Spread Spectrum Review/Recap Lecture 22. Overview Relationship between bandwidth of a signal (before and after and encoding SS) Benefits of SS FHSS Slow.

Spread SpectrumReview/Recap

Lecture 22

Overview Relationship between bandwidth of a signal

(before and after and encoding SS) Benefits of SS FHSS Slow and Fast FHSS DSSS Relationship between Bit Rate of a Signal

(Before and after DSSS encoding) CDMA

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Bandwidth Before and After SS Encoding

Q:- What is the relationship between the bandwidth of a signal before and after it has been encoded using spread spectrum?

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Definition of Spread Spectrum

Spread spectrum is a modulation method applied to digitally modulated signals that increases the transmit signal bandwidth to a value much larger than is needed to transmit the underlying information bits.

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Spread Spectrum Signal Characteristics

1. They are difficult to intercept for unauthorized person.

2. They are easily hidden, it is difficult to even detect their presence in many cases.

3. They are resistant to jamming.4. They have an asynchronous multiple-access

capability.5. They provide a measure of immunity to distortion

due to multipath propagation.

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Spread Spectrum Conditions

• The signal occupies a bandwidth much larger than is needed for the information signal.

• The spread spectrum modulation is done using a spreading code, which is independent of the data in the signal.

• Despreading at the receiver is done by correlating the received signal with a synchronized copy of the spreading code.

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Processing Gain :

The spread spectrum increases the bandwidth of the message signal by a factor N, called the processing gain where β is the message signal bandwidth, βss is the corresponding SS signal bandwidth.

, N > 1

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Spread Spectrum Versus Narrowband TechnologyNarrowband wireless communications can be defined as wireless communications using a single frequency center with no redundancy to communicate information at high power levels chosen to overpower interference in that frequency band.

Spread spectrum wireless communications can be defined as wireless communications using a range of frequencies to communicate information at low power levels.

Spread spectrum has also been defined as a wireless communications technology that uses more bandwidth than is required to deliver information. Spread spectrum also uses low power and can do so because all interference does not need to be overcome, due to the redundancy and/or error correction.

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Spread Spectrum Versus Narrowband Technology

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Spread Spectrum

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Spread spectrum

Characteristics of Spread Spectrum

Bandwidth of the transmitted signal W is much greater than the original message bandwidth (or the signaling rate R)

Transmission bandwidth is independent of the message. Applied code is known both to the transmitter and receiver

Interference and noise immunity of SS system is larger, the larger the processing gain

Multiple SS systems can co-exist in the same band (=CDMA). Increased user independence (decreased interference) for (1) higher processing gain and higher (2) code orthogonality

Spreading sequence can be very long -> enables low transmitted PSD-> low probability of interception (especially in military communications)

Narrow band Signal (data)

Wideband signal(transmitted SS

signal)

/ /c b cL W R T T

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Characteristics of Spread Spectrum (cont.)

Processing gain, in general

Large Lc improves noise immunity, but requires a larger transmission bandwidth

Note that DS-spread spectrum is a repetition FEC-coded systems

Jamming margin Tells the magnitude of additional interference and noise that

can be injected to the channel without hazarding system operation. Example:

, 10/ (1/ ) /(1/ ) / , 10 log ( )c c b b c c dB cL W R T T T T L L

[ ( ) ]J c sys despM L L SNR

30dB,available processing gain

2dB,margin for system losses

10dB,required SNR after despreading (at the RX)

18dB,additional interference and noise can deteriorate

receive

c

sys

desp

j

L

L

SNR

M

d SNR by this amount 15

Characteristics of Spread Spectrum

Spectral efficiency Eeff: Describes how compactly TX signal fits into the transmission band. For instance for BPSK with some pre-filtering:

Energy efficiency (reception sensitivity): The value of to obtain a specified error rate (often 10-9). For BPSK the error rate is

QPSK-modulation can fit twice the data rate of BPSK in the same bandwidth. Therefore it is more energy efficient than BPSK.

