FTN (Faster Than Nyquist) signaling allows capacity gains but requires substantially complex equalization schemes, even for an ideal AWGN (Additive White Gaussian Noise) channel. Extending conventional FTN receivers for severely time- dispersive channels is not an option, since the receiver complexity becomes prohibitively high. In this paper we address the design of a receiver for FTN signaling over severely time-dispersive channels. To cope with the severe ISI (Inter-Symbol Interference) associated to the combined effects of FTN signaling and the time-dispersive channel we consider an iterative FDE scheme (Frequency-Domain Equalization) combined with a HARQ (Hybrid Automatic Repeat reQuest) especially designed taking into account the characteristics of FTN signals. We also present a simple, yet accurate model for the performance evaluation, considering multiple retransmissions per block. Our performance results show that we can have significant throughput gains, while maintaining essentially the receiver complexity of conventional, Nyquist-rate signaling schemes, even for severely time-dispersive channels.

Prefix-assisted direct-sequence code-division multiple access (DS-CDMA) is a viable transmission technique for high-data-rate wireless broadband systems, capable of coping with highly dispersive channels at the uplink. Packet combining (PC) ARQ with an iterative block decision feedback equalization (IB-DFE) technique can be used to cope with errors. However, extracting the IB-DFE's packet error rate (PER) for MAC-layer simulations can be time consuming. Most works that model the DS-CDMA behavior with PC ARQ use simplified asymptotic PER models in the study of the system performance, which are not precise for low signal-to-noise ratios and for a low number of mobile terminals (MTs). In this paper, the MAC performance is modeled using a discrete-time Markov chain that relies on a PER model. As an example, this paper describes a PER analytical model for an uncoded prefix-assisted DS-CDMA system with PC ARQ that considers the following inputs: the number of MTs accessing the channel, the MTs' number of transmissions, and the channel realizations for a given bit energy over noise ratio. The results show that the model portrays with better accuracy the simulations' results, compared with an alternative asymptotic model.

Random access approaches in Low Earth Orbit (LEO) satellite networks are usually incompatible with the Quality of Service (QoS) requirements for multimedia traffic, especially when hand-held terminals must operate with a very low signal-to-noise ratio. This paper proposes the Satellite Random Network Diversity Multiple Access (SR-NDMA) protocol that handles multimedia traffic under this context through the combination of a random and scheduled access scheme. The protocol uses a multi-packet receiver, for Single Carrier with Frequency Domain Equalization (SC-FDE) in the uplink, that gradually reduces the packet error rate with additional transmissions. The paper proposes analytical performance models for the throughput, delay and energy efficiency - as long as terminals have finite queues. System parameters are defined to enhance the energy efficiency while satisfying the QoS requirements for limited queue and bit-rate constraints. Results show that the proposed system is energy efficient and provides enough QoS to support multimedia services such as video telephony.

Traditionally, Low Earth Orbit (LEO) satellite networks use hard handovers when a Mobile Terminal (MT) moves between the footprints of two satellites. Such approach introduces a transitory disconnection interval, which may limit the capacity to provide demanding real-time requirements. A soft- handover approach that applies distributed packet combining on a satellite diversity scenario is proposed in this paper. Considering inexpensive MT hardware and low transmission powers, the paper analyses the performance gains and the difficulties that are associated to different Doppler and time-of-arrival shifts when the MT signal is broadcast to two satellites during the short handover period where both satellite's footprint overlap. A Single-Carrier with Frequency Domain Equalization (SC-FDE) transmission technique is considered in this paper. It is shown that it is possible to have an inter-satellite handover almost without quality of service (QoS) degradation for a Satellite Network Diversity Multiple Access (S-NDMA) scheme.

