[Show abstract][Hide abstract] ABSTRACT: Energy efficiency (EE) has become an important design goal for future cellular systems. In this paper, we study the EE problem for limited-feedback coordinated beamforming systems providing real-time services with constant date rate. To maximize the EE, we jointly optimize the transmit power among users and the feedback bits of each user to quantize the desired and interfering channel directions, subject to the constraints of outage probability and number of feedback bits. An iterative optimization algorithm is proposed for alternating power and bit allocation by employing Yates’ framework. To further reduce the computational complexity, a suboptimal algorithm is proposed in closed form by decoupling the problem into subproblems under asymptotical analysis. Simulation results show that the suboptimal algorithm performs closely to the iterative optimization algorithm under high-quantization resolution. Moreover, the proposed two algorithms provide substantial EE gain over existing schemes with equal power and bit allocation, and the EE depends on the target data rate and outage probability of each user as well as the circuit power consumption at the base stations.
EURASIP Journal on Wireless Communications and Networking 12/2015; 2015(1). DOI:10.1186/s13638-015-0346-x · 0.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Inter-cell interference leads to severe performance degradation in cellular networks, and the study of multi-user interference channel is the corner stone for solving this problem. Amplitude-space layered interference alignment (IA), as an effective complementation to the vector-space IA, is a promising method to increase the data rate in static interference channels. However, recent studies of layered IA has been focused on analyzing the degrees of freedom (DoF) or the achievable rate under specific channel constraints. In this paper, we propose a layered IA scheme that can work with arbitrary channel coefficients. We develop a layer partitioning method and optimize the active layer assignment through linear programming. An implementation scheme is then introduced with multi-level nested lattice codes where the signal and interference are nested in amplitude space, and the interference from different users is nestedly aligned. The performance of the proposed scheme is finally evaluated in homogeneous and heterogeneous cellular networks with practical settings.
EURASIP Journal on Wireless Communications and Networking 12/2015; 2015(1). DOI:10.1186/s13638-015-0389-z · 0.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper, we design coordinated beamforming at base stations (BSs) to facilitate interference cancelation at users in interference networks, where each BS is equipped with multiple antennas and each user is with a single antenna. By assuming that each user can select the best decoding strategy to mitigate the interference, either canceling the interference after decoding when it is strong or treating it as noise when it is weak, we optimize the beamforming vectors that maximize the sum rate for the networks under different interference scenarios and find the solutions of beamforming with closed-form expressions. The inherent design principles are then analyzed, and the performance gain over passive interference cancelation is demonstrated through simulations in heterogeneous cellular networks.
Journal on Advances in Signal Processing 11/2015; 2015(1). DOI:10.1186/s13634-015-0253-7 · 0.78 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Video on-demand (VOD) service is widely requested, and is becoming a dominant application in wireless networks, where energy efficiency (EE) is a major design goal. Reducing the energy consumption of VOD service with given quality of service (QoS) requirement can improve the EE of wireless systems. Recent finding shows that user mobility is highly predictable, and hence future location information is possible to know. In this paper, we study EE-optimal transmit power and bandwidth allocation for VOD service in orthogonal frequency division multiplexing (OFDM) system by exploiting context information. We consider two kinds of context information, i.e., the predictive average channel gains and the QoS of VOD service. The optimal resource allocation policy that minimizes the average energy consumption for VOD service is proposed. At the beginning of the service, the average transmit power and number of used subcarriers are allocated with future average channel gains. During the procedure of the service, the instantaneous transmit power is allocated to different subcarriers after the instantaneous channel gains becomes available. Simulation results show that the energy consumed for VOD service with the proposed policy is around half of that with an existing policy.
