Wireless Communications: Microwave Fundamentals

Engr. Jefril M. Amboy

Introduction

Microwave communications arise due to the congestion of the radio frequency spectrum.

There is an increasing demand for services as radio technology evolves and advances rapidly.

There is a continuing trend in the use of higher frequencies.

Today the 2, 4 and 6 GHz bands for the heart of many communication systems.

Due to the rapid advancements and demand for spectrum, the microwave region will soon be congested as well.

Challenges

The use of outdated design rules adapted from old analog systems backdrop

Growing public resistance to more radio sites

Increase in physical structures, such as wind farms and high rise buildings

The need to provide capacity for new data services without equivalent additional subscriber revenue.

The transition from TDM to packet-based systems

Unreliability and poor system design

In 1865, James Clerk Maxwell predicated the existence of electromagnetic waves.

In 1887, Heinrich Hertz confirmed the predictions of Maxwell when he discovered and generated electromagnetic waves and demonstrated its existence.

History of Wireless

In 1897, Guglielmo Marconi founded the Wireless Telegraph and Signal Company

The first practical application of wireless was in transatlantic telegraphy

In 95s the first commercial terrestrial point-to-point systems were installed.

However, the lack in expertise and understanding of radio during those times made wireless unreliable.

New thinking needs to be applied to old concepts to revolutionize the way microwave radio networks are planned and operated.

An electromagnetic wave consists of time-varying electric and magnetic fields that are perpendicular to each other. It travels in a direction perpendicular to both the E and M fields, hence the term transverse electromagnetic (TEM) waves.

Ec

B

0 0

1c

E = electric field (V/m) B = magnetic field (T)

All EM waves are generated by moving electric charges. Whenever a charged particle accelerates, it radiates energy.

Antennas are used to generate radio waves. They are basically transducers because they convert the electrical energy into EM radiation.

What are Microwaves?

Microwaves are electromagnetic waves with frequencies ranging from

approximately 500 MHz to 300 GHz.

Why Microwaves?

Due to the inherent high frequency of these electromagnetic waves, their wavelengths are too small compared to the wavelengths of conventional RF signals.

Thus, they are called microwaves, with the prefix micro which means small.

Due to their high frequencies, microwave signals propagate in space in a straight line, just like a beam of light.

A successful transmission of microwaves requires line-of-sight or LOS between the transmitting and receiving ends.

Properties of Microwaves

Microwaves are electromagnetic radiation with high frequencies and thus, short wavelength.

Microwaves travel in straight lines like beams of light.

Microwaves can be reflected by conductive surfaces.

The current created by microwaves travel through a thin outer layer of a conductor.

Microwaves penetrate through the ionosphere.

What is a Microwave Communication System?

A microwave communications system is a form of electronic communications that uses microwaves to transmit and receive information signals.

The simplest form of microwave communications is a microwave link.

A microwave link is a high-frequency radio link specifically designed to provide a signal connection between two points.

Typical Microwave link

What is a Microwave Radio

Microwave radio in this context means a point-to-point fixed digital links that operate in duplex mode.

Duplex mode means that each channel consists of a pair of frequencies for transmission and reception

They are sometimes called low-band channels and high-band channels. The distance between the two frequencies is called T to R spacing.

The actual RF bandwidth depend on the link capacity and modulation scheme used.

Microwave Radio, cont.

The digital baseband modulates an analog RF carrier and then transmitted as an electromagnetic wave.

The transmit and receive frequencies are combined in a single antenna using FDD, where a duplexer provides the coupling for the two signals.

Classifications of Microwave Communication Systems

Based on the nature of Signal Analog Digital

Based on Distance Covered Short Haul Long Haul

Based on Capacity Narrowband Wideband

Advantages of Using Microwaves

Due to the high frequency of microwaves, more data can be sent, thus, increased bandwidth and higher speeds

Due to their small wavelengths, microwave transmission requires relatively smaller antennas

Signals are easily propagated around physical obstacles like mountains, buildings and bodies of water

Improved directive properties reduces the risk of interference

There is much less background noise in microwave frequencies than at RF

Fewer repeaters are necessary for amplification

Advantages of Using Microwaves

Does not require physical facilities like cables and thus, underground facilities are minimized

Microwave systems do not require a right of way

Less fading, and thus, increased reliability and less maintenance

Microwave transmission does not require high transmit power

Delay times are minimum

Minimal crosstalk

Disadvantages of Microwaves

More difficult to analyze and design circuits at microwave frequencies

Difficult to implement measuring techniques

Conventional components behave differently at microwave frequencies

Conventional semiconductor devices do not work properly at microwave frequencies

Transient time is more critical

Requires the use of specialized components

Disadvantages of Microwaves

Transmission distance is limited by line-of-sight

Microwaves are easily diverted/reflected due to their short wavelengths

Can be relatively prone to absorption and attenuation by fog and heavy rains.

Cost of implementing the infrastructures is high

Microwave Frequency Bands

Microwave frequencies include a portion of the UHF.

The microwave frequency spectrum is divided into different bands according to IEEE standards.

Band Designation

Frequency Range (GHz)

L band 1 2 S band 2 4 C band 4 8 X band 8 12 Ku band 12 18 K band 18 26.5 Ka band 26.5 40 Q band 30 50 U band 40 60 V band 50 75 E band 60 90 W band 75 110 F band 90 140 D band 110 170 Submillimeter greater than 300