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## PowerPoint Slideshow about 'DIGITAL SPREAD SPECTRUM SYSTEMS' - thi

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FREQUENCY HOPPING

- Data is sent during the dwell time of a frequency hopping radio
- Modulation is typically Binary FSK
- The frequency shift is small compared to the frequency hop center frequency channels
- If the data is voice as in a tactical military radio or cordless telephone, it is digitized according to some digital voice standard (vocoder)
- Various vocoders have been adopted, but a common speech vocoder is known as CVSD (continuously variable, slope, delta) modulation
- Often, forward error correction (FEC) is employed, however, speech can tolerate considerable disruption before speech becomes unintelligible
- Speech data must be compressed to allow continuous transmission during time transmitter is transitioning to a new frequency

FREQUENCY HOPPINGExample

- CVSD speech ASICs often use 16 kbps, typically, for high quality speech
- If we wish to use employ frequency hopping, how much compression must we use?
- Assume the channel bandwidth (demodulator) can only support 20 kbps
- Then 16K/20K = 0.80 → 80% duty cycle
- If we need to send 100 bits per dwell, what is our hop rate?
- 100 bits (1/20K) = 5 ms (Dwell time)
- 5 ms / 0.8 = 6.25 ms (Hop time) → 160 hps

6.25 ms

100 data bits

5 ms

FREQUENCY HOPPINGClarifying Processing Gain

- A FH transmitter dwells for a period t1(time per hop) at each center frequency
- Hopping takes place over M frequencies
- PG = Td BWss = number of frequencies (M) ( for FH)
- Example:
- Assume contiguous coverage, BWss = 20 MHz
- N = 1000 frequencies
- N = 10 log 1000 = 30 dB
- If 20 MHz / 1000 = 20 kHz channel bandwidth (contiguous)
- PG = 20 MHz / 20 KHz = 1000 = 30 dB
- But not so if channels overlap or are non-contiguous

- The frequency synthesizer output is a sequence of tones of duration Tc, therefore,

ht(t) = Σ2p(t – nTc) cos(nt + n )

n = -

where p(t) is a unit amplitude pulse of duration Tc starting at time t = 0

nt and n are the radian frequency and phase during the nth frequency hop interval

The frequency n is taken from a set of 2k frequencies

- The transmitted signal is the data modulated carrier up-converted to a new frequency ( 0 + n ) for each FH chip

st(t) = [ sd(t) Σ2p(t – nTc) cos(nt + n ) ]

n = -

- The transmitted power spectrum is the frequency convolution of Sd (f) and Ht(f)

- Example:
- FH, 250 hps, 2 ms dwell time, 48 bits per dwell
- Hop time = 1 /250 = 4 ms
- ds = 48 / 2 ms = 24 kbps (signaling rate during a dwell)
- dr = 48 / 4 ms = 12 kbps (channel rate throughput)
- Minimum spacing for FSK tones are:
- 1 / T = 24 kHz (non-coherent FSK)
- 1 / 2T = 48 kHz (coherent FSK)

FREQUENCY SYNTHESIZERS

- There are two fundamental techniques for implementing frequency synthesis:
- Direct
- Indirect
- In the direct implementation, a number of frequencies are mixed together in various combinations to give all of the sum and difference frequencies:

Example:

cos(21) cos(22) = 1/2 cos(2 (1- 2)) + 1/2 cos(2 (1+ 2))

- The selection is made based upon a digital control word as to which filters pass the selected tone
- The direct implementation becomes very difficult when a large number of frequencies must be used
- Size and weight of the filters are major factors in the choice to use this technique

BASIC ADD-AND-DIVIDE FREQUENCY SYNTHESIZER

A control word selects the gate on f2 – fm which are mixed with a reference frequency which usually specifies the frequency separation or spacing

INDIRECT SYNTHESIZERS

- Any synthesizer that employs a phase-locked loop is called an indirect synthesizer
- The output of the phase detector is filtered and drives a variable controlled oscillator (VCO)
- The phase detector drives the oscillator in the direction necessary to make = 0
- Any change causes the VCO to change in the opposite direction, thereby keeping the device locked to the input
- Frequency synthesis is accomplished by adding a divide-by-n block in the feedback path
- The VCO will lock to a multiple of the reference selected by n

BASIC INDIRECT FREQUENCY SYNTHESIZER

The divide-by-n is changed digitally by the code generator to select another output frequency

NUMERICALLY CONTROLLED OSCILLATORS (NCO)

