Radio resource management across multiple protocol layers in satellite networks: a tutorial overview

Paolo Barsocchi, Nedo Celandroni, Franco Davoli, Erina Ferro, Giovanni Giambene, Francisco Javier González Castaño, Alberto Gotta, Jose Ignacio Moreno, Petia Todorova
2005 International Journal of Satellite Communications And Networking  
Satellite transmissions have an important role in telephone communications, television broadcasting, computer communications, maritime navigation, and military command and control. Moreover, in many situations they may be the only possible communication set-up. Recent trends in telecommunications indicate that four major growth market/service areas are: messaging and navigation services (wireless and satellite), mobility services (wireless and satellite), video delivery services (cable and
more » ... lite), and interactive multimedia services (fibre/cable, satellite). The major drawback when using geostationary satellites (GEO) is the long delay, which can have a great impact on the end-to-end delay user requirements. Moreover, atmospheric conditions may seriously affect the data transmitted via satellite. Since the satellite bandwidth is a relatively scarce resource compared to the terrestrial (e.g., the optical transport networks), and the environment is harsher, resource management of the radio segment assumes an important role in the system's efficiency and economy. This tutorial aims to give the basic elements of telecommunications via GEO satellite, with emphasis on the issue of radio resource management. The low earth orbit (LEO) satellite constellations are briefly discussed. Currently, after the IRIDIUM service disaster, LEO constellations are no longer a hot research field; however, in the future they may again play an important role. Aspects considered will include basic transmission and multiple access techniques, channel modelling and fade countermeasures, and their performance analysis done by means of theoretical, experimental, and simulation tools. lower layers depend. Immediately under the application layer, the transport layer must manage allocation problems to satisfy the upper levels' requirements. Nowadays, the TCP/IP stack is so widespread that TCP is the most popular transport protocol; whatever TCP version one uses, it may work inefficiently on a satellite link, due to the effect that the long satellite round-trip time (RTT) has on the congestion window. Our work does not focus on TCP over satellites, but we certainly cannot avoid mentioning it, since it has a great influence on the MAC layer, which runs the algorithms for the bandwidth resource allocation. We will not explicitly deal with the QoS solutions at the network layer, as they are not specific for the satellite environment; nevertheless, there is an interaction in mapping, for instance IP QoS classes onto DVB (Digital Video Broadcasting) classes. The MAC layer must manage the type of access to the satellite link, and the algorithms for utilizing the common resource (i.e., the channel capacity). Most of those algorithms implement some optimisation of the bandwidth allocation, which influences the TCP goodput. Call Admission Control (CAC) is also managed at the MAC level, being itself a way to block requests that cannot be satisfied, according to the QoS specified by the application. At the physical link level, most of the games have already been done, in the sense that, optimised or not, a stream of bits must be transmitted over a satellite link, which is often affected by signal fade due to bad atmospheric conditions. Nevertheless, the original QoS of the data, which at this level is expressed in terms of Bit Error Rate (BER), must be maintained as much as possible. The physical link must deals with several problems, such as the signal fade, which must be counteracted by means of techniques that are transparent to the upper layers, but which interfere with the MAC layer, in the sense that there must be a strong correlation between the two layers in order to select the best countermeasure. In LEO satellite constellations the handoff problem has to be faced as well, as shown in Section 6. Section 2 of this paper briefly presents the satellite networks and the broadband satellite scenario requirements. Section 3 deals with QoS requirements of applications that may use a satellite network. Sections 4, 5 (which are the core Sections), and 6 cover aspects pertaining to the transport, data link and physical layer, respectively. The survey in this paper is far from exhaustive: rather, the goal is to touch upon some of main aspects related to resource allocation in this specific environment for the purpose of QoS management. For the reader's convenience, references are cited in the form [x.y], where x is the number of the Section referred to by y. Digital satellite networks The advantages of combining the high bandwidth, wide area coverage, reconfigurability, and multicast capabilities of satellites with terrestrial networks offer vast new market opportunities. In those areas where terrestrial high-bandwidth communications infrastructure is impractical or non-existent, satellite communications may be the only solution. It is expected that the satellite component will play an important role in the universal delivery of third-generation wireless multimedia services, due to the large coverage area offered. The satellite component can be used to complete the coverage of the terrestrial network in areas 3 where deployment of the latter would be uneconomical or technically infeasible. The UMTS (Universal Mobile Telecommunications System) integrates cellular, cordless, and paging technologies. Integration of the Satellite-UMTS (S-UMTS) component with the third-generation (3G) terrestrial mobile networks is regarded as a key factor for the success of the system, as it poses a valuable complement to the terrestrial UMTS network. S-UMTS is currently in an advanced standardization phase. It is expected to have the same set of features as terrestrial UMTS (T-UMTS) [2.1], in particular concerning the channel allocation process. However, the actual allocation techniques will have to be adapted to the final specifications about the frame length, the allocation frequency and, more generally, to the chosen transmission scheme and its constraints. Moreover, the actual channel allocation policies will have to cope with satellite delays, frequent handoffs, and channel impairments, in order to maintain the required QoS. Some satellite characteristics can be utilized for new services or result in satellite-specific QoS classes (e.g., wide area coverage). The major drawbacks of satellite communications are the high propagation delay, due to their altitude, and the SNR (signal to noise ratio), which can dramatically decrease with adverse atmospheric conditions, particularly in the Ka band, i.e., at frequencies above 14 GHz. Taking into account that different constellations and orbits can be envisioned for the exploitation of S-UMTS services, several considerations regarding the choice of satellite constellation can be made in terms of the characteristics and geographical locations of the targeted users, as well as the desired services. In particular, geostationary satellites can be proven to be the right choice for providing complementary services over regions already served by T-UMTS networks. On the other hand, constellations of satellites in Low or Medium Earth Orbit (LEO or MEO) might prove to be better suited to collect the traffic of users evenly distributed over the entire globe, such as sea vessels and airplanes [2.2]. Geostationary satellites orbit the earth at an altitude of about 36,000 km. This embeds a typical figure of about a quarter-second round-trip (uplink plus downlink) propagation delay in a communication system. Many potential customers of a future global communications system would not tolerate these delays, for instance in voice applications. The performance of protocols with acknowledgements and a time-out-based congestion control mechanism, e.g., TCP, is inherently related to the delay-bandwidth product of the connection. If this product is high, as it is in GEO satellite links, a non-negligible packet loss due to data corruption may cause significant performance degradation. In fact, TCP is unable to distinguish between congestion and corruption losses; thus, any lost packet causes a reduction in TCP packet sending rate, even if the link is not congested [2.3]. In order to reduce the BER (and then the packet loss due to corruption), the employment of FEC (Forward Error Correction) coding is mandatory in satellite transmissions; this allows trading bandwidth for data reliability, as well as a certain gain in power. LEO satellites travel at altitudes ranging from 700 to 2,000 km above the earth. They have the potential to be successful in future global telecommunications systems, due to their greatly reduced propagation delays as well as their ability to communicate with less powerful earth terminals. LEO systems also yield smaller satellite cells, allowing greater frequency reuse and hence higher capacity.
doi:10.1002/sat.820 fatcat:lqvugiz2qraoxmt6afiozk7gr4