04 radio school

7/23/2019 04 Radio School

1/31

1

DT14 SPREAD-SPECTRUM TECHNIQUES

Radio School

Core Unit Radio Systems and Technology

R C U R

DetectorModulator

Channel coder

Speech coder

Channel decoder

Speech decoder

Digital Radio TransmissionDT14 Spread-spectrum

techniques CDMA

DT14


2/31

Index to DT14

bandspreading

CDMA, Code Division MA

code sequences

cellular systems

chip

cluster size 1

coordinated frequency hopping

discontinuous transmission, DTX

DS-CDMADS-CDMA, multipath propagation

DS-CDMA, synchronization

DS-CDMA, traffic capacity

fast frequency hopping

FEC

FH-CDMA

freqency hopping for military use

Gold sequences

interference-limited systems

interference suppression

GPS

jamming margin

matched filter detector

near-far problem

pn-sequences

processing gain

rake receiver

slow frequency hopping

Walsh functions


3/31

3


Digital Radio Transmission

DT14 Spread-spectrumtechniques CDMA

Contents page

1. Overview 4

1.1. Different types of spread spectrum 4

1.2. How DS-CDMA works 5

1.3. CDMA for mobile cellular networks 6

1.4. Characteristics of interference-limited systems 7

1.5. Other uses of band-spreading 8

1.6. Interference suppression 8

2. DS-bandspreading systems 9

2.1. Processing gain and anti-jamming margin 92.2. Transmitter and receiver arrangements 12

3. Technical aspects of DS band-spreading systems 14

3.1. Synchronization 14

3.2. Code sequences 14

3.3. Adjusting the receiver structure to multipath propagation 14

3.4. Use of FEC (Forward Error Control) 17

4. Frequency hopping for military systems 18

4.1. Fast frequency hopping 18

4.2. Slow frequency hopping 20

4.3. DS-CDMA 215. Cellular systems based on band-spreading 22

5.1. Introduction 22

5.2. Band-spreading through channel coding 22

5.3. DS-CDMA 22

5.4 FH-CDMA 25

Appendix 1. Characteristics of pn sequencies 27

Appendix 2. Walsh functions 30


4/31

4


1. Overview1.1. Different types of spread spectrum

It has already been shown in connection with frequency modulation and channel

coding that the detection characteristics can be improved through bandwidthexpansion (modulation gain and coding gain). If overall system considerations

permit bandwidth expansion, spread spectrum techniques can provide more

noticeable improvements in receiver sensitivity when interference is the limiting

factor.

The bandwidth expansion is achieved through applying a modulation (coding) that

is not directly related to the baseband information. Compared to FM, which can

also give considerable bandwidth expansion, there is no limit (or threshold) to the

improvement that can be obtained, so long as synchronism can be maintained

between the transmitter and the receiver. Thus, spread spectrum systems can

continue to discriminate against unwanted interfering signals, not marked by the

right code, even if the desired signal is considerably weaker than the interferingsignals within the wideband radio channel being used.

There are several ways of achieving band spreading. The three most common ones

are:

a. Direct-Sequence Spread SpectrumIt is normally called Direct-Sequence Code Division Multiple Access

(DS-CDMA) in civilian communication applications.

b.Fast Frequency-Hopping Spread SpectrumIt has only been used in military communication systems.

c.Slow Frequency-Hopping Spread SpectrumIt is normally called Frequency-hopping Code Division Multiple Access(FH-CDMA) in civilian communication applications. It is used in military

mobile networks, i.e. the Swedish Truppradio.

The name CDMA refers to the fact that a spread-spectrum system with sufficient

bandwidth expansion can give so large suppression of interference from radio

connections, marked with the wrong code, that enough isolation is obtained

between simultanious connectionsto permit multiple access. CDMA is thus an

alternative toFDMA and TDMA,discussed in previous modules and in module

DM1.

The major reason to apply band-spreading in military applicationsis thediscrimination against hostile interference (jamming) with the intention to disruptthe communication. The suppression of hostile jamming is based on the assumption

that the code can be kept secret, to prevent jamming signals from being marked

with the same code. In military applications, the code is generally determined by a

cryptographic key, similar to that used for encrypted transmissions.

Suppression of jamming was the original use of spread spectrum, which explains

the name of one of the most important characteristics: the Anti-Jamming orJamming Margin. Due to the bandwidth expansion, an acceptable transmissionquality can be obtained even if the interference/jamming (I or J) is considerably

stronger than the desired signal (received power C) at the receiver input. This

corresponds to a negative protection ratio (C/I)min

, normally expressed in dB.
http://dt1e.pdf/http://dt1e.pdf/http://dt1e.pdf/


5/31

5


The definition of the jamming margin (I/C)max

is the maximum relative level of the

interfering signal, I, at the receiver input, for acceptable transmission quality. The

jamming margin, for interference falling within the assigned frequency band, also

corresponds to the system selectivity at a FDMA system.

In true spread spectrum systems, the bandwidth expansion occurs for each informa-

tion bit, which means that also the short term frequency spectrum is spread out.This is obtained with direct sequence and to a large extent also with fast frequency

hopping.

In direct sequenceband-spreading systems, each primary radio symbol is codedwith a chipsequence of much higher rate than the symbol rate. The ratiobetween the chip and symbol rate determines the spreading ratio (ratio betweenthe modulation bandwidth after and before band-spreading). In a basic

direct-sequence system, the spreading ratio is equal to the processing gainwhichis closely related to the jamming margin, seesection 2.