- More recent technique of frequency synthesizers are NCOs, also called “direct digital synthesizers” (DDS)
- DDSs are available as ASICs, see appendix 9 in text
- NCO’s are available as FPGA “cores”, i.e. drop-in modules
- These devices simply have a sinusoid stored into memory that is outputted when selected.
- One such device uses a 32-bit tuning word to provide 0.0291 Hz tuning resolution and can change frequencies 23 million times per second, i.e.43 ns switching time
- These devices can control the phase, often with 5-bits, in increments of 180, 90, 45, 22.5, 11.25 degrees or combinations there of

2

Σ

1

0

MULTIPLE CORRELATORS FOR FREQUENCY HOPPING ACQUISITION

Time Delay

Delay

f1

f2

f3

f4

Let f1 = 101 MHz

f2 = 107 MHz

f3 = 105 MHz

f4 = 103 MHz

Outcomes

REVISITING PROCESSING GAIN

- What is processing gain?
- From Peterson / Ziemer / Borth:

“The amount of performance improvement that is achieved through the use of spread spectrum is defined as processing gain”

- That effectively means that processing gain is the difference between a system using spread spectrum and system performance when not using spread spectrum. . .all else equal
- An approximation is:

Gp = BWss / ri

- Some authors use other definitions
- Some system marketers use improper definitions just to make their system sound superior to competitors
- The particular definition chosen is of little consequence as long as it is understood that real system performance is the primary concern

REVISITING PROCESSING GAIN (Cont.)

- We could define processing gain as:

Gp = td / tc

Where td is the data bit time and tc is the chip time

- In the case of frequency hopping, a jammer or interferer can place all of his energy on a single narrowband signal, therefore, if the signal hops over M frequencies, the jammer must distribute power over all M frequencies with 1/M watts on each frequency
- Therefore, Gp = M = BWss / BWd (frequency hopping)

however, we must assume contiguous, non-overlapping frequencies

- If overlapping occurs, Gp is reduced because the jammer can affect performance in adjacent channels. Thus Gp must be reduced by the amount of the overlap
- If non-contiguous, Gp> M if jammer does not know system channelization since power will be wasted in regions where hopper never transmits

REVISITING PROCESSING GAIN (Cont.)

- Sklar defines processing gain as:

“How much protection spreading can provide against interfering signal with finite power”

- Spread spectrum distributes a relatively low-dimensional signal into a large-dimensional signal space
- The signal is thereby “hidden” so to speak in the signal space since the jammer does not know how to find it
- Dixon, however is not very consistent:

Page 6 – “A signal-to-noise advantage gained by modulation and demodulation process is called process gain”

Page 10 – “What is really meant by Gp in spread spectrum is actually jamming margin”

Gp = BWss / BWinf (which assumes BWinf = Rinf (1 Hz/bit))

REVISITING PROCESSING GAIN (Cont.)

- Note if:

Gp = BWss / BWinf = BWss / Rinf

where Rinf = 1 / Td

Then Gp = TdBWss (time-bandwidth product)

REVISITING PROCESSING GAIN (Cont.)

- Example:

Assume contiguous coverage for a frequency hopping radio

BWss = 20 MHz, N = 1000 frequencies

Gp = N = 10 log 1000 = 30 dB

If

20x106 / 1000 = 20 kHz channelization

Gp = 20x106 / 20x103 = 1000 = 30 dB

But not equivalent if channels overlap or are non-contiguous

COUNTERMEASURES

- To interfere with the enemy’s effective use of the electromagnetic spectrum
- Communications jamming involves the disruption of information, i.e. voice, video, digital command/control signals
- Rule One: Jam receiver, not the transmitter

Electronic Attack (EA)

JAMMING MARGIN

- In general, the major factors which influence communicating in a jamming environment are:
- Processing Gain
- Antenna gain (Tx, Rx, and jammer)
- Power (Tx and jammer)
- Receiver sensitivity and performance
- Geometrical channel
- Item 5 deals with issues such as directivity and line-of-sight features. Adaptive array processing and null steering are used to gain directivity advantages over a jammer or group of jammers

dT

GR

Tx

PT

Rx

GJ

dJ

J

PJ

SIGNAL-TO-JAMMING RATIO- Assume the jammer power dominates thermal noise (AWGN)
- The free-space propagation equation is:

(S/J)R = PTGTGRdJ2 / PJGJdT2

- GR is the ratio of gain in the direction of the communication transmitter to gain in the jammer direction

SIGNAL-TO-JAMMING RATIO (Cont.)

