Designing Satellite Transponder To Support High Data Computer Science Essay

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Satellite Transponders, though robust in nature, like any other communication infrastructure, has not been spared from evolving in sync with the ever changing technologies. The evolution of existing satellite systems, fuelled by the desire for better communication systems such as, broadband services, interference free transmissions, high speed internet, etc, has seen many satellite technologies race towards the Next Generation Network (NGN) infrastructure concept. Emerging applications such as broad-band internet, satellite-HDTV, live video, telemedicine, interactive gaming, video conferencing, and others have not only strained the existing telecommunication infrastructure and hence the new developments towards higher frequencies, but have also placed a heavy burden on designers for purer (interference free) transmissions. Limitations do exist in current Interference Suppression methods in tracking and tackling the interference problem; hence this project seeks to address the issues mentioned above and provides solutions to minimise interference, support broadband networks and IP services. This project proposes two ways of tackling the above mentioned problems namely: (1) by way of a proposed Improved Two-Stage Adaptive Interference Suppression method module, which comprises, basically of a Threshold calculator and a set of Interference Canceller (notch/clipper) combination in feed forward cascade connection (fig 3) for the Adaptive Interference Suppression Method using Transform Domain Approach in an On-Board Filter Bank for Satellite Communications, (2) by proposing to incorporate a small optical transponder unit within the rate conversion circuitry of the transponder to enhance data transfer and transmission (Fig 4).

KEY WORDS

Propagation Impairments; Mitigation Techniques; Satellite Transponders; adaptive interference suppression; optical transponder;

INTRODUCTION

It is necessary that the satellite transponder has the ability to suppress interferences and any other incidental noises for reliable communications in real applications; hence they are required to overcome intentional and unintentional interferences in order to maintain communication links [1]. Though robust in nature, like any other communication infrastructure, the need for progressive change is a must lest they be rendered obsolete in this ever changing environment. Transmitting data at high speeds has become fashionable hence low uploads and downloads are a sign of old and archaic systems. This project is divided into: (I) Summaries from at least six journals, (II) Existing suppression technologies, (III) The proposed Improved Two-Stage Adaptive Interference Suppression method and (IV) Incorporation of an Optical Link (V) Conclusions.

(I) JOURNAL SUMMARIES

Six journals dealing with transponders in general were used as reference literature. The following are summaries of these journals which formed the core of our references:

Communication Issues in Satellite Links: A comprehensive survey (Dec 2008) pages 1-6.

This paper explored and focused on issues that dealt with developments in multiple spot beams by transponders along with power allocation strategies and implementation of routing protocols in satellite networks with fully processed payload architectures [2]. Communications issues that related to the establishment of satellite links such as: Transponder design, Power allocation strategies, Satellite Constellations and their limitations were also discussed. However, the paper did not address issues of high data rate satellite link to provide broadband services. It was observed that multicast tree update intervals for improved throughput needed further investigation [2].

Transponders: Effectiveness of Propagation Impairments Mitigation Techniques at Q/V Band (2008) pages 1-6

The paper presents, in part some selected applications for Q/V band, both potential and experimental ones. Then, the applications of some PIMT techniques to a specific Q/V-band satellite communication scenario are discussed and some initial choices for their application outlined. The main focus of the paper was to study and design a payload to be used to fully characterize the channel at Q/V bands and to test novel adaptive interference and fading mitigation techniques such as ACM (Adaptive Coding and Modulation). Frequency scaling error at Q/V band was found to be difficult to predict and Tx-Site Diversity very complex to perform [6]. This paper did not identify the difficulty of realization of physical components to support Q/V band.

Adaptive Interference Suppression Methods Using Transform Domain Approach in an On-Board Filter Bank for Satellite Communications.

This paper, presented a proposed adaptive interference suppression methods (transform domain) for each sub-channel signal in the on-board filter bank with their related system designs and concrete structure. Three kinds of algorithms to compute the threshold level adaptively were evaluated with notch filter and clipper under various interfering conditions. The notch filter and the clipper were combinations with the three algorithms, giving a total of six combinations, were used as suppression methods and evaluated in this paper [1].

