7EC3 Wireless communication Unit 1 notes updated upto ... Communication Propagation Phenomena...

COMPUCOM INSTITUTE OF TECHNOLOGY & MANAGEMENT,

JAIPUR (DEPARTMENT OF ELECTRONICS & COMMUNICATION)

Notes

Wireless

Communication (Subject Code: 7EC3)

Prepared By: LOKESH KUMAR ARYA

Class: B. Tech. IV Year, VII Semester

Wireless Communication Propagation Phenomena

Prepared by: LOKESH KUMAR ARYA

Syllabus UNIT 1: PROPAGATION PHENOMENA - Fundamentals of fading, Multipath channels, Spread Spectrum signals: Direct-sequence spread spectrum signals, p-n sequences, Frequency-hopped spread spectrum signals, Code-division multiplexing.

Beyond the Syllabus Practical application of wireless technology

Learning Objectives

This unit gives the detailed knowledge about basic problems in wireless communication and to overcome this different techniques are explained



Unit 1

Propagation phenomena

1. Fundamentals of Fading:

1.1 Multipath Propagation:

In wireless telecommunications, multipath is the propagation phenomenon that results in radio signals

reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric

reflection and refraction, and reflection from water bodies and terrestrial objects such as mountains and buildings.

The effects of multipath include constructive and destructive interference, and phase shifting of the signal. In digital

radio communications (such as GSM) multipath can cause errors and affect the quality of communications. We

discuss all the related issues in this chapter.

Multipath signals are received in a terrestrial environment, i.e., where different forms of propagation are

present and the signals arrive at the receiver from transmitter via a variety of paths. Therefore there would be

multipath interference, causing multipath fading. Adding the effect of movement of either Tx or Rx or the

surrounding clutter to it, the received overall signal amplitude or phase changes over a small amount of time. Mainly

this causes the fading.

1.2 Fading:

The term fading, or, small-scale fading, means rapid fluctuations of the amplitudes, phases, or multipath

delays of a radio signal over a short period or short travel distance. This might be so severe that large scale radio

propagation loss effects might be ignored.

1.2.1 Multipath Fading Effects:

In principle, the following are the main multipath effects:

1. Rapid changes in signal strength over a small travel distance or time interval.

2. Random frequency modulation due to varying Doppler shifts on different multipath signals.

3. Time dispersion or echoes caused by multipath propagation delays.

1.2.2 Factors Influencing Fading:

The following physical factors influence small-scale fading in the radio propagation channel:

(1) Multipath propagation {Multipath is the propagation phenomenon that results in radio signals reaching the

receiving antenna by two or more paths. The effects of multipath include constructive and destructive

interference, and phase shifting of the signal.

(2) Speed of the mobile {The relative motion between the base station and the mobile results in random

frequency modulation due to different doppler shifts on each of the multipath components.



(3) Speed of surrounding objects {If objects in the radio channel are in motion, they induce a time varying

Doppler shift on multipath components. If the surrounding objects move at a greater rate than the mobile,

then this effect dominates fading.

(4) Transmission Bandwidth of the signal {If the transmitted radio signal bandwidth is greater than the

\bandwidth" of the multipath channel (quantified by coherence bandwidth), the received signal will be

distorted.

1.2.3 Types of Small-Scale Fading:

The type of fading experienced by the signal through a mobile channel depends on the relation between the

signal parameters (bandwidth, symbol period) and the channel parameters (rms delay spread and Doppler spread).

Hence we have four different types of fading. There are two types of fading due to the time dispersive nature of the

channel.

a) Fading Effects due to Multipath Time Delay Spread:

i. Flat Fading:

Such types of fading occur when the bandwidth of the transmitted signal is less than the coherence bandwidth of the

channel. Equivalently if the symbol period of the signal is more than the rms delay spread of the channel, then the

fading is at fading.

So we can say that at fading occurs when

BS << BC (1.1)

Where BS is the signal bandwidth and BC is the coherence bandwidth. Also

TS >>στ (1.2)

Where TS is the symbol period and στ is the rms delay spread. And in such a case, mobile channel has a constant

gain and linear phase response over its bandwidth.

ii. Frequency Selective Fading:

Frequency selective fading occurs when the signal bandwidth is more than the coherence bandwidth of the mobile

radio channel or equivalently the symbols duration of the signal is less than the rms delay spread.

BS >> BC (1.3)

and

TS <<στ (1.4)

At the receiver, we obtain multiple copies of the transmitted signal, all attenuated and delayed in time. The channel

introduces inter symbol interference. A rule of thumb for a channel to have at fading is if

στ /TS<= 0.1 (1.5)

b) Fading Effects due to Doppler Spread:

i. Fast Fading:

In a fast fading channel, the channel impulse response changes rapidly within the symbol duration of the signal. Due

to Doppler spreading, signal undergoes frequency dispersion leading to distortion. Therefore a signal undergoes fast

fading if



TS >> TC (1.6)

where TC is the coherence time and

BS >> BD (1.7)

where BD is the Doppler spread. Transmission involving very low data rates suffers from fast fading.

ii. Slow Fading:

In such a channel, the rate of the change of the channel impulse response is much less than the transmitted signal.

