Mobile Communication-Systems II: From Cellular to Mobile Services Prof. Dr.-Ing. Rolf Kraemer Lehrstuhl für Systeme
Lecture Overview Quick Repetition of Basics GSM: Architecture and Features GPRS: Extended GSM System Architecture DECT: Digital Enhanced Cordless Telephony Tetra: Terrestrial Trunked Radio UMTS: Universal Mobile Communication System HSDPA: High Speed Downlink Packet Access Mobile IP Mobile TCP Mobile Application Frameworks
Development of Mobile Access Speed Source: VTC-2007, Fettweis
Number of Cellular Phones 07.6.2007 Mobiltelefone: erstmals über drei Milliarden Anschlüsse weltweit Die Zahl der Mobilfunk-Anschlüsse wird in diesem Jahr weltweit erstmals die 3-Milliarden-Grenze überschreiten. Damit werden sich die Verträge für Handys und Prepaid-Karten innerhalb von sechs Jahren fast verdreifacht haben. Dies ergab eine Studie des europäischen Marktforschungsinstituts EITO im Auftrag des [1] Bitkom. Ende 2007 wird somit statistisch fast jeder zweite Mensch auf der Erde mobil telefonieren. Dabei übersteigt in manchen Industrieländern wie Italien, Schweden oder auch Deutschland dank des Trends zum Zweitoder Dritthandy inzwischen die Zahl der Geräte die Zahl der Einwohner.
Mobile phone subscribers worldwide 1200 1000 Subscribers [million] 800 600 400 GSM total TDMA total CDMA total PDC total Analogue total Total wireless Prediction (1998) 200 0 1996 1997 1998 1999 2000 2001 2002 year
Wireless systems: overview of the development 1981: NMT 450 1986: NMT 900 cellular phones 1992: GSM 1994: DCS 1800 1991: CDMA 1983: AMPS 1991: D-AMPS 1993: PDC 1982: Inmarsat-A 1988: Inmarsat-C satellites 1992: Inmarsat-B Inmarsat-M 1998: Iridium cordless phones 1980: CT0 1984: CT1 1987: CT1+ 1989: CT 2 wireless LAN 1991: DECT 199x: proprietary 1997: IEEE 802.11 1999: 802.11b, Bluetooth analogue digital 2000: GPRS 4G fourth generation: when and how? 2001: IMT-2000 200?: Fourth Generation (Internet based) 2000: IEEE 802.11a
Foundation: ITU-R - Recommendations for IMT-2000 M.687-2 IMT-2000 concepts and goals M.816-1 M.817 M.818-1 M.819-2 framework for services IMT-2000 network architectures satellites in IMT-2000 IMT-2000 for developing countries M.1034-1 requirements for the radio interface(s) M.1035 M.1036 framework for radio interface(s) and radio sub-system functions spectrum considerations M.1078 security in IMT-2000 M.1079 speech/voiceband data performance M.1167 framework for satellites M.1168 framework for management M.1223 evaluation of security mechanisms M.1224 vocabulary for IMT-2000 M.1225 evaluation of transmission technologies... http://www.itu.int/imt
Development of mobile telecommunication systems CDMA TDMA FDMA CT0/1 AMPS NMT CT2 IS-136 TDMA D-AMPS GSM PDC IS-95 cdmaone GPRS cdma2000 1X EDGE IMT-FT DECT IMT-SC IS-136HS UWC-136 1G 2G 2.5G 3G IMT-DS UTRA FDD / W-CDMA IMT-TC UTRA TDD / TD-CDMA IMT-TC TD-SCDMA IMT-MC cdma2000 1X EV-DO 1X EV-DV (3X)
Worldwide wireless subscribers (old prediction 1998) 700 600 500 400 300 200 Americas Europe Japan others total 100 0 1996 1997 1998 1999 2000 2001
Mobile phones per 100 people 1999 Germany Greece Spain Belgium France Netherlands Great Britain Switzerland Ireland Austria Portugal Luxemburg Italy Denmark Norway Sweden Finland 0 10 20 30 40 50 60 2002: 50 70 % penetration in Western Europe
Worldwide cellular subscriber growth 1200 1000 Subscribers [million] 800 600 400 200 0 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 Note that the curve starts to flatten in 2000
Cellular subscribers per region (June 2002) Middle East; 1,6 Africa; 3,1 Americas (incl. USA/Canada); 22 Asia Pacific; 36,9 Europe; 36,4
Simple reference model used here Application Application Transport Transport Network Network Network Network Data Link Data Link Data Link Data Link Physical Physical Physical Physical Radio Medium
