I n f o r m a t i o n T r a n s m i s s i o n - PowerPoint PPT Presentation

I n f o r m a t i o n T r a n s m i s s i o n C h a p t e r 4 , C h a n n e l s OVE EDFORS Electrical and information technology

L e a r n i n g o u t c o m e s ● A f t e r t h i s l e c t u r e t h e s t u d e n t s h o u l d – u n d e r s t a n d t h e b a s i c p r o p e r t i e s o f w i r e d c h a n n e l s , s u c h a s c a b l e s a n d o p t i c a l fi b e r s , – k n o w t h e b a s i c p r o p e r t i e s o f w i r e l e s s c h a n n e l s , i n c l u d i n g p r o p a g a t i o n l o s s i n f r e e s p a c e a n d a n t e n n a g a i n s , – u n d e r s t a n d h o w n o i s e e n t e r s t h e s y s t e m a n d h o w i t i s c h a r a c t e r i z e d , – u n d e r s t a n d t h e b a s i c p r i n c i p l e s o f h o w m o v e m e n t s a n d m u l t i p l e w i r e l e s s p r o p a g a t i o n p a t h s c r e a t e D o p p l e r e f f e c t s a n d f a d i n g ( v a r i a t i o n s i n s i g n a l s t r e n g t h ) , a n d – b e f a m i l i a r w i t h t h e p r i n c i p l e o f t h e m a g n e t i c r e c o r d i n g c h a n n e l ( f o r s t o r i n g d a t a ) . O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 2

Wh e r e a r e w e i n t h e B I G P I C T U R E ? Lecture relates to pages Models of transmission and 105–117 in textbook. storage media. O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 3

Wi r e s , c a b l e s a n d fi b e r s » Coaxial cable » Twisted pair » Used for high frequency » Standard telephone line transmission » Shielded and controlled properties O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 4

Mo d e l o f a t r a n s m i s s i o n l i n e ( w i r e ) Model of short (unit length) section of line: - resistive loss - inductance from wires - “short circuit” resistance - capacitance between wires Model of entire wireline ... unit length sections in series O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 5

Wi r e s , c a b l e s a n d fi b e r s • Wires and cables have quite high attenuation • Where the propagation “constant” is given by • Sinusoid in – sinusoid out, but with an attenuation and a phase shift NOTE: Due to something called the skin effect , the resistance R is frequency dependent at high frequencies, so that O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 6

A t t e n u a t i o n o f a w i r e p a i r ( p h o n e l i n e ) • For longer wire lengths the attenuation is huge Received power [dB] at higher frequencies. • They are already in place, so let’s use them… Frequency [Hz] O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 7

P r o p a g a t i o n i n a fi b e r Fibers have low attenuation (< 0.5 dB/km). Reflections inside the fiber lead to dispersion – the light pulse will Smear out in time. O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 8

R a d i o C h a n n e l s – F r e e s p a c e O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 9

F r e e - s p a c e l o s s An isotropic antenna radiates equally in If we assume RX antenna to be isotropic: all directions. d Attenuation between two isotropic antennas in free space is (free-space loss): O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 1 0

A n t e n n a g a i n • An antenna will collect its power from an effective area A . The larger antenna, the more power it will collect • Similarly, it will focus its transmit power in a certain direction where the power density then will be higher O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 1 1

F r e e - s p a c e l o s s , F r i i s ’ l a w Received power, with antenna gains G TX and G RX : If we write the expression in dB ... In free space, the received power decays with distance at a rate of 20 dB/decade O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 1 2

N o i s e s o u r c e s The total noise situation in a receiver depends on several noise sources Noise picked up Wanted by the antenna signal Output signal Analog Detector with requirement circuits on quality Thermal noise in circuits O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 1 3

R e c e i v e r n o i s e : N o i s e s o u r c e s ( 1 ) The noise power spectral density of a noise source is usually given in one of the following ways: 1) Directly [W/Hz] 2) Noise temperature [Kelvin] The noise power N is also determined by the bandwidth B of the receiver Noise Here k is Boltzmann’s constant (1.38x10 -23 W/Hz) and T K is the is the temperature of the noise source in Kelvin. O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 1 4

R e c e i v e r n o i s e : N o i s e s o u r c e s ( 2 ) Antenna example N a Model Noise temperature Noise free of antenna 1600 K antenna Power spectral density of antenna noise is Multiply with bandwidth to get noise power O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 1 5

D i s t r i b u t i o n o f t h e n o i s e ● T h e n o i s e i s m o s t o f t e n a s s u m e d t o h a v e a G a u s s i a n d i s t r i b u t i o n ● Wi t h t h i s d i s t r i b u t i o n i t i s p o s s i b l e t o c a l c u l a t e t h e p r o b a b i l i t y t h a t a n o i s e s a m p l e e x c e e d s a c e r t a i n l e v e l . O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 1 6

Mu l t i - p a t h p r o p a g a t i o n , T w o w a v e s Wave 1 Wave 1 + Wave 2 Wave 2 At least in this case, we can see that the interference pattern changes on the wavelength scale. O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 1 7

S m a l l - s c a l e f a d i n g Illustration of interference pattern from above Received power [log scale] Movement A B Position Transmitter A B Reflector O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 1 8

S m a l l - s c a l e f a d i n g - R a y l e i g h f a d i n g Amplitude distribution when mean amplitude is 1, 2, 4, and 10. O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 1 9

D o p p l e r s h i f t s Frequency of received signal: v r     f f 0 c where the Doppler shift is v       f c r cos Receiving antenna moves with 0 speed v r at an angle θ relative to the propagation direction The maximal Doppler shift is of the incoming wave, which v has frequency f 0 .   f c max 0 O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 2 0

Mo r e t h a n o n e i n c o m i n g w a v e Incoming waves from several Spectrum of received signal directions when a f 0 Hz signal is transmitted. (relative to movement or RX) RX movement 2 1 3 2 4 1 RX 4 3 All waves of equal strength in this example, for simplicity. O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 2 1

Ma g n e t i c r e c o r d i n g • Store magnetic field with different orientation Figure source: http://hyperphysics.phy-astr.gsu.edu O v e E d f o r s E I T A 3 0 - C h a p t e r 4 ( P a r t 1 ) 2 2