Spread Spectrum

Question 1

Spread spectrum definition ?

Answer 1

Methods by which electromagnetic energy generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth

Question 2

Why are spread spectrum techniques used ?

Answer 2

For establishing secure communications, increasing resistance to natural interference and jamming to prevent detection, to limit the power flux density in satellite down links

Question 3

What is power flux density ?

Answer 3

Power flux density is the quantity that describes the rate at which energy is transferred by electromagnetic radiation through a real or virtual surface, per unit area per unit wavelength

Question 4

Why do the military use spread spectrum techniques

Answer 4

Due to following features, the LPI, low probability of intercept
And
Anti Jam or AJ

Question 5

What is sacrificed to gain signal to noise performance ?

Answer 5

Bandwidth

Question 6

What two qualities do spread spectrum signals tend to have as they are spread out over such a wide bandwidth?

Answer 6

They tend to be noise like and difficult to detect

Question 7

Due to the width of a spread spectrum signal, they require a much lower power density than narrow band signals, what advantage does this give spread spectrum signals?

Answer 7

They can occupy the same Radio frequency as a narrow band signal with little or no interference

Question 8

What is a fundamental premise of spread spectrum techniques ?

Answer 8

In channels with narrow band noise, increasing the transmitted signals bandwidth results in an increased probability the the information with be received correctly

Question 9

What is the performance increase in spread spectrum systems referred to as ?

What does that describe ?

Answer 9

Processing gain

Which is used to describe the received signals fidelity gained at the cost of bandwidth

Question 10

What is processing gain ?

Answer 10

The ratio of the radio frequency bandwidth to the information rate of the transmitted signal

Question 11

What does processing gain also relate to

Answer 11

The tolerance level of interference of a signal, which if reached will cease useful communications

Question 12

What are the common types of spread spectrum techniques?

Answer 12

Direct sequence 
Frequency hopping 
Time hopping 
Hybrid 
Stacked carrier

Question 13

How does a direct sequence spread spectrum technique work ?

Answer 13

It phase modulates a sine wave pseudorandomly with a continuous string of pseudo noise code symbols called chips.

Question 14

What are the features of pseudo noise (PN) code symbols or chips?

Answer 14

The chip rate is much higher than the information signal but rate.

Question 15

How are pseudo noise (PN) codes generated ?

Answer 15

By a shift register and module 2 added to the data for transmission

Question 16

What is special about the dsss signal structure and the sequence of chips that is produced by a terminal and received by the receiver ?

Answer 16

It is unique to each communications link.

It is known as a priori

Question 17

How does the receive terminal decode the signal ?

Answer 17

It uses the same PN pseudo noise sequence to reverse the Pn code, so as to reconstruct the original information

Question 18

What term describes the techniques of transmitting several direct sequence spread spectrum signals ?

Answer 18

Code division multiple access

Question 19

What are the most common types of codes used to spread spectrum signals

Answer 19

Maximum length Lrs (linear recursive sequence)

Gold codes

Composite codes

Question 20

What is the most common sequence used in dsss, and what is the most useful ?

Answer 20

A LRS, linear reclusive sequence,

While a maximal length sequence is the most useful form of a Lrs

Question 21

How are gold codes generated ?

Answer 21

They are created by adding two maximal length Lrs register generators.

The two outputs are modulo 2 added together to create the gold code

Question 22

How are composite codes generated ?

Answer 22

They are generated by modulo 2 two maximal length sequences.

The length will be equal to the least common multiple of the periods of the two sequences

Question 23

Why do Direct sequence spread spectrum signals have a low probability of intercept ?

Answer 23

Because the power density is very low, which means the signal to noise ratio at the receiver will be very low, and may be below the noise floor level

Question 24

What is frequency hopping spread spectrum technique?

Answer 24

It is used to generate many possible carrier frequencies over a large RF bandwidth

Allowing it to hop from frequency to frequency over a wide RF bandwidth

Question 25

In frequency hop spread spectrum, what is a function of the PN pseudo noise code ?

Answer 25

The Order in which frequencies are occupied

Question 26

What determines the rate of hopping in frequency hop spread spectrum ?

Answer 26

The information rate

Question 27

How many carrier frequencies are selected at any one time

Answer 27

Only one frequency can be occupied at one time, and all the frequencies will eventually be selected during a predetermined interval

Question 28

What determines the band width if a frequency hop spread spectrum signal ?

Answer 28

The highest and lowest frequencies of the frequency hopped carriers

Question 29

In direct sequence spread spectrum techniques, how is the bandwidth determined ?

Answer 29

By the chip rate

Question 30

How are frequency hoppers classified

Answer 30

As either slow or fast

Question 31

What are the advantage to fast frequency hop spread spectrum systems ?

Answer 31

They are less vulnerable to jamming as they change frequency several times within each traffic bit

Question 32

What are the advantages and disadvantages of a slow frequency hop spread spectrum systems ?

Answer 32

They are less expensive to produce and less susceptible to system errors However they are more susceptible to jamming

Question 33

How does time hop spread spectrum techniques work ?

Answer 33

They us a pseudonoise sequence to produce an on off keyed transmission. The information and the PN sequence are combined using modulo 2 addition. The transmitter will turn on for a 1 and off for a 0

Question 34

What is time hop spread spectrum often combined with, in order to improve performance ?

Answer 34

Direct sequence or frequency hopping

Question 35

What are hybrid spread spectrum techniques ?

Answer 35

Hey did signals are generated using two or more spread spectrum techniques

Question 36

What are the most common hybrid signals ?

Answer 36

Frequency hopping and direct sequence Time hopping and frequency hopping Time hopping and direct sequence

Question 37

What is a stacked carrier signal ?

Answer 37

It consists of numerous modulated carriers that transmit the same data

Question 38

Why are stacked carrier signals primarily designed?

Answer 38

To over come signal fading problems within HF communications.

Question 39

What are the advantages of stacked carrier signals ?

Answer 39

They improve reliability, as the effects of fading, jamming and inference are reduced