Multi-cell Systems Research Articles

One-shot cooperative beamforming for downlink multicell systems

We propose a one-shot (non-iterative) cooperative beamforming scheme for downlink multicell systems. Unlike previous non-iterative beamforming schemes, the proposed cooperative beamforming strives to balance maximizing the desired signal power while minimizing the generated interference power to neighbors by maximizing the network-wide average sum rate. Based on the average sum rate analysis, we derive what we term a “global selfishness” that steers the egoistic-altruistic balance of the network to maximize average sum rate. The global selfishness enables an autonomous decision on the cooperative beamforming vector in each cell. The main advantage of our approach is that cooperative beamforming solutions are analytically derived not only for an ideal two-cell network scenario but also for a practical three-sectored cellular network scenario. The simulation results verify that the proposed one-shot cooperative beamforming outperforms other conventional non-iterative schemes especially in interference-limited regions, which implies that it is very effective for performance improvement of edge users.

Threshold Based Limited Feedback Bit Partitioning for Interference Channels in Multicell Systems

Threshold Based Limited Feedback Bit Partitioning for Interference Channels in Multicell Systems

Uplink Pilot Design for Multi-Cell Massive MIMO Networks

In massive multiple-input multiple-output (MIMO) systems, the effect of inter-user interference and noise can be suppressed through simple signal processing techniques. However, pilot contamination, which results from the use of a correlated pilot, causes performance bottleneck for massive MIMO networks. In this letter, a pilot design algorithm based on alternating minimization is developed to alleviate the pilot contamination in multi-cell massive MIMO systems. Contrary to the existing pilot reuse protocols that restrict the pilot length to be an integer multiple of the number of users, the proposed method designs a pilot with an arbitrary length. Hence, pilot sequence can be designed more flexibly to maximize spectral efficiency.

Decentralized transmit beamforming scheme with zero forcing and user selection for multicell system dynamical systems

Conventional distributed beamforming schemes in multicell system often ignore two practical conditions, (1) the coexistence of intra-cell interference (InCI) and inter-cell interference (ICI), (2) the number of users is larger than the number of base station (BS) antennas. In this paper, we consider these two conditions and analyze the decentralized beamforming of the multicell system from the viewpoint of zero forcing (ZF). A novel user selection method based on ZF algorithm is proposed to divide the users into two categories, i.e. one of the categories is eligible to participate in ZF algorithm and the other is not. The proposed criterion of partitioning users is based on the degree of parallelity between local channels. In addition, each BS utilizes only the local channel state informations (CSIs) to derive its beamforming vector. The simulation results demonstrate that our proposed algorithm can achieve higher rates and are more robust against InCI and ICI than the traditional ones under the different number of transmitting antennas and users.

Distributed Linear Precoding and User Selection in Coordinated Multicell Systems

In this paper, we tackle the problem of semidistributed user selection with distributed linear precoding for sum-rate maximization in multiuser multicell systems. A set of adjacent base stations (BSs) forms a cluster to perform coordinated transmission to cell-edge users, and coordination is carried out through a central processing unit (CU). However, the message exchange between BSs and the CU is limited to scheduling control signaling, and no user data or channel state information (CSI) exchange is allowed. In the considered multicell coordinated approach, each BS has its own set of cell-edge users and transmits only to one intended user while interference to nonintended users at other BSs is suppressed by signal steering (precoding). We use two distributed linear precoding schemes, namely, distributed zero forcing and distributed virtual signal-to-interference-plus-noise ratio (DVSINR). Considering multiple users per cell and the backhaul limitations, the BSs rely on local CSI to solve the user selection problem. First, we investigate how the signal-to-noise ratio regime and the number of antennas at the BSs impact the effective channel gain (the magnitude of the channels after precoding) and its relationship with multiuser diversity. Considering that user selection must be based on the type of implemented precoding, we develop metrics of compatibility (estimations of the effective channel gains) that can be computed from local CSI at each BS and reported to the CU for scheduling decisions. Based on such metrics, we design user selection algorithms that can find a set of users that potentially maximizes the sum rate. Numerical results show the effectiveness of the proposed metrics and algorithms for different configurations of users and antennas at the BSs.

Interference Mitigation Through Inter-Cell Interference Coordination Using Virtual PRB Allocation in 4G Networks

This paper proposes a distributed and coordinated resource allocation algorithm in multi-cell MIMO-OFDMA systems. Firstly, each cell classifies mobile users into cell-edge and cell-center users bas...

Constraints on plate tectonics initiation from scaling laws for single-cell convection

The Earth is the only planet known to have plate tectonics, while other planets are covered with a stagnant lid. On the Earth, the initiation of subduction, which is thought to be the fundamental process for plate tectonics initiation, is caused not only by the negative buoyancy of the lithosphere but also by the forces from plate motions. However, for planets which do not have plate tectonics, the very first episode of lithospheric failure has to be caused by forces other than plate motions. Sublithospheric convection has been proposed as a possible mechanism that provides lithospheric instability through inducing stresses in the lithosphere, and lithospheric failure can occur when the yield stress is below a critical value. We test the applicability of scaling laws for the critical yield stress obtained in single-cell convection simulations to strongly time-dependent multi-cell systems. We show that with an appropriate choice of characteristic aspect ratio for the convective system, the scaling laws from single-cell simulations can be used to evaluate the conditions on the terrestrial planets in the inner Solar System for plate tectonics to exist. In agreement with previous studies, the estimated values for critical yield stress and coefficient of friction are much lower than the expected values for the Earth’s lithosphere.

Pilot distribution optimization in multi-cellular large scale MIMO systems

Salient characteristics of a wireless communication system deploying a great number of antennas at the base station (BS), namely a massive MIMO system, are investigated in this work. The asymptotic performance of the linear zero-forcing precoding scheme is found, both in terms of signal-to-interference-plus-noise ratio (SINR) and bit-error rate (BER), and shown to be equivalent to the matched-filter beamforming performance. Furthermore, analysis of the massive MIMO system downlink is carried out from the viewpoint of uncoded BER performance, including some realistic adverse effects, such as interference from neighbouring cells, channel estimation errors due to background thermal noise, and pilot contamination. The latter has been shown to be the only impairment that remains in the MIMO multicell system with infinite number of BS antennas. For such scenario, we derive expressions for the asymptotic BER, i.e. in the limit of infinite number of antennas at BS. A quite simple and efficient method for optimizing the massive MIMO system performance under different optimization metrics is proposed, which consists of simply distributing the pilot sequences among the users of the cell in an efficient manner. As a result, a user rate gain of six times regarding the random strategy has been achieved for the downlink with unitary reuse factor, while the user rate increases twice for reuse factor of three. These benefits are achieved by only knowing the powers and the long-term fading coefficients of users in adjacent cells, for each pilot sequence.

Hierarchy precoder design for multi‐cell multiuser multiple‐input–multiple‐output wireless networks with interference alignment

A hierarchy precoding approach is proposed in this study for multi-cell multiuser systems with any number of base stations and that of users, which is suitable for any number of data streams. The key feature of this approach is aligning the inter-user interferences within the same cell to the room spanned by the inter-cell interferences, by which both the inter-cell and inter-user interferences are cancelled simultaneously. Then, the inter-stream interference for each user can be easily tackled. It is found that the interference alignment-based hierarchy precoder achieves to the full freedom of degree. With interference-free transmissions achieved by the proposed precoder, the transmit power is optimised in an analytical expression by maximising the sum rate and minimising the sum weighted mean square error. Extensive simulations demonstrate the effectiveness of the proposed method.

Decentralized Linear Transceiver Design and Signaling Strategies for Sum Power Minimization in Multi-Cell MIMO Systems

This paper considers linear downlink transceiver design for the sum power minimization problem with per-user rate constraints in a multi-cell multi-user MIMO system. This non-convex problem is divided into transmit and receive beamforming optimization steps which are iteratively repeated such that the sum power converges. The transmit beamformers are solved using a successive convex approximation method (SCA), whereas the receive beamformers are computed via the linear minimum mean square error (MMSE) criterion. In addition to centralized design, we propose two decentralized algorithms where the transmit and receive beamforming designs are facilitated by a combination of pilot and backhaul signaling. In the first algorithm, sum power is minimized by computing the transmit beamformers at each base station (BS) and the receive beamformers at each user via iterative over-the-air optimization process. BS side processing requires local effective channel state information (CSI) acquired from precoded uplink pilots. In the second algorithm, the transmit and receive beamformers are iteratively optimized at each BS requiring only local CSI obtained from antenna-specific uplink pilots. In both algorithms, transmit beamforming optimization is decoupled among the BSs via primal decomposition method, which requires scalar backhaul information exchange for decentralized processing. The performance of the proposed algorithms is evaluated via numerical examples under different system settings in static and time-correlated channel conditions.

Downlink performance of cell edge using cooperative BS for multicell cellular network

We consider the downlink of a multicell system comprised of base stations (BSs) and user terminals equipped with multiple antennas respectively on the condition that arbitrary BS cooperation and distance dependent propagation path loss are assumed. In this paper, we consider homogeneous networks for the rectangular coordinates and show the cell edge performance of cellular networks based on distance from their cell center, i.e., BS. We focus on the downlink capacity of edge users in the cellular networks and show that BS cooperation can improve the spectral efficiency. The BSs cooperate for their transmission to the cell edge users in order to improve their signal-to-interference-plus-noise ratio (SINR) for inter-cell interference (ICI) cancelation in downlink multicell systems. When fractional frequency reuse (FFR) is applied to the cell edge, it is conjectured that BS cooperation, or a coordinated multipoint (CoMP), will further improve the system performance. Simulation results show that the proposed scheme outperforms the reference schemes in terms of the cell edge SINR with a minimal impact on the path loss exponent in the networks.

Distributed Energy Spectral Efficiency Optimization for Partial/Full Interference Alignment in Multi-user Multi-relay Multi-cell MIMO Systems

The energy spectral efficiency maximization (ESEM) problem of a multi-user, multi-relay, multi-cell system is considered, where all the network nodes are equipped with multi-antenna transceivers. To deal with the potentially excessive interference originating from a plethora of geographically distributed transmission sources, a pair of transmission protocols based on interference alignment (IA) are conceived. The first, termed the full-IA, avoids all intra-cell interference (ICI) and other-cell interference by finding the perfect interference-nulling receive beamforming matrices (RxBFMs). The second protocol, termed partial-IA, only attempts to null the ICI. Employing the RxBFMs computed by either of these protocols mathematically decomposes the channel into a multiplicity of non-interfering multiple-input--single-output channels, which we term as spatial multiplexing components (SMCs). The problem of finding the optimal SMCs as well as their power control variables for the ESEM problem considered is formally defined and converted into a convex optimization form with carefully selected variable relaxations and transformations. Thus, the optimal SMCs and power control variables can be distributively computed using both the classic dual decomposition and subgradient methods. Our results indicate that indeed, the ESEM algorithm performs better than the baseline equal power allocation algorithm in terms of its ESE. Furthermore, surprisingly the partial-IA outperforms the full-IA in all cases considered, which is because the partial-IA is less restrictive in terms of the number of available transmit dimensions at the transmitters. Given the typical cell sizes considered in this paper, the path-loss sufficiently attenuates the majority of the interference, and thus the full-IA over-compensates, when trying to avoid all possible sources of interference.

Downlink Hybrid Information and Energy Transfer With Massive MIMO

We consider a downlink massive MIMO system, where the base station simultaneously sends information and energy to information users and energy users, respectively. The aim is to maximize the minimum harvested energy among the energy users while meeting the rate requirements of information users. With perfect channel state information (CSI), the problem is solved by obtaining the asymptotically optimal power allocation of information users and the combination coefficients of the energy precoder. For the CSI estimation in time-division duplex systems, orthogonal pilot sequences are employed by information users during the uplink, and one common pilot sequence is shared by all energy users. It is shown that the energy-harvesting performance of such a shared pilot scheme is always better than that of the orthogonal pilot scheme. Further, exploiting the intercell interference in multicell systems, a joint precoder is proposed for cooperative energy transfer, for which both the centralized and distributed implementations are given. Results indicate that the cooperative energy transfer always outperforms the noncooperative scheme with either perfect or estimated CSI.

Robust rank‐two beamforming for multicell multigroup multicast

In practical cellular communication systems, perfect channel state information is not available at the base stations. Assuming that the channel uncertainty is bounded within a known spherical region, the authors consider in this study robust rank-two multicast beamforming for multiple groups of users in multicell systems. Worst-case robust beamforming is designed to guarantee that the predefined signal-to-interference-plus-noise ratio targets of all users are met for any channel instance in the bounded region. The Alamouti code is employed to double the degrees of freedom in the beamformer design. The authors apply the semi-definite relaxation (SDR) technique to address the formulated non-convex robust rank-two beamforming problem. Analytical results show that the SDR is tight when the number of users in each multicast group is no more than two and the channel uncertainty is sufficiently small. Finally, extensive simulations are carried out to validate the proposed beamforming design and the analytical studies.

Sectorization based pilot reuse for improving net spectral efficiency in the multicell massive MIMO system

A sectorization method using the uniform circular array (UCA) is proposed to improve the net spectral efficiency (SE) of the multicell massive MIMO system, which is an important index for evaluating the performance of a communication system. We derive the ergodic achievable uplink net SE per cell of a general sectorized system and obtain its deterministic approximation based on the large random matrix theory. Different weight matrices are considered for the sectorized system and the one with the best performance is utilized for further analysis. The consistency of the deterministic approximation with the result of Monte-Carlo simulation is proved at the same time. At last, numerical results indicate that the net SE per cell can be greatly improved compared to the conventional multicell massive MIMO system, which validates the effectiveness of the sectorization method. Moreover, comparisons with other pilot reuse methods are also made in this paper.

Statistical Beamforming for Interference Mitigation in Multi-cell Massive MIMO Systems

AbstractThis paper proposes a statistical beamforming approach to mitigate interference in multi-cell massive multiple-input multiple-output systems. The proposed approach projects the signal subspace onto the null space of the interference to form the weight vector with the statistical channel state information. In contrast to the existing methods which only utilize the partial null space of the interference, the proposed approach can collect more signal when mitigating the interference. In addition, the large system analysis demonstrates that the interference is eliminated with uniform rectangular arrays. Numerical results show that the proposed approach can achieve higher sum rate than the existing methods and verify the performance analysis.

Overhearing Protocol Design Exploiting Intercell Interference in Cooperative Green Networks

The conventional two-way relaying (TWR) protocol requires that the transmitter (receiver) in one direction must be the receiver (transmitter) in the other direction, a limitation precluding the application of the TWR for sophisticated real-world wireless networks. In this paper, the authors study a more general multicell system consisting of a downlink (DL) traffic in one cell and an uplink (UL) traffic in an adjacent cell, with a multiantenna relay located in the cell edge and shared by both cells. For the coexistence of DL and UL transmissions, the authors propose exploiting the overheard signals from the adjacent cell (commonly known as the intercell interference) to improve the quality of signal reception in both cells. To reduce the power consumption to suit for green networks, the authors design the optimum relay precoder to minimize the total power at the relay yet satisfying the rate constraints for both the DL and UL traffic flows. The original precoder design is a nonconvex problem that is difficult to solve. To make the problem tractable, the authors transform the nonconvex problem to an equivalent quadratically constrained quadratic program (QCQP), which is then solved by the semidefinite relaxation (SDR) technique. Finally, simulations validate the effectiveness of the authors proposed protocol together with the optimized relay precoder.

Effects of antenna array characteristics on in-band full-duplex relays for broadband cellular communications

Abstract This paper considers a broadband multicell system with key characteristics that base-stations and relays employ beamforming via large antenna arrays, as well as in-band full duplex relay operation. The signal to interference plus noise ratio gain (which is in excess of the maximum required for such a system) could be exploited either for reducing self-interference or the backhaul and access link transmitted powers. The presented results indicate that with a moderate front-to-back-ratio scenario for the antenna array, “small” antenna arrays with 8 elements can lower the requirement of full duplex self-interference cancellation by up to 55 dB, while large antenna arrays of 1024 elements will produce 100 dB reduction. Alternatively, for the same scenario, both the backhaul and access transmitted powers could be reduced by ∼55 dB and ∼25 dB with 1024 elements.

Energy Efficient Coordinated Beamforming for Multicell System: Duality-Based Algorithm Design and Massive MIMO Transition

In this paper, we investigate joint beamforming and power allocation in multicell multiple-input single-output (MISO) downlink networks. Our goal is to maximize the utility function defined as the ratio between the system weighted sum rate and the total power consumption subject to the users’ quality of service requirements and per-base-station (BS) power constraints. The considered problem is nonconvex and its objective is in a fractional form. To circumvent this problem, we first resort to an virtual uplink formulations of the the primal problem by introducing an auxiliary variable and applying the uplink-downlink duality theory. By exploiting the analytic structure of the optimal beamformers in the dual uplink problem, an efficient algorithm is then developed to solve the considered problem. Furthermore, to reduce further the exchange overhead between coordinated BSs in a large-scale antenna system, an effective coordinated power allocation solution only based on statistical channel state information is reached by deriving the asymptotic optimization problem, which is used to obtain the power allocation in a long-term timescale. Numerical results validate the effectiveness of our proposed schemes and show that both the spectral efficiency and the energy efficiency can be simultaneously improved over traditional downlink coordinated schemes, especially in the middle-high transmit power region.

Estimation accuracy of multi‐cell massive multiple‐input multiple‐output systems in correlated Rician fading channel

The performance analysis of a multi-user multi-cell massive multiple-input multiple-output system that applies a conventional channel estimation technique, namely the linear minimum mean square error over correlated Rician fading is investigated. Based on the analysis, it is found that increasing the line-of-sight component can enhance the estimation accuracy.