Infrequency hopping,the codes define the differerent frequency hopping patterns.The optimum interference suppressing capability is obtained with random patterns,

which results in collisions, as sometimes more than one connection occupy the

same time-frequency slot. This limits the mutual isolation and thus the CDMA

capability.

At fast frequency hopping, the hops occur at least as fast as the rate of the inputsignal to the transmitter modulator.

Atslow frequency hopping(FH-CDMA), the hopping rate is so slow that manyinformation bits, using normal narrowband modulation, are sent during the duration

of each frequency hop. The problem is that it is necessary for good interference

suppression that the information in each source bit is spread out over the whole

assigned frequency band. This is achieved by adding FEC channel coding with

interleaving. This is the same hopping arrangement, that is discussed in modulesDT10 (channel coding) andDM1 (GSM). Frequency hopping was motivated in the

GSM system mainly by the need to introduce frequency diversity to support

channel coding with interleaving in connection with quasi-stationary propagation

channels.

This module deals primarily with DS-CDMA, as this is the preferred type of spread

spectum for cellular radio. A more detailed coverage is given in module DM3.

1.2. How DS-CDMA works

The ability to detect the desired signal against a background of strong interference

is based on the incoming desired signal being marked with a specific code known

to the receiver, and not used by any other of the simultanious connections in the

system. All interfering signals into the receiver (besides the wideband input noise

No), not marked with this code, are considerably suppressed by the receiver signal

processing, see section 2. The degree ofsuppression is determined by the

spreading ratio,which determines the processing gain. The processing gain isclosely related to the jamming margin.

An additional condition for maximum receiver sensitivity is that the timing of the

locally generated code sequency matches the coding on the received wanted signal.

Input signals with wrong timing are suppressed, even is the code is right. Thismeans that if the propagation channel has a large delay spread, only part of the

multi-path signal can be detected with full sensitivity by a simple receiver with one

detection channel.
http://dt10e.pdf/http://../DM/DM1E.pdfhttp://dt10e.pdf/http://../DM/DM1E.pdf


6/31

6


In a simple, one-channel receiver, the detector will try to match its code timing to

the largest multi-path component of the wanted signal. With more advanced signal

processing (rake structure, see section 3.3), several signal components can be

combined coherently in a multi-channel receiver, resulting in better utilization of

the received signal power and also in a diversity gain (multi-path diversity against

frequency-selective fading). To obtain diversity gain, it is necessary that the modu-

lation bandwidth is wide enough to enable the different propagation paths to be

distinguished in time by the receiver signal processing. The same condition can be

expressed as the need for the system bandwidth to exceed the correlation

bandwidth of the propagation channel to make frequency diversity possible.

Such a receiver, which can simultaneously receive signals with different

propagation delays and/or different codes, can also be used for base-stationdiversitywith soft handover in the outward direction. Se figure 1.1.

Figure 1.1

The same baseband signal with suitable spreading is sent from several base stationswith overlapping coverage. The terminal receiver decodes and combines signals

from several base stations simultaneously. It is necessary that the terminal receiver

knows the spreading codes used by the base transmitters involved.

1.3. CDMA for mobile cellular networks

An interesting question is whether CDMA can give better spectrum efficiency than

other types of multiple access, such as FDMA and TDMA. At CDMA the maxi-

mum number of simultanious connections is determined by the fact that all users

beside the studied one (wanted signal) will contribute to the total interference level.If this grows too much, the transmission quality will be unsufficient. On one hand

the bandspreading consumes more spectrum, on the other hand the jamming

margin allows several connections to share the same band.

Macrodiversity. Soft handover.

T: Base-station transmitterR: Terminal receiver

The receiver

knows thespreading codes used bythe transmitters.

(Cf. fig. 3.1)


7/31

7


In a cellular system based on DS-CDMA, enough bandspreading is generally used

so that all cells can share a common frequency band (cluster size 1). This means

that the number of cochannel interferers will be much larger than at a

corresponding FDMA/TDMA system, which must use fairly large cluster size. In a

suitable designed, so called interference-limited system, advantage is taken of the

very efficient interference averaging, which results in a small difference betweenthe worst-case interference level and the average level. The cluster size is therefore

nearly determined by the average level of the interference. The interference

averaging gives a considerable improvement in frequency economy compared to

traditional FDMA and TDMA systems, where the needed cluster size is more or

less determined by the worst case interference level, which is much higher than the

average level. This is the main reason why a DS-CDMA or a FH-CDMA system

typically gives better frequency economy than a FDMA or a TDMA system.

A major additional advantage of DS-CDMA is very efficient dynamic resource

allocation (bandwidth-on-demand), see section 1.4 below. This is an importantconsideration in future systems, which must handle non-speech signals. These

signals often have a high degree of burstiness, i.e. during a connection the maxi-

mum source data rate is much higher than the average rate. This is one of the main

reasons why DS-CDMA will be usd for the next generation cellular system,

UMTS.

FH-CDMA is not quite as efficient in this respect, but instead has the advantage

that coordinated hopping, which eliminates collisions, can be used within eachcell, giving in principle perfect isolation between the connections in each cell. This

is called an orthogonal arrangement. (To obtain maximum suppression of

interference from other cells, hopping patterns used at different cells shall be

mutually random. An option at GSM is frequency hopping, which is arranged so

that the hopping patterns are coordinated within each cell, but random between

differerent cells.) In a basic DS-CDMA system (without interference cancellation)

orthogonality cannot be obtained in the inward directions (see page 15).

