Question: What characteristic of the Raptor forward error correction code, specified for MBMS, allows the decoder to recover the original data with a minimal overhead of encoding symbols?
Option 1: The use of a fixed number of repair symbols for each block of source symbols.
Option 2: The transmission of only modified source symbols along with repair symbols.
Option 3: The ability to generate as many encoding symbols as needed on-the-fly, enabling recovery from any set slightly larger than the original source symbol count.
Option 4: The requirement for an equal number of source and repair symbols for successful decoding.
Explanation: This option correctly highlights the key feature of Raptor codes as fountain codes, which is their ability to generate encoding symbols on demand. This allows for efficient recovery of the original data at the decoder with just a slight overhead in terms of the number of received encoding symbols compared to source symbols, making it particularly suitable for applications like MBMS where reliability and efficiency are crucial.