We can consider a slow faded channel a channel in which channel is almost constant over at least one symbol

duration. Hence

TS << TC (1.8)

and

BS >> BD (1.9)

We observe that the velocity of the user plays an important role in deciding whether the signal experiences fast or

slow fading.

1.2.4 Doppler Shift:

Illustration of Doppler effect

The Doppler effect (or Doppler shift) is the change in frequency of a wave for an observer moving relative to the

source of the wave. In classical physics (waves in a medium), the relationship between the observed frequency f and

the emitted frequency fo is given by:

(1.10)

Where v is the velocity of waves in the medium, vs is the velocity of the source relative to the medium and vr is the

velocity of the receiver relative to the medium. In mobile communication, the above equation can be slightly

changed according to our convenience since the source (BS) is fixed and located at a remote elevated level from

ground.



Consider a mobile moving at a constant velocity v, along a path segment length d between points A and B, while it

receives signals from a remote BS source S. The difference in path lengths traveled by the wave from source S to the

mobile at points A and B is ∆l = d cos Ө = v∆t cos Ө, where ∆t is the time required for the mobile to travel from A

to B, and Ө is assumed to be the same at points A and B since the source is assumed to be very far away. The phase

change in the received signal due to the difference in path lengths is therefore

(1.11)

and hence the apparent change in frequency, or Doppler shift (fd) is

(1.12)

2. Multipath channel:

2.1 Types of multipath channels:

Many multipath models have been proposed to explain the observed statistical nature of a practical mobile

channel. Both the first order and second order statistics have been examined in order to find out the effective way to

model and combat the channel effects. The most popular of these models are Rayleigh model, which describes the

NLoS propagation. The Rayleigh model is used to model the statistical time varying nature of the received envelope

of a flat fading envelope. Below, we discuss about the main first order and second order statistical models.

i. AWGN channel:

Additive white Gaussian noise (AWGN) is a channel model in which the only impairment to

communication is a linear addition of wideband or white noise with a constant spectral density (expressed as watts

per hertz of bandwidth) and a Gaussian distribution of amplitude. The model does not account for fading, frequency

selectivity, interference, nonlinearity or dispersion. However, it produces simple and tractable mathematical models

which are useful for gaining insight into the underlying behavior of a system before these other phenomena are

considered. Wideband Gaussian noise comes from many natural sources, such as the thermal vibrations of atoms in

conductors (referred to as thermal noise or Johnson-Nyquist noise), shot noise, black body radiation from the earth

and other warm objects, and from celestial sources such as the Sun. The AWGN channel is a good model for many

satellite and deep space communication links. It is not a good model for most terrestrial links because of multipath,

terrain blocking, interference, etc. However, for terrestrial path modeling, AWGN is commonly used to simulate

background noise of the channel under study, in addition to multipath, terrain blocking, interference, ground clutter

and self interference that modern radio systems encounter in terrestrial operation.

ii. Rayleigh Fading channel:

(1.13)

Where r = amplitude of received signal



σ2 = average power of received signal

(1.14)

2.2 Multipath Channel Parameters:

To compare the different multipath channels and to quantify them, we define some parameters. They all

can be determined from the power delay profile. These parameters can be broadly divided in to two types.

2.2.1 Time Dispersion Parameters:

These parameters include the mean excess delay, rms delay spread and excess delay spread. The mean excess delay

is the first moment of the power delay profile and is defined as

(1.15)

where ak is the amplitude, _k is the excess delay and P(τk) is the power of the individual multipath signals.

The mean square excess delay spread is defined as



(1.16)

Since the rms delay spread is the square root of the second central moment of the power delay profile, it can be

written as

(1.17)

As a rule of thumb, for a channel to be at fading the following condition must be satisfied

(1.18)

where TS is the symbol duration. For this case, no equalizer is required at the receiver.

2.2.2 Frequency Dispersion Parameters:

To characterize the channel in the frequency domain, we have the following parameters.

a) Coherence bandwidth:

It is a statistical measure of the range of frequencies over which the channel can be considered to pass all

the frequency components with almost equal gain and linear phase. When this condition is satisfied then we say the

channel to be at.

Practically, coherence bandwidth is the minimum separation over which the two frequency components are

affected differently. If the coherence bandwidth is considered to be the bandwidth over which the frequency

correlation function is above 0.9, then it is approximated as

BC≈1/(50στ) (1.19)

However, if the coherence bandwidth is considered to be the bandwidth over which the frequency

correlation function is above 0.5, then it is defined as

BC≈1/(5στ) (1.20)

The coherence bandwidth describes the time dispersive nature of the channel in the local area. A more

convenient parameter to study the time variation of the channel is the coherence time. This variation may be due to

the relative motion between the mobile and the base station or the motion of the objects in the channel.

b) Coherence time:

This is a statistical measure of the time duration over which the channel impulse response is almost

invariant. When channel behaves like this, it is said to be slow faded. Essentially it is the minimum time duration

over which two received signals are affected differently. For an example, if the coherence time is considered to be

the bandwidth over which the time correlation is above 0.5, then it can be approximated as

(1.21)

where fm is the maximum doppler spread given be fm = v/λ



3. Spread spectrum signals:

3.1 Principles of spread-spectrum communications:

Digital transmission schemes which provide satisfactory performance and an adequate bit rate can be

arranged into two categories.

