JPH0410731A - Demodulating device - Google Patents

Abstract

PURPOSE:To maintain effective modulating characteristics even if frequency offset exists by providing the demodulating device with a Doppler detection part for deciding whether the frequency offset due to Doppler effect exists in a signal of a loop or not and a level detecting part for detecting an input signal level and using the output of the Doppler detection part as a control signal for an oscillation circuit (NCO) for generating controlled frequency corresponding to numerical control based upon the output of the level detection part. CONSTITUTION:The demodulating device is provided with the Doppler detection part 64 for detecting the component of the frequency offset due to Doppler effect from a signal in the PLL loop and the level detection part 65 for detecting a receiving input level from a complex base band signal. The NCO 63 is controlled by both the outputs of the detection part 64 and a loop filter 62 in accordance with the detection signal of the detection part 65. Even when the level is dropped due to fading or the like, the optimum control of the NCO oscillation frequency against the frequency offset due to Doppler effect makes it possible to follow a phase. Thus, the demodulating characteristics can be prevented from being deteriorated.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to PSK (ph
as e sh i f t k e y i
n g ) This relates to a demodulation device that demodulates a signal.

[Prior Art] FIG. 2 is a block diagram showing the configuration of a PSK signal demodulation device. In the figure, (1) is a received intermediate frequency signal, (2) is a frequency converter, and (3) is a Local oscillator, (4
) is a low-pass filter (hereinafter abbreviated as LPF), (
5) is an analog-to-digital converter (hereinafter abbreviated as A/D), (6) is a demodulator, (7) is a complex baseband signal, and 8) is a demodulated output signal.

The received intermediate frequency signal (1) of the PSK signal is converted into a complex baseband signal (7) in a frequency converter (2). This complex baseband signal (7) is processed by LPF (
After unnecessary high frequency components are removed by A/D
It is converted into a digital signal in step (5).

FIG. 3 is a block diagram showing the configuration of a conventional demodulator (6), in which the same symbols as in FIG. 2 indicate the same parts, (60) is a phase control section, (61) is a phase error detection section, (62 ) is a loop filter, (63) is an NCO (numerical
y controlled oscillator
).

At the input point of the demodulator (6), the complex baseband signal (
7) is in the form of a digital signal, and is divided into 70), which represents the amplitude of the real part, and (71>, which does not represent the amplitude of the imaginary part).

By the way, since there is an error (frequency offset) between the local oscillation frequency on the transmitting side and the local oscillating frequency on the receiving side, the phase of the complex baseband signal (7) depends on the phase change due to the desired modulation method. In addition, a phase rotation occurs at a speed proportional to the frequency offset, and the amount of phase change caused by this is corrected by NC0 (63). In other words, if the oscillation frequency of the NCO (63) matches the frequency offset between transmitting and receiving, the NCO (63)
The output phase of is always the complex baseband signal (70), (7
1), and therefore the phase of the demodulated output signal (8) matches the phase of the signal (7) represented by
) can be obtained.

In reality, there is a phase difference between the output phase of NC0 (63) and the phase of the complex baseband signal (7), so the phase error detection unit (61) -
Loop filter (62) NGO (63) - phase control section (
60), the feedback control loop, i.e. the phase opening
It constitutes a power loop (PLL).

[Problems to be Solved by the Invention] Since the conventional demodulator described above is configured as described above, it is difficult to solve the problem for communication between fixed stations in which there is only a frequency offset due to the local oscillation frequency difference between transmitting and receiving. However, in mobile communications, level fluctuations occur due to fading, etc., and with conventional equipment, if there is a frequency offset due to the Doppler effect in addition to the stationary frequency offset, the received signal may be affected due to fading etc. If the level drops and PLI- becomes difficult to operate, the phase difference between the input signal and NCO will make it difficult to pull in the PLL when the received signal level recovers, resulting in burst errors occurring for a long period of time. There were some problems.

Figure 4 shows the relationship between the fluctuation of the frequency offset and the output frequency of the NGO. Before time to, the frequency offset and the output of the NCO are almost equal due to the loop filter output, but from to to t4, the level increases. When I feel depressed, P
LL will be unable to operate normally, and NGO will hold the output at to. When the level is restored at t4, the frequency offset and the output of the NCO are lf4 f.

Furthermore, as shown in FIGS. 5 and 6, there is a phase difference of 1QcQNl.

Due to this large phase difference and the characteristics of the PLL after t, cycle slips and the like occur during re-pulling, resulting in prolonged burst errors and deterioration of demodulation characteristics.

The present invention was made to solve this problem, and an object of the present invention is to provide a demodulator that can effectively control the NCO even if there is a frequency offset due to the Doppler effect when the received signal level decreases. It is said that

[Means for Solving the Problems] A demodulation device according to the present invention includes a Doppler detection unit that detects a frequency offset component I due to the Doppler effect from a signal in a PLL loop, and a complex base A level detection section for detecting a received input level from a bunt signal is provided, and the NGO is controlled by both the output of the Doppler detection section and the output of the loop filter in accordance with the detection signal of the level detection section.

[Effect] Even while the level is dropping due to fading etc.
NGO against frequency offset due to Doppler effect
By optimally controlling the oscillation frequency of , it is possible to track the phase, and it is possible to prevent the deterioration of the demodulation characteristics as shown in .

