Davide Chiarotto

Spectrum Leasing via Cooperative Opportunistic Routing Techniques

A licensed multihop network that coexists with a set of unlicensed nodes is considered. Coexistence is regulated via a spectrum leasing mechanism that is based on cooperation and opportunistic routing. Specifically, the primary network consists of a source and a destination communicating via a number of primary relay nodes. In each transmission block, the next hop is selected in an on-line fashion based on the channel conditions (and thus the decoding outcome) in the previous transmissions, according to the idea of opportunistic routing. The secondary nodes may serve as extra relays, and hence potential next hops, for the primary network, but only in exchange for spectrum leasing. Namely, in return for their forwarding of primary packets, secondary nodes are awarded spectral resources for transmission of their own traffic. Secondary nodes enforce Quality-of-Service requirements in terms of rate and reliability when deciding whether or not to cooperate. Four policies that exploit spectrum leasing via opportunistic routing in different ways are proposed. These policies are designed to span different operating points in the trade-off between gains in throughput and overall energy expenditure for the primary network. Analysis is carried out for networks with a linear geometry and quasi-static Rayleigh fading statistics by using Markov chain tools. Different multiplexing techniques are considered for multiplexing of the primary and secondary traffic at the secondary nodes, namely orthogonal multiplexing (such as time, frequency or orthogonal code division multiplexing) and superposition coding. The optimality in terms of both throughput and primary energy consumption of superposition coding over all possible multiplexing strategies, for the given routing techniques, is proved. Finally, numerical results demonstrate the advantages of the proposed spectrum leasing solution based on opportunistic routing and illustrate the trade-offs between primary throughput and energy consumption.

Throughput and Energy Efficiency of Opportunistic Routing with Type-I HARQ in Linear Multihop Networks

Opportunistic routing is a well-known technique that exploits the broadcast nature of wireless transmissions and path diversity to form the route in an adaptive manner based on current channel conditions. This paper studies the throughput advantages of opportunistic routing over conventional multihop routing for linear multihop wireless networks with type-I Hybrid Automatic Repeat reQuest (HARQ) and quasi-static Rayleigh fading channels. The end-to-end throughput of opportunistic routing is derived using Markov chain tools and accounting for fading statistics. Both fixed-rate and optimal-rate transmissions are considered. Moreover, an investigation of the throughput using standard information-theoretic performance metrics for asymptotic signal-to-noise ratio regimes is provided. Specifically, the multiplexing gain and energy efficiency (i.e., minimum energy per bit) of both opportunistic and multihop routing are analyzed. Numerical results are given to corroborate the analysis.

Performance Analysis of an Opportunistic Relay Selection Protocol for Multi-Hop Networks

We analyze the performance of an interference-aware opportunistic relay selection protocol for multi-hop line networks which is based on the following simple rule: a node always transmits if it has a packet, except when its successive node on the line is transmitting. We derive analytically the saturation throughput and the end-to-end delay for two and three hop networks, and present simulation results for higher numbers of hops. In the case of three hops, we determine the throughput-optimal relay positions.

On the Impact of Channel Estimation Errors on MAC Protocols for MIMO Ad Hoc Networks

In this paper, we evaluate the performance of a Medium Access Control (MAC) protocol for MIMO ad hoc networks under imperfect channel estimation. To this end, we also present an analysis of channel estimation errors using correlator-based and Minimum Mean-Square Error (MMSE) channel estimators. Unlike similar works, we specifically focus on a scenario where the presence of several simultaneous, symbol-asynchronous signals makes the problem more complicated than in traditional channel estimation. In particular, we show that there is direct dependence of the channel estimation error on the instantaneous channel matrix. The model we propose in this paper makes it possible to quickly evaluate the performance of channel estimation schemes as a function of the system parameters. In this light, we include the effect of channel estimation errors in an ad hoc networking protocol simulator and thoroughly evaluate their impact. Our results show that there exists a significant interplay between the performance of MAC protocols for MIMO networks and the accuracy of channel estimation. Moreover, we show that interesting tradeoffs arise between MAC- and physical level-related parameters.

On the Statistics and MAC Implications of Channel Estimation Errors in MIMO Ad Hoc Networks

In this paper, we propose an analytical technique to evaluate the statistics of the channel estimation error in a simple multi-user ad hoc networking scenario. This problem is very relevant in situations where advanced PHY techniques are used (e.g., MIMO or interference cancellation) and channel state information may be needed. The presence of several simultaneous and non-orthogonal signals makes the problems significantly more complicated than in traditional channel estimation. In particular, there is direct dependence of the channel estimation error on the instantaneous channel matrix. The proposed model makes it possible to quickly evaluate the performance of channel estimation schemes as a function of the system parameters. In this light, we include the effect of channel estimation errors in an ad hoc networking protocol simulator and thoroughly evaluate their impact. Our results show that there exists a significant interplay between the performance of MAC protocols for MIMO networks and the accuracy of channel estimation. Moreover, we show that relevant tradeoffs arise between MAC- and PHY-level parameters which lead to the definition of design guidelines.

