How does the modulation of the transponder work?
About PCM


  I do not understand which are the advantages of using PCM [in a transponder]


Pulse Code Modulation (PCM) is a method used in codecs, while modulation is a method used in modems. PCM with modulation in the name is a misnomer, PCM doesn't relate to modulation in any way.

PCM is used in audio processing to take an analog signal and encode it into a digital stream by sampling, quantization and companding. It is for instance used for audio storage on a Compact Disc and in a Waveform file (.wav). It can be used for transmission, but CPM is only the portion of the chain which prepares the digital data to be sent.

Some forms of PCM are oriented for audio compression (e.g. DPCM or ADPCM), this aspect might have some interest regarding the larger mode S data blocks, but this compression is lossy, meaning some data would not be transmitted, which is indeed not an option for SSR.

To clear any doubt about PCM (and associated encoding delta modulation, delta-sigma, and differential PCM) not being a modulation technique, from Introduction to Analog and Digital Communications:


  A point that needs to be stressed one last time: In reality, PCM, DM,
  and DPCM are source-encoding strategies, whose purpose is to convert
  analog signals into digital form. For actual transmission of the
  encoded data over a communication channel, the discrete form of
  pulse-amplitude modulation (PAM) is typically used.


Afaik, there are no PCM application in a transponder or in the secondary surveillance radar to which the transponder replies. It would be interesting to see the document where PCM is mentioned in the context of SSR or modulation.

About transponder modulation


  How does the modulation of the transponder work?


Usually a SSR (secondary surveillance radar) ground station or an anti-collision system interrogates aircraft transponders by sending interrogation pulses on 1030 MHz. The transponders answer on 1090 MHz. There are several modes which are used to obtain different information from a compatible transponder. Civil aviation modes are:


Mode A to get the 12-bit identification/squawk code.
Mode C to get the 12-bit identification and the altitude.
Mode S to get 24-bit ICAO identification and various data. In this mode, the transponder also regularly broadcasts data spontaneously for ACAS and/or ADS-B uses.


Modes A/C can be identified by the time elapsed between pulses P1 and P3 (more here) and mode S by the presence of pulse P6 datablock. Transponders answer if they are compatible with the interrogation mode only.

Type of modulation

The basic type of modulation used for everything except the datablock of mode S interrogation (pulse P6) is the On-Off Keying (OOK), a type of Amplitude Shift Keying (ASK) for which amplitude can have two values. Bits of information are transmitted one by one, the carrier is sent or interrupted, according to the bit value:

  

OOK is a simple class of modulations, used to transmit information at low speed.

Mode S

Mode S (Select) allows the interrogator to specify the ID of the interrogated transponder, to prevent all other transponders to answer. This protocol requires a significantly more information both for the interrogation and the reply. This information is not transmitted as simple pulses using OOK, some form of synchronization must be added, specially when long chain of 0s or 1s are transmitted.

Instead of representing 0 by the absence of carrier and 1 by the presence of carrier (OOK), bits in the P6 datablock are represented by a presence of carrier which some characteristic is changed:


For interrogation, pulse P6 contains information for the Synchronization Phase Reversal (SPR) signal, the interrogation command, the target transponder ID, and 24 bits of the cyclic redundancy check (CRC).

P6 bits are sent using a variant of Differential Phase Shift Keying, the Differential Binary Phase Shift Keying (DBPSK) with phase shift being 0° for 0 and 180° for 1, i.e. consisting in inverting the carrier when bit value is 1:


DBPSK used for in interrogation data

The bit rate is 4 Mbps. In addition, the difference of amplitude of pulse P5 (sent in all directions) and pulse SPR (sent only forwards) is sensed to mute transponders not in front of the interrogator antenna.
Reply data blocks are encoded using Pulse Position Modulation (PPM) which consists in sending a pulse of carrier with a time shift dependent on the value. This ensures a return to zero which is a form of synchronization. For binary values, the pulse is sent either in the first half of the time slot for the bit (for bit 1), or the second half (for bit 0).

PPM with only two values is therefore equivalent to first coding data using Manchester code, then modulating the carrier using OOK already used for mode A/C. This choice was done to simplify airborne electronics when mode S was adopted by FAA.


PPM used for reply data

The bit rate is 1 Mbps. The content of the data block depends on the type of interrogation, or the type of automatic broadcast used. Example for ADS-B squitter sent automatically:


Source


More on ATC interrogation and reply formats here.