Anastasios Stamoulis

Differential space-time coding for frequency-selective channels

We introduce two space-time transmission schemes which allow full-rate and full-diversity noncoherent communications using two transmit antennas over fading frequency-selective channels. The first scheme operates in the frequency domain where it combines differential Alamouti (seeIEEE J. Select. Areas Commun., vol.16, p.1451-58, Nov. 1998) space-time block-coding (STBC) with OFDM. The second scheme operates in the time domain and employs differential time-reversal STBC to guarantee blind channel identifiability without the need for temporal oversampling or multiple receive antennas.

Space-time processing for broadband wireless access

We present an overview of research activities on space-time coding for broadband wireless transmission performed at AT&T Shannon Laboratory over the past two years. The emphasis is on physical layer modem algorithms such as channel estimation, equalization, and interference cancellation. However, we also discuss the impact of space-time coding gains at the physical layer on throughput at or above the networking layer. Furthermore, we describe a flexible graphical user interface attached to our physical layer simulation engine in order to explore the performance of space-time codes under a variety of practical transmission scenarios. Simulation results for the EDGE cellular system and the 802.11 wireless LAN environment are presented.

Space-time block-coded OFDMA with linear precoding for multirate services

Relying on space-time linearly precoded orthogonal frequency-division multiple access (OFDMA) and exploiting both transmit and receive antenna diversity, we design herein multirate transceivers that guarantee deterministic symbol recovery with diversity gains regardless of the (possibly unknown) frequency-selective finite impulse response (FIR) channels and multiuser interference. Our approach is based on a three-level design of user codes: the top level (based on OFDMA) handles multiuser interference, the middle level (based on space-time block coding) results in space-time diversity gains, and the lower level (based on linear precoding) mitigates intersymbol interference (ISI). In a multiuser/multirate setting, with two transmit and a single receive antenna, our designs achieve guaranteed diversity gains, whereas the use of two receive antennas could potentially double the capacity of the system (in terms of maximum number of users or achievable transmission rates) under favorable conditions (such as no frequency offset). Simulations illustrate the merits of our approach.

Estimation of fast fading channels in OFDM

In this paper, we investigate OFDM transmission over fast fading channels. In such scenarios, the Doppler spread of the channel and synchronization errors cause intercarrier interference (ICI) and complicate channel estimation because the channel cannot be assumed constant during the transmission of an OFDM block. Channel estimation in rapidly time-varying scenarios becomes critical, an we propose a scheme for estimating channel parameters varying within a transmission block. Along with the channel estimation scheme, we also examine the issue of pilot tone placement and show that in time-varying channels it may be better to group pilot tones together into clumps equispaced onto the FFT grid; this placement technique is in contrast to the common wisdom for time-invariant channels. Finally we provide numerical results.

Impact of space-time block codes on 802.11 network throughput

By employing more than one antenna at the transmitter and by properly coding data across the transmit antennas, physical layers (PHYs) with space-time block codes (STBCs) promise increased data rates with minimal decoding complexity at the receiver. This paper presents a comprehensive study of how the STBC gains at the PHY translate to significant network performance improvement in 802.11a wireless local area networks. We base our study on a detailed, across-all-layers, simulation of an 802.11a system. We have extended the network simulator with an implementation of the 802.11a PHY, which allows us to assess the impact of STBC not only at the PHY layer, but at the higher layers as well. An extensive set of simulations illustrates the merits of transmit diversity (in the form of STBC) and sheds light on how performance can be improved for transmission control protocol (TCP) traffic. Essentially, STBC presents to TCP a smoother wireless channel; this is corroborated by a brief theoretical analysis as well.

Intercarrier interference in MIMO OFDM

We examine multicarrier transmission over time-varying channels. We first develop the model for such a transmission scheme and focus particularly on OFDM-based schemes. We analyze the impact of time-variation within a transmission block which could arise both from Doppler spread of the channel and from synchronization errors. We propose a time-domain approach to mitigate the effects of such time-variations. This approach reduces to the familiar single-tap frequency-domain equalizer when the channel is block time-invariant. We also develop this in the context of multiple transmit and receive antennas and specialize the receiver to space-time block-coded systems. Finally we provide numerical results.