1.4. Characteristics of interference-limited systems

The transmission quality, i.e. the b.e.r., is determined by the ratio between the

wanted signal (C) and the combined interference (I) from all the other connections

in the receiver input. Therefore, all features that have an influence on the level of I

(relative to the value of the wanted signal C and assuming a fixed spreading ratio)

will have an impact on the normalized system capacity (normalized with respect to

a given system bandwidth) and thus the frequency economy.

One consequence is the need to control the signal levels into the receiver (mainly

the base receiver) very accurately. Otherwise a nearby terminal with low

propagation loss will prevent detection of a weak signal from a terminal at the

outskirts of the cell. This is referred to as the near-far problem.

On the other hand all system facilities, which reduce the total interference level,

improve the frequency economy. A simple case, often used at two-way speech

connections is Discontinuous Transmission(DTX). It means that the transmitteris cut off during speech pauses. For normal, balanced speech each direction is

active less than half the time. Therefore, DTX gives two times improvement in

system capacity in a typical case.


8/31

8


This concept is exteded further as fast dynamic demand assignmentof channelcapacity (bandwith-on-demand). As is discussed in more detail in moduleDM 3,

the needed level of C at the receiver input, and thus the corresponding transmit

power, is proportional to the instantanious source data rate. (For a given bandwidth

after spreading, the processing gain and thus the jamming margin is inversely

proportional to the source data rate.) Demand assignment of channel capacity is

simply achieved through adaption of the transmit power to the source data rate.

Therefore, no central allocation of channel resources is necessary. This is one of

the most important advantages of DS-CDMA in connection with future cellular

systems.

1.5 Other uses of band-spreading

As a DS-CDMA detector strongly discriminates against received signals with the

wrong timing relative to the local code, very accurate measurements of propagation

delays are possible. The accuracy of the delay measurement is roughly inversely

proportional to the modulation bandwidth (roughly corresponding to the chip

length). Propagation delay measurements of signals coming from different trans-

mitters with known positions can be used for navigation or position dermination.An example is the Global Positioning System, GPS.

The wide bandwidth of the transmitted radio signal also means that the signal

power per hertz is low. This makes hostile interception of the signal moredifficult. If the bandwidth is wide enough, the signal will be masked by the inputnoise of a surveillance receiver. For the same reason, spread spectrum facilitates

the coexistence of uncoordinated radio servicesin the same frequency band.

1.6. Interference suppression

An advanced attachment to a DS-CDMA system is interference cancellation or

joint detection. The principle is to use very advanced and complicated signal

processing to analyse the mixture of wanted signal and interference signals in the

receiver input. This information is used to improve the isolation between the

wanted signal and the interference. The ideal situation is to establish full

orthogonality between the wanted signal and the interference. This procedure can

only work if the receiver knows the code markings of all the interfering signals,that shall be cancelled, and also can estimate the propagation channels for these

signals.

The simplest procedure is successive cancellation of one interferer at a time,

starting with the largest one. To the input is added out-of-phase a cancelling signal

corresponding to the strongest interferer. After that, the next largest one is detected,

and a suitable cancelling signal subtracted. The procedure continues until the

remaining interfering level is low enough, so that the reduced I/C ratio falls below

the AJ-margin and thus can be handled by the basic CDMA system.

It is planned to use interference suppression in the two system alternatives chosen

for UMTS (see moduleDM 3).
http://../DM/DM3E.pdfhttp://../DM/DM3E.pdfhttp://../DM/DM3E.pdfhttp://../DM/DM3E.pdf


9/31

9


2. DS-CDMA2.1. Processing gain and anti-jamming margin

In direct-sequence spread spectrum systems, the output signal from the normal

transmitter modulator is mixed with a local oscillator signal modulated with a codesequence (pseudo-noise or pn sequence) (see Fig. 2.1). Often2 ASK (antipodal

modulation) is used. The bandwidth expansion is obtained by replacing each

incoming information bit with a code sequence comprising M chips. We assume

that that the chip modulation is also 2 ASK. An example of suitable sequences is

maximum-length shift-register sequences (see Appendix 1). If the information bit

rate is di, the duration of each information bit will be T

i= 1/d

i.

If the antipodal modulation is used, in rough terms the modulation bandwidth

before spreading will be W = 1/Ti= d

i. After coding, the signal will comprise a

sequence of chips of length Tc

= Ti/M. The modulation bandwidth will be

B = 1/Tc

= MW, that is, the bandwidth expansion will be B/W = M. This

corresponds to a processing gain, Gp, of M times, or 10 log M dB.

Fig. 2.1

Spread spectrum through direct modulation with p-n sequence

Information bit stream:

0

0

0 01 1 0 1 1 1 0 10

1 1 0 1 1 1 0 0 1 1 0 0 0

0 1 0 0 0 1 1 0 0 1 1 1"1":

"0":

Modulation bandwidth:

One chip

Eachinformation bit iscoded with anM-chip codesequence.

