Anders Høst-Madsen

Capacity bounds and power allocation for wireless relay channels

We consider three-node wireless relay channels in a Rayleigh-fading environment. Assuming transmitter channel state information (CSI), we study upper bounds and lower bounds on the outage capacity and the ergodic capacity. Our studies take into account practical constraints on the transmission/reception duplexing at the relay node and on the synchronization between the source node and the relay node. We also explore power allocation. Compared to the direct transmission and traditional multihop protocols, our results reveal that optimum relay channel signaling can significantly outperform multihop protocols, and that power allocation has a significant impact on the performance.

On the capacity of MIMO relay channels

We study the capacity of multiple-input multiple- output (MIMO) relay channels. We first consider the Gaussian MIMO relay channel with fixed channel conditions, and derive upper bounds and lower bounds that can be obtained numerically by convex programming. We present algorithms to compute the bounds. Next, we generalize the study to the Rayleigh fading case. We find an upper bound and a lower bound on the ergodic capacity. It is somewhat surprising that the upper bound can meet the lower bound under certain regularity conditions (not necessarily degradedness), and therefore the capacity can be characterized exactly; previously this has been proven only for the degraded Gaussian relay channel. We investigate sufficient conditions for achieving the ergodic capacity; and in particular, for the case where all nodes have the same number of antennas, the capacity can be achieved under certain signal-to-noise ratio (SNR) conditions. Numerical results are also provided to illustrate the bounds on the ergodic capacity of the MIMO relay channel over Rayleigh fading. Finally, we present a potential application of the MIMO relay channel for cooperative communications in ad hoc networks.

The multiplexing gain of wireless networks

At high SNR the capacity of a point-to point MIMO system with N T transmit antenna and NR receive antenna is min{N T, NR} log(SNR) + O(1). The factor in front of the log is called the multiplexing gain. In this paper we consider a network with 2N nodes (N source destination pairs) that each have only a single antenna. These single antenna nodes could cooperate to form larger virtual arrays, usually called cooperative diversity, user cooperation, or coded cooperation. The question we ask is: how large a multiplexing gain is possible. We prove that for N = 2 the multiplexing gain is 1, and consider generalizations to larger networks

On the capacity of wireless relaying

We consider wireless relaying: one or more nodes in a wireless (ad-hoc) network assist other nodes in their transmission by partially retransmitting messages. A characteristic of wireless relays - as compared to the work by T.M. Cover and A.A. El Gamal on the relay channel (see IEEE Trans. on Inf. Theory, vol.25, no.5, p.572-84, 1979) - is that they cannot transmit and receive simultaneously at the same frequency. We derive new upper and lower bounds for the capacity of this wireless relay channel. We then apply these result to a 4 terminal network, and show that the gain (considering outage capacity) of using wireless relaying is of the order of 8-9 dB.

Cooperative diversity for wireless ad hoc networks

Cooperative diversity is a novel technique proposed for conveying information in wireless ad hoc networks, where closely located single-antenna network nodes cooperatively transmit and/or receive by forming virtual antenna arrays. For its building blocks, the relay channel and the two-transmitter, two-receiver cooperative channel, we survey the latest advances made in determining the theoretical capacity bounds and describe the best practical code designs reported so far. Both theory and practice predict that cooperative communication can provide increased capacity and power savings in ad hoc networks.

Assessment of Heart Rate Variability and Respiratory Sinus Arrhythmia via Doppler Radar

An investigation of heart rate variability (HRV) and respiratory sinus arrhythmia (RSA) indices using data obtained from Doppler radar cardiopulmonary remote sensing is presented in this paper. High accuracy in extracting the HRV and RSA indices was achieved using a direct-conversion quadrature radar system with linear demodulation method. This method was validated using data obtained from 12 human subjects in seated and supine positions. For supine position measurements, all standard deviation of normal beat-to-beat interval indices from Doppler radar and electrocardiogram reference differed less than plusmn9 ms of each other, while all the root mean square of differences of successive normal beat-to-beat interval indices differed less than plusmn76 ms. The measurements from subjects in seated and supine positions with normal RSA demonstrated that the results from radar correlated well with both respiratory piezoresistor chest belts. Higher level of HRV and RSA was detected in seated position, as expected.

Through-the-Wall Radar Life Detection and Monitoring

Technology that can be used to unobtrusively detect and monitor the presence of human subjects from a distance and through barriers can be a powerful tool for law enforcement, military, and health monitoring applications. To this end, ultra-wide band radar has shown promise for real-time subject imaging, and compact Doppler radar solutions have demonstrated potential for providing non-invasive detection and monitoring of cardiopulmonary activity for multiple subjects. These technologies work through walls and other obstructions, and can even leverage the presence of ambient radio signals to provide a covert means to detect, isolate, and physiologically monitor multiple human subjects from a remote position. Practical applications ranging from counter-terrorism to health monitoring require systems that are accurate, affordable, and easy to use. Current research efforts addressing these challenges through radio, signal processing, and sensor networking will be presented.

Detection of Multiple Heartbeats Using Doppler Radar

Doppler radar life sensing has shown promise in medical and security applications. The current paper considers the problem of determining the number of persons in a given area (e.g., a room) using the Doppler shift due to heartbeat. The signal is weak and time-varying, and therefore poses a complicated signal processing problem. We develop a generalized likelihood ratio test (GLRT) based on a model of the heartbeat, and show that this can be used to distinguish between the presence of 2, 1, or 0 subjects, even with a single antenna. We further extend this to N antennas. The results show that one can expect to detect up to 2N-1 subjects using this technique

Effects of sampling and quantization on single-tone frequency estimation

The effects of sampling and quantization on frequency estimation for a single sinusoid are investigated. The Cramer-Rao bound for 1-bit quantization is derived and compared with the limit of infinite quantization. It is found that 1-bit quantization gives a slightly worse performance, however, with a dramatic increase of variance at certain frequencies. This can be avoided by using four times oversampling. The effect of sampling when using nonideal antialiasing lowpass filters is therefore investigated through derivation of the Cramer-Rao lower bounds. Finally, fast estimators for 1-bit quantization, in particular, correlation-based estimators, are derived, and their performance is investigated. The paper is concluded with simulation results for four times oversampled 1-bit quantization.

Performance of blind and group-blind multiuser detectors

In blind (or group-blind) linear multiuser detection, the detector is estimated from the received signals, with the prior knowledge of only the signature waveform of the desired user (or the signature waveforms of some but not all users). The performance of a number of such estimated linear detectors, including the direct-matrix-inversion (DMI) blind linear minimum mean square error (MMSE) detector, the subspace blind linear MMSE detector, and the form-I and form-II group-blind linear hybrid detectors, are analyzed. Asymptotic limit theorems for each of the estimates of these detectors (when the signal sample size is large) are established, based on which approximate expressions for the average output signal-to-interference-plus-noise ratios (SINRs) and bit-error rates (BERs) are given. To gain insights on these analytical results, the performance of these detectors in an equicorrelated code-division multiple-acces (CDMA) system is compared. Examples are provided to demonstrate the excellent match between the theory developed here and the simulation results.