One of the advantages of employing adaptive algorithms in the transform domain through the FFT/IFFT, as opposed to adaptive algorithms of the time domain, such as least mean squares (LMS) algorithm, for the above method, was their ability to respond to fast rate system and the relative ease of removal of the out- of - bandwidth components [1]. The proposed interference suppression methods also has the ability to calculate the threshold level adaptively in the threshold calculator depending on the interfering condition; the interferences being removed by the interference canceller composed of a notch filter and a clipper. The proposed strategies were evaluated using Monte-Carlo simulations under several interfered environments [1]. This paper although it highlighted interference issues, did not address the problems that relate to high speed broad band services in satellite networks.

Transponders: Research and Analysis for the Development of Telecommunication Payloads in Q/v (2009) pages 1-11

The paper focuses on identifying pre-operative experimental missions aimed at fully verifying the feasibility of future Q/V bands satellite telecommunication applications and the minimization of implementation risks for operative system characterized by a series of technological challenges. The major challenges highlighted in this paper were the realization of suitable Q band HPA with 32 W output RF power level and a Q/V band Tx/Rx single beam Antenna which required a specific development and qualification campaign [5]. This paper, though proposing a process for realization of Q/V band for satellite communications, did not address interference issues in satellite networks that relate to the use of Q/V transmission.

AMERHIS Next Generation Global IP Services in the Space (2010) pages 169 - 176

The paper gave a Special treatment to AMERHIS generation satellite systems, covering a description of AMERHIS satellite systems family. The focus being the production of a single multi-task On-Board Processing (OBP) system that incorporates a whole family of solutions for Global IP services in the sky. The key feature of AmerHis was that the payload was regenerative and provided unique connectivity possibilities via its 'switchboard in the sky' functionality. The need to reduce complexity, provide high speed data transmission at competitive prices for network access services was observed as a real challenge [4]. This paper did not address interference issues and high frequency band (Q/V) support.

Small Optical Transponder for Small Satellites (2010) pages 558 -561

The focus in this paper was to develop high-data-rate space optical communications link from small satellites to ground using Small on- Board Optical Transponder. The development concept of the SOTA (Small Optical Transponder) and its application to small satellites were proposed; recent trends of the data rates of small satellites are covered and a description of the requirements for the optical communication systems given. The data rate between the SOTA and an optical ground station (OGS) and the configuration of the optical communication system and issues related to small optical communication systems onboard small satellites are discussed. Noted also were Transponder weight limitations, antenna size, acquisition and tracking problems [3]. This paper though addressing the issue of high data rates, did not address the other issues of interference.

(II) EXISTING SUPPRESSION TECHNOLOGIES

Various methods to minimise the effects of interference have been studied including the one mentioned in (3) above; the majority of which have been implemented in real time situations. The following are therefore brief discussions of a few of them.

Noise Reduction and Echo Cancellation System

Noise reduction and echo cancellation fundamentals were dealt with in a previous paper [7]. In this paper two fundamental problem in hand free telecommunications systems were identified as: echo cancellation and ambient noise reduction and a system for audio echo cancellation based on transform domain LMS adaptive filter theory were presented. A noise reduction filter was introduced in order to achieve better performance. The algorithm used had limitations though easy to implement; was found to be slow and also introduced high computational complexity to the system.

