Frequency-reuse planning of the down-link of distributed antenna systems with maximum-ratio-combining (MRC) receivers

Distributed antenna systems (DAS) have been shown to considerably outperform conventional cellular systems in terms of capacity improvement and interference resilience. However, the influence of frequency reuse planning on the performance of DAS remains relatively unknown. To partially fill this gap, this paper presents a comparative analysis of the down-link of DAS versus conventional cellular systems using different values of frequency reuse factor. The analysis assumes Rayleigh fading channels and it also considers maximum-ratio-combining (MRC) receivers at the user terminals to exploit diversity both in the transmission and reception links. Numerical evaluation of the analytical expressions shows that, in general, for most of the cases DAS can achieve better performance figures than conventional cellular systems using considerably smaller values of frequency reuse factor. Conversely, DAS can significantly improve the throughput (2x-3x) and power consumption (6-10 dB) of conventional systems when using the same frequency reuse factor. An interesting result shows that in some particular cases DAS outperform conventional cellular systems no matter the frequency reuse factor used by the latter one, which indicates an effective capacity gain provided by the combined operation of DAS and MRC receivers

Multi-Objective and Financial Portfolio Optimization of Carrier-Sense Multiple Access Protocols with Cooperative Diversity

8th International Workshop on Multiple Access Communications (MACOM2015), Helsinki, Finland.This paper presents a trade-off design and optimization of a class of wireless carrier-sense multiple access protocols where collision-free transmissions are assisted by the potential cooperative retransmissions of inactive terminals with a correct copy of the original transmission. Terminals are enabled with a decode-and-forward relaying protocol. The analysis is focused on asymmetrical settings, where terminals experience different channel and queuing statistics. This work is based on multi-objective and financial portfolio optimization tools. Each packet transmission is thus regarded not only as a network resource, but also as a financial asset with different values of return and risk (or variance of the return). The objective of this financial optimization is to find the transmission policy that simultaneously maximizes return and minimizes risk in the network. The work is focused on the characterization of the boundaries (envelope) of different types of trade-off performance regions: the conventional throughput region, sum-throughput vs. fairness, sum-throughput vs. power, and return vs. risk regions. Fairness is evaluated by means of the Gini-index, which is a metric commonly used in economics to measure income inequality. Transmit power is directly linked to the global transmission rate. The protocol is shown to outperform non-cooperative solutions under different network conditions that are here discussed

A cross-layer approach to the downlink performance analysis and optimization of distributed antenna systems in multi-cell environments

Distributed antenna systems (DAS) constitute one of the most attractive schemes to efficiently achieve the stringent quality of service demands of next generation wireless networks. However, existing works have been only focused on the analysis of the downlink of single-user DAS multi-cell networks with fixed node configurations and without considering the effects of upper layer algorithms. This paper partially fills this gap by presenting the performance analysis and optimization of the downlink of DAS systems in multi-cell environments with single and multiple users and with basic resource management algorithms. The cross-layer analysis and optimization is first carried out using a simplified system-level simulation tool which provides enough flexibility for the understanding of the basic rules of operation of the proposed algorithms. After this, the most promising algorithms are implemented in a full version of a system level simulator which has been especially developed for the accurate analysis and validation of 3G (third generation) and B3G (beyond 3G) radio access technologies. Our results show that under the appropriate allocation and management policies, distributed antenna systems efficiently counteract the main impairments of current cellular architectures

Collision resolution algorithms for RFID applications

This paper presented a review of modern contention resolution algorithms which combine signal processing tools and scheduling mechanisms, and which are suitable for implementation in RFID systems. We have concluded that in order to minimize the number of transmissions and retransmissions of the contending tags, retransmission diversity algorithms with random selection of orthogonal ID sequences provide an attractive option. Due to the operating frequencies of RFID systems, the most probable method to achieve MPR capabilities is through reader cooperation, which remains as an open research area. Another contribution of this paper is the identification of tag collisions as a method to improve privacy and security. However, despite such security enhancement schemes, single packet transmissions are still susceptible to severe privacy and security attacks that must be solved either by other layers or through a further modification of the MAC procedures

