SOURCE GENERATOR LOW-PASS SIGNAL FILTER TRANSMITTER RECEIVER Figure 2.25. JPL's first attempt at a stored reference spread-spectrum design is shown here. This particular system uses one unmodulated noise signal N2 for synchronizing the receiver's pseudonoise generator and another N for carrying data. Most SR-SS systems do not use a separate signal for SS signal synchronization. (Redrawn from [167].) Early Spread-Spectrum Systems 29 31 Time Displacement Length of Volue of Sequence Value of MULTIPLIER OUTPUT (PN SIGNAL) 32767 32767 TYPICAL 32767 OUTPUT 131071 13107 262 43 104 857 Sm -1 Figure 2.26(a). The type-I PN generator uses a multiplier to combine the outputs of binary (+1or-1) signal shapers which in turn are driven by the outputs of relatively prime frequency dividers operating on the same sinusoid. the component square waves and the resultant PN product signal are shown here. (b) JPL's type-II generator was an m-stage linear-feedback shift register which produced binary (0 or 1) sequences of max- imum period. the output of the mth and nth stages are added modulo 2 to produce the input of the first stage S1. (c) This first list of connections for the type-II generator was produced at JPL by hand and computer search. (Diagrams and table redrawn from [167].) size and weight. The type-II PN generator was based on the equation where t represents time, m and n are integers (m represents a time displacement greater than n), and the functions x(t+m), x(t), and x(t + n) may equal 1 If the correct values of m and n are chosen, the period before repeat is 2m - 1 The correlation function of all type-II PN generators consists of a triangle of height unity and of a width equal to twice the shift time standing on a block of height (2m - 1)-¹. The Historical Origins of Spread-Spectrum Communications This origination of an almost perfect spike-like autocorrelation function, accompanied by descriptions of shift register hardware, positive results of baseband synchronization experiments at -20 dB SNR's and a table of suit- a