Wireless Communication systems & Propagation

Chapter 5 Wireless Communication systems & Propagation

Chapter Outlines • Chapter 5 Wireless Communication Systems & propagation • The Friis Transmission Equation • Antenna Noise Temperature • Radar • Propagation Path Loss • Free Space Propagation • Plane Earth Propagation • Attenuation

Introduction Wireless communications involves the transfer of information between two points without direct connection  sound, infrared, optical or RF energy. Most modern wireless systems rely on RF or microwave signals, usually in the UHF to millimeter wave freq range. But why high freq?  spectrum crowding, need for higher data rates  majority of today’s wireless systems operate at freq ranging from 800MHz to few GHz. E.g. broadcast radio and TV, cellular telephones, DBS TV service, WLAN, GPS and RFID.

Introduction (Cont’d..) • Characterizing the wireless systems: • Point to point radio systems  single transmitter with single receiver  use high gain antennas in fixed positions to max received power and minimize interference with other radios (nearby frequencies). • Point to multipoint systems  connect a central station to a large number of possible receivers  commercial AM and FM radio and broadcast tv  Uses an antenna with broad beam to reach many listeners and viewers.

Introduction (Cont’d..) • Multipoint to multipoint systems  simultaneous communication between individual users (maybe not in fixed location)  generally not connect two users directly, but rely on a grid of base stations to provide desired interconnections between users. E.g. cellular telephone systems and WLAN. • Can also be characterize in terms of directionality of communication: • Simplex system  communication occurs in one direction, from tx to rx. E.g. broadcast TV, radio and paging systems.

Introduction (Cont’d..) • Half Duplex system  communication in two directions, but not simultaneously. E.g. early mobile radios (walkie-talkie) ..which rely on push to talk function with different intervals of transmitting and receiving. • Full Duplex systems  simultaneous two-way transmission and reception. E.g. cellular telephone and point to point radio systems  require ‘duplexing’ techniques : 1. using separate freq bands for transmit and receive, 2. users to transmit and receive in certain predefined time intervals.

5.1 The Friis Transmission Equation The Friis transmission equation describes how well the energy is exchanged between transmitter and receiver. Consider a pair of horn antennas with the same polarization and aligned each other.

The Friis Transmission Equation (Cont’d..) The radiated power density from Horn 1 at the location of Horn 2 is : The power received by Horn 2 is product of this power density and capture area A2, written as : The power received at Horn 1 resulting from power emitted by Horn 2 :

The Friis Transmission Equation (Cont’d..) The reciprocity property – the transmission pattern is the same as receive pattern, and the ratio of received power to radiated power will be the same, regardless which pair is transmitting or receiving. Therefore, or Since the directivity and area are independent each other, the ratio must be equal to constant :-

The Friis Transmission Equation (Cont’d..) Generally, We find, r – receiver t – transmitter The ratio is also valid even the antennas are not in line :

The Friis Transmission Equation (Cont’d..) Replace the effective area with receiving area to get : Finally consider, To get:

The Friis Transmission Equation (Cont’d..) This result is known as Friis transmission equation, which addresses on how much power is received by an antenna. Practically, it can be interpreted as the max possible received power, whereby with lot of factors to reduce the received power in actual radio system: • impedance mismatch at either antenna • polarization mismatch between the antennas • propagation effects  leads to attenuation or depolarization • mutlipath effects  partial cancellation of the received field.

The Friis Transmission Equation (Cont’d..) Important Notes!! The received power decreases as 1/R2 as the separation between transmitter and receiver increases. It seems large for large distance, but it is much better than the exponential decrease in power due to losses in a wired communication link (coax lines, waveguides, even fiber optic lines)  the attenuation power on Tline varies as e-2αz , with α is attenuation constant of the line  at large distance, the exp function decreases faster than an algebraic dependence like 1/R2 . For long distance communication, radio links perform better than wired links.

Example 1 Consider a pair of half wavelength dipole antennas, separated by 1 km and aligned for maximum power transfer as shown. The transmission antenna is driven with 1 kW of power at 1 GHz. Assuming antennas are 100% efficient, determine the receiving antenna’s output power.