• In applications like satellite communications, these schemes provide efficient usage of the limited power available.

• In applications such as mobile wireless, where the schemes achieve efficient usage of the limited bandwidth

available for the service in demand.

However, both schemes are narrowband and vulnerable to hostile jamming and radio interference. The

novelty of the spread-spectrum concept is that it provides protection against such attacks. This concept is based upon

exchanging bandwidth expansion for anti-jamming capability.

The bandwidth expansion in spread spectrum is acquired through a coding process that is independent of

the message being sent or the modulation being used. The spread spectrum, unlike FM, does not combat interference

originated from thermal noise. The trade-off between signal-to-noise ratio (SNR) and data bit rate (or bandwidth) in

the spread-spectrum scheme can be demonstrated by the following.

General model of spread spectrum digital communication

A spread spectrum modulation produces a transmitted spectrum much wider than the minimum bandwidth

required. There are many ways to generate spread spectrum signals. We are going to introduce some of the most

common spread spectrum techniques such as direct sequence (DS), frequency hop (FH), time hop (TH), and

multicarrier (MC). Of course, one can also mix these spread spectrum techniques to form hybrids which have the

advantages of different techniques. Spread spectrum originates from military needs and finds most applications in

hostile communication environments.

In some situations it is required that a communication signal be difficult to detect, and difficult to

demodulate even when detected. Here the word ‘detect’ is used in the sense of ‘to discover the presence of’. The

signal is required to have a low probability of intercept - LPI.

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

signal has properties which help to achieve these ends. Spread spectrum signals may be divided into two main

groups - direct sequence spread spectrum (DSSS), and frequency hopping spread spectrum (FHSS). This experiment

is concerned with demonstrating some of the principles of the first.



Advantages of Spread Spectrum (SS) Techniques:

a) Reduced interference: In SS systems, interference from undesired sources is considerably reduced due to

the processing gain of the system.

b) Low susceptibility to multi-path fading: Because of its inherent frequency diversity properties, a spread

spectrum system offers resistance to degradation in signal quality due to multi-path fading. This is particularly

beneficial for designing mobile communication systems.

c) Co-existence of multiple systems: With proper design of pseudo-random sequences, multiple spread

spectrum systems can co-exist.

d) Immunity to jamming: An important feature of spread spectrum is its ability to withstand strong

interference, sometimes generated by an enemy to block the communication link. This is one reason for

extensive use of the concepts of spectrum spreading in military communications.

3.2 Types of spread spectrum techniques:

a) Direct sequence spread spectrum

b) Frequency hopping spread spectrum

c) Time hopping

d) Hybrid spread spectrum

a) Direct sequence spread spectrum:

Principle of DSSS:

Consider the frequency translation of a baseband message (of bandwidth B Hz) to a higher part of the spectrum,

using DSBSC modulation. The resulting signal occupies a bandwidth of 2B Hz, and would typically override the

noise occupying the same part of the spectrum. This makes it easy to find with a spectrum analyzer (for example),

and so the probability of intercept is high. A local carrier, synchronized with that at the transmitter, is required at the

receiver for synchronous demodulation. The recovered signal-to-noise ratio is 3 dB better than that measured at its

original location in the spectrum. This 3 dB improvement comes from the fact that the contributions from each

sideband add coherently, whereas the noise does not. This can be called a 3 dB ‘processing gain’, and is related to

the fact that the transmission bandwidth and message bandwidth are in the ratio of 2:1.

In a spread spectrum system literally thousands of different carriers are used, to generate thousands of DSBSC

signals each derived from the same message. These carriers are spread over a wide bandwidth (much wider than 2B

Hz), and so the resulting DSBSC signals will be spread over the same bandwidth.

If the total transmitted power is similar to that of the single DSBSC case, then the power of an individual

DSBSC in the spread spectrum case is thousands of times less. In fact, over the bandwidth occupied by one of these

DSBSC signals, it would be literally ‘buried in the noise’, and difficult to find with a spectrum analyzer (for

example).



Instead of the total transmitted power being concentrated in a band of width 2B Hz, the multiple carriers have

spread it thinly over a very wide bandwidth. The signal-to-noise ratio for each DSBSC is very low (well below 0

dB). To recover the message from the transmitted spread spectrum signal all that a receiver requires is thousands of

local carriers, at the same frequency and of the same relative phase, as all those at the transmitter. All these carriers

come from a pseudo random binary sequence (PRBS) generator.

Given a stable clock, and a long sequence, it may be shown that the spectrum of a pseudo random binary

sequence generator is a good source of these carriers. A second PRBS generator, of the same type, clocked at the

same rate, and appropriately aligned, is sufficient to regenerate all the required local carriers at the receiver

demodulator.

In the spread spectrum context the PRBS signal is generally called a PN – pseudo noise - signal, since its

spectrum approaches that of random noise.

Having the correct sequence at the receiver means that the message contributions from each of the thousands of

minute DSBSC signals combine in phase – coherently - and add up to a finite message output. Otherwise they add

with random phases, resulting in a (very) small, noise-like output.

