Lecture 2: Basics of communications
Can also be called "Propagation constant". Somewhat misleading, as the propagation usually varies strongly.
Alternative names:
Propagation constant, symbol γ, is defined by the ratio between the amplitude at the source, and the amplitude at some distance x. Is a complex quantity, so we use α (attenuation constant) and β (phase constant) to define it. Attenuation constant, is the loss of signal, or attenuation of an electromagnetic wave travelling through a medium. Phase constant is the change in phase per meter, along the path travelled.
Band | Frequency | Wavelength | Propagation via |
Very low frequency, VLF | 3-30 kHz | 100 - 10 km | Guided between the earth and the ionosphere. |
Low frequency, LF | 30 - 300 kHz | 10 - 1 km | Guided between the earth and the D layer of the ionosphere. Surface waves. |
Medium frequency, MF | 300 - 3000 kHz | 1000 - 100 m | Surface waves.E, F layer ionospheric refraction at night, when D layer absorption weakens. |
High frequency, HF (short wave) | 3-30 MHz | 100-10 m | E layer ionospheric refraction. F1, F2 layer ionospheric refraction. |
Very high frequency, VHF | 30-300 MHz | 10-1 m | Sporadic E propagation Extremely rare F1,F2 layer ionospheric refraction during high sunspot activity up to 80 MHz. Generally direct wave. |
Ultra high frequency, UHF | 300-3000 MHz | 100-10 cm | Line-of-sight propagation. Sometimes tropospheric ducting. |
Super high frequency, SHF | 3-30 GHz | 10-1 cm | Direct wave. |
Extremely high frequency, EHF | 30-300 GHz | 10-1 mm | Direct wave limited by absorption. |
The frequencies which we use for mobile communications are ranging from 450 MHz (ICE), the old TV bands, 800-900 MHz (GSM), 1800 (GSM), 1900, 2100 MHz (UMTS), 2400 MHz (Wifi), 2650 MHz (LTE), and 5100 MHz (IEEE802.11a..). While previously frequency band were used for a specific technology, refarming started in 2012 to open for communication technologies in other bands. Examples of such refarming are LTE1800 indicating an operation of LTE in the 1800 band. Back in 2013 Apple surprised the European operators, as the iPhone came with LTE only in the 1800 band, and not, as usual in Europe, in the 2600 band.
* Prerequisite: Ohm's law, current, dielectric constant , conductivity
[Source: Wikipedia] Note: Depending on the convention, either [A m] or are used to indicate the magnetic field. In UNIK4700 and this compendium we use the notation of |
A modulated radio signal can be written in a general form: Any of these three parameters can be varied: amplitude-, frequency- or phase-modulation.
[Source:K.E. Walter, Basics of Mobile Communications]
UMTS as an example (in one of the future lectures)
A4-Signal Strength and Capacity
Why did it take twenty years to fill the gap between Hartley’s law and Shannon’s formula? The only necessary step was to substitute 1+C/N for m in Hartley's law. Why, all of a sudden, did three or more people independently “see the light” almost at the same time? Why did neither Nyquist, nor Hartley or Küpfmüller realize that noise, or more precisely the signal-to-noise ratio play as significant a role for the information transfer capacity of a system as does the bandwidth?
[source: L. Lundheim, Telektronikk 2002]
* Electronic parts of transmitter and receiver (components)
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Radio
[Source:Wikipedia, "interference"]
,
where P is average power
[source: Wikipedia]
Shannon's theorem will be part of next lecture...