Solution to Example 1 For 100% efficiency and antennas optimally aligned, For the λ/2 dipole antennas we have Dmaxt= Dmaxr = 1.64 and at 1 GHz, λ = 0.3m,

Solution to Example 1 (cont’d..) In terms of decibels, So finally,

The Friis Transmission Equation (Cont’d..) The Friss transmission equation can also be known as (in terms of receive and transmit) : Whereby, the product of PtGt can e interpreted equivalently as the power radiated by an isotropic antenna with input power PtGt, or effective isotropic radiated power (EIRP):

The Friis Transmission Equation (Cont’d..) For a given frequency, range and receiver antenna gain, the received power is proportional to EIRP of transmitter, and can only be increased by increasing the EIRP  increase transmit power, or transmit antenna gain or both. In any RF or microwave system, impedance mismatch will reduce the power delivered from a source to a load, where the Friss formula can be multiplied by the impedance mismatch factor,

The Friis Transmission Equation (Cont’d..) Max transmission between two antennas requires both antenna be polarized in the same direction. E.g. if a transmit antenna is vertically polarized, max power will be delivered to a vertically polarized receive antenna, while zero power would be delivered to a horizontally polarized received antenna. The polarization mismatch effects is measured by multiplying the Friss formula by the polarization loss factor,

5.2 Antenna Noise Temperature • NOISE Noise is any unwanted received signal independent of the transmitted signal and man action • EXTERNAL NOISE Cosmic noise, Atmospheric noise • INTERNAL NOISE Cable noise (waveguide or copper), receiver noise (thermal noise)

Antenna Noise Temperature (Cont’d..) Natural and manmade sources of background noise.

Antenna Noise Temperature (Cont’d..) Antenna noise temperature is the sum of all noise source at the antenna Noise power given by: Where k = Boltzman’s constant = J/K = 228.6 dBW/k/Hz T = physical temperature of source in kelvin degree B = noise bandwidth in which the noise power is measured in Hz

Antenna Noise Temperature (Cont’d..) Normally, we have the simple case to measure an available output noise power N0, given by: Illustrating the concept of background temperature. (a) A resistor at temperature T. (b) An antenna in an anechoic chamber at temperature T. (c) An antenna viewing a uniform sky background at temperature T.

Chamber room…

Antenna Noise Temperature (Cont’d..) But when the antenna beamwidth is broad enough that different parts of the antenna pattern see different background temperatures, the temperature now is called as effective noise temperature, Teseen by the antenna. This antenna brightness temperature takes into account the distribution of background temperature, directivity and the power pattern function of the antenna

Antenna Noise Temperature (Cont’d..) System noise temperature, Ts = Tin + Te Where Ts = system temperature Tin = noise temperature of antenna and cable

Antenna Noise Temperature (Cont’d..) So, the system noise temperature Noise power given by:

Antenna Noise Temperature (Cont’d..) If given noise in term of noise figure, to find noise temperature and Where F = noise figure (nf) T0 = ambient temperature

Antenna Noise Temperature (Cont’d..) The G/T ratio is another important parameter where the signal to noise ratio (SNR) at the input of a receiver is proportional to G/Ts. The SNR at the input to the receiver can be calculated as:

Antenna Noise Temperature (Cont’d..) Where SNR is proportional to G/T of the receive antenna. Only Gr/Ts is controllable at the receiver, and others are fixed by the transmitter design and location. G/T can be maximized by increase the gain of antenna  usually minimize reception of noise from hot sources at low elevation angles  but higher gain requires larger and more expensive antenna, and high gain may not be desirable for application of omnidirectional coverage!!

