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Wireless Transmission Fundamentals (Physical Layer) Professor Honggang Wang

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Electromagnetic Spectrum Wireless communication uses 100 kHz to 60 GHz

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The Layered Reference Model 3 Application Transport Network Data Link Physical Medium Data Link Physical Application Transport Network Data Link Physical Data Link Physical Network Radio Often we need to implement a function across multiple layers.

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Outline RF introduction Antennas and signal propagation How do antennas work Propagation properties of RF signals Modulation and channel capacity

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What is Antenna Conductor that carries an electrical signal and radiates an RF signal. The RF signal “is a copy of” the electrical signal in the conductor Also the inverse process: RF signals are “captured” by the antenna and create an electrical signal in the conductor. This signal can be interpreted (i.e. decoded) Efficiency of the antenna depends on its size, relative to the wavelength of the signal. e.g. half a wavelength

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Types of Antennas Antenna is a point source that radiates with the same power level in all directions – omni-directional or isotropic An antenna that transmits equally in all directions (isotropic) Shape of the conductor tends to create a specific radiation pattern Common shape is a straight conductor Shaper antennas can be used to direct the energy in a certain direction Well-know case: a parabolic antenna A parabolic antenna

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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 distance sender transmission detection interference

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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 fading (frequency dependent) Shadowing Reflection at large obstacles Refraction depending on the density of a medium Scattering at small obstacles Diffraction at edges reflectionscatteringdiffractionshadowing refraction

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Propagation Degrades RF Signal Attenuation in free space Signal gets weaker as it travels over longer distance Free space loss- Signal spreads out Refraction and absorption in the atmosphere Obstacle can weaken signal through absorption or reflection. Part of the signal is re-directed. Multiple path effects Multiple copies of the signal interfere with each other Mobility Moving receiver causes another form of self interference Node moves ½ wavelength cause big change in signal strength path loss log (distance) Received Signal Power (dB) location

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Decibels Attenuation = 10 Log10 (Pin/Pout) decibel Attenuation = 20 Log10 (Vin/Vout) decibel Example 1: Pin = 10 mW, Pout=5 mW Attenuation = 10 log 10 (10/5) = 10 log 10 2 = 3 dB Example 2: Pin = 100mW, Pout=1 mW Attenuation = 10 log 10 (100/1) = 10 log = 20 dB

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Shadowing 11 Signal strength loss after passing through obstacles Some sample numbers i.e. reduces to ¼ of signal 10 log(1/4) = -6.02

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

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Multipath propagation Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction 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 signal at sender signal at receiver LOS pulses multipath pulses

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Multipath Effects Receiver receives multiple copies of the signal, each following a different path Copies can either strengthen or weaken each other Depends on whether they are in our out of phase Small changes in location can result in big changes in signal strength Larger difference in path length can cause intersymbol interference (ISI) More significant for higher bit rates (shorter bit times)

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Free-Space Isotropic Signal Propagation 15 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 Loss depends on the frequency: Higher loss with higher frequency Loss increase quickly with distance (d^2) m P r : received power m P t : transmitted power m G r, G t : receiver and transmitter antenna gain m (=c/f): wave length Sometime we write path loss in log scale: Lp = 10 log(Pt) – 10log(Pr)

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Outline RF introduction Antennas and signal propagation How do antennas work Propagation properties of RF signals Modulation and channel capacity

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Signals 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 sine wave as special periodic signal for a carrier: s(t) = A t sin(2 f t t + t )

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

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Multiplexing Multiplexing in 4 dimensions space (s i ) time (t) frequency (f) code (c) Goal: multiple use of a shared medium Important: guard spaces needed! s2s2 s3s3 s1s1 f t c k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 f t c f t c channels k i

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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 k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 f t c

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f t c k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 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

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f Time and frequency multiplex Combination of both methods A channel gets a certain frequency band for a certain amount of time Example: GSM Advantages protection against frequency selective interference but: precise coordination required t c k2k2 k3k3 k4k4 k5k5 k6k6 k1k1

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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 Disadvantages varying user data rates more complex signal regeneration Implemented using spread spectrum technology k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 f t c

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Modulation Digital modulation digital data is translated into an analog signal (baseband) ASK, FSK, PSK differences in spectral efficiency, power efficiency, robustness Analog modulation shifts center frequency of baseband signal up to the radio carrier Basic schemes Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)

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Modulation and Demodulation synchronization decision digital data analog demodulation radio carrier analog baseband signal radio receiver digital modulation digital data analog modulation radio carrier analog baseband signal radio transmitter

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Digital modulation Modulation of digital signals known as Shift Keying Amplitude Shift Keying (ASK): very simple low bandwidth requirements very susceptible to interference Frequency Shift Keying (FSK): needs larger bandwidth Phase Shift Keying (PSK): more complex robust against interference 101 t 101 t 101 t

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Frequency Shift Keying (FSK):

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Advanced Phase Shift Keying BPSK (Binary Phase Shift Keying): 0 = Same phase, 1=Opposite phase A cos(2 π ft), A cos(2 π ft+ π ) low spectral efficiency robust, used e.g. in satellite systems QPSK (Quadrature Phase Shift Keying): 2 bits coded as one symbol symbol determines shift of sine wave needs less bandwidth compared to BPSK 11=A cos(2 π ft+45°), 10=A cos(2 π ft+135°), 00=A cos(2 π ft+225°), 01=A cos(2 π ft+315°) Q I 01 Q I A t

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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 Example: 16-QAM (4 bits = 1 symbol) Symbols 0011 and 0001 have the same phase φ, but different amplitude 0000 and 1000 have different phase, but same amplitude Q I 0010 φ a

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Channel Capacity Capacity = Maximum data rate for a channel Nyquist Theorem: Bandwidth = B Data rate < 2 B Bi-level Encoding: Data rate = 2 × Bandwidth Multilevel: Data rate = 2 × Bandwidth × log 2 M Example: M=4, Capacity = 4 × Bandwidth

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Shannon’s Theorem Bandwidth = B Hz Signal-to-noise ratio = S/N Maximum number of bits/sec = B log2 (1+S/N) Example: Phone wire bandwidth = 3100 Hz S/N = 30 dB 10 Log 10 S/N = 30 Log 10 S/N = 3 S/N = 1000 Capacity = 3100 log 2 (1+1000) = 30,894 bps

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Q/A ???

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