Integration of RFID readers into wireless mobile telecommunication networks

Over the last few years significant work has been done in defining interfaces, standards, reader protocols and middleware platforms for RFID (radio frequency identification) systems. However, current solutions are mainly dedicated to environments where readers or interrogators are located in the same premises that host the middleware and database servers, thus leaving unattended an accurate analysis of how efficiently to integrate them into wireless mobile telecommunication networks. This is particularly critical in cases where readers need to be placed in geographically distant and mobile locations, and where bandwidth limitations of the underlying public wireless networks might restrict the deployment of certain RFID applications. This paper aims at partially filling this gap by first proposing an approximate and heuristic theoretical model for the effects of the traffic generated by a mobile RFID reader upon the communication links to be used, and then by analyzing the possibilities of integrating readers into known mobile telecommunication technologies. Based on the proposed approximation, basic relationships between different RFID parameters such as reader complexity, tag reading rate and the total utilized bandwidth are here found and discussed. Although the models and assumptions used here still need to be validated and/or adjusted to real system scenarios, it is expected that this work will bring further research on the interesting problem of designing RFID systems, where not only variables from different layers of the OSI (open system interconnection) model interact with each other, but many others coming from privacy, security, business and regulatory fields must also be addressed

A Random Access Protocol Incorporating Multi-packet Reception, Retransmission Diversity and Successive Interference Cancellation

This paper presents a random access protocol assisted by a set of signal processing tools that significantly improve the multi-packet reception (MPR) capabilities of the system. A receiver with M antennas is mainly used to resolve collisions with multiplicity K≤MK≤M. The remaining unresolved conflicts (with multiplicity K>MK>M or with decoding errors) are processed by means of protocol-induced retransmissions that create an adaptive multiple-input multiple-output (MIMO) system. This scheme, also known as NDMA (network diversity multiple access) with MPR, can achieve in ideal conditions a maximum throughput of M packets/time-slot. A further improvement is proposed here, where the receiver attempts to recover the information immediately after the reception of each (re)transmission. This is different from conventional NDMA, where this decoding process only occurs once the adaptive MIMO channel is assumed to become full-rank (i.e., once the estimated number of required retransmissions has been collected). The signals that are correctly decoded at every step of the proposed algorithm are used to mitigate interference upon the remaining contending signals by means of successive interference cancellation (SIC). The use of SIC not only improves signal reception, but most importantly reduces the required number of retransmissions to resolve a collision. Significantly high throughput figures (depending on channel/load conditions) that surpass the nominal rate (T>MT>M) are reported. To the best of our knowledge this is the first random access protocol that achieves this throughput figure. Spatial and time correlation, as well as imperfections of SIC operation are also considered. In ideal conditions, the effects of SIC are found to be equivalent to a splitting tree operation. The inclusion of SIC in NDMA-MPR also opens the possibility of backwards compatibility with legacy terminals. The protocol achieves the highest throughput in the literature with minimum feedback complexity (identical to conventional NDMA ). This is a significant result for future highly dense and 5G wireless networks

Distributed Linear Precoding and User Selection in Coordinated Multicell Systems

In this paper, we tackle the problem of semidistributed user selection with distributed linear precoding for sum-rate maximization in multiuser multicell systems. A set of adjacent base stations (BSs) forms a cluster to perform coordinated transmission to cell-edge users, and coordination is carried out through a central processing unit (CU). However, the message exchange between BSs and the CU is limited to scheduling control signaling, and no user data or channel state information (CSI) exchange is allowed. In the considered multicell coordinated approach, each BS has its own set of cell-edge users and transmits only to one intended user while interference to nonintended users at other BSs is suppressed by signal steering (precoding). We use two distributed linear precoding schemes, namely, distributed zero forcing and distributed virtual signal-to-interference-plus-noise ratio (DVSINR). Considering multiple users per cell and the backhaul limitations, the BSs rely on local CSI to solve the user selection problem. First, we investigate how the signal-to-noise ratio regime and the number of antennas at the BSs impact the effective channel gain (the magnitude of the channels after precoding) and its relationship with multiuser diversity. Considering that user selection must be based on the type of implemented precoding, we develop metrics of compatibility (estimations of the effective channel gains) that can be computed from local CSI at each BS and reported to the CU for scheduling decisions. Based on such metrics, we design user selection algorithms that can find a set of users that potentially maximizes the sum rate. Numerical results show the effectiveness of the proposed metrics and algorithms for different configurations of users and antennas at the BSs

Cognitive spectrum portfolio optimisation, approaches and exploitation

Abstract A major challenge for cooperative cognitive radio networks is the creation and optimisation of a suitable spectrum portfolio, utilised by the radio nodes in the process of dynamic spectrum management. This paper presents several optimisation approaches for spectrum portfolios. Their characteristics are discussed regarding a variety of different scenarios, and it is shown how different approaches can complement each other to optimise the overall spectrum management, in particular considering spectrum portfolio optimisation under mobility and QoS constraints. Special consideration of upcoming TV whitespace communication use cases is shown in this discussion