Data transmission system with correlation receiver

Abstract: A receiver for use in a system for communication and telecontrol over the electricity mains network has a correlation stage in which values of correlation coefficients for a received signal are predicted in a processor taking due account of known distortions of the network. Then these coefficients are multiplied, shifted (2) and averaged (4) with recent results in elements 2 to 5 to minimise the effects of noise, the consequent values being multiplied at element 6 by the actual correlation values of the received signal in the bins for comparison at comparator 7.

Description: The present invention relates to a signal receiver for use in communication over noisy lines and more particularly, but not exclusively, for use in communication and telecontrol over the electricity mains network, and also to a method of processing signals in such a signal receiver. European Patent Specification Publication No. 88564 describes a mains telecontrol system in which each transmitted "1" bit is represented by a first pseudo-random sequence of 1024 bit-elements (referred to as "chips"), and each "0" bit by a second such sequence. In the receiver, each sequence of 1024 chips is correlated with two reference sequences stored in the receiver to determine whether the received sequence represents a "1" or a "0". Since the transmitter and receiver are not synchronised, the correlation must be repeated for each phase; in order to expedite the operation, the correlation is performed for eight different phase locations simultaneously. Once the 1024 bit sequence has been received, the receiver contains 16 correlation values each consisting of a number stored in a "bin". To process the correlation values each of them is squared and then the resultant products in each set are added, the two sums being compared in order to determine whether the received sequence represents a "1" or a "0". An object of the present invention is to provide an improved analysis of the correlation results. The present invention provides a line network communication system including a transmitter means capable of transmitting a series of coded data signals on a baseband carrier signal and transmitting a series of data signals represented by signal sequences of first and second kinds on a baseband carrier signal and receiver means capable of receiving a transmitted signal and recovering therefrom said data signals, the receiver means being arranged to operate in a search mode to compare received signals with first and second reference sequences at different relative phases until at least one comparison signal generated as a result of the comparisons indicates a degree of correlation exceeding a threshold value, the reference sequences corresponding to the signal sequences of the first and second kinds respectively, a track mode to verify that said degree of correlation exists for a preset time interval and, in dependence on a successful verification in the tracking mode, a locking mode to maintain a selected phase between the received signals and the reference sequences while said degree of correlation exists permitting recovery of the data signals, characterized in that the receiver includes means to predict correlation coefficients for the received signal sequences of the first and second kinds with the corresponding reference signal sequences at different relative phases, means to store corresponding actual correlation coefficients between a received signal sequence and the first and second reference sequences, and means to effect an analysis process in which the actual correlation coefficients stored are multiplied by the corresponding predicted correlation coefficients for both reference sequences, the two sets of resultant products, each set being related to a respective reference sequence, are individually summed, and the two summed sets are compared to determine the identity of the received signal sequence. By this technique of predicting the correlation coefficients and then using them in the processing of the actual correlation coefficients, account can be taken of known distortions in the communication system. The present invention also provides a receiver for use in a receiver for use in a line network communication system including a transmitter means capable of transmitting a series of data signals represented by signal sequences of first and second kinds on a baseband carrier signal, the receiver being arranged to operate in a search mode to compare received signals with first and second reference sequences at different relative phases until at least one comparison signal generated as a result of the comparisons indicates a degree of correlation exceeding a threshold value, the reference sequences corresponding to the signal sequences of the first and second kinds respectively, to operate in a tracking mode to verify that said degree of correlation exists for a preset time interval, and to operate, in dependence on a successful verification in the tracking mode, in a locking mode to maintain a selected phase between the received signals and the reference sequences while said degree of correlation exists permitting recovery of the data signals, characterized in that the receiver includes means to predict correlation coefficients for the received signal sequences of the first and second kinds with the corresponding reference signal sequences at different relative phases, means to store corresponding actual correlation coefficients between a received signal sequence and the first and second reference sequences, and means to effect an analysis process in which the actual correlation coefficients stored are multiplied by the corresponding predicted correlation coefficients for both reference sequences, the two sets of resultant products, each set being related to a respective reference sequence, are individually summed, and the two summed sets are compared to determine the identity of the received signal sequence. The present invention also provides a method of processing signals in a receiver for use in a line network communication system including a transmitter means capable of transmitting a series of data signals represented by signal sequences of first and second kinds on a baseband carrier signal and receiver means capable of receiving a transmitted signal and recovering therefrom said data signals, the method comprising, in a search mode comparing received signals with first and second reference sequences at different relative phases until at least one comparison signal generated as a result of the comparisons indicates a degree of correlation exceeding a threshold value, the reference sequences corresponding to the signal sequences of the first and second kinds respectively, in a tracking mode verifying that said degree of correlation exists for a preset time interval, and, in dependence on a successful verification in the tracking mode, in a locking mode to maintain a selected phase between the received signals and the reference sequences while said degree of