Igor Stanojev

Spectrum Leasing to Cooperating Secondary Ad Hoc Networks

The concept of cognitive radio (or secondary spectrum access) is currently under investigation as a promising paradigm to achieve efficient use of the frequency resource by allowing the coexistence of licensed (primary) and unlicensed (secondary) users in the same bandwidth. According to the property-rights model of cognitive radio, the primary terminals own a given bandwidth and may decide to lease it for a fraction of time to secondary nodes in exchange for appropriate remuneration. In this paper, we propose and analyze an implementation of this framework, whereby a primary link has the possibility to lease the owned spectrum to an ad hoc network of secondary nodes in exchange for cooperation in the form of distributed space-time coding. On one hand, the primary link attempts to maximize its quality of service in terms of either rate or probability of outage, accounting for the possible contribution from cooperation. On the other hand, nodes in the secondary ad hoc network compete among themselves for transmission within the leased time-slot following a distributed power control mechanism. The investigated model is conveniently cast in the framework of Stackelberg games. We consider both a baseline scenario with full channel state information and information-theoretic transmission strategies, and a more practical model with long-term channel state information and randomized distributed space-time coding. Analysis and numerical results show that spectrum leasing based on trading secondary spectrum access for cooperation is a promising framework for cognitive radio.

Energy efficiency of non-collaborative and collaborative Hybrid-ARQ protocols

In this paper, we consider the energy efficiency of truncated hybrid-ARQ (HARQ) protocols in a single-user link (i.e., non-collaborative HARQ), or with the inclusion of a relay station (i.e., collaborative HARQ). The total energy consumption accounts for both the transmission energy and the energy consumed by the transmitting and receiving electronic circuitry of all involved terminals (source, destination and, possibly, the relay). Using the transmission time and transmission energy of each packet as optimization variables, the overall energy is minimized under an outage probability constraint for HARQ Type I, HARQ chase combining and HARQ incremental redundancy protocols (in the latter case, a tight lower bound is considered). Numerical optimization provides insight into the optimal design choices that enhance energy efficiency with HARQ protocols. It is shown, for instance, that, if the circuitry energy consumption is not negligible, selection of the transmission energy is not only dictated by the outage constraint, but is also significantly affected by the need to reduce the number of retransmissions. Our results also demonstrate the performance limitations of collaborative HARQ protocols in terms of energy efficiency, when circuitry consumption is properly accounted for.

Performance of multi-relay collaborative hybrid-ARQ protocols over fading channels

According to collaborative hybrid automatic-repeat-request (ARQ) protocols, relay stations that have been able to decode the original message from previous transmissions, collaborate with the source in future retransmissions by jointly sending a space-time codeword. In this letter, analysis of the average number of retransmissions and throughput of collaborative hybrid-ARQ Type I (i.e., without memory) and chase combining (i.e., with memory) protocols is provided for any number of relays.

Performance Analysis of Collaborative Hybrid-ARQ Incremental Redundancy Protocols Over Fading Channels

Future wireless communication systems are expected to rely heavily on spatial diversity for mitigation of fading impairments. In scenarios where practical constraints prevent the collocation of multiple antennas on a single user terminal, collaboration between single-antenna nodes becomes the only viable solution. Among cooperative schemes, collaborative ARQ transmission protocols, prescribing cooperation only upon erroneous decoding by the destination, emerge as an interesting solution in terms of achievable spectral efficiency. In this paper, an information theoretical approach is presented for assessing the performance of collaborative hybrid-ARQ protocol based on incremental redundancy. Upper and lower bounds for the expected number of retransmissions and the average throughput are derived in explicit form, for any number of relays. Numerical results are presented to supplement the analysis and give insight into the performance of the considered scheme

Spectrum Leasing via Distributed Cooperation in Cognitive Radio

The concept of cognitive radio (or secondary spectrum access) is currently under investigation as a promising paradigm to achieve efficient use of the frequency resource. In this paper, we consider a decentralized cognitive radio model based on spectrum leasing, whereby a primary (licensed) user leases its bandwidth for a fraction of time to a network of independent secondary (unlicensed) terminals in exchange for cooperation. On one hand, the primary user decides whether to exploit (space-time coded) cooperation from the network of secondary terminals in order to maximize its own transmission rate. On the other hand, secondary terminals accept to cooperate with the primary only if compensated with a large enough fraction of time for their own transmission, towards the goal of maximizing their rate discounted by the overall cost of transmitted power. The considered model is studied in the framework of Stackelberg games, with the primary and the set of secondary users modelled as the (Stackelberg) game leader and the follower, respectively. Numerical results show that spectrum leasing based on trading secondary spectrum access for cooperation is a promising framework for cognitive radio.

