Spread spectrum radio

CDMA trải phổ spread spectrum

to integrated bar code scanner/

palmtop computer/radio modem de-vices for warehousing, digital dispatch, digital cellphone communications, and

‘information society’ city-, area-,

state-or country-wide netwstate-orks fstate-or transmit-ting facsimile, computer data, e-mail

or multimedia data

History of spread spectrum

Early spark-gap wireless transmitters actually used spread spectrum, since their RF bandwidths were much wider than their information bandwidth The first intentional use of spread spectrum was probably by Armstrong in the late 1920s or early 1930s with wide-band frequency modulation (FM) However, the real impetus for spread spectrum came with World War II

Both the allies and the axis powers experimented with simple spread-spec-trum systems Much of what was done

is still shrouded in secrecy, however

The first publicly available patent

Advances in technology have

brought to us a new form of

digital radio service called

‘spread spectrum.’ First developed by

the military as a deterrent to jamming

and eavesdropping (espionage), the

spread-spectrum technique handles

ra-dio signal differently from other forms

of digital radio

In spread-spectrum operation, the

radio signal is spread across a great

bandwidth with the use of a

spread-ing algorithm based upon a

pseudo-noise (PN) code, or a number that each

unit of the system is programmed

with The result is a signal that is

es-sentially ‘buried’ in the noise floor of

the radio band

The receiver is programmed to

ex-amine the bandwidth of the spread

sig-nal and correlate the data (despread it)

The process of correlation also causes

any other signal received to be spread

as the wanted signal is despread This

causes unwanted signals which appear

as noise The result is a signal that is

extremely difficult to detect, does not

interfere with other services and still

passes a great bandwidth of data

Spread spectrum, the art of secure

digital communications, is now being

exploited for commercial and

indus-trial purposes In the future, hardly

anyone will escape being involved, in

some or the other way, with

spread-spectrum communications

Commercial applications for

spread spectrum range from wireless

PC-to-PC local area networks (LANs)

Lamarr, the Hollywood movie actress,

and George Antheil, an avant-garde

music composer This patent was granted in 1942, but the details were a military secret for many years The in-ventors never realised a dime for their invention; they simply turned it over

to the US government for use in the war effort, and commercial use was delayed until the patent had expired Most of the work done in spread spectrum throughout the 1950s, 1960s and 1970s was heavily backed by the military and drowned in secrecy The global positioning system (GPS) is now the world’s single largest spread-spectrum system Most of the details

on GPS are now public information Spread spectrum was first used for commercial purposes in the 1980s when Equatorial Communications of Mountain View, CA, used direct se-quence for multiple-access communi-cations over synchronous satellite tran-sponders In the late 1980s, the US

Fed-SPREAD-SPECTRUM

TECHNOLOGY AND ITS

APPLICATIONS

In the future, hardly anyone will escape being involved, in some or the other way, with spread-spectrum—the art of secure digital communications

Fig 1: Spread-spectrum signals are hard to detect on narrow-band equipment because the signal’s energy is spread over a bandwidth of maybe 100 times the information bandwidth

eral Communications Commission

(FCC) opened up the industrial,

scien-tific and medicine (ISM) frequency

bands for unlicenced spread-spectrum

communications

Applications

Typical applications for

spread-spec-trum radio are:

1 Cellular/PCS base station

inter-connect

2 Last-mile obstacle avoidance

3 Private networks

4 Railroads and transportation

5 Utilities like electricity, oil, gas

and water

6 Banks, hospitals, universities and

corporations

7 Disaster recovery and

special-event PSTN extensions

8 TELCO bypass

9 Rural telephony

10 Videoconferencing

11 LAN/WAN/Internet

connec-tion

How spread spectrum

works

The term ‘spread spectrum’ describes

a modulation technique that makes the

sacrifice of bandwidth in order to gain

signal-to-noise (S/N) performance

Ba-sically, in a spread-spectrum system,

the transmitted signal is spread over a

frequency much wider than the

mini-mum bandwidth required to send the

signal

The fundamental premise is that in

channels with narrow-band noise,

in-creasing the transmitted signal

band-width results in an increased

probabil-ity that the received information will

be correct If total signal power is

in-terpreted as the area under the

spec-tral density curve, signals with

equiva-lent total power may have either a

large signal power concentrated in a

small bandwidth or a small signal

power spread over a large bandwidth

Spread signals are intentionally

made to be much wider-band than the

information they are carrying to make

them more noise-like Because

spread-spectrum signals are noise-like, they

are hard to detect They are also hard

to intercept or demodulate Further,

spread-spectrum signals are harder to

jam (interfere with) than narrow-band signals The low probability of inter-cept and anti-jam features are the rea-sons why the military has used spread spectrum for so many years

