarity of the phase difference to be output as a first phase error; a second phase error measurer for using the first phase error detected by the first phase error measurer to output a second phase error having a weighted value; and a filter and an oscillator for cumulating the received second phase error, and generating a sine/cosine wave proportionate to the cumulated phase error, the sine
LG Electronics Inc.
Liu Shuwang
Wang Ted
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