/ /b T b Fff Re R B RE B

,

2 2log log

1/RF filt c cRF

b

B T LB

k M T M

/ / 1/c b c c b cL T T L T T

1beff

RF b

RE

B T bT 2 2

log log

c c

M M

L L

0/

b bE N

2/

1( 2 ), ( ) exp( 2)

2e b

k

p Q Q k d

22 logkM k M

, : bandwidth for polar mod.

: number of levels

: number of bits

RF filtB

M

k

16

Bandwidth Before and After SS Encoding

Q:- What is the relationship between the bandwidth of a signal before and after it has been encoded using spread spectrum?

Ans:- The bandwidth is wider after the signal has been encoded using spread spectrum17

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SS Benefits

Q:- List three benefits of spread spectrum.

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Benefits of Spread Spectrum

Spread spectrum is being used in more and more applications in data communications.

Security – need a wide BW receiver and precise knowledge and timing of the pseudorandom sequence

Resistance to jamming and interference – jamming signals are usually restricted to one frequency

Band sharing – many signals can use the same frequency band; but… many spread spectrum signals raise the overall background noise level

Precise timing – can be used in radar where accurate knowledge of transmission time is needed 20

SS BenefitsQ:- List three benefits of spread spectrum.Ans:- (1) We can gain immunity from various kinds of noise and multipath distortion. (2) It can also be used for hiding and encrypting signals. Only a recipient who knows the spreading code can recover the encoded information.(3) Several users can independently use the same higher bandwidth with very little interference, using code division multiple access (CDMA).

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Frequency Hopping SS

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FHSS

Q:- What is frequency hopping spread spectrum

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Frequency hopping This dilemma was recognized

prior to WWII. In 1942, Hedy Lamarr and

pianist George Antheil patented a “Secret Communication System”.

Their scheme was for a frequency hopping remote control for torpedo guidance.

Hedy LamarrActress and co-inventor of frequency hopping spread spectrum25

First spread-spectrum patent

By changing the transmitter frequencies in a “random” pattern, the torpedo control signal could not be jammed.

Lamarr proposed using 88 frequencies sequencedfor control.

Frequency switching pattern 26

FHSS Algorithm The initiating party sends a request via a predefined frequency or

control channel. Once the receiving party have received the request, it sends a

pseudo number called seed to the transmitting party via the same channel.

The initiating party uses this seed as a variable in predefined algorithm and calculates the sequence of frequencies that must now be used to transmit the signals. Most often the period of frequency change is predetermined.

The initiating party then sends the first piece of information via the lowest band of newly generated frequency spectrum. Thus acknowledging the receiving party that it has correctly calculated the sequence.

The communication begins, and both the receiving party and the sending party change the frequencies along with the calculated order. Starting at the same point of the time.

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FHSS Algorithm

In FH Data is divided into chunks and transmitted at different frequencies at different times. 28

Frequency hopping spread spectrum

In a frequency hopping spread spectrum system, the carrier frequency is switched in a pseudorandom fashion.

The transmitter and receiver know the pattern and are synchronized.

Dwell time

225 230 235 240 245 250 255 f (MHz)

Tim

e (

ms)

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Dwell TimeFHSS systems include characteristics such as dwell time, hopping sequences, and hop time. These characteristics come together to make up how the FHSS system will function and the actual data throughput that will be available.

The amount of time spent on a specific frequency in an FHSS hopping sequence is known as the dwell time. These channels, 1 MHz of bandwidth each, provide 79 optional frequencies on which to dwell for the specified length of the dwell time.

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The hopping sequence is the list of frequencies through which the FHSS system will hop according to the specified dwell time. The IEEE 802.11 standard, section 14.6.5, states that 1 MHz channels should be used. These channels exist between 2.402 and 2.480 GHz in the United States and most of Europe.

Every station in a Basic Service Set must use the same hopping sequence. Every station must also store a table of all the hopping sequences that are used within the system.

These hopping sequences must have a minimum hop size of 6 MHz in frequency. If the device is currently communicating on the 2.402 GHz frequency, it must hop to 2.408 GHz at the next hop at a minimum.

Hopping sequence

Frequency hopping transmitter

The binary data to be transmitted is applied to a conventional two-tone FSK modulator.

A frequency synthesizer produces a sine wave of a random frequency determined by a pseudorandom code generator.

