1 CSE 6806: Wireless and Mobile Communication Networks

2 The physical layer for Mobile Communications

Electromagnetic Signal Function of time Can also be expressed as a function of frequency –Signal consists of components of different frequencies 3

Time-Domain Concepts Analog signal - signal intensity varies in a smooth fashion over time –No breaks or discontinuities in the signal Digital signal - signal intensity maintains a constant level for some period of time and then changes to another constant level Periodic signal - analog or digital signal pattern that repeats over time –s(t +T ) = s(t ) - ɑ ≤ t ≤ + ɑ where T is the period of the signal 4

Analog and Digital Signal 5

6 Types of signals (a) continuous time/discrete time (b) continuous values/discrete values –analog signal = continuous time, continuous values –digital signal = discrete time, discrete values Signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift  –sine wave as special periodic signal for a carrier: s(t) = A t sin(2  f t t +  t )

Time-Domain Concepts Aperiodic signal - analog or digital signal pattern that doesn't repeat over time Peak amplitude (A) - maximum value or strength of the signal over time; typically measured in volts Frequency (f ) –Rate, in cycles per second, or Hertz (Hz) at which the signal repeats 7

Time-Domain Concepts Period (T ) - amount of time it takes for one repetition of the signal –T = 1/f Phase (  ) - measure of the relative position in time within a single period of a signal Wavelength ( ) - distance occupied by a single cycle of the signal –Or, the distance between two points of corresponding phase of two consecutive cycles 8

Sine Wave Parameters General sine wave –s(t ) = A sin(2  ft +  ) The effect of varying each of the three parameters (see next slide) –(a) A = 1, f = 1 Hz,  = 0; thus T = 1s –(b) Reduced peak amplitude; A=0.5 –(c) Increased frequency; f = 2, thus T = ½ –(d) Phase shift;  =  /4 radians (45 degrees) note: 2  radians = 360° = 1 period 9

Sine Wave Parameters 10

Frequency-Domain Concepts Any electromagnetic signal can be shown to consist of a collection of periodic analog signals (sine waves) at different amplitudes, frequencies, and phases The period of the total signal is equal to the period of the fundamental frequency 11

12 The underlying mathematics tt ideal periodic signal real composition (based on harmonics) Fourier representation of periodic signals

13 Freq. components of Square Wave

14 Freq. components of Square Wave

15 Frequency domain Fundamental frequency: when all frequency components of a signal are integer multiples of one frequency, it is the fundamental frequency Spectrum - range of frequencies that a signal contains Absolute bandwidth - width of the spectrum of a signal Effective bandwidth - narrow band of frequencies that most of the signal’s energy is contained in

Data Rate and Bandwidth The greater the bandwidth, the higher the information-carrying capacity Conclusions –Any digital waveform will have infinite bandwidth –BUT the transmission system will limit the bandwidth that can be transmitted –AND, for any given medium, the greater the bandwidth transmitted, the greater the cost –HOWEVER, limiting the bandwidth creates distortions 16

Data Communication Terms Data - entities that convey meaning, or information Signals - electric or electromagnetic representations of data Transmission - communication of data by the propagation and processing of signals 17

Analog and Digital Data Analog –Video –Audio Digital –Text –Integers 18

Analog Signals A continuously varying electromagnetic wave that may be propagated over a variety of media, depending on frequency Examples of media: –Copper wire media (twisted pair and coaxial cable) –Fiber optic cable –Atmosphere or space propagation Analog signals can propagate analog and digital data 19

Digital Signals A sequence of voltage pulses that may be transmitted over a copper wire medium Generally cheaper than analog signaling Less susceptible to noise interference Suffer more from attenuation Digital signals can propagate analog and digital data 20

Analog Signaling 21

Digital Signaling 22

Reasons for Choosing Data and Signal Combinations Digital data, digital signal –Equipment for encoding is less expensive than digital-to- analog equipment Analog data, digital signal –Conversion permits use of modern digital transmission and switching equipment Digital data, analog signal –Some transmission media will only propagate analog signals –Examples include optical fiber and satellite Analog data, analog signal –Analog data easily converted to analog signal 23

Analog Transmission Transmit analog signals without regard to content Attenuation limits length of transmission link Cascaded amplifiers boost signal’s energy for longer distances but cause distortion –Analog data can tolerate distortion –Introduces errors in digital data 24

Digital Transmission Concerned with the content of the signal Attenuation endangers integrity of data Digital Signal –Repeaters achieve greater distance –Repeaters recover the signal and retransmit Analog signal carrying digital data –Retransmission device recovers the digital data from analog signal –Generates new, clean analog signal 25

About Channel Capacity Impairments, such as noise, limit data rate that can be achieved For digital data, to what extent do impairments limit data rate? Channel Capacity – the maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions 26

27 Signals, channels and systems What is a signal? –Modulation –Bandwidth –Transmission/reception What is a channel? –Bandwidth –Noise –Loss? What is a communication system?

