Mobile Communications Chapter 2 Wireless Transmission Frequencies q

Mobile Communications Chapter 2: Wireless Transmission Frequencies q Signals q Antenna q Signal propagation q Multiplexing q Spread spectrum q Modulation q Cellular systems q 2. 1

Frequencies for communication twisted pair coax cable 1 Mm 300 Hz 10 km 30 k. Hz VLF LF optical transmission 100 m 3 MHz MF HF VLF = Very Low Frequency LF = Low Frequency MF = Medium Frequency HF = High Frequency VHF = Very High Frequency 1 m 300 MHz VHF UHF 10 mm 30 GHz SHF 100 m 3 THz EHF infrared 1 m 300 THz visible light UV UHF = Ultra High Frequency SHF = Super High Frequency EHF = Extra High Frequency UV = Ultraviolet Light Frequency and wave length: = c/f wave length , speed of light c 3 x 108 m/s, frequency f 2. 2

Frequencies for mobile communication q VHF-/UHF-ranges for mobile radio simple, small antenna for cars q deterministic propagation characteristics, reliable connections q q SHF and higher for directed radio links, satellite communication small antenna, beam forming q large bandwidth available q q Wireless LANs use frequencies in UHF to SHF range some systems planned up to EHF q limitations due to absorption by water and oxygen molecules (resonance frequencies) q l weather dependent fading, signal loss caused by heavy rainfall etc. 2. 3

Frequencies and regulations ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences) 2. 4

Signals I physical representation of data q function of time and location q signal parameters: parameters representing the value of data q classification q continuous time/discrete time q continuous values/discrete values q analog signal = continuous time and continuous values q digital signal = discrete time and discrete values q q signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift q sine wave as special periodic signal for a carrier: s(t) = At sin(2 ft t + t) 2. 5

Fourier representation of periodic signals 1 1 0 0 t ideal periodic signal t real composition (based on harmonics) 2. 6

Signals II q Different representations of signals amplitude (amplitude domain) q frequency spectrum (frequency domain) q q phase state diagram (amplitude M and phase in polar coordinates) Q = M sin A [V] t[s] I= M cos f [Hz] Composed signals transferred into frequency domain using Fourier transformation q Digital signals need q infinite frequencies for perfect transmission q modulation with a carrier frequency for transmission (analog signal!) q 2. 7

Antennas: isotropic radiator Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission q Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna q Real antennas always have directive effects (vertically and/or horizontally) q Radiation pattern: measurement of radiation around an antenna q y z z y x x ideal isotropic radiator 2. 8

Antennas: simple dipoles q 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 q /2 Example: Radiation pattern of a simple Hertzian dipole y y x side view (xy-plane) q z z side view (yz-plane) simple dipole x 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) 2. 9

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 directed antenna sectorized antenna top view, 6 sector 2. 10

Antennas: diversity q Grouping of 2 or more antennas q q multi-element antenna arrays Antenna diversity q switched diversity, selection diversity l q receiver chooses antenna with largest output diversity combining combine output power to produce gain l cophasing needed to avoid cancellation l /2 /4 /2 + ground plane 2. 11

Signal propagation ranges Transmission range communication possible q low error rate q Detection range detection of the signal possible q no communication possible q Interference range signal may not be detected q signal adds to the background noise sender transmission q distance detection interference 2. 12

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 q fading (frequency dependent) q shadowing q reflection at large obstacles q refraction depending on the density of a medium q scattering at small obstacles q diffraction at edges shadowing reflection refraction scattering diffraction 2. 13

Real world example 2. 14

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 2. 15

Effects of mobility Channel characteristics change over time and location signal paths change q different delay variations of different signal parts q different phases of signal parts q quick changes in the power received (short term fading) Additional changes in distance to sender q obstacles further away q long term fading power slow changes in the average power received (long term fading) t short term fading 2. 16

Multiplexing in 4 dimensions space (si) q time (t) q frequency (f) q code (c) q channels ki k 1 k 2 k 3 k 4 k 5 k 6 c t Goal: multiple use of a shared medium s 1 f s 2 f c Important: guard spaces needed! t s 3 f 2. 17

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: q no dynamic coordination necessary k 1 k 2 k 3 k 4 k 5 q works also for analog signals k 6 c f Disadvantages: q waste of bandwidth if the traffic is distributed unevenly q inflexible q guard spaces t 2. 18

Time multiplex A channel gets the whole spectrum for a certain amount of time Advantages: q only one carrier in the medium at any time q throughput high even for many users Disadvantages: q precise synchronization necessary k 1 k 2 k 3 k 4 k 5 k 6 c f t 2. 19

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 q protection against frequency selective interference q higher data rates compared to code multiplex q k 1 k 2 k 3 k 4 k 5 k 6 c f but: precise coordination required t 2. 20

Code multiplex Each channel has a unique code All channels use the same spectrum at the same time Advantages: k 1 k 2 k 3 k 4 k 5 k 6 c bandwidth efficient q no coordination and synchronization necessary q good protection against interference and tapping q f Disadvantages: lower user data rates q more complex signal regeneration q Implemented using spread spectrum technology t 2. 21

Modulation Digital modulation digital data is translated into an analog signal (baseband) q ASK, FSK, PSK - main focus in this chapter q differences in spectral efficiency, power efficiency, robustness q Analog modulation q shifts center frequency of baseband signal up to the radio carrier Motivation smaller antennas (e. g. , /4) q Frequency Division Multiplexing q medium characteristics q Basic schemes Amplitude Modulation (AM) q Frequency Modulation (FM) q Phase Modulation (PM) q 2. 22

