Lecture 2 Communication Terminology and Channel Concepts Lecturer Madeeha Owais

1/12/2008 NUST-SEECS 1

• • • •

What is Wireless Communication?

Any form of communication that does not require the transmitter and receiver to be in physical contact through guided media Electromagnetic wave propagated through free-space • • • •

Communication Channel

Refers to the medium used to convey information from a transmitter) to a

receiver sender

(or A path for conveying electrical or electromagnetic signals, usually distinguished from other parallel paths A specific radio frequency, pair or band of frequencies, usually named with a letter, number, or codeword, and often allocated by international agreement e.g. Wi-Fi consists of unlicensed channels 1-13 from 2412MHz to 2484MHz in 5MHz steps 1/12/2008 NUST-SEECS 2

Types of communications channels

• Simplex Communication – Communication is possible in only one direction . The systems in which messages are received but not acknowledged.

• • Paging systems TV broadcast • Duplex Communication – A duplex communication system is a system composed of two connected parties or devices which can communicate with one another in both directions. – (The term

duplex

is not used when describing communication between more than two parties or devices. It is then called multiplex) 1/12/2008 NUST-SEECS 3

Types of communications channels

• Half Duplex Communication – Allow two –way communication ,but use the same radio channel for both transmission and reception( not simultaneously) – At any given time ,a user can only transmit or receive information • "walkie-talkie" style two-way radio 1/12/2008 NUST-SEECS 4

Types of communications channels

•

Full Duplex Communication

– Unlike half-duplex, full duplex systems allows radios transmission and reception to happen simultaneously. • Land-line telephone networks or Mobile phone are full-duplex since they allow both callers to speak and be heard at the same time.

–

How can duplex communication be achieved?

• Simultaneous communication possible by – – providing two separate radio channels (Frequency Division Duplex) or adjacent time slots on a single radio channel (Time Division Duplex) 1/12/2008 NUST-SEECS 5

Types of communications channels

• Frequency Division Duplex – Frequency-division duplexing means that the transmitter and receiver operates at different carrier frequencies.

– A pair of simplex channels with a fixed and known frequency separation is used (5% of the nominal RF frequency) – Based station to mobile user Forward Channel/Downlink – – Mobile user to base station Reverse Channel/Uplink At base station • separate transmit and receive antennas – At subscriber unit • single antenna for transmission/reception • Duplexer inside the unit to enable the antenna to be used for both purposes simultaneously 1/12/2008 NUST-SEECS 6

FDD 1/12/2008

Frequency Separation

Figure and explanation in Chapter 1-Rappaport NUST-SEECS 7

Types of communications channels

• – – – – – – – – Time Division Duplex (TDD) Uses time instead of frequency to provide both forward and reverse channels.

In TDD, multiple users share a signal radio channel by taking turns in the time domain i-e portion of time is used to transmit from BS to mobile and remaining is used to transmit from mobile to BS Individual users are allowed to access the channel in assigned

time slots

. Each duplex channel has both downlink and uplink time slots.

Time separation between downlink and uplink is necessary.

TDD only possible with digital transmission formats and modulation It is very sensitive to timing.

It is for this reason that TDD has been recently used And only for indoor or small area wireless applications where physical coverage distance(radio propagation delay) are much smaller than many kms used in conventional cellular systems.

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TDD 1/12/2008 NUST-SEECS 9

Symmetric Transmission vs. Asymmetric Transmission

• • Data transmission is symmetric if the data in the downlink and the data in the uplink is transmitted at the same data rate. This will probably be the case for voice transmission - the same amount of data is sent both ways.

For internet connections or broadcast data (e.g., streaming video), it is likely that more data will be sent from the server to the mobile device (the downlink).

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Asymmetric Transmission in TDD

• • • • FDD transmission is not so well suited for asymmetric applications as it uses equal frequency bands for the uplink and the downlink (a waste of valuable spectrum).