These two signals are mixed together, filtered and then transmitted.

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Frequency hopping transmitter

Typically the rate of frequency change is much higher than the data rate.

The illustration below shows that the frequency synthesizer changes 4 times for each data bit.

The time period spent on each frequency is called the dwell time (typically < 10 ms)

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Frequency hopping spread spectrum

The resulting signal, whose frequency rapidly jumps around, effectively scatters pieces of the signal all over the band.

Someone else monitoring the spectrum would not recognize that a transmission is being made.

225 230 235 240 245 250 255 f (MHz)34

Frequency hopping receiver

The received signal is mixed using a local oscillator driven by the same pseudorandom sequence.

The output produces the original two-tone FSK signal from which the binary data can be extracted.

Timing is extremely critical in frequency hopping systems in order to maintain synchronization.

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Practical Example: Bluetooth

2.4 GHz – 2.4835 GHz Operating Range 79 Different Radio Channels Hops 1600 times per second for data/voice links Hops 3200 times per second for page and

inquiry scanning 1 Mbps = Rb for Bluetooth Ver 1.1/1.2 3 Mbps = Rb for Bluetooth Ver 2.1 Gaussian Frequency Shift Keying (GFSK)

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Frequency Hopping SS Signal is broadcast over seemingly random series of

radio frequencies A number of channels allocated for the FH signal Width of each channel corresponds to bandwidth

of input signal Signal hops from frequency to frequency at fixed

intervals Transmitter operates in one channel at a time Bits are transmitted using some encoding scheme At each successive interval, a new carrier

frequency is selected 37

Frequency Hopping SS

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Frequency Hopping SS Hopping Sequence

Channel sequence dictated by spreading code Pseudorandom number serves as an index into

a table of frequencies Chip Period

Time spent on each channel FCC regulation maximum dwell time of 400

ms IEEE 802.11 standard 300 ms

Chipping rate Hopping rate 39

Frequency Hopping SS Receiver, hopping between frequencies in

synchronization with transmitter, picks up message

Advantages Eavesdroppers hear only unintelligible blips Attempts to jam signal on one frequency

succeed only at knocking out a few bits

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FHSS Performance Considerations

Large number of frequencies used Results in a system that is quite resistant

to jamming Jamming signal must jam all frequencies With fixed power, this reduces the jamming

power in any one frequency band

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FHSSQ:- What is frequency hopping spread spectrum

Ans:- With frequency hopping spread spectrum (FHSS), the signal is broadcast over a seemingly random series of radio frequencies, hopping from frequency to frequency at fixed intervals. A receiver, hopping between frequencies in synchronization with the transmitter, picks up the message. 42

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Slow and Fast FHSS

Q:- Explain the difference between slow FHSS and fast FHSS.

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Discrete changes of carrier frequency sequence of frequency changes determined via

pseudo random number sequence Two versions

Fast Hopping: several frequencies per user bit Slow Hopping: several user bits per frequency

Advantages frequency selective fading and interference limited

to short period simple implementation uses only small portion of spectrum at any time 45

(Frequency Hopping Spread Spectrum)

FHSS: Example

user data

slowhopping(3 bits/hop)

fasthopping(3 hops/bit)

0 1

tb

0 1 1 tf

f1

f2

f3

t

td

f

f1

f2

f3

t

td

tb: bit period td: dwell time 46

Comparison between slow hopping and fast hopping

Slow hopping Pros: cheaper Cons: less immune to narrowband

interference Fast hopping

Pros: more immune to narrowband interference

Cons: tight synchronization increased complexity

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Slow and Fast FHSS commonly use multiple FSK (MFSK) have frequency shifted every Tc seconds duration of signal element is Ts seconds Slow FHSS has Tc Ts Fast FHSS has Tc < Ts FHSS quite resistant to noise or jamming

with fast FHSS giving better performance

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Slow MFSK FHSS

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Fast MFSK FHSS

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Slow and Fast FHSS

Q:- Explain the difference between slow FHSS and fast FHSS.

Ans:- Slow FHSS = multiple signal elements per hop; fast FHSS = multiple hops per signal element.

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DSSS

Q:- What is direct sequence spread spectrum?

