Wireless Communication By

1. Wireless CommunicationBy Engr. Muhammad Ashraf Bhutta

2. Antennas and Propagation Introduction • An antenna is a transducer that converts radio frequency electric current to electromagnetic waves that are radiated into space • In two-way communication, the same antenna can be used for transmission and reception

3. Fundamental Antenna Concepts • Reciprocity • Radiation Patterns • Isotropic Radiator • Gain • Polarization

4. Reciprocity • In general, the various properties of an antenna apply equally regardless of whether it is used for transmitting or receiving • Transmission/reception efficiency • Gain • Current and voltage distribution • Impedance

5. Radiation Patterns • Radiation pattern • Graphical representation of radiation properties of an antenna • Depicted as a two-dimensional cross section • Reception pattern • Receiving antenna’s equivalent to radiation pattern

6. Antenna Gain • Antenna gain • Power output, in a particular direction, compared to that produced in any direction by an isotropic antenna • Effective area • Related to physical size and shape of the antenna

7. Antenna Gain • Relationship between antenna gain and effective area • G  antenna gain • Ae effective area • f  carrier frequency • c  speed of light (» 3 x 108 m/s) •  carrier wavelength

8. Polarization • Defined as the orientation of the electric field (E-plane) of an electromagnetic wave • Types of polarization • Linear • Horizontal • Vertical • Circular

9. Polarization • Vertically Polarized Antenna • Electric field is perpendicular to the Earth’s surface • e.g., Broadcast tower for AM radio, “whip” antenna on an automobile • Horizontally Polarized Antenna • Electric field is parallel to the Earth’s surface • e.g., Television transmission (U.S.) • Circular Polarized Antenna • Wave radiates energy in both the horizontal and vertical planes and all planes in between

10. Types of Antennas • Isotropic antenna • Idealized • Radiates power equally in all directions • Omnidirectional • Dipole antennas • Half-wave dipole antenna • Hertz antenna • Quarter-wave vertical antenna • Marconi antenna • Parabolic Reflective Antenna • Smart Antenna

13. RF propagation Coverable distance The distance that a wireless link can bridge is depends on: • RF budget • gain • Insertion loss • Receiver sensitivity • Path loss • Environmental Conditions (influencing the path loss) • free space versus non free space • line of sight • Reflections / Interference • Weather

14. RF propagationFree space versus non free space Non-free space • Line of sight required • Objects protrude in the fresnel zone, but do not block the path Free Space • Line of sight • No objects in the fresnel zone • Antenna height is significant • Distance relative short (due to effects of curvature of the earth)

15. RF propagationFirst Fresnel Zone First Fresnel Zone Direct Path = L Reflected path = L + l /2 Food Mart

16. RF PropagationBasic loss formula Propagation Loss d = distance between Tx and Rx antenna [meter] PT = transmit power [mW] PR = receive power [mW] G = antennae gain Pr ~ 1/f2 * D2 which means 2X Frequency = 1/4 Power 2 X Distance = 1/4 Power

17. RF propagationRF Budget The total amount of signal energy that is generated by the transmitter and the active/passive components in the path between the two radios, in relation to the amount of signal required by the receiver to be able to interpret the signal Lp < Pt - Pr + Gt - It + Gr - Ir Where: Pt = Power on transmit Pr = Power on receive Gt = Gain of transmitting antenna It = Insertion loss in the transmit part Gr = Gain of receiving antenna Ir = Insertion loss in the receive part Lp = path loss

18. + Antenna Gain + Antenna Gain - Path Loss over link distance Antenna Antenna RF Cable RF Cable - LOSS Cable/connectors - LOSS Cable/connectors Lightning Protector Lightning Protector pigtail cable pigtail cable + Transmit Power CTRL Bldg. Satellite Town RSL (receive signal level) > sensitivity + Fade Margin Calculate signal in one direction if Antennas and active components are equal WP II WP II RF propagation Simple Path Analysis Concept (alternative)

