Fingerprinting-Based Positioning in Distributed Massive MIMO Systems

Location awareness in wireless networks may enable many applications such as emergency services, autonomous driving and geographic routing. Although there are many available positioning techniques, none of them is adapted to work with massive multiple-in-multiple-out (MIMO) systems, which represent a leading 5G technology candidate. In this paper, we discuss possible solutions for positioning of mobile stations using a vector of signals at the base station, equipped with many antennas distributed over deployment area. Our main proposal is to use fingerprinting techniques based on a vector of received signal strengths. This kind of methods are able to work in highly-cluttered multipath environments, and require just one base station, in contrast to standard range-based and angle-based techniques. We also provide a solution for fingerprinting-based positioning based on Gaussian process regression, and discuss main applications and challenges.Comment: Proc. of IEEE 82nd Vehicular Technology Conference (VTC2015-Fall

Group Sparse Precoding for Cloud-RAN with Multiple User Antennas

Cloud radio access network (C-RAN) has become a promising network architecture to support the massive data traffic in the next generation cellular networks. In a C-RAN, a massive number of low-cost remote antenna ports (RAPs) are connected to a single baseband unit (BBU) pool via high-speed low-latency fronthaul links, which enables efficient resource allocation and interference management. As the RAPs are geographically distributed, the group sparse beamforming schemes attracts extensive studies, where a subset of RAPs is assigned to be active and a high spectral efficiency can be achieved. However, most studies assumes that each user is equipped with a single antenna. How to design the group sparse precoder for the multiple antenna users remains little understood, as it requires the joint optimization of the mutual coupling transmit and receive beamformers. This paper formulates an optimal joint RAP selection and precoding design problem in a C-RAN with multiple antennas at each user. Specifically, we assume a fixed transmit power constraint for each RAP, and investigate the optimal tradeoff between the sum rate and the number of active RAPs. Motivated by the compressive sensing theory, this paper formulates the group sparse precoding problem by inducing the $\ell_0$-norm as a penalty and then uses the reweighted $\ell_1$ heuristic to find a solution. By adopting the idea of block diagonalization precoding, the problem can be formulated as a convex optimization, and an efficient algorithm is proposed based on its Lagrangian dual. Simulation results verify that our proposed algorithm can achieve almost the same sum rate as that obtained from exhaustive search

Cell-Free Massive MIMO versus Small Cells

A Cell-Free Massive MIMO (multiple-input multiple-output) system comprises a very large number of distributed access points (APs)which simultaneously serve a much smaller number of users over the same time/frequency resources based on directly measured channel characteristics. The APs and users have only one antenna each. The APs acquire channel state information through time-division duplex operation and the reception of uplink pilot signals transmitted by the users. The APs perform multiplexing/de-multiplexing through conjugate beamforming on the downlink and matched filtering on the uplink. Closed-form expressions for individual user uplink and downlink throughputs lead to max-min power control algorithms. Max-min power control ensures uniformly good service throughout the area of coverage. A pilot assignment algorithm helps to mitigate the effects of pilot contamination, but power control is far more important in that regard. Cell-Free Massive MIMO has considerably improved performance with respect to a conventional small-cell scheme, whereby each user is served by a dedicated AP, in terms of both 95%-likely per-user throughput and immunity to shadow fading spatial correlation. Under uncorrelated shadow fading conditions, the cell-free scheme provides nearly 5-fold improvement in 95%-likely per-user throughput over the small-cell scheme, and 10-fold improvement when shadow fading is correlated.Comment: EEE Transactions on Wireless Communications, accepted for publicatio

Sigma-Delta-over-Fiber for High-Speed Wireless Communication Systems

Future mobile communication networks aim to increase the communicationspeed, provide better reliability and improve the coverage. It needs to achieveall of these enhancements, while the number of users are increasing drastically.As a result, new base-station (BS) architectures where the signal processingis centralized and wireless access is provided through multiple, carefullycoordinated remote radio units are needed.The sigma-delta-over-fiber (SDoF) is a communication technique that canaddress both requirements and enable very low-complexity, phase coherentremote radio transmission, while transmitting wide-band communication signalswith high quality. This thesis investigates the potential and limitations of SDoFcommunication links as an enabler of future mobile networks.In the first part of the thesis, a multiple-input-multiple-output (MIMO)communication testbed with physically separated antenna elements, distributed-MIMO, is formed by multiple SDoF links. It is shown that the digital upconversion,performed with a shared local-oscillator/clock at the central unit,provides excellent phase coherency between the physically distributed antennaelements. Moreover, the same approach decreases the complexity and thepackage size of the antenna units\ua0significantly by moving the complexity of theBSs to a central unit. The implemented testbed is evaluated through variouscommunication experiments. The results show that distributing the antennaunits of a MIMO communication system can increase the coverage and signalquality.In the second part of the thesis, an ultra-high-speed SDoF (UHS-SDoF) linkis realized by using the state-of-the-art vertical-cavity surface-emitting-lasers(VCSEL). The effects of VCSEL characteristics on such links in terms of signalquality, energy efficiency and potential lifespan is investigated. Furthermore, thepotential and limitations of UHS-SDoF are evaluated with signals having variousparameters. The results show that, low-cost, reliable, energy efficient, highsignal quality SDoF links can be formed by using emerging VCSELs. Therefore,UHS-SDoF is a very promising technique for beyond 10 GHz communicationsystems.In conclusion, this thesis has showed that low-complexity, low-cost, andenergy efficient ultra-high speed communication links and distributed MIMOsystems can be implemented by employing SDoF