JPH0142180B2 - - Google Patents

Description

[Detailed description of the invention] (Technical field to which the invention pertains) The present invention relates to a digital frequency modulation method (phase continuous FSK method with a modulation index of 0.5 including MSK).
The present invention relates to a modulation and demodulation method that uses the essential characteristics of the modulation and demodulation system to suppress deterioration of the identification error rate due to intersymbol interference, noise, etc., even when the transmitted signal is subject to strong band limitations.

(Description of Prior Art) As a method for demodulating digital frequency modulation signals, a method in which a received signal and a signal obtained by delaying the received signal by a certain period of time are demodulated using a sine phase comparator is known as a delayed detection method. , is widely used because of its simple circuit configuration.

By the way, a carrier wave is sent out for each channel.
In SCPC-style wireless communication, narrowing the occupied bandwidth and suppressing out-of-band radiation are essential conditions for effective frequency utilization. In order to satisfy this condition, it is necessary to limit the base band or carrier band on the transmitting side. At this time, if conventional delay detection that performs sine phase comparison is used on the receiving side, the phase transition will be insufficient and the reception characteristics will be significantly degraded.

On the other hand, in order to improve demodulation characteristics in the presence of thermal noise, an error correction method using code constraint between sine phase comparison and cosine phase comparison has been proposed (Japanese Patent Application No. 51-116271, (Sho 53-100882),
This method also loses its improvement effect if strong band limitation is performed.

(Objective of the Invention) The present invention eliminates these drawbacks, and aims to provide a modulation/demodulation method in which the identification error rate is less degraded even when a transmission signal is strongly band-limited.

(Characteristics of the Invention) The present invention performs summation logic conversion on one bit of data code on the transmitting side, and then frequency-modulates the converted output through a baseband limiting filter, and on the receiving side, it performs frequency modulation under a certain delay time. It is characterized by performing delayed detection using cosine phase comparison and decoding.Due to strong transmission and reception band restrictions, it is possible to detect large codes that cannot be identified using the normal sine phase comparison method. This is a modulation/demodulation method that enables normal signal identification even under interference between signals.

(Explanation of the principle) For simplicity, the operating principle of the method of the present invention will be explained using the modulation index.
An explanation will be given using an MSK signal as an example of a 0.5 digital frequency modulation signal.

Generally, in the phase continuous FSK method, that is, the digital frequency modulation method, the transmitted wave s(t) can be expressed as follows.

s(t)=Acos {2π fc t+φm(t)} φ _n (t)=mπ/T∫ ^t _-∞∞ 〓 ⁿ al・g(t−nT)dt where a _l : Data code string 0, 1 Corresponding -1,
Variables that take +1 g(t): Impulse response of the baseband filter placed at the modulator input terminal m: Modulation index (=2△fdT), △fd: Frequency deviation, T: Data code repetition period f _c : Center frequency φ _n (t): Instantaneous phase shift A: Peak value of signal In the case of MSK with modulation index m = 0.5, g(t) is g(t) = 1 (0≦t≦T) 0 (Other than that) Therefore, when the data is 1, the phase of the modulated wave advances linearly by π/2 relative to the phase of the center frequency f _c in T seconds, and when the data is 0, They are linearly delayed by π/2.

Output v(t) obtained by comparing the cosine phase of the received signal and a signal obtained by delaying the received signal by τ seconds at the receiving device
is expressed as the following equation.

v(t)=cos {φ _n (t)−φ _n (t−τ)} Here, it is assumed that 2π f _c τ=2kπ (k: positive integer) is satisfied.

Next, changes in phase φ _n (t) will be explained.

FIG. 1 shows the relative phase transition of a phase-continuous FSK signal, where the horizontal axis is time and the vertical axis is relative phase. In the figure, m=0.5 corresponds to MSK. The turning point of the phase change, that is, the time T,
Intersymbol interference occurs due to band limitation at 2T, 3T, etc. In FIG. 1, the dashed-dotted line indicates a phase transition when severe band limitation is applied. Next, by examining the eye pattern of the differential detection output under such severe band-limiting conditions, we demonstrate the effectiveness of the cosine phase comparison differential detector.

FIG. 2 shows the eye pattern of the detection output for a digital frequency modulated signal with a modulation index of 0.5.

