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.

In this paper, we present a theoretical analysis for obtaining the minimum Euclidean distance between two nonlinearly distorted OFDM signals (Orthogonal Frequency Division Multiplexing). This analysis takes advantage of the Gaussian-like behavior of OFDM signals with a large number of subcarriers and can be employed to provide the asymptotic gain of the optimum receiver. This approach is then employed with several different nonlinear characteristics, showing that in general the optimum performance of nonlinear OFDM is better than linear OFDM.

Recent results indicate that the optimum performance of uncoded OFDM (Orthogonal Frequency Division Multiplexing) in the presence of strong nonlinear distortion effects can be better than with linear transmitters. However, the uncoded OFDM performance is not very useful since typically OFDM schemes are combined with channel coding (also denoted coded OFDM), with substantially different uncoded and coded OFDM performances. In this paper we consider the optimum performance of coded OFDM schemes in the presence of strong nonlinear distortion effects. It is shown that nonlinear distortion effects not only do not lead to performance degradation, but they also might lead to substantial performance gains relatively to coded OFDM with linear transmitters.

In this paper we consider the optimum ML (Maximum-Likelihood) detection for OFDM signals (Orthogonal Frequency Division Multiplexing) with iterative clipping and filtering. It is shown that the nonlinear distortion does not necessarily mean significant performance degradation and, in fact, the optimum performance could even better than the performance with ideal, linear transmitters. We also present sub-optimum receivers that allow remarkable performance improvements, being able to reduce significantly the gap between the optimum performance and the performance of typical OFDM receivers.

The Quality of Service (QoS) requirements for modern broadband wireless systems can be very high, with small error rates and delays. Packets with errors are usually discarded and need to be retransmitted, leading to performance degradation. An alternative to simple retransmissions like Automatic Repeat reQuest (ARQ) schemes, that can minimize the degradation is to combine the signals associated to different transmission attempts, usually called diversity combining techniques. This paper considers the use of Single-Carrier (SC) modulations with Frequency-Domain Equalization (FDE) and low-complexity soft combining ARQ schemes. The time diversity combining technique presented in this paper, allows packets associated to different transmission attempts to be combined in a soft way so as to improve the performance. This technique is employed in a Time Division Multiple Access (TDMA) scheme. An accurate analytical model is proposed for the evaluation of the uplink packet delay in non-saturated traffic condition, and for the system's throughput (goodput) considering a generic packet arrival process. Physical (PHY) layer (packet error rates) and system-level (goodput and packet delay considering both PHY and Medium Access Control (MAC) layers) results are presented and validated by simulations for two distinctive arrival processes: Poisson and Geometric.

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 deals with multi-packet detection for SC modulations (Single-Carrier) with FDE (Frequency- Domain Equalization). We consider iterative frequency-domain receivers that jointly perform equalization, multi-packet separation and channel decoding operations, for up to Qmaxmobile terminals transmitting in one slot. In this paper we propose an accurate analytical model for a p-persistent random access MAC protocol, designed to handle a total number of mobile terminals J above Qmax. It evaluates the uplink non-saturated packet delay and calculates the optimal configuration for the packet transmission probability associated to each mobile terminal that minimizes the delay. Our analytical results are validated using physical and MAC layer simulations.

This paper considers the use of SC modulations (Single-Carrier) with FDE (Frequency-Domain Equalization) with low-complexity soft combining ARQ schemes (Automatic Repeat reQuest). With our technique, packets associated to different transmission attempts are combined in a soft way, allowing improved performances. Its low complexity makes it particularly interesting for the uplink of wireless systems. This paper proposes an accurate analytical model for a TDMA (Time Division Multiple Access) scheme where packet combining ARQ is applied. It evaluates the uplink non-saturated packet delay and goodput for a generic message arrival process. Our analytical results are validated using physical and MAC layer simulations.

This paper presents a multipacket detection technique for dealing with packet collisions in OFDM schemes (orthogonal frequency division multiplexing). Instead of discarding collided packets as it is done in traditional MAC approaches (medium access control) additional packets are transmitted and separated using multiple input multiple output (MIMO) based techniques. Reliable detection and high throughputs can be achieved as long as different interleavers are used for different retransmissions. We also include a method for estimating the users involved in the collision.

