EC312 Lesson 16: Propagation

EC312 Lesson 16: Propagation

Objectives:

(a) Define reflection, refraction and diffraction.

(b) Describe the characteristics of ground waves, sky waves, and space waves.

(d) Compute received power level for a communication system using Friis Free Space equation.

I. Wireless Propagation

Propagation is the study of how to get a signal from Point A to Point B. It sounds simple, but it is THE most fundamental aspect of wireless communications. The wired guys don’t really have to worry about propagation per se, however, wireless transmission requires a fundamental understanding of how electromagnetic waves move through the atmosphere.

1. Two categories of propagation

In general, we can think about radio frequency propagation in two broad categories: Large Scale and Small Scale. Our emphasis in EC312 is on understanding Large Scale propagation, although many engineers have devoted their entire careers to understanding and modeling Small Scale propagation. In fact, you’ve likely experienced Small Scale propagation numerous times without ever realizing. The classic example would be driving down the highway while talking on your cell phone and experiencing a dropped call.

Large Scale Propagation – The behavior of the radio channel over large distances (100’s or 1000’s of λ’s). Received power is directly related to distance between Tx and Rx, and is stationary with respect to time.

Small Scale Propagation – The behavior of the radio channel over a small local area (1-10 λ’s) and/or small time durations. Received power fluctuates rapidly based on position, speed, direction of travel, etc. of the mobile.

Before we deep dive into all the glories of Large Scale Propagation in real-world environments, it behooves us to look at the most basic way we can transmit energy from Point A to Point B in an environment devoid of terrain, mountains, buildings, ground, or atmosphere. Such an environment is known as Free Space, and conveniently, wireless propagation in such an environment is known as Free Space Propagation.

2. Free Space Propagation

Let’s consider the following scenario. You have a brand-new iPhone (or Samsung phone as the case may be), have just signed up for a super-fast LTE plan, and would like to upload a photo, surf the web, browse facebook, or just plain make a phone call. To make that happen, your phone has to transmit that information over the air to the nearest LTE cell tower (cost: $5 Million, that’s why your phone bill is $100/month), which happens to be 5 miles away.

Question: Will your signal make it to the tower and will it have sufficient power to “close the link” and allow you communicate? Or will you suffer the fate of a cellular “dead zone”?

Recall from our previous lecture what happens when we transmit energy out an antenna:

Presume I have a wireless transmitter which is going to output some power: pt. I’m going to connect the output of the transmitter an antenna:

Now, if I had a perfect isotropic antenna, it would radiate that energy equally in all directions, and create a spherical wavefront: it is completely unfocused

Now, a perfect isotropic antenna really doesn’t exist. All real antennas will concentrate the energy of the wave in one or more directions:

Because we’re radiating the same amount of power, but in a smaller area, we say that the antenna has a gain associated with it, due to the fact that it’s concentrating the energy in a smaller area of space. Another way of thinking about this is a ratio of energy densities:

We defined something in the previous lesson called the effective radiated power (ERP or EIRP), which is the gain of an antenna (with respect to an isotropic radiator) multiplied by its input power. This represents the amount of power that the communication system would have produced if the antenna were a perfect isotropic radiator.

Note That: The Antenna Gain/EIRP equation is only valid when we’re in the far field of the antenna, or a distance away of:

D / is the largest physical dimension of the antenna [meters]
λ / is the wavelength [meters]

Note Also That: A lot of people believe very strongly that the Far Field starts at a distance of 10 wavelengths (10λ) away. That is a rough rule-of-thumb that you can use to be safe or as an approximation, but the real far-field distance is based on the above equation.

To propagate the energy away from the transmitter, we need to enlarge our sphere—just like blowing up a balloon. We’re going to keep blowing it up larger and larger, until it intersects with our receiver.

Now, just like a balloon, as we keep blowing it up, the total energy is going to remain constant, however, the energy density is going to decrease. Our density here is going to be based on the surface area of a sphere:

d / Is the distance between transmitter and receiver [meters]

Thus, our energy density at the receiver is:

pd / Is the power density in W/m2

Now, the last thing we need is to turn that power density into an actual received power. So what we need is a “catcher’s mitt” to catch all of that energy for us. Luckily, we call that an antenna. The physically larger the antenna, the more of that energy it can “catch”. That’s why long-range antennas are so large – the energy they’re collecting is spread out over an incredibly large area.

Thus, the problem of determining the received power is just a geometry problem: how much of the power density intersects with the effective area of the receive antenna.

The Effective Area for an antenna is conveniently defined as:

λ / is the wavelength [meters]
gr / Is the gain of the receive antenna

Finally, we can put all this together and find out what our received power is going to be:

/ Is the received power [Watts]
/ Is the trasmit power [Watts]
/ Is the gain of the transmit antenna [linear units]
/ Is the gain of the receive antenna [linear units]
λ / Is the wavelength [meters]
/ Is the distance between transmitter and receiver [meters]

This is known as the Friis Free Space Equation. It is fundamental to understanding how power falls of as a function of distance for wireless communications.

Two most common mistakes: Using dB values instead of linear values, and failing to get the wavelength/distance units correct.

Let’s go back to our example.

Example

Your cell phone transmits at a power level of 500 mW, and an antenna gain of 2.0 dB. The cell tower has an antenna gain of 8.0 dB, and is a distance of 5 miles away. For LTE, you’re transmitting at 700 MHz.

Will your signal make it to the tower and will it have sufficient power to “close the link” and allow you communicate? Or will you suffer the fate of a cellular “dead zone”?

Note that: The Friis Free Space equation is technically only valid for Free Space environments (although many situations will mimic free space). So the question is: what happens when we add back in the mountains, the buildings, the earth, and the atmosphere?

