High throughput satellites in 5 G and MIMO interference limited communications

Classically, thermal noise has been the workhorse of satellite communications due to the long distances to be covered between the satellite and the user terminal (UT). Lately, LDPC (Low-Density Parity-Check) codes allow the noise threshold to be set very close to the Shannon limit for the memory-less satellite channel; thus, solving the noise problem that turbo codes were not able to solve. However, recently, the high target rates in next generation 5G wireless terrestrial system are pushing the required spectral efficiency in Satellite Communications; therefore, shifting the SatCom paradigm towards an interference limited one. This paper revisits the 5G scene and the role of next generation satellite communications, with a special focus on high throughput satellites (HTS) together with the future accompanying MIMO interference mitigation techniques.


Introduction
5G is conceived as a composition of: i) enhanced mobile broadband; ii) ultra-reliable and low latency communications; iii) massive machine type communications [1].However and more importantly it is seen as a new infrastructure that will bring a new unique network and service capability to provide an order of magnitude improvement in different key performance indicators.While too much focus has been set so far in urban scenarios, the 5G ecosystem should continue to adapt their mobile use cases and deployments.Emerging use cases that have been identified are: rural infrastructure, high bandwidth because of many users, but low bandwidth to each one, broadband mobile connectivity.The satellite component can have many things to say in the use cases aiming at helping to bridge the digital divide in parts unreachable by terrestrial, whilst optimizing the value for money to the end-users [2][3].Ubiquitous coverage, together with resilient overlay communications and disaster relief are the three main assets that satellite can complement terrestrial communications.
Satellite orbits can be classified mainly into three main types: Geostationary (GEO) orbits (36.000 km above the equator), Medium earth orbits (from 6000 to 25000 km altitude) and Low earth orbits (<1500 km altitude).Whenever these orbits are combined, hybrid orbits arise.Concerning the services they provide, Fixed Satellite Services (FSS) operate at C band (3)(4)(5)(6), whenever extreme wide coverage and/or resilience to bad weather conditions are needed.This band was the first to be used and currently FSS are moving to Ku band (12)(13)(14)(15)(16)(17)(18) and Ka band (26.5-40GHz), mainly to provide backhauling services to terrestrial operators.Mobile Satellite Service (MSS) operate primarily in L band (1-2 GHz) and S band (2-4 GHz).They offer services to both handheld and vehicle mounted terminals as well as some fixed terminals.In the past the air interfaces and networks have been mainly proprietary (e.g.BEGAN for MSS) and recently satellite systems have adopted standards within the DVB-S, ETSI and ITU.Either in FSS or in MSS, the supported services by SatCom are not limited to basic data speeds (with a 5G target of up to 1Gbps in average per user for mobile broadband) and voice services, but also for machine type communications, broadcast and narrow band services (up to 100 Kbps per user in machine type communications).
So far, the major business and focus are: FSS-DTH (Direct To Home) for new High Definition (HD), Ultra HD, 3D TV.Another important market is IP trunking to connect parts of the backbone, either for internet or for VoIP, at 310 Mbps.Residential broadband is another important data communication application with 2.20 Mbps for the end user that are evolving towards higher rates.Cellular backhauling for connecting cellular radio towers to the core network are currently working at 2 Mbps and, finally, corporate and public VSAT for connecting a site to the IT network of a private or public organization.These applications are carried out through GEO satellites and, while C-band use keeps flat in forthcoming years, Ku and Ka band use increases.

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