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DIGITAL SPREAD SPECTRUM SYSTEMS

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DIGITAL SPREAD SPECTRUM SYSTEMS

ENG-737

Wright State University

James P. Stephens

- 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

- 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

- 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

FREQUENCY HOPPER RECEIVER

st(t)

ht(t)

Sync is usually based on time-of-day and correlation

1 . . . . .k

FREQUENCY HOPPER RECEIVER

- 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

FREQUENCY HOPPER RECEIVER

- 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)

FREQUENCY HOPPER RECEIVER

- 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)

- 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

SIMPLE DIRECT FREQUENCY SYNTHESIZER

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

- 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

- 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

BASIC NUMERICALLY CONTROLLED OSCILLATOR

DIRECT DIGITAL SYNTHESIZER

MULTIPLE CORRELATORS FOR FREQUENCY HOPPING ACQUISITION

3

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

- 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

- 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

- 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))

- Note if:
Gp = BWss / BWinf = BWss / Rinf

where Rinf = 1 / Td

Then Gp = TdBWss (time-bandwidth product)

- 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

- 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)

- 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

GT

dT

GR

Tx

PT

Rx

GJ

dJ

J

PJ

- 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

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

- Noise
- Barrage
- Partial Band
- Narrowband

- Tone
- Single
- Multiple
- Swept
- Pulsed

- Smart
- Synchronized (coherent repeater)
- Non-synchronized (spectral matching)
- Knowledge based

Digital signals are highly susceptible to gradual degradation

BER

SNR (Eb/N0)

Brute

Force

Jamming

Power Required to Jam Victim

Smart /

Responsive

Jamming

Knowledge Required About Victim

G3

G2

G1

WSS

FULL BAND NOISE

G3

G2

G1

WSS

PARTIAL BAND NOISE

G3

G2

G1

WSS

NARROW BAND NOISE

G3

G2

G1

WSS

STEPPED NOISE

G3

G2

G1

WSS

PARTIAL BAND TONES

G3

G2

G1

WSS

STEPPED TONES

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

:

For BPSK

modulation

where

For No+Jo

J/S = jamming/signal ratio

Gp = processing gain

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

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

for 0<p<1

Po for optimal Pe :

if Jo >>No

and 1

and

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

JAMMING STRATEGIES AGAINST FH

- 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

- Use error corrective coding with interleaving

Hopper Link

Jamming Link

Monitor Link

Transmitter

d2

d1

Jammer

d3

Receiver

Th

Jamming time

- 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)

+ tp + (1 - ) Th

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