ECE 6640Digital Communications

Dr. Bradley J. BazuinAssistant Professor

Department of Electrical and Computer EngineeringCollege of Engineering and Applied Sciences

ECE 6640 2

Chapter 12

Chapter 12: Spread-Spectrum Techniques.12.1 Spread-Spectrum Overview. 12.2 Pseudonoise Sequences. 12.3 Direct-Sequence Spread-Spectrum Systems. 12.4 Frequency Hopping Systems. 12.5 Synchronization. 12.6 Jamming Considerations. 12.7 Commercial Applications. 12.8 Cellular Systems.

Chapter Content

• Spread spectrum signals used for the transmission of digital information are distinguished by the characteristic that their bandwidth W is much greater than the information rate R in bits/s. That is, the bandwidth expansion factor Be =W/R for a spread spectrum signal is much greater than unity.

– The large redundancy inherent in spread spectrum signals is required to overcome the severe levels of interference that are encountered in the transmission of digital information over some radio and satellite channels.

• Spread spectrum signals are used for– Combating or suppressing the detrimental effects of interference due to jamming,

interference arising from other users of the channel, and self-interference due to multipath propagation.

– Hiding a signal by transmitting it at low power and, thus, making it difficult for an unintended listener to detect in the presence of background noise.

– Achieving message privacy in the presence of other listeners.• General applications:

– Sonar, radar, GPS, Bluetooth, Military communications, narrowband interference communications (all forms of jamming), multiuser communications (CDMA cell phones), maintain average power in a link (telephony).ECE 6640 3

Spread Spectrum Model

• “Spreading” performed by a pseudorandom sequence generator.– The pattern/sequence must be known to both ends and

synchronization must occur.– Often an initial contact is made (link entry and establishment) and

then a new code or sequence would be used for the communications.

ECE 6640 4Notes and figures are based on or taken from materials in the course textbook: J.G. Proakis and M.Salehi, Digital Communications, 5th ed., McGraw-Hill, 2008.

Popular Classes of Systems

• Direct Sequence Spread Spectrum (DSSS)– A pseudorandom “chipping” sequence is used to spread an

underlying binary data stream. The chip rate defines the spectrralbandwidth.

• Frequency Hopping– A narrower bandwidth communication signal is periodically

moved to different centers frequencies in a defined frequency band in a pseudorandom way.

– Systems may be characterized as slow or fast hopping based on the time spent at each frequency (or the number of symbols transmitted during each dwell time at a frequency).

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Benefits

• Interference suppression– intentional (jamming) or

unintentional (spurious interference)

• Jamming– Jamming bandwidth involves all

signal spectrum locations– Jamming bandwidth is partial

band

• May also become low probability of detection (LPD) or low probability of interception (LPI)

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Energy Density Reduction

• By spreading the energy of the transmitted signal, it may be more difficult to detect (LPD)

• There are cases in DSSS where the effective signal power is below the noise floor, which makes detection and interception difficult (LPI)

• If the signal is difficult to detect or intercept, other forms of tracking or exploitation become unavailable (position fixing as in an E911 system).

ECE 6640 7

Signal Processing Functions

ECE 6640 8Notes and figures are based on or taken from materials in the course textbook:

Bernard Sklar, Digital Communications, Fundamentals and Applications, Prentice Hall PTR, Second Edition, 2001.

DS OR DSSS:DIRECT SEQUENCE SPREAD SPECTRUM

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Direct Sequence Spread Spectrum

• The data signal is “spread” by a spreading code or chipping sequence with a much higher rate or bandwidth.

• When the same spreading code is applied at the receiver, the noise becomes spread and the signal of interest becomes “squared”. – noise is spread, signal becomes despread.

ECE 6640 10

PN Code and Data

• Encoding involves “multiplying” an integer number of PN code chips for each bit of data.

• Typically the chip rate is significantly higher than the data bit rate!– Bandwidth

expansion– Spreading gain

ECE 6640 11Notes and figures are based on or taken from materials in the course textbook: J.G. Proakis and M.Salehi, Digital Communications, 5th ed., McGraw-Hill, 2008.

Autocorrelation

• If we transmit the DSS signal, there is a time delay due to distance. Thus, we must time align the received DSS with the chip sequence in order to receive it.

