Jörg Holfeld

The FUTON prototype: Broadband communication through coordinated multi-point using a novel integrated optical/wireless architecture

The growing relevance of wireless communications has been driving research and development to enable cost-efficient support of very high data rates to be delivered to a large number of users. The high bandwidth targets for future networks are well reflected in the ITUs call for IMT-Advanced. As interference poses the main limitation in todays networks, cooperative signal processing (often referred to as coordinated multi-point, CoMP) is seen as a key enabler to achieve these targets. However, cooperation among base stations in traditional cellular infrastructures is problematic because interconnections (referred to as backhaul) are often very limited in capacity and investigations on more flexible and cost-efficient future architectures are in progress. A promising concept was proposed in the European research project FUTON. It is based on the use of radio-over-fiber (RoF) technology to create a distributed antenna system (DAS) that is capable of supporting the high requirements of future wireless networks. In the scope of the project, a prototype system was built as a proof of concept. In this contribution, we present measurement results from the prototype system that demonstrate the potential of the FUTON architecture.

Joint Detection and CFO Compensation in Asynchronous Multi-User MIMO OFDM Systems

It is well known that carrier frequency offsets caused by inaccuracies of local oscillators between transmitter and receiver stations destroy the orthogonality among OFDM subcarriers and induce additional intercarrier interference. In conjunction with MIMO transmission where different users interfere with each other, this effect strongly degrades the signal detection performance if it is not compensated beforehand. In the uplink of multi-user systems different CFOs between all mobile terminals occur whereas its correction is more difficult. In this paper we consider the uplink of a fully asynchronous system with different transmitter and receiver CFOs which appears in distributed MIMO systems with separate users and base stations. We describe a combined scheme for joint CFO correction and spatial layer detection in frequency domain. The detection performance is shown for some selected equaliser concepts. In this way we present interference cancellation algorithms with low and scalable complexity. Finally, numerical simulation results are provided to compare the detection performance.

Field trial results for LTE-advanced concepts

Coordinated Multi-Point and relaying are two likely candidates for the upcoming LTE-Advanced standard, as both are able to satisfy the ever increasing demands for ubiquitous services with higher data rates. In this paper, we present field trial results for both techniques and discuss the most challenging problems during the implementation process in the EASY-C testbed in downtown Dresden.

Field trial evaluation of UE specific antenna downtilt in an LTE downlink

The benefits of horizontal beamforming in cellular networks are well unterstood, and the technology is already used in commercial products. Recently, vertical beamforming (basically a user specific downtilt (DT)) receives a lot of attention as well. However, available channel models do not allow for an accurate simulation of this transmission scheme. This publication investigates the impact of antenna DT in a typical urban area using field trials. Two models are presented and compared with measurement data in order to study their value and limitations for the evaluation of vertical beamforming, which is an important basis for planning and deploying of such schemes in order to increase cellular downlink (DL) throughput.

Order-recursive precoding for cooperative multi-point transmission

This paper describes an order-recursive algorithm to calculate the Wiener transmit filter for coherent coordinated multi-point transmissions between several decentralized base stations to several decentralized mobile terminals in cellular radio networks. State of the art implementations do not offer the flexibility to handle multiple transmission setups as required for multi-user precoding in conjunction with the proposed reduced complexity. A discussion of all signal processing steps shows how algebraic observations are mapped to physical parameters and how they reflect the spatial eigenmodes. These observations can be integrated into a spatial scheduling process to extend the precoding coefficients recursively with additional spatial streams. Finally, experimental resultqs with a hardware platform for cellular field trials are presented that compare the proposed multiuser interference cancelation technique with a non-cooperative transmission.

Field trial results for CoMP downlink transmissions in cellular systems

This paper presents measurement results for coordinated multi-point downlink transmissions as Long Term Evolution - Advanced concept from a cellular test bed in Dresden, Germany. At first, a description of the real-time prototyping system and its signal processing introduces the major modifications from the existing 3GPP-LTE standard. Then, the cellular propagation and measurement environment of the test bed is characterized. Various spatial setups are compared in conjunction with cooperative and non-cooperative base stations as well as cell-center and cell-edge scenarios. Finally, results concerning the geometry factor, time difference of arrivals and spectral efficiencies complete this field trial.