Speech Enhancement by Adaptive Noise Cancellation in the Wavelet Domain

Adaptive filtering, based on wavelet transform, was proposed for speech enhancement with the aim of using the adaptation process in the lower scales where the value of SNR is higher than the other scales [8]. Excellent results were obtained with the algorithm being computationally efficient. The adaptive filter took the general form shown below in fig 1

Fig 1: An example for adaptive system [8]

An Enhanced V-Blast With Non-Stationary Interference Avoidance Capability

In this paper, a transform domain processing (TDP) and minimum mean square error (MMSE) detector combined with the V-BLAST (Vertical-Bell Laboratories Layered Space-Time) to enhance performance in narrow band interference (NBI) environment were proposed. Fundamentally, the TDP was designed for an interference-free adaptive waveform in frequency-domain rather than in time-domain; crowded regions being avoided via adaptive spectral notching at both the transmitter and the receiver. The adaptive interference-free signal was synthesized by a non-parametric spectral estimator, called Capon's method (CM) which estimated interfered spectral environments. The combined effects of MMSE nulling in V-BLAST detection and CM method in the TDP processing stage mitigated both stationary and non-stationary interferences [9].

(III) PROPOSED IMPROVED TWO-STAGE ADAPTIVE INTERFERENCE SUPPRESSION METHOD

The Adaptive Interference Suppression Methods Using Transform Domain Approach [1] can be applied to mitigate for intentional and unintentional interferences in satellites systems. Figure 2 below, shows a system block diagram of the adaptive interference canceller. An important aspect of the adaptive interference suppressor method (AISM) is its ability to compute the threshold levels which in turn are used by the interference canceller to suppress signal energy bins which exceed that threshold level [1]. Therefore, the foremost problem in AISM is to accurately and quickly estimate the interference spectrum for better performance. In this project, we propose the use of a set of Interference Canceller (notch/clipper) combination in feed forward cascade connection (fig 3) to enhance the performance of AISM. The Notch will help minimise the level of the signal power before the calculation of appropriate threshold for the final interference removal.

Fig 2: Block diagram of adaptive interference suppressor using transform domain approach with 50% overlap processing [1]

Interference canceller 2 (with clipper)

Interference canceller 1 (with Notch filter)

N-point

FFT

N-point

IFFT

Threshold calculator

Fig 3: Modified Block diagram of the Suppression part (Adapted from [1])

Various algorithm/canceller combinations could also be evaluated using the Monte- Carlo simulations under several interfered environments. The project is simply a modification of the suppression part of [1], but could have far reaching results for uses in satellite and mobile wireless systems.

(IV) OPTICAL LINK INCORPORATION

The rate conversion circuitry (Fig 4) could be modified to incorporate a small optical transponder circuit that will be used for optical transmission. The adoption and incorporation of the small optical transponder circuitry might be a bit complicated and hence needs to be thoroughly investigated for compatibility before application. Fig 5 shows the block diagrams of the various component parts [3] that need to be considered. This arrangement will enable different rates to be switched at baseband and recombined before transmission on the various downlinks or inter-satellite links.

On Board Processor Whole link to be replaced by optical link (optical transponder and optical ground station)

high rate optical

transmission high rate optical transmission

Fig 4: Proposed modification of Transponder to incorporate optical transponder

(Adopted from [10])

Figure 5: Configuration for the SOCRATES system. [3]

(V) CONCLUSIONS

A number of issues emanating from the study of satellite transponders, which include issues that relate to interference free transmissions, high speed transmissions and broadband services were highlighted in this project. In general it is impossible to design a perfect transponder that mitigates for all distorting effects of a communication channel, provide high speed transmission and state of the art broadband services in the sky. In this project an attempt has been made to reduce such effects as interference by proposing simple modifications to existing interference mitigation techniques such as the Adaptive Interference Suppression Methods Using Transform Domain Approach. Some modifications to the data rate conversion circuitry in order to incorporate small optical transponder links for high data transmission were also proposed. These changes are not without drawbacks; the main one being increase in the complexity of the transponder hence higher overhead costs.

Thorough investigations for compatibility of these changes before application would need to be carried out.

Notwithstanding the above, the benefits of such a transponder would outweigh the draw backs by far in providing cleaner transmissions for both satellite and mobile systems; propagation delays would further be reduced hence remote rural areas could be connected just as fast with satellite communication as with terrestrial communications thus benefitting rural folk, urban dwellers and the country at large.

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