Mobile Computing and Wireless Networking Lec 02 03/03/2010

Outline Characteristic of wireless channels Antennas and Signal Propagation Multiplexing

ideal periodical digital signal Fourier Transform: Every Signal Can be Decomposed as a Collection of Harmonics Time domain Frequency domain 1 1 t t ideal periodical digital signal decomposition Two representations: time domain; frequency domain Knowing one can recover the other

Try spectrum1.m and spectrum2.m Examples Try spectrum1.m and spectrum2.m

Recap: Modulation Objective Basic schemes encode digital data into analog signals at the right frequency range Basic schemes Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)

Modulation Modulation of digital signals known as Shift Keying Amplitude Shift Keying (ASK): Frequency Shift Keying (FSK): Phase Shift Keying (PSK): 1 t

Example Suppose fc = 1 GHz (fc1 = 1 GHz, fc0 = 900 MHz for FSK) Bit rate is 1 Mbps Encode one bit at a time Bit seq: 1 0 0 1 0 Q: How does the wave look like for each scheme? 1 t t

Phase Shift Keying: BPSK BPSK (Binary Phase Shift Keying): bit value 0: sine wave bit value 1: inverted sine wave very simple PSK Properties robust, used e.g. in satellite systems Q I 1 Q: What is the spectrum usage of BPSK?

Spectral Density of BPSK Spectral Density = bit rate ------------------- width of spectrum used b fc : freq. of carrier Rb =Bb = 1/Tb b fc

Phase Shift Keying: QPSK 11 10 00 01 Q I A t QPSK (Quadrature Phase Shift Keying): 2 bits coded as one symbol symbol determines shift of sine wave often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK

Antennas and Signal Propagation

Antennas: Isotropic Radiator Isotropic radiator: a single point equal radiation in all directions (three dimensional) only a theoretical reference antenna Radiation pattern: measurement of radiation around an antenna z y z ideal isotropic radiator y x x Q: how does power level decrease as a function of d, the distance from the transmitter to the receiver?

Antennas: directed and sectorized Universität Karlsruhe Institut für Telematik Mobilkommunikation SS 1998 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 antenna directed x z x side view (xy-plane) side view (yz-plane) top view (xz-plane) z z antenna sectorized x x top view, 3 sector top view, 6 sector Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Signal propagation ranges Universität Karlsruhe Institut für Telematik Mobilkommunikation SS 1998 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 distance detection interference Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Free-Space Isotropic Signal Propagation In free space, receiving power proportional to 1/d² (d = distance between transmitter and receiver) Suppose transmitted signal is x, received signal y = h x, where h is proportional to 1/d² Pr: received power Pt: transmitted power Gr, Gt: receiver and transmitter antenna gain  (=c/f): wave length Sometime we write path loss in log scale: Lp = 10 log(Pt) – 10log(Pr)

Free Space Signal Propagation 1 t at distance d ?

Real Antennas Q: Assume frequency 1 Ghz,  = ? Real antennas are not isotropic radiators Some simple antennas: quarter wave /4 on car roofs or half wave dipole /2  size of antenna proportional to wavelength for better transmission/receiving /4 /2 Q: Assume frequency 1 Ghz,  = ?

Dipole: Radiation Pattern of a Dipole http://www.tpub.com/content/neets/14182/index.htm http://en.wikipedia.org/wiki/Dipole_antenna

Why Not Digital Signal (revisited) Not good for spectrum usage/sharing The wavelength can be extremely large to build portal devices e.g., T = 1 us -> f=1/T = 1MHz -> wavelength = 3x108/106 = 300m

Signal Propagation Receiving power additionally influenced by shadowing (e.g. through a wall or a door) refraction depending on the density of a medium reflection at large obstacles scattering at small obstacles diffraction at edges diffraction reflection refraction scattering shadow fading

Signal Propagation: Scenarios Details of signal propagation are very complicated We want to understand the key characteristics that are important to our objective

i.e. reduces to ¼ of signal 10 log(1/4) = -6.02 Shadowing Signal strength loss after passing through obstacles Some sample numbers i.e. reduces to ¼ of signal 10 log(1/4) = -6.02

Multipath Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction

Multipath Can Reduce Signal Strength Example: reflection from the ground: received power decreases proportional to 1/d4 instead of 1/d² due to the destructive interference between the direct signal and the signal reflected from the ground ground For detail, see page 9: http://www.eecs.berkeley.edu/~dtse/Chapters_PDF/Fundamentals_Wireless_Communication_chapter2.pdf

Multipath Can Spread Delay signal at sender LOS pulse Time dispersion: signal is dispersed over time multipath pulses signal at receiver LOS: Line Of Sight

Multipath Can Cause ISI dispersed signal can cause interference between “neighbor” symbols, Inter Symbol Interference (ISI) Assume 300 meters delay spread, the arrival time difference is 300/3x108 = 1 ms if symbol rate > 1 Ms/sec, we will have serious ISI In practice, fractional ISI can already substantially increase loss rate signal at sender LOS pulse multipath pulses signal at receiver LOS: Line Of Sight

Universität Karlsruhe Institut für Telematik Mobilkommunikation SS 1998 Multiplexing channels ki Multiplexing in 4 dimensions space (si) time (t) frequency (f) code (c) Goal: multiple use of a shared medium Important: guard spaces needed! k1 k2 k3 k4 k5 k6 c t c s1 t s2 f f c t s3 f Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Universität Karlsruhe Institut für Telematik Mobilkommunikation SS 1998 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 Disadvantages waste of bandwidth if the traffic is distributed unevenly inflexible k1 k2 k3 k4 k5 k6 c f Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Universität Karlsruhe Institut für Telematik Mobilkommunikation SS 1998 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 Disadvantages precise synchronization necessary k1 k2 k3 k4 k5 k6 c f Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Time and frequency multiplex Universität Karlsruhe Institut für Telematik Mobilkommunikation SS 1998 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 but: precise coordination required k1 k2 k3 k4 k5 k6 c f t Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Universität Karlsruhe Institut für Telematik Mobilkommunikation SS 1998 Code multiplex Each channel has a unique code 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 varying user data rates more complex signal regeneration Implemented using spread spectrum technology k1 k2 k3 k4 k5 k6 c f t Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller