Matched filter

Details Matched filter Matched filter

1999-12-14

2003-06-24

Chin, Stephen (Department: 2634)

Pulse or digital communications

Spread spectrum

Direct sequence

C375S142000, C375S147000, C375S149000, C375S152000

Reexamination Certificate

active

06584142

ABSTRACT:

BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a matched filter suitable for use in a radio receiver adopting direct sequence code division multiple access.
(2) Description of Related Art
In direct sequence code division multiple access (DS-CDMA [Direct Sequence Code Division Multiple Access]), the transmitter primarily modulates a data code in, for example, QPSK (Quadrature Phase Shift Keying), spreads a bandwidth thereof using a spreading code, and transmits the data code. On the other hand, the receiver cross-correlates using the same spreading code as in the transmitter to detect a peak of the correlation value, performs acquisition and tracking, and demodulates by correlation detection.
FIG.
11
(
a
) is a block diagram of a transmitting unit of a radio terminal in DS-CDMA using QPSK. In the transmitting unit
30
of the radio terminal in DS-CDMA shown in FIG.
11
(
a
), a data code is QPSK-modulated in a primary modulating unit
30
a
, a bandwidth of the signal is spread with an PN code (Pseudo Noise Code) in a spreading modulating unit
30
b
, an output of the spreading modulating unit
30
b
is up-converted in a frequency converting unit
30
c
, a power thereof is amplified in an RF amplifying unit
30
d
, and a radio signal is sent out to a radio propagation path from an antenna
30
e.
FIG. 12
is a diagram showing a relationship among a data code, a spreading code and a transmit code when the data is spread using QPSK in the primary modulation. Data codes D
i
and D
q
are complex-multiplied by spreading codes C
i
and C
q
in multipliers
36
a
,
36
b
,
36
c
and
36
d
, and obtained results are added in adders
37
a
and
37
b
and outputted as transmit codes S
i
and S
q
The complex-multiplication signifies an operation [(D
i
+j·D
q
)·(C
i
+j·C
q
) with the date codes D
i
and D
q
and the spreading codes C
I
and C
q
, where j represents an imaginary unit (j
2
=−1).
Next, a receiving system will be described. FIG.
11
(
b
) shows a block diagram of a receiving unit of the radio terminal in DS-CDMA using QPSK. In the receiving unit
31
of the radio terminal in DS-CDMA, a weak radio signal whose bandwidth has been spread is received by an antenna
31
a
, the radio signal is amplified with a low noise in an RF amplifying unit
31
b
, an output of the RF amplifying unit
31
b
is down-converted in a frequency converting unit
31
c
. Further, the signal down-converted in the frequency converting unit
31
c
and a spreading replica code generated inside the receiving unit
31
are band-cross-correlated in a despread demodulating unit
31
d
, a narrow-band signal is thereby taken out, and an output of the despread demodulating unit
31
d
is QPSK-demodulated in a primary demodulating unit
31
e.
FIG. 13
is a diagram showing a detailed structure of the receiving unit
31
of the radio terminal in DS-CDMA using QPSK. As shown in
FIG. 13
, a flow of the signal in the frequency converting unit
31
c
, the despread demodulating unit
31
d
and the primary demodulating unit
31
e
is as follows. Namely, an I channel signal is mixed with an output of a local oscillator
38
b
in a frequency converting unit
38
a
, whereas a Q channel signal is mixed with an output of a 90° phase shifter
38
c
in a frequency converting unit
38
d
. Outputs of the frequency converting units
38
a
and
38
d
are converted from analog to digital in A/D (analog/digital) converters
39
a
and
39
b
. These digital signals are branched and inputted to despread demodulating unit
40
. In four matched filters in the despread demodulating unit
40
, the signals are band-cross-correlated, an output of a matched filter
40
a
and an output of a matched filter
40
d
are added, in an adder unit
41
a
, whereby I channel data S
i
is outputted. In a similar manner, a signal obtained by inverting an output of a matched filter
40
b
and an output of a matched filter
40
c
are added in an adder unit
41
b
, Q channel data S