IEEE/CIC International Conference on Communications in China (invited paper); 11/2015
[Show abstract][Hide abstract] ABSTRACT: It is widely accepted that a tradeoff exists between transmit power and average delay. In this paper, we consider wireless systems transmitting randomly arrived traffic over fading channels with statistical quality-of-service requirement, characterized by a delay bound and a delay bound violation probability. We study the relation between the maximal energy efficiency (EE) and the delay bound with given delay violation probability. We prove that the EE-delay tradeoff vanishes if the average total power consumption, including transmit and circuit powers of the base station, linearly increases with the average service/transmission rate. By taking massive multi-input-multi-output (MIMO) system as an example, we show that if the required total power consumption is a linear function of the service rate, the maximal EE is independent of the delay bound. If the required total power is strictly convex in the service rate, then the EE can be improved by extending the delay.
IEEE Online Conference on Green Communications; 11/2015
[Show abstract][Hide abstract] ABSTRACT: This paper optimizes resource allocation that maximizes the energy efficiency (EE) of wireless systems with statistical quality of service (QoS) requirement, where a delay bound and its violation probability need to be guaranteed. To avoid wasting energy when serving random sources over wireless channels, we convert the QoS exponent, a key parameter to characterize statistical QoS guarantee under the framework of effective bandwidth and effective capacity, into multi-state QoS exponents dependent on the queue length. To illustrate how to optimize resource allocation, we consider multi-input-multi-output orthogonal frequency division multiplexing (MIMO-OFDM) systems. A general method to optimize the queue length based bandwidth and power allocation (QRA) policy is proposed, which maximizes the EE under the statistical QoS constraint. A closed-form optimal QRA policy is derived for massive MIMO-OFDM system with infinite antennas serving the first order autoregressive source. The EE limit obtained from infinite delay bound and the achieved EEs of different policies under finite delay bounds are analyzed. Simulation and numerical results show that the EE achieved by the QRA policy approaches the EE limit when the delay bound is large, and is much higher than those achieved by existing policies considering statistical QoS provision when the delay bound is stringent.
IEEE Transactions on Communications 09/2015; DOI:10.1109/TCOMM.2015.2480770 · 1.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: With the increase of cell density and explosive growth of data traffic, user centric is becoming one of the design principles of next-generation cellular networks. One meaning of user centric lies in that no matter where a user is located, its demand in quality of service (QoS) will be guaranteed in high probability. One approach to achieve such an ambitious goal is allowing each user to select several preferred base stations to transmit cooperatively. In this paper, we propose a user-centric downlink cooperative transmission scheme with orthogonal beamforming based limited feedback, where the cooperative clusters of multiple users may overlap and per-cell codebooks are considered. To assist the central unit (CU) for scheduling users with guaranteed QoS and performing adaptive transmission, a method for each user to estimate its signal-to-noise-and-interference ratio is derived. Targeting to ensure the required QoS of multiple users, we propose a method to select the cooperative cluster at each user and provide a method to schedule users based on their service priorities and channel conditions at the CU, where the clusters are selected semidynamically. Simulation results show that the proposed scheme significantly increases the percentage of users with satisfactory QoS demands.
[Show abstract][Hide abstract] ABSTRACT: Improving energy efficiency of wireless systems by exploiting the context
information has received attention recently as the smart phone market keeps
expanding. In this paper, we devise energy-saving resource allocation policy
for multiple base stations serving non-real-time traffic by exploiting three
levels of context information, where the background traffic is assumed to
occupy partial resources. Based on the solution from a total energy
minimization problem with perfect future information,a context-aware BS
sleeping, scheduling and power allocation policy is proposed by estimating the
required future information with three levels of context information.
Simulation results show that our policy provides significant gains over those
without exploiting any context information. Moreover, it is seen that different
levels of context information play different roles in saving energy and
reducing outage in transmission.
