Xiaodai Dong

Low-Complexity Hybrid Precoding in Massive Multiuser MIMO Systems

Massive multiple-input multiple-output (MIMO) is envisioned to offer considerable capacity improvement, but at the cost of high complexity of the hardware. In this paper, we propose a low-complexity hybrid precoding scheme to approach the performance of the traditional baseband zero-forcing (ZF) precoding (referred to as full-complexity ZF), which is considered a virtually optimal linear precoding scheme in massive MIMO systems. The proposed hybrid precoding scheme, named phased-ZF (PZF), essentially applies phase-only control at the RF domain and then performs a low-dimensional baseband ZF precoding based on the effective channel seen from baseband. Heavily quantized RF phase control up to 2 bits of precision is also considered and shown to incur very limited degradation. The proposed scheme is simulated in both ideal Rayleigh fading channels and sparsely scattered millimeter wave (mmWave) channels, both achieving highly desirable performance.

Linear Interpolation in Pilot Symbol Assisted Channel Estimation for OFDM

In this paper, we investigate several efficient interpolation techniques for pilot symbol assisted channel estimation in OFDM. The interpolation methods studied include two dimensional (2-D) separable lowpass sine interpolator with Kaiser window, 2-D separable Deslauriers-Dubuc (DD) interpolation and 2-D discrete Fourier transform (DFT) based lowpass interpolation. The performances of these interpolators are compared with those of the well known minimum mean-square error (MMSE) 2-D separable Wiener filter and the perfect channel state information. It is shown that the Kaiser window based interpolator and DD interpolation are simple, robust, and outperform the 2-D DFT based lowpass interpolation as well as several existing interpolation methods proposed in the literature. These two schemes are suitable candidates for use in 1-D and 2-D channel estimation

Impact of Channel Estimation Error on the Performance of Amplify-and-Forward Two-Way Relaying

In this paper, the impact of channel-state information (CSI) estimation error on the performance of an amplify-and-forward two-way multiple relay network has been investigated. In contrast to the existing literature, which assumes perfect self-interference cancellation, we consider imperfect self-interference cancellation at both sources that exchange information through multiple relays, and maximal-ratio combining is then applied to improve the decision statistics under imperfect signal detection. We derive the effective signal-to-noise ratio (SNR) subject to noisy channel estimation, and based on this SNR, the system outage probability is given. In addition, we derive the closed-form expression of the average system bit error rate (BER) and the asymptotic expressions for both outage probability and BER. Furthermore, instead of employing all relays, we examine the impact of imperfect CSI on a single relay selection (RS) scheme. To mitigate the negative impact of imperfect CSI, we show that power allocation (PA), by minimizing either the outage probability or the BER, can suitably be cast as the geometric-programming problem. Numerical results validate the correctness of the derived expressions and show that the adaptive-PA scheme outperforms the equal-PA scheme under the aggregated effect of imperfect CSI.

Joint Precoding Optimization for Multiuser Multi-Antenna Relaying Downlinks Using Quadratic Programming

This paper studies the optimization problem for joint precoding design in a multi-antenna downlink channel using relaying. We formulate the joint source and relay precoding design by aiming at sum capacity maximization. Since this problem is in general nonconvex, we first convert this problem into standard convex quadratic programs, and then propose an iterative joint precoding optimization algorithm by utilizing efficient quadratic programming approaches. We observe that the iterative method always yields optimal precoding matrices which diagonalize the compound channel of the backward (source-to-relay) and the forward (relay-to-destination) links at high SNR regimes. Motivated by this observation, we further develop an efficient structured precoding design. Simulation results are presented to verify the effectiveness of our proposed precoding schemes.

Short-Term Operation Scheduling in Renewable-Powered Microgrids: A Duality-Based Approach

This paper considers the operation scheduling problem in renewable-powered microgrids, which is used to determine the least-cost unit commitment (UC) and the associated dispatch, while meeting load, environmental, and system operating requirements. The intermittency nature of the renewable energy sources, as well as microgrids capacity to operate either in parallel with, or autonomously of, the traditional power grid, pose new challenges to this classic optimization task. A probability-based concept, probability of self-sufficiency (PSS), is introduced to indicate the probability that the microgrid is capable of meeting local demand in a self-sufficient manner. Furthermore, to the best of our knowledge, we make the first attempt in approaching the mixed-integer UC problem from a convex optimization perspective, which leads to an analytical closed-form characterization of the optimal commitment and dispatch solutions. The simulation results show that 1) the proposed method achieves an efficient performance that incurs no loss of optimality with lower complexity than existing algorithms; 2) an energy storage system (ESS) with suitable capacity contributes to the self-sufficiency target of a microgrid, and the stored energy varies less remarkably as the microgrid tends to operate more independently; 3) the proposed method provides guidelines in deciding the ESS size to achieve a desired PSS.

