Harald Burchardt

Area spectral efficiency performance comparison between VLC and RF femtocell networks

In this paper the average area spectral efficiency (ASE) of indoor visible light communication (VLC) wireless network is investigated for various room geometries (2.5m×5m×3m, 5m×5m×3m and 10m×10m×3m). A comparison is made against state-of-the-art radio frequency (RF) systems employing femtocells indoors. A modified version of the wireless world initiative new radio (WINNER) indoor deployment scenario (office building) is used as the common setup where femtocell access points (APs) and VLC APs are installed alternatively. For the RF system the minimum number of femtocell APs per floor area is four as specified. The requirement for the VLC system is to fulfill the lighting conditions inside the room, characterized by the illuminance distribution in rooms with several possible dimensions. The ASE gains of the VLC system compared to the femtocell network range between 12 and 924 (depending on the number of femtocell APs and floor geometry).

Distributed and Autonomous Resource and Power Allocation for Wireless Networks

In this paper, a distributed and autonomous technique for resource and power allocation in orthogonal frequency division multiple access (OFDMA) femto-cellular networks is presented. Here, resource blocks (RBs) and their corresponding transmit powers are assigned to the user(s) in each cell individually without explicit coordination between femto-base stations (FBSs). The “allocatability” of each resource is determined utilising only locally available information of the following quantities: ; the required rate of the user; : the quality (i.e., strength) of the desired signal; : the frequency-selective fading on each RB; and : the level of interference incident on each RB. Using a fuzzy logic system, the time-averaged values of each of these inputs are combined to determine which RBs are most suitable to be allocated in a particular cell, i.e., which resources can be allocated such that the user requested rate(s) in that cell are satisfied. Fuzzy logic presents a completely novel, low-complexity methodology for inter-cell interference coordination (ICIC). A comprehensive study of this system in a femtocell environment is performed, yielding system performance improvements in terms of throughput, energy efficiency and coverage over state-of-the-art ICIC techniques.

Uplink interference protection and scheduling for energy efficient OFDMA networks

One of the key challenges for future orthogonal frequency division multiple access-based networks is inter-cell interference coordination. With full frequency reuse and small inter-site distances, coping with co-channel interference (CCI) in such networks has become increasingly important. In this article, an uplink interference protection (ULIP) technique to combat CCI is introduced and investigated. The level of uplink interference originating from neighbouring cells (affecting co-channel mobile stations (MSs) in the cell of interest) can be effectively controlled by reducing the transmit power of the interfering MSs. This is done based on the target signal-to-noise-plus-interference ratio (SINR) and tolerable interference of the vulnerable link. Bands are prioritised in order to differentiate those (vulnerable/victim) MSs that are to be protected from interference and those (aggressor/interfering MSs) that are required to sacrifice transmission power to facilitate the protection. Furthermore, MSs are scheduled such that those users with poorer transmission conditions receive the highest interference protection, thus balancing the areal SINR distribution and creating a fairer allocation of the available resources. In addition to interference protection, the individual power reductions also serve to decrease the total system uplink power, resulting in a greener system. It is shown through analytic derivation that the introduction of ULIP guarantees an increase in energy efficiency for all MSs, with the added benefit that gains in overall system throughput are also achievable. Extensive system level simulations validate these findings.

Multicell cooperation: evolution of coordination and cooperation in large-scale networks

Due to the large growth of mobile communications over the past two decades, the supporting cellular systems have continuously needed to expand and evolve in order to meet the ever increasing demand of wireless connections. While simple frequency reuse and power control were enough to manage the demand of these networks at first, multicellular cooperation and coordination have become paramount to the operation of larger and more highly utilized communication systems. This article discusses the development of different techniques for cooperation in large cellular networks, and offers insights into the need for such an evolution. We investigate various branches of multicell cooperation, including user-based cooperation, system-wide optimization, and the opportunity of multiple-base-station transmission. Furthermore, we offer an example of a recently proposed technique designed for multicell cooperation in full frequency reuse orthogonal frequency-division multiple access networks.

Pareto Optimal SINR Scheduling for Femto-Cell Deployment in Wireless Networks

In this paper, Pareto Femto-Cell Scheduling (PFCS), a novel scheduling mechanism for randomly deployed femto-cells, is presented. Here, the signal-to-interference-plus-noise ratio (SINR) targets of femto-users are adapted such that the sufficient conditions for Pareto optimal power control (POPC) are fulfilled. Furthermore, interference from full bandwidth users is managed such that as many mobile stations (MSs) as possible can transmit. Due to the random nature of femto-base station (FBS) deployment, interference graphs are used to group femto-cells (and hence, users) such that target spectral efficiencies can be achieved at Pareto optimum power. Simulation results show that PFCS achieves significant system capacity gains over other SINR-target-based power allocation techniques, while maximising coverage in dense mobile environments. Furthermore, substantial power savings can be achieved.

