Structure of the Lecture

Transcription

1 Structure of the Lecture Chapter 2 Technical Basics: Layer 1 Methods for Medium Access: Layer 2 Representation of digital signals on an analogous medium Signal propagation Characteristics of antennas Chapter 3 Wireless Networks: Bluetooth, WLAN, WirelessMAN, WirelessWAN Mobile Networks: GSM, GPRS, UMTS Satellites and Broadcast Networks Chapter 4 Mobility on the network layer: Mobile IP, Routing, Ad-Hoc Networks Mobility on the transport layer: reliable transmission, flow control, QoS Mobility support on the application layer Page 1

2 Radio Waves Data are represented physically as electromagnetic waves: s(t) = A sin(2 π f t + ϕ) A A: Amplitude f: Frequency (T: Duration of an oscillation) ϕ: Phase ϕ 0 T = 1/f Twisted Pair Coaxial Cable Wave Guide Optical transmission Wavelength Frequency Infrared Visible light Relation between frequency and wavelength:: λ = c/f with wavelength λ, speed of light c 3x10 8 m/s, frequency f Page 2

3 Frequencies for Mobile Communication Twisted Pair Coaxial Cable Wave Guide Optical transmission Wavelength Frequency 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, focusing Large bandwidth available Wireless LANs use frequencies in UHF to SHF spectrum Some systems planned up to EHF Limitations due to absorption by water and oxygen molecules (resonance frequencies), leading to weather dependent fading, signal loss caused by heavy rainfall etc. Infrared Visible light Page 3

4 Frequencies (MHz) and Regulations ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences) Europe USA Japan Cellular Phones Cordless Phones Wireless LANs GSM , / , , / , / UMTS (FDD) , UMTS (TDD) , CT , CT DECT IEEE HIPERLAN , AMPS, TDMA, CDMA , TDMA, CDMA, GSM , PACS , PACS-UB IEEE , PDC , , , PHS JCT IEEE Page 4

5 Signals Classification of signals regarding their characteristics: continuous time/discrete time continuous values/discrete values analog signal = continuous time and continuous values digital signal = discrete time and discrete values Different representations of a signal: Amplitude spectrum (amplitude over time) Frequency spectrum (amplitude or phase over frequency) Phase state diagram (amplitude A and phase ϕ in polar coordinates) A [V] A [V] Q = A sin ϕ (Quadrature) t[s] ϕ ϕ f [Hz] I = A cos ϕ (In-phase) Page 5

6 Modulation For transmission, a modulation of the discrete time and value signal on an analogous carrier frequency The carrier is a periodical sinus signal on a certain frequency: s(t) = A sin(2 π f t + ϕ) X un-modulated signal carrier frequency (sin) modulated signal Modulation in two steps: analogous base-band digital signal data digital analogous modulation modulation Sender carrier frequency Page 6

7 Modulation of Digital Signals value time Digital modulation is possible in several ways, basing on the parameters of an analogous wave: s(t) = A sin(2 π f t + ϕ) Amplitude Frequency Phase Amplitude Shift Keying (ASK) Easy to realize Needs not much bandwidth Susceptible against disturbance (weakening of signal strength over distance) Often used within optical systems Page 7

8 Bandwidth Each periodic signal can be represented by a composition of harmonic signals (Fourier-Transformation): g( t) = 1 2 c + n= 1 a n sin(2πnft) + n= 1 b n cos(2πnft) ideal periodical signal t real composition (based on harmonics) t An analog signal produced by digital modulation can also be seen as a periodic signal - that influences the transmission! Page 8

9 Needed Bandwidth Nyquist: theoretical minimum bandwidth B required for transmitting S symbols per second is given by S/2 Hz Bandwidth: Width of the frequency range necessary for transmission of a given number of symbols. Not (as usually spoken about) an amount of data! Capacity: from a given bandwidth one can compute the maximum possible data rate achievable on the frequency range (by considering the number of possible symbols and the number of bits which can be coded per symbol) That means: we never can transmit data on only one single frequency, we always need a certain frequency range for transmission! The higher the capacity of a transmission channel has to be, the more bandwidth is needed! Page 9

