Antonio Alberto D'Amico

A Tutorial on the Optimization of Amplify-and-Forward MIMO Relay Systems

The remarkable promise of multiple-input multiple-output (MIMO) wireless channels has motivated an intense research activity to characterize the theoretical and practical issues associated with the design of transmit (source) and receive (destination) processing matrices under different operating conditions. This activity was primarily focused on point-to-point (single-hop) communications but more recently there has been an extensive work on two-hop or multi-hop settings in which single or multiple relays are used to deliver the information from the source to the destination. The aim of this tutorial is to provide an up-to-date overview of the fundamental results and practical implementation issues in designing amplify-and-forward MIMO relay systems.

Energy-Detection UWB Receivers with Multiple Energy Measurements

This paper deals with a novel energy-detection receiver for ultra-wideband systems operating in dense multi-path environments. For binary PPM modulation we propose a detection scheme that operates on signal energy measurements taken on small fractions (bins) of the symbol period. Assuming that the fractional energies of the channel response over those bins can be estimated in some way, we look for the decision strategy that minimizes the error probability. This leads us to a receiver structure that generalizes the conventional energy- detection scheme. The new strategy turns out to be superior to the conventional strategy and its performance is found to improve as the bin size decreases. A simple method is proposed to estimate the fractional energies of the channel response exploiting a training sequence. The impact of the estimation errors on the receiver performance is shown to be marginal.

Joint TOA and AOA Estimation for UWB Localization Applications

A joint TOA/AOA estimator is proposed for UWB indoor ranging under LOS operating conditions. The estimator employs an array of antennas, each feeding a demodulator consisting in a squarer and a low-pass filter. Signal samples taken at Nyquist rate at the filter outputs are processed to produce TOA and AOA estimates. Performance is assessed with transmitted pulses with a bandwidth of either 1.5 GHz (type-1 pulses) or 0.5 GHz (type-2 pulses), which correspond to sampling rates of 3 GHz and 1 GHz, respectively. As expected, the estimation accuracy decreases with the pulse bandwidth. Ranging errors of about 10 cm and angular errors of about 1° are achieved at SNR of practical interest with type-1 pulses and two antennas at a distance of 50 cm. With type-2 pulses the errors increase to 35 cm and 3°. Comparisons are made with other schemes discussed in literature.

TOA Estimation with the IEEE 802.15.4a Standard

This paper discusses a ranging algorithm appropriate for impulse radio ultra-wideband signals compliant with the IEEE 802.15.4a standard. Three steps are indicated to locate the so-called ranging marker in the received signal. The first is to find the highest peaks in the channel responses to the transmitted pulses. The second consists in measuring the distance of each peak to the starting time of the response. The last one involves the identification of the first symbol of the physical-layer header. Algorithms to accomplish all these tasks are indicated. They are implemented in digital form and are in keeping with a non-coherent receiver structure. The achievable accuracy depends on the sampling rate. For example, at SNR = 20 dB and with a rate of 1 GHz the ranging errors are on the order of 20 cm.

Code-multiplexed UWB transmitted-reference radio

In traditional transmitted reference (TR) ultrawideband systems the reference component is time-shifted and orthogonal relative to the data-bearing signal. This paves the way to a correlation receiver in which the local template is derived from the incoming waveform using a delay line. As analog delay lines are difficult to implement with current technology, an alternative TR system has recently been proposed in which reference and data components are made orthogonal by a frequency shift rather than a time shift. The resulting receiver has no delay lines and has better performance compared to the traditional scheme. In the present paper we discuss a third way to achieve orthogonality, i.e., by modulating reference and data components with two distinct code sequences. Even in this case the receiver has no delay lines. However, it is simpler to implement and has better performance than the frequency-shift based receiver.

