Wireless Signals and Modulation - Understanding Basic Wireless Theory

18 important questions on Wireless Signals and Modulation - Understanding Basic Wireless Theory

Regarding a wireless signal, explain what the phase is and what's important about it

The phase of a signal is measured in degrees, 180 degree is opposite and 360 degrees full cycle.
It measures the shift of the signal relative to the start of a cycle.
When two signals are received they add together when they are in phase but cancel each other out when they are 180 degree opposite.

What is the wavelength of a wireless signal?

It is the distance the signal travels during one complete cycle.
Lower frequencies have longer wavelength and higher frequencies have shorter wavelengths.

What is the Amplitude of a signal?

It is a way to measure the strength of a signal. It measures the hight from the top peak to the bottom peak.

Watts can be used to measure the power of an RF signal. What is a difficulty when using Watts?

Because it's an absolute power value the values can differ by order of magnitude. There can be a transmitter using 0.00001 mW and one that is using 10 mW. Here one is a million times stronger than the other.

What is a way to transform the exponential range of Watts into a linear one?

The logarithm function can be used. This takes values a magnitude apart, for example 000.1, 00.1, 0.1, 1, and spaces them evenly within a reasonable range.

How does the Base-10 logarithm function, denoted as log10, work?

It computes how many times 10 can be multiplied by itself to equal a number. Log10(10) equals 1 because 10 is used only once to get the result. Log10(100) equals 2 because 10 is used twice; 10x10 is 100.

The decibel (dB) can be used to compare different absolute values. How is this used?

The difference between 2 power levels is logarithmic calculated using:
dB = 10(log10P2-log10P1)
Where P2 is value of interest and P1 is often called the reference value.
This can be rewritten to:
dB = 10log10(P2/P1)
Which is used most often.

What are the 3 laws of dB calculations?

1. Law of Zero: A value of 0 dB means two absolute power values are equal.
2. Law of 3s: A value of 3 dB means that the value of interest (P2) is doube the reference value (P1), -3 means half the reference value.
3. Law of 10s: A value of 10 dB means that the value of interest (p2) is 10 times the reference value (P1), -10 means 1/10 of the reference value.

How can the 3 laws of dB calculation be used to calculate the dB difference between for example the absolute values of 5 mW and 200 mW?

The laws can be used in succession of eachother, where you want to get from the first value to the second value.
5 x2 = 10
10 x2 = 20
20 x10 = 200
x2 = 3dB and x10 = 10 dB so the diffrence is 3+3+10=16 dB.

In wireless networks instead of just using dB and calculating differences between two absolute values often dBm is used.
What is dBm and how is used?

The dBm is dB-milliwatt. Instead of two working values it uses a reference value of 1 mW.
Two absolute working values for example transmitted power and received are then calculated against the reference value. The resulting dBm can be substracted from eachother to get the difference in dB.

Antenna's provide some amount of gain to the resulting RF signal, however, no additional power (mW) is added. Because of this gain cannot be calculated in dBm
How is antenna gain calculated?

It is compared to a reference antenna, then calculated to a value in dB.
reference antenna often is an Isotropic antenna, so the gain is measured in dBi.
An isotropic antenna is a imaginary ideal antenna. A tiny dot which radiates RF equally in every direction.

Cables between the transmitter and the antenna cause some signal loss. How do you know the amount of loss?

This is often supplied by the vendor as dB per foot or meter of cable.

An important parameter in wireless signals is the EIRP value or Effective Isotropic Radiated Power, which is measured in dBm.

How is the EIRP calculated?

It is calculated by combining the values of the transmitter, cable and antenna. For example:
transmitter: 10 dBm
Cable loss: 5 dB
Antenna gain: 8 dBi
10 dBm - 5 dB + 8 dBi = 13 dBm
these different values can be safely combined.

What does it mean when antenna gain is shown in dBd instead of dBi?

It indicates that a Dipole antenna is used as a reference value instead of an isotropic antenna.
A dipole antenna has a gain of 2.14 dBi. This means a dBd value can be converted to dBi by adding 2.14 dbi;
dBi = dBd + 2.14 dBi

Why is EIRP een important value?

It is often used by governments to regulate maximum levels of radiation.

Regarding wireless signals, what is Free Space Path Loss?
And what are 2 important facts about it.

Free Space Path Loss is the amount the signal weakens when it travels from the transmitter to the receiver through the air.

1. It is an exponential function, falls off quickly nearby the transmitter but more slowly farther away.
2. The loss is a function of distance and frequency only.

What is an important value of a wireless receiver which indicates how strong the received signal has to be?

The Sensitivity Level.
This indicates the threshold of signal strength that the receiver would still be able to understand the signal.

What is the Signal to Noise ratio and how is it calculated?

It is the difference between the signal that is received and any additional signals on the same frequency, called noise. It is measuredin dB and higher is better.
It is calculated by substracting the Noise Floor from the RSSI. For example:
RSSI = -54 dBm
Noise Floor = -90 dBm
Signal to Noise Ratio = -59dBm - -90dBm = 36 dB

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