correlation exists permitting recovery of the data signals, the method being characterized by predicting correlation coefficients for the received signal sequences of the first and second kinds with the corresponding reference signal sequences at different relative phases, storing corresponding actual correlation coefficients between a received signal sequence and the first and second reference sequences, and effecting an analysis process in which the actual correlation coefficients stored are multiplied by the corresponding predicted correlation coefficients for both reference sequences, the two sets of resultant products, each set being related to a respective reference sequence, are individually summed, and the two summed sets are compared to determine the identity of the received signal sequence. It should be noted that US-A-4550414 discloses a spread spectrum adaptive code tracker comprising a plurality of correlator channels which are parallel-fed with an incoming spread spectrum signal. The channels are fed with respective reference signals which are mutually phase-displaced replicas of the spread spectrum code. The gains of the various channels are individually weighted and a feedback arrangement is continuously employed to adaptively adjust the weights in such manner that each becomes representative of the degree of correlation present in the corresponding channel; the weights collectively become representative of the correlation function of the incoming signal with the spread spectrum code. At regular intervals during acquisition and tracking the array of channel weights is matched filtered to determine which channel is aligned with the incoming signal. The spread spectrum code associated with this channel is then used to decode the incoming signal. The impulse response of the matched filter is the autocorrelation of the incoming signal, which is known a priori. In order that the invention may more readily be understood, a description is now given, by way of example only, reference being made to the accompanying drawings of which Figures 1 to 5 show representations of correlation functions, and Figure 6 is an appropriate block circuit diagram of equipment embodying the present invention. Figures 1a, 1b and 2 show representations of the correlation process involving sequences of 11 chips length. Figure 2 shows a complete correlation function, some of the centre portion (indicated as M) forming the matched filter. The present invention embodies a "matched filter" method consisting of multiplying the eight bin values by numbers whose magnitudes are determined by the expected values of the bins (see Figure 3). The correlation function in the absence of noise can be predicted exactly, though it changes gradually with time. The filter coefficients are determined by the known shape of the correlation function, and also by any known changes introduced by the communication medium, enabling allowance to be made for a distorting system. Thus, as shown by Figure 3, the sum of the products is positive and at a maximum for no noise. As noise appears in the correlation function, the sum decreases and tends to an average of zero. The operation is performed on the data 0 and data 1 reference sequences, and the results correspond to determine which data bit has been received. Most communication systems have unsynchronised clocks. This means that, even when initial synchronisation of the transmitted and received signals is achieved, advantageously there is provision to monitor the relative drift in the clock frequencies and ensure that the zero offset position of the correlation function is aligned with the centre of the matched filter. The filter itself can be profitably used to do this, as can be seen by Figure 4, which shows a centroid estimate in a noisy correlation function. The centre position of the material filter in three different places is indicated by A, B and C of which B gives the maximum value, thereafter A, with C possibly being negative. The multiplication is performed in three or more positions on the correlation function and the greatest result chosen. This is then averaged with an appropriate number of previous results (eight for instance) to reduce the effects of noise on the estimate. Finally the multiplication is performed at the position represented by this averaged result, and the result is used to determine which data bit is received. The clock drift causes the contents of the bins to change gradually even when there is no noise, and the averaging process yields precision in the zero position estimate of greater than one bin (i.e. one eighth part in this example). For these reasons there need to be more filter coefficients than just one for each bin, and in this case eight per bin are used. Thus when the zero position is estimated as being between two bins the appropriate coefficients can be used, instead of spoiling the result by using an inaccurate matched filter. See Figure 5 for the change in shape of the filter with clock drift, which occurs because it is a sampled version of a continuous correlation function. This Figure illustrates the continous function and four samples per chip, giving four different filter shapes, the example having only five bins. Figure 6 shows a block diagram of a system which operates in accordance with the present invention, the elements within box D defined by the broken line constituting the analysis technique. The incoming signal from the transmission medium (e.g. mains) is correlated with the reference signal. The reference signal for data Ø and data 1 is adjusted in magnitude and phase in processor 1 to take account of any known distortions of the medium, and expected correlation results for both data Ø and data 1 are produced. These expected results are multiplied in shifter/multiplier 2 in the current position and shifted by one chip to left and right of the current position; the results are compared at comparator 3 in magnitude to determine the optimium position. This position is averaged at averager 4 with seven previous positions stored at storage 5 to give a more precise estimate of the correct position and to filter out the effects of noise. The results from processor 1 are then multiplied at shifter/multiplier 6 by the correlation results in this optimium position and the two results for data Ø and data 1 are compared at comparator 7 in magnitude to decide which data bit has been received. In a modification, the functions of at least one or more of the components illustrated in Figure 6 can be implemented by computer software. This system gives an improvement of up to 10 dB signal-to-noise ratio over the conventional system, without taking account of the transmission medium characteristics; further improvement can be obtained by incorporating these characteristics into the filter coefficients. Other embodiments of the present invention may adopt different numbers of bins and different numbers of previous results in the estimation can be used, depending on the noise level, clock drift rate and also processing power available.