Tc

A2

Tc-Tc

NTc-NTc

R() = A2 (1 - | | / Tc) for | | Tc= - A2 /N elsewhere

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Figure 15.1-1

DSS Transmitter System

A. Bruce Carlson, P.B. Crilly,, “Communication Systems, 5th ed., McGraw-Hill, 2010. ISBN: 978-0-073380407-7.

DSS Bandwidth

• The bandwidth is based on the chip rate

repC

c TT

R

repCCc T

fTfTfG 1sinc 2

T

Rx

TfTfGx sinc

tctcEtxtxEtctctxtxEtctxtctxERDSS

Crep

CDSS TT

TT

R

Bandwidth Expansion Factor

• For DS spreading of the signal, the bandwidth has gone from R=1/T to 1/Tc

RW

TTg C

C

bBWex

Correlation Receiver

• Use the PN sequence as an optimal “filter” or correlator sequence

A. Bruce Carlson, P.B. Crilly,, “Communication Systems, 5th ed., McGraw-Hill, 2010. ISBN: 978-0-073380407-7.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Figure 15.1-2

DSS Inherent Anti-Jam

Notes and figures are based on or taken from materials in the course textbook: Bernard Sklar, Digital Communications, Fundamentals and Applications,

Prentice Hall PTR, Second Edition, 2001.

Randomness Properties

• Balance Property– for periods of the code we want the total number of ‘0’’s and ‘1’s

to be balanced (nearly equal).

• Run Property– Minimize long runs of either ‘1’s or ‘0’s. The maximum would be

n for a LFSR, but it only happens once.

• Correlation Property– The correlation is a maximum at one point and insignificant

elsewhere.

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Generating A PN Sequence

• Maximal length linear feedback shift registers!

ECE 6640 19

12 nngthSequenceLe

PN Autocorrelation

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BPSK DSSS Systems

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Digital Code Example

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Processing Gain and Performance

• Algorithmic analysis performed in section 12.3.2

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RW

TTG ss

Chip

bp

DSSS Systems

• The GPS system …• Code-Division Multiple Axis (CDMA) cellular telephone

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GPS Signal Characteristics

• Two Principal Frequencies– L1 Band at 1575.42 MHz with C/A and P(Y) codes– L2 Band at 1227.60 MHz with P(Y) code– L5 Band at 1176.45 MHz with “new” C/A code

• Direct Sequence Spread Spectrum Communications– Data Message at 50 bps consisting of 1500 bit pages (30 sec.)– C/A-code spreads the data using 1023-bit Gold codes at a chipping rate of

1.023 Mcps (C/A – coarse-acquisition code)– P(Y)-code spreads the data using a code that Does not repeat at a chipping

rate of 1.0.23 Mcps (P – precision code)• Code Transmission

– The C/A- and P(Y)-codes are transmitted in quadrature on L1– The P(Y)-code is transmitted on L2

GPS Receiver Characteristics

• Receive up to 12 satellites simultaneously• User Minimum received power

– L1 C/A-Code: –130 dBm– L1 P-Code: –133 dBm– L2 P-Code: –136 dBm– kTB (20 MHz): –101 dBm

-10 -5 0 5 10-160

-150

-140

-130

-120

-110

-100

dBm

/ H

z

Frequancy (MHz)

Thermal Noise PowerC/A Code P(Y) Code

Nominal Power Levels

• Reference noise power: -174 dBm/Hz– C/A code bandwidth 2 MHz: 63 dB– Nominal thermal noise floor in band: -111 dBm– Receiver Noise Figure (estimate 10 dB)

• Minimum detectable signal level: -101 dBm

• C/A Signal: -130 dBm– Coding gain 1023,000/50=2046043.1 dB– (P-Code coding gain is 10x better)

• Margin (-130 + 43.1 +6?) – (-101) = 20.1 dB– Note that initial signal was 29 dB below the noise floor!ECE 6640 27

GPS User Position Computation• GPS uses the triangulation of signals from the

satellites to determine locations on earth. • GPS satellites know their location in space and

receivers can determine their distance from a satellite by using the travel time of a radio message from the satellite to the receiver.

• After calculating its relative position to at least 4 satellites, a GPS receiver can calculate its position using triangulation.

• They also have a database (or almanac) of the current and expected positions for all of the satellites that is frequently updated from earth.

Earth

Satellite

r

u

s

SV to User Distancer = || s – u ||r = c x t

Measured Pseudorange = r + c x tu

Unknownsr(x, y, z), tUse 4 SVs to

solve

Code Division Multiple Access

• CDMA in the US is associated with Qualcomm, San Diego, CA.– Early related standards IS-95 and IS-2000 or CDMA2000

• IS-95 (https://en.wikipedia.org/wiki/IS-95)– In the forward direction, radio signals are transmitted by base stations

(BTS's). All forward transmissions are QPSK with a chip rate of 1,228,800 per second. Each signal is spread with a Walsh code of length 64 and a pseudo-random noise code (PN code) of length 215, yielding a PN roll-over period of 80/3 ms.

– For the reverse direction, radio signals are transmitted by the mobile. Reverse link transmissions are OQPSK in order to operate in the optimal range of the mobile's power amplifier. Like the forward link, the chip rate is 1,228,800 per second and signals are spread with Walsh codes and the pseudo-random noise code, which is also known as a Short Code.

