5. Radio System & Antennas

 

5. Radio System & Antennas



17.12.2018

Radio System & Antennas – Video: Register for Course


 

Radio Wave Frequency

  • A radio wave is an oscillation of EM field originated by an antenna
  • The E field is perpendicular to M field and perpendicular to wave direction
  • The waves propagate at the speed of light.
  • The radio waves are shaped as sine waves. A single sinus cycle in the space domain is called wavelength. Thus the period the wave goes for a single cycle is T = S/V = lambda / C
  • So in time domain the single sinus cycle lasts for a period T. And inverse proportion of T is F, which is the number of cycles in a second.

Frequency Range

  • Max range (and why do we have those boundaries)
  • Types of frequency groups
  • Relation between F & Wave and antenna sizes (why frequency and in result wavelength matters)?
  • Main type of systems on the different frequencies
  • Why and when were the small frequencies used (maybe refer to radars etc. – like on the picture from intro view)

Logaritmic Calculations

  • In the wireless world, we use decibels and logaritmic scales a lot (e.g. power, gains, signal strength)
  • To move from the linear values to dB, we simply use 10log of the values.
  • There are two main „rules” 3 and 10
  • The signal power in the wireless systems is typically reflected in dBm with reference to the 1mW (a reference point)
  • This table shows some typical values
  • As an example – the mobile phones and wifi devices typically use 23dBm – which is 200mW of EM wave power

DB Analogy

While discussing power, it is worth mentionig how do we handle power in wireless systems. Channel can attenuate signal milion fold, so if we transmit 1W a 0.001 mW is received. It is very inconviniet to use such wide range of values and that is why we use logarithmic scale, called decibels.

In order to understand how decibels work, we will use a reference to the sound volume.
Lets start with single speaker, which generates unit volume. This is our reference, so decibel value is equal zero.

If we double the numer of speakers, the decibels increase by 3. This is thumb rule numer one in decibels -> double power means +3 decibels.

So, if we add another two speakers, decibels increase again by 3. Notice, that in first case, 3dB corresponds to 1 additional speaker, but now, 3 decibels mean 2 speakers – this is not linear scale.

If we arrange 10 speakers and therefor get the volume of 10, we obtain 10 dB. This is thumb rule numer two -> 10fold power means +10  decibels

Let’s make it 10fold again, so we get 100 speakers! This is getting loud. As we learned before, 10fold increase is +10 db, so we end up with 20db.

You can already see some benefits, because scale from 1 to 100 changed into 0 to 20.

It gets even prettier if we make one more step. Let’s create a powerfull setup of 1000 speakers.

It is impossible to show it on our linear scale right now, but we can easily handle it with decibels. It is just additional 10 dB!

Radio System Altogether

  • Now, lets put all the elements of the radio system together
  • The three main components are: Transmitter, Receiver and the Radio Channel inbetween
  • We start with a transmitter, which is An electronic device which generates a radio frequency alternating current that is feed to the antenna
  • The transmitter is equipped with a Tx antenna, that Radiates the alternating current as a radio waves
  • This radio wave originating from Tx antenna is an input to the radio channel (but can also be refered to as output signal from the transmitter)
  • The radio channel A medium that transports the information signal. It can be degrade the input signal via:
  • Attenuation
  • Noise
  • Interference
  • The radio channel outputs the distorted signal (that also can be called an input signal to the receiver) that is received by the Rx antenna that Intercepts radio waves and converts them to tiny alternating currents which are applied to the receiver, which extracts the desired information.
  • The receiver uses:
  • electronic filters to separate the desired radio frequency signal from all the other signals picked up by the antenna,
  • electronic amplifier to increase the power of the signal for further processing,
  • recovers the desired information through demodulation

Antennas

  • To start dicussing the antennas, we have to have a reference point – and that is a source of EM wave being a theoretical point in the space. Isotropic antenna radiates the EM waves with the same intensity in all direction, thus it’s 3D radiation pattern is a sphere and it’s „gain” is 0dBi, meaning that it doesnt increase the power of the input signal at all
  • A practical antenna example is a half wave dipole, which is a resonant elament that radiates the signal omnidirectionally in all directions perperndicular to it’s elements. The key point here is that the antenna has its characteristics with reference to the wavelength – i.e. the shape of the radiation pattern is kept for the waves of the lenght being half of the lenght of the radiating elements. It’s basic gain (for the signals of this wavelength) is 2.15dBi (meaning,its 2.15dB larger than the isotropic atenna) – this means that the signal power that is delivered to the antenna, is increased by 2.15dBi
  • If we increase the antenna length 2x – or use 2x higher freqeuency, we get a full wave dipole, the directivity increases to 4.15dBi. But we cannot go beyond certain lenght, as then the sidelobes start to show up . Sidelobes makes the antenna transmit or receive to /from unwanted directions.

Real Life Antenna

  • Here we show a real life antenna for WLAN (outdoor).
  • We see the top view and sideview and 3D view.
  • We can see that this antenna transmits in the vertical directions with highest power, we see the sidelobes and from the top the antenna transmits in all directions with the same gain – so it s called an omnidirectional antenna. This apptern applies for the 5.8GHz with 8dBi gain.
  • This pattern also applies for the reception – i.e. the received signal power at the antenna is increased by 2.15 dBi when reaching antenna from the given direction. While, when signal is received from the top, it is „zeroed”

Multiple Antennas

  • What is MIMO?
  • We have multiple antennas at the Tx, and multiple antennas at the Rx and a channel inbetween.
  • This is where inputs and outputs comes into play – i.e. we have Multiple inputs (input signals TO the wireless channel) and multiple outputs (output signals FROM the wireless channel) so when we have M antennas – resulting in M input s and N antennas – resulting in N outputs – we have an MxN MIMO channel
  • This means we have MxN individual radio paths
  • Those radio paths can be utilized in different ways – to increase link throughput, reliability or range – depending on the transmitter precoding schemes

Multiple Antennas Mimo

  • Now let’s take a look at the different antenna configurations
  • We can start with SISO – it is when we have single antenna at Tx and single antenna at Rx
  • Then we can have multiple receive antennas, so a single transmitted signal is received independently from two different antennas – and this can be used to improve link reliability by combining those received versions of the signal
  • When we make this opposite direction, there we could have two copies of the signal transmitted to a single receive antenna – and by this means we could also improve link reliabilitye
  • MIMO is the final stage, when we have both multile Tx and Rx antennas, and by this means we could also improve link reliability, or do something else – that in other configurations is not possbile – i.e. transmit 2x different streams – to increase thrpt.

Multiple Antennas Coherence

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