Andreas Lobinger

Load Balancing in Downlink LTE Self-Optimizing Networks

In this paper we present system level simulation results of a self-optimizing load balancing algorithm in a long-term-evolution (LTE) mobile communication system. Based on previous work, we evaluate the network performance of this algorithm that requires the load of a cell as input and controls the handover parameters. We compare the results for different simulation setups: for a basic, regular network setup, a non-regular grid with different cell sizes and also for a realistic scenario based on measurements and realistic traffic setup.

A Mathematical Perspective of Self-Optimizing Wireless Networks

We present a mathematical framework for quantitative investigations of self-optimizing wireless networks (SON) with focus on the 3GPP Long-Term Evaluation (LTE) system. Basic target functions, such as the signal-to-noise ratio distribution, the number of satisfied users, or energy efficiency are derived as a figure of merit, including the impact of adaptation of downlink transmit power adaptation, antenna tilt, and the handover parameter. The framework is exemplified by basic investigations on load balancing.

Coordinating Handover Parameter Optimization and Load Balancing in LTE Self-Optimizing Networks

In this paper we present simulation results of a self-optimizing network in a long-term-evolution (LTE) mobile communication system that uses two optimizing algorithms at the same time: load balancing and handover parameter optimization. Based on previous work, we extend the optimization by a combined use case. We present the interactions of the two SON algorithms and show an example of a coordination system. The coordination system for self optimization observes system performance and controls the SON algorithms. As both SON algorithms deal with the handover decision itself, not only interactions, but also conflicts in the observation and control of the system are to be expected and are observed. The example of a coordination system here is not the optimal solution covering all aspects, but rather a working solution that shows equal performance to the individual algorithms or in the best case combining the strengths of the algorithms and achieving even better performance; although as localized gain, in time and area.

Efficient uplink modeling for dynamic system-level simulations of cellular and mobile networks

A novel theoretical framework for uplink simulations is proposed. It allows investigations which have to cover a very long (real-) time and which at the same time require a certain level of accuracy in terms of radio resource management, quality of service, and mobility. This is of particular importance for simulations of self-organizing networks. For this purpose, conventional system level simulators are not suitable due to slow simulation speeds far beyond real-time. Simpler, snapshot-based tools are lacking the aforementioned accuracy. The runtime improvements are achieved by deriving abstract theoretical models for the MAC layer behavior. The focus in this work is long term evolution, and the most important uplink effects such as fluctuating interference, power control, power limitation, adaptive transmission bandwidth, and control channel limitations are considered. Limitations of the abstract models will be discussed as well. Exemplary results are given at the end to demonstrate the capability of the derived framework.

Non-regular layout for cellular network system simulations

This paper defines a synthetic non-regular Springwald network layout which is easy to take into use in cellular network system simulations. The performance of the non-regular layout was compared with two regular 3GPP simulation scenarios. The benefit of the non-regular layout is that it reflects better the live network deployments and therefore results in more realistic benchmarking metrics. The results shows that the Springwald layout provides a simulation environment where the system level performance is comparable with the reference scenarios, however, with some additional features of the non-regular effects.

Mobility robustness optimization beyond Doppler effect and WSS assumption

Mobility robustness is often understood as making the radio link Doppler-resistent. However, guaranteeing proper cell changes, e.g. handovers, is another, as important and at least as challenging aspect beyond Doppler and wide sense stationarity (WSS) assumption. This paper tries to describe the complex optimization problem with scientific methods, in order to catalyze future academic work in the interesting field of minimizing handover problems. Simulation results will be presented for intra-frequency mobility robustness optimization (MRO) distinguishing network-wide, cell-specific and cell-pair specific optimization.

Downlink eigenbeamformer with combining of eigenbeams

For W-CDMA several closed-loop Tx diversity concepts employing 4 antenna elements at the base station (BS) are currently discussed to be included in 3GPP release 5. Among these, the downlink eigenbeamformer effectively uses long-term as well as short-term channel properties. More specifically, it is based on an eigenanalysis of the long-term spatial covariance matrix at the mobile station (MS). The eigenvectors (vector of complex weighting factors for all antenna elements) with the largest eigenvalues (largest average signal power) are determined and fed back step by step to the BS. This process takes place on the same time scale as the MS physical movement. Accordingly, required operations in the MS for eigenanalysis as well as required long-term feedback bits for representation of eigenvectors are distributed over a very large number of transmission slots. As an extension of simple switching between eigenbeams a linear combination of several eigenbeams is considered. This combining is performed using constellation rotation and progressive refinement techniques applied to the eigenbeam space. Performance of combining methods is compared with an upper bound (beamforming based on perfect short-term knowledge of the downlink channel) and uplink based beamforming methods where only the long-term spatial covariance matrix is known at the BS. Although combining methods require more short-term feedback information then simple switching and thus are more sensitive to feedback delay and restricted feedback rate, they prove to be beneficial up to a vehicle speed of 40 km/h.

Performance of downlink eigenbeamformer with realistic feedback transmission

Several closed-loop transmitter diversity concepts for multiple antennas are currently under discussion for inclusion in the 3GPP W-CDMA standard. The focus is to find a concept using 4 transmitter antennas allowing for finer beamsteering while still being compatible with the current uplink feedback rate of one bit per transmission slot. Among these concepts, the downlink eigenbeamformer effectively uses long-term as well as short-term channel properties. More specifically, it is based on an eigenanalysis of the long-term spatial covariance matrix. The eigenbeams with the largest eigenvalues (largest average SNR) are determined and fed back step by step to the base station (BS). This process takes place on the same time scale as the mobile stations (MS) physical movement. Accordingly, required operations in the MS as well as required long-term feedback bits are distributed over a very large number of transmission slots. In addition, a short term selection between the eigenbeams is carried out at the MS to account for fast fading. Performance in terms of coded block error rate for the downlink eigenbeamformer concept is analysed under a set of spatial channel models. Simulation results show the influence of important parameters like, e.g., the amount of common pilot channel power compared to another transmitter diversity proposal that does not exploit the long term channel statistics. The multiplexing formats between long-term and short-term feedback information are discussed and a detailed complexity analysis is given.

Multi-Layer Traffic Steering: RRC Idle Absolute Priorities & Potential Enhancements

This paper investigates the potentials of traffic steering in the Radio Resource Control (RRC) Idle state by evaluating the Absolute Priorities (AP) framework in a multi-layer Long Term Evolution (LTE) macrocell scenario. Frequency priorities are broadcast on the system information and RRC Idle users can be steered towards higher priority carriers whenever coverage allows it. However, such an approach may overload the prioritized layers. For that purpose, an enhanced scheme is proposed, where priorities are adjusted on a user basis and are provided to the terminal via the connection release signaling. The priority adjustment is based on both the Composite Available Capacity (CAC) and the radio conditions of the candidate layers. Compared to broadcast AP, the proposed scheme achieves better load balancing performance and improves network capacity, given that the User Equipment (UE) inactivity periods are not significantly long. Finally, better alignment between the RRC Connected and Idle mobility procedures is observed, guarantying significant decrease of handovers/reselections and potential battery life savings by minimizing the Inter-Frequency (IF) measurement rate in the RRC Idle.