Patrick Marsch

Uplink CoMP under a Constrained Backhaul and Imperfect Channel Knowledge

Coordinated Multi-Point (CoMP) is known to be a key technology for next generation mobile communications systems, as it allows to overcome the burden of inter-cell interference. Especially in the uplink, it is likely that interference exploitation schemes will be used in the near future, as they can be used with legacy terminals and be based on operator-proprietary signal processing concepts, hence requiring no or little changes in standardization. Major drawbacks, however, are the extent of additional backhaul infrastructure needed, and the sensitivity to imperfect channel knowledge. This paper jointly addresses both issues in a new framework incorporating a multitude of proposed theoretical uplink CoMP concepts, which are then put into perspective with practical CoMP algorithms. This comprehensive analysis provides new insight into the potential value of different uplink CoMP concepts in next generation wireless communications systems, and reveals the subset of schemes that are most likely to be used in practice.

Static Clustering for Cooperative Multi-Point (CoMP) in Mobile Communications

Coordinated multi-point (CoMP) has been selected as a key technology feature of LTE-Advanced, as it enables the exploitation of inter-cell interference in order to significantly increase spectral efficiency, especially at the cell-edge. While first field trials on CoMP schemes have delivered the proof-of-concept and shown that a moderate extent of theoretically predicated CoMP gains can indeed be achieved in practical systems, the implementation of these schemes has revealed many practical challenges. One central question is, for example, how small cooperation clusters can be extracted from large cellular systems, such that major portions of potential CoMP gains can be obtained at minimum signaling overhead. This paper deals with static clustering concepts, and shows that both in a hexagonal cell layout and under a realistic deployment and signal propagation scenario, static clustering concepts can perform close to optimal UE-specific clustering, while being easy to use and requiring negligible signaling overhead.

Future Mobile Communication Networks: Challenges in the Design and Operation

This article highlights particular challenges inherent in the design and operation of future mobile communications systems This paper also discusses the various degrees of freedom involved, and points out which design paradigms appear most promising and which major fields of future research remain.

Vision for Beyond 4G broadband radio systems

Mobile communication systems have evolved over the past decades and each new generation brought new experience to the users enabled by technology innovations, while keeping some well established principles from previous generations. This trend continued up to LTE (Long Term Evolution) Advanced, the predominant 4th generation system which has just been standardized in 3GPP and is being rolled out soon. How will this trend continue to future systems which will be deployed in some 10 years from now which will be advanced enough to be called “Beyond 4G” (B4G)? This article presents how such B4G systems will look like and some key technologies they will rely on including versatile numerology, massive virtual MIMO from many base stations, both centralized and distributed architectures using fiber optics as backbone, advanced interference mitigation, cognitive self organization, and wideband RF radios.

A Decentralized Optimization Approach to Backhaul-Constrained Distributed Antenna Systems

It has been shown that multi-cell co-operations in cellular networks, enabling distributed antenna systems and joint signal processing across cell boundaries, can significantly increase system capacity and fairness. In recent work on this topic, we have proposed an optimization framework and algorithm that applies joint detection in the uplink or joint transmission in the downlink, respectively, to only a selected subset of users. This already yields a large extent of capacity and fairness improvements, while requiring only comparatively small backhaul capacity between co-operating base stations, which is usually the main issue connected to distributed antenna systems. In the following paper, we will introduce a novel concept of partitioning a cellular network and its resources into small subsystems, within which the optimization algorithm can be applied in a decentralized way. While each subsystem requires only very limited system knowledge, and joint detection and transmission is constrained to within subsystems, the performance improvements are still promising and approach those of a centralized optimization scheme in scenarios of strongly constrained backhaul.

On the performance gain of flexible UL/DL TDD with centralized and decentralized resource allocation in dense 5G deployments

Ultra dense small cell deployments and a very large number of applications are expected to be the essential aspects of the newly emerging 5th generation (5G) wireless communication system. To match the diverse quality of service requirements imposed by a variety of applications, dynamic TDD is proposed as a solution by enabling flexible utilization of the spectrum for uplink and downlink of each cell. In this paper, the system performance of flexible (dynamic) TDD is compared to a fixed portioning of resources for uplink and downlink. Further, the degree of centralization for resource management is investigated in the context of dynamic TDD, because multi-cell scheduling will be important for the design of 5G ultra-dense network architecture. The results show that dynamic TDD is indeed a very promising option for 5G networks, and substantially decreases packet outage delays. We find that the performance gap between centralized and decentralized scheduling is small in case of planned deployments. However, centralized scheduling may be beneficial in certain dynamic TDD deployment scenarios with a very asymmetric access point distribution.

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.

Quasi-Orthogonal STBC Using Stretched Constellations for Low Detection Complexity

Space-time block codes (STBC) from orthogonal designs have been proposed by Alamouti and Tarokh-Jafarkhani-Calderbank, achieving full diversity in the order of the number of transmit antennas. However, orthogonal STBC always result in a symbol rate less than one, if complex symbols are transmitted from more than 2 antennas. Jafarkhani, Tirkkonen-Boariu-Hottinen and Papadias-Foschini have suggested to use quasi-orthogonal STBC (QO-STBC), and it has been shown by various authors that these schemes achieve transmission rate 1 and full diversity, if selected symbols are chosen from suitably rotated constellations. The schemes can, however, lead to a high detection complexity, which has been recently addressed through co-ordinate interleaved orthogonal designs (CIOD) and minimum detection complexity (MDC) QO-STBC, though leading to a slight degradation in performance. In this paper, we present a novel QO-STBC scheme that uses stretched, i.e. non-linearly preprocessed symbol constellations, achieving full diversity and a performance comparable to that of CIOD and MDC, while further reducing detection complexity.

Rate Region of the Multi-Cell Multiple Access Channel under Backhaul and Latency Constraints

Recently, multi-cell joint transmission and joint detection schemes have been identified as promising features of next generation mobile communications systems, as they enable to actively exploit inter-cell interference rather than treating it as noise. Both for uplink and downlink, concrete algorithms have been proposed, and strong increases in spectral efficiency and system fairness have been predicted. Besides posing strong requirements towards the time and frequency synchronization of communicating entities, one essential problem connected to multi-cell cooperative signal processing is the large extent of backhaul required between base stations and an increased detection latency. In this paper, we focus on the latter two aspects in the context of a cellular uplink. We consider different schemes of multi-cell cooperative detection and introduce a framework that allows to derive general backhaul- or latency-constrained rate regions for multi-cell multiple access channels (MAC).

Field trial evaluation of compression algorithms for distributed antenna systems

Coordinated multi-point (CoMP) in the cellular uplink, offering large improvements in spectral efficiency and fairness, appears to be an effective option to combat inter-cell interference. Current approaches range from coordinated scheduling to coherent joint detection. A major drawback of coherent joint detection is the large extent of additional backhaul infrastructure required for the exchange of received signals among base stations. Theoretical research on this topic emphasizes the benefits of source coding as a method to reduce the backhaul that is required. This paper complements previous publications through field trial results obtained in a representative urban setup. Different scalar and vector compression algorithms are compared in terms of their complexity as well as the average distortion of the compressed signal. System performance, evaluated in terms of the SINR of the equalized transmit signals, was determined for measurement data to investigate performance of compression algorithms under real-world conditions.