Digital communications system with immunity to frequency selective fading

(54) Title: Procedure for transmitting digital data via the troposphere and device for carrying out the procedure

The present invention relates to a digital communication system in which data is to be transmitted via a frequency or time dispersive medium in which fading of the transmitted signal is caused by multi-way distortion or some other type of non-linear distortion.

Previously known digital communication systems, where transmission is via a frequency or time dispersive medium, utilize many different types of modulation. Various systems use pulse duration modulation, frequency modulation and phase shift modulation, including quaternary phase shift modulation as well as a variety of other methods. In each of these systems, a modulator operates directly on a locally generated sinusoidal signal. Therefore, in the prior art system, the bandwidth of the transmitted signal is dependent only on the spectral spread caused by the modulation of the sine-wave signal. The bandwidth of a frequency dependent fade-inducing signal, which slides in frequency through a transmission band, is often worse than or greater than the bandwidth of the transmitted signal.

When the fade-inducing signal is in phase with the transmitted signal, the latter can be completely obliterated before reaching the receiver.

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The prior art systems exhibit several methods by which one tries to overcome the problem of frequency dependent fading. Many systems utilize a plurality of receiver antennas located at a certain distance from each other so that a signal received by at least one of the antennas is not affected by the frequency-dependent fading-inducing signal. Since, with frequency dependent fading, the entire signal can be obliterated over a relatively large range, such systems cannot completely solve the problem. The only truly effective method has therefore been to significantly prolong the transmission time of each digital data bit in order to increase the likelihood that fading will not occur throughout the bit transmission time. However, this means that the maximum bit transfer rate of the entire digital communication system is reduced. In addition, this method also reduces the average transmitter power in many systems, since the power available for each pulse transmission is spread over a longer time interval.

Furthermore, in many prior art systems, the entire bandwidth allocated to each digital transmission channel is not effectively utilized. Since the bandwidth is only dependent on the existing modulation, only a small portion of the total bandwidth is typically utilized during a transmission interval.

The object of the invention is to provide a digital communication system with great insensitivity to frequency dependent fading, and a system which efficiently utilizes the bandwidth allocated per channel and which utilizes the available transmit power as efficiently as possible under given frequency fading conditions.

The characterizing feature of the invention is apparent from the characterizing portion of claim 1.

In a digital communication system according to the invention, the spread-spectrum oscillator signal is transmitted in the transmitter before a modulation with the desired digital data is achieved. The intermediate frequency oscillator signal is multiplied by a spread spectrum signal from a second oscillator in a mixer. The carrier signal generated thereby is switched on and off by a key pulse circuit at a rate which depends on the transmission characteristics of the medium. The carrier coupled phase shift is modulated by the incoming digital data stream and then transmitted.

Several different types of bandwidth oscillators can be used depending on the desired frequency response, allocated bandwidth and allowed

7511173-2 degree of complication for the input circuits.

The signals are received by a differentially adjustable receiver, which automatically adapts to the received waveforms. The receiver previously stores samples of distorted received pulses for use as a reference in a detector of a type with a coherently matched filter, which utilizes decision feedback to allow coherent detection without channeling filters.

The invention will be described in more detail with reference to five figures in the accompanying drawings, in which Figure 1 is a perspective view of a bidirectional digital communication system utilizing wave propagation in the troposphere, Figure 2 is a block diagram of a carrier generator circuit and a modulator according to the invention, Figure 3 is a block diagram of an alternative embodiment of the system of Figure 2; 4 shows a diagram of the frequency spectrum and the coated bandwidth of a communication system without using the invention, and Figure 5 shows a diagram of the frequency spectrum and the coated bandwidth of the same system using the invention.

Fig. 1 shows a communication system utilizing wave propagation in the troposphere and for which the present invention is particularly useful. Such communication systems utilize the reflectivity of layers in the troposphere 11 to establish connections between distant locations located beyond the horizon from one another. The reflective layers within the troposphere 11 are by no means unchangeable. They may consist of several partially reflective layers, on top of one another, which often change position as well as their reflectivity. The amount of reflective layers in the troposphere causes the different reflective signals to be added at certain locations and frequencies, while subtracting from each other and canceling each other at other locations and frequencies. The movement of the reflecting layers over time causes the fading of received signals at any location to vary as a function of the frequency of the signals. If the spectrum of a fading state coincides in frequency with the spectrum of the signal or is larger in bandwidth and surrounds the spectrum of the signal, the entire signal will be eliminated and no information thereby reaches the receiving station.

