Spread Spectrum Flashcards
Spread spectrum definition ?
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
Why are spread spectrum techniques used ?
For establishing secure communications, increasing resistance to natural interference and jamming to prevent detection, to limit the power flux density in satellite down links
What is power flux density ?
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
Why do the military use spread spectrum techniques
Due to following features, the LPI, low probability of intercept
And
Anti Jam or AJ
What is sacrificed to gain signal to noise performance ?
Bandwidth
What two qualities do spread spectrum signals tend to have as they are spread out over such a wide bandwidth?
They tend to be noise like and difficult to detect
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?
They can occupy the same Radio frequency as a narrow band signal with little or no interference
What is a fundamental premise of spread spectrum techniques ?
In channels with narrow band noise, increasing the transmitted signals bandwidth results in an increased probability the the information with be received correctly
What is the performance increase in spread spectrum systems referred to as ?
What does that describe ?
Processing gain
Which is used to describe the received signals fidelity gained at the cost of bandwidth
What is processing gain ?
The ratio of the radio frequency bandwidth to the information rate of the transmitted signal
What does processing gain also relate to
The tolerance level of interference of a signal, which if reached will cease useful communications
What are the common types of spread spectrum techniques?
Direct sequence Frequency hopping Time hopping Hybrid Stacked carrier
How does a direct sequence spread spectrum technique work ?
It phase modulates a sine wave pseudorandomly with a continuous string of pseudo noise code symbols called chips.
What are the features of pseudo noise (PN) code symbols or chips?
The chip rate is much higher than the information signal but rate.
How are pseudo noise (PN) codes generated ?
By a shift register and module 2 added to the data for transmission
What is special about the dsss signal structure and the sequence of chips that is produced by a terminal and received by the receiver ?
It is unique to each communications link.
It is known as a priori
How does the receive terminal decode the signal ?
It uses the same PN pseudo noise sequence to reverse the Pn code, so as to reconstruct the original information
What term describes the techniques of transmitting several direct sequence spread spectrum signals ?
Code division multiple access
What are the most common types of codes used to spread spectrum signals
Maximum length Lrs (linear recursive sequence)
Gold codes
Composite codes
What is the most common sequence used in dsss, and what is the most useful ?
A LRS, linear reclusive sequence,
While a maximal length sequence is the most useful form of a Lrs
How are gold codes generated ?
They are created by adding two maximal length Lrs register generators.
The two outputs are modulo 2 added together to create the gold code
How are composite codes generated ?
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
Why do Direct sequence spread spectrum signals have a low probability of intercept ?
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
What is frequency hopping spread spectrum technique?
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
In frequency hop spread spectrum, what is a function of the PN pseudo noise code ?
The Order in which frequencies are occupied
What determines the rate of hopping in frequency hop spread spectrum ?
The information rate
How many carrier frequencies are selected at any one time
Only one frequency can be occupied at one time, and all the frequencies will eventually be selected during a predetermined interval
What determines the band width if a frequency hop spread spectrum signal ?
The highest and lowest frequencies of the frequency hopped carriers
In direct sequence spread spectrum techniques, how is the bandwidth determined ?
By the chip rate
How are frequency hoppers classified
As either slow or fast
What are the advantage to fast frequency hop spread spectrum systems ?
They are less vulnerable to jamming as they change frequency several times within each traffic bit
What are the advantages and disadvantages of a slow frequency hop spread spectrum systems ?
They are less expensive to produce and less susceptible to system errors
However they are more susceptible to jamming
How does time hop spread spectrum techniques work ?
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
What is time hop spread spectrum often combined with, in order to improve performance ?
Direct sequence or frequency hopping
What are hybrid spread spectrum techniques ?
Hey did signals are generated using two or more spread spectrum techniques
What are the most common hybrid signals ?
Frequency hopping and direct sequence
Time hopping and frequency hopping
Time hopping and direct sequence
What is a stacked carrier signal ?
It consists of numerous modulated carriers that transmit the same data
Why are stacked carrier signals primarily designed?
To over come signal fading problems within HF communications.
What are the advantages of stacked carrier signals ?
They improve reliability, as the effects of fading, jamming and inference are reduced