SecVLC: Secure Visible Light Communication for Military Vehicular Networks

Technology coined as the vehicular ad hoc network (VANET) is harmonizing with Intelligent Transportation System (ITS) and Intelligent Traffic System (ITF). An application scenario of VANET is the military communication where vehicles move as a convoy on roadways, requiring secure and reliable communication. However, utilization of radio frequency (RF) communication in VANET limits its usage in military applications, due to the scarce frequency band and its vulnerability to security attacks. Visible Light Communication (VLC) has been recently introduced as a more secure alternative, limiting the reception of neighboring nodes with its directional transmission. However, secure vehicular VLC that ensures confidential data transfer among the participating vehicles, is an open problem. In this paper, we propose a secure military light communication protocol (SecVLC) for enabling efficient and secure data sharing. We use the directionality property of VLC to ensure that only target vehicles participate in the communication. Vehicles use full-duplex communication where infra-red (IR) is utilized to share a secret key and VLC is used to receive encrypted data. We experimentally demonstrate the suitability of SecVLC in outdoor scenarios at varying inter-vehicular distances with key metrics of interest, including the security, data packet delivery ratio and delay.

Pareto Optimal Power Control Scheduling for OFDMA Networks

In this paper, a novel scheduling mechanism that enhances both network spectral and energy efficiency is presented. In Pareto Optimal Scheduling (POS), mobile stations (MSs) are scheduled based on path gains such that the sufficient conditions for Pareto optimal power control (POPC) are fulfilled. This is performed in such a manner to maximise the number of concurrently transmitting MSs. Furthermore, a Stepwise Removal (SR) algorithm is introduced for the situation where links do not meet the sufficient conditions for power control. In this case, links are removed in order for other MSs to achieve their signal-to-interference-plus-noise ratio (SINR) targets. The targets of these remaining MSs are updated to prevent losses in system spectral efficiency caused by the link removals. Large network simulation results show that significant gains in spectral efficiency can be achieved over standard power control techniques, while additionally providing substantially improved energy efficiency.

Sum Rate Increase via Variable Interference Protection

The sum rate of spectrum-sharing in decentralized and self-organizing wireless networks is investigated in this paper. Such networks pose the following two fundamental challenges: 1) cochannel interference and 2) the hidden node problem. For a slotted shared wireless medium, where resources are partitioned into time-frequency slots, time-multiplexed receiver initiated busy burst (BB) transmissions solve these problems by establishing an exclusion region around an active receiver by means of receiver feedback. The size of this exclusion region is controlled by an interference threshold that determines whether a user is allowed to transmit on a specific time-frequency resource unit. We propose a novel approach for setting the interference thresholds based on a heuristic derived for a two-link network. First, for two-links, the optimum threshold value is derived that maximizes the sum rate. Second, for multiple links, the new heuristic threshold that only relies on locally available information is derived. It is demonstrated via simulations that heuristic thresholding achieves superior sum rate compared to a fixed system-wide threshold. To complement simulation results, an analytical approach is developed to approximate the probability density function (pdf) of the sum of the interferers in BB setting with fixed threshold with a cumulant-based shifted log normal fitting method.

Distributed and autonomous resource allocation for femto-cellular networks

A distributed and autonomous technique for resource and power allocation in femto-cell networks is presented. Resource blocks (RBs) are assigned to the user(s) in each cell individually without coordination between base stations (BSs). The allocatability of each resource is determined using only local information: · the users required rate; · the quality of the desired signal; · the level of interference incident on each RB; and · the frequency-selective fading on each RB. Using fuzzy logic, these inputs are combined to determine which RBs are most suitable for allocation in a particular cell. A comprehensive study of this system yields a staggering system performance improvement over state-of-the-art interference coordination techniques.

Uplink interference protection and fair scheduling for power efficient OFDMA networks

In this paper a new method for uplink inter-cell interference coordination (ICIC) in a full frequency reuse system is proposed. The technique is named uplink interference protection (ULIP). ULIP exploits existing reference signals transmitted by all base stations (BSs). The fact that path loss and lognormal shadowing can be considered reciprocal in such frequency-division duplex (FDD) systems is exploited. Therefore, no additional signalling is required for a new mobile station (MS) to estimate the level of inter-cell interference it would cause to ongoing uplink transmissions in neighbouring cells. Using the acquired knowledge, MSs can adjust their transmit power to ensure that existing links are not forced into outage. A scheduling scheme is suggested in which a fair allocation of priority resource blocks (RBs), based on user signal-to-interference-plus-noise ratios (SINRs), enhances both cell-edge throughput and user throughput fairness. Through this, also significant system throughput gains are generated. Finally, it is demonstrated that as a side effect of the interference reductions, considerable energy savings can be achieved.