Ismail Guvenc

Enhanced intercell interference coordination challenges in heterogeneous networks

3GPP LTE-Advanced has recently been investigating heterogeneous network (HetNet) deployments as a cost effective way to deal with the unrelenting traffic demand. HetNets consist of a mix of macrocells, remote radio heads, and low-power nodes such as picocells, femtocells, and relays. Leveraging network topology, increasing the proximity between the access network and the end users, has the potential to provide the next significant performance leap in wireless networks, improving spatial spectrum reuse and enhancing indoor coverage. Nevertheless, deployment of a large number of small cells overlaying the macrocells is not without new technical challenges. In this article, we present the concept of heterogeneous networks and also describe the major technical challenges associated with such network architecture. We focus in particular on the standardization activities within the 3GPP related to enhanced intercell interference coordination.

A Survey on TOA Based Wireless Localization and NLOS Mitigation Techniques

Localization of a wireless device using the time-of-arrivals (TOAs) from different base stations has been studied extensively in the literature. Numerous localization algorithms with different accuracies, computational complexities, a-priori knowledge requirements, and different levels of robustness against non-line-of-sight (NLOS) bias effects also have been reported. However, to our best knowledge, a detailed unified survey of different localization and NLOS mitigation algorithms is not available in the literature. This paper aims to give a comprehensive review of these different TOA-based localization algorithms and their technical challenges, and to point out possible future research directions. Firstly, fundamental lower bounds and some practical estimators that achieve close to these bounds are summarized for line-of-sight (LOS) scenarios. Then, after giving the fundamental lower bounds for NLOS systems, different NLOS mitigation techniques are classified and summarized. Simulation results are also provided in order to compare the performance of various techniques. Finally, a table that summarizes the key characteristics of the investigated techniques is provided to conclude the paper.

A hybrid frequency assignment for femtocells and coverage area analysis for co-channel operation

Successful deployment of femtocells requires intelligent cell planning and introduces some unique challenges. In this letter, an efficient frequency assignment technique is proposed for femtocells considering practical issues such as hand-off, coverage, and interference. Moreover, interference-limited coverage area (ILCA) of a co-channel femtocell base station is calculated based on parameters such as the distance to the macrocell BS, path loss exponents, and transmission power levels.

NLOS identification and weighted least-squares localization for UWB systems using multipath channel statistics

Non-line-of-sight (NLOS) identification and mitigation carry significant importance in wireless localization systems. In this paper, we propose a novel NLOS identification technique based on the multipath channel statistics such as the kurtosis, the mean excess delay spread, and the root-mean-square delay spread. In particular, the IEEE 802.15.4a ultrawideband channel models are used as examples and the above statistics are found to be well modeled by log-normal random variables. Subsequently, a joint likelihood ratio test is developed for line-of-sight (LOS) or NLOS identification. Three different weighted least-squares (WLSs) localization techniques that exploit the statistics of multipath components (MPCs) are analyzed. The basic idea behind the proposed WLS approaches is that smaller weights are given to the measurements which are likely to be biased (based on the MPC information), as opposed to variance-based WLS techniques in the literature. Accuracy gains with respect to the conventional least-squares algorithm are demonstrated via Monte-Carlo simulations and verified by theoretical derivations.

Capacity and Fairness Analysis of Heterogeneous Networks with Range Expansion and Interference Coordination

Range expansion and inter-cell interference coordination (ICIC) can improve the capacity and fairness of heterogeneous network (HetNet) deployments by off-loading macrocell users to low-power nodes. Due to difficulties in analytical treatment, current studies for range expansion and ICIC in HetNets rely mostly on simulations. In this letter, first, off-loading benefits of range expansion in HetNets are captured through cumulative distribution functions (CDFs) of the downlink signal to interference plus noise ratio (SINR) difference between the macrocell and strongest picocell signals. Then, these CDFs are used to investigate the system capacity and fairness as a continuous function of the range expansion bias, and benefits of using ICIC with range expansion are demonstrated through numerical results.Range expansion and inter-cell interference coordination (ICIC) can improve the capacity and fairness of heterogeneous network (HetNet) deployments by off-loading macrocell users to low-power nodes. Due to difficulties in analytical treatment, current studies for range expansion and ICIC in HetNets rely mostly on simulations. In this letter, first, off-loading benefits of range expansion in HetNets are captured through cumulative distribution functions (CDFs) of the downlink signal to interference plus noise ratio (SINR) difference between the macrocell and strongest picocell signals. Then, these CDFs are used to investigate the system capacity and fairness as a continuous function of the range expansion bias, and benefits of using ICIC with range expansion are demonstrated through numerical results.

