- Efficient Spectrum
Utilization
- As has been observed in the past, as the number of users of commercial
two-way radios has grown, channel spacinghas been narrowed, and higher-frequency
spectra have had to be allocated to accommodate the requirements.
- Narrower channel spacing and higher operating frequencies necessitate
tighter frequency tolerance for both the transmitters and the receivers.
- The need to accommodate more users will continue to require higher and
higher frequency accuracies.
- Radio silence interval
- In telecommunications
radio silence is a status in which all fixed or
mobile radio stations in an area
stop transmitting. The radio
stations include anything capable of transmitting a radio signal.
Radio silence generally applies to the military, where any radio transmission
may reveal troop positions,
either audibly from the sound of talking, or by its use as a homing
signal.
- Radio silence can also be maintained for other purposes, such as for highly
sensitive radio astronomy
- Keeping the radio station off for long time requires a long autonomy period
of the local oscillator. The more stable theoscillator is the longer the
autonomy period.
- Secure Communication
- Secure communication describes means by which people can share information
with varying degrees of certainty and which that third parties cannot know what
was said. Other than communication spoken face to face out of possibility of
listening, for example, it is probably safe to say that no communication is
guaranteed secure in this sense, although practical limitations such as
legislation, resources and the sheer volume of communication are limiting
factors for surveillance. Nothing is ever truly secure.
- In a spread spectrum system, the transmitted signal is spread over a
bandwidth that is much wider than the bandwidth required transmitting the
information being sent (e.g., a voice channel of a few kHz bandwidths is spread
over many MHz). This is accomplished by modulating a carrier signal with the
information being sent, using a wideband pseudo noise (PN) encoding signal. A
spread spectrum receiver with the appropriate PN code can demodulate and extract
the information being sent. Those without the PN code may completely miss the
signal, or if they detect the signal, it appears to them as noise.
- Two of the spread spectrum modulation types are:
- Direct sequence, in which the carrier is modulated by a digital code
sequence.
- Frequency hopping, in which the carrier frequency jumps from frequency to
frequency, within some predetermined set, the order of frequencies being
determined by a code sequence. (see special heading on this subject)
- Advantages of spread spectrum systems include the following capabilities:
- Rejection of intentional and unintentional jamming
- Low probability of intercept (LPI)
- Selective addressing
- Multiple access
- High accuracy navigation and ranging
- Frequency Hopping
- It is possible to jam frequency hopping systems with the availability of
fast spectrum analyzers and synthesizers.
- If a jammer is fast enough, it can detect the frequency of transmission and
tune the jammer to that frequency well before the radio hops to the next
frequency.
- However, with a good enough clock, it is possible to defeat such ˇ°followerˇ±
jamming.
- As illustrated below, even a "perfect" follower jammer can be defeated if a
good enough clock is available. (A perfect jammer is defined here as one that
can identify the frequency of a received signal, tune a synthesizer to that
frequency, and transmit the jamming signal in zero time.)
- Because radio waves travel at the speed of light, the
radio-to-jammer-to-radio (R1 to J to R2) and radio-to-radio (R1 to R2)
propagation delays are 3.3 µs per km. Therefore, if the hopping rate is fast
enough for the propagation delay difference to be greater than 1/hop-rate,(i.e.,
if the radios can hop to the next frequency before the jamming signal reaches
the receiver), then the radios are jamming-proof (for follower jammers).
- In the example above, the propagation delays t1, t2, and tR imply that the
message duration tm be less than 30 µs. Since the clock accuracies required by
frequency hopping systems are usually 10% to 20% of tm, the allowed clock error
is about 6 µs. In a military environment, such accuracies can be maintained for
periods of hours and longer only with atomic clocks.
- To summarize: Transmitter s and receivers contain clocks which must be
synchronized; e.g., in a frequency hopping system, the transmitter and receiver
must hop to the same frequency at the same time. The faster the hopping rate,
the higher the jamming resistance, and the more accurate the clocks must be (see
special heading on this subject)
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