Showing papers by "Barry G. Evans published in 2010"

Satellite MIMO Measurement With Colocated Quadrifilar Helix Antennas at the Receiver Terminal

Abstract: An outdoor measurement campaign was undertaken with a dual circularly polarized (CP) contrawound quadrifilar helix antenna (CQHA) at the receiver terminal in a low-elevation land mobile satellite multiple-input-multiple-output (MIMO) system. The CQHA is a combination of two colocated miniaturized quadrifilar helix antennas and feeding networks. Performance of the CQHA was then evaluated and compared to spatially separated reference antennas in terms of measured correlation and capacity. Measurement results indicate that CQHA has the same performance as the reference antennas in non-line-of-sight (NLOS) environment, while in obstructed-LOS (OLOS) areas, proper orientation of the CQHA is required to achieve nearly the same capacity as the reference antennas .

Ground Based and Onboard Based Beamforming for Hybrid Terrestrial-Satellite Mobile System

Abstract: Adaptive beamforming in terrestrial networks is viable due to relaxed complexity constraints. In case of satellite payload, adaptive beamforming is currently not cost effective due to higher onboard complexity requirements, power consumption issues and associated costs. In our earlier proposed Hybrid Terrestrial-Satellite Mobile System (HTSMS) incorporating frequency reuse, Onboard Based Beam-Forming (OBBF) is employed to mitigate Co-Channel Interference (CCI) for a Orthogonal Frequency Division Multiple Access (OFDM) system. The less complex solution is to opt for Ground Based Beam-Forming (GBBF) saving valuable onboard resources. Despite the benefits of GBBF, high feeder link bandwidth is required to support uplink and downlink transmissions. Moreover with GBBF, the satellite payload complexity is a sensitive function of the feed signals transmitted through gateway uplinks and downlinks. Furthermore, performance of GBBF is highly sensitive to the gateway calibration system which must compensate instabilities induced due to payload/gateway component changes over temperature and life as well as propagation amplitude and phase dispersion effects. Hence the choice of beamforming not only depends on complexity but also on performance. In this paper we employ ground based and onboard based beamforming solutions for a Bit Interleaved Coded Modulation-OFDM (BICM-OFDM) in our proposed HTSMS. We also propose a semi static hybrid space/ground beamforming and show that it is a far less complex solution compared to onboard adaptive beamforming. We then investigate the applicability of onboard and ground based approaches and quantify their performance advantages for the HTSMS case in AWGN channel scenario.

Key issues and technologies for a terabit/s satellite

Abstract: In this paper we investigate the next but one generation of fixed satellite systems and the technological challenges that face this generation which we define as operational by 2020. Various technologies and architectures are presented with a view to identifying the most promising to pursue. © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Low complexity time-domain channel and delay spread estimation for OFDM systems

Abstract: A low complexity time-domain channel estimation scheme for OFDM systems is presented. It uses a training symbol (preamble) to estimate the channel impulse response (CIR) in the presence of timing synchronization errors. Its computational complexity is much lower than that of the popular MMSE technique and this can be further reduced if there is prior knowledge of the channel delay spread. Consequently, channel delay spread estimation is also addressed by using threshold setting on the optimized CIR. Computer simulations show that the proposed scheme achieves near-ideal accuracy in quasi-static channels.

Admission control and handover management for high‐speed trains in vehicular geostationary satellite networks with terrestrial gap‐filling

Abstract: Following a recent upgrade, the Digital Video Broadcasting—Return Channel Satellite (DVB-RCS) standard sets up to support terminal mobility. In this scenario, integration with terrestrial systems becomes a primary concern to ensure network connectivity in urban areas. This article proposes an integrated satellite–terrestrial architecture for the provision of broadband services onboard high-speed trains, in which terrestrial cellular networks are seen as viable gap-fillers for discontinuous satellite coverage. We derive an analytical model of the hybrid DVB-RCS-cellular system by exploiting analogies between the mobility pattern predictability of LEO constellations and that of high-speed trains. Terminals whose QoS cannot be guaranteed by the satellite segment are proposed to temporarily divert the connections towards the terrestrial infrastructure, where available. Using an iterative approach based on the Erlang fixed-point approximation, we show performance improvements with respect to stand-alone satellite systems in terms of handover failure probability and overall resource utilization. The analytical model is also validated via our ns2-based DVB-RCS packet-level simulator. Detailed modelling of synchronization and signalling mechanisms confirms the accuracy of the analytical results, and shows that topology and mobility information can contribute to refine radio resource utilization optimality when used jointly. Copyright © 2009 John Wiley & Sons, Ltd.

Improved time diversity for LTE over satellite using split multicode transmission

Abstract: An improved time diversity technique is proposed for deploying the 3GPP LTE air-interface over satellite mobile links. It overcomes the FEC performance loss resulting from the use of a short transmit-time-interval (TTI) in LTE. By using transport block expansion, code block segmentation, advanced rate matching and automatic retransmissions, the FEC coded block is split over as many TTIs as is necessary to break the channel correlation without any reduction in the user data rate.

Self organised cellular networks as the future of wireless communications

Abstract: An introduction into self organizing cellular networks is presented. This topic has generated a lot of research interest over the past few years as operators have identified it as a necessary feature in future wireless communication systems. We review projects which have studied self organization and with knowledge of system model design in computing, we suggest design rules in developing robust and efficient self organizing algorithms. We finally demonstrate a channel assignment example based on the concept of sectorial neighbours where the system autonomously changes its allocation scheme based on external factors in the environment (e.g. geographical location, interfering sectors and demand for resources). Further research directions are also highlighted.

Complex correlation on linear array angle-of-arrival estimation measurement of ultra wideband propagation channels

Abstract: The phase of complex correlation is exploited to provide estimation of the azimuth arrival angle of ultra wideband multipath signals. Regarding the cluster model for uniform linear arrays, spatial correlation between antenna elements plays an important role in the analytical stage. Performance of this estimation is validated by using frequency-domain channel measurements in an anechoic chamber. Five linear array elements are used at the receiver measuring in azimuth between 0 and 360°. Furthermore, six measurement scenario cases including line-of-sight and multipath reflection are created to represent simple indoor environments. In the computation process, the complex spatial correlation coefficients between each array and the reference array are computed at each temporal multipath bin. Associated with the phase difference method, angles-of-arrival of multipath components at each time bin can be obtained. Finally, in order to verify this proposed technique, comparisons with results obtained by the sensor CLEAN algorithm are also presented.

Preamble based Adaptive Beamformer for Hybrid Terrestrial-Satellite Mobile System

Abstract: Convergence behaviour of beamforming in interference mitigation application is critical to overall system performance. In non-blind beamforming, the resources utilised to reach the steady state impacts the overall throughput. Traditionally in the downlink of Orthogonal Frequency Division Multiplexing (OFDM) based systems, a preamble formed of training sequence is transmitted prior to data for the purpose of channel estimation. While enabling superior estimation, the approach compromises on data throughput. On the other hand in the uplink, a preamble approach for channel estimation is not applicable due to interference issues. For beamforming, however, preamble transmission for the uplink becomes viable for improved detection performance. In this paper we propose a pilot reallocation based preamble solution for beamforming where dispersed pilots from OFDM symbols are reallocated to form a preamble at the transmission end. This preamble, transmitted prior to data OFDM symbols, guarantees faster convergence without affecting data throughput. Extending our previous work, we employ this solution in Hybrid Terrestrial Satellite Mobile System (HTSMS) with a channel model specific to the mobile-satellite scenario in order to yield practical results. We also study the impact of the preamble’s pilot density and its length on beamformer’s convergence behaviour and overall performance of HTSMS.

Cross-layer quality-driven adaptation for scheduling heterogeneous multimedia over 3G satellite networks

Abstract: Wireless networks are experiencing a paradigm shift from focusing on the traditional data transfer to accommodating the rapidly increasing multimedia traffic. Hence, their scheduling algorithms have to concern not only network-oriented quality-of-service (QoS) profiles, but also application-oriented QoS targets. This is particularly challenging for satellite multimedia networks that lack fast closed-loop power control and reliable feedbacks. In this paper, we present a cross-layer packet scheduling scheme, namely Hybrid Queuing and Reception Adaptation (HQRA), which performs joint adaptations by considering the traffic information and QoS targets from the applications, the queuing dynamics induced from the network, as well as the end-to-end performance and channel variations from respective users. By jointly optimizing multiple performance criteria at different layers, the scheme enjoys quality-driven, channel-dependant, and network-aware features. HQRA can well accommodate return link diversity and the imperfect feedbacks, whilst ensuring robustness in highly heterogeneous and dynamic satellite environments. We evaluate its performance over diverse network and media configurations in comparison with the state-of-the-art solutions. We observe noticeable performance gains on application-oriented QoS, bandwidth utilization, and objective video quality, together with favorable fairness and scalability measures.

Gradient projection decoding of LDPC codes and algorithmic variations

Abstract: The current study proposes decoding algorithms for low density parity check codes (LDPC), which offer competitive performance-complexity trade-offs relative to some of the most efficient existing decoding techniques. Unlike existing low-complexity algorithms, which are essentially reduced complexity variations of the classical belief propagation algorithm, starting point in the developed algorithms is the gradient projections (GP) decoding technique, proposed by Kasparis and Evans (2007). The first part of this paper is concerned with the GP algorithm itself, and specifically with determining bounds on the step-size parameter, over which convergence is guaranteed. Consequently, the GP algorithm is reformulated as a message passing routine on a Tanner graph and this new formulation allows development of new low-complexity decoding routines. Simulation evaluations, performed mainly for geometry-based LDPC constructions, show that the new variations achieve similar performances and complexities per iteration to the state-of-the-art algorithms. However, the developed algorithms offer the implementation advantages that the memory-storage requirement is significantly reduced, and also that the performance and convergence speed can be finely traded-off by tuning the step-size parameter.

