Timing reproducing device and demodulator

Details Timing reproducing device and demodulator Timing reproducing device and demodulator

2001-08-23

2004-01-27

Kinkead, Arnold (Department: 2817)

Demodulators

Phase shift keying or quadrature amplitude demodulator

C329S307000, C375S324000, C375S326000, C375S327000, C375S376000

Reexamination Certificate

active

06683493

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a demodulator and a timing regenerating device that is used in this demodulator.
BACKGROUND ART
As a conventional system of timing regeneration for a demodulator of a broadband digital radio communication system having a preamble signal analysis function, there has been one as described in Japanese Patent Application Laid-Open No. 8-46658.
This system focuses on the fact that the preamble signal has ½ of the frequency component of the symbol frequency (fs). Based on this, a correlation is obtained at the receiver side between the preamble signal and a ½ symbol frequency component exp[−j&pgr;(fs)t] output from a VCO (Voltage Control Oscillator). A timing phase is estimated from a vector angle of this correlation value.
Further, according to this system, sampling speed (i.e. sample/symbol) of the data is only 2. In the mean time, Japanese Patent Application Laid-Open No. 6-141048 discloses a system of estimating a timing phase from a correlation between a signal (for example, an envelope) after a nonlinear processing and a symbol frequency component exp [−j2&pgr;(fs)t]. According to this system, minimum value of a necessary sampling speed is 4. Therefore, the sampling speed in the system described in Japanese Patent Application Laid-Open No. 8-46658 is ½ of that disclosed in Japanese Patent Application Laid-Open No. 6-141048. As a result, it is possible to realize low power consumption of the receiver.
FIG. 20
is a structure diagram of a demodulator including a timing regenerating device that is similar to the demodulator described in the Japanese Patent Application Laid-Open No. 8-46658.
This demodulator mainly consists of antenna
100
, frequency converting unit
200
, first A/D converter
300
a
, second A/D converter
300
b
, timing regenerating device
400
, and data deciding unit
500
.
The timing regenerating device
400
includes VCO
401
, timing phase difference calculating unit
402
, Ich correlation calculating unit
403
, Qch correlation calculating unit
404
, and vector combination selecting unit
405
.
Detailed structure of the vector combination selecting unit
405
will be explained with reference to FIG.
21
.
The vector combination selecting unit
405
mainly consists of first vector combining unit
406
a
, second vector combining unit
406
b
, third vector combining unit
406
c
, fourth vector combining unit
406
d
, maximum absolute value detecting unit
407
, and selecting unit
408
.
How this demodulator demodulates a received preamble signal will be explained now.
First, the antenna
100
receives the preamble symbol of RF band. The frequency converting unit
200
frequency converts this preamble symbol of the RF band into a preamble symbol of a base band.
FIG. 22
is a signal space diagram showing a preamble symbol of this base band (for example, a “1001” pattern in the QPSK conversion system). In
FIG. 22
, &thgr;c denotes, in degrees, a carrier phase of a reception signal. The preamble symbol shifts between a Nyquist point “A” and a Nyquist point “B” alternately through the origin for each one symbol in the drawing.
The vector angle of the Nyquist point “A” is &thgr;c, and the vector angle of the Nyquist point “B” is (&thgr;c+180). Difference between the vector angles of the Nyquist point “A” and the Nyquist point “B” is 180 degree.
The first A/D converter
300
a
receives the preamble symbol of the base band, samples the in-phase component of the preamble symbol at time t=&tgr;+iT/2 (where i=1, 2, 3, . . . , and &tgr; represents a timing error (−T/2≦&tgr;<T/2), and T represents a symbol frequency), and outputs a sampled preamble data string Ip
i
(i=1, 2, 3, . . . ).
Similarly, the second A/D converter
300
b
receives the preamble symbol of the base band, samples the orthogonal component of the preamble symbol at the time t=&tgr;+iT/2, and outputs a sampled preamble data string Qp
i
(i=1, 2, 3, . . . ). The first A/D converter
300
a
and the second A/D converter
300
b
sample the data based on a sampling clock output from the timing regenerating device
400
.
The timing regenerating device
400
calculates a timing error &tgr; by using the sampled preamble data strings Ip
i
and Qp
i
(i=1, 2, 3, . . . ), and carries out a phase control for canceling the timing error &tgr; to a regeneration sample clock and a regeneration symbol clock. The regeneration symbol clock is a clock of a symbol period having the regeneration sample clock frequency-divided into two.
The data deciding unit
500
receives the significant random data strings Id
i
and Qd
i
(i=1, 2, 3, . . . ) that follow the preambles after the timing error &tgr; has been cancelled by the timing regenerating device
400
, and latches the data at the Nyquist points by the regeneration symbol clock. Then, the data deciding unit
500
decides the data using the latched Nyquist point data, and outputs the demodulated data.
Detail operation of the timing regenerating device
400
will be explained now. First, the Ich correlation calculating unit
403
obtains correlation between each of the in-phase component I (t) and the orthogonal component Q (t) of the preamble symbol shown in
FIG. 22 and a
frequency component exp[−j&pgr;(fs)t] that is ½ of the symbol frequency, respectively. Specifically, the Ich correlation calculating unit
403
performs the calculation shown in the equations (1a) and (1b) with respect to the over-sampled preamble data string Ip
i
(i=1, 2, 3, . . . ):
Ic
i
=Ip
i
×cos &pgr;
i/
2  (1a)
Is
i
=Ip
i
×sin &pgr;
i/
2  (1b)
Then, the Ich correlation calculating unit
403
calculates an average of the obtained results (Ic
i
, Is
i
), thereby to obtain correlation values (CI, SI). Further, the Qch correlation calculating unit
404
performs the calculation shown in the equations (2a) and (2b) with respect to the over-sampled preamble data string QP
i
(i=1, 2, 3, . . . ) in a similar manner:
Qc
i
=Qp
i
×cos &pgr;
i/
2  (2a)
Qs
i
=Qp
i
×sin &pgr;
i/
2  (2b)
Then, the Qch correlation calculating unit
404
calculates an average of the obtained results (Ic
i
, Is
i
), thereby to obtain correlation values (CQ, SQ).
In the equations (1a), (1b), (2a), and (2b), cos &pgr;i/2=1, 0, −1, 0, . . . , and sin &pgr;i/2=0, 1, 0, −1, . . . . Therefore, it is easy to obtain the correlation values (CI, SI) and (CQ, SQ). For example, when averaging with four symbols, the correlation values (CI, SI) can be obtained from the equations (3a) and (3b) as follows:
CI=
(
Ip
i
−Ip
i+2
+Ip
i+4
−Ip
i+6
+Ip
i+8
−Ip
i+10
+Ip
i+12
−Ip
i+14
)/8  (3a)
SI=
(
Ip
i+1
−Ip
i+3
+Ip
i+5
−Ip
i+7
+Ip
i+9
−Ip
i+11
+Ip
i+13
−Ip
i+15
)/8  (3b)
Correlation values (CQ, SQ) can be obtained from the equations (4a) and (4b) as follows:
CQ=
(
Qp
i
−Qp
i+2
+Qp
i+4
−Qp
i+6
+Qp
i+8
−Qp
i+10
+Qp
i+12
−Qp
i+14
)/8  (4a)
SQ=
(
Qp
i+1
−Qp
i+3
+Qp
i+5
−Qp
i+7
+Qp
i+9
−Qp
i+11
+Qp
i+13
−Qp
i+15
)/8  (4b)
The vector angle between the correlation values (CI, SI), and the vector angle between the correlation values (CQ, SQ) both indicate timing phase errors. However, depending on the carrier phase &thgr;c, both the vectors may be pointed in the same direction, opposite directions, or one vector may have a value equal to zero.
For example, for the preamble symbols at A and B that satisfy the range of &thgr;c as (90<&thgr;c<180) or (270<&thgr;c<360) as shown in
FIG. 22
, when the Ich correlation calculating unit
403
samples at the ti

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