Spread-spectrum signals use fast codes that run many times the infor-mation bandwidth or data rate These special ‘spreading’ codes are called

‘pseudo random’ or ‘pseudo noise’

codes They are called ‘pseudo’ be-cause they are not real Gaussian noise

The use of special pseudo-noise codes in spread-spectrum communica-tions makes signals appear wide-band and noise-like It is this very charac-teristic that makes spread-spectrum signals possess the quality of low prob-ability of intercept

Spread-spectrum transmitters use the same transmit power levels as nar-row-band transmitters Because spread-spectrum signals are very wide, they transmit at a much lower spec-tral power density, measured in watts per hertz, than narrow-band transmit-ters The lower transmitted power den-sity characteristic gives spread signals

a big plus Spread and narrow-band signals can occupy the same band, with little or no interference This ca-pability is the main reason for all the

interest in spread spectrum today Since the total integrated signal density or signal-to-noise ratio (SNR)

at the correlator’s input determines whether there will be interference or not, all spread-spectrum systems have

a threshold or tolerance level of inter-ference beyond which useful commu-nication ceases This tolerance or threshold is related to the spread-spec-trum processing gain Processing gain

is essentially the ratio of the RF band-width to the information bandband-width Direct sequence and frequency hopping are the most commonly used methods for the spread-spectrum tech-nology Although the basic idea is the same, these two methods have many distinctive characteristics that result in completely different radio perfor-mances

The carrier of direct-sequence ra-dio stays at a fixed frequency The nar-row-band information is spread out into a much larger (at least ten times) bandwidth by using a pseudo-random chip sequence

Generation of the direct-sequence spread-spectrum signal (spreading) is shown in Fig 2 The narrow-band sig-nal and the spread-spectrum sigsig-nal both use the same amount of transmit power and carry the same information However, the power density of the spread-spectrum signal is much lower than the narrow-band signal (The power density is the amount of power over a certain frequency.) As a result,

it is more difficult to detect the pres-ence of the spread-spectrum signal In this case, the narrow-band signal’s power density is ten times higher than the spread-spectrum signal, assuming the spread ratio as ‘10.’

At the receiving end, the spread-spectrum signal is despread to gener-ate the original narrow-band signal as shown in Fig 3

If there is an interference jammer

in the same band, it will be spread out during despreading As a result, its impact is greatly reduced This is the way the direct-sequence, spread-spec-trum radio fights the interference It spreads out the offending jammer by the spreading factor, which is at least

‘10.’ In other words, the offending

Fig 2: Generation of the direct-sequence spread-spectrum signal (spreading)

Fig 3: At the receiving end, the spread-spectrum signal is despread to generate the original narrow-band signal

jammer’s amplitude is greatly reduced

by at least 90 per cent

Frequency hopping achieves the

same result by using a different carrier

frequency at a different time Its carrier

will hop around within the band, so it

will avoid the jammer at some

frequen-cies The frequency hopper is more

popular and the only way to survive in

the 2.45GHz band because the

leak-ages from the microwave oven (from

2.4 to 2.5 GHz) sometimes exceed 10W

Frequency hopper is not needed in the

915MHz band because there is no legal

big jammer in this frequency

The frequency-hopping technique

is shown in Fig 5 It does not spread

the signal, so there is no processing

gain The processing gain is the

in-crease in power density when the

sig-nal is despread and it improves the

received signal’s SNR In other words,

the frequency hopper needs to put out

more power in order to have the same

SNR as a direct-sequence radio

The frequency hopper is also more

difficult to synchronise the receiver to

the transmitter because both the time

and frequency need to be in tune

Whereas, in a direct-sequence radio,

only the timing of the chips needs to be

synchronised The frequency hopper

will need to spend more time to search the signal and lock to it As a result, the latency time is usually longer Whereas,

a direct-sequence radio can lock in the chip sequence in just a few bits

Usually, to make the initial synchronisation possible, the frequency hopper will park at a fixed frequency before hopping or communication be-gins If the jammer happens to locate at the same frequency as the parking fre-quency, the hopper will not be able to hop at all And once it hops, it will be very difficult to re-synchronise if the receiver ever lost the sync

