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Analytical study ofFHSS and

DSSS

Presented by:

Jyoti Jaiswal(2011jop2630)Lalita devi(2011jop2625)

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Outlines

Spread SpectrumSpread Spectrum Concept

Frequency Hopping Spread Spectrum (FHSS)

Slow and Fast FHSSDirect Sequence Spread Spectrum (DSSS)

System behaviour of FHSS and DSSS

Conclusions

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Spread Spectrum

important encoding method for wireless

communications

analog & digital data with analog signal

spreads data over wide bandwidth

makes jamming and interception harder

two approaches, both in use:

Frequency Hopping

Direct Sequence

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Spread Spectrum Concept

Input fed into channel encoder

Produces narrow bandwidth analog signal around central frequency

Signal modulated using sequence of digits

Spreading code/sequence

Typically generated by pseudonoise/pseudorandom number generator

Increases bandwidth significantly

Spreads spectrum

Receiver uses same sequence to demodulate signal

Demodulated signal fed into channel decoder

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General Model of Spread Spectrum

System

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Gains

Immunity from various noise and multipathdistortion Including jamming

Can hide/encrypt signals Only receiver who knows spreading code can retrievesignal

Several users can share same higher bandwidthwith little interference Cellular telephones

Code division multiplexing (CDM)

Code division multiple access (CDMA)

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Frequency Hopping Spread

Spectrum (FHSS)

Signal broadcast over seemingly random series

of frequencies

Receiver hops between frequencies in sync

with transmitter

Jamming on one frequency affects only a few

bits

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Basic Operation

Typically 2kcarriers frequencies forming 2kchannels

Channel spacing corresponds with bandwidth of

input Each channel used for fixed interval

300 ms in IEEE 802.11

Some number of bits transmitted using some encoding

scheme May be fractions of bit

Sequence dictated by spreading code

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FHSS main parameters

FHSS is defined (in IEEE 802.11) in the 2.4 GHz

band as operating over 79 frequencies ranging from

2.402 GHz to 2.480 GHz (country specific bands

have different frequencies, defined in IEEE 802.11and IEEE 802.11.d).

Each of the frequencies is GFSK modulated, with a

channel width of 1 MHz.

The rates defined are 1 Mbps and 2 Mbps (there are

products in the market operating at 3 Mbps, too)

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Frequency Hopping Example

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Frequency Hopping Spread

Spectrum System (Transmitter)

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Frequency Hopping Spread Spectrum

System (Receiver)

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Slow and Fast FHSS

Frequency shifted every Tc seconds

Duration of signal element is Ts seconds

Slow FHSS has Tc Ts

Fast FHSS has Tc < Ts

Generally fast FHSS gives improved performance in

noise (or jamming)

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Slow Frequency Hop Spread Spectrum Using

MFSK (M=4, k=2)

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Fast Frequency Hop Spread Spectrum Using

MFSK (M=4, k=2)

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Advantages Provide the greatest amount of spreading.

Can be arranged to avoid portions of the spectrum (i.e. thoseoccupied by other systems or being the most affected byfrequency selective fading)

Have a relatively short acquisition time because the chip rate is

considerably less in the frequency hopping system. It is not as much affected by the near far problem as DSSS is

Disadvantages Requires a complex frequency synthesizer in order to generate

the hops

Always requires error correction.

Only the average power is spread; the narrowband interference iseither eliminated completely or not reduced at all.

ADVANTAGES AND

DISADVANTAGES OF FHSS

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Direct Sequence Spread Spectrum

(DSSS) Each bit represented by multiple bits using spreading code

Spreading code spreads signal across wider frequency band

In proportion to number of bits used

10 bit spreading code spreads signal across 10 times bandwidth of 1 bit

code One method:

Combine input with spreading code using XOR

Input bit 1 inverts spreading code bit

Input zero bit doesnt alter spreading code bit

Data rate equal to original spreading code

Performance similar to FHSS

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Example

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DSSS main parameters DSSS is defined (in IEEE 802.11) in the 2.4 GHz band

as operating on one of 14 possible carriers (country

specific bands have different number of frequencies,

defined in IEEE 802.11 and IEEE 802.11.d).