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 2. 23

Digital modulation Modulation of digital signals known as Shift Keying 1 q Amplitude Shift Keying (ASK): 0 1 very simple q low bandwidth requirements q very susceptible to interference q q Frequency Shift Keying (FSK): q t 1 0 1 needs larger bandwidth t q Phase Shift Keying (PSK): more complex q robust against interference 1 0 1 q t 2. 24

Advanced Frequency Shift Keying q q q q 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 low-pass filter GMSK (Gaussian MSK), used in GSM 2. 25

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 t No phase shifts! 2. 26

Advanced Phase Shift Keying Q BPSK (Binary Phase Shift Keying): q q q 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 I 0 Q 11 QPSK (Quadrature Phase Shift Keying): 2 bits coded as one symbol q symbol determines shift of sine wave q needs less bandwidth compared to BPSK q more complex I q Often also transmission of relative, not absolute phase shift: DQPSK Differential QPSK (IS-136, PHS) 00 01 A t 11 10 00 2. 27 01

Quadrature Amplitude Modulation (QAM): combines amplitude and phase modulation q it is possible to code n bits using one symbol q 2 n discrete levels, n=2 identical to QPSK q bit error rate increases with n, but less errors compared to comparable PSK schemes Q 0010 0011 0000 φ a I 1000 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. used in standard 9600 bit/s modems 2. 28

Hierarchical Modulation DVB-T modulates two separate data streams onto a single DVB-T stream q High Priority (HP) embedded within a Low Priority (LP) stream q Multi carrier system, about 2000 or 8000 carriers q QPSK, 16 QAM, 64 QAM q Example: 64 QAM good reception: resolve the entire 64 QAM constellation q poor reception, mobile reception: resolve only QPSK portion q 6 bit per QAM symbol, 2 most significant determine QPSK q HP service coded in QPSK (2 bit), LP uses remaining 4 bit q Q 10 I 00 000010 010101 2. 29

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 protection against narrow band interference power interference spread signal power signal spread interference detection at receiver protection againstf narrowband interference f Side effects: coexistence of several signals without dynamic coordination q tap-proof q Alternatives: Direct Sequence, Frequency Hopping 2. 30

Effects of spreading and interference d. P/df i) user signal broadband interference narrowband interference ii) f sender d. P/df f d. P/df iii) iv) f receiver v) f f 2. 31

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 spread spectrum channels frequency 2. 32

DSSS (Direct Sequence Spread Spectrum) I XOR of the signal with pseudo-random number (chipping sequence) q many chips per bit (e. g. , 128) result in higher bandwidth of the signal Advantages reduces frequency selective fading q in cellular networks q base stations can use the same frequency range l several base stations can detect and recover the signal l soft handover l tb user data 0 1 XOR tc chipping sequence 0110101 Disadvantages q precise power control necessary = resulting signal 01101011001010 tb: bit period tc: chip period 2. 33

DSSS (Direct Sequence Spread Spectrum) II spread spectrum signal user data X chipping sequence transmit signal modulator radio carrier transmitter correlator received signal demodulator radio carrier lowpass filtered signal products X integrator sampled sums data decision chipping sequence receiver 2. 34

FHSS (Frequency Hopping Spread Spectrum) I Discrete changes of carrier frequency q sequence of frequency changes determined via pseudo random number sequence Two versions Fast Hopping: several frequencies per user bit q Slow Hopping: several user bits per frequency q Advantages frequency selective fading and interference limited to short period q simplementation q uses only small portion of spectrum at any time q Disadvantages not as robust as DSSS q simpler to detect q 2. 35

FHSS (Frequency Hopping Spread Spectrum) II tb user data 0 1 f 0 1 1 t td f 3 slow hopping (3 bits/hop) f 2 f 1 f t td f 3 fast hopping (3 hops/bit) f 2 f 1 t tb: bit period td: dwell time 2. 36

FHSS (Frequency Hopping Spread Spectrum) III narrowband signal user data modulator frequency synthesizer transmitter received signal hopping sequence spread transmit signal narrowband signal demodulator frequency synthesizer hopping sequence data demodulator receiver 2. 37

Cell structure Implements space division multiplex: base station covers a certain transmission area (cell) Mobile stations communicate only via the base station Advantages of cell structures: higher capacity, higher number of users q less transmission power needed q more robust, decentralized q base station deals with interference, transmission area etc. locally q Problems: fixed network needed for the base stations q handover (changing from one cell to another) necessary q interference with other cells q Cell sizes from some 100 m in cities to, e. g. , 35 km on the country side (GSM) - even less for higher frequencies 2. 38

Frequency planning I Frequency reuse only with a certain distance between the base stations Standard model using 7 frequencies: f 4 f 3 f 5 f 1 f 2 f 3 f 6 f 7 f 2 f 4 f 5 f 1 Fixed frequency assignment: certain frequencies are assigned to a certain cell q problem: different traffic load in different cells q Dynamic frequency assignment: base station chooses frequencies depending on the frequencies already used in neighbor cells q more capacity in cells with more traffic q assignment can also be based on interference measurements q 2. 39

Frequency planning II f 3 f 1 f 2 f 3 f 1 f 3 f 1 f 2 3 cell cluster f 3 f 2 f 4 f 3 f 6 f 5 f 1 f 2 f 3 f 6 f 7 f 5 f 2 f 4 f 3 f 7 f 5 f 1 f 2 7 cell cluster f 2 f 2 f 1 f h h 3 3 3 h 2 g 2 1 h 3 g 2 g 1 g 1 g 3 g 3 3 cell cluster with 3 sector antennas 2. 40

Cell breathing CDM systems: cell size depends on current load Additional traffic appears as noise to other users If the noise level is too high users drop out of cells 2. 41