CSE 42155431 Mobile Communications Winter 2010 Suprakash Datta
CSE 4215/5431: Mobile Communications Winter 2010 Suprakash Datta datta@cs. yorku. ca Office: CSEB 3043 Phone: 416 -736 -2100 ext 77875 Course page: http: //www. cs. yorku. ca/course/4215 Some slides are adapted from the book website 12/28/2021 CSE 4215, Winter 2010 1
Last class • Introduction to mobile communications • Similarities and differences with wired communication • Review of the TCP/IP architecture 12/28/2021 CSE 4215, Winter 2010 2
Today • The physical layer for mobile communications • Let’s start with the very basic notions 12/28/2021 CSE 4215, Winter 2010 3
Signals, channels and systems • What is a signal? – Baseband signal – Modulation – Bandwidth – Transmission/reception • What is a channel? – Bandwidth – Noise – Loss? • What is a communication system? 12/28/2021 CSE 4215, Winter 2010 4
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 • Periodic signal - analog or digital signal that repeats over time – s(t +T ) = s(t ) -¥< t < +¥ • where T is the period of the signal • 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) 12/28/2021 CSE 4215, Winter 2010 5
Sine Wave Parameters 12/28/2021 CSE 4215, Winter 2010 6
Bandwidth • Of a signal • Of a channel 12/28/2021 CSE 4215, Winter 2010 7
The underlying mathematics Fourier representation of periodic signals 1 1 0 0 t t ideal periodic signal real composition (based on harmonics) What about aperiodic signals ? 12/28/2021 CSE 4215, Winter 2010 8
Frequency domain • Fundamental frequency - when all frequency components of a signal are integer multiples of one frequency, it’s referred to as the fundamental frequency • Spectrum - range of frequencies that a signal contains • Absolute bandwidth - width of the spectrum of a signal • Effective bandwidth (or just bandwidth) narrow band of frequencies that most of the signal’s energy is contained in 12/28/2021 CSE 4215, Winter 2010 9
Transmitting rectangular signals • Observations – 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 12/28/2021 CSE 4215, Winter 2010 10
Bit rates, 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 12/28/2021 CSE 4215, Winter 2010 11
Nyquist Bandwidth • For binary signals (two voltage levels) – C = 2 B • With multilevel signaling – C = 2 B log 2 M • M = number of discrete signal or voltage levels 12/28/2021 CSE 4215, Winter 2010 12
Signal-to-Noise Ratio • 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 a receiver • Signal-to-noise ratio (SNR, or S/N) • A high SNR means a high-quality signal, low number of required intermediate repeaters • SNR sets upper bound on achievable data rate 12/28/2021 CSE 4215, Winter 2010 13
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 12/28/2021 CSE 4215, Winter 2010 14
Example of Nyquist and Shannon Formulations • Spectrum of a channel between 3 MHz and 4 MHz ; SNRd. B = 24 d. B • Using Shannon’s formula 12/28/2021 CSE 4215, Winter 2010 15
Example of Nyquist and Shannon Formulations • How many signaling levels are required? 12/28/2021 CSE 4215, Winter 2010 16
Modulation • Why? • How? 12/28/2021 CSE 4215, Winter 2010 17
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 3 x 108 m/s, frequency f twisted pair coax cable 1 Mm 300 Hz 10 km 30 k. Hz VLF 12/28/2021 LF optical transmission 100 m 3 MHz MF HF 1 m 300 MHz VHF UHF 10 mm 30 GHz SHF EHF CSE 4215, Winter 2010 100 m 3 THz infrared 1 m 300 THz visible light UV 18
Frequencies for wireless communication • VHF-/UHF-ranges for mobile radio – simple, small antenna for cars – deterministic propagation characteristics, reliable connections • SHF and higher for directed radio links, satellite communication – small antenna, beam forming – large bandwidth available • Wireless LANs use frequencies in UHF to SHF range – some systems planned up to EHF – limitations due to absorption by water and oxygen molecules (resonance frequencies) • weather dependent fading, signal loss caused by heavy rainfall etc. 12/28/2021 CSE 4215, Winter 2010 19
Frequencies and regulations • ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences) Examples Europe USA Japan Cellular phones GSM 880 -915, 925960, 1710 -1785, 1805 -1880 UMTS 1920 -1980, 2110 -2170 AMPS, TDMA, CDMA, GSM 824849, 869 -894 TDMA, CDMA, GSM, UMTS 1850 -1910, 1930 -1990 PDC, FOMA 810 -888, 893 -958 PDC 1429 -1453, 1477 -1501 FOMA 1920 -1980, 2110 -2170 Cordless phones CT 1+ 885 -887, 930932 CT 2 864 -868 DECT 1880 -1900 PACS 1850 -1910, 1930 -1990 PACS-UB 1910 -1930 PHS 1895 -1918 JCT 245 -380 Wireless LANs 802. 11 b/g 2412 -2472 802. 11 b/g 2412 -2462 802. 11 b 2412 -2484 802. 11 g 2412 -2472 Other RF systems 27, 128, 418, 433, 868 315, 915 426, 868 12/28/2021 CSE 4215, Winter 2010 20
Multiplexing • Multiplexing in 4 dimensions – – space (si) time (t) frequency (f) code (c) • Goal: multiple use of a shared medium channels ki k 1 k 2 k 3 k 5 k 6 c t s 1 f s 2 f c t • Important: guard spaces needed! s 3 12/28/2021 k 4 CSE 4215, Winter 2010 f 21
