I/Q (quadrature) carrier recovery

Question

I was looking into I/Q demodulation, and when looking into the equations, I realized that the phase/frequency difference between the carrier and the LO in the receiver had an unwanted effect. The constellation diagram is rotated. A couple of hours later on google, I read a little about "carrier recovery", which as far as I understand address exactly this problem.

But how is this done in practice? Especially for high speed data transmission.

I did not find any IC's with carrier recovery build-in.

Answer 1

I'm sure there are many methods to recover frequency and phase of a carrier, but one that is widely used in analog TV, and also in some digital communications, is the one involving a burst PLL.

The signal transmitted can be divided, in the time domain, in units. Each unit contains several regions (also separated in time), which will depend on the application. For instance, in analog TV, each unit is a horizontal line, and contains 3 regions: sync, color burst and luma+chroma information. The sync region is used to distinguish where the units begin and end. The luma+chroma information is the actual video infomation you want to transmit. And the color burst is a "windowed view" at the carrier that was used at the transmitter, to encode the luma+chroma information. During the color burst region, the signal transmitted reflects only the carrier. A multiplexer directs, during each region, the corresponding signal to the output of the transmitter.

The piecewise signal transmitted, for the case of analog NTSC TV, would be this one.

At the receiver, a module finds and separates the sync information embedded in the incoming signal, and uses sync-derived timing to close a gate that directs region 2 of the incoming signal (i.e., the color burst) to a burst PLL. A burst PLL is a heavily damped PLL (it has a loop filter with a long time constant -much longer than the duration of one "unit") that is allowed to modify the voltage that governs its VCO only during the time window while the burst gate is closed. As a result, the output from the burst PLL is a sinusoidal signal that is locked in frequency and phase to the incoming color burst, and that exists always. Even during regions "sync" and (this is the wanted one) "luma+chroma". Thanks to that recovered carrier existing always, it can be used to synchronously demodulate the incoming luma+chroma information.

I've talked about analog TV, color burst, and luma+chroma, but the concept behind a burst PLL is much more general, and it is in fact used in some digital comunications. The following figure shows what could be a general block diagram, not specific to TV, and which deals with the general signals I and Q.

Answer 2

You usually have a known pre-amble on the signal. Since that is known, you can rotate the constellation to the phase you want.

Carrier recovery is getting frequency used and phase, not just the phase.