Wireless Communications Characteristics

1. Wireless CommunicationsCharacteristics 1

2. Contents • Path Loss • Shadowing • Multipath • Doppler Effect 2

3. Fundamentals of Waves • Radio (electro-magnetic) waves travel at a constant speed of c = 3 x 108 m/s in free space • Speed (c), wavelength () and frequency (f) are related by the equation: c =  f or:  = c T T=1/f (period in s) e.g., calculate wavelength of 2.4 GHz signal:  = c / f = 3 x 108 / 2.4 x 109 = 0.125m 3

4. space l wavelength time Speed of light: n = c = 3 x 108 m/s Propagation of Sine Wavein Space velocity l = n T lf = n period 4

5. Propagation of Waves • Waves propagating through an environment may experience various effects: • Dispersion • Reflection • Diffraction • Scattering • Refraction • Absorption • We will consider each of these 5

6. Dispersion • Waves transmitted by a source (e.g. antenna) spread out as they travel further away • Received Signal strength decreases with distance from the transmitter • In free-space, the power received (Pr) is proportional to 1/d2; i.e., Pr = Pt . K / d2 where: Pt is transmitted power K is constant for a given antenna type and size, 6

7. Dispersion - Example • 10m from the transmitter, the received power is 0.2 watts. Calculate the received power 50m from the transmitter. at 10m, d = 10m, Pr = 0.2 W: (Pt . K) = Pr . d2 = 0.2 x 100 = 20 at 50m: Pr = (Pt . K) / d2 = 20 / 2500 = 0.008 watts 7

8. Building Tx Rx Reflection • If a wave is incident upon an object that is much larger than its wavelength, the wave will be reflected • (Recall that the wavelength at 2.4GHz, where most Wireless LANs operate, is 12.5cm) 8

9. Tx Rx Building Diffraction • Diffraction occurs when the surface encountered by the wave has sharp irregularities such as sharp edges • This leads to a bending of the wave 9

10. Scattering location Tx Rx Scattering • Scattering occurs when the wave is incident upon a rough* surface. • The reflected energy is spread out in all directions, due to scattering * a surface is considered rough if (approximately) the deviations are of a comparable size to, or larger than the wavelength of the signal 10

11. Building Rx Tx Refraction • Refraction occurs when the wave passes through a boundary between two dissimilar media. • This leads to bending of the wave at the interface 11

12. Absorption • Absorption occurs when waves pass through any (non-vacuum) medium • It is the conversion of the transmitted energy into another form, usually thermal • For terrestrial networks, absorption in the atmosphere is negligible; • Significant losses due to absorption occur when waves travel through dense media e.g. walls, concrete 12

13. Power Measurements • Power losses in wireless communications are measured in decibels (dB) • Loss in dB = 10 log10 (Pr/Pt) • Losses in dB are additive • Absolute power measurements are expressed in dBm or dBW P (dBW) = 10 log10 [P (W)]; P (dBm) = 10 log10 [P (mW)]; 13

14. Power Measurements - example • A transmitter produces 15W of power. • Express this in a) dBW and b) dBm • P(dBW) = 10 log [ 15W ] = 11.76 dBW • P (dBm) = 10 log [ 15,000 mW ] = 41.76 dBm • A user receives 0.2W from the transmitter • Calculate the power loss in dB • Power loss = 10 log (0.2 / 15) = -18.75 dB Alternatively.. • Power received (in dBW) = 10 log (0.2) = -6.99dB Loss in dB = Pr (dBW) – Pt (dBW) = -6.99 – 11.76 = -18.75dB 14

15. Path Loss (1) • Path Loss is the deterioration of the transmitted signal due to all the effects we have described above • Unless the exact topology is known, it can only be estimated* • There are many different models for Path Loss for different environments * if the topology is known, computer simulations using ray-tracing techniques can be used 15

16. Path Loss (2) • We will look at one model which is made up of two components: • Distance-dependent part • Log-normal random variable: Shadowing • The received power in this model is given by: [Pr (d) – Pr(d0)](dB) = – 10.log10[d/d0] + X (dB) where: d0 is a reference distance* X is a random variable which models shadowing  is the path loss exponent * d0 is often the distance for the antenna far-field 16

17. Shadowing • Shadowing is a method of modeling random variations in the path loss • As a user moves, the path which the signal received by the user takes may vary due to the topology of the environment, in particular the signal may pass through different obstacles (e.g. buildings) • The random variable X (dB) is normally distributed, with a mean of 0 • The standard deviation  varies depending upon the type of environment 17

