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
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?
Lower frequencies have longer wavelength and higher frequencies have shorter wavelengths.
What is the Amplitude of a signal?
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Watts can be used to measure the power of an RF signal. What is a difficulty when using Watts?
What is a way to transform the exponential range of Watts into a linear one?
How does the Base-10 logarithm function, denoted as log10, work?
The decibel (dB) can be used to compare different absolute values. How is this used?
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?
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?
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?
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?
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?
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?
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?
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?
Regarding wireless signals, what is Free Space Path Loss?
And what are 2 important facts about it.
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?
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 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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