The hopper usually costs more and

is more complicated than direct-se-quence radio because it needs extra hopping and synchronising circuits to implement the synchronisation algo-rithm

The frequency hopper, however, is better than direct-sequence radio when dealing with multipath This is because the hopper does not stay at the same frequency and a null at one frequency

is usually not a null at another fre-quency if it is not very close to the original frequency So a hopper can usually survive multipath better than direct-sequence radio The frequency hopper can usually carry more data than direct-sequence radio because the signal is narrow-band

When two signals collide, the stronger one may survive regardless

of the kind of signal In this band, all radio must not exceed the power den-sity limit set by the FCC In other words, all radios are equal when in-terfering with one another The best strategy to prevent interference is to make the important radios close to each other (strengthen the link) and

prevent using frequency hopper be-cause they are guaranteed to interfere with other radios

The hopper itself will also suffer when it interferes with other radio Any system that can suffer more data loss will survive better In general, a voice system can survive an error rate as high

as 10–2, while a data system must have

an error rate lower than 10–4 Voice sys-tem can tolerate more data loss because the human brain can ‘guess’ between the words while a dumb microproces-sor can’t As a result, the frequency hopper is more popular for voice than data communications

Advantages of spread-spectrum wireless systems

Some of the advantages of spread-spectrum wireless systems over con-ventional systems are:

1 No crosstalk interference. Con-ventional cordless phones frequently suffer from crosstalk interference, es-pecially when used in densely popu-lated residential areas (such as apart-ment complexes) This problem disap-pears in spread-spectrum cordless phone systems because:

(i) Crosstalk interference is greatly attenuated due to the processing gain

of the spread-spectrum system as de-scribed earlier

(ii) The effect of the suppressed crosstalk interference can be essentially removed with digital processing where noise below certain threshold results

in negligible bit errors These negli-gible bit errors will have little effect

on voice transmissions

2 Better voice quality/data integ-rity and less static noise. Due to the processing gain and digital processing nature of spread-spectrum technology,

a spread-spectrum based system is more immune to interference and noise This greatly reduces the static noise induced by consumer electron-ics devices that is commonly experi-enced by conventional analogue wire-less system users

3 Lowered susceptibility to multipath fading. Because of its inher-ent frequency diversity properties (thanks to wide spectrum spread), a spread-spectrum system is much less

Fig 4: Direct-sequence spread-spectrum signal

Fig 5: Frequency hopping

susceptible to multipath fading This

makes reception of a spread-spectrum

based cordless phone much less

sensi-tive to the location and pointing

di-rection of the handset than a

conven-tional analogue wireless system

4 Inherent security. In a

spread-spectrum system, a PN sequence is

used to either modulate the signal in

the time domain (direct sequencing)

