The objective of the project is the design, development and validation of key prototype elements for advanced return link (R/L) access techniques that can significantly improve the throughput and/or reduce the transmission delay with respect to currently adopted uplink access schemes. More specifically, the detailed objectives are to:
The challenges posed by the project are the following:
The benefits provided by the testbed consist in a practical demonstration of the performance of the selected multiple access scheme as well as a proof of its practical feasibility.
The main elements of the testbed are a receiver node and a number of transmitting nodes. The receiver node implements the decoding algorithms foreseen by the selected method. One of the transmitting nodes implements the algorithm that is run at the user terminal side, so as to prove the feasibility of both the transmitter and the receiver side.
The system architecture to be developed corresponds to either a random access (RA) system or a dedicated access (DA) system, depending on the result of the simulation campaign that is carried out during the first phase of the project. The RA system is the multi-frequency Asynchronous Contention Resolution Diversity ALOHA (MF-ACRDA). In ACRDA each terminal transmits a number of instances of its message in an uncoordinated way to the satellite. At the ground station, the incoming signal is sampled at a rate higher than the Nyquist rate (oversampling) and the samples are stored. The demodulator operates on the samples covering a decoding window with duration equal to W virtual frames (VF). Each VF has duration . The demodulator starts locating the unique word (UW) (same for all packets) using a correlator. Even in case no clean burst headers are found, the demodulator may still be able to locate some of the UW’s thanks to the power unbalance among the signals. If a UW is detected, the demodulator performs channel estimation and tries to decode the message contained in the burst. If the cyclic redundancy check (CRC) is passed, the packet is declared decoded. At this point the information bits are re-encoded, re-modulated and, after a data-aided channel estimation refinement, the channel effects are included. At this point a noise-free copy of the received burst is available at the receiver. The demodulator uses such replica to remove all the instances of the decoded packet. The information about the location of such replicas is embedded within the header of each of the replicas. Such information uses the transmitter’s local frame as a reference. The process is iterated until either a maximum number of iterations are run or no header can be detected. At this point the decoding window is shifted by a fraction of the VF duration . In this way the oldest samples within the old window are discarded while new samples are included in the new window. At this point the header detection starts again and the iterative decoding carries on. Figure 1 exemplifies some of the concepts just described in a single carrier setup. The same concepts can be easily extended to the case of an MF system.
Figure 1: Example of ACRDA scheme, received signal. The local frames of 5 teminals are shown with two replicas in each. The decoding window at step s and at step s+1 are also shown.
The DA system is a Multi User Detection (MUD) scheme employing spatial Minimum Mean Square Error Successive Interference Cancellation (MMSE SIC). In such scheme all terminals transmit on the same frequency band. The signal transmitted by each of the terminals is picked up by the satellite’s multiple antenna beams. Such setup can be seen as a distributed Multiple Input-Multiple Output (MIMO) system. In order to recover the signals of all terminals, the receiver applies a spatial MMSE-SIC algorithm on the signals received on the different beams. The system and the MUD algorithm are exemplified in Figure 2 below.
Figure 2 Multi beam antenna system and block diagram describing the Spatial MMSE-SIC
The project is divided into two phases. The first phase consists in the study and selection of an advanced multiple access scheme based on a compliancy matrix and a simulation campaign. The second part of the project is dedicated to the SDR implementation of the selected scheme. The main milestones are:
The project is currently in the second phase. Based on the outcome of the first phase the RA scheme has been selected for implementation. The prototype implementation is currently being completed, with the TRR meeting to take place in April.