Pulse or digital communications – Receivers – Automatic frequency control
Reexamination Certificate
1998-04-28
2002-09-24
Corrielus, Jean (Department: 2631)
Pulse or digital communications
Receivers
Automatic frequency control
C375S130000, C375S269000
Reexamination Certificate
active
06456672
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the improvement of a receiver in the field of radio communications.
DESCRIPTION OF THE RELATED ART
Automatic Frequency Control (AFC) circuit using a pilot signal, an automatic frequency control circuit of a conventional automatic frequency control communication system, is set forth in “AUTOMATIC FREQUENCY CONTROL CIRCUIT” in Japanese Unexamined Patent Application No. HEI08-330910. The conventional art is outlined below.
The configuration of a conventional AFC circuit is shown in
FIG. 31
, wherein, a frequency converter
3101
, a pilot signal extraction circuit
3102
, an arc tangent calculation circuit
3103
, a phase-difference calculation circuit
3104
, a linear approximation circuit
3105
, a frequency-error calculation circuit
3106
, a variable oscillator
3107
, and a memory
3108
are illustrated.
The operation is explained next. A digital radio communication system is generally not free from frequency offset in received signals due to the difference in oscillation frequencies between the oscillators used by a transmitter and a receiver, due to temperature change and other factors (this is called as frequency offset). Frequency offset is also caused by the Doppler shift incurred on the mobile unit in mobile communications and on the satellite in satellite communications. Because frequency offset in the received signal can degrade bit error rate, the AFC circuit is applied to the receiver to compensate for the frequency offset in the received signals.
In
FIG. 31
, an input to the AFC circuit is a received signal having frequency offset. After the received signal is input to the AFC circuit, it is input to frequency converter
3101
. Frequency converter
3101
then converts the received signal to an intermediate frequency. Being converted to an intermediate frequency, the received signal is output from the AFC circuit while a pilot signal is extracted by a pilot signal extraction circuit
3102
. A pilot signal is a sequence whose modulation phase is already known to the receiver, and the phase information of the known sequence is already stored in memory
3108
of the receiver, and by comparing the phase information to the phase of the received pilot signal, the frequency error can be obtained disregarding the modulation phase.
The pilot signal extracted by pilot signal extraction circuit
3102
is operated by arc tangent calculation circuit
3103
to obtain its arc tangent. A phase of the pilot signal is obtained through the arc tangent calculation. The phase difference of the pilot signal and phase information stored in memory
3108
are calculated by phase difference calculation circuit
3104
.
Linear approximation circuit
3105
regards the output from phase difference calculation circuit
3104
as the linear function of time, and approximates phase difference linearly. Frequency error calculation circuit
3106
calculates a frequency error based on the inclination of the output from linear approximation circuit
3105
. Based on the frequency error output from frequency error calculation circuit
3106
, variable oscillator
3107
controls the frequency to be provided for frequency converter
3101
so as to eliminate the frequency error.
Based on the frequency error in the received signal output from frequency converter
3101
, variable oscillator
3107
controls the frequency to be provided for frequency converter
3101
to make frequency error zero. Thus, automatic frequency control is achieved.
For the above mentioned AFC circuit, in order for the receiver to obtain the frequency error without removing the modulation phase, a pilot signal having the phase information known to the receiver must be inserted in the transmission data. It is undesirable as it degrades throughput. In addition, such a technique requires a memory in the receiver to calculate a phase difference in the received known sequence. Thus, it is not desirable as an extended circuitry is required, which results in increased power consumption.
Another automatic frequency control circuit in the conventional automatic frequency control communication system is a non-linear AFC circuit, which is expounded in “DIGITAL COMMUNICATIONS BY SATELLITE” (written by V. K. Bhargava, published by Jateck Publishing Co., on May 21, 1986). Given below is the explanation of this conventional art.
FIG. 32
is a configuration of the conventional AFC circuit, wherein, multipliers
3201
and
3202
, a phase converter
3203
, hard limiters
3204
and
3205
, multipliers
3206
and
3207
, and an adder
3208
are shown.
The operation is explained below. In
FIG. 32
, a reference carrier signal is branched out to two. One of which is input to multiplier
3202
and the other is input to phase converter
3203
, where the reference carrier signal is phase shifted by &pgr;/2, and then input to multiplier
3201
. Like the previously explained conventional automatic frequency control circuit, the received signal having frequency offset is input to the AFC circuit shown in FIG.
32
. The received signal having frequency offset is multiplied by the reference carrier signal of the frequency roughly equals the received signal by multipliers
3201
and
3202
.
Signals output from multipliers
3201
and
3202
are input to hard limiters
3204
and
3205
for sign determination. At multiplier
3206
, the input to hard limiter
3205
is multiplied by the output from hard limiter
3204
, while at multiplier
3207
, the input to hard limiter
3204
is multiplied by the output from hard limiter
3205
, respectively. Then at adder
3208
, the output from multiplier
3206
is deducted from the output from multiplier
3207
, to obtain a frequency error signal.
Let us assume that the received signal r (t) and the reference carrier signal R (t) are given by the expressions below.
r
(
t
)=
x
(
t
)cos(&ohgr;
c
t+&thgr;
i
)+
y
(
t
)sin(&ohgr;
c
t+&thgr;
i
)
R
(
t
)=cos(&ohgr;
c
t+&thgr;
0
)
where, &ohgr;
c
denotes a frequency of the received signal, &thgr;
i
denotes a phase of the received signal, &thgr;
0
denotes a phase of the reference carrier signal, x(t) denotes an amplitude of the real part of r(t), and y(t) denotes an amplitude of the imaginary part of r(t).
According to the foregoing “DIGITAL COMMUNICATIONS BY SATELLITE”, the frequency error signal e(t) is expressed approximately using &phgr;=&thgr;
i
−&thgr;
0
, as follows:
e
(
t
)=−
K&phgr;
(
K
being constant)
Then, assuming that the Binary Phase Shift Keying (BPSK) modulation is applied as the modulation method, a baseband signal diagram of a noise-free received signal is shown in FIG.
33
. Let us now assume that the phase of a transmitted signal is 0, a real signal point is point A, and a received signal point is point C of FIG.
33
. That is, &thgr;
i
=&thgr;, &thgr;
0
=0, and the error signal e(t) =−K&thgr;. The phase difference between the real signal point A and the received signal point C becomes a positive value, indicating the frequency of the reference signal is lower than the frequency of the received signal.
When the carrier
oise(CN) ratio of the received signal is low, the position of the received signal point is significantly deviated from its original point due to the influence of noise. The distance from the signal point B (having the opposite polarity to the signal point A) may become shorter than the distance from the real signal point A. The position of the received signal point deviated due to noise is assumed to be C′ in FIG.
33
. Because the distance between C′ and B becomes shorter than the distance between signals C′ and A, &thgr;′ is detected instead of the real value &thgr; as the phase rotation distance stemling from the frequency offset. In the example shown in
FIG. 33
, &thgr; and &thgr;′ differ not only in their values but also in their polarities.
When polarities of &thgr; and &thgr;′ are different, the AFC circuit controls to increase the frequency offset rather than its des
Kojima Toshiharu
Miyake Makoto
Okubo Seiji
Sano Hiroyasu
Uchiki Tatsuya
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