Matteo Butussi

Utility Maximization via Power and Rate Allocation with Outage Constraints in Nakagami-Lognormal Channels

The problem of maximizing a utility function while limiting the outage probability below an appropriate threshold is investigated. A coded-division multi access wireless network under mixed Nakagami-lognormal fading is considered. Solving such a utility maximization problem is difficult because the problem is non-convex and non-geometric with mixed integer and real decision variables and no explicit functions of the constraints are available. In this paper, three methods for the solution of the utility maximization problem are proposed. By the first method, a simple explicit outage approximation is used and the constraint that rates are integers is relaxed yielding a standard convex programming optimization that can be solved quickly but at the price of a reduced accuracy. The second method uses a more accurate outage approximation, which allows one solving the utility maximization problem by the Lagrange duality for non-convex problems and contraction mapping theory. The third method is a combination of the first and the second one. Numerical results show that the first method performs well for average values of the outage requirements, whereas the second one is always more accurate, but is also more computationally expensive. Finally, the third method gives same accuracy as the second one, but has a lower computational complexity only for a small number of transmitters.

Low Complexity Demapping of Rotated and Cyclic Q Delayed Constellations for DVB-T2

The new digital video broadcasting standard DVB-T2 includes the mandatory use of constellation rotation and cyclic Q delay (CRCQD) as a signal-space diversity technique. After transmission on the channel by orthogonal frequency division multiplexing (OFDM), at the receiver the constellation does not exhibit the regular shape of QAM and in particular optimum demapping must be jointly performed on the real-imaginary axis for both hard and soft decoding. We propose a low complexity demapping strategy for the CRCQD modulation. In the proposed approach, only a subset of the constellation points close to the received samples is considered for demapping. The proposed method is compared with existing techniques both in terms of complexity and performance over reference DVB-T2 channels.

Low Complexity Admission in Downlink Beamforming

We study the downlink of a system with multiple antennas at the base stations and propose two different schemes for admission control and beamforming selection when a new user enters the system. To keep the complexity low, only the power control, not the spatial signatures of beamformers for the already admitted users, are adjusted. Numerical examples illustrate the performance compared to joint optimization of all users

Analysis of interpolated channel estimation for mobile OFDM systems

In an OFDM system using pilots for channel estimation, time interpolation among pilots of different OFDM symbols is commonly used to improve the estimate. The estimated channel impulse response may be also windowed to further reduce noise and disturbances. However, for a transmission over a time-varying channel, suboptimal time interpolation, implemented with a filter having only a few taps not matched to the maximum Doppler frequency, degrades channel estimation. In this paper we provide an analysis of the effects of pilot time interpolation over time-varying channels. We show in particular that suboptimal time interpolation yields aliases in the estimated channel impulse response and we derive a closed-form expression of the aliases¿ variance. As aliases can lead to an erroneous estimate of channel length and consequent errors in windowing, we propose a technique to detect aliases and correct the channel length estimate. Parameters of the detection techniques are optimized by an analysis that provides closed-form expressions of the false alarm and miss detection probabilities. Numerical results show the merits of the proposed techniques in a mobile transmission of the terrestrial digital video broadcasting standard (DVB-T).

Power and Rate Control Outage Based in CDMA Wireless Networks under MAI and Heterogeneous Traffic Sources

We characterize the maximum throughput achievable for the up-link of a power-controlled WCDMA wireless system with variable spreading factor. Our system model includes multi access interference caused by users with heterogeneous data sources, and quality of service expressed in terms of outage probability. Inner loop and outer loop power control mechanisms are also explicitly taken into account. We express the system throughput as the sum of the users transmission rates, and propose a mixed integer optimization program where the objective function is the sum of the rates under outage probability constraints. Then, we solve the optimization problem proposing an efficient optimal approach based on two steps: firstly, a modified problem provides feasible solutions, and then the optimal solution is obtained with branch and bound criteria. Numerical results confirm the validity of our approach, and show how the throughput depends on the power control fluctuation, activity of the sources, and quality of service.

