Zoran Utkovski

A distributed differential space-time coding scheme for two-way wireless relay networks

We propose a distributed differential space-time coding scheme for two-way relaying over unknown block Rayleigh fading channels. The accent is on a two-time slots scheme, where in the first stage both terminals transmit their signals to the relays and in the second stage the relays broadcast to the terminals. Although we do not assume channel knowledge neither at the relays nor at the terminals, we show that it is possible for the terminals to subtract their contribution from the broadcasted signal and decide on the desired signal from the other terminal. The performance analysis reveals that the two-time slots differential scheme offers advantage over the traditional four-time slots differential scheme.

Outage margin and power constraints in cognitive radio with multiple antennas

In the commons model for spectrum usage, the cognitive (secondary) users are allowed to use the spectrum as long as the target performance in the primary system is not violated. In this paper we consider primary system that has a target outage performance and the transmission power of the secondary transmitter (STX) should be appropriately not to violate the target outage probability for the primary terminals (PT). We have considered two types of STX, with single antenna (SISO STX) and two antennas (MISO STX) and analyzed the allowed secondary power. In general, the power level allowed for the SISO STX differs from the total power level allowed for the MISO STX. Our analysis shows that the relation between these power levels changes as the direct component (the K-factor) of the Ricean fading in the primary channel changes. For a large K-factor in the primary system, the total power allowed for a MISO STX is higher than the power allowed for a SISO STX system. The situation is reversed when the fading in the primary system has a low value of the K-factor and moves towards Rayleigh fading. This implies that, for example, when the direct component in the primary system is substantial, the usage of multiple antennas in the cognitive system has additional benefit, as it can use a higher power.

Finite-SNR Bounds on the Sum-Rate Capacity of Rayleigh Block-Fading Multiple-Access Channels With No A Priori CSI

We provide nonasymptotic upper and lower bounds on the sum-rate capacity of Rayleigh block-fading multiple-access channels for the set up where a priori channel state information is not available. The upper bound relies on a dual formula for channel capacity and on the assumption that the users can cooperate perfectly. The lower bound is derived assuming a noncooperative scenario where each user employs unitary space-time modulation (independently from the other users). Numerical results show that the gap between the upper and the lower bound is small already at moderate SNR values. This suggests that the sum-rate capacity gains obtainable through user cooperation are minimal for the scenarios considered in the paper.

On the Secondary Capacity of the Communication Protocols

In this paper we consider secondary communication channels, defined via utilization of the protocol overhead information in legacy communication systems, here referred to as primary systems. The secondary communication is carried out by rearranging the order of the packets which are put forward for transmission by the primary system. To motivate the models, we provide several scenarios with wireless primary communication systems. The secondary communication can be seen as a special case of cognitive radio, since additional information is sent by using the same spectrum as the primary system and without affecting the performance of the primary system. The primary system is frame-based and it determines a set of packets that need to be delivered within a frame. The arrangements of the packets within the frame is irrelevant for the primary system and that is creating the opportunity for secondary communication. We define two generic communication models, the permutation model and the combinatorial model, respectively. For both models, we define suitable discrete communication channels, where the error models are obtained by suitably transforming the packet erasures in the primary system. The capacity of these channels is analyzed. It turns out that the capacity results for the combinatorial communication model are related to the fundamental result by Shannon regarding channels with causal side information at the transmitter. The ideas presented in this paper set several interesting directions for future work.

Communication Schemes with Constrained Reordering of Resources

This paper introduces a communication model inspired by two practical scenarios. The first scenario is related to the concept of protocol coding, where information is encoded in the actions taken by an existing communication protocol. We investigate strategies for protocol coding via combinatorial reordering of the labelled user resources (packets, channels) in an existing, primary system. However, the degrees of freedom of the reordering are constrained by the operation of the primary system. The second scenario is related to communication systems with energy harvesting, where the transmitted signals are constrained by the energy that is available through the harvesting process. We have introduced a communication model that covers both scenarios and elicits their key feature, namely the constraints of the primary system or the harvesting process. We have shown how to compute the capacity of the channels pertaining to the communication model when the resources that can be reordered have binary values. The capacity result is valid under arbitrary error model in which errors in each resource (packet) occur independently. Inspired by the information-theoretic analysis, we have shown how to design practical error-correcting codes suited for the communication model. It turns out that the information-theoretic insights are instrumental for devising superior design of error-control codes.