In the present invention, τ=T or 2T. Figure 2a shows a sine phase comparison with a delay time τ = T, which has been conventionally used in delay detection, Figure 2b shows a cosine phase comparison, and Figure 2c shows a cosine phase comparison with a delay time τ = 2T.
The eye pattern of each differential detection output is shown. The normalized 3dB bandwidth (BbT) value of the Gaussian low-pass filter for transmitting baseband band limitation is 0.25,
The normalized 3dB bandwidth (BT) value of the reception band limit is
This is the case when it is set to 1.25. It is shown that the eye pattern for sine phase comparison is degraded due to intersymbol interference due to strong band limitation, but the eye pattern for cosine phase comparison is sufficiently open.

From this, it can be seen that differential detection using sine phase comparison is susceptible to intersymbol interference due to strong band limitations, but delayed detection using cosine phase comparison is less susceptible to intersymbol interference due to band limitations.

In Figure 2, (n-1/2)T, (n+1/2)T, and (n+3/2)T are data transition points, and in the present invention, Figure 2b, that is, τ=T, is the transition point of data. In this case, the opening degree of the eye pattern is the largest at the data transition point, so identification is determined at this point. In the case of c in the figure, that is, τ = 2T, the middle point (n-1) of the data transition points is used. Since the opening degree of the eye pattern is the largest at T, nT, (n+1)T, . . ., identification is determined at this point. At this time, the potential differences for identification determination are shown as E ₁ , E ₂ and E ₃ in FIG.

Here, the phase difference φ _n (t)−φ _n at the time of identification
Examine (t-τ). First, when τ=T,
From Figure 2, since we identify by t=(n+1/2)T (n: integer), the phase difference at the identification point is Similarly, when τ=2T, the discrimination point t=nT (n: integer)
The phase difference at is φ _n (t) − φ _n (t − T) | _t=oT = mπ/T [∫ ^nT _(o-1)T a _o . dt＋∫ ^(n-1)T _(o-2)T a _o −1
dt] = mπ [a _o ≠ a _o-1 ] = 0 (a _o ≠ a _o-1 ) ±(a _o = a _o-1 ). Therefore, the cosine phase comparison output v(t) for each of τ=T and τ=2T is and v(t) | _t=oT = 1 (a _o ≠ a _o-1 ) −1 ( a _o = a _o-1 ), and binary identification is possible.

By the way, if the data code string detected in delayed detection that performs cosine phase comparison is dn, then dn is between modulated data code string bn and dn=bnb _o-1 (indicates addition of mod.2). A relationship exists. However, b _o-1 is b _o delayed by one bit. Therefore, in order to match the detected data code string with the transmitted data code string, the transmitted data code string is subjected to 1-bit summation logic conversion in advance, which is then converted into a modulated data code string, and then digital frequency modulation is performed. Must be sent as a signal. At this time, there is a relationship between a _o and _bo , where a _o is the transmission data code string, and b _o = a _o b _o-1 .

(Explanation based on an embodiment) FIGS. 3 and 4 are block diagrams of an apparatus according to an embodiment of the present invention. First, the transmitting device shown in FIG. 3 will be explained. The signal output of the data code string generator 1 is input to one input terminal of the exclusive OR circuit 2, and the signal output is divided into two, one of which enters a 1-bit delay circuit 3, and the output signal delayed by this circuit. is input to the other input terminal of the exclusive OR circuit 2. 2 and 3 constitute a 1-bit summation logic conversion circuit. The other half of the signal output of the exclusive OR circuit 2 is sent to the FM modulator 5 via the transmission baseband band limiting filter 4.
is input. The output of the carrier wave generator 6 is applied to the FM modulator 5, where it is FM modulated with a modulation index of 0.5 by the output signal of the transmission baseband band limiting filter, and is transmitted from the transmission antenna 7 as a digital frequency modulation signal output.

Next, the receiving device shown in FIG. 4 will be explained. The received signal received by the receiving antenna 15 enters the receiver 16, and its output enters a cosine phase comparison delay detection system. That is, the signal output of the receiver 16 is divided into two, one of which enters the delay circuit 17. Receiver signal output delayed by this circuit and receiver 1
The output signal of 6 is given to a multiplier 18 for cosine phase comparison, and the signal output is sent to a low pass filter 19.
to go into. The output of this low-pass filter 19, ie, the cosine phase comparison delayed detection output, is input to a discrimination generator 20 to obtain a code reproduction output 21.

