Antennas and Propagation From Stallings – Wireless Communications and Networks

Introduction 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 Patterns 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 Antenna gain –Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna) Effective area –Related to physical size and shape of antenna

Antenna Gain Relationship between antenna gain and effective area G = antenna gain A e = effective area f = carrier frequency c = speed of light (» 3 ´ 10 8 m/s) = carrier wavelength

Propagation Modes Ground-wave propagation Sky-wave propagation Line-of-sight propagation

Ground Wave Propagation

Follows contour of the earth Can Propagate considerable distances Frequencies up to 2 MHz Example –AM radio

Sky Wave Propagation

Signal reflected from ionized layer of atmosphere back down to earth Signal can travel a number of hops, back and forth between ionosphere and earth’s surface Reflection effect caused by refraction Examples –Amateur radio –CB radio

Line-of-Sight Propagation

Transmitting and receiving antennas must be within line of sight –Satellite communication – signal above 30 MHz not reflected by ionosphere –Ground communication – antennas within effective line of site due to refraction Refraction – bending of microwaves by the atmosphere –Velocity of electromagnetic wave is a function of the density of the medium –When wave changes medium, speed changes –Wave bends at the boundary between mediums

Line-of-Sight Equations Optical line of sight Effective, or radio, line of sight d = distance between antenna and horizon (km) h = antenna height (m) K = adjustment factor to account for refraction, rule of thumb K = 4/3

Line-of-Sight Equations Maximum distance between two antennas for LOS propagation: h 1 = height of antenna one h 2 = height of antenna two

LOS Wireless Transmission Impairments Attenuation and attenuation distortion Free space loss Noise Atmospheric absorption Multipath Refraction Thermal noise

Attenuation Strength of signal falls off with distance over transmission medium Attenuation factors for unguided media: –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

Free Space Loss Free space loss, ideal isotropic antenna P t = signal power at transmitting antenna P r = signal power at receiving antenna = carrier wavelength d = propagation distance between antennas c = speed of light (» 3 ´ 10 8 m/s) where d and are in the same units (e.g., meters)

Free Space Loss Free space loss equation can be recast:

Free Space Loss Free space loss accounting for gain of other antennas G t = gain of transmitting antenna G r = gain of receiving antenna A t = effective area of transmitting antenna A r = effective area of receiving antenna

Free Space Loss Free space loss accounting for gain of other antennas can be recast as

Categories of Noise Thermal Noise Intermodulation noise Crosstalk Impulse Noise

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

Thermal Noise Amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is: N 0 = noise power density in watts per 1 Hz of bandwidth k = Boltzmann's constant = ´ J/K T = temperature, in kelvins (absolute temperature)

Thermal Noise Noise is assumed to be independent of frequency Thermal noise present in a bandwidth of B Hertz (in watts): or, in decibel-watts

Noise Terminology Intermodulation noise – occurs if signals with different frequencies share the same medium –Interference caused by a signal produced at a frequency that is the sum or difference of original frequencies 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

Expression E b /N 0 Ratio of signal energy per bit to noise power density per Hertz The bit error rate for digital data is a function of E b /N 0 –Given a value for E b /N 0 to achieve a desired error rate, parameters of this formula can be selected –As bit rate R increases, transmitted signal power must increase to maintain required E b /N 0

Other Impairments Atmospheric absorption – water vapor and oxygen contribute to attenuation Multipath – obstacles reflect signals so that multiple copies with varying delays are received Refraction – bending of radio waves as they propagate through the atmosphere

Multipath Propagation

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 in the order of the wavelength of the signal or less

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 Intersymbol interference (ISI) –One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit

Types of Fading Fast fading Slow fading Flat fading Selective fading Rayleigh fading Rician fading

Error Compensation Mechanisms Forward error correction Adaptive equalization Diversity techniques

Forward Error Correction Transmitter adds error-correcting code to data block –Code is a function of the data bits Receiver calculates error-correcting code from incoming data bits –If calculated code matches incoming code, no error occurred –If error-correcting codes don’t match, receiver attempts to determine bits in error and correct

Adaptive Equalization Can be applied to transmissions that carry analog or digital information –Analog voice or video –Digital data, digitized voice or video Used to combat intersymbol interference Involves gathering dispersed symbol energy back into its original time interval Techniques –Lumped analog circuits –Sophisticated digital signal processing algorithms

