Computerized SPREAD SPECTRUM SYSTEMS .


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Computerized SPREAD Range Frameworks. ENG-737. Wright State College James P. Stephens. Recurrence Bouncing . Information is sent amid the stay time of a recurrence jumping radio Balance is normally Paired FSK The recurrence movement is little contrasted with the recurrence bounce focus recurrence channels
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Advanced SPREAD SPECTRUM SYSTEMS ENG-737 Wright State University James P. Stephens

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FREQUENCY HOPPING Data is sent amid the stay time of a recurrence jumping radio Modulation is normally Binary FSK The recurrence move is little contrasted with the recurrence bounce focus recurrence stations If the information is voice as in a strategic military radio or cordless phone, it is digitized by some advanced voice standard (vocoder) Various vocoders have been received, yet a typical discourse vocoder is known as CVSD (persistently factor, incline, delta) balance Often, forward blunder revision (FEC) is utilized, in any case, discourse can endure impressive interruption before discourse gets to be distinctly incomprehensible Speech information must be compacted to permit constant transmission amid time transmitter is transitioning to another recurrence

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FREQUENCY HOPPING Example CVSD discourse ASICs frequently utilize 16 kbps, regularly, for superb discourse If we wish to utilize recurrence bouncing, what amount of pressure must we utilize? Expect the channel data transmission (demodulator) can just bolster 20 kbps Then 16K/20K = 0.80 → 80% obligation cycle If we have to send 100 bits for every stay, what is our bounce rate? 100 bits (1/20K) = 5 ms (Dwell time) 5 ms/0.8 = 6.25 ms (Hop time) → 160 hps 6.25 ms 100 information bits 5 ms

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FREQUENCY HOPPING Clarifying Processing Gain A FH transmitter stays for a period t 1 (time per bounce) at each middle recurrence Hopping happens over M frequencies P G = T d BW ss = number of frequencies (M) ( for FH) Example: Assume bordering scope, BW ss = 20 MHz N = 1000 frequencies N = 10 log 1000 = 30 dB If 20 MHz/1000 = 20 kHz channel transfer speed (coterminous) P G = 20 MHz/20 KHz = 1000 = 30 dB But not all that if channels cover or are non-adjacent

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FREQUENCY HOPPER RECEIVER s (t) h (t) Sync is typically in view of time-of-day and relationship 1 . . . . .k

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FREQUENCY HOPPER RECEIVER The recurrence synthesizer yield is a grouping of tones of term T c , in this manner,  h (t) = Σ 2p(t – nT c ) cos(  n t +  n ) n = -  where p(t) is a unit adequacy beat of length T c beginning at time t = 0  n t and  n are the radian recurrence and stage amid the nth recurrence bounce interim The recurrence  n is taken from an arrangement of 2 k frequencies

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FREQUENCY HOPPER RECEIVER The transmitted flag is the information regulated transporter up-changed over to another recurrence (  0 +  n ) for each FH chip  s (t) = [ s d (t) Σ 2p(t – nT c ) cos(  n t +  n ) ] n = -  The transmitted power range is the recurrence convolution of S d (f) and H t (f)

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FREQUENCY HOPPER RECEIVER Example: FH, 250 hps, 2 ms stay time, 48 bits for each abide Hop time = 1/250 = 4 ms d s = 48/2 ms = 24 kbps (flagging rate amid a stay) d r = 48/4 ms = 12 kbps (channel rate throughput) Minimum dispersing for FSK tones are: 1/T = 24 kHz (non-lucid FSK) 1/2T = 48 kHz (intelligible FSK)

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FREQUENCY SYNTHESIZERS There are two crucial procedures for actualizing recurrence blend: Direct Indirect In the immediate usage, various frequencies are combined in different mixes to give the greater part of the entirety and distinction frequencies: Example: cos(2  1 ) cos(2  2 ) = 1/2 cos(2  ( 1 -  2 )) + 1/2 cos(2  ( 1 +  2 )) The choice is made based upon a computerized control word as to which channels pass the chose condition The immediate execution turns out to be exceptionally troublesome when an expansive number of frequencies must be utilized Size and weight of the channels are main considerations in the decision to utilize this method

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SIMPLE DIRECT FREQUENCY SYNTHESIZER

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BASIC ADD-AND-DIVIDE FREQUENCY SYNTHESIZER A control word chooses the entryway on f 2 – f m which are blended with a reference recurrence which more often than not determines the recurrence partition or separating

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INDIRECT SYNTHESIZERS Any synthesizer that utilizes a stage bolted circle is called an aberrant synthesizer The yield of the stage identifier is sifted and drives a variable controlled oscillator (VCO) The stage finder drives the oscillator toward the path important to roll out  = 0 Any improvement causes the VCO to alter in the inverse course, consequently keeping the gadget bolted to the information Frequency amalgamation is proficient by including a gap by-n hinder in the criticism way The VCO will bolt to a numerous of the reference chose by n

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BASIC INDIRECT FREQUENCY SYNTHESIZER The gap by-n is changed carefully by the code generator to choose another yield recurrence

