Fotis Foukalas

Spectral efficiency of cognitive radio networks under interference constraint and QoS guarantees

We study the problem of maximizing spectral efficiency of cognitive radio network deployments subject to an interference constraint and under specific quality of service (QoS) guarantees. The interference constraint corresponds to the upper limit of the received power that can be tolerated at the licensed users due to transmissions from unlicensed users. The QoS guarantees stem from the requirements imposed by the applications running at the users terminals. A cross-layer design is adopted that maps the users requirements into delay related QoS guarantees at the data link layer and error probability QoS guarantees at the physical layer. The obtained numerical results provide important insights regarding the impact of the considered constraint and guarantees on the achievable spectral efficiency of cognitive radio networks.

A Study on the Performance of Adaptive Modulation and Cross-Layer Design in Cognitive Radio for Fading Channels

Cognitive radio is considered as one of the main enablers for provisioning dynamic and flexible spectrum/channel allocation in wireless communications. On the other hand several physical layer mechanisms such as adaptive modulation, multiple-input multiple output systems, advanced channel coding and/or combinations of them enhance the capacity of wireless networks. However little effort has been put till now in studying the performance gains of physical layer mechanisms with the presence of cognition capabilities. The incorporation of cognitive mechanisms demands more detailed studies for assessing the impact on the spectral efficiency. To this direction, cross-layer combination of such a physical layer with upper layers should be also considered as a case study in a cognitive wireless environment. In this work we present a study on the spectral efficiency of adaptive modulation and coding which is one of the most promising schemes of applying cognitive radio at the physical layer. Besides, we study a cross-layer combination of adaptive modulation with upper layers in the same cognitive context. We prove that the performance gain of cognitive radio over such a physical layer is not negligible.

Joint power control and spectrum sensing for capacity maximisation in spectrum sharing systems

We study the problem of joint power control and spectrum sensing for maximising the capacity at the secondary user while protecting the primary users transmissions in spectrum sharing cognitive radio systems. Power control regulates the transmission power of the secondary user and spectrum sensing regulates the sensing time and the sensing threshold that care for the primary users protection. This problem is a capacity maximisation problem that we formulate and solve using an iterative greedy algorithm due to its complex form. The solution of the proposed algorithm leads to the global optimal solution that represents the optimal triplet values of transmission power, sensing time and threshold. The obtained results show the potential capacity maximisation that is achieved by the proposed joint design as long as the primary users protection is provided. Finally the convergence behaviour of the proposed algorithm is assessed in terms of the needed iterations for enhancing the capacity of spectrum sharing systems.

Capacity optimization through sensing threshold adaptation for cognitive radio networks

We propose capacity optimization through sensing threshold adaptation for sensing-based cognitive radio networks. The objective function of the proposed optimization is the maximization of the capacity at the secondary user subject to transmit power and sensing threshold constraints for protecting the primary user. After proving the concavity of capacity on sensing threshold, the problem is solved using the Lagrange duality decomposition method in conjunction with a subgradient iterative algorithm. The numerical results show that the proposed optimization can lead to significant capacity maximization for the secondary user as long as this is affordable to the primary user.

On the performance of adaptive modulation in cognitive radio networks

We study the performance of cognitive radio networks (CRNs) when incorporating adaptive modulation at the physical layer. Three types of CRNs are considered, namely opportunistic spectrum access (OSA), spectrum sharing (SS) and sensing-based SS. We obtain closed-form expressions for the average spectral efficiency achieved at the secondary network and the optimal power allocation for both continuous and discrete rate types of adaptive modulation assuming perfect channel state information. The obtained numerical results show the achievable performance gain in terms of average spectral efficiency and the impact on power allocation when adaptive modulation is implemented at the physical layer that is due to the effect of the cut-off level that is determined from the received signal-to-noise ratio for each CRN type. The performance assessment is taking place for different target bit error rate values and fading regions, thereby providing useful performance insights for various possible implementations.