[Examples] Examples of the present invention will be described below with reference to the drawings. 1st
The figure is a block diagram showing an embodiment of the demodulator of the present invention. In the figure, the same reference numerals as in FIGS. 2 and 3 indicate the same or corresponding parts, (64) is a Doppler detection section, (65)
is the level detection section, (620) is the loop filter (62)
(640) and (644) are the output signals of the Doppler detection section (64), and (650) is the detection signal of the level detection section (65).

Also in the embodiment shown in FIG.
Phase error detection section (61) - loop filter (62) -N
The operation of the PLL constituted by the CO(6B)-phase control unit (60) loop is similar to the conventional device shown in FIG. While this PLL is locked, the output (620) of the loop filter (62) becomes a signal instructing the NCO (63) to set the frequency, but if there is a frequency offset due to the Doppler effect, the PLL
Due to the response time constant of
2) appears in the output (620). Doppler detection unit (
In step 64), the output is input and averaged to grasp the frequency offset fluctuation situation.

In case of failures such as blocking or fading, P L
In order to prevent L from operating normally, such a state is detected by the level detection section (65), and the loop filter (62), NCO (63), and Doppler detection section (64) are controlled by this detection signal. In other words, the level detection section (6
Based on the output (650) of the loop filter (650)
2) characteristics such as time constant are changed.

The Doppler detection unit (64) outputs (620) while the output (650) of the level detection unit (65) is at a predetermined level or higher.
is averaged by the averaging processing unit (641) and outputs the signal (6
44), and the level detection section (65) output (6
50) is lower than a predetermined value, it is determined that the output (620) is unreliable, and a numerical value based on the past performance of the output (644,) is output.

The NCO (63) outputs the output (65) of the level detection section (65).
0) is higher than a predetermined value, the output (620) of the loop filter 62) is used for control; otherwise, the output (644) of the Doppler detector (64) is used for control.

In Fig. 7, before 1o and after t4 is the output of the loop filter, and between to and t4 is the Doppler detector (64).
By using the output of NCO (63) according to the output of the level detection section (65), the output of NC0 (6B) will follow the frequency offset even when the level is dropping. Regarding the phase as well, as shown in FIG. 8, the output phase of NC0 (63) at t4 becomes approximately the same value as the phase of the baseband signal, improving the pull-in characteristic at the time of level recovery.

If a time delay occurs in the detection operation in the level detection section (65), the timing can be matched by giving a time delay to the input to the phase control section (60).

Although the above description has been made for two-phase PSK signals, it goes without saying that the present invention can be applied to polyphase PSK signals in general.

[Effects of the Invention] As described above, the present invention has the advantage that good demodulation characteristics can be maintained even if there is a frequency offset due to the Doppler effect when the received input level is reduced due to blocking or fading.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a block diagram showing the configuration of a PSK signal demodulation device,
FIG. 3 is a block diagram showing a conventional demodulation device, FIG. 4 is a diagram showing the frequency relationship between carrier and NCO with respect to input signal level changes in the conventional device, and FIG. FIG. 6 is a diagram showing the amount of phase change depending on the output frequency of the NCO in a conventional device. FIG. 7 is a diagram showing the amount of phase change depending on the output frequency of the NCO in a conventional device. FIG. 8 is a diagram showing the amount of phase change depending on the output frequency of the NCO in an embodiment of the present invention. 1... Received intermediate frequency signal, 2... Frequency converter,
3... Local oscillation unit, 5... Analog-digital converter, 6... Demodulator, 7 Complex baseband signal, 8...
...Demodulated output signal, 60...Phase control section, 61...
Phase error detection section, 62... Loop filter, 63...
-NC0164...Doppler detection section, 65...Level detection section. Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (2)

[Claims] (1) In a demodulator that performs demodulation by inputting a complex baseband signal obtained by frequency converting a received intermediate frequency signal of a PSK signal into a complex baseband signal and further converting this into a digital signal, Oscillator circuit that generates the control frequency (
(hereinafter referred to as NCO), a phase control unit that generates a demodulated output signal corresponding to the complex baseband signal by comparing the phases of the input complex baseband signal and the oscillation frequency signal of the NCO; A phase error detection section that detects a phase error between the complex baseband signal and the oscillation frequency signal of the NCO, a loop filter that averages the output of this phase error detection section, and an oscillation frequency of the NCO that is determined by the output of the loop filter. means for performing feedback control; a Doppler detection unit that determines whether a frequency offset due to the Doppler effect exists in the received frequency offset estimated from the output of the loop filter; and detecting an input signal level from the complex baseband signal. means for controlling the characteristics of the loop filter based on the detection output of the level detection section and the determination result of the Doppler detection section; Means for outputting to the NCO either a signal averaged by inputting the output of the signal or a signal generated based on the past history of the signal; A demodulation device comprising: means for using either the output of the loop filter or the output of the Doppler detection section as a control signal. (2) The Doppler detection unit is characterized by having means for inputting the output of the loop filter, detecting the frequency shift due to the Doppler effect and the change period of the Doppler effect, and averaging these detected values. The demodulator according to claim 1, wherein: