Wireless networks contain an inherent distributed spatial diversity that can be exploited by the use of relaying. Relay networks take advantage of the broadcast-oriented nature of radio and require node-based, rather than link-based protocols. Prior work on relay networks has studied performance limits either with unrealistic assumptions, complicated protocols, or only a single relay. In this paper, a practical approach to networks comprising multiple relays operating over orthogonal time slots is proposed based on a generalization of hybrid-automatic repeat request (ARQ). In contrast with conventional hybrid-ARQ, retransmitted packets do not need to come from the original source radio but could instead be sent by relays that overhear the transmission. An information theoretic framework is exposed that establishes the performance limits of such systems in a block fading environment, and numerical results are presented for some representative topologies and protocols. The results indicate a significant improvement in the energy-latency tradeoff when compared with conventional multihop protocols implemented as a cascade of point-to-point links.

Practical relay networks: a generalization of hybrid-ARQ

The class of low-density parity-check (LDPC) codes is attractive, since such codes can be decoded using practical message-passing algorithms, and their performance is known to approach the Shannon limits for suitably large block lengths. For the intermediate block lengths relevant in applications, however, many LDPC codes exhibit a so-called ?error floor,? corresponding to a significant flattening in the curve that relates signal-to-noise ratio (SNR) to the bit-error rate (BER) level. Previous work has linked this behavior to combinatorial substructures within the Tanner graph associated with an LDPC code, known as (fully) absorbing sets. These fully absorbing sets correspond to a particular type of near-codewords or trapping sets that are stable under bit-flipping operations, and exert the dominant effect on the low BER behavior of structured LDPC codes. This paper provides a detailed theoretical analysis of these (fully) absorbing sets for the class of Cp, ? array-based LDPC codes, including the characterization of all minimal (fully) absorbing sets for the array-based LDPC codes for ? = 2,3,4, and moreover, it provides the development of techniques to enumerate them exactly. Theoretical results of this type provide a foundation for predicting and extrapolating the error floor behavior of LDPC codes.

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Analysis of Absorbing Sets and Fully Absorbing Sets of Array-Based LDPC Codes

The great advances made in the wireless technology have enabled the deployment of wireless communication networks in some of the harshest environments such as volcanoes, hurricane-affected regions, and underground mines. In such challenging environments suffering from the lack of infrastructure, traditional routing is not efficient and sometimes not even feasible. Moreover, the exponential growth of the number of wireless connected devices has created the need for a new routing paradigm that could benefit from the potentials offered by these heterogeneous wireless devices. Hence, in order to overcome the traditional routing limitations, and to increase the capacity of current dynamic heterogeneous wireless networks, the opportunistic routing paradigm has been proposed and developed in recent research works. Motivated by the great interest that has been attributed to this new paradigm within the last decade, we provide a comprehensive survey of the existing literature related to opportunistic routing. We first study the main design building blocks of opportunistic routing. Then, we provide a taxonomy for opportunistic routing proposals, based on their routing objectives as well as the optimization tools and approaches used in the routing design. Hence, five opportunistic routing classes are defined and studied in this paper, namely, geographic opportunistic routing, link-state-aware opportunistic routing, probabilistic opportunistic routing, optimization-based opportunistic routing, and cross-layer opportunistic routing. We also review the main protocols proposed in the literature for each class. Finally, we identify and discuss the main future research directions related to the opportunistic routing design, optimization, and deployment.

A Survey on Opportunistic Routing in Wireless Communication Networks

Satellite communication systems are a promising solution to extend and complement terrestrial networks in unserved or under-served areas, as reflected by recent commercial and standardization endeavors. In particular, 3GPP recently initiated a study item for new radio, i.e., 5G, non-terrestrial networks aimed at deploying satellite systems either as a stand-alone solution or as an integration to terrestrial networks in mobile broadband and machine-type communication scenarios. However, typical satellite channel impairments, as large path losses, delays, and Doppler shifts, pose severe challenges to the realization of a satellite-based NR network. In this paper, based on the architecture options currently being discussed in the standardization fora, we discuss and assess the impact of the satellite channel characteristics on the physical and medium access control layers, both in terms of transmitted waveforms and procedures for enhanced mobile broadband and narrowband-Internet of Things applications. The proposed analysis shows that the main technical challenges are related to the PHY/MAC procedures, in particular random access, timing advance, and hybrid automatic repeat request and depending on the considered service and architecture, different solutions are proposed.

Architectures and Key Technical Challenges for 5G Systems Incorporating Satellites

Opportunistic routing has recently attracted much attention as it is considered a promising direction for improving the performance of wireless ad hoc and sensor networks. With opportunistic routing, intermediate nodes collaborate on packet forwarding in a localized and consistent manner. Opportunistic routing greatly increases transmission reliability and network throughput by taking advantage of the broadcast nature of the wireless medium. In this article we first illustrate the basic idea behind opportunistic routing, and then categorize current research work based on different criteria. We illustrate how different protocols work, and discuss their merits and drawbacks. Finally, we point out potential issues and future directions in opportunistic routing for wireless ad hoc and sensor networks.

Opportunistic routing for wireless ad hoc and sensor networks: Present and future directions

We introduce analytical characterization of the nonlinear interference that results when passing more than one high-order modulation carrier through the same nonlinear transponder high-power amplifier (HPA). A Volterra filter is proposed which is novel in its implementation of this analytical characterization and modeling of intersymbol interference (ISI) and adjacent channel interference (ACI). Our focus is on adaptive algorithms with pilot-based training so that the solutions are completely blind to unknown transponder HPA characteristics, and can rapidly respond to varying operating back-off level. Furthermore, two families of adaptive solutions are provided to compensate for nonlinear ISI and ACI. The first set performs adaptive channel inversion and then applies equalization. The second set of solutions performs adaptive channel identification and then applies cancellation. The effectiveness of the proposed analysis and techniques is demonstrated via extensive simulations for high-order QAM and APSK modulations. Also included is the coded performance with selected LDPC codes designed for the DVB-S2 standard. Finally, computational complexity is assessed and performance impact is quantified when complexity is reduced by decreasing the number of Volterra coefficients.

Analysis and compensation for nonlinear interference of two high-order modulation carriers over satellite link

A new pulse shape for CPM is introduced which is obtained by a linear combination of well-known RC and REC pulse shapes. The new pulse shape addresses the tradeoff between the width of the PSD main lobe and the rate of decay of the side lobe to improve the coded performance of multi-carrier systems affected by ACI. Also, a methodology is proposed to design and evaluate the performance of the new pulse shape for multi-carrier, coded systems based on the modulation constrained capacity. Furthermore, a binary convolutional code and the CPM modulator are concatenated using an S-random bit interleaver to lower the error floor. Finally, Laurent representation of the new pulse shape is suggested such that by retaining only the principal pulses at the receiver, complexity of the receiver can be reduced.

Phase pulse system and method for bandwidth and energy efficient continuous phase modulation

To satisfy the aggressive demand for higher satellite throughput, industry trend is moving toward sharing the transponder amplifier by multiple carriers, each employing high-order modulations that are spectrally compact. This trend, in conjunction with the inherently nonlinear nature of the amplifier when driven efficiently closer to saturation, creates significant intermodulation distortion that needs to be appropriately compensated. This paper presents a powerful compensation technique that is capable of mitigating the nonlinear intermodulation distortion and is placed at the transmitter or gateway. It is a novel multicarrier data predistortion technique that successively modifies the transmitted symbols to drive multicarrier distortion vector toward zero. This distortion vector results from passing the transmitted symbols from the multiple carriers, intrinsically accessible at the gateway, through the nonlinear satellite channel model. The novel successive predistortion technique and methods of estimating the distortion are described in detail. It is demonstrated using extensive computer simulations that the proposed multicarrier predistortion technique is capable of achieving near-optimum performance, even when only a simple linear receiver is employed and no exchange of data is assumed between receivers.

Multicarrier Successive Predistortion for Nonlinear Satellite Systems

This paper proposes and analyzes a new cross-layer protocol for ad hoc and sensor networks that unifies the concepts of geographic random forwarding (GeRaF) and hybrid-ARQ. The protocol is given the descriptive name hybrid ARQ-based intra-cluster geographically informed relaying (HARBINGER). Like GeRaF, HARBINGER assumes that each node knows its own position and that messages are addressed by location. As is common for sensor networks, the nodes cycle on-and-off according to a sleep schedule. Unlike GeRaF, which returns to the initial transmission state If no active node is within range, the nodes in HARBINGER combine transmissions thereby achieving an additional time-diversity benefit. With HARBINGER, a lower density of active nodes achieves almost the same delay and energy efficiency as GeRaF, implying that a lower duty cycle sleep schedule could be used to prolong the useful lifetime of the network. The paper gives detailed analysis of a version of the protocol (Fast-HARBINGER) whereby the sleep states of the network are synchronized with the data packet transmission rate of the network.

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Geographic random forwarding with hybrid-ARQ for ad hoc networks with rapid sleep cycles

An aspect of the present invention is drawn to a receiver operable to receive a first signal transmitted on a first carrier and to receive a second signal transmitted on a second carrier. The receiver includes a first filter, a second filter and a nonlinear compensator. The first filter is arranged to receive the first signal and to generate a first filtered signal. The second filter is arranged to receive the second signal and to generate a second filtered signal. The nonlinear compensator is arranged to output a first compensating signal based on the first filtered signal and the second filtered signal and to output a second compensating signal based on the first filtered signal and the second filtered signal. Further, the nonlinear compensator can reduce one of nonlinear interference within the first filtered signal and nonlinear interference between the first filtered signal and the second filtered signal.

Rohit Iyer Seshadri

Papers

Analysis and compensation for nonlinear interference of two high-order modulation carriers over satellite link

Phase pulse system and method for bandwidth and energy efficient continuous phase modulation

Multicarrier Successive Predistortion for Nonlinear Satellite Systems

Geographic random forwarding with hybrid-ARQ for ad hoc networks with rapid sleep cycles

System and method for compensating for nonlinear interference cancellation in multi-carrier transmission