Cyclic prefixed single carrier transmission in ultra-wideband communications

This letter proposes cyclic prefixed single carrier transmission with frequency domain equalization (SC-FDE) as an alternative physical layer solution for UWB communications. The performance of SC-FDE over IEEE 802.15.3a UWB channel models is analyzed, simulated and compared with that of impulse based single carrier UWB (SC-UWB) and multicarrier UWB employing orthogonal frequency division multiplexing (OFDM-UWB). The impact of channel coding on the performance of OFDM and SC-FDE in UWB is also studied. Our results demonstrate performance advantage of the SC-FDE scheme, especially when implementation issues such as low complexity and low power consumption for UWB are taken into consideration. The performance of SC-FDE with diversity combining using oversampling is also investigated

Hybrid Block Diagonalization for Massive Multiuser MIMO Systems

For a massive multiple-input multiple-output (MIMO) system, restricting the number of RF chains to far less than the number of antenna elements can significantly reduce the implementation cost compared to the full complexity RF chain configuration. In this paper, we consider the downlink communication of a massive multiuser MIMO (MU-MIMO) system and propose a low-complexity hybrid block diagonalization (Hy-BD) scheme to approach the capacity performance of the traditional BD processing method. We aim to harvest the large array gain through the phase-only RF precoding and combining and then digital BD processing is performed on the equivalent baseband channel. The proposed Hy-BD scheme is examined in both the large Rayleigh fading channels and millimeter wave (mmWave) channels. A performance analysis is further conducted for single-path channels and large number of transmit and receive antennas. Finally, simulation results demonstrate that our Hy-BD scheme, with a lower implementation and computational complexity, achieves a capacity performance that is close to (sometimes even higher than) that of the traditional high-dimensional BD processing.

Opportunistic Multiple Relay Selection With Outdated Channel State Information

The quality of channel state information (CSI) plays a significant role in the performance of relay selection (RS) schemes in cooperative communications. Particularly in a highly mobile environment when there exists delay between the selection and data transmission instants, the quality of CSI often degrades, given to what is known as outdated CSI phenomena. In this paper, we first outline several single RS schemes that achieve full diversity when using perfect CSI and prove that the diversity order of these schemes reduces to unity if outdated CSI exists. To deal with the performance deterioration due to the outdated CSI phenomena, we propose a new multiple RS scheme for both amplify-and-forward (AaF) and decode-and-forward (DaF) relay systems, namely, the plus normalized threshold opportunistic RS . In particular, we first opportunistically select best relays from all candidate relays with respect to end-to-end signal-to-noise ratios (SNRs) for AaF and relay-to-destination SNRs for DaF. Then, the ratios of the SNRs on the rest of the candidate relays to that of the th highest SNR are tested against a normalized threshold , and only those relays passing this test are selected in addition to the best relays. The performance of the scheme is theoretically analyzed in terms of outage probability, and its asymptotic diversity order is also examined. In addition, we outline a distributed selection protocol that makes no assumption of the global CSI at each relay. Numerical results confirm the analysis and show the advantage of the proposed scheme against existing counterparts in highly dynamic networks.

Comparison of Frequency Offset and Timing Offset Effects on the Performance of SC-FDE and OFDM Over UWB Channels

In this paper, the effects of carrier frequency offset (CFO) and sampling time offset (STO) on the performance of single-carrier block transmission with frequency-domain equalization (SC-FDE) and orthogonal frequency-division multiplexing (OFDM) over ultrawideband (UWB) channels are investigated. The signal-to-interference-plus-noise ratio (SINR) of SC-FDE in the presence of CFO is derived and compared with that of OFDM. The effects of CFO on the bit error rate (BER) performance of both systems are also simulated and compared. Two forms of STO are considered, i.e., a constant timing shift from its optimum sampling timing instant and a random timing jitter. We show through analysis that although SC-FDE is reasonably robust to a constant timing offset, it is fairly sensitive to random timing jitter. The BER performances of SC-FDE in the presence of a constant STO and random timing jitter are simulated and compared with OFDM.

Power Allocation for Multi-Pair Massive MIMO Two-Way AF Relaying With Linear Processing

In this paper, we consider a multi-pair two-way amplify-and-forward relaying system where multiple sources exchange information via a relay node equipped with large-scale antenna arrays. Given that channel estimation is non-ideal, and that the relay employs either maximum-ratio combining/maximum-ratio transmission (MRC/MRT) or zero-forcing reception/zero-forcing transmission (ZFR/ZFT) beamforming, we derive two corresponding closed-form lower bound expressions for the ergodic achievable rate of each pair sources. The closed-form expressions enable us to design an optimal power allocation (OPA) scheme that maximizes the sum spectral efficiency under certain practical constraints. As the antenna array size tends to infinity and the signal-to-noise ratios become very large, asymptotically OPA schemes in simple closed-form are derived. The capacity lower bounds are verified to be accurate predictors of the system performance by simulations, and the proposed OPA outperforms equal power allocation (EPA). It is also found that in the asymptotic regime, when MRC/MRT is used at the relay and the link end-to-end large-scale fading factors among all pairs are equal, the optimal power allocated to a user is inverse to the large-scale fading factor of the channel from the user to the relay, while OPA approaches EPA when ZFR/ZFT is adopted.