Modulation bandwidth after spreading = B

W d I

Ti

i

=

G B

Wp=

Ti

Tc

Processing gain =

"Spread spectrum" B

T

M

TM W

c i

= 1

Ti

TM

Tc i= 1

B >>W
http://dt2e.pdf/http://dt2e.pdf/


10/31

10


Fig.2. 2

An approximate derivation of this expression is shown in Fig. 2.2. The starting

point is a conventional2-ASK/2-PSK link with a matched receiver. The bandwidth

of the matched filter is approximately W 1/Ti. The spreading is obtained by

antipodal modulation of the transmitter oscillator using a pn-sequence with M times

higher data rate, making the bandwidth M times greater. In the receiver, the LO is

modulated with the same pn-sequence, which is syncronized in time to the

corresponding sequence enbedded in the received signal.

The result is that exactly inverse operations are performed by the transmitter

spreading mixer and the receiver despreading mixer. The bandwidth of the desired

output signal from the receiver mixer is thus despread to the original bandwidth W.

The original, non-spread signal spectrum from the transmitter modulator has thus

been regenerated and can pass through the matched filter of bandwidth W without

any attenuation. All other signals that have not been coded with the right pn-

sequence included time shift will have their bandwidth expanded to at least B when

they pass through the receiver mixer (convolution of the input spectrum and the LO

spectrum). Consequently, at most a fraction, W/B = 1/M, of the interfering signals

can pass the matched filter. As an example a spreading factor of 1 000 corresponds

to a processing gain Gp

= 30 dB. However, owing to implementation difficulties, in

practice a somewhat lower value is obtained than the theoretical value of Gp

indicated above.

Inasmuch as the signal from the receiver LO has a noise-like characteristic, the

interfering signal from the matched filter will also be noise-like - regardless of the

structure of the interfering signal in the receiver input, especially if contributions

from many chips are added in the filter (impulse response much longer than a chip).

Closer analysis reveals that if the bandwidth expansion is enough, the noise from

the matched filter for all types of interfering signals that are not correctly coded is

very similar to Gaussian noise. Thus, the receiver detector is similarly influenced by

the noise density of the interfering signal and the thermal noise No

, which means

that we can apply the same detector characteristic as previously derived for white

Gaussian noise. An example is given in Fig. 2.3.

Spread spectrum technique. Processing Gain G

input signal

PSK(FSK)

Mod.

PSK(FSK)

Demod.

Codegenerator

Signalbandwidth

p

I = Jammer

(Hostile orwithin system)

I

C

No

B

BB

B

W

W

B

Code key

Output signal

d b si /

W di

B W

B B

sign

jam

=

G B

Wp=m

u

m u=

B>>W

p

u : Propagation delay

ModulatedLO

Attenuation C 0dBAttenuation J G r

r

p

p

p


11/31

11


Fig. 2.3

Since the interfering signal occupies a bandwidth of at least B in the input to the

detector filter, the power density of the interfering signal, Jo, in the worst case will

be J/B. The influence of Joon the detector can be regarded as equivalent to thecorresponding noise, N

o. The required value of C/J

owill be the same as the

required value of C/Nofor the basic arrangement without spread spectrum. From

this we get the expression for the jamming margin, J/C, as shown in Fig. 2.3. This

is a rough estimate. In a detailed analysis we would have to take into account the

auto and cross correlation characteristics of the code sequences and also the fact

that the amplitude distribution of the incoming interfering signals to the detector

can somewhat deviate from Gaussian.

Other modulation types than antipodal, which have different ratios between modu-

lation bandwidth and modulation data rate, can be used for the basic modulation

and the spreading modulation. The relations, derived above, are still valid as long

as the processing gain is defined as the ratio beween the bandwidths after andbefore spreading. As an example, at the systems to be used for UMTS, the basic

modulation will be either4QAM or16QAM.

Part of the bandwidth expansion can be achieved through coding. If so, the coding

gain is added to the basic value of the processing gain before adding channel

coding (see section 3.4). The basic processing gain relates to the source data rate -

not the rate after the channel coder.

Jamming Margin = J/C

J J

Bo=

W di

C

N

C

J

C

JB

E

Nd

o o

i

oi = =

C

J

E

N

d

Bi

o

i= J

CG

E

NdBp

dB

i

o dB

=( )

d kb s W kHz i= 10 10 /Information bit rate:

2-ASK modulation. Maximum permissible bit error rate of 1%.

Spreading factor: B/W=500, that is, B=5 MHz

Required

Processing Gain,

Jamming margin, J/C = 27-5 = 22 dB

Example:

p-nsequence

(Noisedensity,Watt/Hz)

C

E

NdBi

o =5

G B

dp

i

=

C E d

J J B

J N

i i

o

o o

==

Gp= 500 times or 27 dB
http://dt2e.pdf/http://dt2e.pdf/http://dt2e.pdf/http://dt2e.pdf/


12/31

12


2.2 Transmitter and receiver arrangements

If antipodal modulation is used, the information and spreading bits are defined in

terms of the polarity of the code sequence (see Fig. 2.1). This means that a com-

mon modulator can be used in the transmitter for both the source information and

spreading code (see Fig. 2.4). An NRZ signal, determined by modulus-2 additionof the source signal and the spreading sequence, is applied to the phase modulator.

(The bit rate of the spread sequence should be synchronized to and be a whole-

number multiple of the information bit rate.)