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Direct Sequence Spread Spectrum Spread spectrum increases the bandwidth of the signal compared to narrow band by spreading the signal.

There are two major types of spread spectrum techniques: FHSS and DSSS.

FHSS spreads the signal by hopping from one frequency to another across a bandwidth of 83 Mhz.

DSSS spreads the signal by adding redundant bits to the signal prior to transmission which spreads the signal across 22 Mhz.

The process of adding redundant information to the signal is called Processing Gain .

The redundant information bits are called Pseudorandom Numbers (PN).

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DSSS

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DSSS works by combining information bits (data signal) with higher data rate bit sequence (pseudorandom number (PN)).

The PN is also called a Chipping Code (eg., the Barker chipping code)

The bits resulting from combining the information bits with the chipping code are called chips - the result- which is then transmitted.

The higher processing gain (more chips) increases the signal's resistance to interference by spreading it across a greater number of frequencies.

IEEE has set their minimum processing gain to 11. The number of chips in the chipping code equates to the signal spreading ratio.

Doubling the chipping speed doubles the signal spread and the required bandwidth.

Signal Spreading

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The Spreader employs an encoding scheme (Barker or Complementary Code Keying (CCK).

The spread signal is then modulated by a carrier employing either Differential Binary Phase Shift Keying (DBPSK), or Differential Quadrature Phase Shift Keying (DQPSK).

The Correlator reverses this process in order to recover the original data.

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Fourteen channels are identified, however, the FCC specifies only 11 channels for non-licensed (ISM band) use in the US.

Each channels is a contiguous band of frequencies 22 Mhz wide with each channel separated by 5 MHz.

Channel 1 = 2.401 – 2.423 (2.412 plus/minus 11 Mhz).

Channel 2 = 2.406 – 2.429 (2.417 plus/minus 11 Mhz).

Only Channels 1, 6 and 11 do not overlap

DSSS Channels

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Spectrum Mask A spectrum Mask represents the maximum power output for the channel at various frequencies.

From the center channel frequency, 11 MHz and 22 MHZ the signal must be attenuated 30 dB.

From the center channel frequency, outside 22 MHZ, the signal is attenuated 50 dB.

±

± ±

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DSSS Frequency Assignments

Channel 12.412 GHz

Channel 62.437 GHz

Channel 112.462 GHz

25 MHz25 MHz

The Center DSSS frequencies of each channel are only 5 Mhz apart but each channel is 22 Mhz wide therefore adjacent channels will overlap.

DSSS systems with overlapping channels in the same physical space would cause interference between systems.

Co-located DSSS systems should have frequencies which are at least 5 channels apart, e.g., Channels 1 and 6, Channels 2 and 7, etc.

Channels 1, 6 and 11 are the only theoretically non-overlapping channels.

602.401 GHz 2.473 GHz

Channel 1 Channel 6 Channel 11

22 MHz

3 MHz

f

P

DSSS Non-overlapping Channels

Each channel is 22 MHz wide. In order for two bands not to overlap (interfere), there must be five channels between them.

A maximum of three channels may be co-located (as shown) without overlap (interference).

The transmitter spreads the signal sequence across the 22 Mhz wide channel so only a few chips will be impacted by interference.

DSSS Encoding and Modulation

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DSSS Encoding and Modulation

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DSSS (802.11b) employs two types of encoding schemes and two types of modulation schemes depending upon the speed of transmission.

Encoding Schemes

Barker Chipping Code: Spreads 1 data bit across 11 redundant bits at both 1 Mbps and 2 Mbps

Complementary Code Keying (CCK):

Maps 4 data bits into a unique redundant 8 bits for 5.5 Mbps

Maps 8 data bits into a unique redundant 8 bits for 11 Mbps.

Modulation Schemes

Differential Binary Phase Shift Keying (DBPSK): Two phase shifts with each phase shift representing one transmitted bit.

Differential Quadrature Phase Shift Keying (DQPSK): Four phase shifts with each phase shift representing two bits.

DSSS Encoding

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Barker Chipping Code

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802.11 adopted an 11 bit Barker chipping code. Transmission.

