Spread Spectrum Questions And Answers
1. What is
spread spectrum communication, and how does it work?
If a system
meets all of the above criteria, it is classified as a spread spectrum
communication system.
1. The signal
takes up far more bandwidth than is required to transmit the information.
2. Spreading
is performed via a data-independent spreading signal, also known as a code
signal.
3. The
original data is recovered by comparing the received spread signal to a
synchronized copy of the spreading signal that was used to spread the data at
the receiver.
2.
Describe the advantages of spread spectrum systems?
Both military and civilian uses benefit from spread spectrum systems. The
advantages of using a spread spectrum system are stated below.
1) Suppression of interference
2) Multiple entry points
3) Lowering the energy density
4) High-resolution time and
5) Message privacy
3. What
does Pseudonoise Sequence mean?
A coded sequence of 1s and 0s with specific autocorrelation qualities is
known as a pseudo-noise (PN) sequence.
Both the transmitter and receiver are aware of the PN sequence, which is
a deterministic, periodic signal. It appears to have captured white noise's
statistical features. As a result, to an unauthorized listener, it looks to be
a random signal.
4. What are the qualities of randomness?
A random binary sequence is one in
which the existence of either the binary sign 1 or the binary symbol 0 is
equally likely. Many of the qualities of a genuinely random binary sequence are
shared by PN sequences. As a test for the appearance of randomness, every
periodic binary sequence can be subjected to three fundamental features. They
are 1) The attribute of balance 2) Correlation property and 3) Run property
5. Identify the property of balance.
The number of 1s is always one more
than the number of 0s throughout each period of the sequence. The Balance
property is what it is called.
6. Identify the Run Property.
One-half of the runs of 1s and 0s
in each period of the sequence is one length, one-fourth are two lengths,
one-eighth are three lengths, and so on. The property is called 'Run Property'.
7. Describe the property of correlation.
A sequence's autocorrelation
function is binary-valued and periodic. The correlation property is the name
for this property.
8. How do you make a pseudo-noise (PN) sequence?
In spread spectrum communications,
the class of sequences utilized is frequently periodic, meaning that a series
of 1s and 0s repeats itself perfectly with a defined time. A frequently used
periodic PN sequence is represented by the maximum length sequence, a form of
cyclic coding.
Maximum length sequences, also
known as PN sequences, may be easily constructed using shift register circuits
with feedback from one or more stages. N = 2m–1, where m is the
number of shift register stages, is the length of the PN sequence.
9. What are some of the drawbacks of using a spread spectrum
system?
1. Increased bandwidth for
transmission
2. The complexity of the system
3. Delay in processing
10. Explain how spread spectrum modulation methods are
classified.
I. Types of averaging systems
2. Avoidance type systems 1. Direct
Sequence Spread Spectrum (DS-SS) System
1. Time hopping system 2. Frequency-hopping
Spread Spectrum (DS-SS) system 3. Chirp
4. Hybrid Techniques
11. Define the DS-SS (Direct Sequence Spread Spectrum)
technique.
The employment of a PN sequence to
modulate a phase shift keyed signal allows immediate spreading of the
transmission bandwidth in Direct sequence spread spectrum (DS-SS) systems.
12. What are the Direct Sequence Spread Spectrum (DS-SS)
system's performance parameters?
1) Processing gain is an essential
performance parameter of the Direct Sequence Spread Spectrum (DS-SS)
technology. 2) Error Probability, and 3) Jamming Margin
13. Specify
the amount of processing gain.
The gain produced by processing a spread spectrum signal over an unspread
signal is the processing gain of a DS-SS system. It's also known as the ratio
of the spread-spectrum signal's bandwidth to the bandwidth of the unspread
signal.
