Daniele Veronesi

CTH15-4: Multiple Frequency Offset Compensation in Cooperative Wireless Systems

Multipath fading can be effectively combatted in wireless networks of single-antenna nodes by employing cooperative transmission techniques. In a wide variety of these cooperative techniques, multiple nodes encode a common data stream and then simultaneously put a signal onto the physical channel to exploit spatial diversity in the channel. For such a cooperative system, often it is difficult or undesirable from a complexity perspective to accurately lock the multiple transmitting nodes to a common oscillator, and thus the problem arises of multiple mismatches between the local oscillators of the transmit nodes and that of the receiver. Previously, such a situation has only been addressed through standard channel estimation; however, such techniques are not effective except for very small oscillator offsets. Here, a novel minimum mean square error (MMSE) decision feedback equalizer (DFE) that explicitly accounts for the frequency offsets is proposed for the receiver, and its performance is compared to standard approaches.

Characterization of in-home MIMO power line channels

In this paper a statistical description of in-home MIMO power line channels is derived, based on channel measurements in the 0 to 100 MHz range. In particular, it is shown that the power delay profile has a statistical distribution that could be well described by Weibull and Gaussian distributions. It is also confirmed that the channel attenuation has a linear behavior as a function of the frequency and that the average channel attenuation and the root mean square of the delay spread are inversely correlated. Furthermore, a statistical description of the MIMO power line channel coefficients is provided by using a suitable definition of the correlation among the channel coefficients. Based on the extracted statistical descriptions a new model for MIMO power line channels is proposed. Finally, the consistency between the measured and the generated power line channels is shown.

Multiple Frequency Offsets Estimation and Compensation for Cooperative Networks

For a cooperative wireless network using delay diversity, the equivalent channel seen at the receiver is multipath and time-varying, as each transmitter has a local oscillator with a random frequency offset. The authors propose to use orthogonal frequency division multiplexing (OFDM) modulation, in order to efficiently implement equalization in the frequency domain; inter-carrier interference arising from multiple frequency offsets is instead compensated by a linear equalizer. For the estimate of the time-varying channel parameters a new technique was proposed that reduces the number of parameters to be estimated and exploits the multiple frequency-offsets.

Noise correlation and its effect on capacity of inhome MIMO power line channels

In this paper we study the characteristics of in-home multi-input multi-output (MIMO) power line channels in the 0 – 100 MHz frequency band. Channel measurements are obtained in several US homes by transmitting two independent signals over two wire pairs (line-neutral and line-protective earth) and receiving on all three wire pairs (line-neutral, line-protective earth and neutral-protective earth). Noise measurements on all three wire pairs are also performed and spectral properties of noise in MIMO power line channels are presented. In particular, noise is characterized in terms of correlation among the receive ports. We analyze the effect of noise correlation on the eigenspread of the channel matrix, and also its effect on MIMO channel capacity. It is shown that while noise correlation contributes to an increase in the eigen-spread and therefore correlation of a composite channel, it ultimately improves channel capacity. The capacity of MIMO power line channels is compared for 2×2 and 2×3 MIMO configurations, showing the gain of a third receive port. Finally, capacity gains of various MIMO configurations over the single-input single-output (SISO) power line systems are provided.

Cross-Layer Optimization for Multimedia Traffic in CDMA Cellular Networks

We consider the uplink transmission of multimedia services in a cellular network where multiple access is implemented by code division multiple access (CDMA) and the base station performs successive interference cancellation (SIC) to enhance performance. We propose a cross-layer optimization technique that operates both at the physical layer, by selecting the detection order at the SIC receiver, and at the medium access control layer, by selecting the power/rate for each mobile terminal. The optimization objective is the maximization of the overall weighted network throughput with the satisfaction of the quality of service criteria for multimedia communications. The resulting problem turns out to be NP-complete and we resort to a discrete stochastic approximation (DSA) approach for its solution. Concerning DSA, an efficient implementation is proposed in order to reduce memory occupation and improve the convergence of the algorithm. In a UMTS cellular environment, numerical results show that the optimization provides a significant performance advantage over existing techniques at the cost of an increase of computational complexity and memory occupation.

Resource allocation and power control in a TDD OFDM-based system for 4G cellular networks

Time division duplex (TDD) systems based on orthogonal frequency division multiplexing (OFDM) have recently been proposed as potential candidates for next generation (4G) cellular networks. In this paper, as a multiple access scheme we propose an adaptive frequency division multiple access (AFDMA), where resources are assigned to users exploiting channel knowledge at the transmitter. In order to reduce the system overhead, the algorithm can also work on group of sub-carriers and OFDM symbols. The proposed solution is shown, through numerical results, to be superior to conventional allocation strategies, namely time division multiple access (TDMA) and FDMA. Furthermore, we also investigate a new power control algorithm based on a single-user discrete bit loading technique. Numerical results are reported for both downlink and uplink transmissions in a multi-cell environment supporting real-time traffic

Stopping Rules for Duo-Binary Turbo Codes and Application to HomePlug AV

This paper focuses on the design of stopping rule criteria for duo-binary turbo codes. In particular, single-binary stopping rules are generalized and iteration/semi-iteration based criteria are developed. The proposed solutions are tested using the framework of HomePlug AV, which is the last generation technology for the in-home distribution of audio and video data over existing power-line wiring.

Sum Power Minimization Under Rate Constraints in Multiuser OFDM Systems

For a multi-cell system using OFDM, this paper considers the multiuser power control problem in a frequency-selective interference channel. In particular, the aim of this work is to develop an iterative algorithm for user power control and bit loading on OFDM subcarriers, which minimizes the sum power under a constraint on the user bit rates. The performance of the proposed algorithm is compared against two previous algorithms in terms of sum power, probability of convergence within a fixed number of iterations, and mean number of iterations required for convergence.

Minimum sum-power design of a cooperative system using the decode and forward protocol

The aim of this paper is to design both transmitters and receiver of a cooperative network, such that the equivalent communication link achieves a given target rate using the minimal overall transmit sum power; the decode-and-forward protocol is considered. Successively, we compare, in terms of consumed energy, the cooperative communication and the direct communication. From this comparison, we determine the condition that makes cooperative communications more convenient than direct communications.

Receiver windowing for the HomePlug AV system

In this paper we investigate the design of the receiver windowing for the OFDM-based HomePlug AV system, using the OPERA power-line channel models. In particular, the aim of this work is twofold: i) to analyze the anticipation of the demodulation window, based on a new interference free- like condition; ii) to identify the receiver window shape and length, by comparing the performance of some receiver windows under the considered scenarios. Results, in terms of throughput versus signal to narrow-band In this paper we investigate the design of the receiver windowing for the OFDM-based HomePlug AV system, using the OPERA power-line channel models. In particular, the aim of this work is twofold: i) to analyze the anticipation of the demodulation window, based on a new interference free- like condition; ii) to identify the receiver window shape and length, by comparing the performance of some receiver windows under the considered scenarios. Results, in terms of throughput versus signal to narrow-band noise power ratio, show that the proposed receiver window allows for a significant increase in throughput, with respect to a traditional rectangular windowing, when narrow-band noise is present.noise power ratio, show that the proposed receiver window allows for a significant increase in throughput, with respect to a traditional rectangular windowing, when narrow-band noise is present.