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NUMERICALLY CONTROLLED OSCILLATORS (NCO) More late strategy of recurrence synthesizers are NCOs, likewise called "coordinate advanced synthesizers" (DDS) DDSs are accessible as ASICs, see addendum 9 in content NCO\'s are accessible as FPGA "centers", i.e. drop-in modules These gadgets essentially have a sinusoid put away into memory that is yielded when chosen. One such gadget utilizes a 32-bit tuning word to give 0.0291 Hz tuning determination and can change frequencies 23 million times each second, i.e.43 ns exchanging time These gadgets can control the stage, frequently with 5-bits, in augmentations of 180, 90, 45, 22.5, 11.25 degrees or mixes there of

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BASIC NUMERICALLY CONTROLLED OSCILLATOR

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DIRECT DIGITAL SYNTHESIZER

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MULTIPLE CORRELATORS FOR FREQUENCY HOPPING ACQUISITION

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3 2 Σ 1 0 MULTIPLE CORRELATORS FOR FREQUENCY HOPPING ACQUISITION Time Delay f 1 f 2 f 3 f 4 Let f 1 = 101 MHz f 2 = 107 MHz f 3 = 105 MHz f 4 = 103 MHz Outcomes

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REVISITING PROCESSING GAIN What is preparing pick up? From Peterson/Ziemer/Borth: " The measure of execution change that is accomplished using spread range is characterized as handling increase " That successfully implies that preparing addition is the distinction between a framework utilizing spread range and framework execution when not utilizing spread range. . .all else measure up to An estimation is: G p = BW ss/r i Some creators utilize different definitions Some framework advertisers utilize inappropriate definitions just to make their framework sound better than contenders The specific definition picked is of little outcome the length of it is comprehended that genuine framework execution is the essential concern

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REVISITING PROCESSING GAIN (Cont.) We could characterize handling pick up as: G p = t d/t c Where t d is the information bit time and t c is the chip time For the situation of recurrence jumping, a jammer or interferer can put the greater part of his vitality on a solitary narrowband flag, accordingly, if the flag bounces over M frequencies, the jammer must appropriate control over all M frequencies with 1/M watts on every recurrence Therefore, G p = M = BW ss/BW d (recurrence jumping) however, we should expect adjoining, non-covering frequencies If covering happens, G p is lessened in light of the fact that the jammer can influence execution in contiguous channels. Along these lines G p must be lessened by the measure of the cover If non-coterminous, G p > M if jammer does not know framework channelization since power will be squandered in locales where container never transmits

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REVISITING PROCESSING GAIN (Cont.) Sklar characterizes preparing pick up as: " How much insurance spreading can give against meddling sign limited power " Spread range circulates a moderately low-dimensional flag into an extensive dimensional flag space The flag is accordingly "concealed" so to talk in the flag space since the jammer does not know how to discover it Dixon, however is not exceptionally reliable: Page 6 – " A flag to-commotion advantage picked up by balance and demodulation process is called handle pick up " Page 10 – " What is truly implied by G p in spread range is really sticking edge " G p = BW ss/BW inf (which accept BW inf = R inf (1 Hz/bit))

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REVISITING PROCESSING GAIN (Cont.) Note if: G p = BW ss/BW inf = BW ss/R inf where R inf = 1/T d Then G p = T d BW ss (time-transmission capacity item)

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REVISITING PROCESSING GAIN (Cont.) Example: Assume adjoining scope for a recurrence bouncing radio BW ss = 20 MHz, N = 1000 frequencies G p = N = 10 log 1000 = 30 dB If 20x10 6/1000 = 20 kHz channelization G p = 20x10 6/20x10 3 = 1000 = 30 dB But not proportionate if channels cover or are non-bordering

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COUNTERMEASURES To meddle with the foe\'s compelling utilization of the electromagnetic range Communications sticking includes the disturbance of data, i.e. voice, video, advanced summon/control signals Rule One : Jam recipient, not the transmitter Electronic Attack (EA)

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JAMMING MARGIN when all is said in done, the central point which impact conveying in a sticking situation are: Processing Gain Antenna pick up (Tx, Rx, and jammer) Power (Tx and jammer) Receiver affectability and execution Geometrical channel Item 5 manages issues, for example, directivity and observable pathway highlights. Versatile cluster preparing and invalid controlling are utilized to pick up directivity favorable circumstances over a jammer or gathering of jammers

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G T d T G R Tx P T Rx G J d J P J SIGNAL-TO-JAMMING RATIO Assume the jammer control rules warm clamor (AWGN) The free-space proliferation condition is: (S/J) R = P T G T G R d J 2/P J G J d T 2 G R is the proportion of pick up toward the correspondence transmitter to pick up in the jammer course

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SIGNAL-TO-JAMMING RATIO (Cont.) Since, (E b/J o ) = (S/J) R P G Where, (S/J) R = the got flag vitality to-commotion control ghastly thickness proportion Then, (E b/J o ) min required to accomplish a worthy P E execution must fulfill: (E b/J o ) min  P T G T G R P G d J 2/P J G J d T 2 Therefore, to enhance execution we can build P T , G T , G R , P G

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