Lars Thiele

Coordinated multipoint: Concepts, performance, and field trial results

Coordinated multipoint or cooperative MIMO is one of the promising concepts to improve cell edge user data rate and spectral efficiency beyond what is possible with MIMOOFDM in the first versions of LTE or WiMAX. Interference can be exploited or mitigated by cooperation between sectors or different sites. Significant gains can be shown for both the uplink and downlink. A range of technical challenges were identified and partially addressed, such as backhaul traffic, synchronization and feedback design. This article also shows the principal feasibility of COMP in two field testbeds with multiple sites and different backhaul solutions between the sites. These activities have been carried out by a powerful consortium consisting of universities, chip manufacturers, equipment vendors, and network operators.

Coordinated Multipoint Trials in the Downlink

Coordinated multi-point (CoMP) is a new class of transmission schemes for interference reduction in the next generation of mobile networks. We have implemented and tested a distributed CoMP transmission approach in the downlink of an LTE-Advanced trial system operating in real time over 20 MHz bandwidth. Enabling features such as network synchronization, celland user-specific pilots, feedback of multicell channel state information and synchronous data exchange between the base stations have been implemented. Interferencelimited transmission experiments have been conducted using optimum combining with interference-aware link adaptation and cross-wise interference cancellation between the cells. The benefits of CoMP transmission have been studied over multi-cell channels recorded in an urban macro-cell scenario.

QuaDRiGa: A 3-D Multi-Cell Channel Model With Time Evolution for Enabling Virtual Field Trials

Channel models are important tools to evaluate the performance of new concepts in mobile communications. However, there is a tradeoff between complexity and accuracy. In this paper, we extend the popular Wireless World Initiative for New Radio (WINNER) channel model with new features to make it as realistic as possible. Our approach enables more realistic evaluation results at an early stage of algorithm development. The new model supports 3-D propagation, 3-D antenna patterns, time evolving channel traces of arbitrary length, scenario transitions and variable terminal speeds. We validated the model by measurements in a coherent LTE advanced testbed in downtown Berlin, Germany. We then reproduced the same scenario in the model and compared several channel parameters (delay spread, path gain, K-factor, geometry factor and capacity). The results match very well and we can accurately predict the performance for an urban macro-cell setup with commercial high-gain antennas. At the same time, the computational complexity does not increase significantly and we can use all existing WINNER parameter tables. These artificial channels, having equivalent characteristics as measured data, enable virtual field trials long before prototypes are available.

Interference-aware scheduling in the multiuser MIMO-OFDM downlink

With the introduction of orthogonal frequency- division multiplexing and multiple antennas in cellular networks, there are new opportunities to adapt the transmission to propagation and interference conditions. In this article we describe a practical approach using space-frequency-selective multiuser MIMO scheduling. Frequency-selective feedback is provided on achievable data rates for preferred single- and multistream transmission modes. The base station selects the best mode while providing instantaneous fairness. We observe that multiuser transmission increases the probability of using multistream transmission. Besides the benefits from optimal combining at the physical layer, there is an additional gain at the MAC layer since the estimation of achievable rates becomes more precise. Altogether, 93 percent of the theoretical throughput can be realized by synchronizing the base stations and providing cell-specific reference signals. We have implemented essential functions of the approach in real time on an experimental 3GPP LTE prototype in 20 MHz bandwidth. Feasibility of the key features is proven in laboratory and field trials.

Capacity Measurements in a Cooperative MIMO Network

Cooperation between adjacent base stations (BSs) may reduce intercell interference in future cellular networks and enhance network capacity in this way. The potential of cooperation is evaluated in this paper based on measurements conducted at 5.2 GHz in a campus environment with four overlapping cells. For each individual link of the four BS and five terminal locations, a 10 times 16 antenna configuration with cross-polarized elements is used. Results show that the direct signal is much stronger than scattering due to the small intersite distances. The polarized line-of-sight (LOS) signal creates two degrees of freedom in the spatial domain, which is also confirmed by a 3-D propagation model. Rich multipath scattering is characteristic for the rest of the links where the direct signal is blocked. Results for a multilink scenario where all users are jointly served by all BSs confirm that such cooperation leads to enhanced rank of the compound channel matrix. The spectral efficiency is five times higher with cooperation, compared with a conventional frequency reuse system, where different radio resources are assigned to adjacent cells. Hence, our measurements confirm the benefits of cooperation in cellular systems.