This paper deals with multi-packet detection for SC modulations (Single-Carrier) with FDE (Frequency-Domain Equalization). We consider iterative frequency- domain receivers that jointly perform the equalization, multi-packet separation and channel decoding operations. Our receivers allow a high throughput provided that the number of mobile terminals that is trying to access a given slot is equal to or below the multi-packet detection capacity Qmax. We propose a new MAC protocol that allows a total number of mobile terminals J above Qmaxby controlling the number of mobile terminals contending to transmit. The paper evaluates the uplink saturated system throughput and proposes an optimal configuration for the packet transmission probability associated to each mobile terminal. Our analytical results are validated using physical and MAC layer simulations.

This paper considers the use of SC-FDE techniques (single-carrier with frequency-domain equalization) with low-complexity soft combining ARQ schemes (automatic repeat request). With our technique, packets associated to different transmission attempts are combined in a soft way, allowing improved performances. Moreover, the transmitter and receiver complexities are low and we can have excellent performances, especially when we have deep fading effects in slow-varying channels. By combining the advantages of SC-FDE schemes with soft combining ARQ, the proposed technique is particularly interesting for the uplink of wireless systems.

The traditional approach for MAC protocols (Medium Access Control) is to discard all packets involved in a collision and request their retransmission, which precludes high throughputs. However, since the signals associated to collisions have important information concerning the packets involved, with proper retransmissions we can efficiently resolve collisions. In this paper we propose a multipacket detection technique for OFDM schemes (Orthogonal Frequency Division Multiplexing) that allows an efficient packet separation in the presence of collisions. This technique allows high throughputs, since the total number of transmissions can be equal to the number of packets involved in the collision. By using different interleavers for different retransmissions we can successfully separate packets involved in a collision even when the channel remains fixed for the retransmissions.

Usually, packets involved in a collision are lost, requiring their retransmission. However, the signal associated to collisions has important information concerning the packets involved. In fact, with proper retransmissions we can efficiently resolve collisions. In this paper we propose a frequency-domain multi- packet detection technique for SC-FDE schemes (Single- Carrier with Frequency-Domain Equalization) that allows an efficient packet separation in the presence of successive collisions. This technique allows high throughputs, since the total number of transmissions is equal to the number of packets involved in the collision, even when the channel remains fixed for the retransmissions. Since we consider SC-FDE schemes and the complexity is concentrated in the receiver, this technique particularly appealing for the uplink of broadband wireless systems.

Usually, packets involved in a collision are lost, requiring the retransmission of all packets. However, if we do not discard collided packets and we use proper retransmissions we can efficiently resolve collisions. In this paper we propose a technique that allows an efficient packet separation in the presence of successive collisions. We consider an SC (Single-Carrier) modulation with FDE (Frequency-Domain Equalization) and we propose a frequency-domain multi-packet detection scheme. Since our technique requires uncorrelated channels for different retransmissions, we also propose a PS technique (Packet-Shift) for retransmissions using the same channel. Our technique allows high throughputs, since the total number of transmissions is equal to the number of packets involved in the collision. Moreover, the complexity is concentrated in the receiver, making this technique particularly appealing to the uplink of broadband wireless systems. By employing the PS scheme we can use the same channel for the retransmissions, with only a small performance degradation.

In this paper, we consider SC-FDE schemes (Single Carrier with Frequency-Domain Equalization) employing OQPSK (Offset Quaternary Phase Shift Keying) signals with quasi-constant envelope. To allow high spectral efficiency, different frequency channels are separated by the symbol rate, regardless of their bandwidth (which is much higher than the symbol rate for quasi-constant envelope OQPSK signals), and we consider a multi-channel frequency-domain receiver with ACI (Adjacent Channel Interference) suppression. Our performance results show that it is possible to have high spectral efficiency with quasi-constant envelope OQPSK signals. This technique is especially interesting for the uplink of future broadband wireless systems since we can employ low-cost, grossly nonlinear power amplifiers at the mobile terminals and almost all signal processing is shifted to the base station.