5G will have to support a multitude of new applications with a wide variety of requirements, including higher peak and user data rates, reduced latency, enhanced indoor coverage, increased number of devices, and so on. The expected traffic growth in 10 or more years from now can be satisfied by the combined use of more spectrum, higher spectral efficiency, and densification of cells. The focus of the present article is on advanced techniques for higher spectral efficiency and improved coverage for cell edge users. We propose a smart combination of small cells, joint transmission coordinated multipoint (JT CoMP), and massive MIMO to enhance the spectral efficiency with affordable complexity. We review recent achievements in the transition from theoretical to practical concepts and note future research directions. We show in measurements with macro-plus-small-cell scenarios that spectral efficiency can be improved by flexible clustering and efficient user selection, and that adaptive feedback compression is beneficial to reduce the overhead significantly. Moreover, we show in measurements that fast feedback reporting combined with advanced channel prediction are able to mitigate the impairment effects of JT CoMP

In this paper, we investigate an efficient MAC protocol for interference coordination in small cells embedded in a macro-cell mobile network. We perform joint transmission (JT)-coordinated multi-point (CoMP), where cells in the cluster include adjacent macro-cells as suggested by the small-cell terminal. Most gains compared to uncoordinated transmission come from selecting the right users in these macro-cells. For minimizing the feedback overhead, we propose successive channel state information (CSI) requests. Thereby we replace users in the set until all of them experience consistently enhanced performance when using JT-CoMP. Gains can be further increased by MIMO mode switching in each cell and frequency-selective scheduling. We tested our algorithms over coherently measured multi-cell channels in the LTE-Advanced testbed in Berlin where small cells were embedded at several places. Using these algorithms, performance becomes comparable to a popular bound assuming no more intra-cluster interference while feedback overhead and complexity are substantially reduced

In this paper, we provide an overview on backhaul requirements in mobile networks featuring inter-site cooperation. We consider a heterogeneous deployment, where small cells are embedded into macro-cells. For reducing inter-cell interference, we consider intra- and inter-site joint transmission (JT)-coordinated multi-point (CoMP) in fully flexible cooperation clusters. Backhaul traffic for a macro-cell site scales linearly with the number of small cells and multiplicative by introducing cooperation. All features in LTE-Advanced together push the backhaul traffic up to 100 Gbit/s per site. JT-CoMP is sensitive to the feedback delay.We consider an exemplary rural fiber to the home (FTTH) network as mobile backhaul and find that delays between adjacent sites are below 1 ms. Impairment effects due to the delay can be reduced by feedback compression, shorter delays in the air interface and Doppler-delay based channel prediction

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