or select the carrier frequency

(fre-quency hopping) Due to the

pseudo-random nature of the PN sequence, the

signal in the air is ‘randomised.’ Only

a receiver that has exactly the same

pseudo-random sequence and

syn-chronous timing can despread and

re-trieve the original signal

Conse-quently, a spread-spectrum system

provides signal security that is not

available to conventional analogue

wireless systems

5 Co-existence. A spread-spectrum

system is less susceptible to

interfer-ence than other non-spread-spectrum

systems In addition, with proper

de-signing of pseudo-random sequences,

multiple spread-spectrum systems can

coexist without causing severe

inter-ference to other systems This further

increases the system capacity for

spread-spectrum systems or devices

6 Longer operating distances. A

spread-spectrum device operated in

the ISM band is allowed to have higher

transmit power due to its

non-inter-fering nature Because of the higher

transmit power, the operating distance

of such a device can be significantly

longer than for a traditional analogue

wireless communication device

7 Hard to detect. Spread-spectrum

signals are transmitted over a much

wider bandwidth than conventional

narrow-band transmissions—20 to 254

times the bandwidth of narrow-band

transmissions Since the

communica-tion band is spread, these can be

trans-mitted at a low power without

suffer-ing interference from background

noise This is because when

despreading takes place, the noise at

one frequency is rejected, leaving the

desired signal

8 Hard to intercept or demodulate.

The very foundation of the spreading

technique is the code used to spread

the signal Without knowing the code,

it is impossible to decipher the trans-mission Also, because the codes are

so long (and quick), simply viewing the code would still be next to impos-sible to solve the code, hence intercep-tion is very hard

9 Harder to jam than narrow bands. The most important feature of the spread-spectrum technique is its ability to reject interference At first glance, it may be considered that spread-spectrum transmission would

be most affected by interference How-ever, any signal is spread in the band-width, and after it passes through the correlator, the bandwidth signal is equal to its original bandwidth plus the bandwidth of local interference

An interference signal with 2MHz bandwidth being input into a direct-sequence receiver whose signal is 10MHz wide gives 12MHz output from the correlator The wider the in-terference bandwidth, the wider the output signal Thus the wider the in-put signal, the less the effect on the system because the power density of the signal after processing is lower, and less power falls in the band-pass

filter

Conversely, it may be guessed that the most effective interference to a direct-sequence receiver is one with the narrow-est bandwidth (a continuous-wave carrier) This is the most effective because power density

in the correlator output due to narrow-band signals is higher than due to wide-band signals

10 Use for ranging and

ra-dar. The spread-spectrum technique can be used to construct precise rang-ing and radar systems The spread car-rier, modulated with the pseudo noise sequence, permits the receiver to mea-sure very precisely the time when the signal was sent; thus, spread-spectrum technique can be used to time the dis-tance to an object, as in the case of radar reflection Both applications have been commonly used in the aerospace field for many years

Modulation

For direct-sequence systems, the en-coding signal is used to modulate a carrier, usually by phase-shift keying (e.g., biphase or quadriphase) at the code rate

Frequency-hopping systems gener-ate their wide band by transmitting at different frequencies, hopping from one frequency to another according to the code sequence Typically, such a system may have a few thousand fre-quencies to choose from, and unlike direct-sequence signal, it has only one output rather than symmetrically dis-tributed outputs

The important thing to note is that

Fig 6: Frequency hopping

Fig 7: FHSS spectrum

both direct sequencing and frequency

hopping generate wide-band signals

controlled by the code-sequence

gen-erator For direct- sequence systems the

code is direct-carrier modulation,

while frequency hopping commands

the carrier frequency

Considering direct sequencing,

bal-ance modulation is an important tool

in any suppressed carrier system, used

to generate the transmitted signal

Bal-anced modulation helps to hide the

signal, and there is no power wasted

in transmitting a carrier that would

contribute to interference rejection or

information transfer When a signal

has poor balance in either code or

carrier, spikes are seen in its

spec-trum With these spikes, or spurs, the

signal is easily detectable, since once

these spikes are noticed above the

noise, it is obvious to look for the

hidden signal

Information modulation in

spread-spectrum systems is possible in most

of the conventional ways; both

ampli-tude modulation (AM) and angle

modulation are satisfactory Normally,

AM is not used for spread-spectrum

signals because it tends to be

detect-able when examining the spectrum

Frequency modulation (FM) is more

useful because it is a constant

enve-lope signal, but information is still

readily observed In both AM and FM,

no knowledge of the code is needed

to receive the transmitted information

Clock modulation is actually

fre-quency modulation of the code clock

It is usually avoided (e.g., for

fre-quency hopping) because the loss in

correlation due to phase slippage

be-tween received and local clocks can

degrade the performance For direct

sequence, an FM demodulator tuned

to radio frequency (RF) carrier plus/

minus the clock could recover the data

Another technique is code

modifi-cation, where the code is changed such that the information is embedded in

it, then modulated by phase transitions

on an RF carrier

Demodulation

Once the signal is coded, modulated and then sent, the receiver must de-modulate the signal This is usually done in two steps:

1 Spectrum-spreading (e.g., direct-sequence or frequency-hopping) modulation is removed

2 The remaining information-bear-ing signal is demodulated by multi-plying with a local reference identical

in structure and synchronised with the received signal

Coding technique in spread spectrum

In order to transmit anything, codes used for data transmission have to be

considered However, here we will not discuss the coding of information (like error-correction coding) but codes that act as noise-like carriers for the infor-mation being transferred These codes are much longer than those for the usual areas of data transfer, as these are intended for bandwidth spreading

Coding can be of three types:

1 Maximal sequences

2 Composite code sequences

3 Error detection and correction codes (EDACs)

The properties of codes used in spread-spectrum systems are:

1 Protection against interference.

Coding enables a bandwidth trade, for processing gain against interfering sig-nals

2 Provision for privacy. Coding enables protection of signals from eavesdropping, so even the code is se-cure

3 Noise-effect reduction. Error-de-tection and correction codes can reduce

the effects of noise and interference One such coding method is maxi-mal sequences Maximaxi-mal codes can be generated by a given shift register or

a delay element of given length In bi-nary shift register sequence generators, the maximum sequence length is 2n-1 chips, where ‘n’ is the number of stages

in the shift register

A shift register generator consists

of a shift register in conjunction with appropriate logic, which feeds back a logical combination of the state of two

or more of its stages to its input The output, and contents of its ‘n’ stages

at any clock time, is a function of the outputs of the stages fed back at the proceeding sample time Some codes can be 7 to 236–1 chips long

Use of EDACs is mandatory for fre-quency-hopping systems to overcome the high rates of error induced by par-tial band jamming These codes’ use-fulness has a threshold that must be exceeded before satisfactory perfor-mance is achieved

In direct-sequence systems, EDAC

is not advisable because of the effect it has on the code, increasing the appar-ent data transmission rate, and may increase the jamming threshold Some demodulators can operate error detec-tion with approximately the same ac-curacy as an EDAC, so it may not be worthwhile to include a complex cod-ing/decoding scheme in the system

Applications of spread spectrum

Wireless local area network (WLAN).

A WLAN is a flexible data communi-cation system implemented as an ex-tension to or an alternative for a wired local area network WLANs transmit and receive data over the air, minimising the need for wired connec-tions Thus, WLANs combine data con-nectivity with user mobility and en-able moven-able LANs

Most WLAN systems use spread-spectrum technique (both frequency hopping and direct sequence) WLANs are being used in health care, retail, manufacturing, warehousing, aca-demic and other arenas These indus-tries have profited from the produc-tivity gains of using handheld

termi-In CDMA spread-spectrum transmission, user

channels are created by assigning different codes to

different users This type of system provides privacy

by controlling distribution of user-unique code

sequences.

nals and notebook computers to

trans-mit real-time information to

centralised hosts for processing

WLANs offer productivity,

conve-nience and cost advantages over wired

networks

Space systems. In space stations,

which are continuously accessible to

interference, spread-spectrum methods

have proved effective This is

espe-cially true for communication satellites

In general, satellites do not employ

processing on-board as it adds to the

complexity and would limit the

num-ber of satellite users A simple

repeat-ing satellite is used, so all the

spread-spectrum modulation and

demodula-tion must be done on the ground With

no on-board processing, the satellite is

forced to transmit an uplink

interfer-ence signal, which reduces the

space-craft transmitter power to send the

de-sired signal Another disadvantage of

no on-board processing is that every

receiver would have to acquire a

spread-spectrum demodulator

Global positioning system (GPS).