Distributed Base Station Cooperation via Block-Diagonalization and Dual-Decomposition

It has been recently shown that base station cooperation may yield great capacity improvement in downlink multiple antenna cellular networks. However, the proposed solutions assume that there is a central processing unit which coordinates the information exchange and determines the optimal resource allocation of the overall cellular network. Whilst the benefits of base station cooperation are large, computational burden of the central unit can be significant. Thus distributed solutions are desirable. This paper suggests a distributed solution for base station cooperation via block-diagonalization and dual-decomposition to maximize the weighted sum network capacity under per-antenna power constraint. The block-diagonalization pre-coding matrix is determined separately at each base station. It enables the full potential of base station cooperation by determining a trade-off between inter-cell interference mitigation, spatial multiplexing and macro diversity. The power allocation problem is formulated as a network utility maximization (NUM) problem. By looking at its Lagrangian dual problem, the decomposable structure of the optimization problem is revealed. This leads to a distributed and iterative algorithm that converges to the global optimum. The advantage of macro diversity in addition to inter-cell interference mitigation and spatial multiplexing in base station cooperation context is studied and shows superior performance in terms of a higher capacity increase with lower variance.

Field trials using coordinated multi-point transmission in the downlink

We report on field trials using CoMP transmission in the downlink of a mobile radio network. Two new features enable over-the-air CoMP transmission from physically separated base stations and terminals. These are distributed synchronization and a fast virtual local area network. Using VLAN tags, terminals feed back the multi-cell channel state information to their serving bases where it is multiplexed with shared data. Both are multicast to other cooperative base stations over the backhaul. In our trials, two terminals are served in two overlapping cells and placed in specific indoor, outdoor-to-indoor and outdoor scenarios. We have realized both intra-site as well as inter-site CoMP. While outage is indeed a big problem at the cell edge with full frequency reuse, with CoMP it is not observed anymore. Average throughput gains by factors 4 to 22 are observed when using CoMP compared to interference-limited transmission while between 27 and 78% of the isolated cell throughput is measured in both cells simultaneously.

Multi-Cell Channel Estimation using Virtual Pilots

Multicellular radio systems are often limited due to the presence of cochannel interference. Proposed physical layer concepts, e.g. coordinated joint transmission and interference rejection combining, try to strengthen the signal while combating the interference. However, the performance may be limited by the available channel knowledge. We provide a concept for multi-cell channel estimation in the downlink applicable for for both physical layer concepts. This concept uses virtual pilots based on block-orthogonal sequences, e.g. Hadamard.

A Geometric Polarization Rotation Model for the 3-D Spatial Channel Model

It is common to use channel models such as the 3GPP spatial channel model (SCM), the WINNER model or ray tracing to evaluate multiple-antenna multiple-user techniques in wireless communications. Cross-polarized antennas can enhance the channel rank and thus the throughput of such systems especially in case of a line-of-sight (LOS) connection. This requires an exact model of the polarization characteristics. To increase the accuracy of the existing channel models, we propose a new method that predicts the polarization state of a microwave link based on findings in the field of optics. We verified the method by cross-polarized multiple-input-multiple-output (MIMO) measurements at 2.6 GHz with 16 transmitters and ten receivers in an urban macrocell environment under strong LOS conditions in downtown Berlin, Germany. Comparisons of simulation and measurement results show that the coefficients of the polarized LOS channel can be predicted very well by the new method. Measured capacities at 10-dB signal-to-noise ratio (SNR) were in between 14.2 and 19.1 b/s/Hz-values that can be predicted by the channel model with more than 90% accuracy. This increase in modeling accuracy is an important feature for many applications such as heterogeneous networks, space-to-ground satellite communications, and cooperative communications.

Green transmission technologies for balancing the energy efficiency and spectrum efficiency trade-off

As 4G wireless networks are vastly and rapidly deployed worldwide, 5G with its advanced vision of all connected world and zero distance communications is already at the corner. Along with the super quality of user experience brought by these new networks, the shockingly increasing energy consumption of wireless networks has become a worrying economic issue for operators and a big challenge for sustainable development. Green Transmission Technologies (GTT) is a project focusing on the energy-efficient design of physical-layer transmission technologies and MAC-layer radio resource management in wireless networks. In particular, fundamental tradeoffs between spectrum efficiency and energy efficiency have been identified and explored for energy-efficiency-oriented design and optimization. In this article, four selected GTT solutions are introduced, focusing on how they utilize the degrees of freedom in different resource domains, as well as how they balance the tradeoff between energy and spectrum efficiency. On top of the elaboration of separated solutions, the GTT toolbox is introduced as a systematic tool and unified simulation platform to integrate the proposed GTT solutions together.