42. Mobile communication is burdened with particular propagation
complications, making reliable wireless communication more
difficult than fixed communication between and carefully
positioned antennas.
• The antenna height at a mobile terminal is usually very small,
typically less than a few meters. Hence, the antenna is expected to
have very little 'clearance', so obstacles and reflecting surfaces in
the vicinity of the antenna have a substantial influence on the
characteristics of the propagation path.
• The propagation characteristics change from place to place and,
if the terminal moves, from time to time.
43.
44.
45.
46.
47.
48.
49. Spread Spectrum
An increasingly popular form of encoding for wireless
communications is known as spread spectrum.
• The use of spread spectrum makes jamming & interception
more difficult and improves reception.
• The basic idea of spread spectrum is to modulate the signal so
as to increase significantly the bandwidth (spread the spectrum)
of the signal to be transmitted.
50. Spread Spectrum
A comment about pseudorandom numberspseudorandom numbers:
These numbers are generated by an algorithm using
some initial value called the seed. The algorithm is
deterministic and therefore produces sequences of
numbers that are not statistically random.
• The important point is that unless you know the
algorithm and the seed, it is impractical to predict the
sequence. Hence, only a receiver that shares this
information with a transmitter will be able to successfully
decode the signal.
51.
52. Frequency Hopping Spread Spectrum
The signal is broadcast over a seemingly random series of
radio frequencies, hopping from frequency to frequency at
split-second intervals.
A receiver, hopping between frequencies in synchronization
with the transmitter, picks up the message.
55. Direct Sequence Spread Spectrum
Each bit in the original signal is represented by multiple bits
in the transmitted signal using a spreading code known as
chipping code.
The chipping code spreads the signal across a wider
frequency band in direct proportion to the number of bits
used.
Therefore, a 10-bit chipping code spreads the signal across
a frequency band that is 10 times greater than a 1-bit
chipping code.
56. Direct Sequence Spread Spectrum
Note that an information bit of 1 inverts the pseudorandom bits in the
combination, while an information bit of 0 causes the pseudorandom
bits to be transmitted without inversion.
57. Direct Sequence Spread Spectrum
The amount of spreading that is achieved is a direct result of the data rate
of the pseudorandom stream; the greater the data rate of the
pseudorandom input, the greater the amount of spreading.
58. Direct Sequence Spread Spectrum
In this case, the information stream and the pseudorandom stream are
both converted to analog signals and then combined, rather than
performing the exclusive-or of the two streams and then modulating.