A Survey on Multicarrier Communications: Prototype Filters, Lattice Structures, and Implementation Aspects

Due to their numerous advantages, communications over multicarrier schemes constitute an appealing approach for broadband wireless systems. Especially, the strong penetration of orthogonal frequency division multiplexing (OFDM) into the communications standards has triggered heavy investigation on multicarrier systems, leading to re-consideration of different approaches as an alternative to OFDM. The goal of the present survey is not only to provide a unified review of waveform design options for multicarrier schemes, but also to pave the way for the evolution of the multicarrier schemes from the current state of the art to future technologies. In particular, a generalized framework on multicarrier schemes is presented, based on what to transmit, i.e., symbols, how to transmit, i.e., filters, and where/when to transmit, i.e., lattice. Capitalizing on this framework, different variations of orthogonal, bi-orthogonal, and non-orthogonal multicarrier schemes are discussed. In addition, filter designs for various multicarrier systems are reviewed considering four different design perspectives: energy concentration, rapid decay, spectrum nulling, and channel/hardware characteristics. Subsequently, evaluation tools which may be used to compare different filters in multicarrier schemes are studied. Finally, multicarrier schemes are evaluated from the perspective of practical implementation aspects, such as lattice adaptation, equalization, synchronization, multiple antennas, and hardware impairments.

TOA estimation for IR-UWB systems with different transceiver types

In this paper, performances of stored-reference, transmitted-reference (TR), and energy-detection (ED)-based time-of-arrival estimation techniques are analyzed for impulse-radio ultra-wideband (IR-UWB) systems at sub-Nyquist sampling rates. First, an additive white Gaussian noise channel is considered to emphasize certain fundamental issues related to these different transceivers. In particular, energy collection characteristics and decision statistics are presented. Probability of accurate peak detection is analyzed for each transceiver, and receiver operating characteristics for the leading edge are derived. Effects of number of pulses per symbol and number of averaging symbols are investigated in detail. Then, realistic multipath channels are addressed, and various maximum-likelihood estimation approaches are investigated. A new estimator that jointly exploits the noise statistics and power delay profile of the channel is proposed, and a Bayesian estimator that (ideally) gives a lower bound is analyzed. Simulation results show that while ED and TR have better energy collection capabilities at low-rate sampling, they suffer from distributing the energy over time.

On the Expanded Region of Picocells in Heterogeneous Networks

In order to expand the downlink (DL) coverage areas of picocells in the presence of an umbrella macrocell, the concept of range expansion has been recently proposed, in which a positive range expansion bias (REB) is added to the DL received signal strengths (RSSs) of picocell pilot signals at user equipments (UEs). Although range expansion may increase DL footprints of picocells, it also results in severe DL inter-cell interference in picocell expanded regions (ERs), because ER picocell user equipments (PUEs) are not connected to the cells that provide the strongest DL RSSs. In this paper, we derive closed-form formulas to calculate appropriate REBs for two different range expansion strategies, investigate both DL and uplink (UL) inter-cell interference coordination (ICIC) to enhance picocell performance, and propose a new macrocell-picocell cooperative scheduling scheme to mitigate both DL and UL interference caused by macrocells to ER PUEs. Simulation results provide insights on REB selection approaches at picocells, and demonstrate the benefits of the proposed macrocell-picocell cooperative scheduling scheme over alternative approaches.

Mobility management challenges in 3GPP heterogeneous networks

In this article we provide a comprehensive review of the handover process in heterogeneous networks (HetNets), and identify technical challenges in mobility management. In this line, we evaluate the mobility performance of HetNets with the 3rd Generation Partnership Project (3GPP) Release-10 range expansion and enhanced inter-cell interference coordination (eICIC) features such as almost blank subframes (ABSFs). Simulation assumptions and parameters of a related study item in 3GPP are used to investigate the impact of various handover parameters on mobility performance. In addition, we propose a mobility-based inter-cell interference coordination (MB-ICIC) scheme, in which picocells configure coordinated resources so that macrocells can schedule their high-mobility UEs in these resources without co-channel interference from picocells. MB-ICIC also benefits low-mobility UEs, since handover parameters can now be more flexibly optimized. Simulations using the 3GPP simulation assumptions are performed to evaluate the performance of MB-ICIC under several scenarios.

Handling CCI and ICI in OFDMA femtocell networks through frequency scheduling

Femtocells have a strong potential for increasing the efficiency and coverage of next-generation broadband wireless networks. In this paper, a co-channel framework for the coexistence of orthogonal frequency division multiple access (OFDMA) based macrocell and femtocell wireless networks is proposed. It is based on utilizing the resource blocks of macrocell-associated mobile stations (mMSs) that are far away to a femtocell base station (fBS), therefore avoiding strong interference that may occur between a femtocell and close-by mMSs. An avoidance method that jointly utilizes the spectrum sensing results as well as scheduling information obtained from the macrocell base station (mBS) is introduced. Moreover, the impact of inter-carrier interference (ICI) from the mMSs in the uplink is discussed and evaluated through simulations.