Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
1999-02-01
2002-09-03
Phu, Phuong (Department: 2631)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C375S152000, C375S143000, C327S058000, C708S314000
Reexamination Certificate
active
06445756
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a peak detecting circuit applicable to a spread spectrum technique used for a radio frequency communication system, a distance measurement system or a positioning system and, more particularly, to a correlation peak detecting circuit for detecting a correlation peak in a spread spectrum signal.
2. Description of the Related Art
FIG. 1
is a block diagram of a digital matched filter
200
used in a conventional spread spectrum circuit. The digital matched filter
200
shown in
FIG. 1
comprises a shift register
201
, a plurality of multipliers
202
each of which multiplies a signal input from the shift register
201
by a tap coefficient and a summing circuit
203
which sums output signals of the multipliers
202
. Normally, the tap coefficient is either +1 or −1 when a correlation of a pseudo noise (PN) signal is obtained.
As appreciated from
FIG. 1
, since the digital matched filter
200
is constituted by a sequence circuit including the shift register
201
and other parts, a time discrete signal which may be produced by sampling by an A/D converter and the like must be input to the digital matched filter
200
.
When the PN signal is subjected to a discrete signal producing process such as a sampling process, the autocorrelation characteristic of the processed PN signal differs from the original characteristic.
FIGS. 2A
,
2
B and
2
C show examples of correlation characteristics represented by a correlation output with respect to a phase shift &Dgr;&tgr; from a reference phase.
FIG. 2A
shows an example of an original correlation characteristic;
FIG. 2B
shows a correlation characteristic when a discrete PN signal is input;
FIG. 2C
shows a correlation characteristic when a discrete PN signal produced by sampling under a bandwidth restriction is input. It should be noted that, in
FIG. 2B
, two samples are taken per one chip timing. For the sake of convenience, two samples are taken per one chip timing for all cases described below.
FIGS. 3A
to
3
E show examples of outputs of a matched filter when a discrete PN signal is input to the matched filter.
FIG. 3A
shows an example of output of an analog matched filter. As shown in
FIG. 3A
, the analog matched filter outputs a signal pulse for each single cycle of the input PN signal.
FIG. 3B
shows an example of output of a digital matched filter. In the example of
FIG. 3B
, the envelope of the signals output from the digital matched filter is the same as that of the analog matched filter shown in FIG.
3
A. However, the output itself is discrete. This is because a shift register of the digital matched filter also performs a discrete shifting process in synchronization with a sampling clock of the A/D converter.
Accordingly, a largest peak signal and two peak signals having a level which is one half of the largest peak signal must always be obtained from the correlation output using the digital matched filter for each cycle as shown in
FIG. 3B
irrespective of a sampling timing. However, in practice, the signal to be input to the digital matched filter is influenced by a bandwidth restriction.
FIG. 4A
shows an example of the PN signal transmitted by a sender.
FIG. 4B
shows a correlation characteristic when the PN signal shown in
FIG. 4A
is input to the digital matched filter.
As shown in
FIG. 4B
, the PN signal input to the digital matched filter is blunted or dulled due to a bandwidth restriction according to a legal regulation or a system performance. The blunted or dulled signal is subjected to a sampling process, and a waveform as shown in
FIG. 4C
or
4
D is obtained and input to the digital matched filter.
Accordingly, the correlation characteristic of the sampled PN signal becomes different from that of the input signal shown in FIG.
4
B. Thus, in a case of the digital matched filter, the output of the digital matched filter becomes as shown in FIG.
3
C. Naturally, the output characteristic of the digital matched filter can be a characteristic as shown in either
FIG. 3D
or
FIG. 3D
which has an envelop equivalent to the characteristic shown in FIG.
3
C.
As appreciated from the above-mentioned example, when the PN signal subjected to the bandwidth restriction is sampled and is input to the digital matched filter, the correlation output varies according to the sample timing and the output signal pulses do not always represent a value of the correlation peak. Additionally, there is a problem in that a time when a largest peak pulse from among the signal pulses appears is varied.
Accordingly, in a communication system merely using a digital matched filter, it is required to set a threshold value for detecting the correlation peak to match a lowest value of the correlation output. Additionally, a time when a correlation peak appears cannot be estimated. This results in deterioration in the transmission characteristic.
In the spread spectrum communication, an information signal is transmitted by being multiplied by a PN signal having a sufficiently high speed. On a receiver side, the information signal spread by the PN signal is processed by a matched filter or a sliding correlator so as to enable a demodulation process.
Particularly, in a case in which the reception signal is input to the matched filter, a code system the same as the PN signal used for spreading the information signal on the sender side is set to a set of coefficients used by the matched filter. Thereby, when the spread information signal is input to the matched filter, the matched filter outputs peak signals having a sharp peak as shown in FIG.
5
A. The receiver side detects a time when the peak signal appears so as to detect a phase of the received signal.
However,
FIG. 5A
shows an ideal case, and, in practice, the received signal is influenced by a bandwidth restriction and the waveform of the received signal is blunted or dulled. Accordingly, the correlation characteristic becomes as shown in FIG.
5
B.
In order to constitute a matched filter, an analog system using a SAW filter and the like or a digital system can be used. The digital system has an advantage over the analog system with respect to cost and size since the digital system can be achieved by an integrated circuit.
FIG. 6
shows a structure of a conventional digital matched filter (DMF). The digital matched filter shown in
FIG. 6
comprises a plurality of delay elements
211
, a plurality of multipliers
212
and an adder
213
which sums outputs of the amplifiers
212
. Each of the delay elements
211
delays an inputting timing corresponding to a single cycle. Each of the multipliers
212
multiplies an output of the corresponding delay element by a coefficient hi (i=1 to m). The coefficient hi takes either a value of +1 or −1.
Since the DMF is constituted by a digital circuit, the signal input thereto is a discrete signal which is obtained by sampling the received signal at every predetermined time. Additionally, the received signal is quantized in response to a dynamic range of the input signal. Hereinafter, an i-th sampled signal with respect to a reference time is represented by X
i
.
The signal input to the DMF is delayed by a multi-bit shift register, and the following signals are output from the respective shift registers, where m is a number of shift registers.
{X
i−1
, X
i−2
, X
i−3
, . . . , X
i−m
}
The output of each of the shift registers is multiplied by the respective coefficient, and summed by the adder
203
. Accordingly, the output signal y
i
of the DMF is represented as follows.
Y
i
=
∑
j
=
1
m
h
i
x
i
-
j
Accordingly, the output of the DMF is also the discrete signal Y
i
. The output characteristic of the output Y
i
is a train of discrete signals as shown in
FIG. 5C
or FIG.
5
D. That is, the train of signals shown in
FIG. 5C
or
FIG. 5D
is obtained by sampling the correlation characteristic shown in FIG.
5
B. In the conventional technique, it is determined that
Phu Phuong
Ricoh & Company, Ltd.
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