Adjacent downlink satellite interference usually occurs because of the presence of side lobes in addition to the desired main lobe in the radiation pattern of the satellite ground station antenna. If the angular separation between two adjacent satellites is not sufficiently large, it is quiet possible that the power radiated through the antenna's radiation pattern, whose main lobe is directed towards the intended satellite, interferes with the received signal from the adjacent satellite. This problem becomes more difficult if both satellites transmit at the same frequency. The SARSAT (Search and Rescue Satellite Aided Tracking) system is designed to provide distress alert and location data in order to assist in search and rescue operation. SARSAT uplink detects and locates distress beacons (406MHz) activated at distress location. The system calculates the location of distress event using Doppler processing techniques. Processed data is repeatedly retransmitted through SARSAT downlink to Local User Terminals (LUT) when satellites are in view and also stored in on-board memory. The downlink of all SARSAT LEO (Low Earth Orbit) satellites uses the same 1544.5 MHz frequency. In case those satellites are within the main lobe of the local user terminal antenna, transmissions from adjacent satellites act as interference to each other. This can result in missed bursts or no solutions received at LUT, consequently no data provided about the distress location∗. Modulation index application for adjacent satellite downlink interference identification, for periods where the interference duration may be significant, is presented in this paper. Measurement results recorded at SARSAT terminals are provided.

Modulation index application for satellite adjacent downlink interference identification

This paper examines the problem of rapid frame and symbol synchronization techniques intended particularly for constant envelope modulation formats M–CPFSK with modulation index h=1/M which are used in strictly bandwidth limited narrowband industrial applications. The data aided and non data aided versions of the algorithm based on digital frequency discrimination are discussed and compared against the synchronization techniques found in literature. Sample wise pattern correlation technique for joint frame and symbol synchronization is also studied. With the focus on a practical digital implementation the advantages and disadvantages of the described approaches are discussed.

Simplified Frame and Symbol Synchronization for 4–CPFSK with h=0.25

A data-aided synchronization method for jointly estimating the symbol timing and carrier frequency offset has been proposed for 4-ary CPFSK (continuous phase frequency shift keying) modulation with h = 0.25. The proposed algorithm is based on a special preamble and has a feedforward structure that is suitable for digital realizations. Simulation results indicate the timing and frequency recovery algorithm can be employed with a short preamble of 16 symbols and is well suited for burst mode transmissions.

Joint frequency and timing recovery for pulse shaped. 4-CPFSK with h = 0.25

Continuous Phase Modulation (CPM) has been widely used in satellite communication and mobile communication systems due to its good constant envelope characteristics and high bandwidth and power utilization. CPM signals can be divided into Single-h CPM signals and Multi-h CPM signals according to the amount of modulation indexes. For CPM signals, the modulation index is the most important modulation parameter, which can affect the change of signal phase information, and its size can not only affect the speed of signal phase change, but also affect the spectral efficiency of the signal. In addition, in order to realize the practical application of CPM demodulation, correct estimation of the modulation index is a very important prerequisite.Therefore, this paper proposes a modulation index estimation based on the first-order cyclic moment, which has good universality.

Modulation Index Estimation of CPM Signals Based on First-Order Cyclic Moments

Continuous Phase Modulation (CPM) signal has been widely used in modern satellite, mobile communication and military communication system due to its good spectrum and power utilization and constant envelope characteristics. The premise of demodulation or jamming of CPM signal intercepted in military communication confrontation is the accurate estimation of signal parameters. Aiming at the difficulty and complexity of multi-h CPM signal modulation index estimation, this paper puts forward a kind of blind estimation algorithm of the modulation index based on first order cyclic moment and the second-order cyclic cumulants, using the first and second order cyclic properties in frequency domain on the spectral properties to signal parameter estimation. The estimation algorithm is suitable for both single h CPM signals and multi-h CPM signals, where the multi-h CPM requires the synchronization of the guidance sequence. Simulation results show that the algorithm has better performance under low signal-to-noise ratio with less symbols required. When the number of symbols is 128 and the signal-to-noise ratio is 15 dB, the accurate recognition rate of the multi-index CPM signal can reach 98% and the recognition rate can reach 99% with the allowable error of 1/32.

A modulation index estimation algorithm for multi-h CPM signals

Modulation index is an important parameter of CPFSK (continuous phase frequency shift keying) signals, which affects the system performance significantly. In noncooperative communication systems, this index is unknown to the receiver and must be estimated. This paper proposed a fast modulation index estimation algorithm for CPFSK signals based on short time Fourier transform (STFT). Simulations show the estimation results fit the actual value well. Furthermore, this paper evaluated the performance of timing synchronization and BER using the estimated modulation index, which is near to the performance using the real index.

Modulation index estimation for CPFSK signals and its application to timing synchronization

Continuous phase frequency shift keying (CPFSK) is a power and bandwidth efficient modulation. This paper proposes a novel method for carrier recovery in CPFSK demodulation. Its complexity is very low, and it is also easy to implement, especially for software implementation. The proposed method has an excellent bit error rate performance.

http://ieeexplore.ieee.org/iel5/7138/19221/00890915.pdf

Zhan Yafeng

Papers

Modulation index estimation for CPFSK signals and its application to timing synchronization

A novel carrier recovery method for CPFSK demodulation