II. Large Scale Propagation – Three Modes

Large scale propagation is composed of three modes listed below in order of their dominance:

· Reflection – Energy reflects off large (relative to λ) conductive objects.

· Diffraction – Bending of energy around objects.

· Scattering – Diffuse re-radiation of energy off rough (smaller than λ) objects.

Let’s look at these briefly one at a time.

1. Reflection The angle of reflection is equal to the angle of incidence. Reflection introduces a 180˚ phase shift.

Refraction (with reflection): The radio wave is bent due to the changing index of refraction of the material.

2. Diffraction Diffraction can be explained by Huygen’s principle – all points on a wavefront serve as point sources for secondary wavefronts:

Consider a Transmitter and Receiver where an object is blocking the direct line-of-sight path between Tx and Rx. The signal can diffract around the object such that energy can get to the Rx even though it is shadowed.

Note: The more deeply the receiver is shadowed, the lower the received power. At some point, the receiver won’t be able to receive any signal.

3. Rough Surface Scattering Sometimes called Diffuse Scattering or Diffuse Reflection, scattering happens when energy impacts a rough surface and is re-radiated in numerous directions.

/
Image Source: http://www.ee.washington.edu/research/ersl/ResearchLinks/WavePropRough.htm

So those are the basic modes of propagation. What happens when we add in a real earth and a real atmosphere?

The earth and the earth’s atmosphere have the greatest impact on signals in the VLF – HF range (3 kHz – 30 MHz). It’s not that the earth and atmosphere don’t affect signals at higher frequencies (in fact, they do!) it’s just at those higher frequencies start to come into play and dominate the effects of the earth/atmosphere.

Let’s look at what happens to these lower frequencies first before moving on to the higher frequencies.

Radio Band / Frequency / Propagation Via
VLF / Very Low Frequency / 3 - 30 kHz / - Guided between the earth and the ionosphere
LF / Low Frequency / 30 - 300 kHz / - Guided between the earth and the ionosphere
- Ground Waves
MF / Medium Frequency / 300 - 3000 kHz / - Ground waves
- E layer ionospheric refraction at night, when D layer absorption disappears
HF / High Frequency (Short Wave) / 3 - 30 MHz / - E layer ionospheric refraction
- F layer ionospheric refraction
VHF / Very High Frequency / 30 - 300 MHz / - Line-of-sight
UHF / Ultra High Frequency / 300 - 3000 MHz / - Line-of-sight
SHF / Super High Frequency / 3 - 30 GHz / - Line-of-sight
EHF / Extremely High Frequency / 30 - 300 GHz / - Line-of-sight limited by absorption

For VLF-HF communications, there are two basic modes a radio wave travels from the transmitter to a receiving antenna:

· Ground wave

· Space wave

· Sky wave

The frequency of the radio wave is the most important factor in determining the performance of each type of propagation.

4. Ground Wave Propagation A ground wave is a radio wave that travels along the earth’s surface (also referred to as a surface wave). A ground wave must be vertically polarized.

Lower frequencies travel efficiently as ground waves because they are diffracted by the surface of the earth. Ground waves thus follow the curvature of the earth and can travel beyond the horizon, for hundreds of miles. Ground wave propagation is strongest in the Low and Medium frequency ranges. Ground wave propagation constitutes the main signal path for signals in the frequency range from 30 kHz-3 MHz.

5. Sky Wave Propagation Sky waves are radiated by an antenna into the upper atmosphere where they are reflected back to earth. This bending is caused by refraction in the ionosphere. The air molecules of the ionosphere are subject to severe radiation from the sun. Ultraviolet radiation causes the molecules to ionize, or separate into charged particles, positive and negative ions.

The ionosphere is composed of 3 layers, D, E and F (although F is subdivided into F1 and F2).

The direction of reflection depends on the angle at which the radio wave enters the atmosphere and the different degrees of ionization of the layers, as well as the frequency of the transmission.

A receiver that lies between the end of the ground wave propagation and the first sky wave reflection will not receive the transmitted signal. This region is termed the skip zone.

6. Space Wave Propagation A space wave refers to the radio wave that travels directly in a straight line from the transmitting antenna (Line-of-sight). These waves are not refracted, and do not follow the curvature of the earth.

The chief limitation of a space wave is that it is limited to line-of-sight distances. The range of space wave propagation is limited by the curvature of the earth and height of the antennas above the earth’s surface.

If an antenna is at height h, the distance, d, to the radio horizon (which is the maximum range for space wave communications from that antenna) is given by the formula

In this formula, the height of the antenna is in feet, and the distance to the horizon is in miles.

In the picture below, where we have one antenna of height and a second antenna of height , the maximum separation at which they can still communicate by line-of-sight is

D =

Example

What is the longest line-of-sight communication range between a transmitter whose transmitting antenna is 350 feet high and a receiver whose receiving antenna is 25 feet high?

III. Log-Normal Model

Ok, so that covers the VLF – HF range of the frequency spectrum. Those bands are mostly used for narrowband, long-distance communication. Most terrestrial wireless communications operate in the “sweet spot” of the wireless spectrum (aka the “beachfront property”) in the VHF and UHF bands. In this range, the earth and atmosphere play a far smaller role, and propagation becomes dominated by the specific local environment.

Let’s consider the following scenario. Suppose we convince the ECE Department to build a cell tower on the top of Rickover Hall, and you’re driving down McNair Road. The signal you receive will be a combination of Reflection, Diffraction, and Scattering, as shown in the image below. The problem is that we call it “mobile” radio for a reason: you want to be able to drive, move about the local environment, and communicate on your cell phone at the same time.