28

29 Bandwidth Of a signal Of a channel

30 Bit rates, channel capacity Noise limits data rate that can be achieved For digital data, to what extent do these impairments limit data rate? Channel Capacity: The maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions

31 Nyquist Bandwidth For binary signals (two voltage levels) C = 2B (in bps) Data rate = B Hz With multilevel signaling C = 2B log 2 M (in bps) M = number of discrete signal or voltage levels

32 Signal-to-Noise Ratio Ratio of the power in a signal to the power contained in the noise that is present at a particular point in the transmission Typically measured at a receiver A high SNR means a high-quality signal, low number of required intermediate repeaters SNR sets upper bound on achievable data rate

33 Shannon Capacity Formula Equation : Represents theoretical maximum that can be achieved In practice, only much lower rates achieved –Formula assumes white noise (thermal noise) –Impulse noise is not accounted for –Attenuation distortion or delay distortion not accounted for

34 Example of Nyquist and Shannon Formulations Spectrum of a channel between 3 MHz and 4 MHz ; SNR dB = 24 dB Using Shannon’s formula, Channel capacity

35 Example of Nyquist and Shannon Formulations How many signaling levels are required?

Classifications of Transmission Media Transmission Medium –Physical path between transmitter and receiver Guided Media –Waves are guided along a solid medium –E.g., copper twisted pair, copper coaxial cable, optical fiber Unguided Media –Provides means of transmission but does not guide electromagnetic signals –Usually referred to as wireless transmission –E.g., atmosphere, outer space

Unguided Media Transmission and reception are achieved by means of an antenna Configurations for wireless transmission –Directional –Omnidirectional

General Frequency Ranges Microwave frequency range –1 GHz to 100 GHz –Directional beams possible –Suitable for point-to-point transmission –Used for satellite communications Radio frequency range –30 MHz to 1 GHz –Suitable for omnidirectional applications Infrared frequency range –Roughly, 3x10 11 to 2x10 14 Hz –Useful in local point-to-point multipoint applications within confined areas

Terrestrial Microwave Description of common microwave antenna –Parabolic "dish", 3 m in diameter –Fixed rigidly and focuses a narrow beam –Achieves line-of-sight transmission to receiving antenna –Located at substantial heights above ground level Applications –Long haul telecommunications service Alternative to Coaxial cable or optical fiber –Short point-to-point links between buildings CCTV Linking multiple LANs

Satellite Microwave Description of communication satellite –Microwave relay station –Used to link two or more ground-based microwave transmitter/receivers –Receives transmissions on one frequency band (uplink), amplifies or repeats the signal, and transmits it on another frequency (downlink) Applications –Television distribution –Long-distance telephone transmission –Private business networks

Broadcast Radio Description of broadcast radio antennas –Omnidirectional –Antennas not required to be dish-shaped –Antennas need not be rigidly mounted to a precise alignment Applications –Broadcast radio VHF and part of the UHF band; 30 MHZ to 1GHz Covers FM radio and UHF and VHF television

42 Frequencies for wireless communication VLF = Very Low FrequencyUHF = Ultra High Frequency LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency Frequency and wave length –  = c/f –wave length, speed of light c  3x10 8 m/s, frequency f 1 Mm 300 Hz 10 km 30 kHz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100  m 3 THz 1  m 300 THz visible light VLFLFMFHFVHFUHFSHFEHFinfraredUV optical transmission coax cabletwisted pair

43 Modulation Why? How?

44 Multiplexing in 4 dimensions –space (s i ) –time (t) –frequency (f) –code (c) Goal: multiple use of a shared medium Important: guard spaces needed! s2s2 s3s3 s1s1 Multiplexing f t c k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 f t c f t c channels k i

45 Frequency multiplexing Separation of the whole spectrum into smaller frequency bands A channel gets a certain band of the spectrum for the whole time Advantages –no dynamic coordination necessary –works also for analog signals Disadvantages –waste of bandwidth if the traffic is distributed unevenly –inflexible k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 f t c

46 f t c k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 Time division multiplexing A channel gets the whole spectrum for a certain amount of time Advantages –only one carrier in the medium at any time –throughput high even for many users Disadvantages –precise synchronization necessary