On the other hand, TDD does not have this fixed structure, and its flexible slots assignment is well-suited to asymmetric applications, e.g., the internet For example, TDD can be configured to provide 384kbps for the downlink (the direction of the major data transfer), and 64kbps for the uplink (where the traffic largely comprises requests for information and acknowledgements) Asymmetric transmission to be used in 3G Technologies 1/12/2008 NUST-SEECS 11

Decibel

• The

decibel

(

dB

) is a logarithmic unit of measurement that expresses the magnitude of a physical quantity (usually power or intensity) relative to a specified or implied

reference level

.

• Since it expresses a ratio of two quantities with the same unit, it is a dimensionless unit. A decibel is one tenth of a

bel

(

B

).

1/12/2008 NUST-SEECS 12

Other related absolute measurements

• •

dBm

– or

dBmW

dB(1 mW) —> power measurement relative to 1 milliwatt.

– X dBm = X dBW + 30.

dBW

– dB(1 W) —> similar to dBm, except the reference level is 1 watt. – – – 0 dBW = +30 dBm; −30 dBW = 0 dBm; X dBW = X dBm − 30.

1/12/2008 NUST-SEECS 13

Presentation Credits for part that follows

… •

William Stallings.

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• •

Antenna Basics

An antenna is an electrical conductor or system of conductors – Transmission - radiates electromagnetic energy into space – Reception - collects electromagnetic energy from space In two-way communication, the same antenna can be used for transmission and reception • • • Radiation pattern – Graphical representation of radiation properties of an antenna – Depicted as two-dimensional cross section Beam width (or half-power beam width) – Measure of directivity of antenna Reception pattern – Receiving antenna’s equivalent to radiation pattern

Types of Antennas

• • • Isotropic antenna (idealized) – Radiates power equally in all directions Dipole antennas – Half-wave dipole antenna (or Hertz antenna) – Quarter-wave vertical antenna (or Marconi antenna) Parabolic Reflective Antenna

Antenna Gain and Effective Area

•

Antenna gain

– Power output, in a particular direction, compared to that produced in any direction by a perfect omni-directional antenna (isotropic antenna) – Self reading at Antenna Gain http://radarproblems.com/chapters/ch06.dir/ch06pr.dir/c06p6.dir/c06p6.htm

•

Antenna Effective area or Effective Aperture

– Functionally equivalent area from which an antenna directed toward the source of the received signal gathers or absorbs the energy of an incident electromagnetic wave .

– Related to physical size and shape of antenna – Relationship to antenna gain

A eff =(λ 2 *G ant )/(4π)

Unit of effective area of antenna: m 2 G ant : antenna gain (not in decibels) λ: wavelength 1/12/2008 NUST-SEECS 17

Wireless Transmission Impairments

• • • • • • • • • • Path loss or Attenuation Free space loss Noise Atmospheric gaseous absorption Rain effects Multipath Reflection Refraction Diffraction Scattering 1/12/2008 NUST-SEECS 18

• • • • • • •

Path loss or Attenuation

Reduction of signal strength/intensity during transmission It is normal when a signal is sent from one point to another.

Strength/power loss is a function of transmission method and circuit medium.

Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal Signal must maintain a level sufficiently higher than noise to be received without error Attenuation is greater at higher frequencies, causing distortion

What can cause energy reduction?

Everything. It is important to understand that most of the calculation will show theoretical information. It is very difficult to predict the real reduction of energy since there are many factors that can cause reduction in energy. Terrain contours, environment (urban or rural, vegetation and foliage), propagation medium (dry or moist air), the distance between the transmitter and the receiver, and the height and location of antennas. 1/12/2008 NUST-SEECS 19

Free Space Loss

• • • • “The reduction in power density (attenuation) of an electromagnetic wave as it propagates through space" In other words it means how much energy an electromagnetic wave lose when its traversing through space.

Free space is a region where these is nothing - the vacuum of outer space is a fair approximation for most purposes.

There are no obstacles to get in the way, no gases to absorb energy, nothing to scatter the radio waves.

Unless you are into space communications, free space is not something you are likely to encounter, but it is important to understand what happens to a radio wave when there is nothing to disturb it.