The selected carrier (channel) is PSK modulated witha channel width of 22 MHz.

The rates defined in IEEE 802.11 are 1 Mbps and 2 Mbps.

IEEE 802.11.b adds to DSSS the rates of 5.5 Mbps

and 11 Mbps (in the 2.4 GHz band), while keeping

the channel width at 22 MHz.

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Transmitter

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Receiver

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Using BPSK Example

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DSSS Performance Considerations

Let us assume a simple jamming signal at the center frequency of the DSSS system.

The jamming signal has the form

and the received signal is

The despreader at the receiver multiplies by c(t), so the

signal componentdue to the jamming signal is

Where

The jamming power passed by the filter is

The jamming power has been reduced by a factor of (Tc /T) through the use of

spread spectrum

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The inverse of this factor is the gain in signal-to-noise ratio

Where

Rc is the spreading bit rate,

R is the data rate,

Wd is the signal bandwidth

Ws the spread spectrum signal bandwidth.

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Approximate

Spectrum of

DSSS Signal

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ADVANTAGES AND

DISADVANTAGES OF DSSS

ADVANTAGES

Simple hard ware implementation

Best noise and anti jam performance

Best discrimination against multi path

Do not require a high speed fast setting frequencysynthesizer DISADVANTAGES

Requires wide band channel with little phase distortion

Long acquisition time.

Fast code generator needed.

Nearfar problem

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a.- Collocation / Aggregate Rate

DSSS biggest advantage over FHSS is its capability to provide rates of up to 11 Mbps.

When covering the whole 2.4 GHz band, three systems may be installed, providing

an aggregate rate of 33 Mbps. (Overall efficiency: 33Mbps/83.5MHz = 0.39 bits/Hz).

Additional systems, if installed, will share the spectrum with the already installed

systems, lowering the overall aggregate rate / throughput because of collision

occurrences.

In a 2.4 GHz FHSS synchronized environment, up to 12 systems can be collocated,

providing an overall aggregate rate of 36 MHz (efficiency: 36Mbps/83.5MHz= 0.43

bits/Hz).

b.- Contiguous band

IEEE 802.11 DSSS needs 22MHz, contiguous. If such a band is not available, the

system can not be operated. FHSS does not require contiguous band for correctoperation. If some frequencies are not available (administrative reasons, multipath

effects, noises, etc.), FHSS system could be set to use sequences that do not include

the unavailable frequencies.

Systems Behavior

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c.- Coverage

11 Mbps DSSS and 3 Mbps FHSS, cover more or less the same distances.

d.- Near / far problem

Present in DSSS, not critical in FHSS.

e.- Multipath sensitivity

DSSS is extremely sensitive, especially when operated at 11Mbps. To minimizemultipath effects, point-to-multipoint topology systems have to be limited to

small environments such as offices, while long distance systems are restricted to

the use of directional antennas, limiting the DSSS technology to point-to-point

topology applications.

f.- Bluetooth interferenceFHSS are significantly less sensitive to Bluetooth interference.

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DSSS provides 11 Mbps capacity links, but it is a sensitive technology(collocation, multipath, near/far, Bluetooth).

The most limiting factor, multipath, may be minimized by using the technology

for short distances or in point to point applications.

FHSS provides only 3 Mbps capacity links, but it is a very robust technology, withexcellent behavior in harsh environment characterized by large areas of

coverage, multiple collocated cells, noises, multipath, Bluetoooth presence, etc.

The technology allows easy cellular point- to-multipoint deployment, providing

excellent reliability.

Conclusions

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References

1. Data and computer communication william stallings eigth edition

2. Principles of communication systems taub and schilling second

edition

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