47 f Time and frequency multiplex Combination of both TDM and FDM A channel gets a certain frequency band for a certain amount of time Example: GSM Advantages –better protection against tapping –protection against frequency selective interference but: precise coordination required t c k2k2 k3k3 k4k4 k5k5 k6k6 k1k1

48 Code multiplex Each channel has a unique code All channels use the same spectrum at the same time Advantages –bandwidth efficient –no coordination and synchronization necessary –good protection against interference and tapping Disadvantages –varying user data rates –more complex signal regeneration k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 f t c

49 Example Lack of coordination requirement is an advantage

50 Modulation Digital modulation –digital data is translated into an analog signal (baseband) –differences in spectral efficiency, power efficiency, robustness Analog modulation –shifts center frequency of baseband signal up to the radio carrier Motivation –smaller antennas (e.g., /4) –Frequency Division Multiplexing –medium characteristics Basic schemes –Amplitude Modulation (AM) –Frequency Modulation (FM) –Phase Modulation (PM)

51 Modulation and demodulation synchronization decision digital data analog demodulation radio carrier analog baseband signal radio receiver digital modulation digital data analog modulation radio carrier analog baseband signal radio transmitter

52 Digital modulation Modulation of digital signals known as Shift Keying Amplitude Shift Keying (ASK): –very simple –low bandwidth requirements –very susceptible to interference Frequency Shift Keying (FSK): –needs larger bandwidth Phase Shift Keying (PSK): –more complex –robust against interference 101 t 101 t 101 t

53 Signal propagation basics Many different effects have to be considered

54 Signal propagation ranges Transmission range –communication possible –low error rate Detection range –detection of the signal possible –no communication possible Interference range –signal may not be detected –signal adds to the background noise distance sender transmission detection interference

55 Signal propagation Propagation in free space always like light (straight line) Receiving power proportional to 1/d² in vacuum – much more in real environments (d = distance between sender and receiver) Receiving power additionally influenced by –fading (frequency dependent) –shadowing –reflection at large obstacles –refraction depending on the density of a medium –scattering at small obstacles –diffraction at edges reflectionscatteringdiffractionshadowing refraction

Multipath Propagation 56

57 Multipath propagation Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction Time dispersion: signal is dispersed over time The signal reaches a receiver directly and phase shifted –distorted signal depending on the phases of the different parts signal at sender signal at receiver LOS pulses multipath pulses

58 Atmospheric absorption Water vapor and oxygen contribute most Water vapor: peak attenuation near 22GHz, low below 15Ghz Oxygen: absorption peak near 60GHz, lower below 30 GHz. Rain and fog may scatter (thus attenuate) radio waves. Low frequency band usage helps…

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

60 Ground Wave Propagation

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

62 Sky Wave Propagation

63 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

64 Line-of-Sight Propagation

65 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

66 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

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

68 LOS Wireless Transmission Impairments Attenuation and attenuation distortion Free space loss Atmospheric absorption Multipath (diffraction, reflection, refraction…) Noise Thermal noise

69 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

70 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)

71 Free Space Loss Free space loss equation can be recast:

72 Spread Spectrum What? Why? How?

73 Spreading and frequency selective fading frequency channel quality narrow band signal guard space frequency channel quality 1 spread spectrum narrowband channels spread spectrum channels

74 Spread Spectrum

75 Spread Spectrum - sender 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

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

77 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

DSSS illustration

DSSS Using BPSK

80 Frequency Hoping 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

81 FHSS - contd 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

82 FHSS - illustration

83 FHSS details - 1 Discrete changes of carrier frequency –sequence of frequency changes determined via pseudo random number sequence Two versions –Fast Hopping: several frequencies per user bit –Slow Hopping: several user bits per frequency Advantages –frequency selective fading and interference limited to short period –simple implementation –uses only small portion of spectrum at any time Disadvantages –not as robust as DSSS –simpler to detect

84 FHSS - iIIustration user data slow hopping (3 bits/hop) fast hopping (3 hops/bit) 01 tbtb 011t f f1f1 f2f2 f3f3 t tdtd f f1f1 f2f2 f3f3 t tdtd t b : bit periodt d : dwell time

85 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

86 FHSS and Retransmissions What happens when a packet is corrupt and has to be retransmitted? IEEE : max time of each hop: 400ms, max packet length: 30 ms.

87 FHSS and WLAN access points IEEE FHSS WLAN specifies 78 hopping channels separated by 1 MHz in 3 groups (0,3,6,9,…, 75), (1,4,7,…, 76), (2,5,8,…,77) Allows installation of 3 AP’s in the same area.

88 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

89 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

90 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 DSSS