Influence of mobile communication to the layer model Application layer Transport layer Network layer Data link layer Physical layer service location new applications, multimedia adaptive applications congestion and flow control quality of service addressing, routing, device location hand-over authentication media access multiplexing media access control encryption modulation interference attenuation frequency
Frequencies for communication twisted pair coax cable optical transmission 1 Mm 300 Hz 10 km 30 khz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 µm 3 THz 1 µm 300 THz VLF LF MF HF VHF UHF SHF EHF infrared visible light UV VLF = Very Low Frequency UHF = 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: λ = c/f wave length λ, speed of light c 3 x 10 8 m/s, frequency f
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 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. WLAN uses unlicensed spectrum in ISM-bands (Industrial, Scientific, Medical) in the 2.4 GHz and 5.2 to 5.8 GHz range)
Frequencies 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 Others GSM 450-457, 479-486/460-467,489-496, 890-915/935-960, 1710-1785/1805-1880 UMTS (FDD) 1920-1980, 2110-2190 UMTS (TDD) 1900-1920, 2020-2025 CT1+ 885-887, 930-932 CT2 864-868 DECT 1880-1900 IEEE 802.11 2400-2483 HIPERLAN 2 5150-5350, 5470-5725 RF-Control 27, 128, 418, 433, 868 AMPS, TDMA, CDMA 824-849, 869-894 TDMA, CDMA, GSM 1850-1910, 1930-1990 PACS 1850-1910, 1930-1990 PACS-UB 1910-1930 902-928 IEEE 802.11 2400-2483 5150-5350, 5725-5825 RF-Control 315, 915 PDC 810-826, 940-956, 1429-1465, 1477-1513 PHS 1895-1918 JCT 254-380 IEEE 802.11 2471-2497 5150-5250 RF-Control 426, 868
Signals I physical representation of data function of time and location signal parameters: parameters representing the value of data classification continuous time/discrete time continuous values/discrete values analog signal = continuous time and continuous values digital signal = discrete time and discrete values signal parameters of periodic signals: period T, frequency f = 1/T, amplitude A, phase shift ϕ sine wave as special periodic signal for a carrier: s(t) = A t sin(2 π f t t + ϕ t )
Fourier representation of periodic signals g( t) = 1 2 c + n= 1 a n sin(2πnft ) + n= 1 b n cos(2πnft ) 1 1 0 ideal periodic signal t 0 real composition (based on harmonics) t
Signals II Different representations of signals amplitude (amplitude domain) frequency spectrum (frequency domain) phase state diagram (amplitude M and phase ϕ in polar coordinates) A [V] A [V] Q = M sin ϕ t[s] ϕ ϕ Composed signals transferred into frequency domain using Fourier transformation Digital signals need infinite frequencies for perfect transmission f [Hz] modulation with a carrier frequency for transmission (analog signal!) I= M cos ϕ
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 Is used as reference for measuring of antennas (EIRP = Equivalent Isotropic Radiated Power) y z z x y x ideal isotropic radiator
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 Metallic Surface 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) Gain measure in db ( 10*log 10 P1/P2)
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 sector top view, 6 sector
Grouping of 2 or more antennas multi-element antenna arrays Antenna diversity Antennas: diversity switched diversity, selection diversity receiver chooses antenna with largest output diversity combining combine output power to produce gain co-phasing needed to avoid cancellation λ/4 λ/2 λ/4 λ/2 λ/2 λ/2 + + ground plane
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 detection interference distance
Signal propagation Propagation in free space always like light (straight line) Receiving power proportional to 1/d² (d = distance between sender and receiver) Receiving power additionally influenced by fading (frequency dependent; H 2 O resonance at 2.5 GHz; O 2 Resonance at 60 GHz) 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
Real world example
