A regenerative payload for satellite multimedia communications

We describe a large-capacity on-board processing payload, designed for next generation satellite systems. It provides broadband multimedia services to mobile and fixed users, and assigns data rates ranging from a few Kbps up to 2 Mbps for an aggregate traffic capacity of about 9 Gbps. This regenerative payload is the key element of the EuroSkyWay communication system     

Cost-effective networking via digital satellite communications

This paper presents the major issues in forming a cost-effective and efficient network from a set of earth stations sharing the satellite resource. The discussion is in the context of satisfying the performance requirements for transmission of data, voice, and image presented to earth stations in digital form. Multiple access techniques have significant impacts on the network economy. Although wide-band single-carrier time-division multiple access (TDMA) uses the satellite capacity efficiently and is cost-effective for networks with large traffic volumes per node, it requires expensive earth stations and is not cost-effective for networks with small traffic volumes per node. The concept of multicarrier-TDMA (MC-TDMA) is described and its economic advantage over TDMA is demonstrated for networks with dispersed points of traffic origination. Application of demand assignment to MC-TDMA is also presented. Some implementation issues are also discussed.     

Communication satellite processing repeaters

Two kinds of processing repeaters which may find application on communication satellites in the near future are described. The type I repeater allows access only if the transmitted signal contains a predetermined code structure. This serves three purposes: first, unauthorized users are excluded, second, co-channel interfering signals are not retransmitted on the downlink, so as not to waste a portion of the satellite transmitter power, and, third, removal of the interference signal at the satellite avoids having to do this at the ground receiving terminal. Thus such a repeater would find particular application where there are a large number of ground receiving stations. The type 2 repeater routes signals received at its N input terminals to be transmitted at N output terminals, thus acting as a "switchboard in the sky." Two purposes are served: first, the satellite capacity is more fully utilized in the presence of fluctuating traffic demands, and, second, single-frequency transmission and reception are possible for user ground stations, thus simplifying these stations and still allowing communication to any station in the network. It is established when the type 1 repeater is able to increase the satellite communication capability beyond that of a simple repeater and further that fairly simple filtering is sufficient on the satellite. For the type 2 repeater, a proposed frequency control plan minimizes the filtering required on the satellite where frequency division multiplex is used and reduces the amount of switching required on the satellite to N single-pole N-throw switches where time division multiplex is used without the requirement of any memory on the satellite. A summary of present-day translating repeaters, as used in the Intelsats III and IV and DSCS II satellites, is included.     

Automatic timing alignment for regenerative repeaters

A novel circuit technique that provides automatic timing alighnment of digital regenerative repeaters is described. The scheme employs correlation feedback to maintain the correct clock and data phase relationship. Theoretical and practical results are presented.     

A digital transmission system for global maritime satellite communications

A digital transmission system for global maritime satellite communications has been designed, and experimental communications equipment has been developed, taking account of its potential application to the INMARSAT system where analog modulation is currently used for telephone signal transmission. This paper discusses possible digital technologies to realize an efficient digital transmission system, and presents a concept for the designed system. The paper also describes the hardware configuration and the performance characteristics of the experimental equipment.     

Low complexity concatenated coding schemes for digital satellite communications

A study of reduced complexity concatenated coding schemes, for commercial digital satellite systems with low-cost earth terminals, is reported. The study explored trade-offs between coding gain, overall rate and decoder complexity, and compared concatenated schemes with single codes. It concentrated on short block and constraint length inner codes, with soft decision decoding, concatenated with a range of Reed-Solomon outer codes. The dimension of the inner code was matched to the outer code symbol size, and appropriate interleaving between the inner and outer codes was used. Very useful coding gains were achieved with relatively high-rate, low-complexity schemes. For example, concatenating the soft decision decoded (9,8) single parity check inner code with the CCSDS recommended standard Reed-Solomon outer code gives a coding gain of 4.8dB at a bit error probability of 10^?5, with an overall rate of 0-78.     

Intermodulation distortion in frequency multiplexed satellite repeaters

Expressions have been derived that describe the output spectra (i.e., signal plus distortion) of a saturating amplifier when the input consists of a large number of independent carriers whose amplitudes, bandwidths, and center frequencies may be independently specified. The resulting equations have been programmed on the GE 635 computer using the fast Fourier transform algorithm, permitting rapid computation of the output spectra for very complex input signal formats. Program outputs have been checked against the results of previous authors, e.g., Cahn [1] and against a measured transfer characteristic of a Hughes model 24H traveling wave tube, with good agreement. The program has been found very useful in design optimization studies of channelized communications repeaters with up to 60 channels.     

