Structure of the Lecture Chapter 2 Technical Basics: Laer Methods for Medium Access: Laer 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 Mobilit on the network laer: Mobile IP, Routing, Ad-Hoc Networks Mobilit on the transport laer: reliable transmission, flow control, QoS Mobilit support on the application laer Page Radio Waves Data are represented phsicall as electromagnetic waves: s(t) = A sin(2 π f t + ) A Twisted Pair A: Amplitude f: Frequenc (T: Duration of an oscillation) : Phase Coaial Cable Wave Guide 0 Infrared T = /f Optical transmission Wavelength Frequenc Visible light Relation between frequenc and wavelength:: λ = c/f with wavelength λ, speed of light c 30 8 m/s, frequenc f Page 2 Frequencies for Mobile 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, focusing Large bandwidth available Wireless LANs use frequencies in UHF to SHF spectrum Some sstems planned up to EHF Limitations due to absorption b water and ogen molecules (resonance frequencies), leading to weather dependent fading, signal loss caused b heav rainfall etc. Page 3 Frequencies (MH) and Regulations ITU-R holds auctions for new frequencies, manages frequenc bands worldwide (WRC, World Radio Conferences) Cellular Phones Cordless Phones Wireless LANs Europe USA Japan GSM 450-457, 479-486/460-467,489-496, 890-95/935-960, 70-785/805-880 UMTS (FDD) 920-980, 20-290 UMTS (TDD) 900-920, 2020-2025 CT+ 885-887, 930-932 CT2 864-868 DECT 880-900 IEEE 802. 2400-2483 HIPERLAN 2 550-5350, 5470-5725 AMPS, TDMA, CDMA 824-849, 869-894 TDMA, CDMA, GSM 850-90, 930-990 PACS 850-90, 930-990 PACS-UB 90-930 902-928 IEEE 802. 2400-2483 550-5350, 5725-5825 PDC 80-826, 940-956, 429-465, 477-53 PHS 895-98 JCT 254-380 IEEE 802. 247-2497 550-5250 Page 4
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) Frequenc spectrum (amplitude or phase over frequenc) Phase state diagram (amplitude A and phase in polar coordinates) A [V] t[s] A [V] f [H] Q = A sin (Quadrature) I = A cos (In-phase) Modulation For transmission, a modulation of the discrete time and value signal on an analogous carrier frequenc The carrier is a periodical sinus signal on a certain frequenc: s(t) = A sin(2 π f t + ) X un-modulated signal carrier frequenc (sin) modulated signal Modulation in two steps: analogous base-band digital signal data digital analogous 0000 modulation modulation Sender carrier frequenc Page 5 Page 6 Modulation of Digital Signals Problem of Modulation value 0 0 time Digital modulation is possible in several was, basing on the parameters of an analogous wave: Each periodic signal can be represented b a composition of harmonic signals (Fourier-Transformation): g( t) = c + a sin(2πnft) + b cos(2πnft) 2 n n= n n= s(t) = A sin(2 π f t + ) Amplitude Frequenc Phase Amplitude Shift Keing (ASK) 0 ideal periodical signal t 0 real composition (based on harmonics) t Eas to realie Needs not much bandwidth Susceptible against disturbance (weakening of signal strength over distance) Often used within optical sstems An analog signal produced b digital modulation can also be seen as a periodic signal - that influences the transmission! Page 7 Page 8
Needed Bandwidth Modulation of Digital Signals value 0 0 ASK could have produced a signal as shown above. Interpret it as a periodic signal (mabe with infinite period) and Fourier transform tells ou that our signal is transmitted as set of harmonics. That means: it is not onl transmitted on the carrier frequenc f T, but we also spread into the neighbored frequenc ranges (where f B is the frequenc of the data signal): time Digital modulation is possible in several was, basing on the parameters of an analogous wave: s(t) = A sin(2 π f t + ) Frequenc Shift Keing (FSK) Amplitude Frequenc Phase Bandwidth: Width of the needed frequenc range for transmission on a carrier frequenc Page 9 Waste of frequencies Needs high bandwidth Used in data transmission using phone lines (modems) Resulting signal (frequenc domain): Page 0 Modulation of Digital Signals Minimum Shift Keing (MSK) value Phase Shift Keing (PSK) 0 0 time Digital modulation is possible in several was, basing on the parameters of an analogous wave: s(t) = A sin(2 π f t + ) Amplitude Frequenc Phase Enhancement of FSK Increases bandwidth efficienc (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 Keing 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 frequenc, original or inverted, is chosen The frequenc of one carrier is twice the frequenc of the other 80 phase shift Comple demodulation Robust against disturbances Often used in mobile transmissions Resulting signal (frequenc domain): Even higher bandwidth efficienc using a Gaussian low-pass filter GMSK (Gaussian MSK), e.g. used in GSM and DECT Page Page 2