10 Bandwidth in Radio Communication Now encode your information on a carrier frequency f T, here using ASK: Forurier transform now tells you: the signal is not longer propagating as a single frequency f T, but is a burst of frequencies Resulting frequency spectrum: It is not only transmitted on the carrier frequency, but we also spread into the neighbored frequency ranges (where f B is the bandwidth of the data signal, i.e. the maximum frequency which with the amplitude level is changing): That means: depending on a signal s bandwidth the needed communication channel bandwidth can be computed. Page 10

11 Modulation of Digital Signals value time Digital modulation is possible in several ways, basing on the parameters of an analogous wave: s(t) = A sin(2 π f t + ϕ) Frequency Shift Keying (FSK) Amplitude Frequency Phase Waste of frequencies Needs high bandwidth Used in data transmission using phone lines (modems) Resulting signal (frequency domain): Page 11

12 Modulation of Digital Signals value time Digital modulation is possible in several ways, basing on the parameters of an analogous wave: s(t) = A sin(2 π f t + ϕ) Phase Shift Keying (PSK) Amplitude Frequency Phase 180 phase shift Complex demodulation Robust against disturbances Often used in mobile transmissions Resulting signal (frequency domain): Page 12

13 Minimum Shift Keying (MSK) Enhancement of FSK Increases bandwidth efficiency (transmission rate / needed bandwidth) and reduces disturbances of neighbored frequencies FSK: bandwidth depends on the distance between the carrier frequencies Special pre-computation reduces the distance: Minimum Shift Keying Bits are separated into even and odd bits, the duration of a bit is doubled Depending on the bit values (even, odd) the higher or the lower carrier frequency, original or inverted, is chosen The frequency of one carrier is twice the frequency of the other Even higher bandwidth efficiency using a Gaussian low-pass filter (i.e. restrict the needed frequency range by cutting away high frequencies) GMSK (Gaussian MSK), e.g. used in GSM and DECT Page 13

14 Minimum Shift Keying (MSK) data even bits odd bits Bit even odd Signal h l l h value f 1 f 2 lower frequency higher frequency h: high frequency l: low frequency +: original signal -: inverted signal MSK signal t No phase shifts! Page 14

15 Advanced Phase Shift Keying Shifting is not restricted to two phases: it can be done between M phases, with the only requirement for M is to be power to 2. This allows to transmit several bits in parallel. Example: QPSK (Quaternary Phase Shift Keying) Shifting between 4 phases 4 phase allow for 4 states: code 2 bits at once Thus: doubling transmission rate Q = A sinϕ ϕ I = A cosϕ A = Amplitude of the signal I = In-Phase, signal component (in phase with carrier signal) Q = Quadrature-Phase, quadrature component (vertically to phase of carrier) Page 15

16 Advanced Phase Shift Keying Quadrature Amplitude Modulation (QAM) Combination of ASK and QPSK n bit can be coded at once (n=2 is the simple QPSK) Bit error rate increases with increasing n, but less than if we would only use PSK with higher number of phases QAM: 4 bit per signal: 0011 and 0001 are in phase, but have different amplitudes 0000 und 0010 have same amplitude, but different phases Today: in use are methods like 256-QAM Page 16

17 Antennas Antennas are converting electrical signals into electromagnetic waves (and vice versa) Passive device, i.e. power of radiated signal is not higher than the power of the transmitter Principle: oscillatory circuit oscillating electrical field saved in a condenser, and magnetic field induced in a coil Modified circuit produces an oscillating electrical and magnetic field when feeding power in it (resp. Converts incoming fields into output power) Antennas are giving a gain i.e. larger antennas are producing signals of larger power and are more sensible for receiving weak signals. Thus, better antennas can increase the signal quality and by this the range of a signal Page 17

18 Antennas: isotropic Radiator 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 y z z x y x ideal isotropic radiator Page 18

19 Antennas: simple Dipoles Real antennas are not isotropic radiators but, e.g., dipoles with lengths λ/4 on car roofs or λ/2 as Hertzian dipole shape of antenna proportional to wavelength λ/4 λ/2 Example: Radiation pattern of a simple Hertzian dipole y y z x z x Simple dipole side view (xy-plane) side view (yz-plane) 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) Page 19

20 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 x directed antenna side view (xy-plane) side view (yz-plane) top view (xz-plane) z z x x sectorized antenna top view, 3 sectors top view, 6 sectors New trend: beamforming antennas depending on the location of a station which should be served, the antenna can focus its radiation to this station Page 20