1/12/2008 NUST-SEECS 20

•

Free Space Loss

Free space loss, ideal isotropic antenna • • • •

P

tx = signal power at transmitting antenna

P

rx  = signal power at receiving antenna = carrier wavelength

r

= propagation distance between antennas where

r

and  are in the same units (e.g., meters • • • Free Space Loss = 32.5 + 20log(d) + 20log(f) dB, Where d is the distance in km and f is the frequency in MHz Read derivation from Free Space Propagation http://www.mike-willis.com/Tutorial/free_space.htm

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Free Space Loss

1/12/2008 NUST-SEECS 22

Categories of Noise

• • • • • Thermal Noise Intermodulation noise Crosstalk Impulse Noise In wireless systems(unguided media),thermal noise is major concern.

• Rest of the categories are prevalent significantly in guided media transmission.

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• • • • •

Thermal Noise

Thermal noise due to agitation of electrons Present in all electronic devices and transmission media & cannot be eliminated.

Function of temperature Particularly significant for satellite communication because of weakness of signal received by satellite earth stations.

Amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is:

N

0 =kT(W/Hz) • • •

N

0 = noise power density in watts per 1 Hz of bandwidth k = Boltzmann's constant = 1.3803 ´ 10 -23 J/K

T

= temperature, in kelvins (absolute temperature) 1/12/2008 NUST-SEECS 24

Thermal noise present in a bandwidth of

B

or, in decibel-watts Hertz (in watts):

N N

   k

TB

10  228 log .

6 k  10 log

T

dBW   10 log

T

10  log 10

B

log

B

• • •

Other Categories

Inter-modulation noise

–special type of crosstalk. Signals from two circuits inter-modulate and form a new signal that falls into a frequency band that is reserved for another signal

Crosstalk

– unwanted coupling between signal paths

Impulse noise

– irregular pulses or noise spikes – – Short duration and of relatively high amplitude Caused by external electromagnetic disturbances, or faults and flaws in the communications system 1/12/2008 NUST-SEECS 25

•

Fading

is the distortion that a carrier-modulated telecommunication signal experiences over certain propagation media.

Types of Fading shall be studied in detail in later part of course.

Small scale and Large Scale Fading NUST-SEECS 26 1/12/2008

• • • • • •

Phenomena that cause fading:

Multipath – obstacles reflect signals so that multiple copies with varying delays are received Reflection - occurs when signal encounters a surface that is large relative to the wavelength of the signal Diffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave Scattering – occurs when incoming signal hits an object whose size is the order of the wavelength of the signal or less and where number of obstacles per unit volume is large Refraction – bending of radio waves as they propagate through the atmosphere Atmospheric absorption – water vapor and oxygen contribute to attenuation 1/12/2008 NUST-SEECS 27

•

The Effects of Multipath Propagation

Multiple copies of a signal may arrive at different phases – If phases add destructively, the signal level relative to noise declines, making detection more difficult – The received signal is made up of a sum of attenuated, phase shifted and time delayed versions of transmitted signal • Intersymbol interference (ISI) – One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequen t bit.

The Effects of Multipath Propagation

• The received signal is made up of a sum of attenuated, phase shifted and time delayed versions of transmitted signal Perfect channel White noise Phase jitter

Statistical Channel Models

• • • AWGN Channel Model Rayleigh Channel Model Rician Channel Model 1/12/2008 NUST-SEECS 30

Additive White Gaussian Noise Channel

• • • • The Gaussian channel can be considered the ideal channel, and it is impaired by additive white Gaussian noise(AWGN) developed internally by the receiver There may also be contributions from interferers.

The AWGN channel is the “best case” channel.

The ideal Gaussian channel is very difficult to achieve in mobile radio environment 1/12/2008 NUST-SEECS 31

Rayleigh Channel

• • • • • The Rayleigh channel is at the other end of the line, often referred to as a worst-case channel.

In a non-line-of-sight multipath propagation direct wave from the transmitter to the receiver is blocked.

The multipath reception has many components.

Each of the waves(multipath component) has a different phase and this phase can be considered as an independent uniform distribution, with the phase associated with each wave being equally likely to take on any value.