In accordance with the present invention, a first transmitter / receiver 12 transmits via an antenna 14 a digital quaternary phase shift modulated signal with a bandwidth which occupies most of the allocated bandwidth per channel. The transmitted signals are reflected at the separate ones

7511173-2 reflecting the surfaces within the troposphere 11 and being received by a second transmitter / receiver 18 via an antenna 16. The two transmitters / receivers may be located at a considerable distance from one another, such as beyond the horizon. Distances of several hundred kilometers are typical for applications of this type. Transmission can also take place in the opposite direction from the second transmitter / receiver 18 to the first transmitter / receiver 12. Each transmitter / receiver comprises reception and transmission equipment according to the invention.

Fig. 2 shows a block diagram of a carrier generator circuit 20, a modulator 30 and a transmitter 32 constructed in accordance with the invention. In the carrier generator circuit 20, an intermediate frequency oscillator 22 generates a sine wave signal of the desired intermediate frequency. Typically, and in the described and preferred embodiment, a frequency of 70 MHz is used. A spread spectrum oscillator 23 generates a second signal at a significantly lower frequency than the frequency of the mid-frequency oscillator 22. In the preferred embodiment, the output of the spreading oscillator 23 is a square wave with a frequency of 1.25 MHz. The signals generated by the intermediate frequency oscillator 22 and by the spread spectrum oscillator 23 are multiplied by one another in a mixer 24. The carrier signal thus generated occurs at the output of the mixer 24.

The carrier signal is switched on and off via a gate 26 by means of a key pulse circuit 28. The resulting pulse bursts in the carrier signal are quarterly phase shifted by the incoming data25 current in the QPSK modulator 30 at a rate of two bits for each keyed pulse. Transmitter 32 amplifies the digitally modulated pulse bursts of carrier signals and superimposes them on an internal oscillator signal in and for transmission at a frequency suitable for transmission via the troposphere. Preferably, a frequency of between is used

300 MHz and 10 GHz.

Prior art systems utilize only a single sine wave oscillator to generate the subcarrier signal. The spectrum of such a signal after switching on and off by means of a key pulse circuit is shown in Fig. 4 and is the known spectrum (sinx) / x for an output signal with a single frequency from the key pulse circuit. and F_u constitutes the lower and upper limit, respectively, of the bandwidth allocated to the particular transmission channel. P is the maximum peak power, which is the maximum allowed for this system in operation. As is known in the past, the points representing half the power of the peak power in the about 40 Fq centered spectrum, depending on the length of time, are below

7511173-2 to which the carrier is connected. The longer the carrier is switched on, the narrower the spread between the half-power points' in the spectrum. As the width of the spectrum increases, the time required to transmit each data bit increases, and consequently, the data transmission rate decreases. Also, providing a single peak in the allocated bandwidth does not minimize the effect of frequency-dependent fading. About the width of the device shown in FIG. 4 the displayed spectrum is increased by the use of a truncated key pulse, the average transmission power is reduced to a corresponding degree and accordingly the reliability of the system also decreases.

Fig. 5 shows the spectrum obtained using the invention. In the spectrum shown in FIG. 5, the time during which the carrier is engaged is equal to that of the spectrum in FIG. reached. In addition, the increased utilization of the allocated bandwidth is achieved without reducing the average transmitter power. Two main carrier peaks occur, one on each side of the center frequency Fq, which is the frequency of the melanic frequency oscillator 22 shown in FIG. 2. Each main peak is offset from F_q with AF, which is the primary frequency of the spread spectrum oscillator 23 in Fig. 2.

There are other frequency components in the frequency spectrum of the circuit shown in FIG. 2, which are caused by the harmonics included in the square wave band signal.

Fig. 3 shows an alternative embodiment of the carrier generator circuit and the transmission circuit. In the alternative carrier generator circuit 21, the spread spectrum oscillator 23 in Fig. 2 has been replaced by a sequence generator 36. The sequence generator 36 at its output generates a predetermined sequence of binary numbers. The binary bits or numbers in the sequence are so selected that they give rise to a desired output spectrum depending on application needs. For example, a Barker code sequence or pseudo-Barker code sequence is very useful for reducing sidebands at the spectral head peaks. In the preferred embodiment, the sequence generator 36 comprises a read only memory in which the predetermined sequence is stored. The fixed memory is preferably addressed by a continuously circulating binary counter.

The signals can be received at the two transmitters / receivers by a differentially adaptable receiver as described in US Patent No. 3,794,921. Such a receiver automatically adapts

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7511173-2 to the spectral response of the received signals at each type of transmitted waveform and nonlinear frequency dependent distortion.

It should be noted that only preferred embodiments of the invention have been described and that a variety of skilled modifications and modifications may be made without departing from the scope of the invention.