Time-Packing as Enabler of Optical Feeder Link Adaptation in High Throughput Satellite Systems

Abstract

This paper carries out a theoretical study of the data rate that a High Throughput Satellite (HTS) system with fullyregenerative payload may achieve when using an intensity modulation/direct detection optical feeder link. A low-order M -ary Pulse Amplitude Modulation (M-PAM) with time-packing is used to modulate the intensity of the laser diode beam, making use of an external Mach-Zehnder modulator. These M -PAM symbols are recovered in the satellite with the aid of a photodetector, and are then encapsulated into the 5G radio frame of the access link. The modulation order and the overlapping factor of the uncoded transmission are jointly selected to tackle the impact that slowly-varying weather conditions introduce. Moreover, the intersymbol interference that time-packing introduces is mitigated in reception using a Viterbi equalizer. As expected, time-packing enables a finer link adaptation granularity than the one that adaptive modulation can provide on an optical feeder link without coding, enabling to adjust the spectral efficiency according to slowly-varying attenuation that thin cloud layers introduce.

Linear Time-Packing Detectors for Optical Feeder Link in High Throughput Satellite Systems

“…For this purpose, we study the BER and Throughput that truncated Minimum Mean Square Error (MMSE) and adaptive MMSE equalization (LMS) can achieve, and compare them with the performance of the Viterbi equalizer with limited trellis states. For simplicity, the roll-off factor of the Square-Root Raised-Cosine (SRRC) filters and the number of trellis states are set to ρ = 0.15 and N s = 4096, respectively [7]. As expected, when the modulation order increases, the throughput with linear equalization improves with respect to the Viterbi decoder with limited trellis states.…”

“…By doing so, the Peak-to-Average Power Ratio (PAPR) of the transmitted signal and the energy consumption that is needed for detection can be reduced but, in return, the spectral efficiency of the communication channel can be notably affected if the ISI that time-packing generates cannot be notably mitigated [12]. Based on the results presented in [7], we have that the number of channel coefficients that introduces notable ISI grows with the order of the M -PAM modulation. Nevertheless, in order to keep the implementation complexity of the detector low, our goal consists in mitigating only part of the time-packing ISI with Viterbi decoding, maintaining some residual ISI that will make the received Signal-to-Interference-plus-Noise Ratio (SINR)…”

“…Unfortunately, in order to capitalize this gain, the complexity of the detector must grow notably when the roll-off factor of the Sinc-pulses decreases, the overlapping factor between adjacent Sinc-pulses grows, and/or the order of the modulation scheme augments [5], [6]. For this reason, it is necessary to study the performance of low-complexity linear detectors, as the performance of Maximum-Likelihood Sequence Estimation (MLSE) impairs notably when the number of trellis states in the Viterbi algorithm is limited [7].…”

On the Use of NB-IoT over GEO Satellite Systems with Time-Packed Optical Feeder Links for Over-the-Air Firmware/Software Updates of Machine-Type Terminals

“…The optimal strategy for cancelling it consist on resorting the Maximum-Likelihood Sequence Decoding (MLSD) [18], which can be implemented in an efficient way by mean of the Viterbi Algorithm [19]. However, to increase the throughput is required to use low roll-off factors and large overlapping factors which increase the complexity of the Viterbi Algorithm notably [9]. As a result alternative strategies have to be applied.…”

“…Moreover, if time-packing encoding is applied on the real-valued electrical signal that is used to modulate the intensity of the LD beam, the data throughput of the optical feeder link can be increased even further, without the necessity of using a wider communication bandwidth. This effect is obtained by shrinking the separation between adjacent transmitted pulses [8], mitigating part of the Inter-Symbol Interference (ISI) power that time-packing introduces with the aid of a linear equalizer that is placed on-board the GEO satellite before symbol detection [9]. It is important to highlight that the impact of the residual ISI, which remains in the forward link after the GEO satellite relaying, can be further mitigated with the proper selection of the Modulation and Coding Schemes (MCS) to communication with the NB-IoT terminal [10].…”