Ming Zhao

Distributed wireless communication system: a new architecture for future public wireless access

The distributed wireless communication system (DWCS) is a new architecture for a wireless access system with distributed antennas, distributed processors, and distributed control. With distributed antennas, the system capacity can be expanded through dense frequency reuse, and the transmission power can be greatly decreased. With distributed processors control, the system works like a software or network radio, so different standards can coexist, and the system capacity can be increased by coprocessing of signals to and from multiple antennas.

Power Allocation in OFDM-Based Cognitive Radio Systems

In this paper, we investigate the optimal power allocation strategy that aims at maximizing the capacity in OFDM- based cognitive radio systems. We show that the traditional water-filling algorithm applied in general OFDM systems needs to be modified due to the per subchannel power constraints in such systems. An iterative partitioned water-filling algorithm is proposed and proved to be optimal based on the convex optimization theory.

Novel techniques to improve downlink multiple access capacity for Beyond 3G

In future public mobile access with high data rates, one of the main challenges we face is spectral efficiency. In this article we will focus on the following new spectrally efficient downlink multiple access techniques that may be essential parts of Chinas Beyond 3G system development: dynamic code-division multiplexing, an adaptive multi-input multi-output technique in distributed wireless communications systems, and interleaver pattern division multi-access.

Power Allocation and Time-Domain Artificial Noise Design for Wiretap OFDM with Discrete Inputs

Optimal power allocation for orthogonal frequency division multiplexing (OFDM) wiretap channels with Gaussian channel inputs has already been studied in some previous works from an information theoretical viewpoint. However, these results are not sufficient for practical system designs. One reason is that discrete channel inputs, such as quadrature amplitude modulation (QAM) signals, instead of Gaussian channel inputs, are deployed in current practical wireless systems to maintain moderate peak transmission power and receiver complexity. In this paper, we investigate the power allocation and artificial noise design for OFDM wiretap channels with discrete channel inputs. We first prove that the secrecy rate function for discrete channel inputs is nonconcave with respect to the transmission power. To resolve the corresponding nonconvex secrecy rate maximization problem, we develop a low-complexity power allocation algorithm, which yields a duality gap diminishing in the order of O(1/√N), where N is the number of subcarriers of OFDM. We then show that independent frequency-domain artificial noise cannot improve the secrecy rate of single-antenna wiretap channels. Towards this end, we propose a novel time-domain artificial noise design which exploits temporal degrees of freedom provided by the cyclic prefix of OFDM systems to jam the eavesdropper and boost the secrecy rate even with a single antenna at the transmitter. Numerical results are provided to illustrate the performance of the proposed design schemes.

Opportunistic channel selection approach under collision probability constraint in cognitive radio systems

In this work, we consider a cognitive radio system with multiple primary channels and one secondary user, and then we introduce a channel-usage pattern model and some basic concepts in this system. Based on this system model and the basic concepts, we propose two opportunistic channel selection algorithms to optimize the throughput of the secondary user: minimum collision rate channel selection algorithm and minimum handoff rate channel selection algorithm. According to the two algorithms, we, respectively, present the channel selection scheme based on minimum collision rate algorithm (CSS-MCRA) and the channel selection scheme based on minimum handoff rate algorithm (CSS-MHRA) under the constraint that the collision probability is bounded below collision tolerable level. Theoretical analysis and simulation results both show that, on one hand, both CSS-MCRA scheme and CSS-MHRA can follow the constraint of collision tolerable level; on the other hand, the performance of CSS-MCRA scheme is better than that of CSS-MHRA scheme if handoff latency is zero or very low, while the performance of CSS-MHRA scheme is better than that of CSS-MCRA scheme if handoff latency is long enough.

