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PreSim: Precoding Simulator

 

1.     Short Description

The Precoding Simulator (PreSim) is a full chain simulator, including all the blocks of a multi-beam satellite forward link, from the gateway (ground segment) to the satellite (space segment) and then to the users (user segment).

The main objective of PreSim is to include in a generic and modular simulation framework a number of precoding techniques, necessary to tackle the multi-user interference (MUI) arising in the multi-beam system, which realizes a full frequency reuse (FFR) scheme. The precoding techniques considered for the simulator range from the most classical channel-based approaches, such as zero-forcing (ZF) and minimum-mean-square-error (MMSE) precoding, to more sophisticated ones, such as frame-based precoding and symbol-based precoding. However, the modular structure of the simulator allows the incorporation of any precoding technique.

 

2.     Generic Structure of the Simulator

Fig. 1 displays the generic block scheme of the simulator. As visible, this is composed by three main parts: the ground segment, the space segment, and the user segment.

The blocks related to the ground segment model the overall transmission chain present at the gateway, from the generation of the bit streams to be transmitted to the actual shape filters. It is worth mentioning the presence of a block for the pilot sequences insertion, which has to be compliant to the super-frame structure of DVB-S2X standard. However, the most important block of this part is the precoder, managing the precoding operation.

The space segment part manages the on-board satellite processing. An important component is the travelling-wave-tube-amplifier (TWTA), whose non-linear characteristics constitutes a critical impairment in the overall communication chain. The amplification block is preceded by an automatic-gain-control (AGC) block, which applies a power back-off scheme.

After the satellite channel, modeling the MUI and the additive-white-gaussian-noise (AWGN), the user segment includes the blocks associated with the receivers. As visible, equalization schemes are applicable. The channel estimation block is essential in order to obtain the channel state information (CSI), to be feedbacked at the transmitter side and used for the precoder calculation.

 

3.     Sample Results

In this section some results obtained through PreSim are shown. More specifically, the reported results highlight the deterioration induced by the on-board non-linear amplification on the precoded system (using ZF precoding) with respect to the non precoded case.

                                                Figure 1: Block scheme of the simulator

 

Fig. 2 shows a comparison of the peak-to-average-power-ratio (PAPR) of the transmitted waveforms between the ZF precoded and the non precoded case. A 32-APSK modulation scheme is considered. The PAPR is plotted versus the number of transmitting feeds, and the curves are obtained both before and after the transmitting shape filters. In both cases the PAPR shows an increase when precoded is applied.

In Fig. 3 the transmitted constellation (32-APSK) is shown before and after the precoding block. The model of satellite channel considered for these results is based on Bessel functions. It is evident how the constellation is modified after precoding, and this explains the increase in the PAPR.

Finally, Fig. 4 shows a performance comparison, in terms of total degradation, between a multi-beam (7 beams) ZF precoded system and a single beam one. For these results, an equalization scheme is used in the user segment (in the corresponding block) in order to mitigate the distortion induced by the non-linear channel. It is apparent that the precoded system shows a higher total degradation and requires a higher output back-off (OBO). For the precoded system, the total degradation curve is shown also in absence of an equalization scheme.

           Figure 2: PAPR of 32-APSK signals vs. Number of transmitting elements.  

 

Figure 3: Tx constellations before (left) and after (right) precoding.

Figure 4: Total Degradation vs. OBO curves, comparing a  multibeam ZF-precoded system and a single-beam one

4.     Related Publications

 

-          D. Spano, D. Christopoulos, S. Andrenacci, S. Chatzinotas, J. Krause, B. Ottersten, “Total Degradation Analysis of Precoded Signals onto Non-linear Satellite Channels”, Ka and Broadband Communications Conference (KaConf), October 2015.

-          D. Christopoulos, S. Chatzinotas, B. Ottersten, “Weighted Fair Multicast Multigroup Beamforming under Per-antenna Power Constraints”, IEEE Transactions on Signal Processing, 2014.

-          D. Christopoulos, S. Chatzinotas, G. Zheng, J. Grotz, B. Ottersten, “Linear and nonlinear techniques for multibeam joint processing in satellite communications EURASIP Journal on Wireless Communications & Networking, 2012.

-          D. Christopoulos, S. Chatzinotas, B. Ottersten, “User scheduling for coordinated dual satellite systems with linear precoding,” in Proc. of IEEE Int. Conf. on Commun (ICC), 2013.

-          D. Christopoulos, S. Chatzinotas, B. Ottersten, “Frame Based Precoding in Satellite Communications: A Multicast Approach”, Proc. of IEEE, Advanced Satellite Multimedia Systems Conference, 2014.

-          G. Zheng, S. Chatzinotas, B. Ottersten, “Generic optimization of linear precoding in multibeam satellite systems,” IEEE Trans. Wireless Commun., 2012.

-          M. Alodeh, S. Chatzinotas, B. Ottersten, “Constructive Multiuser Interference in Symbol Level Precoding for the MISO Downlink Channel”, IEEE Transactions on Signal Processing, 2015.

-          M. Alodeh, S. Chatzinotas, B. Ottersten, “Energy Efficient Symbol-Level Precoding in Multiuser MISO Channels”, 16th IEEE International Workshop on Signal Processing Advances in Wireless Communications, 2015.

-          M. Alodeh, S. Chatzinotas, B. Ottersten, “Symbol Based Precoding in the Downlink of Cognitive MISO Channel”, CROWNCOM 2015.

-          M. Alodeh, S. Chatzinotas, B. Ottersten, “A Multicast Approach for Constructive Interference Precoding in MISO Downlink Channel”, ISIT 2014.

-          M. Alodeh, S. Chatzinotas, B. Ottersten, “Data Aware User Selection in Cognitive Downlink MISO Precoding Systems”, International Symposuim on Signal Processing and Information Technology, 2013.

-          A. Gharanjik,  B. Shankar, P.D. Arapoglou, M. Bengtsson, B. Ottersten, “Precoding Design and User Selection for Multibeam Satellite Channels”, SPAWC 2015.

-          A. Gharanjik,  B. Shankar, P.D. Arapoglou, M. Bengtsson, “Robust Precoding Design for Multibeam Downlink Satellite Channel with Phase Uncertainty”, ICASSP 2015.

-          R. Piazza, B. Shankar, T. Berheide, M. Graesslin, S. Cioni, “Performance Analysis of Fractionally Spaced Equalization in Non-linear Multicarrier Satellite Channels”, AIAA 2014.