18. Path Loss - example • Figures for the path loss exponent , and standard deviation  are determined by empirical measurements • Examples: • Grocery Store:  = 1.8  = 5.2 dB • Offices  = 2.4 to 3.0  = 7 to 14 dB 18

19. Rx Tx Multipath • In nearly all environments, the received signal comprises multiple copies of the original signal, which have propagated over different paths • This phenomenon is referred to as multipath • It can be caused by reflection, diffraction, scattering and/or refraction of the transmitted signal 19

20. Small Scale fading • Variations due to shadowing occur over relatively large distances – often many meters • Signals in multipath environments also undergo small scale fading – variations that occur over the wavelength of the signal • This is due to the different multipath components combining either constructively or destructively 20

21. Small-scale fading (2) • Offset of only a fraction of a wavelength can lead to large change in signal level: 21

22. Large scale fading Received signal strength (log scale) Path loss Small scale fading Distance between transmitter and receiver Combined Path Loss & Fading 22

23. transmitted pulse received pulses Tm Delay Spread • Due to the different paths taken by the multipath components, they may arrive at different times Sender time Receiver time 23

24. Delay Spread (2) • The Delay SpreadTm is defined as the difference between times-of-arrival of the first and last multipath components • Typical values are as follows: 24

25. time time Sender Ts Ts Receiver Tm Inter-Symbol Interference • If the symbol period TS is smaller than the delay spread, i.e. TS < Tm, Inter-Symbol Interference (ISI) will occur • The receiver cannot determine which symbol each multipath component belongs to: 25

26. Coherence Bandwidth • The Coherence Bandwidth BCis a statistical measure of the range of frequencies over which the attenuation of the channel is approximately constant • Two frequency components f1and f2 will experience similar attenuation if (f1– f2) << BC • Coherence Bandwidth is approximately related to the Delay Spread by: BC (Hz) = 1/TM e.g. in a particular factory environment, TM = 120ns, BC = 1/(120 x 10-9) = 8.33 MHz 26

27. Coherence Bandwidth (2) • If the transmitted signal has a bandwidth much smaller than the Coherence Bandwidth, i.e. BU << BC, all frequency components will be attenuated similarly. • This is called Flat Fading • Else, it will undergo Frequency-selective fading, with different components attenuated differently. This causes distortion of the signal 27

28. Effect of Motion: Doppler Effect • Frequency components undergo a Doppler Shift if either the receiver or transmitter are moving • The maximum shift occurs when either transmitter or receiver moves directly towards the other: i.e. fd v/ , where v is the velocity of the moving body • In multipath environments, this becomes a Doppler Spread, since the different components will be affected differently 28

29. Doppler Shift - example • A user of a Wireless system operating at 5.2 GHz is moving at 3.6 km/h. Calculate the maximum frequency shift that can occur. • Answer: The maximum frequency shift occurs when the user is moving directly towards or away from the transmitter. (fd)max = v/ , v = 3.6 km/h = 1 m/s  = 3 x 108 / 5.2 x 109 = 0.058m (fd)max = 1 / 0.058 = 17.3 Hz 29

30. Coherence Time • The Coherence Time TC is a statistical measure of the time duration for which the channel impulse response is essentially invariant i.e., Signals received at t1 and t2 will experience similar attenuation if (t1– t2) << TC • TC is related to the Doppler shift by the following equation: TC = 1 / fd/v 30

31. Fast vs. Slow Fading • The Doppler effect causes variations in the channel over time • If the Doppler spread (fd) is small, and the Coherence Time is large compared with the symbol time (i.e., TC > TS), the channel remains (roughly) constant over many symbol periods and is said to be a Slow Fading channel • Alternatively, if the Coherence Time is small compared with the symbol time, i.e. TC < TS, the channel varies over a symbol period and is said to be a Fast Fading channel 31

32. Summary (1) • Path loss refers to the attenuation in the average received signal strength and its variation in space. It includes: • Attenuation due to distance (dispersion) • Shadowing, the random variation in received signal attenuation caused by the presence of large objects (e.g. buildings) • Multipath refers to multiple copies of the original signal received by the receiver • It causes variability in space and time called small-scale fading 32

33. Summary (2) • A channel can be characterized by the following: • Multipath profile • Delay Spread • Coherence Bandwidth • Flat fading versus frequency-selective • Doppler profile • Doppler spread • Coherence Time • Slow versus fast fading • Inter-Symbol Interference: occurs if the symbol time is smaller than the delay spread 33