Deep-space Ka-band link priority data protection: Preemptive retransmission vs. margin.

In this paper the performance of two preemptive retransmission schemes for protection of priority data over deep-space Ka-band links is evaluated. The first scheme merges the correctly received bit from each transmission to create the most complete set of priority data for each pass (bit merge). The second scheme (symbol combining) combines the soft symbols received from each transmission of the priority data to increase the priority data's signal to noise ratio (SNR), thus increasing the likelihood of their correct reception. These performances were then compared to an equivalent margin scheme in which the data rate used for the transmission of the priority data is reduced by a factor equal to the number of transmissions of the priority data. The performance of each scheme was evaluated through emulation using Water Vapor Radiometer (WVR) and Advanced Water Vapor Radiometer (AWVR) sky brightness temperature measurements along with models for Deep Space Network (DSN) Ka-band capable antenna using Mars Reconnaissance Orbiter's DSN tracking schedule and geometry. The results indicate that “bit merge” and “reduced rate” schemes perform roughly the same in terms of availability of the priority data (at least 97% compared to 93% without any data protections scheme) but for “bit merge” priority data losses occur over fewer passes than for the “reduced rate” scheme. The “symbol combining” scheme is superior to both the “bit merge” and the “reduced rate” schemes with at least 98.5% priority data availability with losses over a small number of passes. Receivers have a minimum demodulation threshold SNR and combining algorithms have combining losses; therefore, the performance of the “symbol combining” scheme was also evaluated with respect to these two factors. The results indicate that even with current receivers, the “symbol combining” scheme performs extremely well and the scheme is relatively robust with respect to combining losses.