Mathematical Problems in Engineering

Volume 2015 (2015), Article ID 364234, 15 pages

http://dx.doi.org/10.1155/2015/364234

## Network Coded Multicast over Multibeam Satellite Systems

Department of Telecommunications and Systems Engineering, Engineering School, Autonomous University of Barcelona, 08193 Barcelona, Spain

Received 23 February 2015; Accepted 27 May 2015

Academic Editor: Zhen-Lai Han

Copyright © 2015 R. Alegre-Godoy and M. A. Vazquez-Castro. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

We propose a multicast scheme for multibeam satellite systems exploiting both the multiuser and spatial diversity inherent in this type of systems while taking into account realistic physical distributions of User Terminals (UTs) over the coverage. Our proposed scheme makes use of the well-known Adaptive Coding and Modulation (ACM) feature in Digital Video Broadcasting over Satellite, 2nd Generation (DVB-S2) and Extension (DVB-S2X) standards but also incorporates a set of innovative features. First, multilink reception, that is, receivers that can syntonize different frequencies and/or polarizations, together with Network Coding (NC) is used to enable decoding of signals from adjacent beams (spatial diversity). Second, efficient and fair allocation of resources is achieved through Proportionally Fair Multicast (PFM) scheduling. Our results, obtained over realistic nonuniform UTs distributions, show average system multicast throughput gains up to 88% with regard to state-of-the-art multicast schemes. Furthermore, a complete cross-layer architecture is proposed, fully compliant with the standard providing Quality of Service (QoS) guarantees.

#### 1. Introduction

Recent studies are showing an increasing demand for the efficient distribution of personalized contents in Internet based networks [1]. This has led to the deployment of satellite platforms delivering high throughputs (HTS systems) such as Ka-SAT [2] or constellations of communication satellites such as the O3b system [3]. Recent works even consider collocating two satellites covering the same region in order to cope with the user needs [4, 5]. Beyond the aforementioned satellite physical aspects, it is also possible to satisfy the user needs by improving the logical mechanisms delivering multimedia contents. Multicasting is one of the cornerstones for the effective dissemination and distribution of personalized multimedia contents in broadband networks and the focus of this paper. Applications such as audio/video streaming, online gaming, file distribution, and file downloading are based on multicast-like transmissions.

In wireless networks, including multibeam satellite networks, the main challenge when multicasting is how to address the heterogeneous channels conditions of the User Terminals (UTs), that is, the presence of multiuser diversity. In clear-sky conditions, the difference in Signal to Interference plus Noise Ratio (SINR) between a UT located at the center of the beam and a UT located at the edge is typically 2-3 dB. When Adaptive Coding and Modulation (ACM) is adopted at the physical layer as in Digital Video Broadcasting over Satellite, 2nd Generation (DVB-S2) [6] and DVB-S2 Extension (DVB-S2X) [7] standards, this difference in SINR is translated into a spectral efficiency difference of 11%–25% [6, 8]. The differences in spectral efficiency can be much higher if we consider a beam partially affected by a rain event.

Traditional colouring schemes in multibeam systems allow many opportunities for exploiting spatial diversity. A UT can potentially access a number of orthogonal transmissions from the adjacent beams [8]. In that case, UTs in the border of the beam would be more advantaged than those in the center of the beam. Since current UTs are syntonized at a single frequency, orthogonal transmissions are not exploited. A multilink receiver, that is, a receiver able to syntonize different frequencies and polarizations, could access and decode the signal meant for adjacent beams. As mentioned in [8], the design of this type of receivers is perfectly possible with the current technology. UTs distribution also plays a fundamental role. In real life, users are not uniformly distributed but concentrated in specific areas such as cities. The concentration of terminals in specific areas of the coverage also affects the performance of cellular based systems [9, 10].

In this paper, we take into account these three aspects, multiuser and spatial diversity and UTs distribution to design a multicasting scheme for the efficient delivery of broadband contents.

*(a) Related Works on Satellite Multicast*. Multicast in multibeam satellite systems has been little investigated from the scheduling and resources allocation point of view. The authors in [11] propose to choose a fixed Reed Solomon code and a fixed rate out of a set of possible rates in order to accomplish a certain degree of reliability. Following a similar approach to [11], works in [12, 13] propose to choose a modulation and codification (MODCOD) which ensures reception to a subset of the UTs in the multicast group. The rest could only decode with a certain probability. In [14], a traditional approach is adopted and information is multicasted according to the channel conditions of the worst UT in the multicast group. With respect to these works, our approach takes advantage of multiuser diversity in order to select in each time-slot the optimal MODCOD rather than assuming a fixed scheme. To the best of our knowledge, the authors in [15] provide the most similar approach to our work since the scheme they propose is based on ACM. In particular, authors propose a Network Utility Maximization (NUM) to trade delay and rate also accounting for Quality of Service (QoS) and multiuser diversity. With respect to this work, we introduce the novelty of multilink reception UTs together with Network Coding (NC) which enables decoding orthogonal transmissions.

Furthermore, our paper breaks the traditional approach of assuming uniform UTs distributions and provides results and analysis for nonuniform distributions which are close to reality.

