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![31RF - Cellcom courseDr. Moshe Ran08/01/14
Generating PN Sequences
• Take m=2 =>L=3
• cn=[1,1,0,1,1,0, . . .],
usually written as
bipolar cn=[1,1,-1,1,1,-1,
. . .]
m Stages connected
to modulo-2 adder
2 1,2
3 1,3
4 1,4
5 1,4
6 1,6
8 1,5,6,7
+
Output
( )
−≤≤−
=
=
= ∑=
+
11/1
01
1
1
LmL
m
cc
L
mR
L
n
mnnc](https://image.slidesharecdn.com/dsssfinal-140731222131-phpapp01/75/Dsss-final-31-2048.jpg)
Uploaded byAJAL A J
Dsss final
The document discusses spread spectrum communication techniques, specifically focusing on Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS). It explains their properties, advantages, differences, and applications, highlighting DSSS's ability to reduce noise and interference through the use of pseudo-noise sequences. The document also evaluates system performance aspects such as throughput, security, and the handling of multi-path environments.
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Dsss final
- 1. 1 AJAL.A.J Assistant Professor –Deptof ECE, Federal Institute of Science And Technology (FISAT) TM MAIL: ec2reach@gmail.com
- 2. 08/01/14 2 Spread Spectrum Spread spectrum isa communication technique that spreads a narrowband communication signal over a wide range of frequencies for transmission then de-spreads it into the original data bandwidth at the receive. Spread spectrum is characterized by: wide bandwidth and low power Jamming and interference have less effect on Spread spectrum because it is: Resembles noise Hard to detect Hard to intercept
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- 4. 4 Spread Spectrum SystemConcept DATA SOURCE JAMMER L I N K S E L E C T O R TR #1 TR #2 TR #K COMMON CLOCKS AND KEYS RCV #1 RCV #2 RCV #K D I V E R SI T Y C O M B I N E R DATA USER ( )1n t ( )2n t ( )Kn t ( )km t ( )2m t ( )1m t ( )J t
- 5. 08/01/14 5 (The radio carrier signal is “spread out” on a specific channel ) SpreadSpectrum Frequency Hopping (FHSS) ( The radio carrier hops around the band. ) Direct Sequence (DSSS)
- 6. 08/01/14 6 Rogoff 'snoise wheel model used in spread spectrum communication systems
- 7. 08/01/14 7 Rogoff 'snoise wheel model
- 8. 8 Spectra in theDirect Sequence Spread Spectrum System
- 9. 08/01/14 9 Direct SequenceSpread Spectrum Modulation technique ,Also known as direct sequence code division multiple access (DS-CDMA) The name 'spread spectrum' comes from the fact that the carrier signals occur over the full bandwidth (spectrum) of a device's transmitting frequency. A RF carrier and pseudo-random pulse train are mixed to make a noise like wide-band signal.
- 10. 08/01/14 10 DSSS (DirectSequence Spread Spectrum) • XOR the signal with pseudonoise (PN) sequence (chipping sequence) • Advantages – reduces frequency selective fading – in cellular networks • base stations can use the same frequency range • several base stations can detect and recover the signal • But, needs precise power control user data chipping sequence resulting signal 0 1 0 1 10 1 0101 0 0 1 11 XOR 0 1 10 0 1011 0 1 0 01 = Tb Tc
- 11. 08/01/14 11 user data m(t) chipping sequence,c(t) X DSSS (Direct Sequence Spread Spectrum) modulator radio carrier Spread spectrum Signal y(t)=m(t)c(t) transmit signal Transmitter demodulator received signal radio carrier X Chipping sequence, c(t) Receiver integrator products decision data sampled sums correlator
- 12. 12 Direct sequence contrasts with the other spread spectrum process, known as frequency hopping spread spectrum, in which a broad slice of the bandwidth spectrum is divided into many possible broadcast frequencies. In general, frequency-hopping devices use less power and are cheaper, but the performance of DS-CDMA systems is usually better and more reliable.
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- 15. 08/01/14 15 DSSS BarkerCode modulation Source: Intersil
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- 17. 08/01/14 17 Direct Sequence Datasignal multiplied by Pseudo Random Noise Code(PN Code) • Low cross-correlation value • Anti-jamming • Main problem: Near-Far effect –In cellular, it can do power control by BS –In non-cellular, it need Frequency Hopping
- 18. 08/01/14 18 Detecting DS/SSPSK Signals X Bipolar, NRZ m(t) PN sequence, c(t) X sqrt(2)cos(ωct + θ) Spread spectrum Signal y(t)=m(t)c(t) transmit signal transmitter X received signal X c(t) receiver integrator z(t) decision data sqrt(2)cos(ωct + θ) LPF w(t) x(t)
- 19. 08/01/14 19 Optimum Detectionof DS/SS PSK • Recall, bipolar signaling (PSK) and white noise give the optimum error probability • Not effected by spreading – Wideband noise not affected by spreading – Narrowband noise reduced by spreading
- 20. 08/01/14 20 Signal Spectra •Effective noise power is channel noise power plus jamming (NB) signal power divided by N 10Processing Gain 10logss ss b c B B T N B B T = = = ÷ Tb Tc
- 21. 08/01/14 21 Multiple AccessPerformance • Assume K users in the same frequency band, • Interested in user 1, other users interfere 4 1 3 5 2 6
- 22. 08/01/14 22 Comparison ofSpectrum 30 kHz Analog Cellular Voice Channel 6 MHz TV Channel 28 - 100 MHz Unlicensed Spread Spectrum Devices 1000 - 3000 MHz Ultra-Wideband Devices
- 23. 08/01/14 23 A DSSSgenerator: • To generate a spread spectrum signal one requires: 1. A modulated signal somewhere in the RF spectrum 2. A PN sequence to spread it
- 24. 08/01/14 24 original RFcarrier = ω0 ωs = is the sequence clock
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- 29. 08/01/14 29 Pseudo-Noise (PN)sequence • A pseudo-noise (PN) sequence is a periodic binary sequence with a noise-like waveform. • It is generated by using linear feedback shift register. • The main advantages of using PN sequences are antijamming, multipath protection, multiple access, message privacy, identification … etc.
