Analogue & Digital Radio Transmission...........?

Question

Hi, please could someone explain how radio signals are sent digitally? I know that analogue signals are modulated onto a carrier wave and then sent to a receiver. How does this work for digital radio signals as these will be made up of one's an zero's so how is this transmitted? Do you still use an a.c. voltage and an antenna? Also, are most radio stations in the UK still analogue?

Answer 1

Same way as in analogue, i.e. a carrier wave is modulated with a signal. Opposed to analogue, though, only discrete values of the modulation are considered valid. As poornakumar described, QPSK (or QAM) is often used, but by no means compulsory - classical FM (or AM as a degenerate case of QAM) works as well. Which modulation is actually used for a specific case depends on the expected (broadcast radio) or measured (mobile comunications) SNR - the better the SNR, the more bits can be squeezed into one symbol.



As for radio stations in the UK - if they're in any way like those in Germany, then definitely yes for acoustical only radio (i.e. radio, as opposed to TV). The industry has been trying for 20 years now to shut off the analogue stations and switch to digital - so far, without success. From my personal guess, DAB is a dead end - even for cars, internet radio is starting to roll up the field and starting to replace DAB before that one even really got off the ground.

Answer 2

Apart from FSK, PSK (phase shift keying) has become most popular & now the mainstay of systems. In this, the phase of a Sine wave (of constant amplitude & frequency as there should be no ambiguity that would interfere with phase information, ϕ) carrier is varied, one phase to represent a "0" & the shifted phase as "1". This two level shift is called Bi-PSK or BPSK. Sine wave with ϕ=0º (unchanged) is the 0 bit while ϕ=180º or π Radians, is 1 bit. At the receiving end, there is a phase discriminator (now a simpler design in which the incoming signal is matched against a combination of Cosine & Sine waveforms generated in a very high Frequency Synthesizer). If the phase is same it outputs a 0 bit and if it isn't it outputs 1 bit.

Two bits are combined too and that results in a combination of 4 or 2²: [00], [01], [11], [10], each combination assigned to a particular phase like

[00] = 45º or π/4,

[10] = 135º or π -π/4,

[11] = 225º or π+π/4,

[01] = 315º (= -45º) or 2π -π/4 (= - π/4).

The circuitry renders the phase transition at the transmitting & the receiving end.

It is called QPSK (Quadrature PSK) when the phases are equi-spaced at π/2 Rad (= -90º).

8 PSK is achieved when 2³ levels of discrete phase levels are assigned spaced π/4 Rad apart.

16 PSK (2⁴ to bunch 4 bits as a "symbol"), ... 256 PSK (2⁸) are used even.

You must have noticed that the bits are bunched into k bits where the number of discrete levels of phase is 2ᵏ. At each increase in k the symbol packs double in number for the same carrier frequency that translates to halving the Band Width requirements which is the secret of a m-PSK system.

A 256PSK can reduce the BW (per symbol) requirement to 1/8 ͭ ͪ of the BW, as 8 bits are bunched into a symbol for 1 phase shift.

Yet if you view the diagram (of phases in a Polar representation), called "constellation" chart you'll notice that the points on the periphery of a unit circle fall too close & errors creeping in propagation, might bring two adjacent symbols close enough to have "inter-symbol interference " with one symbol being interpreted as the next one by the receiver. Only precision can save it from error; but there is a limit till which the system designer can go.

So, instead of placing the points or symbols (on constellation diagram) on a unit circle, they are stacked as points in a square array or pattern. It is another scheme, involving the symbols assigned at specific Amplitudes in addition, as

A Cosϕ+B Sinϕ.

{ There are many manufacturer/vendors who can sell solutions to one's requirement of digital communications, for Satellite Communications. I noticed that "Qualcomm" is the leader }

Answer 3

It is simple, though there are several methods of modulation. the most used method is frequency shift keying (FSK). It uses 2 frequencies to represent 1 and 0. (like FM).But in digital transmission it requires a very small bandwidth.

Yes most stations are analog. there are digital channels as well.