Direct Sequence Spread Spectrum Example:

Direct Sequence Spread Spectrum Transmitter/receiver:

To generate a spread spectrum signal one requires:

1. A modulated signal somewhere in the RF spectrum

2. A PN sequence to spread it



There are two bandwidths involved here: that of the modulated signal, and the spreading sequence. The

first will be very much less than the second. The output spread spectrum signal will be spread either side of the

original RF carrier (ω0) by an amount equal to the bandwidth of the PN sequence. Most of the energy of the

sequence will lie in the range DC to ωs, where ω

s is the sequence clock. The longer the sequence the more spectral

components will lie in this range. It is necessary and usual that ω0

>> ωs, although in the experiment to follow the

difference will not be large. The modulated signal can be of any type, but typically digitally-derived, such as binary

phase shift keyed - BPSK. In this case the arrangement of Figure 1 can be expanded to that of Figure 2. A digital

message is preferred in an operational spread spectrum system, since it makes the task of the eavesdropper even

more difficult.

The input multiplier performs the de-spreading of the received signal, and the second multiplier translates

the modulated signal down to baseband. The filter output would probably require further processing - not shown - to

‘clean up’ the waveform to binary format.

The PN sequence at the receiver acts as a ‘key’ to the transmission. It must not only have the same clock

and bit pattern; it must be aligned properly with the sequence at the transmitter.



Direct Sequence Spread Spectrum Using BPSK Example

Approximate Spectrum of DSSS Signal:



Processing gain:

To achieve most of the claims made for the spread spectrum it is necessary that the bandwidth over which

the message is spread be very much greater than the bandwidth of the message itself. Each DSBSC of the DSSS

signal is at a level below the noise, but each is processed by the synchronous demodulator to give a 3 dB SNR

improvement. The total improvement is proportional to the number of individual DSBSC components. In fact the

processing gain of the system is equal to the ratio of DSSS bandwidth to message bandwidth.

b) Frequency hopping spread spectrum:

Another common method to spread the transmission spectrum of a data signal is to (pseudo) randomly hop

the data signal over different carrier frequencies. This spreading method is called frequency hop spread spectrum

(FH-SS). Usually, the available band is divided into non-overlapping frequency bins. The data signal occupies one

and only one bin for a duration Tc and hops to another bin afterward. When the hopping rate is faster than the

symbol rate, the FH scheme is referred to as fast hopping. Otherwise, it is referred to as slow hopping. A typical FH-

SS transmitter and the corresponding receiver are shown in Figures respectively.

Frequency hopping entails the transmission carrier frequency hopping between available channels within

the spread-spectrum band. A narrow spectral band and an individual carrier frequency at the centre of the band

define each transmitted channel. Successive carrier frequencies are chosen in accordance with the pseudo-random

phases of the spreading code sequence. There are two widely used FH schemes: (1) Fast frequency hopping where

one complete, or a fraction of the data symbol, is transmitted within the duration between carrier hops.

Consequently, for a binary system, the frequency hopping rate may exceed the data bit rate. (2) On the other hand, in

a slow frequency hopping system, more than one symbol is transmitted in the interim time between frequency hops.

Figure illustrates how the carrier frequency hops with time. Let time duration between hops be Th and data bit

duration be denoted by Tb, then:

Th ≤ Tb for fast hopping (1.22)

Th > Tb for slow hopping (1.23)



The basic FH modulation system, depicted in Figure comprises a digital phase or frequency shift keying

modulator and a frequency synthesizer. The latter generates carrier frequencies according to the pseudo-random

phases of the spreading code sequence that is then mixed with the data carrier to originate the FH signal.

In the basic FH receiver, shown in Figure (b), the received FH signal is first filtered using a wideband

bandpass filter and then mixed with a replica of the FH carrier. The mixer output is applied to the appropriate

demodulator. A coherent demodulator may be used when a PSK carrier is received.

Frequency Hopping Spread Spectrum System (Transmitter)

The incoming signal is multiplied by the signal from the PN generator identical to the one at the transmitter.

Resulting signal from the mixer is a binary FSK, which is then demodulated in a "regular" way. Error correction is

then applied in order to recover the original signal. The timing synchronization is accomplished through the use of

early-late gates, which control the clock frequency

Frequency Hopping Spread Spectrum System (Receiver)

The FH receiver is usually non-coherent. A typical non-coherent receiver architecture is represented in figure.



Slow versus Fast Hopping:

As mentioned earlier, the hop period (also called the chip period Tc) may be greater or less than the symbol

duration. The bandwidth expansion factor (and consequently the processing gain) is related only to the number of

hop frequencies N, not the hop period. Thus, we are free to choose the hop period based on other considerations.

Specifically, the hop frequency should be chosen based on implementation and performance considerations. First let

us consider the case where Tc > Ts, which is called slow hopping. Additionally, let us assume that FSK modulation

is used. Figure 4.4 plots the frequency occupancy versus time considering both the data modulation and frequency

hopping. In this example Tc = 4Ts, or the frequency is hopped every four symbols, N = 6 and M = 4 (Tb = Ts/2).

Further, in the figure we have defined B as the bandwidth of the MFSK signal and W as the spread bandwidth. As

can be seen, every Ts seconds, the frequency is changed to one of 4 symbols based on the data.

Additionally, every Tc seconds, the center frequency of these symbols is changed based on the frequency hopping

pattern. At the receiver the pseudorandom hopping is removed, leaving only the data modulation as shown in Figure.

