Mihaly Varga

Link performance prediction methods for cooperative relaying in wireless networks

Next generation wireless networks are likely to include cooperative relaying in their design. The performance of certain cooperation schemes in realistic scenarios should be evaluated by system level simulations (SLS). Moreover, the link adaptation (LA) and radio resource management become more complex, since cooperation schemes involve transmissions on more than one link and joint processing at the receiver. Both LA and SLS require quality metrics of the composite cooperative links, which capture the performance of the transmission scheme. This paper proposes performance prediction methods which rely on mutual information based metrics for two cooperation schemes which use one relay. Simulations show that, yet simple, these methods are accurate in predicting the coded block error rate (BLER).

Vertical Handover Decision Algorithm for Heterogeneous Cellular-WLAN Networks

Abstract Multi-access and heterogeneous wireless communications are considered to be one of the solutions for providing generalized mobility, high system efficiency and improved user experience, which are important characteristics of the Next Generation Networks. This paper proposes a Vertical Handover (VHO) decision algorithm for heterogeneous network architectures which integrate both cellular networks and Wireless Local Area Networks (WLANs). The cellular-WLAN and WLAN-WLAN VHO decisions are taken based on parameters which characterize both the coverage and the traffic load of the WLANs. Computer simulations performed in complex scenarios show that the proposed algorithm ensures better performance compared to “classical” VHO decision algorithms.

Link adaptation algorithm for distributed coded transmissions in relay-aided OFDMA systems

We propose a link adaptation algorithm for cooperative transmissions in an OFDMA-based wireless system. The algorithm aims at maximizing the spectral efficiency of a relay-aided communication link, while satisfying the block error rate constraints at both the relay and the destination nodes. The optimal solution would be to perform an exhaustive search over a high-dimensional space determined by all possible combinations of modulations, code rates and information block lengths on the individual links; clearly, such an approach has an intractable complexity. Our solution is to use a link performance prediction method and a trellis diagram representation such that the resulting algorithm outputs a link configuration that conveys as many as possible information bits and also fulfills the block error rate constraints. The proposed link adaptation algorithm has linear complexity with the number of available resource blocks, but still provides a very good performance, as shown by simulation results.

Link adaptation algorithm for distributed coded transmissions in cooperative OFDMA systems

This paper proposes a link adaptation algorithm for cooperative transmissions in the down-link connection of an OFDMA-based wireless system. The algorithm aims at maximizing the spectral efficiency of a relay-aided communication link, while satisfying the block error rate constraints at both the relay and the destination nodes. The optimal solution would be to perform an exhaustive search over a high-dimensional space determined by all possible combinations of modulations, code rates and information block lengths on the individual channels of the cooperative link; clearly, such an approach has an intractable complexity. Our solution is to use a link performance prediction method and a trellis diagram representation such that the resulting algorithm outputs the link configuration that conveys as many information bits as possible and also fulfilling the block error rate constraints. The proposed link adaptation algorithm has linear complexity with the number of available resource blocks, while still provides a very good performance, as shown by simulation results.

Comparison between measured and computed values of the mean mutual information per coded bits in OFDM based wireless transmissions

To provide a high system performance, the link adaptation algorithms need an accurate method to predict the Block Error Rate (BLER) performance of a transmission on a given realization of the wireless multipath propagation channel. A promising BLER prediction technique is based on mean mutual information of the received bits (MI). The main objective of this paper is to compare the MI values obtained at the reception of a real physical transmission, with the ones provided by the available analytical models. For this we implemented a transmission chain on an Universal Software Radio Peripheral (USRP) device. The obtained results shows relatively good coincidence, but as might be expected, in some cases the measured values slightly differ from the computed ones.

Link adaptation for cooperative communications with repetition coding

The employment of relay-nodes (RN) to assist the base station-user terminal (BS-UT) transmissions in wireless systems could lead to significantly increased performance when compared to the direct BS-UT transmissions, provided that a PER smaller than a target-PER should be ensured. Nevertheless, the performance of the cooperative BS-RN-UT link is depending on the coded-cooperation algorithm employed and on the quality of the three channels involved; moreover it requires additional time-frequency resources. Therefore, the metric used to assess the improvement brought by cooperative transmissions should be the average number of correctly received information bits on QAM symbols. This paper studies the efficiency provided by a cooperative BS-RN-UT link that employs the repetition coding algorithm, with respect to the SNR values of the constituent channels. The performance is evaluated within a generic transmission structure and compared to the spectral efficiency of the non-cooperative BS-UT link, establishing the SNR domains when the cooperative approach would perform better than the non-cooperative one. The employment of these results in the link adaptation and relay-activation processes of a relay-enhanced transmission are also discussed.

Game Theory Based Radio Resource Management Algorithm for Packet Access Cellular Networks

Abstract The goal of Radio Resource Management (RRM) mechanisms is to allocate the transmission resources to the users such that the transmission requests are satisfied while several constraints are fulfilled. These constraints refer to low complexity and power consumption and high spectral efficiency and can be met by multidimensional optimization. This paper proposes a Game Theory (GT) based suboptimal solution to this multidimensional optimization problem. The results obtained by computer simulations show that the proposed RRM algorithm brings significant improvement in what concerns the average delay and the throughput, compared to other RRM algorithms, at the expense of somewhat increased complexity.

Load balancing solution for heterogeneous wireless networks based on the knapsack problem

This paper proposes a practical approach to load balancing in heterogeneous wireless networks, which is based on the adaptation of the multiple knapsack problem to this specific scenario. Several algorithms capable of solving the knapsack problem are considered, namely a brute force, a dynamic programming and a greedy approach. The algorithms are evaluated by computer simulations in terms of the capability of finding the optimal solution and of computational complexity. The best algorithm identified, capable to generate near to optimal solutions while keeping low the computational complexity, is integrated into a load balancing mechanism intended to be used in real communication networks. The test scenario and the results obtained by practical experiments are presented and analyzed.

Load balancing algorithms in heterogeneous networks for intelligent transportation systems

Nowadays the Always Best Connected paradigm becomes more and more important, but fulfilling the requirements of this paradigm is not an easy task. Providing ubiquitous connectivity in the particular case of intelligent transportation systems requires, in most of the cases, the joint usage of the transmission capacities offered by several wireless access technologies. This paper proposes a Game Theory based load balancing algorithm capable to distribute efficiently the data flows generated by various services used by the passengers of the transportation vehicles over the transmission channels of several wireless networks. The paper proposes also two reference load balancing algorithms, the Round Robin and the Multiple Knapsack algorithms, for assessing the performance improvement brought by the Game Theory based algorithm. Simulations performed in a realistic scenario validate the good performance of the proposed algorithm and show that this algorithm is a good alternative for load balancing operations.

On the reliability performance of distributed Alamouti-SFBC schemes

This paper analyzes the block error rate (BLER) performance provided by a virtual MIMO scheme which implements a modified Alamouti Space-Frequency Block Code (SFBC) encoding-decoding algorithm using two fixed Decode & Forward relays that perform selective relaying, and single-antenna devices. It also analyzes the performance of the classical relay-based distributed Alamouti scheme with single-antenna devices and, as baseline, by the non-cooperative Alamouti scheme which uses an antenna array at the source. We present the theoretical derivations of the BLER provided by the three schemes vs. the SNRs of the employed channels, using an approximation of the BLER ensured by the channel code used on each constituent link and validate them by computer simulation, both for the downlink and uplink transmissions of a generic cellular network, over Rayleigh block-faded channels, for various positions of the user terminal vs. the base station and relays.