Fig. 2.4

As can be seen from Fig. 2.2, one way of despreading the received signal is

through the modulation applied to the local oscillator. The receiver mixer correlates

the received signal with the LO signal. Another procedure involves the use of an

unmodulated LO combined with a detector arrangement incorporating a filter, that

is matched to the code sequence (matched filter-receiver). One possibility is to use

a matched transversal filter whose outputs correspond to the train of + and -

bits in the code sequence (see Fig. 2.5).

A

C+

B

C

B

A

Infosource

NRZgenerator

2-ASK

modulatorNRZgenerator

Codegenerator

Code key

Tc

Ti

modulus 2

Transmitter for pn-sequence CDMA


13/31

13


Fig. 2.5

A fixed sequence can be realized by a SAW filter; a more advanced convolver-type

SAW structure can be used for a filter adjustable to different codes. At modest

bandwidths, the matched filter can also be realized using digital signal processing.

The advantage of a matched filter-receiver is that it facilitates synchronization of

the detector to the incoming signal. The correct synchronization in this arrange-ment is obtained by sampling the output from the matched filter at the right instant.

The matched filter can simultaneously process signal components having different

propagation delays. These occur at different times in the output and can be

distinguished provided that the difference in the propagation time is at least Tc.

(The usual way of implement the optimum receiver for multipath signals is the

RAKE structure,see Fig. 3.1).

Detection using a matched filter

PSKmod.

mod.LO

Unmod.LO

Matchedfilter

Env.det

Instant

Matched filter(SAW line)

Autocorrelation function

(Maximum-lenght sequence

+ + - + - + + + +- - - + - -

Tc

1

M

T M Tc=

1

+ -+ -+ + + + + - - + - -

0 T 2T

+1-1

-

P-n sequence

S1

C I

TS t S t dt

T

= ( ) ( ) 10

1


14/31

14


3. Technical aspects DS bandspreadingsystems

3.1 Synchronization

The usual despreading arrangements at a spread spectrum system is based on a

local oscillator, modulated by the chip sequence. For optimum reception of the

wanted signal, the the correct code must be used and also the pn-sequence shifted

to the correct time to match the propagation delay. The synchronization arrange-

ment is often the most complex subsystem in the receiver. The synchronization

initially involves seeking of the correct code delay (locking in) and then

maintaining the synchronization during the connection.

A short sequence with good correlation characteristics, e.g. a maximum-length

sequence, is often used for the initial locking in. The autocorrelation function

determines the output signal from the correlator. A strong output signal from the

correlator is only obtained when the relative time shifts of the received signal and

the sequence applied to the local oscillator match. During the locking-in phase, the

phase of the code sequence that modulates the local oscillator slides slowly in

relation to the corresponding sequence in the received signal. The right setting is

indicated by a strong signal from the correlator. Once the correlation peak is

detected, the slide ceases and a feedback loop is switched in to maintain the

synchronization.

3.2 Code sequences

Despite their good autocorrelation characteristics,maximum-length sequences (pn-

sequences) have some limitations in connection with spread spectrum systems. In

military applications, where the main requirement is high resistance to jamming, it

is essential that the code sequences used cannot be cracked by the enemy. In this

context, it is clearly a drawback of maximum-length sequences that their integral

structure is such that if only a small part of the sequence is known (twice the

number of stages in the generating shift register), the entire sequence can be

calculated. A sequence with a much more complex structure can be obtained, for

example using a nonlinear combination of two maximum-length sequences (non-

linear requires a different mathematical operation from modulus-2 addition).

Several types of code types can be used for DS-CDMA. Often the total band-

spreading is obtained by a combination of more than one spreading process, and

part of the bandspreading is obtained by FEC channel coding.

a. Different time squences of the same basic pn-code are assigned to different

connections. Time shifted versions of the same pn-sequence have optimum auto-

correlation characteristics. (See appendix 1).

b. Differerent pn-sequencies are used to code different connections. The sequences

are generated by shift registers with different feedback structures. Despite their

good autocorrelation characteristics, maximum-length sequences have relatively

poor cross-correlation characteristics, i.e. the correlation curve may have promi-

nent side lobes. In the calculation of the processing gain in section 2.1,it was

assumed that the cross-correlation characteristics were nearly perfect, so that the

effect of interfering signals coded with incorrect sequences was equivalent tothat of noise. However, this does not apply if the cross-correlation has prominent

side lobes. The largest secondary peak of the cross-correlation function is

typically only 10 dB lower than the main correlation peak.


15/31

15


For long maximum-length sequences of a certain length there are many

variations (different feedback arrangements of the signals from the shift-register

cells). For instance, for a shift register having a length of 10 there are 60 diffe-

rent sequence variants. If a suitable pair of sequences is chosen, their cross-

correlation characteristics will be much better than in the typical case given

above. If these sequences are added modulus 2 with different relative time-

shifts, a large number of new sequences are obtained which also have good

cross-correlation characteristics. The Gold sequences (see Appendix 1) are the

best known sequences of this type.

c. Sequencies are used with improved orthogonality characteristics. The simplest

class of such function is the Walsh functions, see appendix 2. The 64-symbol

class of Walsh functions is used at the US DS-CDMA standard IS-95, see

module DM3. The limitation is that full orthogonality is only obtained if there is

very good time synchronization (at the receiver input) between the sequencies

used for the mutual isolation of simultanious connections. The timing is so

critical that it cannot be obtained in the inward direction. Even in the outward

direction, the isolation is somewhat degraded at the input to the terminal recei-ver, if there is multi-path propagation with large delay spread.

