Pulse or digital communications – Receivers – Automatic frequency control
Reexamination Certificate
1999-08-16
2003-10-07
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Automatic frequency control
C375S324000, C375S326000
Reexamination Certificate
active
06631174
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an automatic frequency control apparatus capable of improving transmission quality in the digital modulation/demodulation system employed in satellite communications, mobile communications, and mobile satellite communications. More specifically this invention relates to an automatic frequency control apparatus capable of improving receiver performance by periodically inserting a known signal into an outgoing signal and removing a frequency deviation generated in the known signal during transaction between the transmitter and the receiver.
BACKGROUND OF THE INVENTION
In recent years, in the fields of satellite communications, mobile communications, and mobile satellite communications, research is actively being performed on digital modulation/demodulation. Especially, in the mobile communications, an incoming signal is received which is generally subjected to severe fading, so that now feasibility of various types of demodulation system enabling stable operations even in the fading environment are being examined. Under such circumstances, a technology in which a known signal to be utilized for measuring a distortion due to fading in a channel is periodically inserted into the outgoing signal, the distortion due to fading is estimated and compensated from the known signal has been gathering hot attentions as a technology enabling coherent detection even in the fading environment. When performing quasi-coherent detection using such a system, it is required to estimate and compensate the distortion due to fading with high precision, and further the frequency deviation between the outgoing carrier wave and a reference signal for performing quasi-coherent detection in a receiver should be small.
However, when performing coherent detection in the fading environment, if stability and precision of a frequency of an oscillator in a transmitter/receiver are not sufficient, it is impossible to estimate and compensate the distortion due to fading with high precision unless some other specific processing is disadvantageously executed.
In the field of mobile communications, signal transaction is carried out between two mobile stations or between a stationary station and a mobile station, so that, when two stations are relatively moving, a frequency deviation occurs in the transmitted electric wave due to the Doppler effect. Even if the precision of the oscillator in the transmitter or the receiver is high, a frequency deviation disadvantageously occurs between the outgoing carrier wave and a reference signal for performing quasi-coherent detection in a receiver.
As a technology for solving the problems as described above, there is, for instance, a technology described in “Frequency Offset Compensation Method for QAM in Land Mobile Radio Communications” (Kato, Sasaoka, Collection of reports, Institute of Electronics, Information and Communication Engineers (B-II), J74-B-II, No.9, pp493-496 (1991-9)).
FIG. 19
is a view showing configuration of a conventional type of automatic frequency control apparatus described in the above document. In
FIG. 19
, designated at the reference numeral
80
is a known-signal distortion detecting section, designated at
89
is a inter-known-signal phase difference estimating section, designated at
890
is a inter-known-signal phase difference computing section, designated at
891
is a averaging section, and designated at
85
is a inter-one-symbol phase difference computing section.
Operations of the conventional type of automatic frequency control apparatus as described above are explained below.
FIG. 20
is a view showing a format of an incoming signal when a one-symbol known signal is periodically inserted in the signal. For instance, a transmitter transmits a signal formatted by periodically inserting a one-symbol known signal (herein, a known pilot signal) into a (N
F
−1) symbol data signal as shown in FIG.
20
. It is assumed herein that the one-symbol pilot signal is inserted at a time t=kN
F
T
S
. Herein k indicates a natural number, N
F
indicates an interval at which the pilot signal is to be inserted (insertion interval), and T
S
indicates a symbol duration.
When such a signal is received by the known-signal distortion detecting section
80
, the inter-known-signal phase difference computing section
890
in the inter-known-signal phase difference estimating section
89
computes a phase deviation between the pilot signals inserted at an interval of N
F
. Further, the averaging section
891
calculates the average of the phase deviations outputted from the inter-known-signal phase difference computing section
890
and outputs the average <&thgr;(kN
F
)> of the phase deviations &thgr;(kN
F
).
Then the inter-one-symbol phase difference computing section
85
computes a phase rotation &thgr;
S
in one-symbol as expressed, for instance, by the equation (1) from the average of phase deviations.
θ
S
=
⟨
θ
(
kN
F
)
⟩
N
F
(
1
)
Then the inter-one-symbol phase difference computing section
85
executes integration processing by means of iterative addition of one-symbol cycles as expressed, for instance, by the equation (2) by using the computed phase rotation &thgr;
S
.
&thgr;(
kN
F
+i
)=&thgr;(
kN
F
+i−
1)+&thgr;
S
(2)
Finally the inter-one-symbol phase difference computing section
85
rotates the phase of the incoming signal to remove the frequency deviation. Namely, the inter-one-symbol phase difference computing section
85
rotates the phase for each one-symbol, as expressed by the equation (3), for a digital baseband signal r(kN
F
+i) in the I channel and Q channel to remove the frequency deviation.
r
R
(
kN
F
+i
)=
r
(
kN
F
+i
)exp[−
j&thgr;
(
kN
F
+i
)], 0
≦i≦N
F
−1 (3)
Assuming that the modulating symbol rate is R
S
(symbol/s), a frequency deviation detection range −f
DET
[Hz] to f
DET
[Hz] in the inter-known-signal phase difference estimating section
89
−f
DET
[Hz] to f
DET
[Hz] is as expressed by equation (4):
f
DET
=
R
S
2
N
F
(
4
)
As understood from the equation (4), by reducing the insertion interval N
F
the range of detectable frequency deviation can be made wider. However, when it is considered that a function of the inter-known-signal phase difference estimating section
89
is equivalent to that of a filter, when the range of frequency deviation detection is made wider by reducing the insertion interval N
F
, the same effect as that provided when a frequency band of a filter is equivalently extended is provided. Namely, an estimation error for a frequency deviation due to noises or the like becomes larger, and precision in estimating a frequency deviation is degraded. Accordingly, to improve the precision in estimating a frequency deviation, it is better that the insertion interval N
F
is large.
In the automatic frequency control apparatus as described in the above mentioned document, however, the amount of phase rotation due to a frequency deviation is computed by using only the known signals inserted at an insertion interval of N
F
, so that it is necessary to increase the insertion interval N
F
in order to improve the precision in estimation of a frequency deviation. Due to this fact, the detection range of frequency deviation becomes disadvantageously narrower. On the other hand, if the insertion interval N
F
is decreased in order to widen the detection range of frequency deviation, the frame efficiency becomes lower, and there occurs a new problem that the precision in estimation of the frequency deviation is degraded.
Further, in the conventional type of automatic frequency control apparatus, a distortion of a channel is detected by using only a one-symbol known signal, so that a period of time required for high precision in estimating a frequency becomes longer in an environment with a low C/N (carrier to noise power ratio
Asahara Takashi
Kojima Toshiharu
Chin Stephen
Mitsubishi Denki & Kabushiki Kaisha
Yeh Edith
LandOfFree
Automatic frequency control apparatus does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Automatic frequency control apparatus, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Automatic frequency control apparatus will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3159626