Random Access in C-RAN for User Activity Detection With Limited-Capacity Fronthaul

Cloud-radio access network (C-RAN) is characterized by a hierarchical structure, in which the baseband-processing functionalities of remote radio heads (RRHs) are implemented by means of cloud computing at a central unit (CU). A key limitation of C-RANs is given by the capacity constraints of the fronthaul links connecting RRHs to the CU. In this letter, the impact of this architectural constraint is investigated for the fundamental functions of random access and active user equipment (UE) identification in the presence of a potentially massive number of UEs. In particular, the standard C-RAN approach based on quantize-and-forward and centralized detection is compared to a scheme based on an alternative CU-RRH functional split that enables local detection. Both techniques leverage Bayesian sparse detection. Numerical results illustrate the relative merits of the two schemes as a function of the system parameters.

The Impact of Services on Economic Complexity: Service Sophistication as Route for Economic Growth

Economic complexity reflects the amount of knowledge that is embedded in the productive structure of an economy. By combining tools from network science and econometrics, a robust and stable relationship between a country’s productive structure and its economic growth has been established. Here we report that not only goods but also services are important for predicting the rate at which countries will grow. By adopting a terminology which classifies manufactured goods and delivered services as products, we investigate the influence of services on the country’s productive structure. In particular, we provide evidence that complexity indices for services are in general higher than those for goods, which is reflected in a general tendency to rank countries with developed service sector higher than countries with economy centred on manufacturing of goods. By focusing on country dynamics based on experimental data, we investigate the impact of services on the economic complexity of countries measured in the product space (consisting of both goods and services). Importantly, we show that diversification of service exports and its sophistication can provide an additional route for economic growth in both developing and developed countries.

Protocol coding with reordering of user resources: Capacity results for the Z-channel

We consider protocol coding that gives a rise to secondary communication channels, defined by combinatorial ordering of the user resources (packets, channels) in a primary (legacy) communication system. In general, the capacity analysis of the secondary communication channel depends on the way the errors are introduced in the communication. Here we extend the previous results for the capacity of secondary communication channels obtained for the binary erasure channel model, to the case of the Z-channel model. This error model is of practical relevance in secondary communication channels because they are often asymmetric, i.e the probability that a packet will not be detected is much higher than the probability that noise can produce detection of a valid packet, since packet existence is detected through very robust preamble/synchronization sequences. The capacity results are obtained by modelling the secondary channel by a cascade of channels, which proves to be an effective framework for capacity analysis.

Non-coherent two-way relaying: Rate bounds for the high SNR regime

Rate bounds for the non-coherent two-way relaying channel under the high SNR assumption are derived. No channel knowledge is assumed at neither the terminals nor at the relays. An upper and lower bound on the achievable rates are derived and shown that they differ only by a constant in the high SNR regime. As corollary from the analysis, the degrees of freedom of the non-coherent two-way channel are derived. Additionally, it is shown that the degrees of freedom can be achieved by an AF scheme.

A Recursive Construction of High-Dimensional Grassmann Space-Time Codes for Non-Coherent MIMO Systems

This paper presents a method for construction of high-dimensional Grassmann space-time codes for non-coherent block fading MIMO channels, aiming at codes with large block lengths compared to the number of transmit antennas. The construction method is recursive, starting from a low-dimensional coherent or non-coherent code. The performance analysis of the constructed codes supports the theoretical prediction of performance improvement when increasing the code block length. Additionally, the code structure could potentially offer a possibility for complexity reduction of the decoding procedure.