Spectrum leasing via cooperative opportunistic routing

A spectrum leasing mechanism is proposed for the coexistence between a primary and a secondary network that is based on cooperation and opportunistic routing. The primary network consists of a source and a destination communicating via a number of primary relay nodes. In each transmission slot, the next hop is selected in an on-line fashion based on the decoding outcomes in the previous transmissions according to the idea of opportunistic routing. The secondary nodes may serve as potential next hops for the primary network, but only in exchange for leasing of spectral resources so as to satisfy secondary quality-of-service constraints. Four policies based on spectrum leasing via opportunistic routing are proposed that provide different tradeoffs between gains in throughput and overall energy expenditure for the primary network. Analysis is carried out for networks with a linear geometry and quasi-static Rayleigh fading statistics by using Markov chain tools.

On the impact of transmit waveforms on channel estimation inaccuracies in distributed MIMO ad hoc networks

In this paper, we present a general analysis to evaluate the performance of correlator-based channel estimation in distributed MIMO ad hoc networks. This analysis is required to characterize the behavior of networks employing advanced physical-level (PHY) techniques, which call for precise assessment of channel effects in order to be operated correctly. To this end, we assume that several non-orthogonal signals are simultaneously received in a coordinated (i.e., frame-synchronous) but symbol-asynchronous fashion, and undergo flat, independent Rayleigh fading. We show that, in this scenario, channel estimates are affected not only by noise, but also by a displacement depending on the current channel realization. Such additional component can be modeled as a Gaussian random variable, of which we calculate the mean and variance, highlighting its dependence on the number of training symbols and the shape of transmit impulse. In particular, we show that an interesting interplay exists between MAC parameters (e.g., throughput) and the type of impulse chosen in an ad hoc networking protocol simulator.

On the tradeoff between MAC-level performance metrics in MIMO ad hoc networks with imperfect channel estimation

Classic approaches to channel estimation in MIMO ad hoc networks focus on the accuracy of the estimation techniques as applied to point-to-point links. However, when considering more complex networks, a number of further issues arise that have a significant impact on the performance of channel estimation. More specifically, ad hoc networks usually feature many simultaneous non-orthogonal communications whose impact on channel estimation as well as on higher layers of the protocol stack has not yet been thoroughly studied. In this paper we present some results on the effects of multiuser transmissions on channel state information accuracy in MIMO ad hoc networks, and evaluate the impact of imperfect channel estimation on the performance of MAC protocols. In particular, we consider a cross-layer MAC which accounts for the structure and behavior of the underlying PHY to make decisions on channel access; we then show the interplay between PHY parameters (such as the length of the training sequences used for channel estimation or the number of antennas) and MAClevel performance metrics such as throughput, efficiency, and success ratio. Our results show that relevant tradeoffs arise in this context and allow, e.g., to tune the MAC layer behavior by controlling the channel estimation accuracy.

Spectrum leasing via cooperative opportunistic routing in distributed ad hoc networks: Optimal and heuristic policies

A spectrum leasing strategy is considered for the coexistence of a licensed multihop network and a set of unlicensed nodes. The primary network consists of a source, a destination and a set of additional primary nodes that can act as relays. In addition, the secondary nodes can be used as extra relays and hence potential next hops following the principle of opportunistic routing. Secondary cooperation is guaranteed via the “spectrum leasing via cooperation” mechanism, whereby a cooperating node is granted spectral resources subject to a Quality of Service (QoS) constraint. The objective of this work is to find optimal as well as efficient heuristic routing policies based on the idea outlined above of spectrum leasing via cooperative opportunistic routing. The optimal policy is obtained by casting the problem in the framework of stochastic routing. The optimal performance is then numerically compared with two proposed heuristic routing schemes, which are shown to perform close to optimal solutions and as well being tunable in terms of end-to-end throughput vs primary energy consumption.

Soft capacity of OFDMA networks is suitable for soft QoS multimedia traffic

Multimedia traffic is expected to be widespread in next generation wireless networks, which will be likely based on Orthogonal Frequency Division Multiple Access. While multimedia content is heavily demanding in terms of network resources, it is also inherently adaptable at the application layer, thereby imposing soft QoS constraints, rather than strict requirements on a specific data rate. In this paper, we specifically investigate the suitability of such a medium access control rationale for this kind of traffic. It turns out that, if properly managed, next generation networks can accommodate several multimedia users, thanks to a proper exploitation of user and frequency diversity. However, on the application side a great deal of attention should be paid to take advantage of scalability of the video flows and adaptability of this kind of traffic, to exploit the network capacity at its fullest.