LTE and WiMAX satellite systems: Improving FEC performance using split multicode transmission

Abstract: In this paper we investigate the degradation in FEC performance of LTE and WiMAX air-interfaces in satellite mobile channels due to lack of sufficient time diversity caused by the use of a short transmit time interval (TTI). We then propose a generalized ‘split multicode transmission’ technique to overcome this problem without any reduction in the achievable user data rate. Simulation results are presented using the MAESTRO satellite wideband channel for rural areas.

Low complexity time-domain channel estimation for OFDM using training symbols

Abstract: A low complexity time domain channel estimation technique for OFDM systems is proposed. It uses training symbols to estimate the channel impulse responses (CIR) via circular cross-correlation. A CIR search window technique is also proposed to optimise the accuracy of synchronisation and channel estimation. The output of the search window can also be used for the estimation of channel delay spread. Simulation results show that the proposed method provides near-ideal accuracy with a complexity much lower than the MMSE technique.

Deploying IP Multimedia Subsystem (IMS) Services over Next Generation Networks (NGNs): The IU-ATC Integrated Test Bed

Abstract: The IP Multimedia Subsystem (IMS) is the Third Generation Partnership Project’s (3GPP) standardized service platform that enables the deployment of rich and personalized services over fixed and mobile networks whilst allowing end-users ubiquitous access to services such as voice, video, presence and online gaming anytime and anywhere. However, the delivery of these services to the end-users is highly dependent on the available or preferred access network which could range from fixed broadband access to mobile 4G connections. Although the IMS was initially developed as the core network for Third Generation (3G) systems, it has now been adopted as the service platform for the Long Term Evolution (LTE) and System Architecture Evolution (SAE). As this transition of 3G to 4G and beyond evolves, there is an immediate need for a research testbed that facilitates the research, development and early trials of the integration of these technologies. This has motivated us to integrate the IMS based Advanced Next Generation Network (ANGN) testbed at the University of Surrey (UniS), U.K. with the 4G Access Network Testbed at IIT Madras, India via an academic transnational network link to form a fully functional telecommunications mobile network. In this paper, we discuss the rationales, motivations and objectives behind the integrated testbed whilst also investigating how it can be extended to support 4G and future technologies such as LTE/SAE and WiMAX. The testbed as a whole plays a key as role in the future of IMS development as it provides a fully functional platform similar to commercial networks for researchers to investigate and demonstrate the feasibility of their proposal in a realistic environment.

SNR estimation for ACM in DVB-RCS satellite systems

Abstract: • Mitigates slow fading (e.g., Rain fades) • Improves link availability • Provides capacity gains • Dynamically adapts modulation and coding scheme (mode/Mod-Cod subset) • Efficiency relies on accuracy of SNR estimator • Different estimators for DVB-S2 and DVB-RCS analyzed in the literature [Aroumont_2006]

Sub-optimal joint beamforming on MISO cognitive radio

Abstract: In this paper, we discuss joint beamforming schemes for the downlink of a multiple input single output coexisting radio system, where both transmitters know all the channel state information. Considering the opportunistic case where both systems have equal rights to access the spectrum, we propose sub-optimal methods for several performance criteria, including sum mean squared error, continuous sum throughout, and discrete sum throughput. Simulation results show that the proposed precoding algorithms achieve the better performance trade-offs between two radio systems and increase the spectrum efficiency at the cost of full channel state information.

Link Performance of LTE vs. WiMAX for Satellite Mobile Broadband

Abstract: In this paper, we investigate and compare the performance of the IEEE mobile WiMAX and the 3GPP LTE terrestrial air interfaces over GEO satellite links with amplifier nonlinearity and mobile wideband fading, in order to have an insight into which would be a better candidate for ubiquitous mobile broadband delivery. Both air-interfaces are compared in terms of achievable single-link capacities for a 5MHz channel. Link-level computer simulations are also carried out to establish their block-error-rate performance. Although both standards share a lot of similarity, simulation results show that they can differ in performance as a result of their subcarrier multiplexing schemes and the variants of OFDMA implemented. © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Selective DF relaying for mobile satellite broadcasting

Abstract: This paper addresses mobile terminal (MT) relaying in order to improve the performance of mobile satellite broadcasting. It considers selective decode-and-forward (SDF) relaying with BICM-OFDM air interface. The BER upper bound was derived for the threshold-based SDF relaying in Rician fading channels. The suboptimal SNR threshold was then obtained by minimising this analytical upper bound. This scheme is compared in simulation with a conventional CRC and set threshold schemes. It is shown that the proposed suboptimal-SNR threshold scheme gives up to 1 dB performance gain compared to the conventional CRC-based scheme in the moderate E b /N o range (i.e., 2 dB b /N o b /N o of 10 dB.