ELEG 5693 Wireless Communications Propagation and Noise Part I

Transcription

1 Department of Electrical Engineering University of Arkansas ELEG 5693 Wireless Communications ropagation and Noise art I Dr. Jingxian Wu wuj@uark.edu

2 OULINE 2 Wireless channel ath loss Shadowing Small scale fading Channel classifications Noise and interference Simulation model

3 CHANNEL: OAGAION ENVIONMEN 3

4 CHANNEL: OAGAION MODES 4 Free-space (line-of-sight) here is a clear transmission path between transmitter (x) and receiver (x). E.g. satellite eflection he bouncing of electromagnetic waves from surrounding objects Size of reflecting objects must be large compared to the wavelength of signal eflecting surface must be smooth compared to the wavelength of signal E.g. ground, building, walls, windows, lakes

5 CHANNEL: OAGAION MODES (CON D) Diffraction he bending of electromagnetic waves around objects (such as buildings), or through objects (such as trees). Due to diffraction, signals can propagate Around curved surface of Earth Beyond LOS horizon Behind obstructions Scattering Electromagnetic waveforms incident upon rough or complex surfaces are scattered in many directions elfection: smooth surface, one direction efraction (not common in terrestrial wireless communication). Electromagnetic waves bend as they move from one medium to another. 5

6 CHANNEL 6 Channel: a collection of propagation effects and other signal impairments caused by the environments Wireless channel: propagation, noise and interference ropagation effects: line-of-sight, reflection, diffraction, scattering Induced by the transmission of desired signals itself Impairments: large scale fading (path loss, shadowing), small scale fading Unique to wireless communication Noise and interference: unwanted electrical signals interfering with desired signal. hermal noise (movements of electrons) Automobile ignition, electrical machinery, etc. Interferences from other users operating on the same frequency. resent in all communication systems

7 OULINE 7 Wireless channel ath loss Shadowing Small scale fading Channel classifications Noise and interference Simulation model

8 AH LOSS 8 ath loss: the power loss during signal propagation from x to x Only a portion of the power from x will be captured by the receiver. ath loss is defined as the ratio between the signal power at transmitter () and signal power at receiver (). L p Lp( db) 10log10 10log10 10log10

9 AHLOSS: MODELS 9 Free-space path loss model Clear, unobstructed LOS. Doesn t consider surrounding objects. Highly simplified model E.g. satellite ay-tracing models Consider the effects of reflection, diffraction from surrounding environment More complex, depends on site geometry General model Simple flexible mathematical model Captures the essence of path loss under various conditions Good for high level analysis

10 AHLOSS: FEE-SACE 10 Isotropic radiation Signal strength are equal in all directions Isotropic antenna: an antenna transmits equally in all directions. x At distance from x antenna, the power per unit area 4 he surface area of a sphere with radius 2 : x power ower at x 4 2 A e Effective area of x antenna

11 AHLOSS: FEE-SACE 11 Friis equation: path-loss equation for general antennas G G 2 4 : Distance between x and x. G : c / f : wavelength. G : ransmit antenna gain eceive antenna gain For system with fixed, G and G f

12 AHLOSS: FEE-SACE 12 Example In order to operate properly, a receiver must capture the signal power of at least -90dBm. Assuming a 100-miliwatt transmitter and free-space path loss. he antenna gain at x and x are 3dB. What is the service area radius of the x for a signal frequency of 800MHz? Sol: 100mW G ( mw) ( dbm)/10 9 ( dbm) 10log10 ( mw) ( mw) 1( mw) ( db) G ( db) 3dB G G 10 3/10 2 G G G G c 4 f G G km

13 AHLOSS: AY-ACING MODELS 13 ay-tracing model Besides LOS, also considers the effects of reflection, diffraction and scattering considers the effects from each ray

14 AH LOSS: AY ACING MODEL 14 Example: plane-earth reflection d :Distance between two paths x signal from LOS: E t) A cos(2 ft ) 0( 0 x signal from reflection: E r ( t) A0 cos2 f ( t d / c) A cos2 ft 2f d / c 2d / 0 : phase difference caused by distance difference : due to reflection and distance difference At the receiver: E total ( t) E0( t) E ( t) r

15 AH LOSS: AY ACING MODEL 15 he phase difference between the two rays will result in construction or destruction effects of the x signal LOS eflection otal

16 AH LOSS: GENEAL MODEL 16 A general model capturing the essence of path loss in various environment r 0 1 r 0 0 r : reference distance (typically 1 meter) 0 : measured path loss at reference distance n n : path loss exponent (can be obtained through measurement) r : distance between transmitter and receiver n: path loss exponent r he larger the value of n, the faster the x power falls off with the increase of distance. Free space: n = 2; rolling rural: n = 3.5; suburban: 4; dense urban: 4.5

17 AH LOSS: GENEAL MODEL 17 Usually represented in the unit of db L p n r 0 Lp( db) 10log10 0 n10log r0 10 r r 0 n=4 Fix Lp n=2

18 AH LOSS: GENEAL MODEL 18 Example: At a distance r 10 0 meter from x, the measured power is /. 0 2mW he path loss exponent is n = 2.9. he appropriate operation of the receiver requires the signal power at receiver must be at lease -90mW. What is the service radius? (using the general path loss model) Sol. ( dbm) ( mw) ( dbm)/ log10 ( mw) ( mw) 1( mw) r r ( / 0) 2mW 9 10 mw n km

19 OULINE 19 Wireless channel ath loss Shadowing Small scale fading Channel classifications Noise and interference Simulation model

20 SHADOWING 20 Shadowing: Caused by large obstructions that are distant from MS Analogy: the shadow of light due to mountain. Effects of shadowing is random due to random # and type of obstructions. he existence of shadowing is verified through field measurement. Consider the effects of path loss and shadowing 1 r0 r 0 n S path loss shadowing S : models the effects of shadowing. andom variable.

21 SHADOWING 21 db representation r0 10log10 10log10 n10log10 10log r 10 0 S S( db) 10log10 S : follows Gaussian distribution (normal distribution) f S ( db) ( x) 1 exp ( x m 2 db 2 2 db db ) 2 S(dB) follows normal distribution the log of S follows normal distribution he distribution of S is called lognormal distribution Shadowing is called lognormal shadowing

22 SHADOWING 22 path loss with shadowing (random) path loss only combined effects of path loss and shadowing (red curve)