GPS is a satellite-based navigation

sys-tem developed and operated by the US

Department of Defense The idea

be-hind GPS is to transmit

spread-spec-trum signals that allow range

measure-ment from an unknown satellite

loca-tion With knowledge of the

transmit-ter location and the distance to the

sat-ellite, the receiver can locate itself on a

sphere whose radius is the distance

measured After receiving signals and

making range measurement on other

satellites, the receiver can calculate its

position based on the intersection of

several spheres

GPS permits users to determine

their 3-D position, velocity and time

This service is available for military

and commercial users round the clock,

in all weather, anywhere in the world

GPS uses NAVSTAR (NAVigation

Satellite Timing And Ranging)

satel-lites The constellation consists of 21

operational satellites and three active

spares This provides a GPS receiver

with four to twelve usable satellites ‘in

view’ at any time A minimum of four

satellites allow the GPS card to

com-pute latitude, longitude, altitude and

GPS system time The NAVSTAR

sat-ellites orbit the earth at an altitude of 10,898 Nautical miles in six 55-degree orbital planes, with four satellites in each plane The orbital period of each satellite is approximately 12 hours

The GPS satellite signal contains information to identify the satellite, as also positioning, timing, ranging data and satellite status The satellites are identified by the space vehicle ber or the pseudo-random code num-ber They transmit on two L-band fre-quencies: 1.57542 GHz (L1) and 1.22760 GHz (L2) The L1 signal has a sequence encoded on the carrier frequency by a modulation technique that contains two codes, a precision (P) code and a course/acquisition (C/A) code The L2 code contains only P code, which is encrypted for military and authorised commercial users

Personal communications

The advantages of using spread spec-trum in data and voice communica-tions are:

1 Spread-spectrum signals can be overlaid onto bands where other sys-tems are already operating, with mini-mal performance impact to or from the other systems

2 The anti-interference character-istics of spread-spectrum signals are important in environments where sig-nal interference can be harsh, such as networks operating on manufacturing floors

3 Cellular systems designed with code-division multiple-access (CDMA) spread-spectrum technology offer greater operational flexibility and pos-sibly a greater overall system capacity than systems built on frequency-divi-sion multiple-access (FDMA) or time-division multiple-access (TDMA) methods

4 The anti-mutipath characteristics

of spread-spectrum signaling and re-ception techniques are desirable in ap-plications where multipath is likely to

be prevalent

For these reasons, many companies have begun developing spread-spec-trum systems Voice-orientated digital cellular and personal communication service providers are using CDMA

CDMA implemented with

direct-sequence spread-spectrum signaling is

among the most promising

multiplex-ing technologies for cellular telephony

services

The advantages of direct-sequence

spread-spectrum signaling for these

services include superior operation in

multipath environments, flexibility in

allocation of channels, privacy and the

ability to operate asynchronously Also

among the attractive features of

CDMA spread-spectrum is the ability

to share bandwidth with narrow-band

communication without undue

degra-dation of either system’s performance

In CDMA spread-spectrum

trans-mission, user channels are created by

assigning different codes to different

users This type of system provides

pri-vacy by controlling distribution of

user-unique code sequences

Spread-spectrum systems exhibit

unique qualities that cannot be

ob-tained from conventional narrow-band

systems There are many research

av-enues exploring these unique qualities

Spread-spectrum technology can

alleviate the problems of conventional cordless telephones However, because the technology was initially developed for military applications, it could not

be readily applied for commercial use due to its high cost and large size

As this technology and the com-ponents continue to develop, inte-grated circuit (IC) technology has un-dergone drastic advancement This has made commercial use of spread-spec-trum technology a realistic proposi-tion

In conjunction with increased cordless phone usage, data applica-tions are also increasingly finding their way into homes and small offices This

is due in part to the maturity of multi-media technologies and applications and arrival of the information era where global information through powerful networking vehicles (such as the Internet) is penetrating many homes and offices

These developments have created demands for wireless data for homes and small offices where wiring can

of-ten be either very costly or very in-convenient ISM-band sprespec-trum devices can be designed to ad-dress the wireless data needs of home and small-office users

The road ahead

The factors that will determine the com-mercial success of spread-spectrum technology are its maturity, advance-ment of baseband as well as RF IC tech-nologies, and system integration to of-fer the best value to end users

As these elements continue to ad-vance, spread-spectrum technology will find more and more commercial applications ranging from cordless te-lephony to wireless LAN and wireless data, digital cellular telephony and even personal communication services The end users will be the ultimate ben-eficiaries as the quality of these prod-ucts improves while the cost contin-ues to decline z

The author is a lecturer in mechanical engineer-ing at N.L Polytechnic College, Mettupalayam, Tamil Nadu