Processing Gain (PG) = Spread Signal Bandwidth Unspread Signal Bandwidth
Also, PG = ๐๐๐๐, where Tb is the bit duration, Tc
is the Chip duration
14. Calculate the DS-SS BPSK system's likelihood of error.
The DS-SS BPSK system's probability
of error is
Pe = 1/2๐๐๐๐√(๐ธ๐/๐ฝ๐๐)
where Eb - energy per bit
J - Average interference power
Tc - Chip duration
15. Define a Jamming Margin
The average interference power J
divided by the average signal power Ps is known as the jamming margin.
Jamming Margin = ๐ฝ/๐๐ = ๐๐บ/๐ธ๐/๐0
where PG - Processing gain
๐ธ๐/๐0 - bit energy
to noise density ratio
16. Define the FH-SS (frequency hopping spread spectrum)
system.
The spectrum of the transmitted
signal is spread sequentially in frequency hopping spread spectrum (FH-SS)
systems by randomly hopping the data modulated carrier from one frequency to
the next.
17. Identify and describe the different forms of frequency
hopping.
Frequency hopping may be described
in two ways, both of which are independent of technology.
1. Slow frequency hopping
We have a slow hopped signal in a
frequency hopping (FH) system if the hopping is done at the symbol rate. The
chip rate is identical to the symbol rate in this case.
2. Fast Frequency hopping
If there are numerous hops per
symbol in a frequency hopping (FH) system, we get a quickly hopped signal. The
chip rate is higher than the symbol rate in this case.
18. Define diversity in frequency hopping.
Multiple broadcasts
of the same signal at various frequencies that are spaced apart in time are
referred to as diversity. A signal is built with several duplicate copies, each
broadcast on a different frequency, in frequency hopping with diversity. This
signal has a better chance of surviving than a single similar signal.
19. Compare and contrast slow and fast hopping systems.
20. Define the term "synchronization."
Synchronization is the process of
ensuring that the locally produced carrier at the receiver is in frequency and
phase synchronism with the carrier at the transmitter. For successful
functioning in spread spectrum communication systems, the broadcast and
received PN codes must be perfectly aligned.
21. Identify and define the steps in the synchronization
process.
Two methods are commonly used to
synchronize the locally generated spreading signal with the incoming spread
spectrum signal, They are namely:
1) Acquisition
The acquisition is the initial
stage, which entails aligning the two propagating signals imperfectly.
2) Tracking
The second stage, tracking, takes
over once the received spread-spectrum signal has been captured for fine
alignment.
22. List the acquisition and tracking schemes
There are three distinct sorts of acquisition schemes. They
are:
1. Serial Search Acquisition
2. Parallel Search Acquisition
3. Sequential Search Acquisition
Tracking comprises precision chip synchronization as well as
carrier phase tracking for coherent demodulation. The most popular tracking
loops are:
1. Delay-locked loop (DLL)
2. Tau-dither loop (TDL)
23. What is the definition of jamming?
Intentional disturbance in a
communication system is referred to as jamming. The signaling waveform should
be built in such a way that the jammer cannot obtain a significant jamming
advantage by selecting a waveform and approach.
24. Create a design for an Anti-Jam (AJ) communication
system.
1. The employment of direct
sequence and frequency hopping spread spectrum methods to create frequency
diversity.
2. Using time hoping to create
variety in time.
3. The utilization of a narrow beam
antenna for spatial discrimination.
25. Define ๐ฑ/๐บ Ratio
The (๐ฝ/๐)๐๐๐๐ ratio is a
merit metric that indicates how resistant a system is to interference. It is
provided by:
(๐ฝ/๐)๐๐๐๐ = ๐๐บ/(๐ธ๐/๐ฝ๐)๐๐๐๐
where J - Average Received Jammer Power
S - Received Signal Power
PG - Processing Gain
(๐ธ๐/๐ฝ๐)๐๐๐๐ - bit
energy per jammer noise power spectral density required for maintaining
the link at a specified error probability.
26. Define Antijam margin.
The safety buffer against a certain hazard is known as the
anti-jam (AJ) margin. It's described as
