Wireless networks - Lecture 2: Introduction to Wireless communication

Wireless networks - Lecture 2: Introduction to Wireless communication. The main topics covered in this chapter include: wireless transmission; digital data analog signals; noises; atmospheric noise like thunderstorms; attenuation and other impairments;...

Wireless Networks

Lecture 2 Introduction to Wireless Communication

Dr Ghalib A Shah

Last Lecture Review

Trans mis s ion in Wireles s  Domain 

► obtained by converting analog or digital data 

into analog or digital s ignal, bandwidth = [0,  fmax)

► band­limited s ignal whos e minimum frequency is  

different from zero, bandwidth = [f1, f2)

 in wired networks, new wiring can be added to

accommodate new applications/users – one wire for telephone, one for cable, one for LAN, etc.

 antenna size must correspond to signal’s wavelength

► 1 MHz signal  few 100 m-s high antenna;

► 1 GHz signal  few cm-s high antenna

 characteristics of wireless-signal propagation heavily

depend on signal’s frequency

Signal Encoding/Modulation

 We are concerned with transmitting digital data

 Some transmission media will only propagate

analog signals e.g., optical fiber and unguided media

 Therefore, we will discuss transmitting digital

data using analog signals

 The most familiar use of this transformation is

transmitting digital data through the public

 Each pulse in digital signal is a signal element.

 Binary data are transmitted by encoding each data bit into signal

elements.

 There can be one-to-one correspondence between data elements

and signal elements or one-to-multiple/multiple-to-one

 Data rate: the rate in bits/sec that data are transmitted

 The duration or length of bit is the amount of

time it takes for the transmitter to emit the bit For data rate R, bit time is 1/R

Carrier and Information Signals

 carrier signal: In radio frequency systems an

analog signal is always used as the main

airborne signal

 Information Signal: On top of this signal

another signal, analog or digital, is added that carries the information

 Modulation: This combination of signals is

called the modulation

 Modulation is how an information signal is

added to a carrier signal

 This is the superimposing of the information

onto the carrier

 In an RF system a modulator generates this

information signal

 Then it is passed to the transmitter and out the

antenna

 Then at the other end the signal is

demodulated

 The way to think of this is like a letter

► The envelope is the carrier and the letter is the

PM

Types of Encoding

 There are three forms of Encoding

► ASK – Amplitude-Shift Keying

► FSK – Frequency-Shift Keying

► PSK – Phase-Shift Keying

Amplitude Shift-Keying (ASK)

 ASK changes the height of the sine wave as

time goes by

 The two binary values are represented by two

different amplitudes of the carrier frequency

 One binary digit represented by presence of carrier, at

constant amplitude

 Other binary digit represented by absence of carrier

 Susceptible to sudden gain changes

 Inefficient modulation technique

 On voice-grade lines, used up to 1200 bps

 Used to transmit digital data over optical fiber

Binary Frequency Shift-Keying (BFSK)

 FSK changes the frequency of the sine wave as

time goes by, without changing the height

 Two binary digits represented by two different

frequencies near the carrier frequency

Binary Frequency-Shift Keying (BFSK)

 Less susceptible to error than ASK

 On voice-grade lines, used up to 1200bps

 Used for high-frequency (3 to 30 MHz) radio

transmission

 Can be used at higher frequencies on LANs

that use coaxial cable

Multiple Frequency-Shift Keying (MFSK)

 More than two frequencies are used

 More bandwidth efficient and less susceptible to error

 To match data rate of input bit stream, each output

signal element is held for:

Multiple Frequency-Shift Keying (MFSK)

binary 

0 binary 

t f

t f

1

binary 

0 binary 

 In general when you see phase modulation

schemes explained B stands for binary, which

is only 2 points Q stands for quadrature,

which is 4 points and 16 and 64 represent the higher number of points in the modulation

schemes

 Every time the number of points is increased

the speed is increased, but interference

tolerance is reduced

 This is one of the reasons for automatic speed

reduction in the face of interference

 Going from binary - 2 to 64 requires a really

clean signal

 Noise consists of all undesired radio signals,

whether manmade or natural

 Noise makes the reception of useful

information difficult

 The radio signal’s strength is of little use, if the

noise power is greater than the received signal power

 This is why the signal to noise ratio is important

 Amount of thermal noise to be found in a 

bandwidth of 1Hz in any device or conductor  is:

• N0 = noise power density in watts per 1 Hz of  bandwidth

W/Hz  

k

N

  log  

10 dBW

  6 228

Manmade Noise

 Manmade noise is part of modern life

 It is generated almost anywhere that there is

electrical activity, such as automobile ignition systems, power lines, motors, arc welders,

fluorescent lights, and so on

 Each occurrence is small, but there are so

many that together they can completely hide a weak signal that would be above the natural

noise in a less populated area

Natural Noise

 Naturally occurring noise has two main sources

► Atmospheric noise, such as thunderstorms, from 0

to 5 MHz

► Galactic noise, such as stars, at all higher

frequencies

 Both of these sources generate sharp pulses of

electromagnetic energy over all frequencies

 The pulses are propagated according to the

same laws as the desirable signals being

Noise Remedy

 Increasing receiver amplification cannot

improve the signal to noise ratio since both

signal and noise will be amplified equally and the ratio will remain the same

 If the signal coming out is smaller than the

signal going in, it is loss that appears as heat

 Attenuators produce loss

 Causes of loss or attenuation in RF systems

and the environments through which they

transmit include

► Water, regardless of how it appears or where it is

found including inside connections

► When water is encountered in the air as the signal

passes through, the form of the moisture matters

► At frequencies above 10 GHz attenuation from rain

becomes significant

► Examples of the affect outside include

• Rain causes about 08 dB of loss per mile for 2.4 GHz and 5.8 GHz

• Fog causes about 03 dB per mile for 2.4 GHz

• For 5.8 GHz the loss is about 11 dB per mile

• Ice changes the effective design of an antenna, therefore changing its performance

 If the signal gets larger before it exits the

device, it is gain

 RF amplifiers produce gain

 Gain is an active process in most cases, in

other words it requires a power source

 Gain can also be the combination of signals

from different directions appearing together,

such as the main signal and a reflected signal

 However, the total gain cannot exceed the

 Wireless Transmission

► Why baseband signal can not be transmitted?

► Need bandpass signals whose minimum frequency is higher

than 0

► Modulator produces bandpass by superomposing basband

signal over higher frequency signals

• AM, FM, PM

 Digital data analog signals

► Some transmission media like optical fibers and unguided

propagate only analog signals

► For example public telephone network

• Atmospheric noise like thunderstorms

• Galatic noise such as stars

► Manmade noise