In contrast to slow hopping, with fast frequency hopping Tc < Ts. That is, frequency hopping occurs faster than the

modulation. This is depicted in Figure 4.6 where Tc = Ts 2 , N=6, and M=4. In this case coherent modulation is

extremely difficult since it would require extremely fast carrier synchronization. Thus, non-coherent FSK is

universally used with fast hopping. The despread or de-hopped signal is plotted in Figure 4.5 which shows that the

despread data is the same as in slow hopping. Fast hopping, although more difficult to implement, offers some

advantages over slow hopping. First, unlike slow hopping, fast hopping provides frequency diversity at the symbol

level which provides substantial benefit in fading channels or versus narrowband jamming. Slow hopping can obtain

these same benefits through error correction coding as we will see later, but fast hopping offers this benefit before

coding is applied, which can provide better performance, especially when punctured codes are used. The down side

to fast hopping is that the separate integration periods collected each hop must be combined non-coherently since

non-coherent demodulation is used. As a result, a loss in performance is experiences as compared to standard

noncoherent FSK modulation.

Slow Frequency Hop Spread Spectrum Using MFSK (M=4, k=2)



Fast Frequency Hop Spread Spectrum Using MFSK (M=4, k=2)

Processing Gain:

As discussed in the previous chapter, processing gain is a measure which provides a short-hand description

of the benefits of a particular spread spectrum format. Like with DS/SS there are multiple definitions available in the

literature with subtle differences. As with DS/SS we will formally define processing gain as the ratio of the

bandwidth to the bit rate

PG =W/Rb (1.24)

while we will define the number of non-overlapping frequency bands available for hopping as the bandwidth

expansion factor N. However, note that in most cases of interest, the two will be equivalent and the processing gain

will be equal to the common definition PG = N.

3.3 Pseudo-Random Noise Codes:

A PN code used for DS-spreading exists of NDS units called chips; these chips can have two values: -1/1

(polar) or 0/1. As we combine every data symbol with a complete PN code, the DS processing gain is equal to the

code-length. To be usable for direct-sequence spreading, a PN code must meet the following constraints:

The sequences must be building from 2-leveled numbers.

· The codes must have a sharp (1-chip wide) autocorrelation peak to enable code synchronization.

· The codes must have a low cross-correlation value, the lower this cross-correlation, the more users we can

allow in the system. This holds for both full-code correlation and partial-code correlation. The latter because in

most situations there will not be a full-period correlation of two codes, it is more likely that codes will only

correlate partially (due to random-access nature).



This last requirement stands for good spectral density properties (equally spreading the energy over the whole

frequency-band). Codes that can be found in practical DS-systems are: Walsh-Hadamard codes, M-sequences, Gold

codes and Kasami-codes. These code sets can be roughly divided into two classes: orthogonal codes and non-

orthogonal codes. Walsh sequences fall in the first category, while the other group contains the so-called shift-

register sequences. We first spell out some desired properties we want the spreading sequences to possess:

1. Sequence elements should behave like iid random variables, i.e., the sequence should be pseudorandom.

2. It should be easy to distinguish a spreading signal from a time-shifted version of it.

3. It should be easy to distinguish a spreading signal from other spreading signals, including time shifted versions of

them, in the set.

4. It should be easy for the transmitter and the intended receiver to generate the spreading sequence.

5. It should be difficult for any unintended receiver to acquire and regenerate the spreading sequence.

We would like the spreading sequences to approximate random sequences so that we are actually spreading

the spectrum of the data signal. This explains the first desired property. The second property is used for sequence

acquisition and multipath. The third property is obviously needed for CDMA systems. The fourth property indicates

the practical consideration in sequence design. A sequence is of no use no matter how well it behaves if an excessive

amount of hardware is needed to generate it. The fifth property is important when transmission security is our main

concern. Due to the restriction imposed by the fourth property above, spreading sequences are usually generated by

feedback shift registers in practice since shift registers are easy to build. The spreading sequences generated by

feedback shift registers are periodic and they are usually pseudo-random. We will not consider the fifth property on

transmission security in this brief treatment. The second and third properties are our major concerns here, i.e., we

focus on the distinguish ability of spreading signals.

m-sequence:

Maximal length pseudo random sequence generator

Several spreading codes are popular for use in practical spread spectrum systems. Some of these are

Maximal Sequence (m-sequence) length codes, Gold codes, Kasami codes and Barker codes. In this section will be

briefly discussed about the m-sequences. These are longest codes that can be generated by a shift register of a



specific length, say, L. An L-stage shift register and a few EX-OR gates can be used to generate an m-sequence of

length 2L -1. Figure shows an m-sequence generator using n memory elements, such as flip-flops. If we keep on

clocking such a sequence generator, the sequence will repeat, but after 2L -1 bits. The number of 1-s in the complete

sequence and the number of 0-s will differ by one. That is, if L = 8, there will be 128 one-s and 127 zero-s in one

complete cycle of the sequence. Further, the auto-correlation of an m-sequence is -1 except for relative shifts of (0 ±

1) chips. This behavior of the auto correlation function is somewhat similar to that of thermal noise as the auto

correlation shows the degree of correspondence between the code and its phase-shifted version. Hence, the m-

sequences are also known as, pseudo-noise or PN sequences.

