Data demodulator for correcting bit errors

Details Data demodulator for correcting bit errors Data demodulator for correcting bit errors

2000-03-15

2001-06-05

Ghebretinsae, Temesghen (Department: 2734)

Pulse or digital communications

Receivers

Particular pulse demodulator or detector

C329S311000

Reexamination Certificate

active

06243429

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a data demodulator and more particularly to a data demodulator feasible for a receiver included in a digital radio communication apparatus.
2. Description of the Background Art
Generally, a digital radio communication apparatus using, e.g., a PSK (Pulse Shift Keying) modulation system includes a receiver for receiving a radio wave with an antenna. A radio wave coming in through the antenna is input to a first or front end band-pass filter for removing spurious signal components. The band-pass filter has its output connected to a mixer, which is adapted to multiply the output of the band-pass filter by a preselected frequency signal. The mixer has its output connected to a decision circuit, which is adapted to convert the resulting output of the mixer to an IF (Intermediate Frequency) signal that can be demodulated. The IF signal is fed to a second band-pass filter so as to further reduce noise components, i.e., to increase the ratio of a necessary modulated component to the entire signal. The second band-pass filter has its output connected to a decision circuit, which is adapted to demodulate the output of the second band-pass filter to thereby output a demodulated digital signal. The decision circuit may be adapted to use synchronous detection, delay detection or similar conventional technology detection scheme.
FIG. 1A
shows the frequency spectrum of a specific modulated wave received via the antenna.
FIGS. 1B
,
1
C and
1
D respectively show the outputs of the first band-pass filter, the mixer, and the second band-pass filter in frequency spectrum.
The receiving ability of the above-described receiver depends not only on the detection system and circuit arrangement of the decision circuit, but also on the characteristic of the second band-pass filter expected to attenuate noise outside of a frequency band assigned thereto. It is generally ideal for the second band-pass filter, if provided with, e.g., a Nyquist filter characteristic, to be adapted to pass a modulated component only with the remaining frequency components filtered out, along with its fidelity.
The problem with the above-described conventional receiver will be described specifically with reference to
FIGS. 2A and 2B
.
FIGS. 2A and 2B
respectively show an ordinary band-pass filter characteristic and an ideal band-pass filter characteristic available with the conventional system. The receiver would be able to cancel all noise components ascribable to spurious frequency components if provided with an ideal band-pass filter. However, as shown in
FIG. 2B
, even that receiver cannot cancel a noise component superposed on the pass band of the filter. A bit error rate is determined by a power ratio between the signal component and the noise component that cannot be cancelled. While the bit error rate depends on the modulation system, the conventional demodulation system cannot reduce the noise component contained in the frequency band of the demodulated wave, and is therefore unable to realize a receiver characteristic superior to the theoretical value of any one of the modulation systems.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a data demodulator capable of accurately locating and correcting an error included in the result of decision and therefore promoting accurate demodulation.
A data demodulator in accordance with the present invention includes a first decision circuit for demodulating a received signal to thereby output a first digital signal. A baseband circuit converts the first digital signal to abaseband signal lying in the same frequency band as the received signal. A code processing signal generator has a data rate substantially equal to one-half of the data rate of the received signal, and generates a code processing signal whose transition point is coincident with every two-bit transition point of the received signal. A first mixer superposes the code processing signal on the baseband signal. A second mixer superposes the output of the first mixer on the received signal input to the first decision circuit to thereby demodulate the received signal by spreading. As a result, a noise component superposed on the received signal is spread. A filter has a pass band narrower than the frequency band of the received signal for separating a signal component corresponding to the code processing signal from the output of the second mixer. A second decision circuit demodulates the output of the filter to thereby output a signal for bit error decision. A correction circuit compares the code processing signal and the signal output from the second decision circuit to thereby locate a portion of the first digital signal where a bit error exists, and corrects the bit error to thereby output a second digital signal.
The data demodulator may be provided with a couple of signal processing paths each including the above circuit elements except for the correction circuit in order to further enhance accurate demodulation.


REFERENCES:
patent: 6046630 (2000-04-01), Kim

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