Michel T. Ivrlac

Fading correlations in wireless MIMO communication systems

We investigate the effects of fading correlations on wireless communication systems employing multiple antennas at both the receiver and the transmitter side of the link, so called multiple-input multiple-output (MIMO) systems. It turns out that the amount of transmitter sided channel knowledge plays an important part when dealing with fading correlations. Furthermore, the possible availability of time diversity in a time-selective channel can have essential influence on performance. To study the influence of time-selectivity, the concept of sample-mean outage is introduced and applied to information theoretic measures, like capacity or cutoff rate. It will be shown, that in some cases correlated fading may offer better performance than uncorrelated fading permits, which is due to exploitable antenna gain, that will also be defined in a general form for MIMO systems.

Toward a Circuit Theory of Communication

Electromagnetic field theory provides the physics of radio communications, while information theory approaches the problem from a purely mathematical point of view. While there is a law of conservation of energy in physics, there is no such law in information theory. Consequently, when, in information theory, reference is made (as it frequently is) to terms like energy, power, noise, or antennas, it is by no means guaranteed that their use is consistent with the physics of the communication system. Circuit theoretic multiport concepts can help in bridging the gap between the physics of electromagnetic fields and the mathematical world of information theory, so that important terms like energy or antenna are indeed used consistently through all layers of abstraction. In this paper, we develop circuit theoretic multiport models for radio communication systems. To demonstrate the utility of the circuit theoretic approach, an in-depth analysis is provided on the impact of impedance matching, antenna mutual coupling, and different sources of noise on the performance of the communication system. Interesting insights are developed about the role of impedance matching and the noise properties of the receive amplifiers, as well as the way array gain and channel capacity scale with the number of antennas in different circumstances. One particularly interesting result is that, with arrays of lossless antennas that receive isotropic background noise, efficient multistreaming can be achieved no matter how densely the antennas are packed.

Efficient use of fading correlations in MIMO systems

We investigate the effects of both fading correlations and transmitter channel knowledge in multiple element antenna (MEA) communication systems. While, for independent and identically distributed fades between receive and transmit antennas, pioneering work showed that a huge increase in capacity is possible for MEA compared to a single antenna system, recent contributions warn that fading correlations destroy most of this advantage. While this is true for zero transmitter channel knowledge, we show however that long-term average channel state information enables the transmitter to efficiently use the fading correlations to its advantage and offers the potential to even increase capacity beyond the one possible for independent fading. A conceived transmit technique is presented that efficiently makes use of fading correlations, and also provides optimum choice of digital modulation schemes that carry the information.

On Uplink Intercell Interference in a Cellular System

A new semi-analytical method to analyze intercell interference in a cellular system is introduced. A sound comprehension of the properties of intercell interference is the key to a meaningful assessment of communication systems without exhaustive simulation campaigns. We consider the basic access schemes TDMA, WCDMA and random OFDMA for uplink transmission. We will also give hints, how the methods can be extended to capture other access schemes, and/or additional features. The focus of this work is more on the introduction of the new methodology. In principle, we describe the intercell interference as a random variable which is composed of many other random variables. The distributions, and in particular the mutual dependencies of those have to be carefully studied. Applications and extensions of the ideas will be addressed in future work, only first examples are given here.

Capacity and coding for quantized MIMO systems

In this paper, the effects of receive signal quantization on the channel capacity and the performance of error control coding in multi-input multi-output (MIMO) systems are investigated. The receive antennas of a MIMO system experience a channel-dependent superposition of modulated signals originating from all transmit antennas. A fine-granular analog to digital conversion of the resulting irregular constellation is difficult to obtain in practice. The quantization is therefore likely to be rather coarse. It turns out, however, that the loss in channel capacity due to coarse quantization is surprisingly small. On the other hand, existing coding schemes do not seem to tolerate coarse quantization gracefully, producing rather high error floors. Closing the huge gap between information theoretic opportunities offered by coarse quantized MIMO systems on the one hand, and the actual poor performance of coding schemes which are designed without signal quantization in mind, remains a challenging task.

Quantifying diversity and correlation in Rayleigh fading MIMO communication systems

The performance of MIMO systems and the usefulness of the dedicated signal processing and channel coding techniques is strongly affected by the presence of fading correlation. For instance, space-time block coding is favored by low fading correlation, while beam-forming techniques are advantageous at high amounts of correlation. In this paper, we define an easy to compute quantitative description of fading correlation and diversity present in a MIMO channel. This definition offers the possibility to build equivalence classes of channels which offers the same amount of diversity or correlation. It turns out that the channels from such an equivalence class performs essentially equivalent with respect to channel capacity or throughput.

Effective use of long-term transmit channel state information in multi-user MIMO communication systems

We propose a novel approach to the design of linear transmit processing for the downlink of multi-user multi-input multi-output (MU-MIMO) communication over fading wireless channels. Our purpose is to address the open problem of downlink transmit processing in MU-MIMO systems with only long-term CSI at the transmitter. In contrast to existing schemes, only long-term channel state information is required at the transmitter. For this to work, the MU-MIMO channel must experience correlated fading. The approach consists of two steps: 1) design of transmit processing which can deal with correlated fading channels but is allowed to use full CSI at first; 2) conversion of the transmit processing to use long-term CSI only. As a solution for the first step, we propose a joint receive and transmit mean square error minimization scheme, while the second step is performed by an innovative subspace-based procedure. Performance evaluation shows the significant potential of the proposed approach.

Intercell-Interference in the Gaussian MISO Broadcast Channel

Intercell interference (ICI) is the substantial difference between a cellular and a non-cellular communication system. Effective modeling of the cellular downlink as a Gaussian broadcast channel requires that the ICI powers at the user positions are known to the basestation (BS), as it otherwise, cannot attempt to approach (or even know) its capacity region. However, ICI depends on the transmit processing of the BSs in the neighboring cells, which furthermore is subject to change quickly due to temporal scheduling. The BS therefore only has limited knowledge about the true ICI powers. In this paper, the implications of this lack of knowledge about ICI power on the achievable sum- rate of a Gaussian MISO (multi-input, single-output) broadcast channel is examined. Four different approaches on dealing with unpredictable ICI are discussed and their performance compared to the single-cell (or non-cellular) Gaussian broadcast channel.

Joint optimization of radio parameters in HSDPA

In this paper we propose two contributions to the field of cross-layer optimization for third generation (3G) mobile communication systems. The first chapter derives an analytical bijective system model for the quality of service parameters throughput and delay in an HSDPA system. Link level simulations reveal an excellent match of the new model components. In the second part, the derived model is used to design a cross-layer optimization technique, serving a set of quality of service (QoS) demands with minimum necessary transmit power. The optimized system yields the same QoS performance with a significantly lower amount of transmit power, which in future investigations will enable scheduling algorithms to maximize the system capacity.

Joint transmit and receive multi-user MIMO decomposition approach for the downlink of multi-user MIMO systems

We propose a joint transmit and receive decomposition approach for the downlink of multi-user MIMO systems. This approach makes use of the assumption of part of the receiver structure in computing the solution for the transmit processing. With this assumption, this approach decomposes a multi-user MIMO downlink channel into multiple parallel independent single-user MIMO downlink channels. A new degree of freedom is obtained by making the number of data streams variable. Sample simulation results are provided and these show the potential of the proposed scheme.