Yun Rui

A Unified Energy Efficiency and Spectral Efficiency Tradeoff Metric in Wireless Networks

In this letter, we propose a unified metric for energy efficiency (EE) and spectral efficiency (SE) tradeoff design in wireless networks. Different from previous EE-based or SE-based design, we simultaneously optimize both EE and SE. First, we formulate our design problem as a multi-object optimization (MOO) problem, in which the Pareto optimal set is characterized. After normalizing EE and SE to make them comparable, we then convert the MOO problem into a single-object optimization (SOO) problem by the weighted product scalarization method. We further show that the objective function of this SOO problem, i.e., our proposed EE and SE tradeoff (EST) metric, is quasi-concave with the transmit power and a unique globally optimal solution is derived. Numerical results validate the effectiveness of the proposed unified EST metric.

Mode Selection and Power Optimization for Energy Efficiency in Uplink Virtual MIMO Systems

Driven by green communications, energy-efficient transmission is becoming an important design criterion for wireless systems, aiming to extend the life cycle of batteries in mobile devices. In this paper, we tackle the energy efficiency (EE) issue in uplink virtual multiple-input multiple-output (MIMO) systems, which requires the optimization of two interlaced parameters: the number of constituent mobile users in the virtual MIMO and their corresponding power allocation. The former parameter is a structural parameter defining the size of the virtual MIMO (usually known as the transmission mode) and its optimization relies on the method of enumeration. The difficulty is further aggravated by the fact that the EE is a non-convex function of power, even for a given transmission mode. By exploiting the fact that increasing the number of active users can increase the number of contributors to the total EE on one hand but reducing the diversity order for each single user on the other, we can show the existence of an optimal transmission mode and find a simple way for its search. Through in-depth analysis, we show the existence of a unique globally optimal power allocator for the case without power constraints under the assumption of zero-forcing receivers, and further reveal the impact of power constraints upon power allocation, as compared to its global counterpart, aiming to provide a powerful means for power-constrained EE optimization. Finally, we establish theories, for isometric networks, to narrow down the search range for possible transmission modes, leading to a significant reduction of computational complexity in optimization. Simulation results are presented to substantiate the proposed schemes and the corresponding theories.

Carrier aggregation for LTE-advanced: uplink multiple access and transmission enhancement features

Multiple access and transmission enhancement in support of carrier aggregation techniques has been actively studied in the 3GPP LTEAdvanced standardization process of the nextgeneration mobile broadband communication systems. By means of carrier aggregation, users can access a total bandwidth of up to 100 MHz in order to meet the IMT-Advanced requirements. This article first introduces and compares several uplink multiple access schemes in LTE-Advanced standard. Technical challenges arising from the use of carrier aggregation is then addressed, with focus on reference signals design for uplink transmission. Meanwhile, various candidate options are summarized and compared in terms of their system performance, computational complexity, and design flexibility. The article further proceeds to investigate the link performance with carrier aggregation, taking into account the presence of frequency offset and fast fading in the uplink channel. Numerical results are also presented, which indicate that advanced signal processing algorithms should be further studied and optimized to ensure reliable operation of the 3GPP LTE-Advanced in high mobility.

Impact of Channel Estimation Error on Bidirectional MABC-AF Relaying With Asymmetric Traffic Requirements

Channel estimation error results in severe performance deterioration in wireless networks. In this paper, we study the impact of imperfect channel estimation (ICE) on the outage performance of bidirectional relaying where the two sources have asymmetric traffic requirements (ATRs). In particular, we focus on the amplify-and-forward (AF) relay-assisted multiple-access broadcast (MABC) protocol, i.e., MABC-AF, which has received much attention due to its high spectrum efficiency and low complexity. In the single-relay scenario, we derive exact and generalized closed-form expressions for system outage probability, which indicate that the system outage is determined by the unidirectional outage in the case of highly asymmetric traffic patterns. For more insights into our approach, the closed-form asymptotic expressions are also evaluated, manifesting the interesting error floor (EF) phenomenon due to ICE. Using these analytical results, we further develop a robust and practical optimum power-allocation (OPA) algorithm that minimizes the system outage probability under aggregate and individual node power constraints. In the multirelay scenario, by taking into account the traffic knowledge, a novel relay selection criterion is proposed for asymmetric MABC-AF, followed by its impact on the system outage probability in the presence of ICE. Numerical results validate the accuracy of our analytical results and highlight the effect of the proposed OPA algorithm under various traffic requirements and channel estimation errors. Furthermore, the results also show that the proposed relay selection criterion is superior to the classical max–min criterion with ATRs, and the performance improvement becomes remarkable as channel estimation error increases.

Analysis of CFO effects on and phase compensation method for SC-FDMA systems

Discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S-OFDM) based single carrier-frequency division multiple access (SC-FDMA) is accepted as the uplink transmission scheme by the 3GPP-long term evolution standardization organization. In this paper, the effects of carrier frequency offset (CFO) on DFT-S-OFDM system are analyzed for both single user and multiple user accessing scenarios. According to the analysis, the CFO results in the attenuation, linear phase rotation, inter-symbol interference (ISI) and inter-sub-carrier interference (ICI) on the demodulated symbols of each desired user, and the multi-user interference (MUI) among uplink users. The theoretical values of ISI, ICI, and signal to interference ratio (SIR) are presented for the DFT-S-OFDM system with full and partial bandwidth detection respectively. A phase compensation method is proposed to reduce the effect of linear phase rotation on the demodulated symbols. Finally, the analytical values are verified by the simulation results.

