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Goose > Spread Spectrum > Flashcards

Spread Spectrum Flashcards

1
Q

Spread spectrum definition ?

A

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

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2
Q

Why are spread spectrum techniques used ?

A

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

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3
Q

What is power flux density ?

A

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

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4
Q

Why do the military use spread spectrum techniques

A

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

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5
Q

What is sacrificed to gain signal to noise performance ?

A

Bandwidth

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6
Q

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

A

They tend to be noise like and difficult to detect

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7
Q

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?

A

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

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8
Q

What is a fundamental premise of spread spectrum techniques ?

A

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

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9
Q

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

What does that describe ?

A

Processing gain

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

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10
Q

What is processing gain ?

A

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

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11
Q

What does processing gain also relate to

A

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

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12
Q

What are the common types of spread spectrum techniques?

A 
Direct sequence 
Frequency hopping 
Time hopping 
Hybrid 
Stacked carrier
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13
Q

How does a direct sequence spread spectrum technique work ?

A

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

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14
Q

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

A

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

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15
Q

How are pseudo noise (PN) codes generated ?

A

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

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16
Q

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

A

It is unique to each communications link.

It is known as a priori

17
Q

How does the receive terminal decode the signal ?

A

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

18
Q

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

A

Code division multiple access

19
Q

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

A

Maximum length Lrs (linear recursive sequence)

Gold codes

Composite codes

20
Q

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

A

A LRS, linear reclusive sequence,

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

21
Q

How are gold codes generated ?

A

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

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

22
Q

How are composite codes generated ?

A

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

23
Q

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

A

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

24
Q

What is frequency hopping spread spectrum technique?

A

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

25
Q

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

A

The Order in which frequencies are occupied

26
Q

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

A

The information rate

27
Q

How many carrier frequencies are selected at any one time

A

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

28
Q

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

A

The highest and lowest frequencies of the frequency hopped carriers

29
Q

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

A

By the chip rate

30
Q

How are frequency hoppers classified

A

As either slow or fast

31
Q

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

A

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

32
Q

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

A

They are less expensive to produce and less susceptible to system errors

However they are more susceptible to jamming

33
Q

How does time hop spread spectrum techniques work ?

A

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

34
Q

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

A

Direct sequence or frequency hopping

35
Q

What are hybrid spread spectrum techniques ?

A

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

36
Q

What are the most common hybrid signals ?

A

Frequency hopping and direct sequence

Time hopping and frequency hopping

Time hopping and direct sequence

37
Q

What is a stacked carrier signal ?

A

It consists of numerous modulated carriers that transmit the same data

38
Q

Why are stacked carrier signals primarily designed?

A

To over come signal fading problems within HF communications.

39
Q

What are the advantages of stacked carrier signals ?

A

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

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