Multipath propagation Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction LOS pulses multipath pulses signal at sender signal at receiver Time dispersion: signal is dispersed over time (delay spread) 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
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 slow changes in the average power received (long term fading) power short term fading long term fading t
Multiplexing Multiplexing in 4 dimensions space (s i ) time (t) frequency (f) code (c) channels k i k 1 c t k 2 k 3 k 4 k 5 k 6 SM c t Goal: multiple use of a shared medium s 1 f s 2 f Important: guard spaces needed! c t s 3 f
Frequency multiplex 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 c k 1 k 2 k 3 k 4 k 5 k 6 Disadvantages: waste of bandwidth if the traffic is distributed unevenly inflexible guard spaces t f
Time multiplex 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 Disadvantages: precise synchronization necessary c f t
Time and frequency multiplex Combination of both methods 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 higher data rates as compared to code multiplex but: precise coordination required c k 1 k 2 k 3 k 4 k 5 k 6 f t
Code multiplex Each channel has a unique code k 1 k 2 k 3 k 4 k 5 k 6 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: lower user data rates more complex signal regeneration Implemented using spread spectrum technology t c f
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., λ/4) Frequency Multiplexing medium characteristics Basic schemes Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)
Modulation and demodulation analog baseband digital signal data digital analog 101101001 modulation modulation radio transmitter radio carrier analog demodulation analog baseband signal synchronization decision digital data 101101001 radio receiver radio carrier
Digital modulation Modulation of digital signals known as Shift Keying Amplitude Shift Keying (ASK): very simple low bandwidth requirements very susceptible to interference 1 0 1 t Frequency Shift Keying (FSK): needs larger bandwidth 1 0 1 t Phase Shift Keying (PSK): more complex robust against interference 1 0 1 t
Advanced Phase Shift Keying BPSK (Binary Phase Shift Keying): Q bit value 0: sine wave bit value 1: inverted sine wave very simple PSK 1 0 I low spectral efficiency robust, used e.g. in satellite systems 10 Q 11 QPSK (Quadrature Phase Shift Keying): 2 bits coded as one symbol I symbol determines shift of sine wave needs less bandwidth compared to BPSK 00 01 more complex Often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK (IS-136, PHS) A t 11 10 00 01
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 0010 0001 Example: 16-QAM (4 bits = 1 symbol) 0011 0000 I 1000 Symbols 0011 and 0001 have the same phase, but different amplitude. 0000 and 1000 have different phase, but same amplitude. used in standard 9600 bit/s modems
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 (OFDM) QPSK, 16 QAM, 64QAM Example: 64QAM good reception: resolve the entire 64QAM 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 10 00 Q 000010 010101 I
Multi Carrier Modulation (MCM) With Multi Carrier Modulation (MCM) the data stream is spilt into several concurrent communication streams using different frequencies Example of MCM are ADSL where each frequency is further modulated using BPSK or QAM For IEEE802.11a/g and Hiperlan-2 OFDM is used OFDM uses orthogonal frequencies to avoid inter carier interference It uses long symbols to reduce ISI and to avoid equalization With a symbol rate of n split to c carriers the symbol duration can be increased by n/c
OFDM model for transmission T s = T = N(2 (k-1) /R b ) R b bit rate (bps) FFT
Fourier transform of a single puls F -T/2 +T/2 F*T
OFDM model for transmission (cont d) Modulation factor Constant phase ourier ransform s( t) S( f ) = = e j2π f N 1 2 N n= 2 o t a n N 1 2 N n= 2 T a sin n T t Π 2 T e j2π n f t (( f n f ) π T) ( f n f ) π T How do we select an appropiate value for f?
