Noisy signal communication

Abstract

A signal to be transmitted by transmitter T is generated by a source S. Prior to transmission, random noise is added to the signal by a first noise source N, the output noisy binary PSK signal prefera

Description

Noisy signal communication In typical communication systems, random noise is always present and it tends to impede the reception of the wanted information and it tends to impede the reception of the wanted information signal. It is usually found that random noise is composed of randomly occurring voltages which are unrelated in phase or frequency. It is sometimes known as white Gaussian noise because it covers a very wide range of frequencies and the noise voltages follow a Gaussian distribution. The most usual criterion of performance for a communication system is the ratio of signal power to noise power in the system and it is defined as the signal-to-noise ratio (S / N) at the output of the detector. The evaluation of the noise power in the system is based on the total value within some bandwidth of the receiver used or the bandwidth of the information transmission under consideration. For the best performance of a communication system the (S / N) ratio should be as large as possible. However, in this invention a totally different and unusual technique is used to maximise the noise~to-signal ratio (N / S), by artificially adding random noise to the signal from a suitable noise generator before noise to the signal from a suitable noise generator before transmission. Furthermore, in some systems additional random noise will also be added naturally along the channel during transmission from various sources in space for example. This communication system therefore uses a noisy signal operating well below noise level and it achieves this result without spreading its information bandwidth -as in a spread spectrum system. In the latter case, there is a direct exchange of bandwidth B with (S / N) ratio in accordance with Shannon's relationship C / B = 1.443(S / N) and so for a given value of C, by spreading the information bandwidth B considerably, the value of (S / N) decreases by a very large amount and is obviously below the noise level by several decibels. Figure 1 is a block schematic diagram of the noisy communication system. Figure 2 is a block schematic diagram of the active noise filter. Description In the noisy signal communication system shown in Figure 1, the source S generates the message signal which is ‘processed by the transmitter T. Before transmission random noise is added to the signal by a first noise source N to ensure that the output signal from the transmitter is noisy with a (N / S) ratio well above 0 dB. -It is then sent along a channel C which may be a cable or just free space. During transmission more random noise will be picked up in some systems as mentioned earlier and it can be represented by a second noise source N prior to reception by the receiver R. The receiver will typically have RF / IF stages and its output is fed into filter F. ‘The active noise filter F shown in Figure 2 was patented as GB 2538226 A, to operate between the final IF stage and the demodulation stage of the receiver. By using sum and difference signals, the filter removes random noise and interference from an input binary PSK signal. The sum signal removes the random noise and interference from the difference signal and the output of the filter contains only the digital information transmitted. The filter comprises a first upconverter 1 with a highly stable 6 source 4 at the intermediate frequency fi . A second upconverter combines a time-delayed version of the input signal with the output of the first upconverter 1 to provide a sum signal at lency 3f in which phase shifts due to transmitted data have removed. been The sum signal is then fed into a phase-locked loop 5,8,9,10 which effectively extracts the phase noise component of the input signal and imposes the phase noise on its VCO 5. The VCO controls a downconverter 3 which receives the output of the first upconverter 1 as an input signal and outputs a difference signal at the intermediate frequency fi with the phase noise removed. It can be shown analytically that minimising the mean square error (MSE) between the input and output signals of the filter is tantamount to maximising the output (S / N) ratio, by virtually removing all the noise power received. The filter therefore behaves as an ideal matched filter and it is specifically designed for use with a binary PSK signal. Furthermore, DPSK can also be used for practical purposes without further noise degradation. The filter can be constructed in analogue or digital form and with a two-stage or four-stage filter, the (S / N) improvement should be sufficient for practical purposes. Finally, if binary PSK is used the demodulator D recovers the phase reference signal by using a Costas loop for example and it then extracts the digital information required subsequently. Alternatively, if DPSK is employed , the previous bit is used as the reference signal without the need for a separate phase reference and the digital information is obtained at the output O. Applications One of the advantages of the noisy signal communication system is its ability to conserve transmitter power if required in such cases as data systems, radio systems, radar systems and in the use of space satellites. Additionally, it also provides bandwidth conservation because it only uses the information bandwidth conservation because it only uses the information bandwidth employed and does not spread it as in a spread spectrum system. Jesandly, it provides intrinsic security because it makes etection of a transmitted signal virtually impossible by detection of a transmitted signal virtually impossible by an eavesdropper and only a friendly receiver with all the a priori information available will be able to do so. Furthermore, in military systems, additional security can be provided by the use of time-hopping or frequency-hopping techniques if required. Thirdly, in cryptographic systems which are private, public or 1 are private, "keys" or the Ta aa a el quantum, techniques such as the use of "keys" or the factorisation problem will not be relevant for the noisy communication system, because it is intrinsically secure, even over a long distance as illustrated earlier. Fourthly, the noisy communication system can also be used solely noisy for the purpose of conveying a "key" securely from a sender to ion system can "key" securel) for the purpose of conveying a "key" securely from a sender to a receiver in any cryptographic system, without the need for using quantum information.

Claims

Claims 1. A communication system which employs a noisy signal prior to transmission by using a first noise source which adds random noise artificially from a noise generator to transmit the noisy binary PSK signal well below noise level. Additionally, it may also use a second noise source by picking up random noise during further transmission along a channel from natural sources, in order to ensure a suitable noise-to-signal ratio (N / S) so as to provide maximum security by converting the message information transmitted into a noise-l1ike form and this condition is achieved without spreading the information bandwidth or by using cryptographic techniques. Furthermore, the message information can only be recovered by the use of a friendly receiver with a priori information and an active noise filter specifically designed for removing the random noise received.

2. A communication system as in Claim 1 which employs a noisy binary PSK signal well below noise level for security without using cryptographic techniques, in order to conserve power or bandwidth on transmission over long distances if required.

3. A communication system as in Claim 2 which can be used solely for the purpose of conveying a "key" securely in any cryptographic system which is classical or quantum.

4. A communication system as in Claim 1 or Claim 2 which employs an active noise filter specifically designed as an ideal matched filter for removing the received random noise and it achieves this by using sum and difference signals. The sum signal removes the random noise by using a negative feed-forward phase-locked loop and the difference signal provides the output digital information transmitted.

5. A communication system as claimed in any of the previous claims and substantially described herein with reference to Figure 1 and Figure 2 accompanying this application.

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