Differential Distributed Space-Time Coding with Imperfect Synchronization

Differential distributed space-time coding (D-DSTC) has been considered to improve both diversity and data-rate in cooperative communications in the absence of channel information. However, conventionally, it is assumed that relays are perfectly synchronized in the symbol level. In practice, this assumption is easily violated due to the distributed nature of the relay networks. This paper proposes a new differential encoding and decoding process for D-DSTC systems with two relays. The proposed method is robust against synchronization errors and does not require any channel information at the destination. Moreover, the maximum possible diversity and symbol-by-symbol decoding are attained. Simulation results are provided to show the performance of the proposed method for various synchronization errors and the fact that our algorithm is not sensitive to synchronization error.Comment: to appear in IEEE Globecom, 201

Multihop Diversity in Wideband OFDM Systems: The Impact of Spatial Reuse and Frequency Selectivity

The goal of this paper is to establish which practical routing schemes for wireless networks are most suitable for wideband systems in the power-limited regime, which is, for example, a practically relevant mode of operation for the analysis of ultrawideband (UWB) mesh networks. For this purpose, we study the tradeoff between energy efficiency and spectral efficiency (known as the power-bandwidth tradeoff) in a wideband linear multihop network in which transmissions employ orthogonal frequency-division multiplexing (OFDM) modulation and are affected by quasi-static, frequency-selective fading. Considering open-loop (fixed-rate) and closed-loop (rate-adaptive) multihop relaying techniques, we characterize the impact of routing with spatial reuse on the statistical properties of the end-to-end conditional mutual information (conditioned on the specific values of the channel fading parameters and therefore treated as a random variable) and on the energy and spectral efficiency measures of the wideband regime. Our analysis particularly deals with the convergence of these end-to-end performance measures in the case of large number of hops, i.e., the phenomenon first observed in \cite{Oyman06b} and named as ``multihop diversity''. Our results demonstrate the realizability of the multihop diversity advantages in the case of routing with spatial reuse for wideband OFDM systems under wireless channel effects such as path-loss and quasi-static frequency-selective multipath fading.Comment: 6 pages, to be published in Proc. 2008 IEEE International Symposium on Spread Spectrum Techniques and Applications (IEEE ISSSTA'08), Bologna, Ital

OFDM Communication with Cooperative Relays

Signal fading due to multi-path propagation is one of the major impairments to meet the demands of next generation wireless networks for high data rate services. To mitigate the fading effects, time, frequency, and spatial diversity techniques or their hybrid can be used. Among different types of diversity techniques, spatial diversity is of special interest as is does not incur system losses in terms of delay and bandwidth efficiency.TelecommunicationsElectrical Engineering, Mathematics and Computer Scienc

Design of distributed space-time block codes for relay networks

The fading effect often faced in wireless communications can cause severe attenuation in signal strength. To solve this problem, diversity techniques (in terms of spatial/time/frequency) have been considered. For example, spatial diversity can be achieved by using multiple antennas at the transmitter or the receiver or both. One important architecture that can efficiently exploit the multiple antennas is the space-time block coding (STBC). The realization of STBC requires more than one antenna at the transmitter. Unfortunately, the use of multiple antennas is not practical in many wireless devices due to the size limitation. Recently, the “cooperative diversity”, also known as “user diversity”, enables single-antenna mobiles in a multi-user environment to share their antennas and generate a virtual multiple-antenna transmitter that allows them to achieve transmit diversity. To apply concept of the STBC schemes to the cooperative communications, Laneman et al. suggest the use of “conventional” orthogonal STBC in a “distributed” fashion for practical implementation of user cooperation. The pioneering works on distributed STBC (DSTBC) assume flat fading channels. This can be achieved by using multi-carrier techniques such as orthogonal frequency division multiplex (OFDM) to divide a whole spectrum into a set of narrower bands. Hence, the channel can be considered flat in each sub-band. However, for current wireless communications with single-carrier transmission, the frequency selective channels cannot be avoided. Thus, in this dissertation, I will consider the application of DSTBC to frequency selective fading channels. In the first part of my thesis, I present a new design of DSTBC to achieve full rate transmission and channel decoupling property as in conventional STBC by using zero-padding (ZP). Several receiver techniques in frequency domain are studied for the signal detection of the proposed DSTBC. The extension from ZP to unique-word (UW) will be proposed in the second part. Exploiting the properties of the UW, I will present in the third part of my thesis a method of channel estimation for relay networks

Técnicas de cooperação entre estações base para sistemas celulares

Mestrado em Engenharia Electrónica e TelecomunicaçõesA cooperação entre células é uma das áreas de pesquisa em maior crescimento, sendo uma solução promissora para sistemas celulares sem fio, por forma a amenizar a interferência entre as células, melhorar a equidade do sistema e aumentar a capacidade nos anos vindouros. Esta tecnologia já está em estudo no LTE-Advanced sob o conceito de coordenação multiponto (CoOMP). Esta dissertação insere-se na área de comunicações sem fios e tem como principal objectivo, estudar, implementar e avaliar o desempenho de esquemas de cooperação entre estações base, projectados para os futuros sistemas de comunicações móveis de portadora múltipla (OFDM/A). Especificamente, o sistema cooperativo estudado é constituído por duas estações base equipadas com um agregado de antenas, ligadas a uma unidade de processamento central, e dois terminais móveis equipados cada um com apenas uma antena. O sistema referido foi implementado de acordo com as especificações do LTE e avaliado em diversos cenários de propagação. As técnicas desenvolvidas permitem contornar os problemas relacionados com a má qualidade de canal entre emissor e receptor, melhorando o seu desempenho, especificamente ao nível da taxa de erros de transmissão.Multicell cooperation is one of the fastest growing areas of research, and it is a promising solution for cellular wireless systems to mitigate intercell interference, improve system fairness and increase capacity in the years to come. This technology is already under study in LTE-Advanced under the coordinated multipoint (CoOMP) concept. This dissertation is inserted in the wireless communications area, with its main objective being the study, implementation and evaluation of the performance of cooperative schemes between base stations designed for the future mobile communication multiple carrier systems (OFDM/A). Specifically, the cooperative system studied consists of two base stations, each with multiple antenna, connected to a central processing unit, and two mobile terminals, each equipped with only one antenna. The system referred to was implemented in accordance with the specifications of LTE and was tested in various different propagation situations. The developed techniques ensure the mitigation of problems related to interference between the portable terminals namely at the cell edges, improving specifically the bit error rate performance