Furuzan Atay Onat

Threshold Selection for SNR-based Selective Digital Relaying in Cooperative Wireless Networks

This paper studies selective relaying schemes based on signal-to-noise-ratio (SNR) to minimize the end-to-end (e2e) bit error rate (BER) in cooperative digital relaying systems using BPSK modulation. In the SNR-based selective relaying, the relay either retransmits or remains silent depending on the SNRs of the source-relay, relay-destination, and source-destination links. Different models assuming the availability of different sets of instantaneous and average SNR information at the relay are studied. For each model, the optimal strategy to minimize the e2e BER is a different threshold rule on the source-relay SNR, if the link SNRs are uncorrelated in time and space. Approximations for the optimal threshold values that minimize the e2e BER and the resulting performance are derived analytically for BPSK modulation. Using the derived threshold the e2e BER can be reduced significantly compared to simple digital relaying. By studying the performance under different models, it is shown that knowledge of the instantaneous source-destination SNR at the relay can be exploited. The gain from this knowledge is higher when the average source-destination SNR is large. However, knowledge of the instantaneous relay-destination SNR at the relay does not change performance significantly.

Efficient Cooperative Diversity Schemes and Radio Resource Allocation for IEEE 802.16j

This paper studies various cooperative diversity schemes for orthogonal frequency division multiple access (OFDMA)- time division duplex (TDD) based two-hop cellular networks in low mobility scenarios, where the instantaneous channel state information is available at the base station. A user scheduling and radio resource allocation technique is developed in order to efficiently integrate various cooperative diversity schemes for the emerging IEEE 802.16j based systems. The analysis of the system with this scheduler shows that a simple cooperative diversity scheme which dynamically selects the best scheme between conventional relaying and direct transmission is promising in terms of throughput and implementation complexity. The conventional relaying refers to the scheme where the destination relies solely on the signals received through the relay.

Asymptotic BER analysis of threshold digital relaying schemes in cooperative wireless systems

Threshold relaying is an effective technique to achieve cooperative diversity in uncoded cooperative wireless networks, which suffer from error propagation due to detection errors at the relays. This paper analyzes the asymptotic end-to-end (e2e) bit error rate (BER) of threshold digital relaying. A three node network with a source, destination and relay and with links experiencing independent Rayleigh fading is considered. It is shown that, as the average link signal-to-noise ratios (SNR) are increased simultaneously, the optimal threshold that minimizes the e2e BER increases as log(SNR). The resulting e2e BER decreases as log(SNR)/SNR2. Moreover, any threshold of the form log(cSNR), where c is a positive constant, achieves the same order of e2e BER as the one achieved by the optimal threshold and provides dual diversity. A value of c that performs very close to the optimal threshold is also proposed.

Fast track article: Generating random graphs for the simulation of wireless ad hoc, actuator, sensor, and internet networks

In this paper, we consider generation of graphs that represent specific scenarios that appear in wireless ad hoc, actuator, sensor and Internet networks. Most simulation studies for these networks use connected random unit disk graphs generated by placing nodes randomly and independently from each other. However, in real life usually networks are created in a cooperative manner; certain restrictions are imposed during the placement of a new node in order to improve network connectivity and functionality. This article is an initial study on how constrained connected random unit graphs (C-CRUG) can be generated by fast algorithms and what kind of desirable characteristics can be achieved compared to completely random graphs, especially for sparse node densities.

Optimum Threshold for SNR-Based Selective Digital Relaying Schemes in Cooperative Wireless Networks

We study selective digital relaying schemes where the relay may choose to retransmit or to remain silent based on the qualities of the links between the source, relay and the destination. We first analyze a baseline scheme, called static relaying, where the relaying decisions are based only on the average signal-to-noise ratio (SNR) values of all the links. The second scheme, dynamic relaying, allows the relay to make decisions based on the instantaneous SNR of the source-relay link and average SNRs of the relay-destination and source-destination links. We show that, in dynamic relaying the optimal strategy to minimize the average end-to-end bit error rate is a threshold rule on the instantaneous SNR of the source-relay channel. In this case, the optimal threshold value is a function of average SNR of relay-destination and source-destination channels. We derive closed-form expressions for the optimal threshold and the bit error performance achieved by this threshold. We show that dynamic relaying can provide significant performance advantage over static relaying.

