Dimitrios N. Skoutas

OVSF codes assignment and reassignment at the forward link of W-CDMA 3G systems

We examine the problem of assigning and reassigning orthogonal variable spreading factor (OVSF) codes at the forward link in 3G mobile communications systems employing W-CDMA. Efficient channelization code management results in high code utilization and increased system capacity. We introduce a new code selection scheme, which, by means of a very simple measure, attempts to mitigate the problem of scattered codes that fragments the capacity of the OVSF code tree. The main advantage of this new scheme is its ability to combine effectively the merits of other previously proposed schemes (see Tseng, Y.-C. et al., Proc. IEEE GLOBECOM, vol.1, p.562-6,2001). Thus, our proposed scheme is as fast as the random or ordered strategies, while simulation results have shown that it is as efficient, in terms of capacity improvement, as the crowded-first scheme.

Management of channelization codes at the forward link of WCDMA

We examine the orthogonal variable spreading factor (OVSF) code tree management problem at the forward link of 3G WCDMA systems. Considering single code operation, we evaluate the performance of a code selection scheme called recursive fewer codes blocked scheme (RFCB). RFCB manages to mitigate code blocking and thus minimizes the fragmentation of the OVSF code tree capacity. Simulation results demonstrate the efficiency of the RFCB scheme compared to previously proposed schemes.

A Survey on Buffer-Aided Relay Selection

Relays receive and retransmit signals between one or more sources and one or more destinations. Cooperative relaying is a novel technique for wireless communications that increases throughput and extends the coverage of networks. The task of relay selection serves as a building block to realize cooperative relaying. Recently, relays with buffers have been incorporated into cooperative relaying providing extra degrees of freedom in selection, thus improving various performance metrics, such as outage probability, power reduction, and throughput, at the expense of tolerating an increase in packet delay. In this survey, we review and classify various buffer-aided relay selection policies and discuss their importance through applications. The classification is mainly based on the following aspects: 1) duplexing capabilities, 2) channel state information (CSI), 3) transmission strategies, 4) relay mode, and 5) performance metrics. Relay selection policies for enhanced physical-layer security and cognitive communications with reduced interference are also discussed. Then, a framework for modeling such algorithms is presented based on Markov Chain theory. In addition, performance evaluation is conducted for various buffer-aided relay selection algorithms. To provide a broad perspective on the role of buffer-aided relay selection, various issues relevant to fifth-generation (5G) networks are discussed. Finally, we draw conclusion and discuss current challenges, possible future directions, and emerging technologies.

A Buffer-Aided Successive Opportunistic Relay Selection Scheme With Power Adaptation and Inter-Relay Interference Cancellation for Cooperative Diversity Systems

In this paper, we present a relay selection scheme which combines the spectral efficiency of successive opportunistic relaying with the robustness of single-link relay selection. More specifically, we propose a scheme that minimizes the total energy expenditure per time slot under an inter-relay interference cancellation scheme. The new relay selection policy is analyzed in terms of outage probability and diversity by modeling the evolution of relay buffers as a Markov Chain. We construct the state transition matrix of the Markov Chain and obtain its stationary distribution, which in turn, yields the outage probability. The proposed scheme outperforms relevant state-of-the-art relay selection schemes in terms of throughput, diversity, energy efficiency and average delay, as demonstrated via representative numerical examples.

Buffer-aided successive opportunistic relaying with inter-relay interference cancellation

In this paper we consider a simple cooperative network consisting of a source, a destination and a cluster of decode-and-forward relays characterized by the half-duplex constraint. At each time-slot the source and (possibly) one of the relays transmit a packet to another relay and the destination, respectively. When the source and a relay transmit simultaneously, inter-relay interference is introduced at the receiving relay. In this work, with the aid of buffers at the relays, we mitigate the detrimental effect of inter-relay interference through either interference cancellation or mitigation. More specifically, we propose the min-power opportunistic relaying protocol that minimizes the total energy expenditure per time slot under an inter-relay interference cancellation scheme. The min-power relay-pair selection scheme, apart from minimizing the energy expenditure, also provides better throughput and lower outage probability than existing works in the literature. The performance of the proposed scheme is demonstrated via illustrative examples and simulations in terms of outage probability and average throughput.

