Three Types of Spread Spectrum Communications
Frequency hopping.
The signal is rapidly switched between different frequencies within the hopping
bandwidth pseudo-randomly, and the receiver knows before hand where to find the signal
at any given time.
Time hopping.
The signal is transmitted in short bursts pseudo-randomly, and the receiver knows
beforehand when to expect the burst.
Direct sequence.
The digital data is directly coded at a much higher frequency. The code is generated
pseudo-randomly, the receiver knows how to generate the same code, and correlates the
received signal with that code to extract the data.
Direct Sequence Spread Spectrum
FIG:-1
CDMA is a Direct Sequence Spread Spectrum system. The CDMA system works directly
on 64 kbit/sec digital signals. These signals can be digitized voice, ISDN channels,
modem data, etc.
Figure 1 shows a simplified Direct Sequence Spread Spectrum system. For clarity,
the figure shows one channel operating in one direction only.