Threshold Based Relay Selection in Cooperative Wireless Networks

This paper considers two-hop cooperative relaying using multiple relays and proposes a threshold based relay selection protocol. In this protocol the relays are selected among those having received SNR higher than a threshold value. The relay selection is performed by the destination based on the received SNRs at the destination during the last hop. The exact bit error rate of this protocol is derived and it is shown that it achieves full diversity order. Unlike some other full diversity achieving protocols in the literature, the requirement that the exact/average SNRs of the source-relay links be known at the destination is eliminated using an appropriate SNR threshold.

Threshold-based relay selection for detect-and-forward relaying in cooperative wireless networks

This paper studies two-hop cooperative demodulate-and-forward relaying using multiple relays in wireless networks. A threshold based relay selection scheme is considered, in which the reliable relays are determined by comparing source-relay SNR to a threshold, and one of the reliable relays is selected by the destination based on relay-destination SNR. The exact bit error rate of this scheme is derived, and a simple threshold function is proposed. It is shown that the network achieves full diversity order () under the proposed threshold, where is the number of relays in the network. Unlike some other full diversity achieving protocols in the literature, the requirement that the instantaneous/average SNRs of the source-relay links be known at the destination is eliminated using the appropriate SNR threshold.

Generating Random Graphs for Wireless Actuator Networks

In this paper, we consider graphs created by actuators (people, robots, vehicles etc.) in sensor-actuator networks. Most simulation studies for wireless ad hoc and sensor networks use connected random unit disk graphs generated by placing nodes randomly and independently from each other. However, in real life networks are created by actuators in a cooperative manner. Usually certain restrictions are imposed during the placement of a new node in order to improve network connectivity and functionality. This article is an initial study on how connected actuator graphs (CAG) can be generated by fast algorithms and what kind of desirable characteristics can be achieved compared to completely random graphs, especially for sparse node densities. We describe several CAG generation schemes where the next node (actuator) position is selected based on the distribution of the nodes already placed. In our Minimum Degree Proximity algorithm (MIN-DPA), a new node is placed to be a neighbor of an existing node with the lowest degree (number of neighbors). In our Maximum Degree Proximity algorithm (MAX-DPA), a new node cannot be placed to increase the degree of any existing node over a pre-specified parameter limit. We show that these new algorithms are significantly faster than the well-known random unit graph generation scheme for sparse graphs. The graphs generated by these new schemes are not necessarily drawn from the same distribution as those generated by the independent node placement. Thus, we explore their properties by studying their average node degree and partition patterns.

Enabling Partial Forwarding by Decoding-Based One and Two-Stage Selective Cooperation

With selection decode-and-forward (SDF) relaying, an accurate forwarding decision is essential to avoid error propagation and achieve full cooperation diversity. The methods in literature are either based on CRC or SNR thresholds, both resulting in poor end-to-end performance in practical wireless scenarios with non-block fading and channel coding. We propose to improve performance by accurate and frequent adaptation with partial forwarding. To achieve this in channel-coded systems, we introduce a decoding-based forwarding decision at the relay and propose (1) minimum path difference (MPD) as a non-overhead and accurate decoding-based decision metric and (2) MPD-based SDF protocols with one and two decision stages. Our performance study validates that these protocols significantly improve SDFs end-to-end BER under realistic assumptions.

WLC35-6: Relay-Assisted Spatial Multiplexing in Wireless Fixed Relay Networks

Fixed relays are expected to be a part of future infrastructure-based wireless networks. Besides coverage extension, such relays can form advanced architectures due to the flexibility in their power expenditure and physical size. This paper explores the potential benefits of multi-antenna relays for spatial multiplexing of independent data sources sending data to a common multi-antenna destination such as a base station or an access point. Overall, the system resembles a horizontally coded layered space-time architecture. In particular, we consider zero forcing decision feedback (ZF-DF) type MIMO receivers and study their outage performance under various (non-selective and selective) digital relaying protocols. For diversity relaying protocol, we propose two schemes, Joint ZF-DF and Parallel ZF-DF, for joint processing (combining and decoding) of the direct user signals and the signal from the relay. We show that with the proposed selective diversity relaying protocols and joint ZF-DF processing, the outage probability of the system can be decreased significantly.