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 k 1 k 2 k 3 k 4 k 5 k 6 c f • Disadvantages – waste of bandwidth if the traffic is distributed unevenly – inflexible t 12/28/2021 CSE 4215, Winter 2010 22
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 k 1 k 2 k 3 k 4 k 5 k 6 c f • Disadvantages – precise synchronization necessary t 12/28/2021 CSE 4215, Winter 2010 23
Time and frequency multiplex • Combination of both methods • A channel gets a certain frequency band for a certain amount of time • Example: GSM • Advantages k 1 – better protection against tapping – protection against frequency selective interference k 2 k 3 k 4 k 5 c f • but: precise coordination required t 12/28/2021 k 6 CSE 4215, Winter 2010 24
Code multiplex • Each channel has a unique code k 1 k 2 k 3 k 4 • All channels use the same spectrum at the same time • Advantages k 5 k 6 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 • Implemented using spread spectrum technology 12/28/2021 CSE 4215, Winter 2010 25
Example • Lack of coordination requirement is an advantage. 12/28/2021 CSE 4215, Winter 2010 26
Aside: Digital Communications • • What is coding? What is source coding? What are line codes? What is channel coding? 12/28/2021 CSE 4215, Winter 2010 27
Transceivers • How are signals sent and received in wireless communications? 12/28/2021 CSE 4215, Winter 2010 28
Antennas: isotropic radiator • Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission • Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna • Real antennas always have directive effects (vertically and/or horizontally) • Radiation pattern: measurement of radiation around an antenna z y x 12/28/2021 CSE 4215, Winter 2010 x ideal isotropic radiator 29
Antennas: simple dipoles • Real antennas are not isotropic radiators but, e. g. , dipoles with lengths /4 on car roofs or /2 as Hertzian dipole shape of antenna proportional to wavelength /4 /2 • Example: Radiation pattern of a simple Hertzian dipole y y x side view (xy-plane) z z side view (yz-plane) x simple dipole top view (xz-plane) • Gain: maximum power in the direction of the main lobe compared to the power of an isotropic radiator (with the same average power) 12/28/2021 CSE 4215, Winter 2010 30
Antennas: directed and sectorized • Often used for microwave connections or base stations for mobile phones (e. g. , radio coverage of a valley) y y z x z side view (xy-plane) x side view (yz-plane) top view (xz-plane) z z x x top view, 3 sector 12/28/2021 directed antenna sectorized antenna top view, 6 sector CSE 4215, Winter 2010 31
Antennas: diversity • Grouping of 2 or more antennas – multi-element antenna arrays • Antenna diversity – switched diversity, selection diversity • receiver chooses antenna with largest output – diversity combining • combine output power to produce gain • cophasing needed to avoid cancellation /2 /4 /2 /2 + + ground plane 12/28/2021 CSE 4215, Winter 2010 32
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 12/28/2021 CSE 4215, Winter 2010 33
Antenna Gain • Relationship between antenna gain and effective area • • • 12/28/2021 G = antenna gain Ae = effective area f = carrier frequency c = speed of light (» 3 ´ 108 m/s) = carrier wavelength CSE 4215, Winter 2010 34
Back to modulation • Digital modulation – digital data is translated into an analog signal (baseband) – ASK, FSK, PSK - main focus in this chapter – 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) 12/28/2021 CSE 4215, Winter 2010 35
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 12/28/2021 CSE 4215, Winter 2010 36
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 12/28/2021 CSE 4215, Winter 2010 t 1 0 1 t 37
Advanced Frequency Shift Keying • bandwidth needed for FSK depends on the distance between the carrier frequencies • special pre-computation avoids sudden phase shifts MSK (Minimum Shift Keying) – bit separated into even and odd bits, the duration of each bit is doubled – depending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosen – the frequency of one carrier is twice the frequency of the other – Equivalent to offset QPSK • even higher bandwidth efficiency using a Gaussian lowpass filter GMSK (Gaussian MSK), used in GSM 12/28/2021 CSE 4215, Winter 2010 38
Example of MSK 1 0 1 0 bit data even 0101 even bits odd 0011 odd bits signal value hnnh - - ++ low frequency h: high frequency n: low frequency +: original signal -: inverted signal high frequency MSK signal 12/28/2021 t No phase shifts! CSE 4215, Winter 2010 39
Advanced Phase Shift Keying • BPSK (Binary Phase Shift Keying): – – – • bit value 0: sine wave bit value 1: inverted sine wave very simple PSK low spectral efficiency robust, used e. g. in satellite systems 1 10 QPSK (Quadrature Phase Shift Keying): – 2 bits coded as one symbol – symbol determines shift of sine wave – needs less bandwidth compared to BPSK – more complex • Q I 0 Q 11 I 00 01 A Often also transmission of relative, not absolute phase shift: DQPSK Differential QPSK (IS-136, PHS) t 11 12/28/2021 CSE 4215, Winter 2010 10 00 01 40
