Michal Cierny

On Number of Almost Blank Subframes in Heterogeneous Cellular Networks

In heterogeneous cellular scenarios with macrocells, femtocells or picocells users may suffer from significant co-channel cross-tier interference. To manage this interference 3GPP introduced almost blank subframe (ABSF), a subframe in which the interferer tier is not allowed to transmit data. Vulnerable users thus get a chance to be scheduled in ABSFs with reduced cross-tier interference. We analyze downlink scenarios using stochastic geometry and formulate a condition for the required number of ABSFs based on base station placement statistics and user throughput requirement. The result is a semi-analytical formula that serves as a good initial estimate and offers an easy way to analyze impact of network parameters. We show that while in macro/femto scenario the residue ABSF interference can be well managed, in macro/pico scenario it affects the number of required ABSFs strongly. The effect of ABSFs is subsequently demonstrated via user throughput simulations. Especially in the macro/pico scenario, we find that using ABSFs is advantageous for the system since victim users no longer suffer from poor performance for the price of relatively small drop in higher throughput percentiles.

Device-to-Device Link Admission Policy Based on Social Interaction Information

We propose an admission policy for device-to-device (D2D) communication link based on a social interaction model. We utilize prior information on user interactive statistics such as contact frequency and contact duration among potential D2D pairs to determine the success rate of message delivery. Targeting a particular delivery success rate, we further propose methods to identify admissible D2D links according to their statistical channel state information. The social-interaction-based D2D admission policy provides a real-life guideline for practical D2D underlay operation in cellular networks.

Time domain bi-level downlink power control for cross-tier interference mitigation in HetNet

We develop and analyze a time-domain bi-level power control scheme using Almost Blank Subframe (ABS) for downlink cross-tier interference mitigation. Even for only one femto cell and macro cell, the optimization problem given rate constraint is nonconvex. We prove that by relaxing the integer requirement for subframe division, the optimal strategy is using either power control or ABS individually, and a joint scheme is not needed in the optimal solution. Imposing the integer subframe number requirement, this conclusion is no longer true. However, we show that the optimal solution also assumes simple structure by choosing from one of three feasible solutions. Simulation results show that the joint power control and ABS scheme outperforms traditional single-level power control scheme for downlink cross-tier interference mitigation with low computational complexity.

Impact of Base Station Time Synchronization Mismatch on Almost Blank Subframes

This letter analyzes the impact of imperfect timing synchronization of cellular base stations on feasibility of almost blank subframes, an interference management concept for heterogeneous networks. We claim that the most significant impact of timing synchronization mismatch lays in potential disturbance of a control channel that is located in the beginning of the subframe. We use stochastic geometry to analyze the impact and discover that the timing requirement from time division duplex system suffices also for frequency division duplex system that uses almost blank subframes.

Higher Rank Interference Effect on Weak Beamforming or OSTBC Terminals

Spatial multiplexing is one of the multi-antenna techniques employed by current generation wireless systems. Based on existing research, spatial multiplexing is known to be susceptible to interference. However, it is not well known what the effect of spatial multiplexing on other transmissions is, i.e., how does user performance depend on whether a neighboring cochannel interferer applies a single (spatial) stream or a multi stream transmission. This work attempts to alleviate this shortcoming by analyzing the impact of interference rank (number of spatial streams) on a beamforming and orthogonal space-time block coded user transmission. We generalize existing analytical results on signal-to-interference-plus-noise-ratio distribution and outage probability under arbitrary number of unequal power interferers. Our results are in close form and include interference rank as a parameter, allowing for a thorough study of its impact. Analysis shows that higher rank interference causes lower outage probability, and can support higher outage threshold especially in the case of beamforming.

Inter-cell interference management in OFDMA TDD downlink using sounding/silencing protocol

Modern cellular communication systems are based on frequency reuse 1. Although reuse 1 promises high peak data rates and significantly simplified network planning, the cell edge or cell overlap areas suffer from excessive inter-cell interference. In this work we address interference management for downlink part of these systems. Part of the radio resources is primarily reserved for protected access of vulnerable users. The protected access is based on a novel sounding/silencing protocol that takes advantage of instantaneous signal to interference plus noise ratio prediction and has power to silence strong interferers with low priority. The accompanied signaling scheme is developed for time division duplex systems. Our results in an indoor scenario with partly overlapping closed subscriber groups show that even with realistic control signal reception model the proportion of zero throughput users can be reduced by more than 50%.

Exclusion regions via handshaking protocol for inter-cell interference management

We study the definition of exclusion region through handshake control bursts. The RTS/CTS (request to send/clear to send) bursts that typically take part in the handshake between the radio transmitter and receiver are a perfect medium for sounding their surroundings. Therefore, they can be utilized for setting exclusion regions - geographical areas protecting the active receivers from interference. This combination is particularly interesting for low-cost interference management among indoor cellular wireless networks with no deployment coordination. We propose an interconnection of these principles, creating thus a synchronized protocol that defines the size of exclusion regions via interference prediction from received powers of RTS and CTS control bursts. Performance of the protocol is evaluated analytically for a setup with two links and full load and by the means of simulations for a higher number of links and variable load. Both capacity and fairness are higher than with uncoordinated deployment. Within the simulated scenario the results show that predicting the interference from RTS bursts is sufficient, leading to smaller overhead and no dependence on channel reciprocity.