Voice link over spread spectrum radio

  
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Communication systems using spread spectrum techniques, compare to narrow-band systems are more resistant for noises, signal fading, Doppler effect. Because of code division of signals (CDMA) we can more effective use frequency band and we can increase privacy of transmision. Wide-band systems can be used paralell with narrow-band systems using th
Voice link over spread spectrum radio - schematic

e same frequency. Process gain allows to reduce output power of transmitter. Major methods used in spread spectrum systems are: Rfequency hopping, time hopping, direct sequence or their hybrids. Spread spectrum signals can support any users signal by conventional analogue or digital modulation. However using AM modulation will destroy signals uniform power spectral density, and it will make more difficult to correlate PN sequence in receiver. Analogue modulated signals are possible to be demodulated without any knowledge of PN code. Frequency modulation is better with frequency hopping spread spectrum. When we use direct sequence spread spectrum, the most suitable will be digital modulation of PN code. We used bipolar phase shift keying. Analogue acoustic signal is converted to digital. Digitalised voice is EX-ORed with digital PN (pseudo-noice) sequence. Modified PN code drives DBM (double balanced mixer) configured as biphase shift keyer. (spreading spectrum). After amplification goes to antenna. In receiver wideband signal arter amplification is EX-Ored with original PN code (despreading spectrum). Narrow-band signal is BPSK modulated. After demodulation, digitalized voice is converted to analog. Main condition is to know PN code, its frequency and phase with precision better than ½ byte. o synchronisate PN codes three identical channels are used. Except main branch, we have 2 additional branches. Signal in LATE branch...



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