Advanced tracking loops to support low rate coded uplinks

Conventional second-order Doppler tracking loops impose a tradeoff between phase noise and tracking performance. In order to maintain adequate bit-error rate, the loop bandwidth must be narrow enough that phase noise is small. However, this limits the track range, track rate, and acquisition time of the loop. The tradeoff between bandwidth and tracking capabilities can prevent communication during intervals of strong Doppler dynamics. Furthermore, this tradeoff can also be problematic for deep-space scenarios in which carrier to noise ratio is poor and loop bandwidth must be extremely narrow. In such scenarios, tracking even modest Doppler is prohibitive. This limitation is one reason that coding has not been applied to emergency-mode uplinks. High-order loops can mitigate this tradeoff. Typically, high-order loops are typically only employed for downlink communications. The JHU/APL Frontier Software Defined Radio is capable of implementing both very narrow loops, as well as high-order loops, within the existing hardware and firmware architecture; only software changes are necessary. The present paper describes a case study and preliminary results for carrier tracking loops that can support coded emergency-mode uplinks.