Energy efficient opportunistic cooperative transmission with different ratio combinings: from a new perspective

In cooperative communication, different ratio combining approaches can be utilized at the receiver based on the channel state information (CSI) available to the destination. In this paper, we focus on optimal energy allocation under amplify-and-forward (AF) cooperative communications when various ratio combining methods are utilized. More specifically, we consider a suite of important and fundamental problems, including signal-to-noise ratio (SNR) maximization under a weighted total energy constraint, weighted total energy minimization under an SNR constraint, weighted total energy minimization under an outage probability constraint and outage probability minimization under instantaneous weighted total energy constraint, and analyze the relationship among these problems. We first consider maximal ratio combining (MRC) and derive exact analytical solutions. To reveal the key factors in the resource allocation problem, we further transform the exact results with multiple parameters into more intuitive results with only two quantities. The resulting solutions provide a new perspective to understand optimal energy-efficient opportunistic cooperative transmissions. We then consider fixed ratio combining (FRC) and show that an explicit analytical solution does not exist to the original problem. To this end, based on the convexity proofs for the objective functions, we utilize numerical convex optimizations to obtain the unique solution. Both numerical and simulation studies are conducted to validate our theoretical analysis.

Cross-layer design for cooperative MIMO systems with relay selection and imperfect CSI

Based on imperfect channel state information (CSI) and relay selection, a cross-layer optimization scheme is developed for cooperative MIMO system, and the corresponding system performance is investigated in Rayleigh fading channel. By the theoretical analysis and mathematical manipulation, the average spectral efficiency (SE), packet error rate (PER) and packet loss rate (PLR) are derived. As a result, closed-form expressions of the average SE, PER and PLR are obtained, respectively. According to these, a cross-layer design (CLD) scheme subject to the average PER constraints is proposed to avoid the SE performance loss caused by the conventional instantaneous PER constraints. The scheme optimizes the adaptive thresholds by maximizing the average SE of the system, and an iteration method joint the Lagrange multiplier and Newton methods is presented to solve the above optimization problem. Simulation results verify the validity of the theoretical analysis. The results show that the proposed CLD scheme can improve the SE while target PLR is maintained. The CLD scheme under average PER constraints has higher SE than the conventional CLD scheme under instantaneous PER constraints.

Performance of orthogonal STBC-MIMO with variable-power adaptive modulation and delayed feedback in Nakagami fading channels

A variable-power (VP) adaptive modulation (AM) scheme with space-time block code (STBC) is developed in MIMO systems, and the corresponding performance is investigated in Nakagami fading channels. The optimum switching thresholds are derived so that the spectrum efficiency (SE) under an average power and a target bit error rate (BER) constraint is maximized. By the switching thresholds, average BER and SE are further derived. As a result, closed-form expressions are obtained. Besides, the exact and approximate BER expressions of VP AM and constant-power (CP) AM systems with delayed feedback are respectively derived. Simulation results show that the theoretical SE and BER can match the corresponding simulation well. The results indicate that VP-AM with STBC provide higher spectrum efficiency than the CP counterparts, and has slighter bite error rate performance degradation when the normalized delay is less than 0.01.

Power Scaling Method for Uplink Power Control with Carrier Aggregation

According to 3GPP RAN1 conference, power scaling for uplink power control should be applied equally. However, the detail of power scaling is a remaining issue. In this contribution, we clarify some cases of power scaling and propose several options of power scaling on PUSCHs. It is not clear that whether the equal power scaling is relative to the allocated transmission power after or before power truncation is applied in the CC-specific PC formula. Since link adaptation at the eNodeB is typically performed assuming that the UE will transmit with the allocated transmission power, distribution of power after scaling should be as close as possible to the distribution of power before scaling. From these criteria, two new methods for power scaling are introduced. One of these two methods (option 3) has the optimal performance such that to apply power scaling on all PUSCHs without UCI in the sense that it is as equal as possible. Whereas another method (option 4) has less complexity such that each type of CCs which is treated as a whole would be applied equal power scaling. From the results of simulation, it shows that equal power scaling could have the optimal average performance. Therefore, the equal power scaling rules should be hold whatever the scaling algorithms are adopted for uplink power control.

Performance Analysis of Distributed Antenna Systems with Antenna Selection over MIMO Rayleigh Fading Channel

The downlink performance of distributed antenna systems (DAS) with antennas selection is investigated in Rayleigh fading multicell environment, and the corresponding system capacity and bit error rate (BER) analysis are presented. Based on the moment generating function, the probability density function (PDF) and cumulative distribution function (CDF) of the effective signal to interference plus noise ratio (SINR) of the system are first derived, respectively. Then, with the available CDF and PDF, the accurate closed-form expressions of average channel capacity and average BER are further derived for exact performance evaluation. To simplify the expression, a simple closed-form approximate expression of average channel capacity is obtained by means of Taylor series expansion, with the performance results close to the accurate expression. Besides, the system outage capacity is analyzed, and an accurate closed-form expression of outage capacity probability is derived. These theoretical expressions can provide good performance evaluation for DAS downlink. It can be shown by simulation that the theoretical analysis and simulation are consistent, and DAS with antenna selection outperforms that with conventional blanket transmission. Moreover, the system performance can be effectively improved as the number of receive antennas increases.