Claims: A line network communication system including a transmitter means capable of transmitting a series of data signals represented by signal sequences of first and second kinds on a baseband carrier signal and receiver means capable of receiving a transmitted signal and recovering therefrom said data signals, the receiver means being arranged to operate in a search mode to compare received signals with first and second reference sequences at different relative phases until at least one comparison signal generated as a result of the comparisons indicates a degree of correlation exceeding a threshold value, the reference sequences corresponding to the signal sequences of the first and second kinds respectively, a track mode to verify that said degree of correlation exists for a preset time interval and, in dependence on a successful verification in the tracking mode, a locking mode to maintain a selected phase between the received signals and the reference sequences while said degree of correlation exists permitting recovery of the data signals, characterized in that the receiver includes means (1) to predict correlation coefficients for the received signal sequences of the first and second kinds with the corresponding reference signal sequences at different relative phases, means to store corresponding actual correlation coefficients between a received signal sequence and the first and second reference sequences, and means (6,7) to effect an analysis process in which the actual correlation coefficients stored are multiplied by the corresponding predicted correlation coefficients for both reference sequences, the two sets of resultant products, each set being related to a respective reference sequence, are individually summed, and the two summed sets are compared to determine the identity of the received signal sequence. A line network communication system according to claim 1 wherein, in the analysis process, the predicted correlation coefficients are used as a matched filter for the actual correlation coefficients, thereby to monitor clock drift. A receiver for use in a line network communication system including a transmitter means capable of transmitting a series of data signals represented by signal sequences of first and second kinds on a baseband carrier signal, the receiver being arranged to operate in a search mode to compare received signals with first and second reference sequences at different relative phases until at least one comparison signal generated as a result of the comparisons indicates a degree of correlation exceeding a threshold value, the reference sequences corresponding to the signal sequences of the first and second kinds respectively, to operate in a tracking mode to verify that said degree of correlation exists for a preset tune interval, and to operate, in dependence on a successful verification in the tracking mode, in a locking mode to maintain a selected phase between the received signals and the reference sequences while said degree of correlation exists permitting recovery of the data signals, characterized in that the receiver includes means (1) to predict correlation coefficients for the received signal sequences of the first and second kinds with the corresponding reference signal sequences at different relative phases, means to store corresponding actual correlation coefficients between a received signal sequence and the first and second reference sequences, and means (6,7) to effect an analysis process in which the actual correlation coefficients stored are multiplied by the corresponding predicted correlation coefficients for both reference sequences, the two sets of resultant products, each set being related to a respective reference sequence, are individually summed, and the two summed sets are compared to determine the identity of the received signal sequence. A receiver according to claim 3 wherein, in the analysis process, the predicted correlation coefficients are used as a matched filter for the actual correlation coefficients, thereby to monitor clock drift. A method of processing signals in a receiver for use in a line network communication system including a transmitter means capable of transmitting a series of data signals represented by signal sequences of first and second kinds on a baseband carrier signal and receiver means capable of receiving a transmitted signal and recovering therefrom said data signals, the method comprising, in a search mode comparing received signals with first and second reference sequences at different relative phases until at least one comparison signal generated as a result of the comparisons indicates a degree of correlation exceeding a threshold value, the reference sequences corresponding to the signal sequences of the first and second kinds respectively, in a tracking mode verifying that said degree of correlation exists for a preset time interval, and, in dependence on a successful verification in the tracking mode, in a locking mode to maintain a selected phase between the received signals and the reference sequences while said degree of correlation exists permitting recovery of the data signals, the method being characterized by predicting correlation coefficients for the received signal sequences of the first and second kinds with the corresponding reference signal sequences at different relative phases, storing corresponding actual correlation coefficients between a received signal sequence and the first and second reference sequences, and effecting an analysis process in which the actual correlation coefficients stored are multiplied by the corresponding predicted correlation coefficients for both reference sequences, the two sets of resultant products, each set being related to a respective reference sequence, are individually summed, and the two summed sets are compared to determine the identity of the received signal sequence.

Inventors: BURR ALISTER GRAHAM, YOUNG NICHOLAS ANDREW, BURR, ALISTER GRAHAM, YOUNG, NICHOLAS ANDREW
Applicants: EMI PLC THORN, THORN EMI PLC
Application year: 1987
Main classification: H04L23
IPC codes: H04B1, H04B3, H04L23