The Barker sequence, 10110111000, was chosen to spread each 1 and 0 signal.

The Barker sequence has six 1s and five 0s. Each data bit, 1 and 0, is modulo-2 (XOR) added to the eleven bit Barker sequence.

If a one is encoded all the bits change. If a zero is encoded all bits stay the same.

Reception.A zero bit corresponds to an eleven bit sequence of six 1s.A one bit corresponds to an eleven bit sequence of six 0s.

Barker Sequence

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Direct Sequence Spread Spectrum ….

Complementary Code Keying(CCK)

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Barker encoding along with DBPSK and DQPSK modulation schemes allow 802.11b to transmit data at 1 and 2 Mbps

Complementary Code Keying (CCK) allows 802.11b to transmit data at 5.5 and 11 Mbps.

CCK employs an 8 bit chipping code.

The 8 chipping bit pattern is generated based upon the data to be transmitted.

At 5.5 Mbps, 4 bits of incoming data is mapped into a unique 8 bit chipping pattern.

At 11 Mbps, 8 bits of data is mapped into a unique 8 bit chipping pattern.

Complementary Code Keying (CCK)

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To transmit 5.5 Mbps 4 data bits is mapped into 8 CCK chipping bits..

The unique 8 chipping bits is determined by the bit pattern of the 4 data bits to be transmitted. The data bit pattern is:

b0, b1, b2, b3

b2 and b3 determine the unique pattern of the 8 bit CCK chipping code.

Note: j represents the imaginary number, sqrt(-1), and appears on the imaginary or quadrature axis of the complex plane.


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To transmit 5.5 Mbps 4 data bits is mapped into 8 CCK chipping bits..


b0, b1, b2, b3

b0 and b1 determine the DQPSK phase rotation that is to be applied to the chip sequence.

Each phase change is relative to the last chip transmitted.


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To transmit 11 Mbps 8 data bits is mapped into 8 CCK chipping bits.


b0, b1, b2, b3, b4, b5, b6 ,b7

b2, b3, b4 ,b5, b6 and b7 selects one unique pattern of the 8 bit CCK chipping code out of 64 possible sequences.

b0 and b1 are used to select the phase rotation sequence.

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DSSS Modulation

Differential Binary Phase Shift Keying (DBPSK)

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A Zero phase shift from the previous symbol is interpreted as a 0.A 180 degree phase shift from the previous symbol is interpreted as a 1.

Differential Quadrature Phase Shift Keying (DQPSK)

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A Zero phase shift from the previous symbol is interpreted as a 00.

A 90 degree phase shift from the previous symbol is interpreted as a 01.



DSSS Summary

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1 Barker Coding 11 chips encoding 1 bit DBPSK

2 Barker Coding 11 chips encoding 1 bit DQPSK

5.5 CCK Coding 8 chips encode 8 bits DQPSK

11 CCK Coding 8 chips encode 4 bits DQPSK

Data Rate Encoding Modulation

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Direct Sequence Spread Spectrum

Another method of realizing spread spectrum is called direct sequence spread spectrum (DSSS).

In a DSSS system the message bit stream is modified by a higher rate pseudonoise (PN) sequence (called a chip sequence).

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Direct Sequence Spread Spectrum –DSSS

In direct-sequence spread spectrum (DSSS), the serial binary data is XORed with a pseudo-random binary code which has a bit rate faster than the binary data rate, and the result is used to phase-modulate a carrier.

chipping rate – bit rate of the pseudorandom code the faster you change the phase of a carrier, the more BW the

signal takes up – looks like noiseUNMODULATED CARRIER

SLOW SPEED PSK

HIGH SPEED PSK

many clock (chipping rate) pulses in one data bit time

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DSSSdata

Pseudo RandomSequence

data PRS

1 1 0 1

UNMODULATED CARRIER

SLOW SPEED PSK

HIGH SPEED PSK

“chip”

time of one data bit

frequency

power

carrier modulated by the data

carrier modulated by the data PRS

XOR

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Direct Sequence Spread Spectrum (cont’d)

Observations A signal that would normally occupy a few kHz

BW is spread out 10 to 10,000 times its BW. The fast phase modulation spreads the energy

of the signal over a wide BW – appears as noise in a conventional receiver.