Optimized admission control scheme for coexisting femtocell, wireless and wireline networks

The most important challenge for the implementation of the Future Internet is to make the heterogeneity of access technologies transparent to the end user. Compared to the general case where the interworking networks are independent, the case of femtocells interworking with pre-existing wireless networks poses more challenges due to the sharing of the same backhaul capacity. Therefore, while a user is practically able to initiate the same service through multiple network interfaces, he is allocated capacity from the same capacity pool. However, while the femtocell inherits the QoS mechanisms of cellular networks and is able to provide a reliable CAC, this does not apply to the IP-based networks and that may drastically affect the performance of the femtocell. Hence, we propose an integrated Dynamic Service Admission Control (DSAC) framework for coexisting femtocell, wireless and wireline network environments. In particular, DSAC is able to provide QoS guarantees as a conventional capacity partitioning scheme while at the same time offers better performance in terms of acceptance probability and capacity utilization especially when short term variations of traffic load composition occur.

Joint relay-pair selection for buffer-aided successive opportunistic relaying

In this work, we propose a buffer-aided successive opportunistic relay scheme where each time a relay-pair is selected with the target of rate maximization. Due to overlapping transmissions by the source and the relay which transmits to the destination, inter-relay interference arises. The efficient mitigation of inter-relay interference either through cancellation or avoidance, requires increased channel state information. Thus, in order to reduce the implementation complexity induced by centralized selection, distributed switch-and-stay combining is combined with buffer-aided successive opportunistic relaying. In this way, by applying a rate threshold we can avoid increased relay-pair switching and channel state information acquisition. The efficiency of the proposed scheme is demonstrated through simulations and comparisons with other state-of-the-art relay selection policies.

Capacity Maximization through Energy-Aware Multi-Mode Relaying

In future wireless mobile networks, data rate and quality of service are expected to be comparable to those of wired deployments. To achieve this target, novel architectures must be adopted, successfully countering the disadvantages of the wireless transmission. Inspired by that, cooperative relaying was proposed because of the various gains it introduces to the network. In this work we propose a scheme consisting of multi-mode decode and forward relays facilitating the communication between a base station and a user terminal (UT). By equipping the relays with two interfaces, we can exploit the plethora of the available wireless protocols. Also, instead of performing multi-relay transmissions, we adopt an opportunistic relaying scheme due to its simplicity and outage-optimality. Additionally, we incorporate successive transmissions to improve the spectral efficiency, thus recovering the half-duplex loss in capacity due to the two-hop transmission. However, as inter-relay interference arises from successive transmissions, we propose mitigation techniques through interference cancellation and out-band transmissions using the multi-mode relays. At the same time, an energy-aware mechanism is implemented in the selected relay’s transmission, opting for power reduction, as the channel state information is acquired prior to the signal’s forwarding to the UT. Finally, we give numerical results by comparing the proposed energy-aware multi-mode relaying (EA-MMR) scheme, with two other schemes in terms of average end-to-end capacity, outage probability, delay distribution and power gain.

Distributed joint relay-pair selection for buffer-aided successive opportunistic relaying

In this work, we propose a buffer-aided successive opportunistic relay scheme where each time a relay-pair is selected with the target of rate maximization. Due to overlapping transmissions by the source and the relay which transmits to the destination, inter-relay interference arises. The efficient mitigation of inter-relay interference either through cancellation or avoidance, requires increased channel state information. Thus, in order to reduce the implementation complexity induced by centralized selection, distributed switch-and-stay combining is combined with buffer-aided successive opportunistic relaying. In this way, by applying a rate threshold we can avoid increased relay-pair switching and channel state information acquisition. The efficiency of the proposed scheme is demonstrated through simulations and comparisons with other state-of-the-art relay selection policies.

Relay selection for secure 5G green communications

In this article, we present relay selection policies in applications with secrecy requirements which are of interest in the fifth generation (5G) of wireless networks. More specifically, we provide a classification of relays based on their distinct communication attributes, such as processing, multiple antennas, storage, channel estimation, density and security level. In addition, we discuss the level of efficiency exhibited by each relay class, regarding their impact in delay-critical applications and green communications applications, while aiming at a specific security level at the physical layer. Then, relay selection policies are proposed taking into consideration the goals set by each application. Numerical evaluation of the proposed policies in terms of the average secrecy rate, average delay and power reduction show improved performance compared to other state-of-the-art solutions.