In order to meet the expectations of future generation mobile networks, the mmwave spectrum has been considered along with new spectrum utilization methods that will ensure wider bandwidths and improved spectrum utilization. Spectrum sharing enables mobile operators to share all available resources, at anytime and anywhere. By doing so there is a significant increase in the inter-operator interference. At the same time, the propagation characteristics of mmwave frequencies and the deployment of directional antenna beamforming can contribute towards lower interference and higher SINR levels. This paper investigates the performance of a multioperator network that uses spectrum sharing and how it compares to the traditional exclusive license model. While previous works on this topic considered more theoretic assumptions and methodologies, this paper looks at this problem from a more realistic perspective. It employs a channel model obtained by ray tracing a real world environment and utilizes detailed antenna array modelling based on measurements of a real patch antenna. Our simulation results for a multi-operator mmwave mobile network show that spectrum sharing outperforms the exclusive license model in terms of capacity for the majority of users. However, users with low signal reception do not benefit from the availability of higher bandwidth links and an exclusive license approach consists of a better solution due to the lower interference levels involved. Given that extended simulations for higher network layers were not implemented, the purpose of this paper is to provide an accurate comparison between the performances of the models under investigation and not to test their actual performance. Spectrum sharing proved to be beneficial for high SINR users. Performance gain for the lowest SINR users is unlikely unless coordination techniques or other interference mitigation mechanisms are employed.

This paper investigates the extent to which spectrum sharing in millimeter-wave (mmWave) networks with multiple cellular operators is a viable alternative to traditional dedicated spectrum allocation. Specifically, we develop a general mathematical framework to characterize the performance gain that can be obtained when spectrum sharing is used, as a function of the underlying beamforming, operator coordination, bandwidth, and infrastructure sharing scenarios. The framework is based on joint beamforming and cell association optimization, with the objective of maximizing the long-term throughput of the users. Our asymptotic and non-asymptotic performance analyses reveal five key points: 1) spectrum sharing with light on-demand intra- and inter-operator coordination is feasible, especially at higher mmWave frequencies (for example, 73 GHz); 2) directional communications at the user equipment substantially alleviate the potential disadvantages of spectrum sharing (such as higher multiuser interference); 3) large numbers of antenna elements can reduce the need for coordination and simplify the implementation of spectrum sharing; 4) while inter-operator coordination can be neglected in the large-antenna regime, intra-operator coordination can still bring gains by balancing the network load; and 5) critical control signals among base stations, operators, and user equipment should be protected from the adverse effects of spectrum sharing, for example by means of exclusive resource allocation. The results of this paper, and their extensions obtained by relaxing some ideal assumptions, can provide important insights for future standardization and spectrum policy.

Millimeter wave (mmWave) links will offer high capacity but are poor at penetrating into or diffracting around solid objects. Thus, we consider a hybrid cellular network with traditional sub-6 GHz macrocells coexisting with denser mmWave small cells, where a mobile user can connect to either opportunistically. We develop a general analytical model to characterize and derive the uplink and downlink cell association in the view of the signal-to-interference-and-noise-ratio and rate coverage probabilities in such a mixed deployment. We offer extensive validation of these analytical results (which rely on several simplifying assumptions) with simulation results. Using the analytical results, different decoupled uplink and downlink cell association strategies are investigated and their superiority is shown compared with the traditional coupled approach. Finally, small cell biasing in mmWave is studied, and we show that unprecedented biasing values are desirable due to the wide bandwidth.

Until the 4thGeneration (4G) cellular 3GPP systems, a user equipment's (UE) cell association has been based on the downlink received power from the strongest base station. Recent work has shown that - with an increasing degree of heterogeneity in emerging 5G systems - such an approach is dramatically suboptimal, advocating for an independent association of the downlink and uplink where the downlink is served by the macro cell and the uplink by the nearest small cell. In this paper, we advance prior art by explicitly considering the cell-load as well as the available backhaul capacity during the association process. We introduce a novel association algorithm and prove its superiority w.r.t. prior art by means of simulations that are based on Vodafone's small cell trial network and employing a high resolution pathloss prediction and realistic user distributions. We also study the effect that different power control settings have on the performance of our algorithm.