*(b) Related Works on NC for Multibeam Satellite Systems*. In the past few years, a number of works have studied the implementation of NC in multibeam satellite systems. In [16], an overview of possible satellite scenarios where NC can be applied is provided. For the particular case of multibeam satellite systems, NC is proposed as a mechanism to reduce retransmissions. Works in [17, 18] take advantage of the orthogonal transmissions available using multilink reception. However, the focus is on unicast transmissions and NC is used to provide enhanced reliability and flexibility rather than increasing the throughput. Our previous works in [19, 20] assessed the feasibility of using NC for multicasting in multibeam satellite systems. As a result, it was identified that the multilink reception approach together with NC coding could bring important benefits subject to the location of the UTs. These papers describe a preliminary concept and lack of a(i)method to decide when to use the multilink reception with NC feature,(ii)scheduling policy selecting the optimal MODCOD for the multicast service.

*(c) Contributions of the Paper*. This paper proposes a full multicasting scheme, that is, scheduling policy, packet scheduling architecture, and algorithm to decide if the multilink reception with NC feature must be used or not. Our work presents the following novel results with respect to use of NC technology:(i)A technique for the joint use of multilink reception, NC, and Proportionally Fair Multicast (PFM).And it also presents the following novel results in the field of satellite multicasting: (i)Introduce and adapt the PFM scheduling concept in [21]. More specifically, we provide MODCOD selection and use of the multilink reception with NC feature when suitable.(ii)Its associated cross-layer packet scheduling architecture with respect to the Internet Engineering Task Force (IETF) differentiated services at IP level model (Diffserv).(iii)A scheme providing multicast throughput gains employing the same resources as a traditional multicast scheme demonstrated via analysis and simulations over theoretical and realistic nonuniform UTs distributions.The rest of the paper is organized as follows: Section 2 introduces the multibeam satellite system model. Section 3 describes the proposed multicast scheme. The packet scheduling architecture is introduced in Section 4. Finally, Section 5 provides numerical evaluation of the system performance and Section 6 draws conclusions on the work done.

#### 2. System Model

##### 2.1. Multibeam Satellite System Model

We assume a multibeam and multigateway satellite system with beams, polarizations, and frequency reuse factor . The number of colours of the system is . Forward link transmissions are based on DVB-S2/DVB-S2X with ACM. Each gateway (GW) is associated with a subset of the overall number of beams (or cluster). GWs receive channel state information (CSI) messages from the UTs through a feedback channel.

Let a GW of the system serve the subset of beams . Each beam has assigned a number of UTs requesting the same multicast service. Let us derive the SINR for a UT in , . First, let the number of cochannel beams of the overall system be . Now, we define as the forward link channel matrix which can be decomposed as . Matrix accounts for the atmospheric, propagation, space, and ground system effects and is defined aswhere , with being the output back-off of the satellite high power amplifier, the satellite repeater losses, the propagation losses, and the UT antenna gain. Matrix accounts for the square root of the satellite antennas gains towards the concrete position of the UT and is defined aswhere stands for the square root of antenna gain for antenna towards the location of the UT at beam . Therefore, each element accounts for all the gains and losses from satellite antenna towards UT location at beam . The received signal at UT can be expressed aswhere is the satellite transmitted power, is the Gaussian noise (zero mean complex circular noise of variance ), and and are the transmitted and interfering symbols, respectively. Assuming constant transmitting power, the SINR can be extracted directly from (3) and is given byUnder the ACM specification of DVB-S2 and DVB-S2X, SINR values are mapped to spectral efficiencies (or equivalently MODCODs) as follows:where is the spectral efficiency for UT in and , are mapping functions that relate SINRs and spectral efficiencies for DVB-S2 and DVB-S2X standards, respectively.

##### 2.2. Multilink Reception System Model

The work in [8] introduces and models multilink multibeam systems. Such systems assume the use of multilink receivers, that is, receivers which can syntonize different frequencies or polarizations to simultaneously decode orthogonal transmissions from adjacent beams.

The main concept is as follows. In a multibeam system with colours, UTs can potentially decode up to transmissions, 1 transmission from the own beam and transmissions from adjacent beams in orthogonal frequencies and/or polarizations. This effect is produced because the antenna gain of each spot-beam is so high, that even UTs outside of the beam observe values of SINR that lie within the range of available MODCODs and can decode the signal. To do so,(i)UTs must have multilink reception capabilities, for example, a terminal with a single antenna, one Low Noise Block downconverter (LNB), and multiple reception chains to detect and decode different polarizations and bands;(ii)UTs must observe a value of SINR higher than or equal to the one required to decode the MODCOD transmitted in the orthogonal beam. Conversely, the GW can lower the MODCOD transmitted in a beam to let a number of UTs outside of the beam decode the signal.In our multicasting scheme, we assume that UTs can decode their own transmission and one out of the orthogonal transmissions, more specifically, the transmission with strongest SINR or equivalently the transmission from the closest adjacent beam. This enables an extra path to reach each UT and the opportunity to exploit spatial diversity. Let subscript denote the adjacent beam whose signal intends to decode UT in and let be the SINR observed from this adjacent beam. is obtained particularizing (4) with beam , that is, substituting with and computing the interference power from the cochannel beams of . The spectral efficiency achievable from such adjacent beam is denoted as and obtained particularizing (5) with .

Figure 1 shows the MODCODs achievable for different locations (in coordinates [22]) within a beam of a 70-beam system. More specifically, the top plot shows, per each location, the adjacent beam providing better SINR (the location and the adjacent beam with the strongest SINR are plotted in the same color and dashed lines separate the different areas). The mid and bottom plots show the achievable MODCODs from the own and determined adjacent beam transmissions, respectively. It can be observed that any point of the beam can decode an orthogonal transmission and that locations close to the edge of the beam and in the beam overlapping areas can decode it with a high order MODCOD. Locations in the center of the beam could only decode signals employing low spectral efficiency MODCODs.