On the Interplay Between Edge Caching and HARQ in Fog-RAN

In a Fog Radio Access Network (Fog-RAN), edge caching is combined with cloud-aided transmission in order to compensate for the limited hit probability of the caches at the base stations (BSs). Unlike the typical wired scenarios studied in the networking literature in which entire files are typically cached, recent research has suggested that fractional caching at the BSs of a wireless system can be beneficial. This paper investigates the benefits of fractional caching in a scenario with a cloud processor connected via a wireless fronthaul link to a BS, which serves a number of mobile users on a wireless downlink channel using orthogonal spectral resources. The fronthaul and downlink channels occupy orthogonal frequency bands. The end-to-end delivery latency for given requests of the users depends on the HARQ processes run on the two links to counteract fading-induced outages. An analytical framework based on theory of Markov chains with rewards is provided that enables the optimization of fractional edge caching at the BSs. Numerical results demonstrate meaningful advantages for fractional caching due to the interplay between caching and HARQ transmission. The gains are observed in the typical case in which the performance is limited by the wireless downlink channel and the file popularity distribution is not too skewed

Performance analysis of collaborative hybrid-arq protocols over fading channels

Impairments due to multipath signal propagation on the performance of wireless communications systems can be efficiently mitigated with time, frequency or spatial diversity. To exploit spatial diversity, multiple-antenna technology has been thoroughly investigated and emerged as one of the most mature communications areas. However, the need for smaller user terminals, which results in insufficient spacing for antenna collocation, tends to limit the practical implementation of this technology. Without compromising terminal dimensions, future wireless networks will therefore have to exploit their broadcast nature and rely on collaboration between single-antenna terminals for exploiting spatial diversity. Among cooperative schemes, Collaborative ARQ transmission protocols, prescribing cooperation only when needed, i.e., upon erroneous decoding by the destination, emerge as an interesting solution in terms of achievable spectral efficiency. In this thesis, an information theoretical approach is presented for assessing the performance of Collaborative Hybrid-ARQ protocols based on Space-Time Block Coding. The expected number of retransmissions and the average throughput for Collaborative Hybrid-ARQ Type I and Chase Combining are derived in explicit form, while lower and upper bound are investigated for Collaborative Hybrid-ARQ Incremental Redundancy protocol, for any number of relays. Numerical results are presented to supplement the analysis and give insight into the performance of the considered scheme. Moreover, the issue of practical implementation of Space-Time Block Coding is investigated

Cooperative retransmission protocols in fading channels : issues, solutions and applications

Future wireless systems are expected to extensively rely on cooperation between terminals, mimicking MIMO scenarios when terminal dimensions limit implementation of multiple antenna technology. On this line, cooperative retransmission protocols are considered as particularly promising technology due to their opportunistic and flexible exploitation of both spatial and time diversity. In this dissertation, some of the major issues that hinder the practical implementation of this technology are identified and pertaining solutions are proposed and analyzed. Potentials of cooperative and cooperative retransmission protocols for a practical implementation of dynamic spectrum access paradigm are also recognized and investigated. Detailed contributions follow. While conventionally regarded as energy efficient communications paradigms, both cooperative and retransmission concepts increase circuitry energy and may lead to energy overconsumption as in, e.g., sensor networks. In this context, advantages of cooperative retransmission protocols are reexamined in this dissertation and their limitation for short transmission ranges observed. An optimization effort is provided for extending an energy- efficient applicability of these protocols. Underlying assumption of altruistic relaying has always been a major stumbling block for implementation of cooperative technologies. In this dissertation, provision is made to alleviate this assumption and opportunistic mechanisms are designed that incentivize relaying via a spectrum leasing approach. Mechanisms are provided for both cooperative and cooperative retransmission protocols, obtaining a meaningful upsurge of spectral efficiency for all involved nodes (source-destination link and the relays). It is further recognized in this dissertation that the proposed relaying-incentivizing schemes have an additional and certainly not less important application, that is in dynamic spectrum access for property-rights cognitive-radio implementation. Provided solutions avoid commons-model cognitive-radio strict sensing requirements and regulatory and taxonomy issues of a property-rights model

Cooperative jamming via spectrum leasing

Abstract—Secure communication rates can be facilitated or enhanced via deployment of cooperative jammers in a multi-terminal environment. Such an approach typically assumes ded-icated 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 improve-ments 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 maintain-ing the Stackelberg framework. I

Recruiting Multi-Antenna Transmitters as Cooperative Jammers: An Auction-Theoretic Approach

Abstract—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. I

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 efciency. 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. 1