Modulation/Demodulation Techniques

The SIM3 hardware wallet utilizes a sophisticated modulation technique to encode transaction data into a format suitable for USSD transmission. This technique is critical for overcoming the inherent limitations associated with USSD's narrow data payload capacity and unordered nature of message delivery.

Modulation Process

  • During modulation, each data block from the signed transaction is encoded with sequential identifiers and checksums. The sequential identifiers act as indices to maintain the correct order of data blocks, while checksums are integrated using error-detecting codes, such as cyclic redundancy check (CRC), to validate the integrity of each block upon reception. The encoded data blocks are then mapped to the frequency spectrum allocated for USSD communication, a process which may employ techniques like Quadrature Amplitude Modulation (QAM) or Phase Shift Keying (PSK), adapted for the digital signal processing capabilities of the mobile network.

Demodulation at USSD Server

  • Once received by the USSD server, the demodulation process begins. The server employs a decoding algorithm that interprets the frequency-encoded data, extracting the sequential identifiers and conducting integrity checks using the embedded checksums. The error-detecting codes allow the server to identify any blocks that may have been corrupted or lost during transmission, triggering error-handling protocols or retransmission requests as necessary.

Reassembly of Transaction Blocks

  • The demodulation process includes the reassembly of the ordered data blocks into the original transaction structure. This reassembly is sensitive to the sequencing information, ensuring that each block is restored to its correct position within the transaction. The server then conducts a final integrity check against the entire transaction data, comparing it against a master checksum generated at the modulation stage.

Error Correction and Data Integrity

  • To further enhance the reliability of the data transmission, the demodulation algorithm incorporates error correction codes (ECC) using the Turbo Codes, which enable the recovery of the original data even in the event of transmission errors. This forward error correction (FEC) is essential for maintaining data integrity in the variable conditions of mobile network communication.

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