Physical Layer Fundamentals

Physical Layer Fundamentals
Wireless Information Networking Group (WING) EEL 6591 Wireless Networks Physical Layer Fundamentals

Outline Fundamental system architecture Mobile radio propagation
Wireless Information Networking Group (WING) Outline Fundamental system architecture Mobile radio propagation large-scale small scale Link budget analysis

Wireless Information Networking Group (WING)
Seven Layer Model

Nutshell Typical Communications Model
Wireless Information Networking Group (WING) Nutshell Typical Communications Model

Fundamental System Archiecture
Wireless Information Networking Group (WING) Fundamental System Archiecture

Source Coding Source Encoding: compression, according to statistics of 0’s and 1’s, map blocks of bits to more regular “shorter” blocks! Get rid of redundancy Source Decoding: inverse of source encoding More can be found in Proakis, Digital Communications More theoretical: Thomas Cover, Elements of Information Theory

Channel Coding Channel Encoding: According to channel conditions, add redundancy for more efficient transmission, interleaving may be used too. Channel decoding: the inverse Observation: source encoding attempts to eliminate “useless information”, while channel encoding add “useful information”, both deal with redundancies! More: Proakis, Digital Communications Wicker: Error Control Systems

Modulation/Demodulation
Wireless Information Networking Group (WING) Modulation/Demodulation Modulation: maps blocks of bits to well-defined waveforms or symbols (a set of signals for better transmission), then shifts transmission to the carrier frequency band (the band you have right to transmit) Demodulation: the inverse of modulation Demodulation vs. Detection: Detection is to recover the modulated signal from the “distorted noisy” received signals

Signal Types Basic form: A signal is a time function
Wireless Information Networking Group (WING) Signal Types Basic form: A signal is a time function Continuous signal: varying continuously with time, e.g., speech Discrete signal: varying at discrete time instant or keeping constant value in certain time interval, e.g., Morse code, flash lights Periodic signal: Pattern repeated over time Aperiodic signal: Pattern not repeated over time, e.g., speech

Frequency Domain Concepts
Wireless Information Networking Group (WING) Frequency Domain Concepts Signal usually made up of many frequencies Components are sine waves Can be shown (Fourier analysis) that any signal is made up of component sine waves Can plot frequency domain functions Time domain representation is equivalent to frequency domain representation: they contain the same information! Frequency domain representation is easier for design

Fourier Representation
Wireless Information Networking Group (WING) Fourier Representation

Received Signals Any receiver can only receive signals in certain frequency range (channel concept, filter), corresponding to finite number of terms in the Fourier series approximation: physically: finite number of harmonics mathematically: finite number of terms Transmitted signal design: allocate as many terms as possible in the intended receiver’s receiving range Adjacent channel interference: signal leaks to your neighbor’s channel band

Fundamental Design Transmit end: force most transmission power in the allocated band (channel) Receive end: tune the receiver (filter) to receive most power only transmitted

Analog vs Digital Analog: Continuous values within some interval, the transmitted signal has actual meaning, e.g., AM and FM radio Digital: Digital=DSP+Analog, raw digital bits are processed and mapped to well-known signal set for better transmission, the final transmitted signal is still analog! You could not “hear” though!

Information Carriers s(t) = A sin (2pft+ ) Amplitude: A
Wireless Information Networking Group (WING) Information Carriers s(t) = A sin (2pft+ ) Amplitude: A Frequency: f – f =1/T, T---period Phase:  , angle (2pft+ )

Analog Transmission The received signal: r(t) = s(t) + n(t)
Wireless Information Networking Group (WING) Analog Transmission The received signal: r(t) = s(t) + n(t)

Digital Transmission

QPSK Quadrature Phase Shift Keying
Wireless Information Networking Group (WING) QPSK Quadrature Phase Shift Keying

QAM Quadrature Amplitude Modulation (QAM)
Wireless Information Networking Group (WING) QAM Quadrature Amplitude Modulation (QAM)

Signal Constellation

Channel Impairments Attenuation and attenuation distortion: signal power attenuates with distance Delay distortion: multipath in wireless environments (symbol interference) Co-channel Interference: I Thermal noise: N

Shannon Capacity Shannon Capacity Theorem: For a noisy channel of BW B with signal-to-noise ratio (SNR), the maximum transmission rate is C = B log2 (1+SNR) Capacity increases as BW or signal power increases: Shout as you can! SNR should be replaced by SIR in wireless communications: interference comes into play, shouts may be good for awhile, not good for the whole system performance

Shannon Capacity Shannon Theorem does not give any way to reach that capacity Current transmission schemes transmit much lower rate than Shannon capacity Turbo codes: iterative coding schemes using feedback information for transmission and detection Space-time coding Sailing towards Shannon capacity!