19. 50 ft.LMR 400 3.4 dB 50 ft.LMR 400 3.4 dB 24 dBi 24 dBi parabolic For a Reliable link - the signal arriving at the receiver - RSL - should be greater than the Sensitivity of the Radio (-82dBm for 11 Mbit) This EXTRA signal strength is FADE MARGIN FADE MARGIN can be equated to UPTIME Minimum Fade Margin = 10 dB Links subject to interference (city) = 15dB Links with Adverse Weather = 20dB Calculate RSL > -82 + 10 = -72dBm .7 dB .7 dB 1.3 dB 1.3 dB Tx =15 dBm Rx = -82 dBm WP II WP II RF propagation RSL and FADE MARGIN

20. 16 Km = - 124 dB 50 ft.LMR 400 3.4 dB 50 ft.LMR 400 3.4 dB 24 dBi parabolic 24 dBi RSL > PTx - Cable Loss + Antenna Gain - Path loss + Antenna Gain - Cable Loss This lets us know that if the Fresnel zone is clear, the Link should work. If RSL < than -72 then MORE GAIN is needed, using Higher Gain Antennas or Lower loss Cables or Amplifiers (not a Agere Systems provided option) + 15 dBm - 2 dB - 3.4 dB + 24 dBi - 124 dB + 24 dBi - 3.4 dB - 2 dB - 71.8 dB > -72 .7 dB .7 dB 1.3 dB 1.3 dB Rx = -82 dBm Tx =15 dBm WP II WP II RF propagation Sample Calculation

21. Fresnel Zone Clearance Antenna Height Antenna Height Obstacle Clearance Earth Curvature RF PropagationAntenna Height requirements • Fresnel Zone Clearance = 0.6 first Fresnel distance (Clear Path for Signal at mid point) • 57 feet for 40 Km path • 30 feet for 10 Km path • Clearance for Earth’s Curvature • 13 feet for 10 Km path • 200 feet for 40 Km path Midpoint clearance = 0.6F + Earth curvature + 10' when K=1 First Fresnel Distance (meters) F1= 17.3 [(d1*d2)/(f*D)]1/2 where D=path length Km, f=frequency (GHz) , d1= distance from Antenna1(Km) , d2 = distance from Antenna 2 (Km) Earth Curvature h = (d1*d2) /2 where h = change in vertical distance from Horizontal line (meters), d1&d2 distance from antennas 1&2 respectively

22. Path 6cm ( 1/2 ) longer RF Propagation Reflections • Signals arrive 180° out of phase ( 1/2 ) from reflective surface • Cancel at antenna - Try moving Antenna to change geometry of link - 6cm is the difference in-phase to out of phase

23. RF propagationEnvironmental conditions Weather • Snow • Ice and snow when attached to the antenna has negative impact • heavy rain on flat panels • When rain creates a “water film” it will negatively impact performance • Rainfall in the path has little impact • Storm • Can lead to misalignment • Lightning • Surge protector will protect the equipment against static discharges that result of lightning. It cannot protect the system against a direct hit by lightning, but will protect the building from fire in such a case

24. Propagation Characteristics of mobile radio channels • In an ideal radio channel, the received signal would consist of only a single direct path signal, which would be a perfect reconstruction of the transmitted signal. • In real the received signal consists of a combination of attenuated, reflected, refracted, and diffracted replicas of the transmitted signal • .It can cause a shift in the carrier frequency if the transmitter, or receiver is moving (Doppler effect).

25. Attenuation • Attenuation is the drop in the signal power when transmitting from one point to another. • It can be caused by the transmission path length, obstructions in the signal path, and multipath effects. • Figure on next slide shows some of the radio propagation effects that cause attenuation. • Any objects that obstruct the line of sight signal from the transmitter to the receiver can cause attenuation. 

27. Shadowing of the signal can occur whenever there is an obstruction between the transmitter and receiver. • It is generally caused by buildings and hills, and is the most important environmental attenuation factor. • Shadowing is most severe in heavily built up areas, due to the shadowing from buildings. • Radio signals diffract off the boundaries of obstructions, thus preventing total shadowing of the signals behind hills and buildings. • However, the amount of diffraction is dependent on the radio frequency used, with low frequencies diffracting more then high frequency signals. • Thus high frequency signals, especially, Ultra High Frequencies (UHF), and microwave signals require line of sight for adequate signal strength. • To over come the problem of shadowing, transmitters are usually elevated as high as possible to minimise the number of obstructions

28. Multipath Effects Rayleigh fading • In a radio link, the RF signal from the transmitter may be reflected from objects such as hills, buildings, or vehicles. • This gives rise to multiple transmission paths at the receiver. Figure in next slide show some of the possible ways in which multipath signals can occur.