Cell association in cellular networks has traditionally been based on the downlink received signal power only, despite the fact that uplink and downlink transmission powers and interference levels differed significantly. This approach was adequate in homogeneous networks with macro base stations all having similar transmission power levels. However, with the growth of heterogeneous networks where there is a big disparity in the transmit power of the different base station types, this approach is highly inefficient. In this paper, we study the notion of Downlink and Uplink Decoupling (DUDe) where the downlink cell association is based on the downlink received power while the uplink is based on the pathloss. We present the motivation and assess the gains of this 5G design approach with simulations that are based on Vodafone's LTE field trial network in a dense urban area, employing a high resolution ray-tracing pathloss prediction and realistic traffic maps based on live network measurements.

The performance of any transmission scheme is coupled with the receive strategy. Herein the behavior of transmissions based on interference alignment scheme is investigated under different receive strategies. Moreover, interference alignment is compared with different state-of-art transmission schemes under the assumption of intrabase station and inter-base station coordination. The performance of the above-mentioned techniques is assessed with a fully 3GPP compliant downlink LTE simulator, which gives a very realistic picture of the behavior in real systems. The results have shown that interference alignment is underperforming with respect to the baseline using base station coordination, when a realistic number of base stations is considered. Moreover, the gains of interference alignment are also highly dependent on the receiver type, whereas the baselines show relatively low differences with different receivers.

In cellular networks coordination among base stations (BSs) has been recognized as an important solution to handle inter-cell interference and increase spectral efficiency. In frequency division duplex systems one of the main issue that sensibly degrades the performance of coordinated multipoint (CoMP) transmission techniques is the imperfect channel state information (CSI) due to the limited bandwidth available for the feedback transmission. In this paper we focus on a CoMP scenario with data and CSI sharing among the BSs and we consider a feedback transmission scheme where each user equipment (UE) quantizes the different channels by using codebooks designed for a single-cell scenario. Due to the different propagation characteristics of the channels between a UE and each BS and by considering a constraint on the number of available feedback bits, we propose two practical algorithms depending on the large-scale fading to a) select the subset of BSs from whom the UE prefers to be served and b) optimize the number of feedback bits allocated to each channel. The developed techniques allow a UE to send more feedback bits to the BSs with a stronger signal and numerical results show the merits of the proposed approach.

Motivated by the difficulty in mapping theoretic Coordinated Multi-Point (CoMP) studies into real-world solutions, herein we propose a novel architectural solution for CoMP-based systems. The basic idea is to dynamically allocate virtual processing resources in a base band unit pool, to sets of transmission points, these processing resources being used for jointly transmitting to a given set of users. This proposal corresponds in dropping the cell as the elementary processing unit, and tailoring the system architecture to optimize the service for each user. Such an architecture allows a much easier implementation of advanced CoMP solutions.

In the evolution of wireless systems to the next generation, it is becoming increasingly important to offer substantial improvements, not only in terms of peak and average cell rates but also in terms of outage performance. To this end, interference management by means of coordination between different base stations and coverage/capacity improvements by means of cooperation with relays are considered potential candidate technologies. The introduction of multiuser MIMO processing principles and cross-layer optimized resource allocation may offer promising performance gains for coordination and cooperation but their feasibility depends on overhead signaling constraints and system deployment assumptions. In this paper, we present two realistic approaches for coordination and cooperation, analyze their performance merits and discuss their complexity requirements.

In this paper we propose a two-step distributed scheduling algorithm for relay-assisted cellular networks where a given user can be either served by the base station or by a relay, in a opportunistic way. Such a distributed approach allows a reduced feedback signaling with respect to the centralized case, especially when a simple scalar feedback is not sufficient for estimating the channel quality. As a result of the reduced feedback signaling requirements the system becomes more scalable, as new relays can be deployed where required without need of a careful network planning. We study the effectiveness of the proposal by means of a multicell simulator.