- 30. 08/01/14 30 PN SequenceGeneration • Codes are periodic and generated by a shift register and XOR • Maximum-length (ML) shift register sequences, m-stage shift register, length: n = 2m – 1 bits R(τ) -1/n Tc τ −> -nTc nTc + Output
- 31. 31RF - CellcomcourseDr. Moshe Ran08/01/14 Generating PN Sequences • Take m=2 =>L=3 • cn=[1,1,0,1,1,0, . . .], usually written as bipolar cn=[1,1,-1,1,1,-1, . . .] m Stages connected to modulo-2 adder 2 1,2 3 1,3 4 1,4 5 1,4 6 1,6 8 1,5,6,7 + Output ( ) −≤≤− = = = ∑= + 11/1 01 1 1 LmL m cc L mR L n mnnc
- 32. Problems withProblems withmm-sequences-sequences Cross-correlations with otherCross-correlations with other mm-sequences-sequences generated by different input sequences can begenerated by different input sequences can be quite highquite high Easy to guess connection setup in 2Easy to guess connection setup in 2mm samplessamples so not too secureso not too secure In practice, Gold codes or Kasami sequencesIn practice, Gold codes or Kasami sequences which combine the output of m-sequences arewhich combine the output of m-sequences are used.used.
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- 38. 08/01/14 38 Figure -Signals used to modulate the carrier in FHSS and DSSS (Dwell time in FHSS is represented)
- 39. 08/01/14 39 Systems Behavior •The following issues will be studied in parallel for FHSS and DSSS systems: 1.- Systems Collocation 2.- Noise and Interference Immunity 3.- The Near / Far problem 4.- Multipath Immunity 5.- Time and frequency diversity 6.- Throughput 7.- Security 8.- Bluetooth interference
- 40. 08/01/14 40 1.- SystemsCollocation • The issue: How many independent systems may operate simultaneously without interference? In DSSS systems, collocation could be based on the use of different spreading codes (sequences( for each active system
- 41. 08/01/14 41 2.- Noiseand Interference Immunity • The issue: Capability of the system to operate without errors when other radio signals are present in the same band. • FHSS systems operate with SNR (Signal to Noise Ratio) of about 18 dB. • DSSS systems, because of the more efficient modulation technique used (PSK), can operate with SNR as low as 12 dB.
- 42. 08/01/14 42 3.- Near/ Far problem • The issue: The problems generated to a FH / D SSS receiver by other active transmitters located in its proximity, are known as Near / Far problems.
- 43. 08/01/14 43 4.- Multipath •The issue: Environments with reflective surfaces (such as buildings, office walls, etc.) generate multiple possible paths between transmitter and receiver and therefore the receiver receives multiple copies of the original (transmitted) signal.
- 44. 08/01/14 44 5.- Timeand frequency diversity • Both DSSS and FHSS retransmit lost packets, until the receiving part acknowledges correct reception. A packet could be lost because of noises FHSS systems use “frequency diversity" . )packets are retransmitted on different frequencies / hops(.
- 45. 08/01/14 45 6.- Throughput •The issue: What amount of data is actually carried by the system (measured in bps). 6.1.- Single system throughput 6.2.-Aggregate throughput of collocated systems
- 46. 08/01/14 46 7.- Security •The issue: Protecting the transmission against eavesdropping
- 47. 08/01/14 47 8.- Bluetoothinterference • FHSS are significantly less sensitive to Bluetooth interference.
- 48. 48 Frequency Hopping Vs.Direct Sequence FH systems use a radio carrier that “hops” from frequency to frequency in a pattern known to both transmitter and receiver – Easy to implement – Resistance to noise – Limited throughput (2-3 Mbps @ 2.4 GHz) DS systems use a carrier that remains fixed to a specific frequency band. The data signal is spread onto a much larger range of frequencies (at a much lower power level) using a specific encoding scheme. – Much higher throughput than FH (11 Mbps) – Better range – Less resistant to noise (made up for by redundancy – it transmits at least 10 fully redundant copies of the original signal at the same time)
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