On increasing the down-link capacity of a regenerative satellite repeater in point-to-point communications

A suggestion is presented for increasing the down-link capacity in a satellite communication system by using a regenerative satellite repeater. This idea is based on Shannon's work on the capacity of two-way communication channels. A possible implementation of the idea is outlined, based on TDMA operation, demodulation and storage at the satellite, and modulo 2 addition of the baseband binary streams.     

A modular multistage approach to digital fdm demultiplexing for mobile SCPC satellite communications

A hierarchical multistage method (HMM) for digital demultiplexing of an FDM signal composed of L adjacent SCPC signals is described; L is preferably a power of two, here L=32. This HMM approach to FDM demultiplexing applies bandpass sampling and is based on the processing of complex-valued signals by linear-phase FIR filters, where at any stage of processing the respective signals are always oversampled by two. The simulation results fully confirm the predicted system performance. An electrical demonstration model constructed by cascading six identical specially designed signal processors is being built.     

The use of digital circuit multiplication equipment in satellite communications

The use, within satellite communications, of low rate encoding (LRE) techniques, based on 24, 32 and 40 kb/s ADPCM coding, coupled with digital speech interpolation (DSI) to form a digital circuit multiplication equipment (DCME), is addressed in this paper. The need for a system simulation tool, in order to plan for and correctly use the DCME concept is identified. Results obtained with this simulation tool are presented. The simulation model makes it possible to predict the behaviour of the system from a quality point of view, with external conditions simulated to be very close to actual operating conditions.     

Advanced RF switching and on-board regeneration technologies for digital satellite communications

In this paper, functional blocks of a generic on-board processing payload are identified, and some key technology implementation issues for on-board RF and/or baseband switching, signal regeneration and processing are addressed. The design and performance characteristics of a microwave switch matrix and a 120 Mb/s coherent quadrature phase shift keying demodulator for satellite applications are presented. This hardware is realized with monolithic microwave integration technology. Significant performance improvement and reduction in size and weight, as well as enhanced subsystem reliability, have been demonstrated, making these technologies attractive for flexible communications satellite payload architectures of the future. Transmission analysis is performed to quantify link budget advantages that could be obtained by using on-board regeneration.     

In-orbit testing of digital regenerative satellite: The italsat planned test procedures

ITALSAT is the Italian pre-operational satellite, funded by the National Council of Research, to be launched in late 1989. It will carry three payloads, the most innovative and challenging being a multi-spot regenerative package providing the interconnection among six spots at 147Mb/s with SS TDMA access scheme. In-orbit tests (IOT) will be performed on this payload in order to verify, to the maximum possible extent, the radioelectric characteristics and the behaviour of the modems and baseband switching equipment. The importance of these measurements is two-fold: incentive payment schemes can be applied for better-than-specified characteristics and the information obtained by the tests will permit gathering the information needed for the establishment to a domestic operational system. The non-transparent structure limits applicability of many classical RF measurements: the opportunity of adding some on-board hardware to permit the testing of the most important parameters has been suggested. The design objectives have been to minimize the on-board additional hardware complexity and to concentrate the test equipments on ground, in the IOT earth-station. This paper illustrates the planned additional satellite hardware and the test procedures to be implemented for the in orbit testing.     

A DTN approach to satellite communications

Satellite communications pose serious challenges to transport layer performance, mainly because of long propagation delays (especially in geosynchronous systems) and the possi ble presence of random errors on the satellite link. Solutions that cope with these impairments usually rely upon either the adoption of enhanced versions of transport protocols, or the insertion of intermediate agents, like PEPs (Performance Enhancing Proxies). An alternative approach is to adopt the DTN (Delay/Disruption Tolerant Networking) architecture based on the introduction of the new "bundle" layer in the protocol stack. The aim of the paper is to discuss the advantages and disadvantages of the DTN approach compared to the more conventional solutions mentioned. DTN performance is assessed by considering both fully connected networks and networks where continuous connectivity cannot be guaranteed during data transfer and/or at start-up. Performance is evaluated by using the TATPA testbed (Testbed for Advanced Transport Protocols and Architectures), which is based on a cluster of Linux PCs running the DTNperf application, developed to this end by the authors and now included in the official DTN package. Comparative results show that DTN, coupled with TCP Hybla, outperforms NewReno, offering a performance very close to PEPsal and end- to-end Hybla, which are among the most effective solutions on satellite channels. In addition to these encouraging results, DTN offers an intrinsic greater robustness in intermittent or disruptive environments.     