Quadrature Amplitude Modulation • . Quadrature Amplitude Modulation (QAM) – combines amplitude and phase modulation – it is possible to code n bits using one symbol – 2 n discrete levels, n=2 identical to QPSK • Bit error rate increases with n, but less errors compared to comparable PSK schemes. Q – Example: 16 -QAM (4 bits = 1 symbol) – Symbols 0011 and 0001 have the same phase φ, but different amplitude a. 0000 and 1000 have different phase, but same amplitude 12/28/2021 CSE 4215, Winter 2010 0011 0000 φ I a 1000 41
Hierarchical Modulation • DVB-T modulates two separate data streams onto a single DVB-T stream • High Priority (HP) embedded within a Low Priority (LP) stream • Multi carrier system, about 2000 or 8000 carriers • QPSK, 16 QAM, 64 QAM Q • Example: 64 QAM – good reception: resolve the entire 64 QAM constellation – poor reception, mobile reception: resolve only QPSK portion – 6 bit per QAM symbol, 2 most significant determine QPSK – HP service coded in QPSK (2 bit), LP uses remaining 4 bit 12/28/2021 CSE 4215, Winter 2010 10 I 00 000010 010101 42
Signal propagation basics Many different effects have to be considered 12/28/2021 CSE 4215, Winter 2010 43
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 12/28/2021 CSE 4215, Winter 2010 sender transmission distance detection interference 44
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 12/28/2021 reflection refraction scattering CSE 4215, Winter 2010 diffraction 45
Real world example 12/28/2021 CSE 4215, Winter 2010 46
Propagation Modes • Ground-wave propagation • Sky-wave propagation • Line-of-sight propagation 12/28/2021 CSE 4215, Winter 2010 47
Ground Wave Propagation 12/28/2021 CSE 4215, Winter 2010 48
Ground Wave Propagation • • Follows contour of the earth Can Propagate considerable distances Frequencies up to 2 MHz Example – AM radio 12/28/2021 CSE 4215, Winter 2010 49
Sky Wave Propagation 12/28/2021 CSE 4215, Winter 2010 50
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 12/28/2021 CSE 4215, Winter 2010 51
Line-of-Sight Propagation 12/28/2021 CSE 4215, Winter 2010 52
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 12/28/2021 CSE 4215, Winter 2010 53
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 12/28/2021 CSE 4215, Winter 2010 54
Line-of-Sight Equations • Maximum distance between two antennas for LOS propagation: • h 1 = height of antenna one • h 2 = height of antenna two 12/28/2021 CSE 4215, Winter 2010 55
LOS Wireless Transmission Impairments • • Attenuation and attenuation distortion Free space loss Atmospheric absorption Multipath (diffraction, reflection, refraction…) • Noise • Thermal noise 12/28/2021 CSE 4215, Winter 2010 56
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 12/28/2021 CSE 4215, Winter 2010 57
Free Space Loss • Free space loss, ideal isotropic antenna • 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) 12/28/2021 CSE 4215, Winter 2010 58
Free Space Loss • Free space loss equation can be recast: 12/28/2021 CSE 4215, Winter 2010 59
Free Space Loss • Free space loss accounting for gain of other antennas • • 12/28/2021 Gt = gain of transmitting antenna Gr = gain of receiving antenna At = effective area of transmitting antenna Ar = effective area of receiving antenna CSE 4215, Winter 2010 60
Free Space Loss • Free space loss accounting for gain of other antennas can be recast as 12/28/2021 CSE 4215, Winter 2010 61
Multipath Propagation 12/28/2021 CSE 4215, Winter 2010 62
Multipath propagation • Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction multipath LOS pulses signal at sender signal at receiver • Time dispersion: signal is dispersed over time – interference with “neighbor” symbols, Inter Symbol Interference (ISI) • The signal reaches a receiver directly and phase shifted – distorted signal depending on the phases of the different parts 12/28/2021 CSE 4215, Winter 2010 63
Atmospheric absorption • Water vapor and oxygen contribute most • Water vapor: peak attenuation near 22 GHz, low below 15 Ghz • Oxygen: absorption peak near 60 GHz, lower below 30 GHz. • Rain and fog may scatter (thus attenuate) radio waves. • Low frequency band usage helps… 12/28/2021 CSE 4215, Winter 2010 64
Effects of mobility • Channel characteristics change over time and location – signal paths change – different delay variations of different signal parts – different phases of signal parts – quick changes in the power received (short term long term power fading) fading • Additional changes in – distance to sender – obstacles further away short term fading – slow changes in the average power received (long term fading) 12/28/2021 CSE 4215, Winter 2010 t 65
Next • Channel effects (e. g. fading), noise • Spread spectrum • Cellular system basics 12/28/2021 CSE 4215, Winter 2010 66
- Slides: 66