Mobile Radio Propagation
Wireless Information Networking Group (WING) Mobile Radio Propagation Signal power will decrease with distance Signals will be distorted Radio Propagation models Link Budget Analysis

Free Space Propagation Model
Wireless Information Networking Group (WING) Free Space Propagation Model Received power at distance d (d0 is a reference distance, d>d0)

Path Loss Formula Path loss is used for link budget analysis
Wireless Information Networking Group (WING) Path Loss Formula Path loss is used for link budget analysis

Practical Model Three propagation mechanisms (in addition to possible line of sight (LoS) path) Reflection Diffraction Scattering Need to survey the propagation environments to choose an appropriate model for PL Ray tracing techniques can be used (like in optics)

Reflection Radio wave tends to reflect from objects having different electrical properties and with wide dimension comparing to the wavelength (earth, high building…) Similar to optical ray: all are E-M waves! Two-ray model: (PL exponent is n=4)

Diffraction Diffraction allows radio signals to propagate around the curved surface or obstructions Caused by the propagation of secondary wavelets into the shadowed region Fresnel zone geometry: Fresnel zones represents successive regions where secondary waves have a path length from transmitter to receiver which is nl/2 greater than the LoS path

Scattering Rough surfaces or comparatively small objects cause radio to scatter, receiver may receive many scattered components Radar Cross Section (RCS) technique can be used RCS: the ratio of power density of signal scattered in the direction of the receiver to the power density of the radio wave incident upon the scattering objects

Long-term Fading Model
Wireless Information Networking Group (WING) Long-term Fading Model Long-term fading (shadowing) model can be used for link budget analysis: persisting fading Surrounding environmental clutter causes random perturbation in the propagation model Log-normal shadowing model (n is the path loss exponent):

Link Budget Analysis: General Idea
Wireless Information Networking Group (WING) Link Budget Analysis: General Idea Problem: Determine the transmitting power budget with the desired area coverage based on the propagation characteristic! (cell design), appropriately assign the power margins for all possible power loss so that the received power can meet the design requirement! Coverage: the received power should NOT be below a minimum threshold with certain probability! Need to find this probability!

Wireless Information Networking Group (WING) Link Budget Analysis: General Idea Detailed derivation at

Wireless Information Networking Group (WING) Link Budget Analysis: General Idea If we require the minimum received signal on the circular boundary to be g, then a=0 and The boundary coverage probability: Reproduce Figure 4.18 (in Rappaport) : From this formula find a, then take a into (4.78) to obtain the plots

Experimental Models Propagation models rely on the understanding of the propagation environments, more than often, experimental models are better! List of models: Longley-Rice Model (point-to-point) Durkin’s Model (simulation-based) Okumura Model (measured data based, curve fitting) Hata Model and PCS Model (curve fitting) Walfisch and Bertoni Model (urban areas) Wideband PCS Microcell Model (piece-wise linear)

Short-term Fading Channels
Wireless Information Networking Group (WING) Short-term Fading Channels Fading causes the signal power attenuation Recall: long-term fading tends to be persistent, caused by fixed infrastructure, say, building, hills, trees etc Short-term fading affects channel in short period of time, not persistent, caused mostly by multipath, moving objects etc Short-term fading may cause rapid change in signal power random frequency shift (Doppler frequency shift) time dispersion

Factors Causing Short-term Fading
Wireless Information Networking Group (WING) Factors Causing Short-term Fading Multipath propagation reflecting objects and small scatters Speed of the mobile relative motion between transmitter and receiver --Doppler shift Speed of surrounding objects blockage and reflection of signals The transmission bandwidth of the signal transmitting BW is greater than receiving BW

Fading Characterization
Wireless Information Networking Group (WING) Fading Characterization Time dispersion average excess delay: due to multipath delay spread: the standard deviation of the excess delay Coherence BW the BW of the frequencies over which the channel can be considered to be flat Coherence time and Doppler spread the time duration over which two signals are considered to be correlated Doppler spread is the inverse of coherence time

Types of Short-term Fading
Wireless Information Networking Group (WING) Types of Short-term Fading Relative to the transmitting signal BW Can classify according to time delay spread Flat fading BW of signal < BW of channel delay spread < symbol period Frequency selective fading BW of signal > BW of channel delay spread > symbol period

Types of Short-term Fading
Wireless Information Networking Group (WING) Types of Short-term Fading Can classify according to Doppler spread Fast fading high Doppler spread coherence time < symbol period channel variations faster than signal variation Slow fading low Doppler spread coherence time > symbol period channel variations slower than signal variation

Quantification for Fading
Wireless Information Networking Group (WING) Quantification for Fading Probability distribution of the received envelope (the magnitude of the received signal) Basic idea: S(t) =X(t)+j Y(t) X(t) and Y(t) are independent and Gaussian with zero mean and equal variance X(t) and Y(t) are superpositions of many small random components coming from multipath, hence Gaussian from Central Limit Theorem signal envelope:

Rayleigh Fading It can be shown that (as an exercise ) that the envelope r is Rayleigh distributed--probability density function (Rayleigh distribution) where s is the rms (root of mean-square) value of the received signal

Rician Fading When the mean of X or Y is not zero (i.e., there is a dominant component such as LoS path), the resulting distribution will be Rician distribution given by where A >0 denotes the peak amplitude of the dominant signal and I0(x) is the modified Bessel function of the first kind and zero-order.

Fading Statistics Level crossing rate: the average rate the fading signal crosses a specific level Fading duration: the average period of time for which the received signal is below the specific level These statistics can be used to design the coding schemes to improve the BER (bit-error-rate)

Reading Rappaport, Chapter 4 & 5
Wireless Information Networking Group (WING) Reading Rappaport, Chapter 4 & 5

Physical Layer Fundamentals

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