q
is thereby outputted. These outputs are QPSK-demodulated in a post-demodulation processing unit
42
.
Next, band-cross-correlation in the despread demodulating unit
40
will be described. The despread demodulating unit
40
generates spreading replica codes C
i
and C
q
in the same sequence as on the transmitter's side to perform despreading, which comprises the matched filters
40
a
,
40
b
,
40
c
and
40
d
. When the two spread codes are cross-correlated, each of the I channel component and the Q channel component is despread two times, four times in total. In the matched filters
40
a
,
40
b
,
40
c
and
40
d
, M(nt) in the following formula (1) is computed.
M
(
nt
)=&Sgr;
T
k=1
R
(k)·
P
(
nt
)·Z
−k
  (1)
Where t is a chip duration, T is the number of taps, R is a spreading replica code, k and n are integers, P(nt) is a received spread code, and Z is a complex number in Z transform. One chip duration t represents a time for which the spread code is switched, designed to be a time of speed several tens to several hundreds times one bit duration. One chip duration t is a reciprocal of a chip rate. The number of taps T represents a length of a spread code. A length of the spread code is, for example, 256 bits, but there can be used a different spread code of, for example, 128 bits or the like. The received spread code P(nt) and the spreading replica code R(k) of 256 bits are EXORed, a result of this is shifted at the chip rate, added and outputted. Accordingly, an output signal D
i
of the A/D converter
39
a
and an output signal C
i
of a spreading replica code generator
44
a
are cross-correlated in the matched filter
40
a
. In the similar manner, an output signal D
i
of the A/D converter
39
a
and an output signal C
q
of the spreading replica code generator
44
a
are cross-correlated in the matched filter
40
b
, an output signal D
q
of the A/D converter
39
b
and an output signal C
i
of the spreading replica code generator
44
a
are cross-correlated in the matched filter
40
c
, and an output signal D
q
of the A/D converter
39
b
and an output signal C
q
of the spreading replica code generator
44
a
are cross-correlated in the matched filter
40
d
. A reason why the number of the matched filter is four is to-prevent degradation of an S/N (Signal/Noise) ratio of a despread signal.
Such a matched filter is a key device necessary to despread a received spread code, there is hence required a low power thereof.
FIG. 14
is a diagram showing a block structure of the matched filter. The matched filter
40
a
(
40
b
,
40
c
or
40
d
) shown in
FIG. 14
cross-correlates a digital signal outputted from the A/D converter
39
a
or
39
b
shown in
FIG. 13
with a spreading replica code generated inside the receiving unit
31
, thereby despreading the signal. The matched filter
40
a
(
40
b
,
40
c
or
40
d
) comprises a spread data path unit
43
, a spreading replica code generator
44
a
, a register for replica code
44
b
, a multiplier unit
45
and an adder unit
46
. The spread data path unit
43
is a shift register that captures a received spread code input at each clock and shifts the code one stage by one stage, which comprises T flip-flops (FF)
43
-
1
,
43
-
2
,
43
-
3
, . . .
43
-(T−2),
43
-(T−1) and
43
-T. Hereinafter, the flip-flop will be abbreviated as FF occasionally. The spreading replica code generator
44
a
generates a spreading replica code identical to one used in the transmitter. The register for replica code
44
b
is a register for computing the spreading replica code generated by the spreading replica code generator
44
a
. The multiplier unit
45
multiplies each of outputs of the flip-flops
43
-
1
,
43
-
2
,
43
-
3
, . . . ,
43
-(T−2),
43
-(T−1) and
43
-T with each output of the register for replica code
44
b
. The adder unit
46
adds outputs from the multiplier unit
45
and outputs an added result. Each of the multiplier unit
45
and the adder unit
46
has the number of taps T,

Affiliated with

Also associated with

Rating

Matched filter does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Matched filter, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Matched filter will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3094469