3. The spread spectrum
technologies were initially
developed for the military
and intelligence
communities to overcome
interception and jamming.
4. Digital (telecommunication) signal is transmitted on a
bandwidth considerably larger than the frequency content
of the original information.
For FIXED TRANSMISSION POWER, a broader spectrum implies:
i. Lower power signal
ii. High spectral redundancy
Low signal power makes the communication signals difficult
to be intercepted and detected
High spectral density makes signals resistant to partial band-
jamming
5. Carrier is an unpredictable/ “pseudo-random” wide-
band signal
Bandwidth of carrier is much wider than the bandwidth of
the data modulation
Reception is accomplished by cross-correlation of the
received wide-band signal with a synchronously
generated replica of the wide-band carrier
9. FHSS uses a narrow band signal (less
than 1 MHz), which modulates a data
signal causing it to „hop‟ in random
and the hopping happens in a
pseudorandom „predictable‟
sequence in a regular time from
frequency to frequency which is
synchronized at both transmitting and
receiving ends.
10. The main difference now is that the
carrier frequency varies over
intervals in accordance to a
predetermined pseudo-random
pattern.
The evasive signal so generated
takes a broader spectrum and
becomes difficult to jam and
intercept.
11.
12. Though PAM, PSK and QAM schemes are more
efficient, M-ary modulation is used due to its less
complex non-coherent detection abilities
PN hopping pattern demands the reciever to
maintain phase coherence with every one of the
frequencies used in the hopping pattern.
FSK is allowed as it allows non-coherent detection
Ignoring the frequency converters, the system is no different from a simple digital communication system with an FSK modulator and demodulator [1]. The difference is created in this system by allowing carrier frequency hopping controlled at the transmitter by the pseudo-noise (PN) generator. To track the hopping carrier the receiver must utilize the same (PN) generator in synchronization with the transmitter PN generator. The mixer in the transmitter section serves as a frequency up-converter while in the receiver section it serves as frequency down-converter.
The motivation of preferring binary or M-ary FSK modulation schemes over the more efficient PAM, PSK and QAM schemes stems from its ability to utilize less complex non-coherent detection. The PN hopping pattern demands the receiver to maintain phase coherence with every one of the frequencies used in the hopping pattern. This requirement is difficult to satisfy during frequency hopping. Therefore FSK is employed in FHSS systems, since it allows non-coherent detection.