If each multipath component is independent, the PDF(power distribution function) of its envelope is Rayleigh 1/12/2008 NUST-SEECS 32

Rician Channel

• • • • The characteristics of a Rician channel are in-between those of a Gaussian and those of a Rayleigh channel.

In a line-of-sight(LOS) situation, the received signal is composed of a random multipath component( with multipath energy from local scatterers),plus a coherent LOS component which has essentially constant power.

The power of LOS component will be usually greater than the total multipath power.The theoretical distribution which applies to this case is the Rice distribution.

The Rician channel can be characterized by a function K.It is defined as 1/12/2008 NUST-SEECS 33

• • K = 0 Rayleigh(i.e,the numerator is 0 and all the received energy derives from scattered paths) K = ∞ The channel is Gaussian and the denominator is zero

• • •

Channel Capacity

Variety of impairments distort the signal and limit the data rate for digital data.

Channel Capacity

-The maximum rate at which data can be transmitted over a given communication path , under given conditions.

Four concepts to relate: –

Date rate

: The rate, in bits per second(bps),at which data can be communicated.

–

Bandwidth

:The bandwidth of the transmitted signal as constrained by the transmitter and the nature of transmission medium, expressed in cycles per second or Hertz –

Noise:

This is the average level of noise over the communication systems –

Error rate

• : This is the rate at which errors occur Error = transmit 1 and receive 0; transmit 0 and receive 1 1/12/2008 NUST-SEECS 35

Relationship between Data Rate and Bandwidth

• • • • • • The greater the bandwidth, the higher the information-carrying capacity BUT the transmission system will limit the bandwidth that can be transmitted.

Limitations arise from physical properties of transmission medium or from deliberate limitations at the transmitter on the bandwidth to prevent interference from other sources.

AND, for any given medium, the greater the bandwidth transmitted, the greater the cost HOWEVER, limiting the bandwidth creates distortions

Trade Off

: Aim is to get as high date rate as possible at a particular limit of error rate for a given bandwidth

Nyquist Bandwidth

• • • • • • • If rate of signal transmission is 2B then signal with frequencies no greater than B is sufficient to carry signal rate Given bandwidth B, highest signal rate is 2B Given binary signal(two voltage levels), data rate supported by B Hz is 2B bps When using more than two voltage levels(each signal representing more than one bit, Nyquist formulation is C= 2B log 2 M,where M is # of discrete signal or voltage levels For a given bandwidth, data rate can be increased by increasing # of different signalling elements however this places an increased burden on the receiver.

Instead of distinguishing one of two possible signal elements during each signal time, it must distinguish one of M possible signal elements.

Noise and other impairments on the transmission line will limit the practical value of M.

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Shannon Capacity Formula

• • • • • • • Consider data rate, noise and error rate

Signal to noise ratio(in decibels):

Ratio of the power in a signal to the power contained in the noise that’s present at a particular point in the transmission.Typically measured at the receiver.

SNR db = 10 log 10 (signal/noise) SNR is important in transmission of digital data because it sets the upper bound on achievable date rate.

Capacity C=B log 2 (1+SNR),C is in bps,B is in Hz Represents theoretical maximum that can be achieved.

This is error free capacity 1/12/2008 NUST-SEECS 38

•

Bit Energy to Noise Ratio

E b /N 0 – Energy per bit over the noise power spectral density – – Related to SNR power ratio Standard quality measure for digital communications • The bit error rate(BER) for digital data is a function of

E b

/

N 0

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• • • • •

Relate E

b

/N

0 SNR=S/N (Watt/Watt) E b =ST b – E b : energy per bit (J) – S: signal power (carrier power) (W) – T b : duration of a bit (s) E b /N 0 =(S/N 0 )*T b = (S/N 0 )*(1/R ) N 0 =N/B – N: total noise power (W) – B: bandwidth (Hz) E b /N 0 =(S/N)(B/R)

to SNR

• The S/N (or E b /N 0 ) at the input to the receiver will determine the system performance 1/12/2008 NUST-SEECS 40