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CSE 6806: Wireless and Mobile Communication Networks 1 The physical layer for Mobile Communications 2 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 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) = At sin(2 ft t + t) 6 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(2ft + ) • 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 The underlying mathematics Fourier representation of periodic signals 1 g (t ) c an sin( 2nft) bn cos( 2nft) 2 n 1 n 1 1 1 0 0 t ideal periodic signal t real composition (based on harmonics) 12 Freq. components of Square Wave 13 Freq. components of Square Wave 14 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 15 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-toanalog 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 Signals, channels and systems • What is a signal? – Modulation – Bandwidth – Transmission/reception • What is a channel? – Bandwidth – Noise – Loss? • What is a communication system? 27 28 Bandwidth • Of a signal • Of a channel 29 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 30 Nyquist Bandwidth • For binary signals (two voltage levels) C = 2B (in bps) • Data rate = B Hz • With multilevel signaling C = 2B log2 M (in bps) • M = number of discrete signal or voltage levels 31 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 signal power ( SNR) dB 10 log 10 noise power • A high SNR means a high-quality signal, low number of required intermediate repeaters • SNR sets upper bound on achievable data rate 32 Shannon Capacity Formula • Equation: C B log 2 1 SNR • 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 33 Example of Nyquist and Shannon Formulations • Spectrum of a channel between 3 MHz and 4 MHz ; SNRdB = 24 dB B 4 MHz 3 MHz 1 MHz SNR dB 24 dB 10 log 10 SNR SNR 251 • Using Shannon’s formula, Channel capacity C 10 log 2 1 251 10 8 8Mbps 6 6 34 Example of Nyquist and Shannon Formulations • How many signaling levels are required? C 2 B log 2 M 8 10 2 10 log 2 M 6 6 4 log 2 M M 16 35 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, 3x1011 to 2x1014 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 Frequencies for wireless communication • • • VLF = Very Low Frequency LF = Low Frequency MF = Medium Frequency UHF = Ultra High Frequency SHF = Super High Frequency EHF = Extra High Frequency • • HF = High Frequency VHF = Very High Frequency UV = Ultraviolet Light • Frequency and wave length – = c/f – wave length , speed of light c 3x108 m/s, frequency f twisted pair coax cable 1 Mm 300 Hz 10 km 30 kHz VLF LF optical transmission 100 m 3 MHz MF HF 1m 300 MHz VHF UHF 10 mm 30 GHz SHF EHF 100 m 3 THz infrared 1 m 300 THz visible light UV 42 Modulation • Why? • How? 43 Multiplexing • Multiplexing in 4 dimensions – – – – space (si) time (t) frequency (f) code (c) • Goal: multiple use of a shared medium channels ki k1 k2 k3 k4 k5 k6 c t c t s1 f s2 f c t • Important: guard spaces needed! s3 f 44 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 k1 k2 k3 k4 k5 k6 c f • Disadvantages – waste of bandwidth if the traffic is distributed unevenly – inflexible t 45 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 k1 k2 k3 k4 k5 k6 c f • Disadvantages – precise synchronization necessary t 46 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 k1 – better protection against tapping – protection against frequency selective interference k2 k3 k4 k5 k6 c f • but: precise coordination required t 47 Code multiplex • Each channel has a unique code k1 k2 • All channels use the same spectrum at the same time • Advantages k3 k4 k5 k6 c – bandwidth efficient – no coordination and synchronization necessary – good protection against interference and tapping f • Disadvantages – varying user data rates – more complex signal regeneration t 48 Example • Lack of coordination requirement is an advantage 49 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) 50 Modulation and demodulation digital data 101101001 digital modulation analog baseband signal analog modulation radio transmitter radio carrier analog demodulation analog baseband signal synchronization decision digital data 101101001 radio receiver radio carrier 51 Digital modulation • Modulation of digital signals known as Shift Keying 1 0 • Amplitude