Patent GB-1320911-A (GB)
Title: Electrical signalling systems
IPC: H04L23/02
Filed: 1969-07-28
Abstract: 1320911 Multiplex transmission PLESSEY TELECOMMUNICATIONS RESEARCH Ltd 21 July 1970 [28 July 1969 30 Dec 1969] 37750/69 and 63366/69 Heading H4R A receiver for signals which are split into groups, each of which contains a predetermined number of linearly independent signal elements with corresponding signal elements in each group being related to each other and forming a series of signal elements, has a correlation detection device tuned to receive each series of signal elements, from the output of which output signals representative of each series of signal elements can be derived. In a system described but not claimed cross talk between the different signal series is reduced or eliminated by interconnection of the correlation detector output by controllable gain amplifiers which cancel such cross talk. An iterative detector according to the invention, Fig. 2, has the correlator outputs fed to integrators I1, I2, I3, the outputs of which are multiplied by the overall signal transfer function of the system;  including the respective signal shaping used to differentiate between the signal elements of the group so that the received signal is built up at the output of the adder 14. The adder output is subtracted from the received signal at the input to the correlation detectors so that the system, after a number of repetitions of the input signal adjusts itself to the condition where the input to the correlators is zero and the output of the integrators is the correct detected signal. Clamps CL1, CL2 etc. can be applied to the integrator outputs to limit the signal excursion, and increase the tolerance to noise while possibly reducing the convergence rate slightly. Alternatively the clamps may be replaced by constraint units, Fig. 3 (not shown), which allow only two possible output signal valves, this increases the convergence rate but restricts the range of signals which may be employed. In a further alternative the integrators and clamps are replaced by stores (ST1-STm) Fig. 4 (not shown) with the stores being updated either sequentially, or in some other order, cyclically or otherwise, e.g. in response to the maximum difference between input and output of the store, or the maximum input to the store, etc. An alternative system, Fig. 8, uses only one correlation detector and treats the signal-elements of the group sequentially. In Fig. 8 the received signal has n samples, covering the period of transmission of the m signal elements, fed into the shift register SR1. The overall-signal transfer function Y is stored, as Yn + m - 1 samples in the shift register SR3 and the detected received signal X stored in the shift register SR2. The transmitted signal is such that the overall signal transfer function of each signal element in a group is the same as any other signal element with a respective time shift, allowing the use of the same stored function in register SR3 for all the signal elements. The correlation detector is formed by the multipliers CDM1-CDMn and the adder A, which correlates the stored sample values of the received signal in register SR1 with the signal transfer function in register SR3. After each correlation operation a correction signal #x is determined, which is used to up date the respective received detected signal value x in shift register SR2, and applied via the respective multiplier 0M1-OMn, where they are multiplied by the appropriate transfer function sample, to be summed and subtracted from the received signal samples at the correlator input to cause the iterative adjustment of the detected signal in the shift register SR2. After the correlator output has been adjusted to zero in respect of each of the signal elements in a group the detected elements are read out of register SR2, the stage of SR2 are reset to zero, and a new set of samples of the received signal are fed into the shift register SR1 ready for decoding a further group of signals. In a system where the overall transfer function is liable to vary with time the values of Y stored in register SR3 may be up dated periodically by connecting the received signal shift register SR1 in a feedback loop as shown in Fig. 7. The detected values of the received signal are used to set the feedback connections z 1 -zg + 1 and subtraction circuits are interposed between each of the last g + 1 register stages to subtract the feedback signal from the signal passing through the shift register. It is shown that the signal fed into the register OSR is an up-dated version of the overall signal transfer function which dan be substituted for that stored in the register SR3 in Fig. 8. In an alternative arrangement for estimating the signal transfer function using the apparatus of Fig. 8, the corrected values of the received signal, i.e. - 1 or + 1 for each signal element, are set into register SR3, the received signal is entered in SR1 and in SR2 are set the last estimated values of Y, the signal transfer function. The estimates of Y are adjusted during the iteration process. The precise mechanism of obtaining the estimate depends on the symmetry of Y and the received signal. Preferably each detection process for establishing a value for X, i.e. x 1 , x 2 , -x m , is followed by a detection process to up date the estimate of Y, i.e. y 1  + y 2 -y n  + m - 1.