Herein, we present a novel approach for decode-forward relay transmissions using multiple antennas. In this approach, multiple independent spatial data streams are sent to the destination directly and/or through different relays simultaneously, where the relays are selected in an opportunistic way. We show the efficiency of the proposed approach through simulations.

In this paper, the design of a highly efficient and flexibly deployable wireless backhaul is addressed as a promising alternative to the typical wired solutions. To this end, a novel approach for relay transmissions with multiple antennas is proposed in the framework of wireless meshed backhaul system design optimization. The link and system level performance are assessed and different mesh topologies and relaying protocols are investigated. The optimality of the approach relies on the reconfigurable /adaptive use of the degrees of freedom offered by the combination of multiple antennas and relays and is promising for both wireless backhaul and access applications.

We consider a multiuser MIMO downlink system with multiple antenna mobile terminals where channel state information at the base station is provided through limited uplink feedback. In a multi-cell environment we compare two transceiver architectures based on zero forcing and unitary beamforming respectively. For both schemes we show how the additional degrees of freedom provided by multiple receive antennas can be exploited to improve system performance, by a proper design of the receive combiner. We propose channel quantization solutions that adapt to the transceiver architectures and exploit MIMO channel statistic. Numerical simulations validate the effectiveness of the proposals in a multi-cell environment where long-term proportional fairness is guaranteed among the users in the network.

We consider a MIMO broadcast channel where both the transmitter and receivers are equipped with multiple antennas. Channel state information at the transmitter (CSIT) is obtained through limited (i.e., finite-bandwidth) feedback from the receivers that index a set of precoding vectors contained in a predefined codebook. We propose a novel transceiver architecture based on zero-forcing beamforming and linear receiver combining. The receiver combining and quantization for CSIT feedback are jointly designed in order to maximize the expected SINR for each user. We provide an analytic characterization of the achievable throughput in the case of many users and show how additional receive antennas or higher multiuser diversity can reduce the required feedback rate to achieve a target throughput.We also propose a design methodology for generating codebooks tailored for arbitrary spatial correlation statistics. The resulting codebooks have a tree structure that can be utilized in time-correlated MIMO channels to significantly reduce feedback overhead. Simulation results show the effectiveness of the overall transceiver design strategy and codebook design methodology compared to prior techniques in a variety of correlation environments.

In this paper we derive two iterative joint precoding and power allocation methods for downlink MC-CDMA systems with multiple transmit antennas at the base station. The first method optimally solves the so called assigned target SINRs problem, i.e. determines the precoding coefficients and the minimum sum-power allocation under a constraint on the SINR of each user. The second method instead solves the maximum sum-rate problem, i.e. maximizes the system throughput under a constraint on the overall transmit power. The main result of this paper is that the proposed joint precoding and power control solutions provide an advantage with respect to suboptimal solutions for a fully loaded system, i.e. when the number of users is equal to the spreading factor times the number of transmit antennas. For lower loads, performance of the various schemes becomes closer.

We consider the MIMO broadcast channel (MIMO-BC) where an array equipped with M antennas transmits distinct information to K users, each equipped with N antennas. We propose a linear precoding technique, called multiuser eigenmode transmission (MET), based on the block diagonalization precoding technique. MET addresses the shortcomings of previous ZF-based beamformers by transmitting to each user on one or more eigenmodes chosen using a greedy algorithm. We consider both the typical sum-power constraint (SPC), and a per-antenna power constraint (PAPC) motivated by array architectures where antennas are powered by separate amplifiers and are either co-located or spatially separated. Numerical results show that the proposed MET technique outperforms previous linear techniques with both SPC and PAPC. Asymptotically as the number of user K increases without bound, we show that block diagonalization with receive antenna selection under PAPC and SPC are asymptotically optimal.

