M7/M7L/M7LT Modular Satellite Modem OperationM7/M7L/M7LT - Rev. 0.05 1-25The IESS 308 mode automatically adjusts the n and k factors dependent on the data rate. The“Overhead” ratio is the ratio of the data rate at the RS encoder output to the data rate at the input. It isdefined as n/k. The modem automatically adjusts to accommodate the new rate.The Reed-Solomon Codec function can be turned on and off under software control, and isindependent of the IBS ESC Data Channel enabling.1.6.4. Turbo Product Codes FECThe addition of the Turbo Product Codes (TPC) option card (Figure 1-20) allows replacement of thestandard Viterbi FEC independently for transmit and receive paths. The performance improvementachieved by the TPC is significant. For example, the BER vs. Eb/No performance of TPC Rate 3/4 isapproximately equal to Rate 1/2 Viterbi and uses over 40% less bandwidth. TPC also provides betterbandwidth/power utilization than either Reed-Solomon concatenated on Viterbi or 8PSK/TCM withReed-Solomon.Figure 1-20 - TCP Option CardThe modem offers extensive TPC capabilities including first generation 4k block TPC, with bothlegacy Datum modem compatibility (PSM500) and Competitive Technology (CT) operating modes,plus a newer second generation 16k block TPC. This second generation TPC offers superiorperformance at the expense of more delay due to larger processing blocks.The Datum Systems’ proprietary TPC “Advanced” mode has been optimized to provide the highestperformance available in any TPC on the market. These modes not only outperform othermanufacturer’s TPC, but also LDPC in many cases.1.6.5. Special Codec CT ModesThe wide range of FECs available today and the possibility of many different operating modes, andparameters for each, can make compatibility with other brands of modems extremely difficult. To aidin FEC compatibility with other brands of modems, the modem offers special “CT” modes, standingfor “Competing Technology”. These modes use the same parameter settings as popular modems byother manufacturers.
