microwave communication

University of Malaya

Dr.HarikrishnanDepartment of Electrical Engineering

e-mail: [email protected]

KEEE 2142Introduction to Communication System

Microwaves are generally described as electromagnetic waves with frequencies that range fromapproximately 500 MHz to 300 GHz or more.

Microwave signals has relatively short wavelengths due to their inherently high frequencies.

The vast majority of electronic communications systems established since the mid-1980s havebeen digital in nature and thus carry voice, video and data information in digital form.

Terrestrial (earth-based) microwave radio relay system using (FM) or digitally modulatedcarriers (PSK or QAM) still provide approximately 35 % of the total information carrying circuit

Microwave Radio Communication

University of Malaya KEEE 2142 HRK 2/15

carriers (PSK or QAM) still provide approximately 35 % of the total information carrying circuitmileage.

There are many different types of microwave systems operating over distances that vary from15 miles to 4000 miles in length.

Intrastate microwave system categorized as short haul because they carry information forrelatively short distances.

Long haul microwave system are those used to carry information for relatively long distances

In a typical microwave radio link, information originates and terminates at the terminal stations,where repeaters are used to relay the information to the next downlink microwave station.

Figure 1 shows a microwave radio link that are interconnected by three repeater stations.

Microwave Radio Communication (contd)


Figure 1

As the described in Figure 1 the geographic location of the stations must be carefully selectedsuch that natural and man-made barriers do not interfere with propagation between stations.


Advantages Disadvantages

Radio systems do not require a right of way acquisition It is more difficult to analyze and design circuits at microwave frequencies

Because of their high operating frequencies, microwave radio systems can carry large quantities of information.

Measuring techniques are more difficult to perfect andimplement at microwave frequencies.

High frequencies mean short wavelengths, which require It is difficult to implement conventional circuit components


A vast majority of existing microwave radio systems are frequency modulation, which of courseanalog. Recently however systems have been developed that use either PSK or QAMmodulation

relatively small antennas. (resistors, capacitors, inductors and so on) at microwave frequencies.

Distances between switching centers are less Transient time is more critical at microwave frequencies

Minimal crosstalk exists between voice channels It is often necessary to use specialized components for microwave frequencies.

Each station requires the purchase or lease of only a small area of land.

Microwave frequencies propagate in a straight line, which limits their use to line of sight applications

Satellite communication systems are similar to terrestrial microwave radio systems. The primarydifference is one propagates within the Earths atmosphere and the other outside.

Frequency modulation (FM) is used in microwave radio systems rather than amplitudemodulation (AM) because AM signals are more sensitive to amplitude nonlinearities.



Transmitter

Receiver

Microwave Radio Communication (contd) The permissible distance between an FM microwave transmitter and its associated microwave

receiver depends on several system variables, such as transmitter output power, receiver noisethreshold, terrain, atmospheric conditions, system capacity and performance expectations.

A single hop microwave system, is inadequate for most practical system applications, wherewith systems that are longer than 40 miles or when geographical obstructions, such asmountain, block the transmission path, repeaters are needed.


Repeater

Microwave Radio Communication (contd) A microwave repeater is a receiver and a transmitter placed back to back or in tandem with the

system. The location of intermediate repeater sites is greatly influenced by the nature of theterrain between and surrounding sites.

The exact distance is determined primarily by line of site path clearance and received signalstrength. For frequencies above 10 GHz, local rainfall patterns could also have a large bearingon path length.

Over time radio path losses vary with atmospheric conditions that can vary significantly, causing


a corresponding reduction in the received signal strength of 20, 30, or 40 or more dB. Thisreduction in signal strength is referred to as radio fade.

Diversity suggests that there is more than one transmission path or method of transmissionavailable between a transmitter and receiver. In microwave systems, the purpose of usingdiversity is to increase the reliability of the system by increasing its availability.

When there is more than one transmission path or method of transmission available, thesystem can select the path or method that produces the highest quality received signal.Generally the highest quality is determined by evaluating the carrier to noise (C/N) ratio at thereceiver input.