Gold sequence:

Another interesting property of an m-sequence is that, the sequence, when added (modulo-2) with a

cyclically shifted version of itself, results in another shifted version of the original sequence. For moderate and large

values of L, multiple sequences exist, which are of the same length. The cross correlation of all these codes are

studied. All these properties of a PN sequence are useful in the design of a spread spectrum system. Sometimes, to

indicate the occurrence of specific patterns of sequences, we define ‘run’ as a series of ones and zero-s, grouped

consecutively. For example, consider a sequence 1011010. We say, the sequence of has three runs of single ‘0’, two

runs of single ‘1’ and one run of two ones. In a maximum length sequence of length and 2L -1, there are exactly 2L-

(p+2) runs of length ‘p’ for both of ones and zeros except that there is only one run containing L one-s and one

containing (L-1) zero-s. There is no run of zero-s of length L or ones of length (L-1). That is, the number of runs of

each length is a decreasing power of two as the run length increases.

If the period of an m-sequence is N chips, N = (2n –1), where ‘n’ is the number of stages in the code

generator. The autocorrelation function of an m-sequence is periodic in nature and it assumes only two values, viz. 1

and (-1/N) when the shift parameter (τ) is an integral multiple of chip duration.

Several properties of PN sequences are used in the design of DS systems. Some features of maximal length pseudo

random periodic sequences (m-sequence or PN sequence) are noted below:

Over one period of the sequence, the number of ‘+1’ differs from the number of ‘-1’ by exactly one. Also

the number of positive runs equals the number of negative runs. Half of the runs of bits in every period of the same

sign (i.e. +1 or -1) are of length 1, one fourth of the runs of bits are of length 2, one eighth of the runs of bits are of

length 3 and so on. The autocorrelation of a periodic sequence is two-valued.

3.4 Code division multiple access (code division multiplexing):

Spread spectrum and CDMA are up-to-date technologies widely used in operational radar, navigation and

telecommunication systems and playing a dominant role in the philosophy of the forthcoming generations of

systems and networks. Code Division Multiple Access (CDMA) is a multiple access technique where different users

share the same physical medium that is the same frequency band at the same time. The main ingredient of CDMA is

the spread spectrum technique which uses high rate signature pulses to enhance the signal bandwidth far beyond

what is necessary for a given data rate.



In a CDMA system, the different users can be identified and hopefully, separated at the receiver by means

of their characteristic individual signature pulses (sometimes called the signature waveforms) that is by their

individual codes.

Classifications of CDMA Schemes:

CDMA can be classified according to the modulation method used to obtain the spread spectrum signal into

four major techniques: DS-CDMA, FH-CDMA, TH-CDMA and hybrid CDMA.

3.4.1 DS-CDMA:

Now-a-days, DS modulation has been used for many commercial communication systems (almost all 3G mobile

cellular systems use DS-CDMA as their prime multiple access air-link architecture) and measurement instruments. It

is reasonable to expect that DS modulation will continue to be a familiar form of spreading modulation scheme in

the years to come due to its unique and desirable features. Characteristic of DS spreading modulation is just exactly

that modulation of a carrier by a code sequence. The use of DS-CDMA can effectively enhance overall bandwidth

efficiency compared with traditional multiple access schemes such as FDMA (Frequency Division Multiple Access)

and TDMA (Time Division Multiple Access). Spectrum is extremely expensive; it has to be purchased from various

governmental licensing authorities at auction and sometimes those auctions have involved billions of US dollars (or

equivalent monetary value in other currencies). It represents a considerable investment by a service carrier.

Therefore, the bandwidth efficiency of a communication technology will be a primary concern for any network

operator. The right selection of a suitable multiple access schemes to provide multi-user services is of ultimate

importance. DSCDMA-based mobile cellular carries more calls than TDMA-based technologies. Generally

speaking, CDMA will carry between 2 and 3 times as many calls simultaneously as TDMA in the same amount of

bandwidth. The major advantage of CDMA is its capability for dynamic allocation of bandwidth. To understand

this, it is important to realize that in this context in CDMA, bandwidth refers to the ability of any user to get data

from one end to the other. It does not refer to the amount of spectrum used by the user because in CDMA every

terminal uses the entire spectrum of its carrier whenever it is transmitting or receiving. On the other hand, TDMA

works by taking a channel with a fixed bandwidth and dividing it into several time slots. Any given mobile terminal

is then given the ability to use one or more of the slots on an ongoing basis if it is in a call.

DS-CDMA’s system model: The block diagram of simple asynchronous DS-CDMA modem in a noiseless channel

is shown in figure. This system supports K users each transmitting its own information. The users are identified by k

= 1, 2, 3, ..., K. This modulation scheme is used in Binary Phase Shift Keying (BPSK).



Block diagram of a simple asychronous DS

CDMA system

Each user’s data signal is denoted by dk(t) and each user is assigned a unique pseudo-random code also

known as a spreading code denoted by Ck(t).

There are two classes of spreading codes in general, binary and complex. For simplicity, the following

discussion considers only binary codes. Each spreading code consists of Q pulses, commonly known as chips. Here,

the wanted signal is the signal of user k = 1 and all the other (K-1) signals are considered to be interfering signals.

