Gordon L. Stüber

Broadband MIMO-OFDM wireless communications

Orthogonal frequency division multiplexing (OFDM) is a popular method for high data rate wireless transmission. OFDM may be combined with antenna arrays at the transmitter and receiver to increase the diversity gain and/or to enhance the system capacity on time-varying and frequency-selective channels, resulting in a multiple-input multiple-output (MIMO) configuration. The paper explores various physical layer research challenges in MIMO-OFDM system design, including physical channel measurements and modeling, analog beam forming techniques using adaptive antenna arrays, space-time techniques for MIMO-OFDM, error control coding techniques, OFDM preamble and packet design, and signal processing algorithms used to perform time and frequency synchronization, channel estimation, and channel tracking in MIMO-OFDM systems. Finally, the paper considers a software radio implementation of MIMO-OFDM.

Overview of radiolocation in CDMA cellular systems

Applications for the location of subscribers of wireless services continue to expand. Consequently, location techniques for wireless technologies are being investigated. With code-division multiple access (CDMA) being deployed by a variety of cellular and PCS providers, developing an approach for location in CDMA networks is imperative. This article discusses the applications of location technology, the methods available for its implementation in CDMA networks, and the problems that are encountered when using CDMA networks for positioning.

Subscriber location in CDMA cellular networks

Subscriber radio location techniques are investigated for code-division multiple-access (CDMA) cellular networks. Two methods are considered for radio location: measured times of arrival (ToA) and angles of arrival (AoA). The ToA measurements are obtained from the code tracking loop in the CDMA receiver, and the AoA measurements at a base station (BS) are assumed to be made with an antenna array. The performance of the two methods is evaluated for both ranging and two-dimensional (2-D) location, while varying the propagation conditions and the number of BSs used for the location estimate.

Interchannel interference analysis of OFDM in a mobile environment

Orthogonal frequency division multiplexing with a suitably chosen guard interval is an effective means of eliminating intersymbol interference for high-rate transmission over fading dispersive channels. Time variations of the channel, however, lead to a loss of subchannel orthogonality, resulting in interchannel interference (ICI). The article examines the effects of ICI through analysis and simulation, in the context of a system design for HDTV digital video broadcasting to mobile receivers. It is shown that ICI can be modeled as an additive Gaussian random process that leads to an error floor which can be determined analytically as a function of the Doppler frequency. Antenna diversity and trellis coding are then examined as methods for reducing this error floor.

Channel Estimation for Amplify and Forward Relay Based Cooperation Diversity Systems

Cooperation diversity schemes have been proposed for cellular networks that permit a base station (or a mobile station) to relay signals to a destination receiver, thereby increasing the network coverage and reliability. The mobile relays either decode and forward (DF) or amplify and forward (AF) the received signal. Most existing analyses of cooperation diversity assumes perfect channel information at the receiver. A realistic assessment should consider the effects of practical channel estimation schemes. This paper considers pilot symbol aided channel estimation for cooperation diversity systems. Since the overall channel in AF systems is different from conventional cellular channels, the channel estimation problem is interesting and challenging and therefore our focus is on AF systems. The paper addresses issues such as the estimator design, pilot symbol spacing based upon realistic channel models, and an approximate bit error rate (BER) analysis that accounts for imperfect channel estimation.

Clipping noise mitigation for OFDM by decision-aided reconstruction

Clipping is often used to reduce the large peak-to-mean envelope power ratio of orthogonal frequency-division multiplexing (OFDM) signals. However, it introduces additional noise that degrades the system performance. A technique called decision-aided reconstruction (DAR) is proposed for mitigating the clipping noise. The performance of the proposed technique is evaluated for additive white Gaussian noise and static intersymbol interference channels. The effect of using a realistic channel estimate is also examined. Results show that the DAR technique can mitigate the clipping noise significantly for OFDM systems that have large block sizes.

A novel ARQ technique using the turbo coding principle

A novel automatic repeat request (ARQ) technique based on the turbo coding principle is presented. The technique uses the log-likelihood ratios generated by the decoder during a previous transmission as a priori information when decoding retransmissions. Simulation results show a significant decrease in frame error rate, especially at low-to-moderate E/sub b//N/sub 0/.

Synchronization for MIMO OFDM systems

This paper proposes a time and frequency synchronization technique for a Q transmit and L receive (Q/spl times/L), MIMO OFDM system. The synchronization is achieved using training symbols which are simultaneously transmitted from Q transmit antennas. The training symbols are directly modulatable orthogonal polyphase sequences. The synchronization algorithm shows satisfactory performance even at a low SNR and in a frequency selective channel. The training sequence structure is specialized such that channel parameters in terms of channel coefficients and noise variance can be estimated once synchronization is achieved.

Residual ISI cancellation for OFDM with applications to HDTV broadcasting

An iterative technique is developed for orthogonal frequency division multiplexing (OFDM) systems to mitigate the residual intersymbol interference (ISI) that exceeds the length of the guard interval. The technique, called residual ISI cancellation (RISIC), uses a combination of tail cancellation and cyclic restoration and is shown to offer large performance improvements. The effects of imperfect channel estimation are also considered. The RISIC algorithm is applied to a typical terrestrial high-definition television (HDTV) broadcasting system that uses a concatenated coding scheme for error control. Results show that the RISIC algorithm can effectively mitigate residual ISI on static or slowly fading ISI channels.

A serial concatenation approach to iterative demodulation and decoding

Iterative demodulation and decoding of convolutionally encoded data is treated as a special case of the previously proposed serial concatenation of interleaved codes. It is shown that by exploiting the recursive nature of the differential modulation schemes (for example, DBPSK, DQPSK, CPM, etc.), large interleaving gains can be achieved similar to serial concatenation schemes. We also show that when memoryless modulation is used, precoding can be used to create a rate-1 recursive inner code in order to obtain interleaving gains without adding redundancy from the inner code.