Michael Grieger

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.

Field trial results for a coordinated multi-point (CoMP) uplink in cellular systems

Theoretical research on coordinated multi-point (CoMP) in the cellular uplink claims large improvements in spectral efficiency and fairness. However, the real-world implementation of CoMP is linked with major challenges such as multi-cell synchronization and multi-cell channel estimation, which have to be addressed to make sure that CoMP finds its way into next generation cellular systems (e.g. LTE-Advanced). In this paper, we provide a proof-of-concept that uplink CoMP concepts do in fact yield significant spectral efficiency gains in an outdoor deployment of two cooperating base stations and two terminals. We further show that the performance gains of CoMP in various interference scenarios corresponds quite well with predictions from theory.

Using “predictor antennas” for long-range prediction of fast fading for moving relays

Channel state information at transmitters is important for advanced transmission schemes. However, feedback and transmission control delays of multiple milliseconds in LTE systems result in severe outdating of this information at vehicular velocities. Channel prediction based on extrapolation of the short-term fading is inadequate in LTE systems at vehicular velocities and high carrier frequencies. We here propose and evaluate a simple scheme which may extend the prediction horizon when used on vehicles: Use an additional antenna, a “predictor antenna”, placed in front of the transmission antennas in the direction of travel. This is of particular interest for use with moving relays: Local access points placed on e.g. buses or trams. A measurement-based study for 20 MHz downlink channels at 2.68 GHz is reported here for both line-of-sight and non-line-of-sight conditions.

Field Trial Results on Different Uplink Coordinated Multi-Point (CoMP) Concepts in Cellular Systems

Coordinated multi-point (CoMP) in the cellular uplink appears to be an effective option to combat inter-cell interference, offering large improvements in spectral efficiency and fairness. However, one major drawback of these schemes is that they typically require a large extent of additional backhaul infrastructure compared to a non-cooperative system. A large amount of theoretical work has been published on this topic, emphasizing the benefit of adapting between different CoMP strategies depending on the channel realization in order to optimize the rate/backhaul trade-off. This paper complements previous publications through field trial results obtained in a representative urban setup. The results yield an insight into practical issues connected to some schemes, while being fairly correlated to theoretical predictions and in fact further emphasizing the gain of adaptive CoMP

Large scale field trial results on different uplink coordinated Multi-Point (CoMP) concepts in an urban environment

Coordinated Multi-Point (CoMP) concepts such as multi-cell joint detection and transmission, promising large improvements in spectral efficiency and fairness, appears to be an effective option to combat inter-cell interference in mobile communications. One major drawback of uplink joint detection is the large additional backhaul required when compared to a non-cooperative system. Theoretical work has demonstrated how distributed interference subtraction can be used as a low backhaul option providing moderate CoMP gain. While a large amount of theoretical work has been carried out on this topic, and previous publications have shown that these schemes work in principle, the scenarios of urban deployment and the extent of capacity gains that can be achieved are still unclear. To this end, we compare potential rate gains through linear and non-linear uplink CoMP schemes for a large scale field trial setup wherein two mobile terminals have been moved through an urban test bed of 12 base stations located at UMTS sites in downtown Dresden.

Large Scale Field Trial Results on Uplink CoMP with Multi Antenna Base Stations

Coordinated Multi-Point (CoMP) appears to be an effective option to combat inter-cell interference in mobile communications. Previous field trials for uplink CoMP have shown that large improvements in spectral efficiency and fairness that are promised by theoretical work can also be achieved in real-world scenarios. However, these results only consider systems with single antenna base stations. We extend this work by presenting field trial results for a system with multi antenna base stations, and we show that this change of the system setup has a strong impact not only on the throughput but also on the relative performance of a cooperative compared to a non-cooperative system. Based on the presented results suggestions for further research and field trials are derived.

On the Performance of Compressed Interference Forwarding for Uplink Base Station Cooperation

The capacity of todays cellular mobile communications systems is mainly limited by inter-cell interference. Multi-cell joint transmission or joint detection schemes are means to overcome this limitation and to actively exploit signal propagation across cell borders rather than treating it as noise. In the cellular uplink, messages of multiple terminals can be detected jointly by cooperating base stations, a concept which has shown to significantly increase the efficiency of spectrum usage. Particularly, the performance of cell-edge users and thus system fairness is improved. A major drawback of multi-cell signal processing is the additional backhaul data rate that is required to exchange information among cooperating base stations. In this paper, we examine the performance of different base station cooperation schemes from an information theoretic point of view. The main contribution is the examination of a novel cooperation scheme that is based on the exchange of compressed transmit signals that are known at a base station after the decoding of a message in a very general system model.

Uplink base station cooperation by iterative distributed interference subtraction

The capacity of todays cellular mobile communications systems is mainly limited by inter-cell interference. Multi-cell joint transmission or joint detection schemes are means to overcome this limitation and to actively exploit signal propagation across cell borders rather than treating it as noise. A major downside to multi-cell signal processing is the additional backhaul rate that is required to exchange information among cooperating base stations. Hence, cooperation schemes are desired that efficiently utilize the available backhaul rate. In the cellular uplink, one promising option to mitigate interference and thus to significantly increase the efficiency of spectrum usage is the exchange of decoded messages among base station, an information that can be used to subtract interference prior to the decoding of further messages. In this work, this distributed interference subtraction scheme is extended by allowing multiple iterative information exchanges among base stations. Possible gains of this strategy in terms of achievable data rates and required backhaul rates are explored from an information theoretical point of view.

Field trial results on uplink joint detection for moving relays

Public transportation vehicles are natural hotspots of wireless communication demand. Potentially, multiple users in the vehicle will compete for scarce spectral resources. At the same time, the direct link of users in the vehicle to the base stations might be rather week due to a high indoor-outdoor penetration loss. A potential solution to these this is the use of multi-antenna relays with antennas outside the vehicle for communication with the base stations and inside the vehicle for communication with the users. In order to increase the throughput on the base station - relay link, especially at cell edges, coordinated signal processing of multiple base stations could be used. In this work, we explore the performance of this approach in an uplink large-scale field trial of a multi-antenna transmitter carried on a measurement bus in an urban cellular environment. For this setup we show achievable data rates using linear and non-linear detection and explore the gain of joint detection in cooperation clusters of up to three base stations.

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.