LLR Compression for BICM Systems Using Large Constellations

Digital video broadcasting (DVB-C2) and other modern communication standards increase diversity by means of a symbol-level interleaver that spans over several codewords. De-interleaving at the receiver requires a large memory, which has a significant impact on the implementation cost. In this paper, we propose a technique that reduces the de-interleaver memory size. By quantizing log-likelihood ratios with bit-specific quantizers and compressing the quantized output, we can significantly reduce the memory size with a negligible increase in computational complexity. Both the quantizer and compressor are designed via a GMI-based maximization procedure. For a typical DVB-C2 scenario, numerical results show that the proposed solution enables a memory saving up to 30%.

Multiple Adaptive Frequency Filtering for OFDM Channel Estimation

In orthogonal frequency division multiplexing (OFDM) systems using pilots, channel estimation is performed on pilots and then interpolated over the time and the frequency axis. As time-interpolated estimates have a different mean square error than pilot estimates we propose the use of multiple adaptive filters for exploiting the frequency correlation of the channel. Each filter operates on a different configuration of pilot and time-interpolated estimates and is adapted independently from the other filters. As a simpler suboptimal solution we consider also the use of a single filter, whose adaptation however takes into account the reliability of the various estimates. Performance results are reported with reference to the DVB-T and DVB-T2 standards showing that the proposed technique performs similar or better than existing approaches at a much lower complexity.

Improved channel duration estimate for mobile OFDM systems

Channel estimation for OFDM system using pilot carriers usually comprises time interpolation of the estimates across more OFDM symbols and then frequency interpolation. In order to improve estimate, the frequency response can be transformed into the time domain and windowed. To this end, an estimate of the channel duration is necessary and this process is particularly sensitive to the time interpolation process. In fact, for a transmission over a time-varying channel, if the time interpolation of the pilot estimates is not accurate, aliases arise in the estimated channel impulse response, thus affecting the channel duration estimate. We propose a technique to detect aliases and correct the estimation of channel duration. Parameters of the detection techniques are optimized by an analysis that provides closed-form expressions of the false alarm and miss detection probabilities. Numerical results show the merits of the proposed techniques in a mobile transmission of the terrestrial digital video broadcasting standard (DVB-T).

Outage-Based Rate Maximization in CDMA Wireless Networks

The problem of maximizing the sum of the transmit rates while limiting the outage probability below an appropriate threshold is investigated for networks where the nodes have limited processing capabilities. We focus on CDMA wireless network whose rates are characterized under mixed Rayleigh- lognormal fading. The outage probability is given implicitly by a complex function so that solving the optimization problem requires substantial computing. In this paper, we propose a novel explicit approximation of this function that allows solving the problem in an affordable manner. We propose two solutions of the maximization problem with the simplified outage probability constraint: one solves the problem using mixed integer-real programming. The other relaxes the constraints that rates be integers yielding a standard convex programming optimization that can be solved much faster. Numerical results show that our approaches perform well for average values of the outage requirements.

Multiple adaptive frequency filtering for OFDM channel estimation

Transmission of pilot symbols in orthogonal frequency division multiplexing (OFDM) systems allows for an efficient least square (LS) channel estimate on pilots and its time/frequency interpolation over data carrier. In order to improve channel estimate, the correlation among pilot estimated and time-interpolated estimates can be exploited. However, while most approaches consider a single smoothing filter over time-interpolated estimates, we propose the use of multiple adaptive filters that take into account the different correlation among pilot estimate and interpolated estimates. In fact, time-interpolated estimates have different reliability than pilot estimates and thus must be weighted differently in the adaptive filtering.We consider both least mean square (LMS) and Kalman filter approach for the filter coefficient adaptation and aim at low-complexity solutions with respect to alternative optimal techniques. Moreover, as a simpler approach we consider also the use of a single adaptive filter updated taking into account the reliability of the various estimates. Performance results are reported with reference to the DVB-T standard, showing the merits of the various techniques both in terms of mean square error of the channel estimate and complexity of the adaptive techniques.