Energy-Efficient and Low-Complexity Uplink Transceiver for Massive Spatial Modulation MIMO

In this paper, we design the uplink transceiver for a new multiuser (MU) massive spatial modulation (SM) multiple-input-multiple-output (SM-MIMO) system over frequency-selective fading channels, where the base station (BS) is equipped with massive antennas, and user equipment (UE) has multiple transmit antennas (TAs) but only one radio-frequency (RF) chain. UE transmit data bits to the BS simultaneously by cyclic-prefix single-carrier (CP-SC) SM. For the uplink MU detection (MUD), we construct a low-complexity generalized approximate message passing detector (GAMPD), which can exploit both the sparsity and prior probability distribution of the transmitted signal and is suitable for hardware implementation because its most complex operation is only matrix-vector multiplication. Its mean square error (MSE) and uncoded bit error rate (BER) performances are also analyzed based on the state evolution (SE). Compared with stagewised linear detectors, GAMPD shows orders-of-magnitude lower complexity. Moreover, simulation results indicated that GAMPD approaches to the performance of maximum-likelihood (ML) detection and outperforms minimum MSE (MMSE) significantly. Finally, to design energy-efficient massive SM-MIMO, we propose a practical algorithm to optimize the key system parameters (e.g., the transmission power, the numbers of the BS antennas or UE, or the TAs at the UE). Numerical results indicate that low BS circuit power consumption and long channel coherence time are the two key prerequisites for the success of massive SM-MIMO.

New Lyapunov function for transient stability analysis and control of power systems with excitation control

Abstract In this paper, a direct method of transient stability analysis for power systems with excitation control is presented using Hamiltonian system theory. A modified transient energy function is regarded as Hamiltonian function which is used to formulate a type of standard Hamiltonian equation. A new Lyapunov function is then constructed, and based on it, a novel excitation control strategy is obtained directly from a nonlinear power system. Numerical results demonstrate the effectiveness of the proposed approach.

Conceptual platform of distributed wireless communication system

Research on next generation wireless communications integrated with current wireline networks is a potential issue considered worldwide. A novel wireless system, called distributed wireless communication system (DWCS), is proposed. Logically, it is divided into four layers. They are distributed antennas, a distributed fiber-optic transmission network, distributed processing network and a distributed core network. To support different radio access standards, the distributed processing network is realized with a software radio technique. Also, some new concepts, such virtual cell and virtual tunnel is proposed for DWCS. It is foreseen that the DWCS has three phases of development and how it conforms with the Internet is considered. Thus, the DWCS is a system that depends not only on wireless technologies, but also on Internet technologies. In terms of our description, the DWCS has the characteristics of an open distributed system with flexibility, scalability and expansibility.

Robust Transmission for Multiuser MIMO Downlink Systems with Imperfect CSIT

The performance of multiuser MIMO (MU-MIMO) downlink systems with preceding and decoding method by minimizing the total mean square error (TMSE) of all data streams of all users (T-MMSE) depends on the accuracy of the channel state information (CSI) available at the transmitter (CSIT). In this paper, a robust transmission scheme is proposed to suppress the interferences among data streams and users caused by imperfect CSIT. At the transmitter, the precoders are obtained through an iterative procedure by minimizing the expectation of the TMSE of all data streams of all users conditioned on the estimated CSI under a total transmit power constraint. At each user, a linear decoder is calculated based on the minimum mean square error (MMSE) criteria according to the realtime CSI. To fulfill it, a new pilot-sending architecture is introduced. Numerical results demonstrate that the proposed scheme is robust not only to the channel uncertainty at the transmitter but also to the channel uncertainty at each user, and can effectively improve the system performance.

Fault missing rate analysis of the arithmetic residue codes based fault-tolerant FIR filter design

Relative to the Triple Modular Redundancy (TMR) scheme, the arithmetic residue codes based fault-tolerant DSP design consumes much less resources. However, the price for the low resource consumption is the fault missing problem. The basic tradeoff is that, smaller modulus used for the fault checking consumes fewer resources, but the fault missing rate is higher. The relationship between the value of modulus and the fault missing rate is analyzed theoretically in this paper for fault-tolerant FIR filter design, and the results are verified by FPGA implemented fault injections.