Protocol reconfiguration using component-based design

Previous modular protocol design and implementation allow a flexible configuration and reconfiguration of protocol layers or full protocol stacks. However, in our days, software engineering technologies introduce new methods for designing and specifying modular software. Such a technology is the component-based software technology. Using those techniques, a software system could be modular. This paper proposed a reconfigurable protocol design and specification approach as well as a protocol reconfiguration management/runtime model based on protocol components that represent distinct protocol functions, which in previous works have been designed and specified as modules. The following content could be considered as a suggestion for a UML profile for protocol components and protocol reconfiguration.

Collision avoidance in TV white spaces: a cross-layer design approach for cognitive radio networks

One of the most promising applications of cognitive radio networks (CRNs) is the efficient exploitation of TV white spaces (TVWSs) for enhancing the performance of wireless networks. In this paper, we propose a cross-layer design (CLD) of carrier sense multiple access with collision avoidance (CSMA/CA) mechanism at the medium access control (MAC) layer with spectrum sensing (SpSe) at the physical layer, for identifying the occupancy status of TV bands. The proposed CLD relies on a Markov chain model with a state pair containing both the SpSe and the CSMA/CA from which we derive the collision probability and the achievable throughput. Analytical and simulation results are obtained for different collision avoidance and SpSe implementation scenarios by varying the contention window, back off stage and probability of detection. The obtained results depict the achievable throughput under different collision avoidance and SpSe implementation scenarios indicating thereby the performance of collision avoidance in TVWSs-based CRNs.

Cross-layer design in opportunistic spectrum access-based cognitive radio networks

Cognitive radio is considered as one of the most important enablers for achieving enhanced spectral efficiency in wireless communications. In this work, we present a cross-layer design for reliable data transmission over a cognitive radio network which combines adaptive modulation at the physical layer and hybrid automatic repeat request at the data link layer. The cognitive radio network follows the principles of opportunistic spectrum access that utilises an optimal power adaptation policy for channel allocation. The obtained numerical results denote that the considered approach achieves significant spectral efficiency improvement and therefore it could be deployed in wireless communication networks that encompass cognitive capabilities. Furthermore, we assess the introduced interference and we show that it can be kept within levels that do not jeopardise our design.

Practical implementation of cross-layer design in broadband wireless access networks

The established success of wireless communications in terms of high data rates provision is undoubtedly owed to several combinations of different mechanisms. Such a combination is the well-known forward error correction (FEC) mechanism that combines automatic-repeat-request (ARQ) protocol and error correction capabilities. Furthermore, in the last decade, a cross-layer design provides the combination of different mechanisms across the layers and thus the performance gain is increased in wireless communications over fading channels. In this paper, we present a practical cross-layer design implementation that combines adaptive modulation and coding (AMC) and hybrid ARQ at the physical and data link layer by using different error correction schemes. Our implementation is based on a cross-layer design of AMC and HARQ. We perform simulations with different channel coding and modulation schemes that are implemented in broadband wireless access networks as IEEE 802.16 and 3GPP HSPA and thus the results obtained show the performance achieved by this practical cross-layer implementation in terms of spectral efficiency in two main broadband wireless access networks available into the market.

On Cross-Layer Design of AMC Based on Rate Compatible Punctured Turbo Codes

This paper extends the work on cross-layer design which combines adaptive modulation and coding at the physical layer and hybrid automatic repeat request protocol at the data link layer. By contrast with previous works on this topic, the present development and the performance analysis as well, is based on rate compatible punctured turbo codes. Rate compatibility provides incremental redundancy in transmission of parity bits for error correction at the data link layer. Turbo coding and iterative decoding gives lower packet error rate values in low signal-to-noise ratio regions of the adaptive modulation and coding (AMC) schemes. Thus, the applied cross-layer design results in AMC schemes can achieve better spectral efficiency than convolutional one while it retains the QoS requirements at the application layer. Numerical results in terms of spectral efficiency for both turbo and convolutional rate compatible punctured codes are presented. For a more comprehensive presentation, the performance of rate compatible LDPC is contrasted with turbo case as well as the performance complexity is discussed for each of the above codes.