Direct Sequence Spread-Direct Sequence Spread-Spectrum with Spectrum with

Frequency Hopping Frequency Hopping

What is spread spectrum?What is spread spectrum? Methods by which energy generated in a Methods by which energy generated in a

particular bandwidth is deliberately particular bandwidth is deliberately spread in the frequency domain, resulting spread in the frequency domain, resulting in a signal with a wider bandwidth. in a signal with a wider bandwidth.

used for a variety of reasons, including used for a variety of reasons, including the establishment of secure the establishment of secure communications, increasing resistance to communications, increasing resistance to natural interference and jamming, and to natural interference and jamming, and to prevent detection. prevent detection.

Why do we need this?Why do we need this?

In some situations it is required that a In some situations it is required that a communication signal be difficult to communication signal be difficult to detect, and difficult to demodulate even detect, and difficult to demodulate even when detected. Here the word ‘detect’ is when detected. Here the word ‘detect’ is used in the sense of ‘to discover the used in the sense of ‘to discover the presence of’.presence of’.

In other situations a signal is required that In other situations a signal is required that is difficult to interfere with, or ‘jam’.is difficult to interfere with, or ‘jam’.

Commercial useCommercial use Initial commercial use of spread spectrum Initial commercial use of spread spectrum

began in the 1980s in the US with three began in the 1980s in the US with three systems: Equatorial Communications System's systems: Equatorial Communications System's very small aperture (VSAT) satellite terminal very small aperture (VSAT) satellite terminal system for newspaper newswire services, Del system for newspaper newswire services, Del Norte Technology's radio navigation system Norte Technology's radio navigation system for navigation of aircraft for crop dusting and for navigation of aircraft for crop dusting and similar applications, and Qualcomm's similar applications, and Qualcomm's OmniTRACS system for communications to OmniTRACS system for communications to trucks trucks

Spread-spectrum telecommunicationsSpread-spectrum telecommunications a technique in which a (telecommunication) a technique in which a (telecommunication)

signal is transmitted on a bandwidth signal is transmitted on a bandwidth considerably larger than the frequency content considerably larger than the frequency content of the original information. of the original information.

a signal structuring technique that employs a signal structuring technique that employs direct sequence, frequency hopping or a hybrid direct sequence, frequency hopping or a hybrid of these, which can be used for multiple access of these, which can be used for multiple access and/or multiple functions. and/or multiple functions.

This technique decreases the potential This technique decreases the potential interference to other receivers while achieving interference to other receivers while achieving privacy. privacy.

Forms of spread spectrumForms of spread spectrum

Frequency-hopping spread spectrum(FHSS)Frequency-hopping spread spectrum(FHSS) direct-sequence spread spectrum (DSSS) direct-sequence spread spectrum (DSSS) time-hopping spread spectrum (THSS)time-hopping spread spectrum (THSS) chirp spread spectrum (CSS)chirp spread spectrum (CSS)

and combinations of these techniquesand combinations of these techniques

Each of these techniques employs pseudorandom Each of these techniques employs pseudorandom number sequences — created using pseudorandom number sequences — created using pseudorandom number generators .number generators .

What is pseudorandom number sequences ?What is pseudorandom number sequences ?>is a sequence of numbers that has been computed >is a sequence of numbers that has been computed by some defined arithmetic process but is effectively by some defined arithmetic process but is effectively a random number sequence for the purpose for a random number sequence for the purpose for which it is required. which it is required.

To determine To determine andand control the spreading pattern of the control the spreading pattern of the signal across the allotted bandwidth signal across the allotted bandwidth

BenefitsBenefits

Resistance to intended or unintended Resistance to intended or unintended jamming.jamming.

Sharing of a single channel among Sharing of a single channel among multiple users.multiple users.

Determination of relative timing between Determination of relative timing between transmitter and receiver.transmitter and receiver.

UsesUses The United States GPS, Russian The United States GPS, Russian GlonassGlonass, and , and

European Galileo satellite navigation systems.European Galileo satellite navigation systems. CDMA cellular phones.CDMA cellular phones. Cordless phones operating in the 2.4 and 5.8 Cordless phones operating in the 2.4 and 5.8

GHz bands.GHz bands. The 802.11 and 802.11b The 802.11 and 802.11b Wi-FiWi-Fi standards. (The standards. (The

faster modes in 802.11g use OFDM, not faster modes in 802.11g use OFDM, not spread spectrum, although it can fall back to spread spectrum, although it can fall back to the slower 802.11b modes.)the slower 802.11b modes.)

