Abstract — It is known that next generation mobile comunications systems will most likely employ multi-cell signal processing-often referred to as network MIMO- in order to improve spectral efficiency and fairness. Many publications exist that predict strong achievable rate improvements, but usually neglecting various practical issues connected to network MIMO. In this paper, we analyse the impact of a constrained backhaul infrastructure and imperfect channel knowledge on uplink network MIMO from an information theoretical point of view. Especially the latter aspect leads to the fact that the channel conditions for which network MIMO is reasonably beneficial are strongly constrained. We observe different base station cooperation schemes in scenarios of maximal 3 base stations and 3 terminals, provide simulation results, and discuss the practicability of the discussed schemes and the implications of our results. I.

A continuously increasing demand for higher spectral efficiencies in mobile communications will require next generation cellular systems to employ a very dense reuse of spectrum in combination with smart interference mitigation or cancellation schemes. Recent publications have revealed a large potential in terms of spectral efficiency and system fairness of multi-cell cooperative schemes, where e.g. multiple base stations jointly receive or transmit signals connected to multiple terminals. The downside of these schemes is typically the large extent of backhaul link capacity required between base stations. In this paper, we focus on the cellular uplink and investigate the information theoretical limits of multi-cell cooperation under a constrained backhaul. We propose a framework that allows to observe different forms of cooperation between base stations in combination with superposition coding, revealing non-negligible performance improvements compared to our previous results.

Abstract — Next generation mobile communications systems will most likely employ network MIMO in order to mitigate inter-cell interference and improve system fairness and spectral efficiency. Critical issues of such schemes are, however, the large extent of backhaul infrastructure required for the information exchange between cooperating base stations, and the availability of channel knowledge at transmitter and receiver. In this paper, we consider a cooperative downlink transmission under a constrained backhaul, limited channel knowledge at base station and terminal side, and a per-antenna power constraint. We derive inner capacity bounds for different cooperation schemes through uplink/downlink duality and provide numerical results showing the superiority of certain cooperation schemes in terms of rate/backhaul tradeoff for different interference scenarios. I.

It is well-known that the main capacity limitation in cellular communication systems is due to inter-cell interference. Multi-cell signal processing, for example joint transmission from multiple base stations to multiple terminals, is known to strongly improve spectral efficiency and system fairness by actively exploiting interference rather than treating it as noise. Specifically, we consider the scenario where downlink multi-cell beamforming is used to obtain perfect fairness, i.e. to provide all involved terminals with the same achievable rate. The aim is to find the power allocation and beamforming matrix achieving the highest possible common rate under per-base-station power constraints. This power-constrained optimization (PCO) problem has so far been solved by iteratively solving rateor SINR-constrained (SCO) transmit power optimization problems. In this paper, we derive a direct and therefore significantly less complex solution of a PCO problem with per-base-station power constraints. 1

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).

Recent work has shown that multi-cell cooperative signal processing in cellular networks can significantly increase system capacity and fairness. For example, multi-cell joint transmission and joint detection can be performed to combat inter-cell interference, often mentioned in the context of distributed antenna systems. Most publications in this field assume that an infinite amount of information can be exchanged between the cooperating base stations, neglecting the main downside of such systems, namely the need for an additional network backhaul. In recent publications, we have thus proposed an optimization framework and algorithm that applies multi-cell signal processing to only a carefully selected subset of users for cellular systems with a strongly constrained backhaul. In this paper, we consider the cellular downlink, and provide a comprehensive summary and extension of our previous and current work. We compare the performance obtained through centralized or decentralized optimization approaches, or through optimal or sub-optimal calculation of precoding matrices, and identify reasonable performance-complexity trade-offs. It is shown that even low-complexity optimization approaches for cellular systems with a strongly constrained backhaul can yield major performance improvements over conventional systems.

Next generation mobile comunications systems will most likely employ multi-cell cooperative signal processing schemes, often referred to as network MIMO, as these are known to effectively combat inter-cell interference and improve system fairness and spectral efficiency. A major downside of such schemes is, however, the large extent of backhaul infrastructure required for the information exchange between cooperating base stations. In this paper, we observe a cooperative downlink transmission from two base stations to two terminals under different extents of available backhaul capacity. We adapt some well-known concepts from the Gaussian interference channel and observe a variety of possible cooperation schemes. We observe that it is beneficial to use an adaptive cooperation concept, where the base stations exchange either the data to be jointly transmitted itself or partially precoded and compressed signals, depending on the instantaneous channel realization.