PHY Enhancements: Non-orthogonal Waveforms and Massive MIMO for High-Mobility
The advantages of non-orthogonal pulse-shapes for multi-carrier transmission over doubly-dispersive channels compared to OFDM have been recognized already several years ago [W. Kozek and A. Molisch, “Nonorthogonal puse shapes for multicarrier communications in doubly dispersive channels”, IEEE Journal on Selected Areas in Communications, 1998]. Since cyclic prefix (CP)-OFDM, as employed by most state-of-the-art wireless communication systems, is spectrally inefficient and prone to inter-carrier-interference at high mobility, optimal pulse-shaping in non-orthogonal multicarrier systems has gained attention in recent years. As channel characteristics (time/frequency-dispersion) in mobile communications can vary significantly depending on the environment, we will develop adaptable transmission schemes that adjust transmission parameters to channel conditions as well as to qualtiy-of-service requirements imposed by the end-users.
Massive MIMO promises order of magnitude spectral efficiency gains by employing hundreds or even more antennas at the base stations to spatially multiplex tens of users. Major requirement to achieve such gains is the availability of accurate channel state information (CSI). In massive MIMO communication, pilot contamination puts deterministic limits on the signal to interference and noise ratio (SINR) and the achievable rate. Several methods to mitigate such effects have been proposed, frequently employing not only pilot signals but also data symbols to improve channel estimates. Pilot contamination becomes even more pronounced in high-mobility situation, since the channel coherence time is short thus restricting the length of pilot signals. Furthermore, channel ageing in time division duplex (TDD) massive MIMO systems due to time variation of the propagation channel, implying outdated CSI during transmission, causes enormous performance losses. This situation is even worse in frequency division duplex (FDD) systems, which have to rely on explicit CSI feedback to obtain channel estimates at the transmitter. We will account for such issues in our investigations and develop robust as well as efficient transceiver architectures for massive MIMO systems operating at high user mobility.
Signal Processing Enhancements
Filter-bank multi-carrier modulation schemes and large-scale full-dimension multiple-input multiple-output antenna arrays both promise efficiency gains by adapting the time-frequency as well as spatial properties of the transmit signal to the channel characteristics. We will develop adaptable transceivers that enable exploiting such gains under practical design constraints.
The rotary unit develop by Sebastian Caban and Martin Lerch during the CD-Lab Wireless Technologies for Sustainable Mobility of Prof. Christoph Mecklenbräucker enables repeatable and controllable measurements at high velocity (400-500 km/h). It will be extended during our CD-lab to support measurements of non-orthogonal multicarrier modulation schemes as well as measurements in the mmWave regime, to enable characterization of the mmWave channel at high mobility.
Innovative Technologies: Millimeter Wave Transmission and Ad-Hoc Networks for High-Mobility
Wireless communications in the mmW-band (around 30-300 GHz) is of interest for 5G mobile networks, since large amounts of untapped spectrum are available in this regime, promising multi-Gbit/s transmission and substantially increased cell capacities. Transmission in the mmW-band, however, comes with its own challenges, such as: hardware complexity constraints impacting beamforming and precoding algorithms; the requirement for highly directive beams to compensate for increased path-loss; significant probability for signal outages due to shadowing and reduced multipath propagation. Especially the latter two issues require careful investigation in the context of dependable high-mobility communications, since accurate beamforming is challenging in highly time-variant scenarios and signal outages cannot be tolerated in safety-relevant applications.
D2D transmission and ad-hoc networking in wireless systems at high mobility are mainly considered in the context of vehicle to infrastructure (V2I) and vehicle to vehicle (V2V) communications for traffic telematics and ITS applications. Such vehicular ad-hoc networks (VANETs) are commonly based on specific infrastructure and communication systems, such as, European Telecommunications Standards Institute (ETSI) ITS G5. Recently, however, interest in mobile communication technology to support VANET is increasing, since this technology is available off-the-shelf enabling cost-effective implementation. It thus appears as an opportunity to design future mobile communication networks to complement dedicated V2X communications, enabling to offload traffic from congested VANETs to improve dependability. Inter-vehicle relaying, for example, can enhance the transmission link between wireless access points and vehicles. This idea will be further developed to utilizing a combination of ad-hoc V2V networking and cellular communications to mutually improve the broadband experience of participating users.
The first important task is to characterize the mmWave channel in mobile situations. For that purpose we will extend our rotary unit to support reception in the mmWave band. In parallel we will develop transceivers and signal processing methods for efficient data transmission in the mmWave regime.
Network Architectures: HetNets and DASs for Masses of Mobile Users
Network densification is recognized by academia as well as industry as essential to cope with future network capacity demands; only through aggressive spatial reuse of the scarce resource bandwidth is it possible to sustain the exponential growth of data traffic, as currently experienced by network operators worldwide. There basically exist two possibilities for network densification: 1) adding autonomous base stations with low transmit power, that is, small cells; 2) providing existing base stations with remotely controlled radio units that are distributed throughout the cells forming DASs. If such radio access equipment is mounted on vehicles, as considered in several recent proposals, then not only users are moving through the network, but the network itself becomes mobile.
While small cells are currently mostly deployed indoors to improve capacity at user hot-spot locations, more recently outdoor roll-outs have garnered industry interest to complement existing macro cell infrastructure. Such deployments, however, face difficulties in providing user mobility, since small cell sizes imply frequent hand-overs, increasing the signalling load of the network and degrading dependability of the wireless connection due to hand-over failures. As an alternative to autonomous small cells, spatially distributed active antenna systems that are centrally controlled by macro base stations promise better support of high mobility users through cooperative approaches. To support highest mobility in small cell networks and distribtued antenna systems, we will develop dynamic autonomous network coordination strategies and multi-point connectivity concepts that provide additional macro-diversity to enhance the dependability of the wireless link.
Future mobile networks will consist of many types of network access equipment serving users under very different channel conditions (indoor/outdoor, static/highly-mobile). Such situations will require dynamic network coordination on multiple layers of the network, involving macro base stations, small cells and active antenna systems. The investigation of such heterogeneous networks and distributed antenna systems is supported by our mobile network simulators The Vienna LTE-A Simulators.