At the DS-CDMA transmitter of user k is 1st multiplied by the spreading code ck(t). This causes the spectrum of the

information signal to be spread across the allocated bandwidth. Next, the signal is modulated onto its carrier before

it is transmitted. The transmitted signal is given by:

(1.25)

where, wc is the carrier frequency in rad sec-1 and A is the amplitude of the carrier signal. At the DS-CDMA

receiver, the composite of all the K user signal is received, consisting of the transmitted signal from user 1 and the

other (K-1) interfering signals. Ignoring the noise, the received signal is given by:

(1.26)

where, Tk is the propagation delay from the transmitter to the receiver of the kth user.

3.4.2 FH-CDMA:

After having discussed the issues on DS-CDMA techniques, let’s look at Frequency-hopping (FH) CDMA.

Compared to the DS-CDMA technique, the FH-CDMA technique is a relatively less widely used CDMA scheme in

real applications. The reason for its less wide acceptance is owing to several factors. First, the FH technique requires

a very accurate reference clock in the whole wireless system which uses the FH-CDMA technique for user

separation. This accurate network-wide reference clock is very costly to implement using currently available digital

technology.



Maybe in the future, the situation will be different with the advancement in micro-electronics technologies. Second,

the hardware to implement an FH-CDMA is still much too complex compared to DS-CDMA under the same

maximum data transmission rate constraint. Therefore, system designers still prefer DS-CDMA to FH-CDMA for

most commercial wireless applications. In Frequency Hopping CDMA (FHCDMA), the transmission bandwidth is

divided into frequency sub-bands where the bandwidth of each sub-band is equal to the bandwidth of the

information signal. A pseudo-random code is then used to select the sub-band in which the information signal is

transmitted and this sub-band changes periodically according to the code. There are two sub-categories of FH-

CDMA: Fast frequency hopping where one complete or a fraction of the data symbol is transmitted within the

duration between carrier hops. Consequently for a binary system, the frequency hopping rate may exceed the data bit

rate. Slow frequency hopping system, >1 symbol is transmitted in the interim time between frequency hops.

Usually, an FH system must have a code generator and a synthesizer which is capable of generating the

corresponding frequencies according to the code generator. As stated earlier, the difficult part of developing an FH

system is to design a fast-settling synthesizer with a sufficiently large number of carrier frequencies. Theoretically

speaking, the output instantaneous frequency the synthesizer generates must be a single frequency. This is one of the

reasons why an FH system is very difficult and costly to implement. In particular, the synthesizer in a fast-hopping,

FH system has to work by switching from one frequency to another in a very fast and stable way, especially when

the data rate is very high. However, a practical system may produce an output spectrum which can be a composite of

the desired frequency, sidebands generated by hopping, as well as some other spurious frequencies generated as by-

products. Figure shows a conceptual block diagram of an FH transmitter. The receiver of the FH system is given in.

The FH-CDMA transmitter shown in figure, consists of the following basic blocks, a data modulator, a mixer

(denoted simply by a multiplier), an FH pattern code generator, a synthesizer and an antenna.

Basic FH modulator

The hopping rate is a very important parameter in an FH-CDMA system which will determine if it is a fast-

hopping or a slow-hopping FH system. At the FH-CDMA receiver as shown in figure, the received signal should

first go through a band pass filter which will be used to reject the image of the carrier frequency produced in the

mixer. For the same purpose, the code generator will produce a replica of the sequence used by the transmitter and

will yield an FH pattern which should be exactly the same as that used in the transmitter in the output of the

synthesizer.



Basic FH receiver

The locally generated FH pattern will be mixed with the received signal to produce a narrowband data-

modulated signal with a fixed carrier frequency which should be equal to the intermediate frequency (IF) ωI. The

output IF signal will be demodulated by a PSK demodulator to recover the transmitted data information. Ideally, the

spectrum generated from an FH system should be perfectly rectangular with spectral lines distributed evenly in

every predetermined frequency channel. The transmitter should also be designed to send the same amount of power

in each frequency. Otherwise, the detection efficiency on different frequencies will be different causing decision

errors at a receiver. As shown in figure, the received frequency-hopping signal is mixed with a locally generated

replica which is offset by a fixed amount (which is equal to a carrier frequency suitable for the reception process at

the receiver, ωI) such that the output from the mixer in the receiver will produce a constant difference frequency or

ωI if transmitter and receiver code sequences are synchronous. Signal that is not a replica of the local reference is

spread by multiplication with the local reference and is never restored into its original narrowband waveform. The

bandwidth of an undesired signal after multiplication with the local reference is approximately equal to the

bandwidth before dispreading.

3.4.3 TH-CDMA:

The 3rd CDMA technique, TH-CDMA is found to be much less widely used than the previous two mainly

due to its implementation difficulties and hardware cost associated with its transmitter which should provide an

extremely high dynamic range and very high switching speed.