OFDM model for transmission (cont d) f = 0.8/T f T We find ICI (Inter-Carrier-Interference) f = 1.2/T f T
OFDM model for transmission (cont d) f = 1/T f T No ICI We have orthogonality between the different subcarriers
Spread spectrum technology Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference Solution: spread the narrow band signal into a broad band signal using a special code power interference spread signal power detection at receiver signal spread interference f f Side effects: coexistence of several signals without dynamic coordination tap-proof Alternatives: Direct Sequence, Frequency Hopping
Effects of spreading and interference dp/df dp/df i) dp/df ii) f sender dp/df f user signal broadband interference narrowband interference dp/df iii) iv) v) f receiver f f
Spreading and frequency selective fading channel quality 1 2 3 4 5 6 narrowband channels frequency narrow band signal guard space channel quality 2 1 2 2 2 2 spread spectrum channels spread spectrum frequency
DSSS (Direct Sequence Spread Spectrum) I XOR of the signal with pseudo-random number (chipping sequence) many chips per bit (e.g., 128, best known 11) result in higher bandwidth of the Spreading Factor s = t b /t c signal Advantages reduces frequency selective fading in cellular networks base stations can use the same frequency range several base stations can detect and recover the signal soft handover Disadvantages precise power control necessary Precise synchronization necessary (multi correlators can take advantage from multi-path propagation (Rakereceiver) t b 0 1 t c 0 1 1 0 1 0 1 0 1 1 0 1 0 1 0 1 1 0 1 0 1 1 0 0 1 0 1 0 t b : bit period t c : chip period user data XOR chipping sequence = resulting signal
DSSS (Direct Sequence Spread Spectrum) II user data X spread spectrum signal modulator transmit signal chipping sequence radio carrier transmitter correlator received signal demodulator lowpass filtered signal X products integrator sampled sums decision data radio carrier chipping sequence receiver
Example Barker Code Good autocorrelation properties Minimal sequence allowed by FCC Coding Robust gain against 10.4 time db delay spread
FHSS (Frequency Hopping Spread Spectrum) I 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
FHSS (Frequency Hopping Spread Spectrum) II t b user data f f 3 f 2 f 1 f f 3 f 2 f 1 0 1 t d t d t b : bit period 0 1 1 t t t t d : dwell time slow hopping (3 bits/hop) fast hopping (3 hops/bit)
FHSS (Frequency Hopping Spread Spectrum) III user data modulator narrowband signal modulator spread transmit signal transmitter frequency synthesizer hopping sequence received signal demodulator narrowband signal demodulator data hopping sequence frequency synthesizer receiver
Example Bluetooth Frequency Hopping Bluetooth uses a slow frequency hopping scheme The frequency is changed every slot (625 µs) so approximately 1600 hops/s For multi-slot packets the frequency is changed with the next packet The hopping sequence is determined by the master (derived from the bluetooth MAC address) During inquiry and paging the the master MAC and timing offset is exchanged with the slaves The slot are enumerated from 0 to 2 27 1 The master uses always the even slot The slot size is 1, 3 or 5
What is Ultra Wideband? Radio technology that modulates impulse based waveforms instead of continuous carrier waves Time-domain behavior Frequency-domain behavior Ultrawideband Communication Impulse Modulation 1 0 1 time 3 frequency 10 GHz (FCC Min=500Mhz) Narrowband Communication Frequency Modulation 0 1 0 1 2.4 GHz
Information Modulation Pulse length ~ 200 ps; Energy concentrated in 2-6 GHz band; Voltage swing ~100 mv; Power ~ 10 µw Pulse Position Modulation (PPM) Pulse Amplitude Modulation (PAM) On-Off Keying (OOK) Bi-Phase Modulation (BPSK)
UWB Spectrum FCC ruling permits UWB spectrum overlay Emitted Signal Power GPS PCS Bluetooth, 802.11b Cordless Phones Microwave Ovens 802.11a -41 dbm/mhz UWB Spectrum Part 15 Limit 1.6 1.9 2.4 3.1 5 Frequency (Ghz) 10.6 FCC ruling issued 2/14/2002 after ~4 years of study & public debate FCC believes current ruling is conservative
Related Standards IEEE 802.15 : Wireless Personal Area Network (WPAN) IEEE 802.15.1 : Bluetooth, 1 Mbps IEEE 802.15.3 : WPAN/high rate, 50 Mbps IEEE 802.15.3a: WPAN/Higher rate, 500 Mbps, UWB IEEE 802.15.3c: WPAN Ultra High Data Rates 2-10 Gb/s IEEE 802.15.4 : WPAN/low-rate, low-power, mw level, 200 kbps IEEE 802.15.4a: WPAN/low-rate, low-power, distance measurement; UWB