Shift Keying (ASK): 1 – very simple – low bandwidth requirements – very susceptible to interference t 1 0 1 • Frequency Shift Keying (FSK): – needs larger bandwidth • Phase Shift Keying (PSK): – more complex – robust against interference t 1 0 1 t 52 Signal propagation basics Many different effects have to be considered 53 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 sender transmission distance detection interference 54 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 shadowing reflection refraction scattering diffraction 55 Multipath Propagation 56 Multipath propagation • Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction multipath LOS pulses pulses signal at sender signal at receiver • 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 57 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… 58 Propagation Modes • Ground-wave propagation • Sky-wave propagation • Line-of-sight propagation 59 Ground Wave Propagation 60 Ground Wave Propagation • • • • Follows contour of the earth Can Propagate considerable distances Frequencies up to 2 MHz Example – AM radio 61 Sky Wave Propagation 62 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 63 Line-of-Sight Propagation 64 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 65 Line-of-Sight Equations • Optical line of sight d 3.57 h • Effective, or radio, line of sight d 3.57 h • d = distance between antenna and horizon (km) • h = antenna height (m) • K = adjustment factor to account for refraction, rule of thumb K = 4/3 66 Line-of-Sight Equations • Maximum distance between two antennas for LOS propagation: 3.57 h1 h2 • h1 = height of antenna one • h2 = height of antenna two 67 LOS Wireless Transmission Impairments • • • • Attenuation and attenuation distortion Free space loss Atmospheric absorption Multipath (diffraction, reflection, refraction…) • Noise • Thermal noise 68 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 69 Free Space Loss • Free space loss, ideal isotropic antenna Pt 4d 4fd 2 2 Pr c 2 2 • Pt = signal power at transmitting antenna • Pr = 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) 70 Free Space Loss • Free space loss equation can be recast: Pt 4d LdB 10 log 20 log Pr 20 log 20 log d 21.98 dB 4fd 20 log 20 log f 20 log d 147.56 dB c 71 Spread Spectrum • What? • Why? • How? 72 Spreading and frequency selective fading channel quality 1 2 5 3 6 narrowband channels 4 frequency narrow band signal guard space channel quality 1 spread spectrum 2 2 2 2 2 spread spectrum channels frequency 73 Spread Spectrum 74 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 pseudorandom number generator • Effect of modulation is to increase bandwidth of signal to be transmitted 75 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 76 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 77 DSSS illustration DSSS Using BPSK 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 80 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 81 FHSS - illustration 82 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 83 FHSS - iIIustration tb user data 0 1 f 0 1 1 t td f3 slow hopping (3 bits/hop) f2 f1 f t td f3 fast hopping (3 hops/bit) f2 f1 t tb: bit period td: dwell time 84 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 85 FHSS and Retransmissions • What happens when a packet is corrupt and has to be retransmitted? • IEEE 802.11: max time of each hop: 400ms, max packet length: 30 ms. 86 FHSS and WLAN access points • IEEE 802.11 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. 87 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 88 CDMA Example • If k=6 and code is a sequence of 1s and -1s – For a ‘1’ bit, A sends code as chip pattern • <c1, c2, c3, c4, c5, c6> – For a ‘0’ bit, A sends complement of code • <-c1, -c2, -c3, -c4, -c5, -c6> • Receiver knows sender’s code and performs electronic decode function Su d d1 c1 d 2 c2 d 3 c3 d 4 c4 d 5 c5 d 6 c6 • <d1, d2, d3, d4, d5, d6> = received chip pattern • <c1, c2, c3, c4, c5, c6> = sender’s code 89 CDMA Example • User A code = <1, –1, –1, 1, –1, 1> – To send a 1 bit = <1, –1, –1, 1, –1, 1> – To send a 0 bit = <–1, 1, 1, –1, 1, –1> • User B code = <1, 1, –1, – 1, 1, 1> – To send a 1 bit = <1, 1, –1, –1, 1, 1> • 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 90 CDMA for DSSS