Block diagram of a TH-CDMA transmitter and receiver

The TH technique in fact, works in a very similar way as a digital modulation scheme called Pulse Position

Modulation (PPM). The TH-SS (Time Hopping Spread Spectrum) technologies are not as popular as the other two

spread spectrum techniques, i.e., the DS-SS (Direct Sequence Spread Spectrum) and FH-SS (Frequency Hopping



Spread Spectrum) techniques. The main reason is implementation difficulties, especially for the pulse generator

which is the core of a TH-SS system and should be able to produce a train of very narrow impulses with its width

being at an order of nano sec. The pulse generator should also provide very good timing accuracy such that the PPM

can be effectively applied to code different SS sequences for multiple accesses. The TH-SS technique seldom works

independently in an SS system (except for the case of an Ultra-wideband (UWB) system, a technology developed

based on the TH technique). Instead, it works with some other SS modulation schemes in particular the FH

technique which has been discussed in the previous study to result in a time-frequency hopping SS scheme.

TH-CDMA's system model:

In the TH-CDMA system, a pseudo-noise sequence defines the transmission moments, rather than the

transmission frequency as FH does. The data signal in time-hopping CDMA is transmitted in rapid bursts at time

intervals determined by the code assigned to the user. The time axis is divided into frames and each frame is divided

into M, time slots. During each frame, the user will transmit in one of the M time slot which of the M time slots is

transmitted depends on the code signal assigned to the user. Since, a user transmits all of its data in one instead of

M, time slots, the frequency it needs for its transmission has increased by a factor M.A. block diagram of a TH-

CDMA system is shown in figure.

figure shows the time-frequency plot of the TH-CDMA system which clarified that TH-CDMA uses the

whole wideband spectrum for short periods instead of parts of the spectrum all of the time.

Time frequecny plot of the TH-CDMA

3.4.4 Hybrid CDMA:

The increasing demand for high data rate transmission for newly evolving wireless communications

systems (3G, beyond 3G and 4) has challenged the researchers to exploit new modulation, diversity and coding

techniques to overcome the limited natural wireless resources: frequency and power. There are many different types

of hybrid CDMA schemes which can be formed by various combinations of DS, FH and TH, together with Multi-

carrier (MC) and Multi-tone (MT) techniques.



3.5 Applications of Spread Spectrum:

A specific example of the use of spread spectrum technology is the North American Code Division Multiple Access

(CDMA) Digital Cellular (IS-95) standard. The CDMA employed in this standard uses a spread spectrum signal

with 1.23-MHz spreading bandwidth. Since in a CDMA system every user is a source of interference to other users,

control of the transmitted power has to be employed (due to near-far problem). Such control is provided by

sophisticated algorithms built into control stations. The standard also recommends use of forward error-correction

coding with interleaving, speech activity detection and variable-rate speech encoding. Walsh code is used to provide

64 orthogonal sequences, giving rise to a set of 64 orthogonal ‘code channels’. The spread signal is sent over the air

interface with QPSK modulation with Root Raised Cosine (RRC) pulse shaping. Other examples of using spread

spectrum technology in commercial applications include satellite communications, wireless LANs based on IEEE

802.11 standard etc.

Applications of wireless technology

Mobile telephones

One of the best-known examples of wireless technology is the mobile phone, also known as a cellular phone, with

more than 4.6 billion mobile cellular subscriptions worldwide as of the end of 2010. These wireless phones use radio

waves to enable their users to make phone calls from many locations worldwide. They can be used within range of

the mobile telephone site used to house the equipment required to transmit and receive the radio signals from these

instruments.

Wireless data communications

Wireless data communications are an essential component of mobile computing. The various available technologies

differ in local availability, coverage range and performance, and in some circumstances, users must be able to

employ multiple connection types and switch between them. To simplify the experience for the user, can be used, or

a deployed to handle the multiple connections as a secure, single. Supporting technologies include:

Wi-Fi is a wireless local area network that enables portable computing devices to connect easily to the

Internet. Standardized as IEEE 802.11 a,b,g,n, Wi-Fi approaches speeds of some types of wired Ethernet.

Wi-Fi has become the de facto standard for access in private homes, within offices, and at public hotspots.

Some businesses charge customers a monthly fee for service, while others have begun offering it for free in

an effort to increase the sales of their goods.



Cellular data service offers coverage within a range of 10-15 miles from the nearest cell site. Speeds have

increased as technologies have evolved, from earlier technologies such as GSM, CDMA and GPRS, to 3G

networks such as W-CDMA, EDGE or CDMA2000.

Mobile Satellite Communications may be used where other wireless connections are unavailable, such as

in largely rural areas or remote locations. Satellite communications are especially important for

transportation, aviation, maritime and military use.

Wireless energy transfer

Wireless energy transfer is a process whereby electrical energy is transmitted from a power source to an electrical

load that does not have a built-in power source, without the use of interconnecting wires.

Computer interface devices

Answering the call of customers frustrated with cord clutter, many manufacturers of computer peripherals turned to

wireless technology to satisfy their consumer base. Originally these units used bulky, highly limited transceivers to

mediate between a computer and a keyboard and mouse; however, more recent generations have used small, high-

quality devices, some even incorporating Bluetooth. These systems have become so ubiquitous that some users have

begun complaining about a lack of wired peripherals. Wireless devices tend to have a slightly slower response time

than their wired counterparts; however, the gap is decreasing.

Concerns about the security of wireless keyboards arose at the end of 2007, when it was revealed that Microsoft's

implementation of encryption in some of its 27 MHz models was highly insecure

7EC3 Wireless communication Unit 1 notes updated upto ... Communication Propagation Phenomena...

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