Antenna Noise Temperature (Cont’d..) • Signal to Noise Ratio or

Example 2 Suppose we have satellite system operates at 12.5GHz, with transmit carrier power 120W, transmit antenna gain 34dB, IF Bandwidth 20 MHz. The receiving dish have gain of 33.5dB, with receiver noise figure 1.1dB, locates 39000km from the satellite. The temperature noise between Tx-Rx are, Tsky = 50K and TG = 50K and Lw/g = 1dB. Find: EIRP of the transmitter G/T for the receive antenna Received carrier power at receiver terminal Signal to Noise Ratio (SNR)

Solution to Example 2 Convert the quantities in dB to numerical values: 34 dB = 2512, 1.1 dB = 1.29, 33.5 dB = 2239 The operating frequency 12.5 GHz, so wavelength 0.024m. So, In dB, convert Pt in dB, PT = 50.8dBm

Solution to Example 2 (Cont’d..) To find G/T, first find noise temperature of the antenna So then G/T for the antenna is:

Solution to Example 2 (Cont’d..) The received carrier power is from Friis formula: Or in dB

Solution to Example 2 (Cont’d..) The SNR at the receiver is: Or in dB

5.3 Radar The operation of monostatic radar (radio detection and ranging) system, (a) A radar antenna transmits a signal to the target.(b) The target scatters this signal, some of which is received by the radar antenna.

Radar (Cont’d..) • The direction of antenna’s main beam determines the location of the target (azimuth and elevation). • The distance or range to the target corresponds to the time between transmitting and receiving EM pulse. • The speed of target, relative to antenna, can be determined by observing any frequency shift in EM energy (doppler effect). The radar equation, σs is the radar cross section

Radar (Cont’d..) A more popular expression in terms of an effective area of the radar antenna is : The strongest receive power occur when the antenna’s main beam is pointing at the target, D(θ,φ) = Dmax. The received power also be detectable over the noise in the system, so radar will have a minimum detectable power.

Example 3 A radar with minimum detectable power specified as 1 pW is 1 km distant from a target with a 1 m2 radar cross section. Operated at 1 GHz the antenna has directivity of 100. Determine how much power must be radiated to enable detection of the target.

Solution to Example 3 Solve the radar equation in terms of Prad1 : At 10 GHz, we have λ = 0.3m, then we get:

Introduction to Propagation • The propagating wave between transmit and receive antennas in radio communication channel subjects to variety of effects (amplitude, phase or frequency) :- • Reflection (from the ground or large objects) • Diffraction (from edges and corners of terrain or buildings) • Scattering (from foliage or other small objects) • Attenuation (from rain or the atmosphere) • Doppler (from moving users) • This list covers the important effects for frequencies above 500 MHz.

Introduction to Propagation (Cont’d) • For frequencies below, about 100 MHz, other propagation effects can be important: • ground surface waves • atmosphere ducting • ionosphere reflection • Generally, propagation effects have the effect of reducing the received signal power, thus limit the usable range or maximum data rate of a wireless system.

Free Space Power Received Plane Earth Distance 5.4 Propagation path loss • Propagation path loss due to movement of the mobile away from RBS (Radio Base Station) • Propagation over land or sea follows law (propagation power law where  has a theoretical value 4 (for flat earth))

5.4.1 Free Space Propagation From Friis equation, the received power decreases as 1/R2 with distance from the transmitter  path loss  only applies to propagation in free space where no reflection, scattering or diffraction along the path between transmitter and receiver. Practically, the Friis equation can be used if there’s essentially a single line of sight (LOS) path between transmitter and receiver  usually implies that at least one of the link antennas has a narrow beamwidth (high gain) e.g. point to point radio links, satellite to satellite links and earth to satellite links.

Free Space Propagation (Cont’d..) A point to point radio link with a single line of sight propagation path A cellular telephone channel having multiple propagation paths.

Free Space Propagation (Cont’d..) Multipath propagation is particularly likely when the antennas have broad beams (low gain) and in close proximity to the ground or other large reflecting structures i.e. buildings, vehicles or foliage. May be no LOS path at all!!  common situation for cellular phone located in a building or vehicle. Communication still possible in multipath or even in the absence of LOS path  but the total signal voltage received will have varying degrees of destructive or constructive interference due to the variable phase delays that occur at different paths  Friis can not be used!

Free Space Propagation (Cont’d..) • For multipath propagation there are several model of propagation such as: • Ground Reflections • Vegetation Propagation • Urban Propagation • Okumura Model • Ionosphere Propagation • Troposphere Propagation

5.4.2 Plane earth propagation dd

Plane earth propagation (cont…) • Triangle ABC • Binomial series, dd = • Same step for triangle BCD

Wireless Communication systems & Propagation