Future satellite communications to military aircraft

This paper outlines some of the techniques being developed to provide affordable, reliable satellite communications suitable for a wide range of military aircraft, from agile platforms such as fast jets and helicopters to surveillance, tanker and transport aircraft. It gives an overview of the development of airborne SHF and also extremely high frequency (EHF) satcom techniques and terminal demonstrators by the DERA (Defence Evaluation and Research Agency) and its partners through the Ministry of Defence's applied research programme. This research is aimed at providing affordable, secure and robust satcoms to a range of UK military aircraft     

Optical technologies for signal processing in satellite repeaters

The evolution of modern communications satellites from the older cable-in-the-sky concept toward more intelligent architectures, exploiting onboard processing (OBP) techniques involving various technologies to improve the system performance and flexibility, is addressed. The key components in advanced communications satellite repeaters compatible with integrated optical device implementation are presented. The relevant device technologies and fabrication techniques are examined, and integrated optical circuit configurations that can be applied to OBP are described with reference to recent experimental data. Particular attention is given to optical beamforming networks. Areas for further research and development are suggested     

Application of 32 and 16 kb/s speech encoding techniques to digital satellite communications

This paper describes the performance of various voice encoding techniques at 32 and 16 kb/s for applying to digital satellite communication systems. The subjective performances of adaptive differential PCM (ADPCM), adaptive predictive coding (APC), subband coding (SBC) and adaptive delta modulation (ADM) are compared under various satellite channel environments, that is, random and burst channel errors in satellite link and an ambient noise in the ship-to-shore direction in a maritime satellite channel. The performance of the voiceband data at 4·8 and 2·4 kb/s is also evaluated for these coders. ADPCM encoding at 32 kb/s is very attractive for conventional fixed satellite systems, keeping the equivalent quality to 64 kb/s PCM. On the other hand, APC encoding at 16 kb/s is also most suitable for maritime satellite communication systems at the sacrifice of a small degradation of speech quality.     

Mobile satellite communications

The paper presents an up-to-date overview as well as new results on mobile satellite communications. Specifically, the following subjects are addressed: worldwide activities; satellite orbits; multiple access techniques; Novstar global positioning system; economic and regulatory issues; future trends. Some of these technical and nontechnical subjects appeal disjoint, but they are unique and significant among the many facets, which effect the development and operation of mobile satellite communication systems     

Antenna and receiver system with digital beamforming for satellite navigation and communications

To demonstrate and investigate the possibilities of active antennas with digital beamforming, a compact breadboard model operating in C-band with planar patches as radiators is fabricated and measured. It was originally designed as a technology demonstrator for navigation purposes, but will be extended for future use in combined navigation and communication systems. Several parameters for beam position, sidelobe reduction, and ing are set and controlled by a suitable user interface. A special calibration and error-correction procedure is developed to compensate for drift of active elements, mutual coupling of the patches, and finite-ground plane effects. The excellent results demonstrate the versatile features of digital beamforming for satellite navigation and communication systems.     

On-board satellite "split TCP" proxy

Several satellite systems currently in operation or under development claim to support broadband Internet applications. In these scenarios, transmission control protocol (TCP) plays a critical role. Unfortunately, when used with satellite links, TCP suffers from a number of well-known performance problems, especially for higher data rates and high altitude satellites with longer delays. In response to these difficulties, the satellite and Internet research communities have developed a large gamut of solutions ranging from architectural modifications to changes in the TCP protocol. Among these, one approach requiring minimal modifications involves splitting the TCP connection in two or more segments with one segment connecting terrestrial nodes across the satellite network. In this paper, we consider an evolution of this idea: placing a TCP proxy on board the satellite that further subdivides the end-to-end connection into separate TCP connections between ground and space. We focus upon the efficient use of buffer resources on board the satellite, while at the same time enhancing TCP performance. We evaluate two TCP protocol versions, TCP NewReno and TCP Westwood. We consider various geosynchronous earth orbit satellite scenarios, with and without the split proxy, and with different channel error conditions (random errors, shadowing, etc.). Using simulation, we show that an on-board proxy provides a number of distinct advantages and can enhance throughput up to threefold for both TCP New Reno and TCP Westwood, in some scenarios, with relatively modest on-board buffering requirements. The main contributions of this paper are: the on-board split proxy concept, the buffer management strategy, the use of a realistic "urban shadowing" model in the evaluation, and the extensive comparison of the recently announced TCP Westwood with the traditional TCP New Reno.