Milan Zivkovic

Preamble-Based SNR Estimation in Frequency Selective Channels for Wireless OFDM Systems

Orthogonal frequency division multiplexing (OFDM) offers high data rates and robust performance in frequency selective channels by link adaptation utilizing information about the channel quality. A crucial parameter required for adaptive transmission is the signal-to-noise ratio (SNR). In this paper, we propose a novel SNR estimation algorithm for wireless OFDM systems based on the reuse of the synchronization preamble. The periodic structure of the preamble is utilized for the computationally efficient SNR estimation algorithm, based on the second-order moments of received preamble samples. The performance of the proposed algorithm is compared with the MMSE algorithm and two preamble-based algorithms found in the literature. It is shown that the proposed algorithm is robust against frequency selectivity and may therefore be used for subchannel SNR estimation.

An improved preamble-based SNR estimation algorithm for OFDM systems

A crucial parameter required for adaptive transmission in orthogonal frequency division multiplexing (OFDM) systems is the signal-to-noise ratio (SNR). In this paper, certain modifications to previously proposed preamble-based SNR estimator for wireless OFDM systems are suggested in order to improve estimation performance. Proposed modifications, related to DFT interpolation, are based on the adaptive selection of significant channel impulse response (CIR) paths utilizing the average noise power estimate obtained in frequency domain. The modified estimator shows performance improvement of average SNR estimation in low SNR regime and considerably outperforms original algorithm for SNR per subcarrier estimation.

Optimization of cooperative spectrum sensing and implementation on software defined radios

Reliable detection of primary user activity by spectrum sensing is a crucial issue of cognitive radio systems. The objective of cooperative spectrum sensing is to combine the detection results of multiple cognitive radios in order to maximize the probability of detecting unused spectrum while meeting a required reliability of detecting primary user activity. In this paper, a Kullback-Leibler distance-based optimization approach for the local decision thresholds of cooperative spectrum sensing is proposed. It is both computationally efficient and scalable with the number of cognitive radios. To validate the concept, real spectrum sensing results are used. The employed practical setup is based on software defined radio and detects a WiMAX-like OFDM signal. The presented numerical results illustrate the feasibility and effectiveness of the approach.

Preamble-based SNR estimation algorithm for wireless MIMO OFDM systems

Orthogonal frequency division multiplexing (OFDM) offers high data rates and robust performance in frequency selective channels by link adaptation utilizing information about the channel quality. Combined with multiple-input multiple-output (MIMO) technique it provides improved link reliability and increased data rate. A crucial parameter required for adaptive transmission is the signal-to-noise ratio (SNR). In this paper, we propose a novel SNR estimation algorithm for wireless 2×2 MIMO OFDM systems based on the reuse of the synchronization preamble. The periodic structure of the preamble is utilized for the computationally efficient estimation of average SNR and SNRs per subcarrier, based on the second-order moments of received preamble samples. The performance of the proposed algorithm is assessed for channels with different levels of frequency selectivity. Simulation results show robust performance of proposed algorithm even in strong frequency selective channels.

OFDM Synchronization in the presence of interference

Timing Synchronization at the receiver of an Orthogonal Frequency Division Multiplexing (OFDM) system is the first step towards extracting the information contained within the received OFDM frame. In this paper we will be looking at the robustness of the Schmidl and Cox timing synchronisation algorithm in a dynamic spectrum access environment where interference is present. Interference with respect to the secondary user refers to the electromagnetic wave transmitted by the primary user in an environment where the primary and the secondary users are sharing the same spectrum. In order to describe the occupied spectrum of the primary user, we will be utilising the fractional bandwidth (FBW) mode. The FBW mode would serve as an indicator describing the position of the secondary user on the shared spectrum. Initial testing was carried out using Matlab and the final implementation and testing were done on Software Defined Radio (SDR) using a testbed developed at the TI institute.

Performance Evaluation of Timing Synchronization in OFDM-Based Cognitive Radio Systems

In this paper, the performance of Schmidl and Cox (S&C) timing estimator in cognitive radio systems under a fractional bandwidth (FBW) scenario is investigated. The synchronization preamble is appropriately modified in order to avoid transmission in the subbands where primary users (PU) are active. However, at the receiver, the timing synchronizer processes all received signal samples within the given OFDM band without any knowledge of a PU presence, thus being affected by interference. Statistical properties of timing metric in the presence of interference caused by PU are derived for an AWGN channel.

Joint Frequency Synchronization and Spectrum Occupancy Characterization in OFDM-Based Cognitive Radio Systems

OFDM-based cognitive radio (CR) systems are shown to be an effective solution for increasing spectrum efficiency by activating the certain group of subcarriers (subbands) in locally available spectrum. However, each CR receiver should synchronize itself to appropriate carrier frequency and to identify currently activated subbands. Moreover, energy and bandwidth efficiency of CR systems can be improved if each CR could provide additional characterization of the local spectral content. In this paper a novel joint frequency synchronization and spectrum occupancy characterization method for OFDM-based CR systems is proposed. The synchronization preamble structure is appropriately modified in order to efficiently perform frequency offset estimation, to identify occupied subbands, and, finally, to provide SNR and interference power estimates as reliable quantitative indicators of spectrum occupancy. The performance of proposed method is evaluated for different amounts of spectrum occupancy and interference levels.

OFDM-Based Dynamic Spectrum Access

Optimal utilization of radio resources (bandwidth, transmit power) in multi-carrier based systems becomes very challenging due to coexistence of various wireless standards within the same frequency band. One possible solution is dynamic spectrum access (DSA) approach that allows several standards or users to opportunistically share the available spectrum resources without introducing mutual interference. Proposed demonstration, implemented in GNU Radio framework, enables interference-free coexistence of two OFDM-based systems within a common frequency band with optimally configured transmission parameters for given system constraints. Furthermore, a highly reconfigurable framework allows for implementation and evaluation of various transmission strategies for different DSA scenarios, different classes of given requirements and various sets of controllable parameters.

Reconfigurable framework for adaptive OFDM transmission

Optimal utilization of radio resources (bandwidth, transmit power) in multicarrier based systems becomes very challenging due to coexistence of various wireless standards within the same frequency band. Proposed demonstration, implemented in GNU Radio framework, enables capacity achieving OFDM-based data transmission between two network nodes with optimally configured transmission parameters for given system constraints. Furthermore, a highly reconfigurable framework allows for implementation and evaluation of various transmission strategies for different classes of given requirements and various sets of controllable parameters.

Optimum discrete signaling over channels with arbitrary noise distribution

General channels with arbitrary noise distributions and a finite set of signaling points are considered in this paper. We aim at finding the capacity-achieving input distribution. As a structural result we first demonstrate that mutual information is a concave function of the input distribution and a convex function of the channel transfer densities. Using the Karush-Kuhn-Tucker theory, capacity achieving distributions are then characterized by constant Kullback-Leibler divergence between each channel transfer density and the mixture hereof built by using the probabilities as weights. If, as a special case, the noise distribution and the signaling points are rotationally symmetric, then the uniform input distribution is optimal. For 2-PAM modulation and certain types of asymmetric noise distributions, including exponential, gamma and Rayleigh, we present extensive numerical evaluations of the optimal input. Furthermore, for 4-QAM we determine the optimal input from a restricted symmetric class of distributions for correlated Gaussian noise.