Power Allocation in Two-Hop Amplify-and-Forward MIMO Relay Systems With QoS Requirements

The problem of minimizing the total power consumption while satisfying different quality-of-service (QoS) requirements in a two-hop multiple-input multiple-output (MIMO) network with a single nonregenerative relay is considered. As shown by Y. Rong [“Multihop nonregenerative MIMO relays: QoS considerations,” IEEE Trans. Signal Process., vol. 59, no. 1, pp. 209-303, 2011] in [1], the optimal processing matrices for both linear and nonlinear transceiver architectures lead to the diagonalization of the source-relay-destination channel so that the power minimization problem reduces to properly allocating the available power over the established links. Unfortunately, finding the solution of this problem is numerically difficult as it is not in a convex form. To overcome this difficulty, existing solutions rely on the computation of upper- and lower-bounds that are hard to obtain or require the relaxation of the QoS constraints. In this work, a novel approach is devised for both linear and nonlinear transceiver architectures, which allows to closely approximate the solutions of the nonconvex power allocation problems with those of convex ones easy to compute in closed-form by means of multistep procedures of reduced complexity. Computer simulations are used to assess the performance of the proposed approach and to make comparisons with alternatives.

GLRT receivers for UWB systems

This letter investigates generalized-likelihood-ratio-test (GLRT) detectors for ultra-wideband (UWB) impulse radio systems employing two alternative signaling schemes. One is the so-called transmitted reference method where, in each time frame, a reference pulse is transmitted prior to the data pulse. The other is the differential transmitted reference scheme, wherein the data pulse received in the previous frame is used as a template waveform in the current frame. The two detectors are compared in terms of bit error rate performance.

Channel estimation for the uplink of a DS-CDMA system

This paper deals with channel estimation in the uplink of a direct-sequence code-division multiple-access system operating in a multipath environment. The parameters of interest are the delays and the complex attenuations incurred by the signal echoes along the propagation paths. We propose an iterative approach for estimating the channel parameters of a new user entering the system. The method is based on the space-alternating generalized expectation-maximization algorithm and exploits a training sequence. In comparison to other estimation algorithms, it reduces a complicated multidimensional optimization problem to a sequence of one-dimensional problems. In addition, it can be effectively used in applications over fast-fading channels. Computer simulations are employed to assess the performance of the proposed scheme. It is found that it is resistant to multiuser interference and has accuracy close to the Cramer-Rao lower bound even with very short training sequences.

Multipath channel estimation for the uplink of a DS-CDMA system

This paper investigates channel estimation in the uplink of a DS-CDMA system. The transmission medium is characterized by the presence of multipath components and the parameters of interest are the delays and complex gains of the propagation paths. We propose a new method for estimating the channel parameters of a user entering the system. The method is based on the maximum likelihood criterion and exploits a training sequence. In comparison with other estimation algorithms it has two advantages. First, it reduces a complicated multidimensional optimization problem to a sequence of one-dimensional problems. Secondly, it has better capability to resolve closely-spaced multipath components. Computer simulations are used to assess its performance. It is found that it is resistant to multiuser interference and can be effectively used in third generation mobile cellular networks.

Tomlinson-Harashima Precoding in MIMO Systems: A Unified Approach to Transceiver Optimization Based on Multiplicative Schur-Convexity

This paper deals with the design of a multicarrier multiple-input multiple-output system which performs nonlinear Tomlinson-Harashima precoding at the transmitter and linear equalization at the receiver. The goal is the joint optimization of the processing matrices at both ends of the wireless link, assuming a constraint on the overall transmit power. We consider different design criteria which require the maximization/minimization of objective functions that depend on the mean square errors over the individual data streams. A unified framework for the solution of a great number of optimization problems is developed, based on the concept of multiplicative Schur-convexity. We show that many widely used design criteria can be easily accommodated in our framework. Our analysis is centered on two theorems which, in the case of cooperative processing among the different subcarriers, provide closed-form solutions to a number of optimization problems. On the other hand, when an independent processing is assumed at each subcarrier closed-form solutions may not exist. Nonetheless, exploiting the results of the two theorems, we show that for a variety of cost functions the optimization problem can be efficiently solved. Theoretical analysis and computer simulations are used to assess the performance of the proposed schemes and make comparisons with linear transceiver architectures. It turns out that nonlinear precoding provides better results than linear prefiltering when the cost function is multiplicatively Schur-convex. In contrast, when the cost function is multiplicatively Schur-concave the nonlinear scheme is equivalent to the linear one.