Naofal Al-Dhahir

Optimum finite-length equalization for multicarrier transceivers

A new criterion for partially-equalizing severe ISI channels to reduce the cyclic prefix overhead of the discrete multitone (DMT) transceiver, assuming a fixed transmission bandwidth, is introduced. The equalized DMT is shown to recover a significant portion of the performance loss incurred because of the use of a moderate-size FFT in the DMT to reduce latency and implementation cost. In particular, equalizers designed using our new criterion result in a higher DMT performance margin than traditional mean-square-error DMT equalizers. Finally, additional promising methods that further enhance the performance of the equalized DMT are investigated.

Single-carrier frequency-domain equalization for space-time block-coded transmissions over frequency-selective fading channels

We propose an Alamouti-like scheme for combining space-time block-coding with single-carrier frequency-domain equalization. With two transmit antennas, the scheme is shown to achieve significant diversity gains at low complexity over frequency-selective fading channels.

The finite-length multi-input multi-output MMSE-DFE

A new theoretical framework is introduced for analyzing the performance of a finite length minimum-mean-square error decision feedback equalizer (MMSE-DFE) in a multi-input multi-output (MIMO) environment. The framework includes transmit and receive diversity systems as special cases and quantifies the diversity performance improvement as a function of the number of transmit/receive antennas and equalizer taps. Fast and parallelizable algorithms for computing the finite-length MIMO MMSE-DFE are presented for three common multi-user detection scenarios.

MMSE decision-feedback equalizers: finite-length results

This paper extends a number of results on the infinite-length minimum-mean-square-error decision Feedback equalizer (MMSE-DFE) reported by Cioffi, Dudevoir, Eyuboglu and Forney (see IEEE Trans. Commun., 1995) to the finite-length case. Cholesky factorization and displacement structure theory are demonstrated to be two powerful analytical tools for analyzing the finite-length MMSE-DFE. Our objective throughout the paper is to establish finite-length analogs of the well-known infinite-length MMSE-DFE results. Similarities and differences between the two cases are examined and delineated. Finally, convergence of our derived finite-length results to their well-established infinite-length counterparts is shown. >

Single-carrier frequency domain equalization

This paper present an alternative promising approach to ISI mitigation by the use of single-carrier (SC) modulation combined with frequency-domain equalization (FDE).

Great expectations: the value of spatial diversity in wireless networks

The effect of spatial diversity on the throughput and reliability of wireless networks is examined. Spatial diversity is realized through multiple independently fading transmit/receive antenna paths in single-user communication and through independently fading links in multiuser communication. Adopting spatial diversity as a central theme, we start by studying its information-theoretic foundations, then we illustrate its benefits across the physical (signal transmission/coding and receiver signal processing) and networking (resource allocation, routing, and applications) layers. Throughout the paper, we discuss engineering intuition and tradeoffs, emphasizing the strong interactions between the various network functionalities.

Efficiently computed reduced-parameter input-aided MMSE equalizers for ML detection: a unified approach

A unified approach for computing the optimum settings of a length-N/sub f/ input-aided equalizer that minimizes the mean-square error between the equalized channel impulse response and a target impulse response of a given length N/sub b/ is presented. This approach offers more insight into the problem, easily accommodates correlation in the input and noise sequences, leads to significant computational savings, and allows us to analyze a variety of constraints on the target impulse response besides the standard unit-tap constraint. In particular, we show that imposing a unit-energy constraint results in a lower mean-square error at a comparable computational complexity. Furthermore, we show that, under the assumed constraint of finite-length filters, the relative delay between the equalizer and the target impulse response plays a crucial role in optimizing performance. We describe a new characterization of the optimum delay and show how to compute it. Finally, we derive reduced-parameter pole-zero models of the equalizer that achieve the high performance of a long all-zero equalizer at a much lower implementation cost.

FIR channel-shortening equalizers for MIMO ISI channels

Finite-length delay-optimized multi-input multi-output (MIMO) equalizers that optimally shorten the impulse response memory of frequency-selective MIMO channels are derived. The MIMO equalizers are designed to minimize the average energy of the error sequence between the equalized MIMO channel impulse response and an MIMO target impulse response (TIR) with shorter memory. Two criteria for optimizing the MIMO TIR are analyzed and compared. The presented analytical framework encompasses a multitude of previously-studied finite-length equalization techniques.

Doppler characterization for LEO satellites

Mobile ground-based terminals observe significant Doppler on the forward channel when communicating through low Earth orbit (LEO) satellites. This paper deals with the analytic derivation of the Doppler shift measured by a user on the surface of Earth on a signal transmitted by a circular orbit LEO satellite. Two simplifications are performed to obtain the analytical expression of the Doppler shift as a function of time. First, during the visibility duration of the satellite at a terminal, the trajectory of the satellite with respect to the Earth is approximated by a great circle arc. Second, the angular velocity of the satellite with respect to the user is assumed to be constant. Numerical results validate the approximations. Another result of our analysis is an expression for the visibility window duration of a satellite at a terminal as a function of the maximum elevation angle. An algorithm for estimating the parameters of the Doppler curve based on a couple of Doppler and Doppler-rate measurements is also presented.

Equalization Techniques for Distributed Space-Time Block Codes With Amplify-and-Forward Relaying

In this paper, we investigate equalization methods for cooperative diversity schemes over frequency-selective fading channels. Specifically, we consider three equalization schemes proposed originally for conventional space-time block codes (STBC) and extend them to distributed STBC in a cooperative transmission scenario with amplify-and-forward relaying. The distributed STBC equalization schemes are named after their original counterparts as distributed time-reversal (D-TR) STBC, distributed single-carrier (D-SC) STBC, and distributed orthogonal frequency division multiplexed (D-OFDM) STBC. The underlying orthogonality of distributed STBC results in decoupled data streams at the receiver side allowing low-complexity implementations. Without loss of generality, we consider a single-relay scenario where the source-to-relay SrarrR, relay-to-destination RrarrD, and source-to-destination SrarrD links experience possibly different channel delay spreads. Under the assumption of perfect power control for the relay terminal and high signal-to-noise ratio (SNR) for the underlying links, our performance analysis demonstrates that D-TR-STBC, D-SC-STBC, and coded D-OFDM-STBC schemes are able to achieve a maximum diversity order of min(L1,L3)+L2+2 where L1, L2, and L3 are the channel memory lengths for SrarrR, SrarrD, and RrarrD links, respectively. This illustrates that the minimum of the multipath diversity orders experienced in SrarrR and RrarrD links becomes the performance bottleneck for the relaying path. For the case of a nonfading relaying path where line-of-sight propagation is possible in either one of these underlying links, we demonstrate that diversity orders of L1+L2+2 and L3+L2+2 are achievable assuming nonfading SrarrR and RrarrD links, respectively. An extensive Monte Carlo simulation study is presented to corroborate the analytical results and to provide detailed performance comparisons among the three candidate equalization schemes