Diversity Techniques Diversity is based on the fact that individual channels experience independent fading events Space diversity – techniques involving physical transmission path Frequency diversity – techniques where the signal is spread out over a larger frequency bandwidth or carried on multiple frequency carriers Time diversity – techniques aimed at spreading the data out over time

Signal Encoding Techniques Stallings – Wireless Communications and Networks Chapter 6

Reasons for Choosing Encoding Techniques Digital data, digital signal –Equipment less complex and expensive than digital-to-analog modulation equipment Analog data, digital signal –Permits use of modern digital transmission and switching equipment

Reasons for Choosing Encoding Techniques Digital data, analog signal –Some transmission media will only propagate analog signals –E.g., optical fiber and unguided media Analog data, analog signal –Analog data in electrical form can be transmitted easily and cheaply –Done with voice transmission over voice-grade lines

Signal Encoding Criteria What determines how successful a receiver will be in interpreting an incoming signal? –Signal-to-noise ratio –Data rate –Bandwidth An increase in data rate increases bit error rate An increase in SNR decreases bit error rate An increase in bandwidth allows an increase in data rate

Factors Used to Compare Encoding Schemes Signal spectrum –With lack of high-frequency components, less bandwidth required –With no dc component, ac coupling via transformer possible –Transfer function of a channel is worse near band edges Clocking –Ease of determining beginning and end of each bit position

Factors Used to Compare Encoding Schemes Signal interference and noise immunity –Performance in the presence of noise Cost and complexity –The higher the signal rate to achieve a given data rate, the greater the cost

Basic Encoding Techniques Digital data to analog signal –Amplitude-shift keying (ASK) Amplitude difference of carrier frequency –Frequency-shift keying (FSK) Frequency difference near carrier frequency –Phase-shift keying (PSK) Phase of carrier signal shifted

Basic Encoding Techniques

Amplitude-Shift Keying One binary digit represented by presence of carrier, at constant amplitude Other binary digit represented by absence of carrier where the carrier signal is Acos(2πf c t)

Amplitude-Shift Keying 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) Two binary digits represented by two different frequencies near the carrier frequency where f 1 and f 2 are offset from carrier frequency f c by equal but opposite amounts

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 but more susceptible to error f i = f c + (2i – 1 – M)f d f c = the carrier frequency f d = the difference frequency M = number of different signal elements = 2 L L = number of bits per signal element

Multiple Frequency-Shift Keying (MFSK) To match data rate of input bit stream, each output signal element is held for: T s =LT seconds where T is the bit period (data rate = 1/T) So, one signal element encodes L bits

Multiple Frequency-Shift Keying (MFSK) Total bandwidth required 2Mf d Minimum frequency separation required 2f d =1/T s Therefore, modulator requires a bandwidth of W d =2 L /LT=M/T s

Multiple Frequency-Shift Keying (MFSK)

Phase-Shift Keying (PSK) Two-level PSK (BPSK) –Uses two phases to represent binary digits

Phase-Shift Keying (PSK) Differential PSK (DPSK) –Phase shift with reference to previous bit Binary 0 – signal burst of same phase as previous signal burst Binary 1 – signal burst of opposite phase to previous signal burst

Phase-Shift Keying (PSK) Four-level PSK (QPSK) –Each element represents more than one bit

Phase-Shift Keying (PSK) Multilevel PSK –Using multiple phase angles with each angle having more than one amplitude, multiple signals elements can be achieved D = modulation rate, baud R = data rate, bps M = number of different signal elements = 2 L L = number of bits per signal element

Performance Bandwidth of modulated signal (B T ) –ASK, PSKB T =(1+r)R –FSKB T =2DF+(1+r)R R = bit rate 0 < r < 1; related to how signal is filtered DF = f 2 -f c =f c -f 1

Performance Bandwidth of modulated signal (B T ) –MPSK –MFSK L = number of bits encoded per signal element M = number of different signal elements

Quadrature Amplitude Modulation QAM is a combination of ASK and PSK –Two different signals sent simultaneously on the same carrier frequency

Quadrature Amplitude Modulation

Reasons for Growth of Digital Techniques Growth in popularity of digital techniques for sending analog data –Repeaters are used instead of amplifiers No additive noise –TDM is used instead of FDM No intermodulation noise –Conversion to digital signaling allows use of more efficient digital switching techniques