ZigBeeZigBee / 802.15.4 / 802.15.4 Automatic meter readingAutomatic meter reading

Spread Spectrum modulation Spread Spectrum modulation techniques present two major techniques present two major advantages for Wireless Local advantages for Wireless Local

Area Networks (WLAN):Area Networks (WLAN):((ranges allocated for use with Spread Spectrum ranges allocated for use with Spread Spectrum

technology is 2.4 GHz to 2.4835 GHz)technology is 2.4 GHz to 2.4835 GHz)

Low power densityLow power density relates to the fact that relates to the fact that the transmitted energy is spread over a the transmitted energy is spread over a wide band, and therefore, the amount of wide band, and therefore, the amount of energy per specific frequency is very low. energy per specific frequency is very low. The effect of the low power density of the The effect of the low power density of the transmitted signal is that such a signal transmitted signal is that such a signal will not disturb (interfere with) the will not disturb (interfere with) the activity of other systems' receivers in the activity of other systems' receivers in the same area. same area. 

RedundancyRedundancy relates to the fact that the relates to the fact that the message is (or may be) present on message is (or may be) present on different frequencies from where it may different frequencies from where it may be recovered in case of errors. The effect be recovered in case of errors. The effect of redundancy is that Spread Spectrum of redundancy is that Spread Spectrum systems present a high resistance to systems present a high resistance to noises and interference, being able to noises and interference, being able to recover their messages even if noises are recover their messages even if noises are present on the medium.  present on the medium. 

A Comparison of Frequency A Comparison of Frequency Hopping and Direct Sequence Hopping and Direct Sequence

Spread SpectrumSpread Spectrum

Why we need to compare the performance of Why we need to compare the performance of this technology? (WLAN’s)this technology? (WLAN’s)

possibility to collocate systems possibility to collocate systems noise and interference immunity noise and interference immunity operation in environments generating radio operation in environments generating radio

reflections reflections data transfer capacity (throughput)data transfer capacity (throughput) size size power consumption (relevant for battery based power consumption (relevant for battery based

note books) note books) price. price. 

Direct Sequence Spread SpectrumDirect Sequence Spread Spectrum> > ModulationModulation technique ,Also known as technique ,Also known as direct sequence code division multiple direct sequence code division multiple access (DS-CDMA) access (DS-CDMA) > The name 'spread spectrum' comes > The name 'spread spectrum' comes from the fact that the carrier signals occur from the fact that the carrier signals occur over the full bandwidth (spectrum) of a over the full bandwidth (spectrum) of a device's transmitting frequency.device's transmitting frequency.

>A RF carrier and pseudo-random pulse >A RF carrier and pseudo-random pulse train are mixed to make a noise like wide-train are mixed to make a noise like wide-band signal.band signal.

Direct sequence contrasts with the other Direct sequence contrasts with the other spread spectrum process, known as spread spectrum process, known as frequency hopping spread spectrum, in frequency hopping spread spectrum, in which a broad slice of the bandwidth which a broad slice of the bandwidth spectrum is divided into many possible spectrum is divided into many possible broadcast frequencies. broadcast frequencies.

In general, frequency-hopping devices In general, frequency-hopping devices use less power and are cheaper, but the use less power and are cheaper, but the performance of DS-CDMA systems is performance of DS-CDMA systems is usually better and more reliable. usually better and more reliable.

DS ModulationDS Modulation

Frequency Hopping spread Frequency Hopping spread spectrumspectrum

It is the repeated switching of frequencies It is the repeated switching of frequencies during radio transmission, often to during radio transmission, often to minimize the effectiveness of "electronic minimize the effectiveness of "electronic warfare" - that is, the unauthorized warfare" - that is, the unauthorized interception or jamming of interception or jamming of telecommunications. telecommunications.

It also is known as frequency- hopping It also is known as frequency- hopping code division multiple access (FH-code division multiple access (FH-CDMA).CDMA).

