## 2 CCSDS 131.2-B Protocol for High-Rate Telemetry
### Transmitter Side  
The definition of the CCSDS 131.2-B is based on the application of efficient coding schemes that address the problem of bandwidth limitations in the timing-varying Ka-band channel. All CCSDS 131.2-B 27 ModCods employ efficient, high-speed data transmission schemes based on an SCCC derived from the Modem for High Order Modulation Schemes (MHOMS) project [18].  
A transmitter compliant with the CCSDS 131.2-B standard executes different functions: error-control coding (based on serially concatenated convolutional coding), frame validation, transfer frame synchronization, physical layer (PL) framing, bit synchronization, and pseudo-randomization. The architecture for an encoder and modulator compliant with the CCSDS 131.2-B is described in the standard [16] and is shown in Figure 1. The figure identifies the functional blocks of the system and shows the logical relationships among them.  
The transmitting system accepts, as inputs, Transfer Frames (TFs) of fixed length, performs functions selected for the mission, and transmits a continuous and contiguous stream of physical channel symbols, obtained from PL frames. In particular, the SCCC encoder can be considered the main part of the system. It applies convolutional encoding employing two consecutive convolutional encoders (CC1 and CC2). Moreover, it applies the _puncturing_ technique, which removes bits after the encoding following specific patterns, and the _interleaving_ technique, which interleaves the data bits. The encoded bits are framed in a PL frame that is finally given as input to a baseband filter, applying a Squared-Root Raised Cosine (SRRC filter, defined in the standard). The input transfer frames have a specific format defined in the standard, which, together with the structure of the obtained PL frames, is crucial for supporting data-aided operations carried out at the receiving side.  
The SCCC encoder of the transmitter encodes the so-called information blocks, composing output blocks with a dimension that is related to the used ModCod. The same ModCod scheme is applied to every 16 consecutive blocks, referred to as Codewords (CWs), that will be grouped to form a PL frame. The length of encoded symbol blocks after encoding and mapping to modulation symbols is constant (8100 symbols), regardless of the applied ModCod scheme; this facilitates frame synchronization at the PL for the receiver  
Figure 1: Transmitter Architecture.  
side. Also, a set of predefined pilot symbols can be inserted inside each CW. Each CW is therefore divided into 15 subsections, each composed of 540 symbols and each followed by 16 pilot symbols. Overall, a PL frame is composed of a 320-symbol PL header, of which the first 256 symbols represent the Frame Marker (FM), and the remaining 64 symbols represent the Frame Descriptor (FD) and \(16\times 8100\)-symbol CWs, eventually interleaved with 240 pilot symbols per CW.  
The well-defined structure of the PL frame is crucial at the receiving side for synchronization. In particular, the FM is used to detect the start of a PL frame, as defined in the standard. The FD instead carries the information about the ACM format of the PL frame and the presence/absence of pilot symbols. These symbols are used on the receiving side for phase synchronization.