Here, it is assumed that the delay time of the delay circuit 17 has the above-mentioned relationship with the modulation index and the data repetition period, and the identification timing in the identification generator 20 is set at the above-described time point corresponding to the delay time.

An example of operation measurement results obtained by such an apparatus will be explained in comparison with a conventional apparatus. FIG. 5 shows a comparison of characteristics between the cosine phase comparison delay detection method according to the above embodiment and the sine phase comparison delay detection method of the conventional device for a digital frequency modulation signal with a modulation index m=0.5. FIG. 5a shows a conventional sine phase comparison delay detection system, in which the delay time τ of the reception delay circuit is T for one bit. 2B and 2C are based on the above embodiment, and the delay time .tau. of the reception delay circuit 17 is T for 1 bit in FIG. 2B and 2T for 2 bits in FIG. 2C.

The horizontal axis in FIG. 5 represents the reciprocal of the normalized 3 dB bandwidth (BbT) of the device baseband band limit. The vertical axis indicates the value of the signal energy to noise density Eb/No per bit required to obtain an error rate of 10 ^-3 .
In addition, the normalized 3dB bandwidth BT of reception band limit = 1.25
The bit rate is 16kbps.

As shown in this figure, it is clear that when strong transmission baseband band limitation is performed, the characteristics of cosine phase comparison differential detection are superior to conventional sine phase comparison differential detection. It can also be seen that cosine phase comparison differential detection with τ = 2T is superior to sine phase comparison differential detection even when the transmission baseband band is not limited.

(Effects of the Invention) As explained above, by using the modulation method according to the present invention, even when the transmitting side is subject to strong band limitation, the deterioration of the identification error rate due to intersymbol interference or noise can be reduced, and a good result can be achieved. Communication quality can be ensured. Also, on the receiving side, stronger band limitations can be tolerated, making it possible to identify delayed detection with a higher signal-to-noise ratio.
This has the excellent effect of improving communication quality.

This system can be widely applied in general, in addition to systems that communicate in a limited frequency band, such as mobile wireless systems and fixed wireless systems.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of phase transition characteristics of a digital frequency modulation signal. Figure 2 is an example of an eye pattern of differential detection output under strong band limitation. FIG. 3 is a block diagram of a transmitting device according to an embodiment of the present invention. FIG. 4 is a block diagram of a receiving apparatus according to an embodiment of the present invention. FIG. 5 is a diagram showing an example of the (BT) ^-1 versus required Eb/No characteristics of the transmission baseband band limit of the demodulated signal. DESCRIPTION OF SYMBOLS 1... Data code string generator, 2... Exclusive OR circuit, 3... 1-bit delay circuit, 4... Transmission baseband band limiting filter, 5... FM modulator, 6... Carrier wave generator, 7... Transmission antenna, 15 ... Reception antenna, 16 ... Receiver, 17 ... Delay circuit, 18 ... Multiplier, 19 ... Low pass filter, 20 ... Identification regenerator, 21 ... Code regeneration output.

Claims (1)

[Claims] 1. In a digital frequency modulation/demodulation method in which a digital information signal with an information repetition period T is subjected to base band limitation on a transmitting side, and then frequency modulated and transmitted, the digital information signal is means for performing 1-bit sum logic conversion; a base band limiting filter for band-limiting and outputting the output signal of this means; and means for frequency modulating the output signal of this filter with a modulation index of 0.5 as a modulation input; The receiving device includes means for receiving an output signal from the transmitting device, and means for comparing the cosine phase of the output signal of this means with a signal obtained by delaying the output signal of this means by an information repetition period T. Then, the change in the phase state of the output signal of this cosine phase comparing means is delayed by T/2 by half the data repetition period T from the turning point of the temporal change in phase when no band limitation is performed. At the time,
A method for modulating and demodulating a digital frequency modulation signal, comprising means for identifying and determining. 2. The means for performing 1-bit summation logic conversion on a digital information signal is configured to take the exclusive OR of the input digital information signal and a signal obtained by delaying the digital modulation signal by 1 bit. A modulation/demodulation method for a digital frequency modulation signal according to the first item.