Transmitting on one frequency for a Transmitting on one frequency for a certain time, then randomly jumping to certain time, then randomly jumping to another, and transmitting again. another, and transmitting again.

FH-CDMA devices use less power and FH-CDMA devices use less power and are generally cheaper, but the are generally cheaper, but the performance of DS-CDMA systems is performance of DS-CDMA systems is usually better and more reliable. usually better and more reliable.

The biggest The biggest advantageadvantage of frequency of frequency hopping lies in the coexistence of several hopping lies in the coexistence of several access points in the same area, something access points in the same area, something not possible with direct sequence. not possible with direct sequence.

FH ModulationFH Modulation

Frequency hopping has two benefits. Electrical Frequency hopping has two benefits. Electrical noisenoise—random electromagnetic signals which are —random electromagnetic signals which are not part of any communications signal—will only not part of any communications signal—will only affect a small part of the signal. Also, the effects affect a small part of the signal. Also, the effects of any other forms of radio communications of any other forms of radio communications operating in narrow bands of the spectrum will be operating in narrow bands of the spectrum will be minimized. Any such interference that occurs will minimized. Any such interference that occurs will result in only a slightly reduced quality of voice result in only a slightly reduced quality of voice transmission, or a small loss of data. Since data transmission, or a small loss of data. Since data networks acknowledge successful receipt of data, networks acknowledge successful receipt of data, any missing pieces will trigger a request to any missing pieces will trigger a request to transmit the lost data. transmit the lost data.

Hybrid DS/FH Spread Spectrum Hybrid DS/FH Spread Spectrum Communication SystemCommunication System

Hybrid DS/FH Spread Spectrum Hybrid DS/FH Spread Spectrum

The DS/FH Spread Spectrum technique is The DS/FH Spread Spectrum technique is a combination of direct-sequence and a combination of direct-sequence and frequency-hopping. One data bit is frequency-hopping. One data bit is divided over frequency-hop channels divided over frequency-hop channels (carrier frequencies). In each frequency-(carrier frequencies). In each frequency-hop channel one complete PN-code of hop channel one complete PN-code of length is multiplied with the data signal length is multiplied with the data signal (see figure, where is taken to be 5). (see figure, where is taken to be 5).

As the FH-sequence and the PN-codes As the FH-sequence and the PN-codes are coupled, an address is a combination are coupled, an address is a combination of an FH-sequence and PN-codes. To of an FH-sequence and PN-codes. To bound the hit-chance (the chance that two bound the hit-chance (the chance that two users share the same frequency channel in users share the same frequency channel in the same time) the frequency-hop the same time) the frequency-hop sequences are chosen in such a way that sequences are chosen in such a way that two transmitters with different FH-two transmitters with different FH-sequences share at most two frequencies sequences share at most two frequencies at the same time (time-shift is random).at the same time (time-shift is random).

Hybrid System: DS/(F)FHHybrid System: DS/(F)FH

AdditionalAdditional

CDMA use technique Spread SpectrumCDMA use technique Spread Spectrum

How spread spectrum generatedHow spread spectrum generated

AdditionalAdditional

occupies a larger bandwidth than occupies a larger bandwidth than necessarynecessary

use of a code which is independent of the use of a code which is independent of the transmitted datatransmitted data

AdditionalAdditionalDirect SequenceDirect Sequence

Data signal multiplied by Pseudo Random Data signal multiplied by Pseudo Random Noise Code(PN Code)Noise Code(PN Code)

Low cross-correlation valueLow cross-correlation value Anti-jammingAnti-jamming Main problem: Near-Far effectMain problem: Near-Far effect

In cellular, it can do power control by BSIn cellular, it can do power control by BS In non-cellular, it need Frequency HoppingIn non-cellular, it need Frequency Hopping

AdditionalAdditionalFrequency HoppingFrequency Hopping

AdditionalAdditional

Disadvantage:Disadvantage: a high processing-gain is hard. There is a high processing-gain is hard. There is

need for a frequency-synthesizer able need for a frequency-synthesizer able perform fast-hopping over the carrier-perform fast-hopping over the carrier-frequencies.frequencies.