Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
1999-08-05
2004-01-13
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Angle modulation
C375S346000
Reexamination Certificate
active
06678337
ABSTRACT:
FIELD OF INVENTION
This invention relates to a method and a device for demodulating a received signal including a pilot signal, and more particularly to, a method and a device for demodulating a received signal including a pilot signal while suppressing the bit error rate (BER) based on a unique word as a pilot signal in mobile satellite communications that incur fast fading on the transmission path.
BACKGROUND OF THE INVENTION
FIG. 1
shows a mobile terminal for mobile satellite communications. This mobile terminal is, for example, a portable telephone, which comprises an antenna
91
, a signal converter
92
, a demodulation circuit
93
, a voice signal generator
94
and a speaker
95
. The antenna
91
receives, through a satellite, a transmit signal as a continuous signal that has a unique word as a pilot signal in multiple sections. The signal converter
92
converts an analogue received signal into a digital signal. The demodulation circuit
93
demodulates a digital signal converted by the signal converter
92
. The voice signal generator
94
converts digital demodulation signal from the demodulation signal
93
into an analogue voice signal. The speaker
95
performs the electrical—acoustical conversion of output of the voice signal generator
94
, generating voice.
In the composition above, an analogue receive signal received by the antenna
91
is converted into a digital signal. The converted digital signal is demodulated by the demodulation circuit
93
, and then the demodulated signal is converted into an analogue voice signal by the voice signal generator
94
. The analogue voice signal converted by the voice signal generator
94
is output as voice from the speaker
95
.
In the satellite communication system, for the purpose of reducing the consumed power and miniaturizing the antenna as well as retaining a low BER even under a low ratio of carrier power to noise power (Eb/NO), an error correction encoding with a large encoding gain, or a high-efficiency encoding technique is used.
FIG. 2
shows the composition of the demodulation circuit
93
in FIG.
1
. The demodulation circuit
93
comprises a delay circuit
101
that receives the receive signal as an input, a data demodulator
102
that is connected to the delay circuit
101
and demodulates data, a unique word demodulator
103
that demodulates a unique word included in a received signal and a data distortion estimator
104
that includes a Wiener filter and that estimates the fading distortion of data in the received signal based on the fading distortion of the unique word. The demodulation circuit
93
is described in Meyr, “Digital Communication Receiver”, John Wiley & Sons, pp.744-747, 1997. This circuit demodulates a received signal compensating a distortion (hereinafter referred to simply as ‘fading distortion’) of the received signal based on a flat fading in multi-path fading that includes a frequency selective fading with a frequency characteristic and a flat fading with no frequency characteristic.
FIG. 3
shows a format burst type transmit/receive signal different from the continuous signal communicated by the mobile terminal in FIG.
1
. For example, one burst comprises multiple data (though data compose data sequence, herein referred to simply as ‘data’) D
1
to D
5
with multiple symbols and multiple unique words (though unique words compose unique-word sequence, herein referred to simply as ‘unique word(s)’) UWm (m=1 to 4) with multiple symbols, and is composed of 100 to 150 symbols as a whole. At four boundaries between data D
1
and D
5
, unique words UWm as pilot signals are inserted. The unique word UWm has multiple symbols (in some cases, a single symbol) according to BER targeted, and the value of phase modulation of each symbol is known in the mobile terminal. The length (number of symbols) of unique words UW
1
to UW
4
is set so that it becomes minimum in the range that a certain error rate can be kept. This format signal is communicated using TDMA (time division multiple access). For example, each symbol is determined by phase-modulating (BPSK: binary phase shift keying) carrier wave with a phase of 0 and &pgr; according to binary data of 1 and −1.
In
FIG. 2
, a continuous-wave received signal with unique words UWm as pilot signals inserted at given intervals is input to an input terminal IN, then supplied to the delay circuit
101
and the unique word demodulator
103
. Unique words UWm in the received signal are demodulated by the unique word demodulator
104
, then input to the data distortion estimator
104
. The unique word demodulator
104
estimates the fading distortion of each symbol of data D
1
to D
5
in the received signal based on the fading distortion of unique word UWm calculated from a known value of the unique word (for example, 0 by phase-demodulating +1, &pgr; by phase-demodulating −1) and a value of demodulated unique word UWm, then outputting it to the data demodulator
102
. The data demodulator
102
demodulates data while compensating data in the received signal delayed for a given time by the delay circuit
101
using the estimation amount of fading distortion, and then the demodulated signal is output from an output terminal OUT. Thus, data can be demodulated compensating the fading distortion of data transmitted as a continuous wave.
FIG. 4
shows another demodulation circuit that can be used in place of the demodulation circuit
93
in FIG.
2
. This circuit is described in S. Sanpei, “Compensation System of Fading Distortion of 16QAM for Overland Mobile Communications”, Technical Report of IEICE B-11, Vol.J72-B-11, No.1, pp.7-15, 1989. It demodulates compensating the fading distortion of a continuous-wave received signal
ĉ
(
k
−
1), ĉ(
k
) and ĉ(
k
+
1)
that one unique word symbol is inserted to every (N−1) information symbols. In
FIG. 4
, when a received signal is input to an input terminal IN, fading distortion estimators
111
,
112
and
113
calculate the estimation values: of (k−1 th, kth and (k+1 th unique words in the received signal delayed sequentially. These estimation values are multiplied by a zero-order or first-order interpolation coefficient:
Q
1
, Q
0
, or
Q.
1
at multipliers
114
,
115
and
116
, then added by an adder
117
. The adder
117
outputs, as the result of addition, c {k+(m/N)} that is the fading distortion of mth information symbol in kth information symbol sequence. The fading distortion of information symbol is brought into 1/c{k+(m/N)} by a reciprocal transformer
118
, then output to a multiplier
120
. The multiplier
120
multiplies information symbol in a received signal delayed by a delay circuit
119
by 1/c {k+(m/N)} output from the reciprocal transformer
118
, thereby demodulating data, which is output from an output terminal OUT. Thus, the received signal can be demodulated compensating the envelope curve and phase of the received signal distorted with the fading distortion.
The demodulation device in
FIG. 2
can demodulate a continuous-wave received signal that unique words are inserted into continuous data at given intervals, at a required BER. However, when it receives a burst signal with a frame format that unique words are inserted into several positions of a 100 to 150 symbol data sequence, since the fading distortion of a data sequence located at both ends is estimated using the fading distortion of the unique word only at one side, the estimation precision of the fading distortion of data at both ends reduces. Therefore, under the condition of fast fading or low Eb/NO ratio, a required BER cannot be obtained. For example, when Eb/NO=2 under the conditions of signal-to-fading intensity C/M=7 dB and Doppler frequency=0.01, a required BER corresponds to a deterioration of 0.5 dB from the theoretical value. Therefore, when the Wiener filter is replaced by a Kalman filter etc. with a higher estimation precision, the amount of operation increases since a matrix-like
Chin Stephen
NEC Corporation
Williams Lawrence
LandOfFree
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