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IS-95 Forward Link

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IS-95 Receiver Link

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Tracking: Early Late and Prompt code taps

DLL Feedback range for code offset.

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FREQUENCY HOPPING

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Frequency Hopping

• In a frequency-hopped (FH) spread spectrum communication system the available channel bandwidth is subdivided into a large number of contiguous frequency slots. In any signaling interval, the transmitted signal occupies one or more of the available frequency slots. The selection of the frequency slot(s) in each signaling interval is made pseudorandomly according to the output from a PN generator.

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Typical FH Modulation Type

• Although PSK modulation gives better performance than FSK in an AWGN channel, it is sometimes difficult to maintain phase coherence in the synthesis of the frequencies used in the hopping pattern and, also, in the propagation of the signal over the channel as the signal is hopped from one frequency to another over a wide bandwidth. Consequently, FSK modulation with noncoherent detection is often employed with FH spread spectrum signals.

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SKLAR FH System

• PN generator picks discrete frequency set• Transmitter and receiver must be synchronized

ECE 6640 36

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Figure 15.2-1

(a) Transmit

(b) Receive

FH-SS System.

A. Bruce Carlson, P.B. Crilly,, “Communication Systems, 5th ed., McGraw-Hill, 2010. ISBN: 978-0-073380407-7.

FH System Block Diagram

• PN generator picks discrete frequency set• Transmitter and receiver must be synchronized

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FH Example

• 8-FSK symbol transmission• Hopping for each Symbol Interval (50 Hz x 3 = 150 bps)• Must meet FSK tone seperation

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Hop Rate

• Repeated or partial symbol period – Fast Hopping– Bandwidth consideration due to time intervals

• Single Symbol • Multiple Symbols – Slow hop

• Challenge of hopping rates– The local oscillator or carrier frequency must retune and become

stable. This takes time.– Use multiple oscillators, select one at a time. Oscillator settling

time is the repetition interval for the oscillator.

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Fast vs. Slow Hopping

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Processing Gain

• From the respect of the hopping frequency band

• However, fundamentally the underlying communication process (FSK based) is being maintained even though the carrier frequency is moving in a broader band. – Entire hops can be eliminated due to narrowband interference.– Encoding and decoding for burst errors is desired …

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FFH Synchronization and Demodulation

• A polyphase filterbank channelizing receiver is perfect for this application. Multiple uniformly spaced frequncy bins can be simultaneously monitored …

• Fast scanning may be too slow (snapshot spectrum analysis)

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FH Serial search

• If the PN sequence is known, once one hop is found a limited number of frequencies can follow. – For faster acquisition a subset of frequencies could be used.

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JAMMING

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Jamming Consideration

• Intentional Jamming– Usually military or some type of adversary

• Inadvertent Jamming– Erroneous radio transmitter operation – unintentional– Harmonic transmission – poor transmitter performance

• Self Jamming– Communication in the same band too close or with incorrect power

levels. Cellular telephone system adjacent channel interference (ACI) or base-to-base interference (BBI). Also the “near-far” problem.

– Too many simultaneous signal users.(each transmitter adds to the local noise plus interference level)

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Basic Jammer Waveforms

• Broad band jammer• Partial band jammer• Stepped partial band jammer• Tone jammer• Stepped tone jammer

• Fast follower jammer• Smart jammer• Pulse jammer

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Jammer Waveforms

• Noise• Pseudo random sequences• Tones• Modulated noise• Signal like noise or a particular signal characteristic

– Jam pilot tones or access channels only– Find signals that “defeat” specific signal features or attributes– Eliminate signal detection or acquisition features required by a

receiver … then you don’t have to worry about the rest of the signal.

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Noise plus Interference

• For noise-like jamming, it is an additive power that reduced the SNR at the receiver.– Jammer power and distance matter. – Communication system “link margin” is available to include

jamming signal power. This becomes the

• Jammer power usually dominates

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Spread spectrum gain

related

CELLULAR PHONE SYSTEMS

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Cellular Systems: Historical

ECE 6640 51… OK the book is getting old ...

Old Cellular Protocols

• AMPS & TDMA: individual 30 kHz bandwidth communication channels– 824-849 MHz mobile station and 869-894 MHz base station– Frequency division duplex – TX and RX on different frequencies

• GSM: 200 kHz bandwidths• CDMA 1.2288 Mcps chip rate and 1.25 MHz bandwidth

ECE 6640 52

Hexagonal Cell Regions

• Reuse 7 configuration – center and surrounding– Initial band divided by seven for

allocation. – Allows a repeated pattern for

assignments.

• New, smaller cells pattern can be embedded to adjust for volume.

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