[Show abstract][Hide abstract] ABSTRACT: We obtain the exact maximum Degrees of Freedom (DoF) region achieved by linear transceivers without symbol extensions of the 2 2 2 multiple-input multiple-output (MIMO) interference network, which is comprised of two sources, two relays and two destinations, each with arbitrary number of antennas. We prove that the maximum achievable DoF region for the networks under some antenna configurations coincide with the cut-set bound of the networks, while there is only one DoF gap from the bound for the networks under other configurations, which can not be bridged by linear transceivers. The basic idea of the proof is to construct a two-tier transceiver, where the first tier decomposes the general networks to several basic networks with special configurations, and the second tier achieves the maximal DoF tuples for these basic networks. A mechanism for constructing the second tier transceiver is proposed, which is referred to as channel scaling assisted interference elimination. Combined with interference avoidance at sources and interference cancelation at destinations depending on the antenna configurations, interference-free transmission can be ensured.
IEEE Transactions on Signal Processing 07/2015; 63(14):1-1. DOI:10.1109/TSP.2015.2424197 · 2.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper, we propose a power control algorithm for uplink and downlink multi-cell massive MIMO systems, where channel estimation error, pilot contamination and correlated channels are taken into account. Moreover, a closed-form power control algorithm under the assumption of equal power allocation for multiple users is provided. The simulation results show that equal power allocation is near optimal to maximize global EE for the system with zero-forcing detection/precoding, and is near optimal for the system with large number of antennas and MRC/MRT.
2015 IEEE 16th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), Stockholm, Sweden; 06/2015
[Show abstract][Hide abstract] ABSTRACT: Coordinated beamforming has been optimized to maximize the sum rate under the transmit power constraints or to minimize the transmit power under the data rate constraints. In this paper, we design a precoder that maximizes the energy efficiency (EE) of multicell multiantenna downlink network with coordination and, in the meantime, ensures the minimal data rate requirement of each user. To find a solution of such a nonconvex optimization problem, we construct a convex subset of the original constraint set and a quasi-concave lower bound of the EE. Then, we propose an iterative algorithm to maximize the lower bound of the EE within the convex subset, which is shown to rapidly converge to a local optimum of the original problem. We evaluate the EE of the proposed algorithm through simulations under different user locations, data rate requirements, and numbers of antennas. The results demonstrate that the proposed precoder performs closely to an upper bound derived from interference-free assumption. When the circuit power consumption is dominant, the proposed precoder is much more energy efficient than the transmit power minimization precoder, as well as a precoder, including an interference alignment (IA) beamformer and the optimized transmit power maximizing the EE.
[Show abstract][Hide abstract] ABSTRACT: Massive multi-input multi-output (MIMO) can support high spectral efficiency
(SE) with simple linear transceivers, and is expected to provide high energy
efficiency (EE). In this paper, we analyze the EE of downlink multi-cell
massive MIMO systems under spatially correlated channel model, where both
transmit and circuit power consumptions, training overhead, channel estimation
and pilot contamination (PC) are taken into account. We obtain the maximal EE
for the systems with maximum-ratio transmission (MRT) and zero-forcing
beamforming (ZFBF) for given number of antennas and users by optimizing the
transmit power. The closed-form expressions of approximated optimal transmit
power and maximal EE, and their scaling laws with the number of antennas M are
derived for the systems with MRT and ZFBF. Our analysis shows that the maximal
EE decreases with M for both systems with and without PC, but with different
descending speeds. For the system without PC, the optimal transmit power should
be configured to increases with M, while for the system with PC, the optimal
transmit power should be configured as a constant independent from M. The
analytical results are validated by simulations under a more realistic
three-dimensional channel model.
[Show abstract][Hide abstract] ABSTRACT: Caching popular contents at the base station (BS) can reduce the backhaul
cost and improve the network throughput. Yet whether locally caching at the BSs
can improve the energy efficiency (EE), a major goal for 5th-generation
cellular networks, remains unclear. Due to the entangled impact of various
factors on EE such as interference level, backhaul capacity, BS density, power
consumption parameters, BS sleeping, content popularity and cache capacity,
another important question is what are the key factors that contribute more to
the EE gain from caching. In this paper, we attempt to explore the potential of
EE of the cache-enabled wireless access networks and identify the key factors.