Traditionally, an unsuccessful packet reception, either due to channel conditions or interference from multiple sources (collisions), is discarded and followed by a new reception of the same packet. Network diversity multiple access (NDMA) handles collisions by using time diversity multipacket reception: the base station forces terminals to transmit P copies of each packet when P terminals collide. This rigidity of having P copies is unsuitable for poor propagation conditions. It might be desirable to have more than P copies of a given packet, adopting a technique for single packet reception called hybrid-ARQ. Hybrid-ARQ effectively decreases the packet error rate. This paper proposes such mechanism for multi-packet reception, called hybrid-ARQ NDMA (H-NDMA), a gated slotted random access protocol, where the access mechanism forces the terminals involved in a collision with reception errors to transmit more than P times. Analytical models for the throughput, delay and energy per useful packet are proposed. The energy performance is analyzed for bounded average delay constraints. The proposed system's performance is evaluated for a single-carrier with frequency domain equalization (SC-FDE) scheme, and compared with the classical NDMA protocol. The comparison highlights that H- NDMA is scalable for an increasing number of terminals and network load.

Traditionally, a packet with errors, either due to channel noise or collisions, is discarded and needs to be retransmitted, leading to performance losses. Hybrid Automatic Retransmission reQuest (H-ARQ) and time diversity multipacket reception approaches, such as Network Diversity Multiple Access(NDMA), improve the system performance by requesting additional retransmissions and combining all the signals received together. However, the high round trip delay time associated to satellite networks introduces limitations in the number of retransmission requests that may be issued by the terminals to fulfill the Quality of Service (QoS) requirements. This paper considers the design of hybrid protocols combining H-ARQ and NDMA for satellite networks with demand-assigned traffic. The satellite NDMA (S-NDMA) protocol is presented and analytical models are proposed for its performance. Energy efficient QoS provisioning is also analyzed. The proposed system's performance is evaluated for a Low Earth Orbit (LEO) network with a Single-Carrier with Frequency Domain Equalization (SCFDE) scheme, and compared to H-NDMA. Results show that the proposed system is energy efficient and can provide enough QoS to support high demand services such as video telephony.

Multipacket Reception (MPR) techniques employing either Direct Sequence Code Division Multiple Access (DS- CDMA) or Network Diversity Multiple Access (NDMA) have great potential to deal with packet collisions from multiple Mobile Terminals (MTs). However, their performance evaluation in prac- tical systems is difficult because lengthy simulations are needed to obtain the packet error rates for specific channel conditions. This paper considers MPR receivers based on the Iterative Block Decision Feedback Equalization (IB-DFE) concept. An analytical tool that computes the Bit Error Rate (BER) and Packet Error Rate (PER) for DS-CDMA is proposed, where its theoretical results have a good accuracy. This analytical approach, that provides a simple and relatively fast way to compute the BER and PER performance for DS-CDMA, is then employed for a comparison with NDMA.

Diversity Combining (DC) and Multipacket Detection (MPD) are efficient techniques that cope with packet errors due to severe propagation conditions and/or collisions. To evaluate their network performance on a network simulator, an accurate characterization of the Bit Error Rate (BER) and Packet Error Rate (PER) can successfully replace the behavior of the PHY layer. This choice is better in terms of computational time than implementing a full PHY layer to be jointly simulated with the upper layers. In this paper, an analytical model is proposed to compute the BER and PER of DC, MPD and hybrid techniques, considering a Single-Carrier with Frequency-Domain Equalization (SC-FDE) scenario. The paper also considers two frequency-domain detection methods: a simple linear receiver and a powerful iterative receiver. The proposed model is simple and versatile, since it can support nonhomogeneous transmitting powers, and different channel conditions concerning retransmissions and channel randomization techniques. Several simulations show that the proposed model provides accurate BER and PER results for a wide range of scenarios.