Spread Spectrum Stallings Wireless Chapter 7

Spread Spectrum Input is fed into a channel encoder –Produces analog signal with narrow bandwidth Signal is further modulated using sequence of digits –Spreading code or spreading sequence –Generated by pseudonoise, or pseudo-random number generator Effect of modulation is to increase bandwidth of signal to be transmitted

Spread Spectrum On receiving end, digit sequence is used to demodulate the spread spectrum signal Signal is fed into a channel decoder to recover data

Spread Spectrum

What can be gained from apparent waste of spectrum? –Immunity from various kinds of noise and multipath distortion –Can be used for hiding and encrypting signals –Several users can independently use the same higher bandwidth with very little interference

Frequency Hopped Spread Spectrum (FHSS) Signal is broadcast over seemingly random series of radio frequencies –A number of channels allocated for the FH signal –Width of each channel corresponds to bandwidth of input signal Signal hops from frequency to frequency at fixed intervals –Transmitter operates in one channel at a time –Bits are transmitted using some encoding scheme –At each successive interval, a new carrier frequency is selected

Frequency Hoping Spread Spectrum Channel sequence dictated by spreading code Receiver, hopping between frequencies in synchronization with transmitter, picks up message Advantages –Eavesdroppers hear only unintelligible blips –Attempts to jam signal on one frequency succeed only at knocking out a few bits

Frequency Hoping Spread Spectrum

FHSS Using MFSK MFSK signal is translated to a new frequency every T c seconds by modulating the MFSK signal with the FHSS carrier signal For data rate of R: –duration of a bit: T = 1/R seconds –duration of signal element: T s = LT seconds T c  T s - slow-frequency-hop spread spectrum T c < T s - fast-frequency-hop spread spectrum

FHSS Performance Considerations Large number of frequencies used Results in a system that is quite resistant to jamming –Jammer must jam all frequencies –With fixed power, this reduces the jamming power in any one frequency band

Direct Sequence Spread Spectrum (DSSS) Each bit in original signal is represented by multiple bits in the transmitted signal Spreading code spreads signal across a wider frequency band –Spread is in direct proportion to number of bits used One technique combines digital information stream with the spreading code bit stream using exclusive-OR (Figure 7.6)

Direct Sequence Spread Spectrum (DSSS)

DSSS Using BPSK Multiply BPSK signal, s d (t) = A d(t) cos(2  f c t) by c(t) [takes values +1, -1] to get s(t) = A d(t)c(t) cos(2  f c t) A = amplitude of signal f c = carrier frequency d(t) = discrete function [+1, -1] At receiver, incoming signal multiplied by c(t) –Since, c(t) x c(t) = 1, incoming signal is recovered

DSSS Using BPSK

Code-Division Multiple Access (CDMA) Basic Principles of CDMA –D = rate of data signal –Break each bit into k chips Chips are a user-specific fixed pattern –Chip data rate of new channel = kD

CDMA Example If k=6 and code is a sequence of 1s and -1s –For a ‘1’ bit, A sends code as chip pattern –For a ‘0’ bit, A sends complement of code Receiver knows sender’s code and performs electronic decode function = received chip pattern = sender’s code

CDMA Example User A code = –To send a 1 bit = –To send a 0 bit = User B code = –To send a 1 bit = Receiver receiving with A’s code –(A’s code) x (received chip pattern) User A ‘1’ bit: 6 -> 1 User A ‘0’ bit: -6 -> 0 User B ‘1’ bit: 0 -> unwanted signal ignored

CDMA for Direct Sequence Spread Spectrum

Categories of Spreading Sequences Spreading Sequence Categories –PN sequences –Orthogonal codes For FHSS systems –PN sequences most common For DSSS systems not employing CDMA –PN sequences most common For DSSS CDMA systems –PN sequences –Orthogonal codes

PN Sequences PN generator produces periodic sequence that appears to be random PN Sequences –Generated by an algorithm using initial seed –Sequence isn’t statistically random but will pass many test of randomness –Sequences referred to as pseudorandom numbers or pseudonoise sequences –Unless algorithm and seed are known, the sequence is impractical to predict

Important PN Properties Randomness –Uniform distribution Balance property Run property –Independence –Correlation property Unpredictability

Linear Feedback Shift Register Implementation

Typical Multiple Spreading Approach Spread data rate by an orthogonal code (channelization code) –Provides mutual orthogonality among all users in the same cell Further spread result by a PN sequence (scrambling code) –Provides mutual randomness (low cross correlation) between users in different cells