Patent GB-1497858-A (GB)
Title: Ssma communications-transmission system
IPC: H04B7/216
Filed: 1971-07-07
Abstract: 1497858 Multiplexing LICENTIA PATENTVERWALTUNGS GmbH 7 July 1971 [7 July 1970] 29758/71 Heading H4M An SSMA transmission is decoded in a receiving station m which receives a signal mix by simulating the transmission functions f of the stations i#m, subtracting the simulated transmission functions f' i  from the received signal mix f e  or a reduced signal mix to form a resulting reduced signal mix f' e  and determining the desired message sequence K m  from the reduced signal mix f' e . In one embodiment (Fig. 1) the received signal mix f e  is fed to a correlator KO 1  from which binary characters k i  are obtained by correlation of the mix f e  with the address of station 1. Subsequently, the address of station 1 is again modulated with the characters k i  and impressed on a carrier in a modulator MO 1 . The signal is a simulated transmission function f' 1  which corresponds with the transmission function f at the transmitter of the station i. Passage of the signal mix f e  through the correlator and modulator imparts a delay to the output f' i . Simultaneously, the received signal mix f is passed through a parallel channel including a delay element V, which imparts a delay equal to that mentioned above. A subtraction element S, subtracts the signal f' from f e  to give a reduced signal mix which forms the input to a next, similar receiver stage which generates a simulated transmission function f' 2 . This is used in a subtraction element S 2  to give a further reduced signal mix for passage to a subsequent stage. As shown, however, there is no further stage, and the further reduced signal mix forms the final reduced signal mix This passes to a correlator KO m  from which an almost error- and disturbance-free message sequence K* m  is derived. Control lines L feed control signals from a control unit (not shown) to the correlators KO 1 , KO 2 , modulators MO 1 , MO 2  and delay elements V 1 , V 2  so as to minimize the power of the signals derived from the substation elements S 1 , S 2 . In a second embodiment (Fig. 2) the received signal mix f e  is fed simultaneously to parallel channels one of which incorporates a delay element V, and the others of which each include a correlator KO 1 , KO 2 ....and a modulator MO 1 , MO 2 ...which function as previously described to derive respective simulated transmission functions f' 1 , f' 2  .... The delays in all channels are equal and the subtraction element S 21  gives as an output the reduced signal mix which is used in a correlator KO m  as previously described. In another embodiment (Fig. 3, not shown) the separate series and parallel arrangements of the foregoing embodiments are combined. In a further embodiment (Fig. 4, not shown) any residual error in the message sequence K* m  is reduced by repeating the procedure followed in the embodiment of Fig. 1. Thus, after processing of the received signal mix f e  in a number of stages identical to those of the embodiment of Fig. 1, a further simulated transmission function (f' m ) is derived from a modulator (MO m ) following the correlator KO m  shown in Fig. 1. This further simulated transmission function (f' m ) passes to a subtraction element (S 41 ) where it is subtracted from the received signal mix f which has been passed through a suitable delay (V 41 ). The output of the subtraction element (S 41 ) is used in a further number of stages to derive simulated transmission functions (f" i )(i#m) which are subtracted from the reduced signal mix (f' e ) to provide a final reduced signal mix (f" e ) which is used to derive an error-and disturbance-free message sequence (K** m ). In other embodiments (Figs. 5 and 6, not shown) one or more feed-back stages are incorporated.