We investigate the downlink throughput of cellular systems where groups of M antennas - either co-located or spatially distributed - transmit to a subset of a total population of K > M users in a coherent, coordinated fashion in order to mitigate intercell interference. We consider two types of coordination: the capacity-achieving technique based on dirty paper coding (DPC), and a simpler technique based on zero-forcing (ZF) beamforming with per-antenna power constraints. During a given frame, a scheduler chooses the subset of the K users in order to maximize the weighted sum rate, where the weights are based on the proportional-fair scheduling algorithm. We consider the weighted average sum throughput among K users per cell in a multi-cell network where coordination is limited to a neighborhood of M antennas. Consequently, the performance of both systems is limited by interference from antennas that are outside of the M coordinated antennas. Compared to a 12-sector baseline which uses the same number of antennas per cell site, the throughput of ZF and DPC achieve respective gains of 1.5 and 1.75.

We consider the problem of designing the vector quantizer for a multiuser eigenmode transmission applied to a MIMO broadcast channel with limited feedback. We propose a hierarchical vector quantization method based on the idea that the CDI can be refined by permitting multiple feedback steps between receivers and transmitter. At each new quantization level a new codebook is built based on the codebook and on the best codeword at the previous level. We firstly give a general definition of a hierarchical quantizer. Then we build a hierarchical quantizer based on the Fourier codebook. Finally, we propose an algorithm for implementing the hierarchical quantization principle. The proposed algorithm improves the channel quantization quality if the channel vector remains almost constant during different feedback steps, whereas allows backward steps to lower levels, in order to track channel variations. Simulations comparison with respect to the Fourier codebook and to the random vector quantization technique, confirm the effectiveness of the proposed approach.

Herein, we present a novel approach for high data rate transmission in a wireless relay network. The system include a source, a destination and multiple relays. All nodes are equipped with multiple antennas. A half-duplex transmission protocol and a decode-forward transmission mode are deployed in the system. Our approach splits the data at the source into multiple independent data streams, and send them to the destination via some selected relays at the same time. The relays are selected using an opportunistic approach based on spatially decomposing the channels and accessing the best modes. Simulation results show the benefits of our technique, particularly with correlated channels.

We propose a user selection algorithm and the supporting feedback scheme, for MIMO Gaussian broadcast channels with arbitrary number of users, transmit beamforming and limited channel state information (CSI) available at the transmitter. The method relies on estimation of the user signal-to-interference-plus-noise ratios (SINR's) at the transmitter, by means of an appropriate finite-rate channel report from the mobiles. The basic principle is that users are added successively only if they are beneficial to the system in terms of increased throughput. Numerical results show improved performance compared to other finite-rate feedback schemes.

In this paper, as an alternative to post-detection techniques, we derive two iterative joint linear precoding and power allocation methods for downlink MC-CDMA systems with multiple transmit antennas at the base station. The first method optimally solves the so called assigned target SINRs problem, i.e. determines the precoding coefficients and the minimum sum-power allocation under a constraint on the SINR of each user. The second method instead solves the maximum sum-rate problem, i.e. maximizes the system throughput under a constraint on the overall transmit power. The main result of this paper is that the proposed joint precoding and power control solutions provide an advantage with respect to suboptimal solutions for a fully loaded system, i.e. when the number of users is equal to the spreading factor times the number of transmit antennas. For lower loads, performance of the various schemes becomes closer.

This paper is focused on low-complexity joint-detection (JD) algorithms, where the computational complexity is reduced using banks of discrete Fourier transformations. Mainly, we have developed a fast hybrid zero-forcing block decision-feedback equalizer (H-ZF-BDFE) combining the Fourier-based JD scheme (IEEE J. Sel. Areas Commun., vol. 19, p. 1461, 2001) with the fast decision-feedback structure (F-ZF-BDFE) IEEE J. Sel. Areas Commun., vol. 19, p. 245, 2001. In a Rayleigh frequency-selective fading channel, the new structure yields an improvement in E/sub b//N/sub 0/ of almost 2 dB against the fast block linear equalizer (BLE) (IEEE J. Sel. Areas Commun., vol. 19, p. 1461, 2001) at a bit error rate (BER)=10/sup -3/. Moreover, it has the same performance as the F-ZF-BDFE, but the computational complexity for the signal processing is reduced by almost 50%.