Also called CDMA – Code Division Multiple Access

used in satellites – many signals can use the same transponder

used in cell phones – many users in same BW79

Direct Sequence Spread Spectrum

Receiver Receiver must know the pseudorandom

sequence of the transmitter and have a synchronizing circuit to get in step with this pseudorandom digital signal.

The receiver using an identically programmed PN sequence compares incoming signals and picks out the one with the highest correlation.

Other signals using different PN sequences appear as noise to the receiver and it doesn’t recognize them. 80

Processing gain The measure of the spreading is called the

processing gain, G, which is the ratio of the DSSS bandwidth, BW, divided by the data rate, fb .

The higher the processing gain, the greater the DSSS signal’s ability to fight interference.

b

BWG

f

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DSSS Signal

The spread signal has the same power as the narrowband signal, but far more sidebandsAmplitudes are very low and just above the random noise levelTransmitter and receiver are using the same PN sequence, so signal will be recognized 82

Benefits of Spread Spectrum

Spread spectrum is being used in more and more applications in data communications.

Security – need a wide BW receiver and precise knowledge and timing of the pseudorandom sequence

Resistance to jamming and interference – jamming signals are usually restricted to one frequency

Band sharing – many signals can use the same frequency band; but… many spread spectrum signals raise the overall background noise level

Precise timing – can be used in radar where accurate knowledge of transmission time is needed

83

DSSS

Q:- What is direct sequence spread spectrum?

Ans:- With direct sequence spread spectrum (DSSS), each bit in the original signal is represented by multiple bits in the transmitted signal, using a spreading code.84

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Bit Rate in DSSS Before and After

Q:- What is the relationship between the bit rate of a signal before and after it has been encoded using DSSS

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DSSS

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Direct Sequence each bit in the original signal is represented by multiple bits in the transmitted

signal, known as a chipping code the chipping code spreads the signal over a wider frequency band in direct

proportion to the number of bits used.

Bit Rate in DSSS Before and After

Q:- What is the relationship between the bit rate of a signal before and after it has been encoded using DSSS

Ans:- For an N-bit spreading code, the bit rate after spreading (usually called the chip rate) is N times the original bit rate.

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CDMA

Q:- What is CDMA?

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CDMA Transceiver Block Diagram

91

Lets walk through an example?

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1x 1= 1

Multiplication

1x-1=-1

-1x 1=-1

-1x

-1= 1 93

CDMA example

Low-Bandwidth Signal:

High-Bandwidth Spreading Code:

...repeated...94

CDMA exampleLow-Bandwidth Signal:

High-Bandwidth Spreading Code:

Mix is a simple multiply

… and transmit. 95

CDMA example

To Decode / Receive, take the signal:

96

CDMA exampleTo Decode / Receive, take the signal:

Multiply by the same Spreading Code:

… to get ...

… which you should recognise as...97

CDMA exampleTo Decode / Receive, take the signal:


… to get ...

98

(Discuss noise)To Decode / Receive, take the signal:


… to get ...

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What if we use the wrong code?

Take the same signal:

Multiply by the wrong Spreading Code:

100




… for example, let's just shift the same code left a bit:

101




… for example, let's just shift the same code left a bit:102



… you get ...

… which clearly hasn't recovered the original signal.Using wrong code is like being off-frequency.


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This obviously shows thattiming is critical. To receive a signal, you not only need to be generating the RIGHT code,

but your TIMING needs to be locked very tightly to the

received signal too.

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CDMA

Q:- What is CDMA?

Ans:- CDMA allows multiple users to transmit over the same wireless channel using spread spectrum. Each user uses a different spreading code. The receiver picks out one signal by matching the spreading code.

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Summary

107

Relationship between bandwidth of a signal (before and after and encoding SS)

Benefits of SS FHSS Slow and Fast FHSS DSSS Relationship between Bit Rate of a Signal

(Before and after DSSS encoding) CDMA

Spread Spectrum Review/Recap Lecture 22. Overview Relationship between bandwidth of a signal (before and after and encoding SS) Benefits of SS FHSS Slow.

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