EE 552/452 Wireless Communications (and Networks) Lecture Notes 2007 PDF

EE 552/452, Spring, 2007 Wireless Communications (and Networks) Zhu Han Department of Electrical and Computer Engineering Class 7 Feb. 6th, 2007  Outline  Review – Free space...

EE 552/452, Spring, 2007 Wireless Communications (and Networks) Zhu Han Department of Electrical and Computer Engineering Class 7 Feb. 6th, 2007  Outline  Review – Free space propagation  Received power is a function of transmit power times gains of transmitter and receiver antennas  Signal strength is proportional to distance to the power of -2 – Reflection:  Cause the signal to decay faster.  Depends on the height of transmitter and receiver antennas  Homework  Conference, moving of classes  Project, TI toolboxes  Diffraction  Scattering  Practical link budget model EE 552/452 Spring 2007  Diffraction  Diffraction occurs when waves hit the edge of an obstacle – “Secondary” waves propagated into the shadowed region – Water wave example – Diffraction is caused by the propagation of secondary wavelets into a shadowed region. – Excess path length results in a phase shift – The field strength of a diffracted wave in the shadowed region is the vector sum of the electric field components of all the secondary wavelets in the space around the obstacle. – Huygen’s principle: all points on a wavefront can be considered as point sources for the production of secondary wavelets, and that these wavelets combine to produce a new wavefront in the direction of propagation. EE 552/452 Spring 2007  Diffraction geometry  Derive of equation 4.54-4.57 EE 552/452 Spring 2007  Diffraction geometry EE 552/452 Spring 2007  Diffraction geometry  Fresnel-Kirchoff distraction parameters, 4.56 EE 552/452 Spring 2007  Fresnel Screens  Fresnel zones relate phase shifts to the positions of obstacles  Equation 4.58  A rule of thumb used for line-of-sight microwave links 55% of the first Fresnel zone is kept clear. EE 552/452 Spring 2007  Fresnel Zones  Bounded by elliptical loci of constant delay  Alternate zones differ in phase by 180 – Line of sight (LOS) corresponds to 1st zone – If LOS is partially blocked, 2nd zone can destructively interfere (diffraction loss) LOS 0 0o  How much power is propagated -10 90 this way? -20 180o – 1st FZ: 5 to 25 dB below dB -30 free space prop. -40 -50 Obstruction -60 Tip of Shadow 1st 2nd Obstruction of Fresnel Zones  EE 552/452 Spring 2007  Fresnel diffraction geometry EE 552/452 Spring 2007  Knife-edge diffraction  Fresnel integral, 4.59 EE 552/452 Spring 2007  Knife-edge diffraction loss  Gain  Exam. 4.7  Exam. 4.8 EE 552/452 Spring 2007  Multiple knife-edge diffraction EE 552/452 Spring 2007  Scattering  Rough surfaces – Lamp posts and trees, scatter all directions – Critical height for bumps is f(,incident angle), 4.62 – Smooth if its minimum to maximum protuberance h is less than critical height. – Scattering loss factor modeled with Gaussian distribution, 4.63, 4.64.  Nearby metal objects (street signs, etc.) – Usually modeled statistically  Large distant objects – Analytical model: Radar Cross Section (RCS) – Bistatic radar equation, 4.66 EE 552/452 Spring 2007  Measured results EE 552/452 Spring 2007  Measured results EE 552/452 Spring 2007  Propagation Models  Large scale models predict behavior averaged over distances >>  – Function of distance & significant environmental features, roughly frequency independent – Breaks down as distance decreases – Useful for modeling the range of a radio system and rough capacity planning, – Experimental rather than the theoretical for previous three models – Path loss models, Outdoor models, Indoor models  Small scale (fading) models describe signal variability on a scale of  – Multipath effects (phase cancellation) dominate, path attenuation considered constant – Frequency and bandwidth dependent – Focus is on modeling “Fading”: rapid change in signal over a short distance or length of time. EE 552/452 Spring 2007  Free Space Path Loss  Path Loss is a measure of attenuation based only on the distance to the transmitter  Free space model only valid in far-field; – Path loss models typically define a “close-in” point d and 0 reference other points from there: 2 d   d  Pr (d ) Pr (d 0 ) 0  PL (d ) [ Pr (d )]dB  PL (d 0 )  2    d   d 0  dB  Log-distance generalizes path loss to account for other environmental factors d – Choose a d in the far field. PL (d )  PL (d 0 )     0  d 0  dB – Measure PL(d ) or calculate Free Space Path Loss. 0 – Take measurements and derive  empirically. EE 552/452 Spring 2007  Typical large-scale path loss EE 552/452 Spring 2007  Log-Normal Shadowing Model  Shadowing occurs when objects block LOS between transmitter and receiver  A simple statistical model can account for unpredictable “shadowing” – PL(d)(dB)=PL(d)+X0, – Add a 0-mean Gaussian RV to Log-Distance PL – Variance  is usually from 3 to 12. – Reason for Gaussian EE 552/452 Spring 2007  Measured large-scale path loss  Determine n and  by mean and variance  Equ. 4.70  Equ. 4.72  Basic of Gaussian distribution EE 552/452 Spring 2007  Area versus Distance coverage model with shadowing model  Percentage for SNR larger than a threshold  Equ. 4.79  Exam. 4.9 EE 552/452 Spring 2007  Questions? EE 552/452 Spring 2007