Jitter-Induced Symbol Slip Rates in Next-Generation Ground Segment Receivers

NASA is in the process of modernizing its communications infrastructure to accompany the development of a Crew Exploration Vehicle (CEV) to replace the Space Shuttle. With this effort comes the opportunity to infuse more advanced coded modulation techniques, including low-density parity-check (LDPC) codes that offer greater coding gains than the current capability. However, in order to take full advantage of these codes, the ground segment receiver synchronization loops must be able to operate at a lower signal-to-noise ratio (SNR) than supported by equipment currently in use. At low SNR, the receiver symbol synchronization loop will be increasingly sensitive to transmitter timing jitter. Excessive timing jitter can cause bit slips in the receiver synchronization loop, which will in turn cause frame losses and potentially lead to receiver and/or decoder loss-of-lock. Therefore, it is necessary to investigate what symbol timing jitter requirements on the satellite transmitter are needed to support the next generation of NASA coded modulation techniques. The work presented here is based on a series of receiver measurements at the Electronics Systems Test Laboratory (ESTL) in Johnson Space Center (JSC), coupled with empirical observations made on the performance of the symbol synchronizer loop. Measurements of ground segment receiver sensitivity to transmitter bit jitter were conducted at ESTL using a satellite transponder and two different commercial off-the-shelf (COTS) Staggered QPSK (SQPSK) receivers. The symbol synchronizer loop transfer functions were characterized for each COTS receiver. Symbol timing jitter was introduced at the transmitter. Effects of sinusoidal (tone) jitter on symbol error rate (SER) degradation and symbol slip probability were measured. These measurements were used to define regions of sensitivity to phase, frequency, and cycle-to-cycle jitter characterizations. An assortment of other band-limited jitter waveforms was then applied within each region to identify peak or root-mean-square measures as a basis for comparability. Results from the ESTL test and analysis indicate that the receiver symbol synchronization loop is more sensitive to certain types of symbol jitter and jitter frequencies, depending on the selection of the loop filter and damping ratio. It is also shown that the target symbol slip probability of less than 10 -12 can be achieved for E s /N 0 < -0.8 dB, provided that the timing jitter does not exceed certain limits and through proper selection of receiver synchronization loop parameters. A symbol timing jitter mask derived from the experimental data is presented, which would allow for less than 0.1 dB degradation in SER performance at low E s /N 0 due to transmit symbol timing jitter.