On the Energy Efficiency of Hybrid-ARQ Protocols in Fading Channels

As the distance between terminals in modern wireless networks tends to decrease, the energy consumption issue, conventionally assumed to be exclusively dominated by the transmission power, needs to be revaluated. In particular, retransmission (ARQ) protocols that typically reduce the transmission energy required to obtain a given error probability on the channel (at the expense of a larger delay), also increase the energy consumed by the circuitry other than the power amplifier. In this paper, the energy efficiency of hybrid-ARQ type I, chase combining and incremental redundancy protocols in Rayleigh fading channels, is analyzed by accounting for the energy consumed by the transmitting and receiving electronic circuitry. It is shown that the advantages of hybrid-ARQ protocols in terms of energy consumption strictly depend on the transmission range.

Cooperative jamming via spectrum leasing

Secure communication rates can be facilitated or enhanced via deployment of cooperative jammers in a multi-terminal environment. Such an approach typically assumes dedicated and/or altruistic jamming nodes, investing their resources for the good of the whole system. In this paper, we demonstrate that jammers can be recruited to provide significant improvements of secrecy rates even when this assumption is alleviated. A game-theoretic framework is proposed where a source node, towards the maximization of its secrecy rate, utilizes the jamming services from a set of non-altruistic nodes, compensating them with a fraction of its bandwidth for transmission of their user data. With the goal of maximizing their user-data transmission rate priced by the invested power, potential cooperative jammers will provide the jamming/transmitting power that is generally proportional to the amount of leased bandwidth. Elaborating initially on a single-jammer scenario, interaction between the source and a cooperative jammer is modeled as the Stackelberg leader-follower game. The scheme is further extended to involve multiple potential jammers, applying competition mechanisms such as the auctioning and power control game, while maintaining the Stackelberg framework.

Recruiting multi-antenna transmitters as cooperative jammers: An auction-theoretic approach

This paper proposes and investigates a distributed mechanism that motivates otherwise non-cooperative terminals to participate as cooperative jammers assisting a source-destination pair that communicates over a wireless medium, in the presence of an eavesdropper from whom the communicated messages need to be kept secret. The cooperation incentive is provided by an opportunity for potential jammers, possibly equipped with multiple antennas, to utilize the spectrum belonging to the ongoing secure transmission for their own data traffic. A fully decentralized framework is put forth through a competition of potential cooperative jammers for spectrum access by trying to make the jamming offer that most improves the secrecy rate of the source-destination pair. Effective arbitration of cooperative jamming is performed using auction theory, with the source in the role of the auctioneer, and the jammers acting as bidders. The proposed scheme can be alternatively seen as a practical basis for the implementation of cognitive radio networks operating according to the property-rights model, i.e., spectrum leasing.

On the optimal number of hops in linear wireless ad hoc networks with hybrid ARQ

In this paper, we study the optimal design of a linear wireless multihop network that employs Hybrid Automatic Repeat reQuest (HARQ) protocols in a quasi-static fading environment. Data transmission is assumed to be delay-tolerant up to a certain predefined maximum delay (in transmission slots) that is larger or equal to the number of hops. Multihop routing is implemented through a time-division scheduling and decode-and-forward relaying. Excess time is exploited for retransmissions using either HARQ Type I or Chase Combining protocols on the hops that result in link-outage. Relying on analysis and numerical optimization, we seek answer to the following question: given the maximum allowed delay and signal-to-noise ratio, what is the optimal number of hops that maximizes the end-to-end throughput?

An Auction-Based Incentive Mechanism for Non-Altruistic Cooperative ARQ via Spectrum-Leasing

We propose and analyze a novel decentralized mechanism that motivates otherwise non-cooperative stations to participate as relays in cooperative ARQ protocol. Cooperation incentive is provided by the possibility for the source to lease a portion of retransmission slot for the traffic of relaying terminals. To further leverage the opportunistic nature of cooperative ARQ and obtain a fully decentralized solution, the (motivated) relaying nodes compete for access to the retransmission slot by trying to make the best retransmission offer. Effective arbitration of cooperative retransmissions is performed using auction theory (bidding), with the source in the role of the auctioneer, the relaying nodes acting as the bidders and the (use of the) retransmission slot as the bidding article. It is noted that the proposed solution can be seen as a practical framework for the implementation of cognitive radio networks running according to the property-rights model (spectrum leasing). Numerical results and analysis confirm the efficient dynamic resource allocation property of the proposed scheme, revealing the relevant gains in terms of expected number of (re)transmissions required for successful data delivery for both the source (primary) and the cooperating (secondary) nodes.