21 In Reality Page 21

22 Antennas: Diversity Grouping of two or more antennas to improve signal quality or transmission capacity We need multi-element antenna arrays Antenna diversity Switched diversity, selection diversity Receiver chooses antenna with highest output power Diversity combining Combine output power to produce gain Co-phasing needed to avoid cancellation (signal is received with different phases) λ/4 λ/2 λ/4 λ/2 λ/2 λ/2 + + ground plane Page 22

23 Signal Propagation Propagation in free space always in straight line Signal power decreases with increasing distance Basic relation: λ P = P G G r t 4π d r t with P r = received signal power P t = transmitted signal power G r = gain of receiver antenna G t = gain of transmitter antenna λ = wave length of carrier frequency d = distance between transmitter and receiver The quality of a received signal not only depends on the signal strength on sender side, but also on the used antennas, and on the carrier frequency Bad luck: the higher the carrier frequency, the lower the wave length and by this also the received power decreases faster with ongoing distance Page 23

24 Signal Propagation Depending on the received signal power, three regions can be identified: Transmission range Communication possible Low error rate Detection range Detection of the signal possible No communication possible (error rate) Interference range Signal may not be detected Signal adds to the background noise Interference Detection Transmission Sender distance In reality: received power is proportional to sending power by 1/d α (d = distance between sender and receiver, α = 2 in free space, in a city, 4-6 indoor) Values for α larger than 2 result from influences of the environment causing attenuation, reflection, scattering, Page 24

25 Influence of the Atmosphere 400 km Ionosphere Ionosphere Electrical loaded ions enable a propagation of radio waves over long distances 50 km 6-18 km 0 km Stratosphere Troposphere Earth Surface Stratosphere Constant temperature and steam; not much influence on radio waves Troposphere variable expansion (6 km at the poles, 18 km at the equator) Weather changes can influence the signal propagation Page 25

26 Frequency and Propagation A ground wave" propagates along earth s surface Propagation over the horizon (1000 km) A diffraction is caused by obstacles Only frequencies up to 2 MHz Diffraction of waves at the ionosphere (depending on the frequency, the ion density, and the angle of incidence) The higher the frequency, the lower the diffraction Propagation over 100 km, only frequencies up to ca. 100 MHz In mobile telecommunication networks and Wireless LANs: Higher frequencies, not influenced by earth s surface or ionosphere Receiving only possible in line of sight, but by obstacles, not only an attenuation Chapter is possible, 2.1: Layer but 1sometimes also an increasing of the range! Page 26

27 Propagation in Natural/Human Environments Factors influencing the propagation: Natural environment: mountains, water, vegetation, rain, snow Artificial environment: buildings, cars, etc. Propagation: Attenuation, shading (rain, vegetation) Diffraction at edges Reflection at large planes Scattering at small obstacles Refraction: refraction index gets smaller with increasing height diffraction attenuation line of sight scattering reflection Antenna diversity: multi-path propagation maybe can improve signal quality! Page 27

28 Multipath Propagation Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction, : signal at sender signal at receiver The signal is dispersed over time (time dispersion) Interference with neighbored signals, Inter-Symbol-Interference Attenuation of the signal due to reflection, diffraction, A signal reaches the receiver directly and phase shifted! depending on the phase of the different received signal parts, destructive interference is possible, in the worst case erase the signal completely Page 28

29 Real World Examples Page 29

30 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 fading) Additional changes in Distance to sender Obstacles further away long term fading Slow changes in the average power received (long term fading) short term fading Page 30

31 Further Problems Reason Multipath Propagation Movement Attenuation Shading Effect Fast Fading Delay spread (Time dispersion) Doppler effect (Frequency dispersion) Path loss Slow Fading Meaning Highly fluctuating signal strength depending on the distance Received signal consists of several components with short temporal differences The phase of the received signal is changed basing on the current distance between sender and receiver Rain/fog are changing the propagation of a signal depending on its frequency Slow fluctuation of the average signal strength Page 31

32 Conclusion Important task on layer 1: modulation of digital data to analogous radio waves ASK, FSK, PSK PSK often is preferred in mobile communications To achieve higher data rates: QPSK, QAM Problems with signal propagation Attenuation of signal by the environment Multipath propagation leads to disturbances of received signals (but also can help to reach places which are not in straight line-of-sight of the transmitter) We have to live with the effects of mobility layer 2 has to deal with the problems Page 32