f f 2 Minimum Shift Keing (MSK) data even bits odd bits lower frequenc higher frequenc 0 0 0 Bit even 0 0 odd 0 0 Signal h l l h value - - + + h: high frequenc l: low frequenc +: original signal -: inverted signal Advanced Phase Shift Keing Shifting is not restricted to two phases: it can be done between M phases, with the onl requirement for M is to be power to 2. This allows to transmit several bits in parallel. Eample: QPSK (Quaternar Phase Shift Keing) Shifting between 4 phases 4 phase allow for 4 states: code 2 bits at once Thus: doubling transmission rate 0 0 00 Q = A sin 0 I = A cos 0 MSK signal t 00 0 00 0 A = Amplitude of the signal I = In-Phase, signal component (in phase with carrier signal) No phase shifts! Q = Quadrature-Phase, quadrature component (verticall to phase of carrier) Page 3 Page 4 Advanced Phase Shift Keing 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 onl use PSK with higher number of phases 00 0 000 0 0 00 00 0 00 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) - onl a theoretical reference antenna Real antennas alwas have directive effects (verticall and/or horiontall) Radiation pattern: measurement of radiation around an antenna 0 00 000 00 0 000 000 00 0000 6-QAM: 4 bit per signal: 00 and 000 are in phase, but have different amplitudes 0000 und 000 have same amplitude, but different phases ideal isotropic radiator Toda: in use are methods like 256-QAM Page 5 Page 6
Antennas: simple Dipoles Real antennas are not isotropic radiators but, e.g., dipoles with lengths λ/4 on car roofs or as Hertian dipole shape of antenna proportional to wavelength Antennas: directed and sectoried Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valle) λ/4 directed antenna Eample: Radiation pattern of a simple Hertian dipole side view (-plane) side view (-plane) top view (-plane) side view (-plane) side view (-plane) top view (-plane) Simple dipole Gain: maimum power in the direction of the main lobe compared to the power of an isotropic radiator (with the same average power) top view, 3 sectors top view, 6 sectors sectoried antenna Page 7 Page 8 Antennas: Diversit Signal Propagation Grouping of 2 or more antennas Multi-element antenna arras Antenna diversit Switched diversit, selection diversit Receiver chooses antenna with largest output Diversit combining Combine output power to produce gain Co-phasing needed to avoid cancellation ground plane λ/4 + λ/4 + Page 9 Propagation in free space alwas in straight line Receiving power is proportional to /d α (d = distance between sender and receiver, α = 2 in free space, 2.7-5 in a cit, 4-6 indoor) Values for α larger than 2 result from influences of the environment causing attenuation, reflection, scattering, Depending on the received signal power, three regions can be identified: Transmission range Communication possible Interference Low error rate Detection Detection range Transmission Detection of the signal possible Sender No communication possible (error rate) Interference range Signal ma not be detected Signal adds to the background noise distance Page 20
Influence of the Atmosphere Frequenc and Propagation 400 km Ionosphere Ionosphere Electrical loaded ions enable a propagation of radio waves over long distances 50 km Stratosphere 6-8 km Troposphere 0 km Earth Surface Stratosphere Constant temperature and steam; not much influence on radio waves Troposphere variable epansion (6 km at the poles, 8 km at the equator) Weather changes can influence the signal propagation Page 2 A ground wave" propagates along earth s surface Propagation over the horion (000 km) A diffraction is caused b obstacles Onl frequencies up to 2 MH Diffraction of waves at the ionosphere (depending on the frequenc, the ion densit, and the angle of incidence) The higher the frequenc, the lower the diffraction Propagation over 00 km, onl frequencies up to ca. 00 MH In mobile telecommunication networks and Wireless LANs: Higher frequencies, not influenced b earth s surface or ionosphere Receiving onl possible in line of sight, but b obstacles, not onl an attenuation Chapter is possible, 2.: Laer but sometimes also an increasing of the range! Page 22 Propagation in Environments Real World Eamples 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 inde gets smaller with increasing height diffraction attenuation line of sight scattering reflection Page 23 Page 24
Multipath Propagation Effects of Mobilit Signal can take man 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-Smbol-Interference Attenuation of the signal due to reflection, diffraction, A signal reaches the receiver directl and phase shifted! depending on the phase of the different received signal parts, destructive interference is possible, in the worst case erase the signal completel Page 25 Channel characteristics change over time and location Signal paths change Different dela 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 awa Slow changes in the average power received (long term fading) short term fading long term fading Page 26 Further Problems Conclusion Reason Effect Meaning Multipath Propagation Movement Attenuation Shading Fast Fading Dela spread (Time dispersion) Doppler effect (Frequenc dispersion) Path loss Slow Fading Highl 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 frequenc Slow fluctuation of the average signal strength Important task on laer : 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 b the environment Multipath propagation leads to disturbances of received signals We have to live with the effects of mobilit laer 2 has to deal with the problems Page 27 Page 28