Since,

(Eb/Jo) = (S/J)RPG

Where,

(S/J)R = the received signal energy-to-noise power spectral density ratio

Then,

(Eb/Jo) min required to achieve an acceptable PE performance must satisfy:

(Eb/Jo) min PTGTGR PG dJ2 / PJGJdT2

Therefore, to improve performance we can increase PT, GT, GR, PG, or dJ

Or decrease PJ, GJ, or dT

JAMMING STRATEGIES

- Noise
- Barrage
- Partial Band
- Narrowband
- Tone
- Single
- Multiple
- Swept
- Pulsed
- Smart
- Synchronized (coherent repeater)
- Non-synchronized (spectral matching)
- Knowledge based

PROBABILITY OF BER VERSUS SNR

Digital signals are highly susceptible to gradual degradation

BER

SNR (Eb/N0)

KNOWLEDGE – POWER RELATIONSHIP IN JAMMING

Brute

Force

Jamming

Power Required to Jam Victim

Smart /

Responsive

Jamming

Knowledge Required About Victim

DSSS IMMUNITY TO WIDEBAND NOISE

Noise jammer rejected by receiver

- Least power efficient technique but more covert than CW
- Requires no knowledge of signal
- High collateral damage (fratricide)
- Jamming power may be adjusted for gradual degradation

DSSS Performance in Broadband Noise Jamming

:

For BPSK

modulation

where

For No+Jo

J/S = jamming/signal ratio

Gp = processing gain

DSSS IMMUNITY TO CW

CW Interferer rejected by receiver

- Requires high power to overcome DSSS processing gain
- More power efficient than wideband noise
- Non-covert, target may employ filter to remove jammer

DSSS Performance in Tone Jamming

N = Processing gain S = signal power

Pt = noise power Tb= data bit duration

= phase angle difference between jammer and target signal

= frequency difference between jammer and target signal

Pj = power of jammer tone

JAMMING STRATEGIES AGAINST DSSS

- Most effective (non-adaptive) technique is provided by single-tone jammer at or near the carrier frequency
- This stresses the carrier suppression of balanced demodulators
- CCM
- Use an adaptive notch filter to delete the tone
- Detect the tone by a PLL and then subtract it from the signal or spatially null the jammer
- Decipher the PN code, replicate it as a jamming signal which will not be eliminated by the processing gain
- Most effective if jammer can become synchronized to the receiver
- CCM
- Make the PN code generators programmable so that the code can be readily changed or use complex, adaptive, codes

JAMMING STRATEGIES AGAINST DSSS (Cont.)

- Determine the carrier frequency and chip rate, then jam with a PN signal having these parameters (spectral matching)
- Less effective than 1) or 2), but more difficult to counter
- CCM - Use an adaptive code rates (ditter)
- Attack the acquisition process using a combination of 1) or 3)
- CCM – Use short code for quick acquisition, then switch to longer code
- Pulse jamming and swept jamming at the carrier frequency
- Generally less effective than other methods
- Can be vary effective against AGC and tracking loops of target receiver if knowledge of receiver design is known
- CCM – Use interleaving and error corrective coding

- Repeater jamming which involves intercepting signal, determining the center frequency, and transmitting a tone at that carrier frequency
- Very effective against slower FH systems
- CCM
- Increase hop rate
- Partial band or multitone
- Jammer places a series of tones across bandwidth where the received power per jamming tone exceeds the system’s received power per hop
- CCM
- Use error corrective coding with interleaving
- Swept frequency
- Increases the BER, but is less effective than 1) or 2)
- CCM
- Use error corrective coding with interleaving

Note: Generally speaking, FH systems are less susceptible to attacks on acquisition than are DSSS

d2

d1

Jammer

d3

Receiver

Th

Jamming time

GEOMETRY FOR FREQUENCY HOP REPEAT JAMMER- Th is the hopping period and is the fraction of hopping period within which the jammer must operate to be effective (Typically 50% of the dwell time)

c c

GEOMETRY FOR FREQUENCY HOP REPEAT JAMMER

For jamming to be effective we must have:

d2 + d3 d1

Propagation time for Jammer

Where,

tp = jammer processing time

c = speed of light (3 x 108 m/sec)

(1 - ) = fraction of dwell to be jammed

Source: Modern Communications Jamming Principles and Techniques - Poisel

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