More advanced functions with similar properties as the Walsh functions will be

used for the WCDMA version of UMTS, see moduleDM3. The advantage with

these improved functions is adaptivity to a large spread of source data rates.

3.3 Adjusting the receiver structure to multipath propagation

Essential properties of a spread spectrum system are related to the criterion that the

receiver code must have exactly the right time relation to the wanted input signal to

be detected. The requirement on exact timing is given by the autocorrelation

function for the code sequence. In the case of good code sequences (maximum

length pn-sequences), the correlation will be insignificant as soon as the relative

time position differs by at least one chip. Broadly speaking, this means that the full

suppression, corresponding to Gp, will be obtained if the time position of the

receiver sequence deviates by more than 1/B from the optimum time setting (where

B is the modulation bandwidth after spreading). This has two important

implications for a system, with a wide modulation bandwidth:

- the impulse response of the radio channel can be measured very accurately

(resolution corresponding to one chip interval)

- only a small part of the radio channels impulse response is used by the data

demodulator if there is extensive time dispersion.

In the case of a simple receiver structure, some improvement to the sensitivity

characteristics can be obtained if the fading depth is reduced when the receiver is

influenced only by reflections having almost the same propagation delay. Greater

improvements in sensitivity can be achieved with a more complicated receiver

structure known as a rake receiver, which has several detection channels matched

to different propagation delays (see below).

With normal cell structures of the small cell or large cell type, a modulation

bandwidth of 3 MHz (time resolution 1/3s or a 100-m length difference) isrequired for a considerable diversity gain. In a system based on micro cells and

pico cells, greater modulation bandwidth would be needed since the time disper-

sion is smaller and therefore the correlation bandwidth greater.
http://../DM/DM3E.pdfhttp://../DM/DM3E.pdfhttp://../DM/DM3E.pdf


16/31

16


Fig. 3.1

The simplest arrangement for reducing the necessary fading margin is that whereby

only a small part (one chip interval) of the impulse response is used at any given

instant but where it is possible, when necessary, to jump to another, stronger

propagation path (corresponds to selection diversity). This is made possible by a

separate receiver channel that continuously measures the impulse response and

locates the strongest propagation paths. (A pilot sequence can be used for this

purpose).

A more effective diversity arrangement is obtained using a rake receiver, whichgathers the energy from several propagation paths (see Fig. 3.1). For coherent

combination of the contributions from different propagation paths to be possible, in

addition to time adjustment of the code, the relative phase positions of the LOs for

the detector channels must also be adjusted. This corresponds to equal-gain type of

diversity (seemodule G2, figure 5.3).

As shown in Fig. 3.1, a rake receiver can also be used to combine signals from twobase stations. This achieves macro diversity with soft handover.

Threshold-detector

Code-sequencegenerator

h t( )

1

2

3

2

T R T1 R

T2

1 2 3t

h(t) = Impulse response for channel

Impulse response R (microdiversity)

T1

TR (macrodiversity)

microdiversity macro+microdiversity

Rake receiver

T
http://../G/G2E.pdfhttp://../G/G2E.pdf


17/31

17


3.4 Use ofFEC (Forward Error Control)

In a spread spectrum system, it is very attractive to obtain part of the spreading

through error-correction coding. The resulting coding gain (time diversity gain)

enables the required E/No(local average) for acceptable transmission quality to be

reduced. (The same applies to E/Jo). For a given spread bandwidth and source datarate, the processing gain is unaffected by the increased data rate from the channel

coder and still given by the ratio dc/d

i. (The coding gain is expressed in relation to

Ei/N

o, where E

iis the received energy per bit from the source). Thus, the coding

gain is gratis, i.e. obtained without any need for additional bandwidth expansion

(see Fig. 3.2), for a given value of Gpand d

i.

Fig. 3.2

Channel coding combined with interleaving is an important addition to military

spread spectrum systems subjected to jamming. Without this arrangement, pulsed

interference is highly effective, especially if a relatively low error rate is required

by the transmission system. The most effective jamming is achieved when the duty

cycle is adjusted such that the relative power of the interference at the input to the

receiver to be jammed is just above the jamming margin. During jamming periods,

the error rate will be close to 50%. A transmission channel subjected to pulsed

interference resembles a channel with fading dips below the sensitivity threshold.

d

d

d

G

G d

d

i

b

c

k

PI c

i dB

=

Spreading

Detector

Modulator

C

J

Detector

ModulatorChannelcoding

Channeldecoding

C

J

didb d c

G d

dPII c

b

=

E

Ni

o

I

With channel coding (same d and d as above)

= Information data rate

= Data rate from channel coder

B = Chip data rate (= radio bandwidth)

= Coding gain

G d

dPI c

i

=

EN

EN G

i

o

II

i

o

I

k ggr

=

( )

1

Legend

Processing gainwith no channel coding

Bandcompre-

ssion

No channel coding

E E d

db i

i

b

=E C TE C T

i i

b b= =

}

db

di d cdi

i c Gc

c

J

C

N

E

d

d

N

E

d

d

d

d

J

C

N

EG G

J

C

E

NG G dB

ggr

o

b ggr

II

c

b

o

i ggr

II

b

i

c

b

ggr

o

i ggr

I

c P

I

i

o

I

c P

I

=

=

=

( ) ( )

=

+ +

Spreading

Spreading

E

N

E

J

C d

J d

i

o

i

o

i

c

=

/

/

J

C

N

E

d

dggr

o

i

I

c

i

=

=


18/31

18


4. Frequency hopping for militarysystems

4.1 Fast frequency hopping

In fast frequency hopping, the available frequency band is divided into a large

number of frequency slots. The spreading is achieved by generating unmodulated

radio pulses whose frequencies (corresponding the available frequency slots) are

determined by a combination of the input signal and code-sequence generator. The

code sequence in turn is determined by the code key.

