Malte Schellmann

Scenarios for 5G mobile and wireless communications: the vision of the METIS project

METIS is the EU flagship 5G project with the objective of laying the foundation for 5G systems and building consensus prior to standardization. The METIS overall approach toward 5G builds on the evolution of existing technologies complemented by new radio concepts that are designed to meet the new and challenging requirements of use cases todays radio access networks cannot support. The integration of these new radio concepts, such as massive MIMO, ultra dense networks, moving networks, and device-to-device, ultra reliable, and massive machine communications, will allow 5G to support the expected increase in mobile data volume while broadening the range of application domains that mobile communications can support beyond 2020. In this article, we describe the scenarios identified for the purpose of driving the 5G research direction. Furthermore, we give initial directions for the technology components (e.g., link level components, multinode/multiantenna, multi-RAT, and multi-layer networks and spectrum handling) that will allow the fulfillment of the requirements of the identified 5G scenarios.

Synchronization of cooperative base stations

We consider synchronization techniques required to enhance the cellular network capacity using base station cooperation. In the physical layer, local oscillators are disciplined by the global positioning system (GPS) and over the backbone network for outdoor and indoor base stations, respectively. In the medium access control (MAC) layer, the data flow can be synchronized by two approaches. The first approach uses so-called time stamps. The data flow through the user plane and through copies of it in each cooperative base station is synchronized using a timing protocol on the interconnects between the base stations. The second approach adds mapping information to the data after the user plane processing is almost finalized. Each forward-error encoded transport block, its modulation and coding scheme and the resources where it will be transmitted are multicast over the interconnect network. Interconnect latency is reduced below 1 ms to enable coherent interference reduction for mobile radio channels.

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.

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.

Implementation concepts for distributed cooperative transmission

Information theory predicts huge performance gains in terms of spectrum efficiency by using cooperative transmission from multiple base stations. Cooperation eliminates inter-cell interference and it enables a higher channel rank. The isolated cell capacity is an upper bound for unlimited backbone capacity, infinite signal processing power and infinite channel feedback.We discuss a combined physical, medium access control (MAC) and network layer approach targeting minimal implementation loss while relaxing assumptions. It is based on frequency-selective channel quality identifier (CQI) reports to the serving station selecting the right user(s) on a given resource with best signal to interference and noise ratio (SINR). Such scheduling is performed independently in each cell. In adjacent cells it forms a group of users whose mutual interference is smaller than on average. The task of cooperative transmission is to further reduce the mutual interference within this group. In a multi-user system, the group shares only part of the spectrum. Feedback can hence be limited to the granted resource blocks. Based on virtual pilot sequences we reduce the pilot overhead and use a suitable mirror feedback scheme in addition. Altogether the feedback overhead is reduced by 2 orders of magnitude.

Capacity Scaling of Multi-User MIMO with Limited Feedback in a Multi-Cell Environment

We demonstrate that the fundamental capacity scaling law of multiple-input multiple-output radio systems, being proportional to the minimum of the number of receive and transmit antennas, holds also for the interference-limited multi-user multi-cell downlink scenario. It can be realized by using a sophisticated combination of physical and medium access control layer algorithms. The algorithms have low complexity and require no coherent channel state information at the transmitter. Instead, limited feedback on the effective channel quality is provided via a low-rate control channel. Our set of algorithms offers a fixed grid of beams at the transmitter, where the terminals can select the best beam set. Further, we use receivers exploiting the instantaneous knowledge of the interference at the terminal side. A score-based scheduler, which asymptotically reaches proportional fairness, is used to switch adaptively between multi-user diversity and multi-user multiplexing, in a frequency-selective manner. We provide many insights into the synergy between these algorithms from multi-cell simulations in a hexagonal cellular deployment.

MU-MIMO with Localized Downlink Base Station Cooperation and Downtilted Antennas

Multi-cellular 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. It is well known that base station cooperation yields great capacity improvement for downlink multi-antenna cellular networks. However, the proposed solutions assume a central processing unit, coordinating the information exchange and thus determining the optimal resource allocation of the overall cellular network. Recently multi-user eigenmode transmission was proposed to relax the constraint of channel state information at the transmitter. It requires the terminals to feed back their dominant eigenmodes only. To reduce the computational costs and the information exchange further, we consider limited localized base station cooperation. We demonstrate potential capacity gains in a cellular orthogonal frequency division multiplexing system using real 3D measured antenna patterns and the scaling with the number of cooperating antenna arrays. Additionally, we use minimum mean square error equalization at the terminal side to combat residual cochannel interference.

On the value of synchronous downlink MIMO-OFDMA systems with linear equalizers

Cellular radio systems are often limited due to the presence of cochannel interference. Basically, radio systems may be operated by utilizing asynchronous or synchronous downlink transmission from all base stations in the system, where synchronization between terminals and their serving base station is mandatory in both concepts. We provide a comparison between the theoretical achievable spectral efficiency in an orthogonal frequency division multiple access systems using different linear equalizers and their resulting performance taking estimation errors into account. It is shown, that the choice of the appropriate receiver depends on the degree of synchronization in the system. We demonstrate that a 3G long term evolution radio system may achieve higher spectral efficiency thanks to interference suppression if fully synchronized data transmission from all base stations is introduced.

Spatial Transmission Mode Switching in Multiuser MIMO-OFDM Systems With User Fairness

Multiantenna radio systems allow accessing the channel in diversity or spatial multiplexing (SMUX) mode. Adequate switching between these modes according to current channel conditions was shown to yield significant performance improvements while requiring little feedback from the receiving side. We present a transmission concept for the downlink of a multiuser multiple-input-multiple-output orthogonal frequency-division-multiplexing (MU-MIMO-OFDM) system aiming at high user rates with limited feedback demands. An extended score-based scheduling (SB) approach ensures fair-resource allocation to the users, whereas transmission mode switching is used to guarantee high user rates. The degree of fairness of the scheduler can be adapted by adequately configuring a weighting function for the scores. Comparison with single-mode schemes reveals substantial throughput gains of the adaptive switching concept. Furthermore, targeting maximum throughput, we show that a considerable proportion of the capacity of the MIMO broadcast channel (BC) can be achieved with a comparatively low amount of required feedback.

Suppressing the out-of-band power radiation in multi-carrier systems: A comparative study

As of today, orthogonal frequency-division multiplexing (OFDM) has been the dominant technique in broadband transmission systems. However, for the application in cognitive radios, the sidelobes of rectangularly pulsed OFDM signals must be well suppressed to satisfy the rigorous spectral masks. In this context, filter-bank based multi-carrier (FBMC) transmission, which allows designing prototype filters for signal pulse shaping, is considered to be more effective. In this article, different sidelobe suppression schemes for OFDM and pulse-shaped FBMC are comparatively investigated with respect to their out-of-band power radiation and their demand in computational complexity. Our results reveal that FBMC, thanks to the excellent time-frequency localization of the pulse shape, allows for a superior isolation of the transmit signal in frequency, enabling to achieve the minimal out-of-band power leakage for the operation in both contiguous and non-contiguous spectrum. Moreover, the additional complexity required for FBMC compared to the other schemes is very moderate.