Research On Key Techniques In Adaptive Satellite Transmission

With the rapid development of broadband multimedia services, many satellitecommunication systems operate on Ku/Ka bands, where the rain attenuation effectsare very serious. In order to combat the effects, the research on adaptive transmissiontechniques for satellite systems has become a hot topic. This paper mainlyinvestigates the key techniques in satellite adaptive transmission, including analysis ofsystem capacity, the selection of adaptive coding and modulation schemes,multi-beam power allocation, code-aided frame synchronization at low signal-to-noiseratios (SNRs) and high-precision real-time code-aided channel estimation techniques.Moreover, a low-complexity adaptive broadband satellite modem is designed andimplemented. The main innovation points and contributions are as follows:1. The capacity of adaptive satellite transmission system is analyzed. Twosubset selection algorithms of adaptive coding and modulation schemes and adynamic power allocation algorithm in multi-beam satellite system are proposed.The proposed algorithms significantly improve the spectral efficiency of adaptivetransmission. The theoretical capacity of adaptive satellite transmission system isanalyzed, which offers a theoretical designing reference. Two subset selectionalgorithms of adaptive coding and modulation schemes based on rain attenuationstatistical model for satellite channel are proposed. The algorithms achieve morecapacity utilizing less adaptive coding and modulation schemes than traditionalalgorithms. A dynamic power allocation algorithm based on PSO is proposed formulti-beam satellite system. The system capacity obtained by using the proposedalgorithm is two times by using static power allocation algorithm.2. A low-complexity code-aided frame synchronization algorithm isproposed. The mean acquisition time and confirming frame loss time forcode-aided frame synchronization are derived. A code-aided frame synchronizationalgorithm with low-complexity is proposed based on LDPC code constrains. At frameacquisition stage, the start of frame is captured according to the fraction of satisfiedconstraints after hard decisions of received signals. At frame tracking stage, the startof frame is tracked by utilizing decoding information from LDPC decoder. The acquisition and tracking performance of the algorithm is better than that of traditionalpilot-aided algorithms. Furthermore, mean acquisition time and confirming frame losstime for the proposed algorithm are derived and simulated. This offers the theoreticalreference in designing and implementing of code-aided frame synchronization.3. Two high-precision code-aided SNR estimation algorithms are proposedfor amplitude phase shift keying (APSK) signals. Cramer-Rao bound (CRB) ofthe proposed SNR estimator for APSK signals is also derived. A code-aided SNRestimation expression is derived based on ML criterion for APSK signals. Consideringthe coupling between decoder and SNR estimator of the above algorithm, an iterativecode-aided SNR estimator based on EM algorithm is proposed for APSK signals.Utilizing a posteriori probabilities of coded APSK signals, the closed-form expressionof the SNR estimator is obtained, which has more excellent performance at low SNRsthan traditional estimators and provides high-precision channel estimation foradaptive transmission system. Finally, CRB of the proposed code-aided SNRestimator for coded APSK signals is derived, which verifies the validity of theproposed algorithms and offers a design reference to code-aided estimators.4. Low-complexity frame synchronization, carrier phase synchronizationand SNR estimation algorithms are proposed. A low-complexity adaptivebroadband modem is designed and realized. The measurement results satisfy thesystem demands. The physical layer frame structure with distributed pilot structureand equal interval structure between UW and pilot sequence are designed and reducethe implementation complexity for synchronization. A differential framesynchronization detector is modified jointly utilizing both frame header and pilotsymbols, which has better detection performance and needs less acquisition time thanthe original detector. Jointly exploiting both UW and pilot symbols, a carrier phasesynchronization algorithm based on linear interpolation is proposed and improvessynchronization accuracy. Low-complexity non-data-aided SNR estimators areproposed. Finally, a low-complexity adaptive broadband modem is designed andrealized. The performance of modem prototype is measured in baseband andintermediate frequency loop with adaptive transmission. The measurement resultssatisfy the system demands.