Code-sequencegenerator

Serial-parallelconverter

Frequencysynthesizer

From information source

Code key

B

f

t

t

hop

"1"

"0"

d I

tWi

hopp

= =0 0 1 1 1 0

Spread spectrum through fast frequency hopping(1 bit transmitted per hop)

thop

Fig. 4.1

For instance, if the available frequency band B is divided into n frequency slots

each having a width of W = B/n, a given hop frequency can be defined by sending

k bits at a time (n = 2k) from the code sequence generator. The width of the

frequency slots is often chosen such that it corresponds to the lowest value that will

give orthogonality (W = di = 1/thopfor the case in which the hop frequencycoincides with the information data rate). Fig. 4.1 applies to a simple transmitter

arrangement, whereby the useful information is sent by2-FSK with the use of two

adjacent frequency slots. The hop sequence determines where these pair of slots is

placed within the allocated bandwith B. The receiver incorporates a corresponding

hopping frequency synthesizer and suitable time synchronization, which results in

synchronized hopping of the receiver and the transmitter.

Since it is hardly possible to achieve phase coherence between adjacent received

pulses (owing to implementation complications of the frequency-hopping

frequency synthesizer and, above all, to the frequency-dependent delay on a radio

channel with time dispersion), transmission of the information is generally based

on orthogonal, noncoherent 2-FSK or MFSK. The digital input signal and the codesequence control the frequency synthesizer in the transmitter.


19/31

19


For example, a 10-bit code sequence and 2-FSK, 211(2048) different hop

frequencies are defined. In the receiver the identical code sequence is fed to the

frequency-synthesizer arrangement. On the basis of this, the receiver can determine

which two frequency slots (f1and f

2) are to be monitored during a hop interval to

determine whether a 0 or a 1 has been applied to the transmitter. A suitable

receiver arrangement is shown in Fig. 4.2 The signal energy in both frequency

slots is compared in the decision circuit, which decides which channel has the

highest energy.

Fig. 4.2

In MFSK (M > 2), during each frequency-hop interval, information is sent about

n = 2log M information bits. Depending on the value of n information bits, a

frequency, fm, is generated from a group of M different frequencies (f

1, f

2... f

M).

The receiver has M parallel channels. The outgoing sequence of n bits corresponds

to the channel which has the highest signal energy. (Basic modulation MFSK).

One considerable advantage of frequency hopping over direct-sequence spread

spectrum is the much easier synchronization. For a given bandwidth, B, the

requirement for synchronization precision in direct pn-sequence is determined bythe chip length, 1/B. In frequency hopping, the frequency band is split into a large

number of frequency slots each having a width of B/n (where n is the number of

frequency slots). This corresponds to a minimum length of radio symbols of n/B.

Compared with direct sequence, the required time precision in the synchronization

will be n times lower.

Above it has been assumed that the frequency slots for 2 FSK and MFSK are

packed together. However, this is not necessary. They can be placed independently

in random frequency slots over the allocated band. Using M frequencies, it is also

possible to send on more than one of these during each hop interval, i.e several

source bits can be transmitted during each hop interval.

HF

IF Env.det.

Env.det.

IF

Frequencysynthesizer

Code sequence "0"

Code sequence "1"

Decision

circuit

Select highest

"0" / "1"Frequencysynthesizer

Receiver for frequency-hopping CDMA

f1-fIF

f2-fIF

f1 or f2


20/31

20


4.2. Slow frequency hopping

In network planning for military mobile and portable communications, the radio

equipment has to meet stringent selectivity requirements. Military radio networks

have to be designed such that they can quickly be reconfigured if sections (nodes)

of the network are knocked out. Therefore, the network must be able to functionwithout fixed base or relay stations. The basic framework for the network is based

entirely on the terminals included. This leads to aone-frequency simplexnetwork.(By way of contrast, civil mobile radio applications use duplex networks or

two-frequency simplex networks with base stations.)

A one-frequency simplex system uses the same frequency band for

communications in both directions. This incurs the risk of mutual interference

between terminals in close proximity to each other, if selectivity is limited between

links in a common, relatively narrow frequency band. The extreme case with

respect to interference levels occurs when terminals on the same vehicle have set

up simultanious connections with remote terminals. This results in very strong

interference because of low isolation between the nearby transmitters andreceivers. In an FDMA system this problem is overcome by suitable spread out of

the radio channels plus the use of highly selective channel filters, giving a

selectivity of 70-110 dB. A direct sequence spread-spectrum system would be

completely jammed by mutual interference in the system. Also the system

selectivity of a system based on fast frequency hop would be unsufficient

(spectrum spreading at the transmitter and receiver caused by transients due to the

very short dwelling time per slot).