By deriving the closed-form expression of the EE, we provide the condition when
the EE can benefit from caching, find the optimal cache capacity that maximizes
the network EE, and analyze the maximal EE gain brought by caching. We show
that caching at the BSs can improve the network EE when power efficient cache
hardware is used. When local caching has EE gain over not caching, caching more
files at the BSs may not provide higher EE. Numerical and simulation results
validate our analysis and show that the caching EE gain is large when the
backhaul capacity is stringent, interference level is low, cached files are
popular, and when caching at pico BSs instead of macro BSs.
[Show abstract][Hide abstract] ABSTRACT: Coordinated multipoint (CoMP) transmission can provide high spectral efficiency for cellular systems if perfect channels are available. In limited feedback systems, except for channel direction, signal-to-noise-plus-interference ratio (SINR) is necessary to assist in user scheduling and adaptive transmission. When the methods to estimate SINR in single-cell systems is applied to CoMP systems, the performance will significantly degrade. In this paper, we estimate the SINR for downlink CoMP systems. We start by showing that the quantization error vector is no longer isotropic when quantizing CoMP channels. This implies that the vector has an inherent structure. We proceed to formulate an optimization problem to find the quantization error vector that maximizes the multiuser interference power under the structure constraint. This way, we can exploit the nonisotropic nature of the quantization error vector and avoid overestimating the SINR. Simulation results show that the proposed method provides high throughput and low outage probability with negligible extra feedback overhead.
[Show abstract][Hide abstract] ABSTRACT: This article provides an overview on research in energy-efficient wireless networks during the past decade and discusses its potential applications toward the fifth generation cellular systems. After analyzing the trade-off between spectrum efficiency and energy efficiency, various research results are summarized within a framework of energy-efficient resource allocation with optimization as a common tool. Then potential energy efficiency improving approaches in both physical layer and deployment aspects are provided. Finally, energy efficiency related open problems in massive multiple-input multiple- output, device-to-device communications, ultra dense networks, and other emerging technologies are identified.
[Show abstract][Hide abstract] ABSTRACT: This paper proposes a codebook for jointly quantizing channel direction
information (CDI) of spatially correlated three-dimensional (3D)
multi-input-multi-output (MIMO) channels. To reduce the dimension for
quantizing the CDI of large antenna arrays, we introduce a special structure to
the codewords by using Tucker decomposition to exploit the unique features of
3D MIMO channels. Specifically, the codeword consists of four parts each with
low dimension individually targeting at a different type of information:
statistical CDIs in horizontal direction and in vertical direction, statistical
power coupling, and instantaneous CDI. The proposed codebook avoids the
redundancy led by existing independent CDI quantization. Analytical results
provide a sufficient condition on 3D MIMO channels to show that the proposed
codebook can achieve the same quantization performance as the well-known
rotated codebook applied to the global channel CDI, but with significant
reduction in the required statistical channel information. Simulation results
validate our analysis and demonstrate that the proposed joint CDI quantization
provides substantial performance gain over independent CDI quantization.
[Show abstract][Hide abstract] ABSTRACT: This paper studies user-specific clustering for downlink cloud radio access network (C-RAN), where a central unit, connected to all base stations (BSs) via limited-capacity backhaul links, coordinates the BSs to form cooperative clusters for every user. By taking into account the training overhead for channel estimation in C-RAN, we design the clustering scheme aimed at maximizing the average net throughput of the network subject to the constraint on backhaul capacity, where a hybrid coordinated multi-point (CoMP) transmission mode is considered. The proposed clustering scheme can be operated in a semi-dynamic manner merely based on large-scale channel information, has low computational complexity, and performs close to the optimal scheme found by exhaustive searching. Under two special cases where the backhaul capacity are very stringent and unlimited, the proposed scheme is then tailored for pure coordinated beamforming (CB) mode and pure joint transmission (JT) mode to further reduce the clustering complexity. Simulation results show that the proposed semi-dynamic clustering schemes are superior to the dynamic clustering scheme due to the reduction of required training overhead.