30. The relative phase of multiple reflected signals can cause constructive or destructive interference at the receiver. This is experienced over very short distances (typically at half wavelength distances), thus is given the term fast fading. These variations can vary from 10-30dB over a short distance. Figure 4 shows the level of attenuation that can occur due to the fading

31. Figure Typical Rayleigh fading while the Mobile Unit is moving (for at 900 MHz)

32. The Rayleigh distribution is commonly used to describe the statistical time varying nature of the received signal power. It describes the probability of the signal level being received due to fading.

33. Frequency Selective Fading In any radio transmission, the channel spectral response is not flat. It has dips or fades in the response due to reflections causing cancellation of certain frequencies at the receiver. Reflections off near-by objects (e.g. ground, buildings, trees, etc) can lead to multipath signals of similar signal power as the direct signal. This can result in deep nulls in the received signal power due to destructive interference. For narrow bandwidth transmissions if the null in the frequency response occurs at the transmission frequency then the entire signal can be lost. This can be partly overcome in two ways. 

34. . This can be partly overcome in two ways.  By transmitting a wide bandwidth signal or spread spectrum as CDMA, any dips in the spectrum only result in a small loss of signal power, rather than a complete loss. Another method is to split the transmission up into many small bandwidth carriers, as is done in a COFDM/OFDM transmission. The original signal is spread over a wide bandwidth and so nulls in the spectrum are likely to only affect a small number of carriers rather than the entire signal. The information in the lost carriers can be recovered by using forward error correction techniques

35. Delay Spread • The received radio signal from a transmitter consists of typically a direct signal, plus reflections off objects such as buildings, mountings, and other structures. • The reflected signals arrive at a later time then the direct signal because of the extra path length, giving rise to a slightly different arrival times, spreading the received energy in time. Delay spread is the time spread between the arrival of the first and last significant multipath signal seen by the receiver. • In a digital system, the delay spread can lead to inter-symbol interference. This is due to the delayed multipath signal overlapping with the following symbols. This can cause significant errors in high bit rate systems, especially when using time division multiplexing (TDMA). Figure 5 shows the effect of inter-symbol interference due to delay spread on the received signal. As the transmitted bit rate is increased the amount of inter-symbol interference also increases. The effect starts to become very significant when the delay spread is greater then ~50% of the bit time

37. Table shows the typical delay spread for various environments. The maximum delay spread in an outdoor environment is approximately 20 us, thus significant inter-symbol interference can occur at bit rates as low as 25 kbps. Environment or cause Inter-symbol interference can be minimized in several ways. One method is to reduce the symbol rate by reducing the data rate for each channel (i.e. split the bandwidth into more channels using frequency division multiplexing, or OFDM). Another is to use a coding scheme that is tolerant to inter-symbol interference such as CDMA. 

38. Doppler Shift When a wave source and a receiver are moving relative to one another the frequency of the received signal will not be the same as the source. When they are moving toward each other the frequency of the received signal is higher then the source, and when they are approaching each other the frequency decreases. This is called the Doppler effect. An example of this is the change of pitch in a car’s horn as it approaches then passes by. This effect becomes important when developing mobile radio systems.  The amount the frequency changes due to the Doppler effect depends on the relative motion between the source and receiver and on the speed of propagation of the wave. The Doppler shift in frequency can be written:

39. (from [12]) fd=fo v/c Where fdis the change in frequency of the source seen at the receiver , fo is the frequency of the source, v is the speed difference between the source and transmitter, and c is the speed of light. For example: Let fo= 1GHz, and v = 60km/hr (16.7m/s) then the Doppler shift will be: This shift of 55Hz in the carrier will generally not effect the transmission. However, Doppler shift can cause significant problems if the transmission technique is sensitive to carrier frequency offsets (for example OFDM) or the relative speed is higher (for example in low earth orbiting satellites).

40. What is function of SMH? What sort of processing is done with SU in outgoing processor at MTP level 2 ? What is the function of Sevice indicator (SI)In SIO?