Traditionally, a packet with errors, either due to channel noise or collisions, is discarded and needs to be retransmitted, leading to performance losses. Network Diversity Multiple Access (NDMA) handles collisions by combining a multipacket detection scheme with time diversity. In NDMA, the base station (BS) forces mobile terminals (MTs) to transmit J copies of each packet when J MTs collide. Diversity combining is limited to J copies of the packets, not allowing it to adapt to severe errors due to channel noise. This paper considers a multipacket detection scheme recently proposed, which reduces the packet error rate (PER) when more than J copies of the packets are available. In this paper, an Hybrid-ARQ NDMA access mechanism is proposed. The access mechanisms forces MTs with reception errors during a collision resolution epoch to transmit more than J times. Analytical models are proposed for the goodput and delay, considering Poisson traffic. The proposed system's performance is evaluated for a Single-Carrier with Frequency Domain Equalization (SC-FDE) scheme, and compared to classical NDMA.

Packet Combining (PC) and Multipacket Reception (MPR) techniques have great potential to deal with lost packets since both take full advantage of the signals associated to all failed transmission attempts. However, their performance evaluation in practical systems is difficult because lengthy simulations are needed to obtain the packet error rates for specific channel conditions. The paper presents an analytical tool to obtain the Bit Error Rate (BER) and Packet Error Rate (PER) performances for Packet Combining (PC), Multipacket Reception (MPR) and the hybrid of both schemes, based on Iterative Block Decision Feedback Equalization (IB-DFE) receivers for Single-Carrier with Frequency-Domain Equalization (SC-FDE). A set of simulation results show that the proposed analytical approach has accurate Bit Error Rate (BER) and Packet Error Rate (PER) results with a simple and relatively fast method.

Traditionally, a packet with errors, either due to channel noise or collisions, is discarded and needs to be retransmitted, leading to performance losses. Two different approaches are considered to cope with lost packets, a low-complexity packet combining ARQ scheme (Automatic Repeat reQuest) and a multipacket detection scheme. These approaches are employed on a SC-FDE scheme (Single-Carrier with Frequency Domain Equalization). Suitable receiver structures are described, employing packet combining, multipacket detection and hybrid schemes. An accurate analytical tool is presented for predicting the performance of the different schemes.

This paper considers the use of low-complexity diversity combining hybrid ARQ schemes (Automatic Repeat reQuest) on a TDMA (Time Division Multiple Access) system. Its low complexity makes it particularly interesting for battery-powered nodes of a wireless sensor network (WSN). TDMA is commonly used for WSNs applications that require high throughput or constrained packet delay. This paper analyzes the energy per useful packet, packet delay and goodput performances of the TDMA system. Analytical models were used to define a constrained energy optimization problem, where the minimum transmission power is calculated based on the goodput and delay constraints. Analytical results were validated through simulations.

This paper considers the use of a code combining hybrid ARQ scheme (Automatic Repeat reQuest) on a TDMA (Time Division Multiple Access) system. A multi-rate turbo-code is proposed to implement the H-ARQ scheme, allowing improved performances. This paper analyzes the energy per packet successfully received, the packet delay and goodput performances for the TDMA system on a HAP (High Altitude Platform). Analytical models were used to define a constrained energy optimization problem, where the minimum transmission power is calculated based on the goodput and delay constraints. Analytical results were validated through simulations for a typical HAP Network uplink channel.

DTIA (Dynamic Topological Information Architecture), an inter-domain routing architecture, assumed the existence of "regions of domains" to offer a scalable solution for the Internet. The way regions can be connected and how traffic flow amongst them is not a trivial aspect and can jeopardize the solution proposed for intra-region traffic. This paper covers the inter-region case showing that it can be performed by using a smaller set of valid paths than for the intra-region case in the source region, but still featuring multipath to the destination and taking advantage of multi-homing. The routing is based on a monotone algebra to converge without the need to change the IP headers or introduce additional signaling or state in the network. The overall system complies with the current business model of the Internet, and it also features one tendency that has been increasingly used: the fact that Autonomous Systems (ASes) are each time more connected with direct (or semi-direct) links instead of in a pure hierarchical structure up to tier-1 ASes.