Frequency diversity microwave system

Free space path loss is often defined as the loss incurred by an electromagnetic wave as itpropagates in a straight line through a vacuum with no absorption or reflection of energy fromnearby objects.

Free space path loss assumes ideal atmospheric conditions, so no electromagnetic energy isactually lost or dissipated.

Microwave Radio Communication (contd) The mathematical expression for free space path loss is :

2

P4 DL pi =

where LP = Free space path loss (unitless)D = Distance (kilometers)f = Frequency (hertz)


When the frequency is given in MHz :

When the frequency is given in GHz :

= Wavelength (meters)

( ) ( )P MHz kmL 32.4 20log f 20 log D= + +

( ) ( )P GHz kmL 96.6 20log f 20log D= + +

(1)

(2)

Microwave Radio Communication (contd) In its simplest form, system gain (GS) is the difference between the nominal output power of a

transmitter (Pt) and the minimum input power to a receiver (Cmin) necessary to achievesatisfactory performance.

System gain must be greater than or equal to the sum of all gains and losses incurred by asignal as it propagates from a transmitter to a receiver.

In essence system gain represents the net loss of a radio system, which is used to predict thereliability of a system for a given set of system parameters.


Ironically, system gain I actually a loss, as the losses a signal experiences as it propagates froma transmitter to a receiver are much higher than the gains.

Therefore, the net system gain always equates to a negative dB value (loss). Because thesystem gain is defined as a net loss, individual losses are represented with positive dB values,while individual gains are represented with negative dB values.

Mathematically system gain in its simplest form:

S t minG P C= (3)

Microwave Radio Communication (contd)where GS = System gain (dB)

Pt = Transmitter output power (dBm or dBW)Cmin = Minimum receiver input power necessary to achieve a given reliability and

quality objective


Microwave Radio Communication (contd)where At = Transmit antenna gain relative to an isotropic radiator (dB)

Ar = Receive antenna gain relative to an isotropic radiator (dB)LP = Free space path loss incurred as a signal propagates from the transmit

antenna to the receive antenna through Earths atmosphere (dB)Lf = Transmission line loss between the distribution network (dB)Lb = Total coupling or branching loss in the channel combining network between the

output of a transmitter and the transmission line (dB)


FM = Fade margin for a given reliability objective (dB)

A more useful expression for system gain is :

Fade margin is sometimes called link margin is a fudge factor that considers the nonideal andless predictable characteristic such as multipath propagation and terrain sensitivity.

Carrier to noise (C/N) ratio is probably the most important parameter considered whenevaluating the performance of a microwave communications system.

( ) ( ) ( ) ( ) ( ) ( ) ( )S t min PdB dB f dB b dB t dB r dBG P C FM dB L L L A A= + + + (4)

Microwave Radio Communication (contd) From:

At room temperature of 290 K :

N kTB=

( )dBmkTB kTN 10log 10 log 10 log B0.001 0.001

= = +

( )dBmN 174dBm 10log B= + (5)


Fade margin is essentially a fudge factor included in system gain equation that considers thenonideal and less predictable characteristic of radio wave propagation, such as multipathpropagation and terrain sensitivity.

Fade Margin (FM)

( ) ( )FM 30log D 10log 6AB 10log 1 R 70= + (6)multipath

effectterrain

sensitivityreliability

objectives

Microwave Radio Communication (contd)where FM = Fade margin

D = distance

f = frequencyR = Reliability constant

A = Roughness factor

B = Terrain area condition


Example 1Refer to Figure 2. For a system gain of 112 dB, a total noise figure of 6.5 dB, an input noise power of -104dBm and a minimum (S/N)out of the FM demodulator of 32 dB, determine the minimum carrier power andthe minimum transmit power. Given that the minimum C/N at the input to the FM receiver is 15 dB.

Example 2For the system shown in Figure 3, determine Gs, Cmin/N, Cmin, N and Pt, given that Lb = 4 dB, Lf = 4.875 dBand At = Ar = 37.8 dB. Given that the minimum C/N at the input to the FM receiver is 23 dB. The terrain ofpropagation is a mountainous and dry environment, with A = 0.25 and B = 0.125.


Figure 2


Figure 3

microwave communication

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