Even with very large bandwidth expansion, the selectivity performance expressed

as the jamming margin is modest in a direct-sequency bandspreading system in

comparison with FDMA. In an FDMA system, typical adjacent channel selectivity

is 70 dB. Substantially better selectivity is obtained at greater frequency separationfrom the interfering signal. It is impossible in practice to get anywhere near these

selectivity characteristics in a direct-sequence or even fast frequency hopping

bandspreading system. Instead, slow frequency hopping, employing typically 100

hops a second, is used. Many information bits are sent by normal narrowband

modulation during each frequency-hop interval.

In slow frequency hopping, the hop sequence is so slow that the same level of

selectivity is obtained as in normal FDMA transmission. Selectivity is reduced by a

negligible amount by the widening of the transmitted spectrum and the receiver

selectivity as a result of the frequency hopping.

Mutual interference between terminals is caused by collisions, i.e blocks will be

completely lost, if the same time-frequency slot is used by more than oneconnection at a given instant. The more terminals there are in close proximity, the

more blocks will be lost and the higher will be the bit error rate. (In some system

configurations it is possible to coordinate the frequency hops such that collisions

can be avoided. This solution is known as orthogonal or coordinated frequency

hopping.)

If a system is to have good tolerance both to mutual interference within the system

and to external jamming, it needs to be designed such that the transmission quality

is acceptable even at a high error rate. If the speech-quality requirement is modest,

suitable speech codecs (e.g. adaptive delta modulators) can be used up to a bit error

rate exceeding 10%. Data transmission with a low bit error rate can be obtained

using low-rate channel coding with interleaving over several hops.
http://../G/G1E.pdfhttp://../G/G1E.pdfhttp://../G/G1E.pdfhttp://../G/G1E.pdf


21/31

21


The disadvantage of slow frequency hopping as regards hostile interference is that

it incurs the risk of repeater jamming. If the time difference between thepropagation path via a hostile jamming installation and over the direct, desired

propagation path is small enough, the enemy will in principle have enough time to

measure the frequency of the transmitted radio signal (each frequency hop) and

generate a jamming signal on the same frequency. However, repeater jamming is

impossible if the additional propagation delay via the jammer is greater than the

duration of individual hops and if also the hopping structure is unknown to the

enemy.

Repeater jamming is clearly possible on a system using slow frequency hopping,

since the hops is of the order of 10 ms. This corresponds to a propagation distance

of 3.000 km. However, the scope for effective jamming in practice is considered to

be limited. The main reason for this is that many radio connections are established

simultaneously within a geographical area. This makes it extremely complicated in

real time to sort out which combination of hops belongs to any individual link, so

that this can then be jammed selectively. The alternative is for the enemy to adopt

wideband (subband) interference, in which case a system based on slow frequency

hopping can jamming is more or less as resistant to jamming as a system, based on

direct sequence or fast hopping.

4.3 DS-CDMA

DS-CDMA has been little used for terrestrial military networks without central

control and therefore based on simplex. As mentioned above, the reason is the

limited system selectivity, corresponding to the AJ-margin.


22/31

22


5. Cellular systems based on bandspeading

5.1. Introduction

Three basic alternatives have been studied:

a. Very low rate channel coding, using orthogonal codes

b. DS-CDMA

c. FH-CDMA

Of these, only DS-CDMA has found commercial applications. It is used in the US

standard IS-95and will be used for UMTS.

Thanks to large bandwidth expansion and other improvements, the required trans-

mission quality can be obtained at much lower values of C/I (global average), than

was possible for the first-generation of digital mobile telephone systems based onTDMA. Several factors contribute to the reduction in the necessary C/I: processing

and coding gain, frequency and antenna diversity and averaging of cochannel

interference.

5.2. Bandspreading through channel coding

Reduction of C/I through increased bandwidth can be achieved through channelcoding. The reduction in the C/I ratio is due to the coding gain. However, the scopefor achieving low values of the C/I ratio is limited by the fast performancedegradation that occurs at high ber from the data detector, i.e. at low values of

E/No. So additional bandspreading is necessary to reach cluster size one, coding

alone is not enough. Channel coding, combined with a sufficient modulation

bandwidth to provide strong frequency diversity and interference averaging,

enables the cluster size to be reduced to three, which simplifies frequency planning

of the cell structure. If a base-station arrangement with sector antennas is used,

each site covers three cells. Therefore, each site can use the same set of

frequencies.A concrete example is the Wideband TDMA system, proposed for

GSM. See module DM1.

5.3. DS-CDMA

The necessary protection ratio in a spread-spectrum system based on DS-CDMA is

determined by the processing gain, Gp, and the required E/N

oratio - which together

determine the antijamming margin (see Section 2).

From section 2.1 we get the following basic relations (to which coding gain might

be added):

Protection ratio:

C/I = Eb

/No

- Gp

dB

Antijamming margin, AJ:

AJ = I/C = Gp- E

b/N

odB
http://../DM/DM3E.pdfhttp://../DM/DM1E.pdfhttp://../DM/DM1E.pdfhttp://../DM/DM3E.pdf


23/31

23


In a cellular system, the major part of the interference comes from the other

connections within the same cell, but in addition there are interference from other

cells, especially as with DS-CDMA all cells share the same radio band (cluster

size 1). Therefore, strong cochannel interference is obtained especially from

adjacent cells. See figure 5.1.

Figure 5.1

CDMA . All cells using the same frequency band

B2

T5

T6

B3

T4

B1

T1

T3

T2

(Base station)

C

I1

I5

I4I6

I3

IC

AJ

04 radio school

Documents