Mengmeng Liu's research while affiliated with Beijing University of Posts and Telecommunications and other places

With the communication services evolution from the fourth generation (4G) to the fifth generation (5G), we are going to face diverse challenges from the new network systems. On the one hand, seamless handoff is expected to integrate universal access among various network mechanisms. On the other hand, a variety of 5G technologies will complement each other to provide ubiquitous high speed wireless connectivity. Because the current wireless network cannot support the handoff among Wireless Access for Vehicular Environment (WAVE), WiMAX, and LTE flexibly, the paper provides an advanced handoff algorithm to solve this problem. Firstly, the received signal strength is classified, and the vehicle speed and data rate under different channel conditions are optimized. Then, the optimal network is selected for handoff. Simulation results show that the proposed algorithm can well adapt to high speed environment, guarantee flexible and reasonable vehicles access to a variety of networks, and prevent ping-pong handoff and link access failure effectively.

The pilot design problem in large-scale multi-input-multioutput orthogonal frequency division multiplexing (MIMO-OFDM) system is investigated from the perspective of compressed sensing (CS). According to the CS theory, the success probability of estimation is dependent on the mutual coherence of the reconstruction matrix. Specifically, the smaller the mutual coherence is, the higher the success probability is. Based on this conclusion, this paper proposes a pilot design algorithm based on alternating projection and obtains a nonorthogonal pilot pattern. Simulation results show that applying the proposed pattern gives the better performance compared to applying conventional orthogonal one in terms of normalized mean square error (NMSE) of the channel estimate. Moreover, the bit error rate (BER) performance of the large-scale MIMO-OFDM system is improved.

In the last decade, the nonstationary properties of channel models have attracted more and more attention for many scenarios, that is, vehicle-to-vehicle (V2V), mobile-to-mobile (M2M), and high-speed train (HST). However, little research has been done on the real-physical channel model. In this paper, we propose a generalized three-dimensional (3D) nonstationary channel model, in which the scatterers are assumed to be distributed around the transmitter (Tx) and receiver (Rx) on a two-sphere model. By employing the von Mises-Fisher distribution, the mean values of the azimuth angle of departure (AAoD) and elevation angle of departure (EAoD) and the azimuth angle of arrival (AAoA) and elevation angle of arrival (EAoA) are tracked by time-variant (TV) Brownian Markov (BM) motion paths, which ensure the nonstationarity of the proposed channel model. Moreover, the TV autocorrelation function (ACF) and Doppler power spectrum density (DPSD) of the proposed nonstationary channel model are calculated by using signal processing tools, for example, fast Fourier transform (FFT) and short-time Fourier transform (STFT). In addition, the simulation results show that the TV scatterer distribution results in a nonstationary nonisotropic channel model, and the proposed model can be employed to simulate the 3D nonstationary channel model.

This paper analyzes the performance of a large-scale multiuser multiple-input multiple-output (MU-MIMO) system with amplify-and-forward (AF) relaying in the presence of co-channel interference. Each user terminal (UT) has a single antenna while the relay is equipped with very large antenna arrays. To evaluate the impact of large number of relay antennas on the performance of the considered system, we derive a lower bound on the achievable rate. We also perform a large system analysis in the regimes of high signal-to-noise ratio (SNR), large number of relay antennas and large number of UTs. Numerical results are presented to verify the theoretical analysis.

This paper analyzes the performance of a large-scale multiuser multiple-input multiple-output (MU-MIMO) system with amplify-and-forward (AF) relaying in the presence of co-channel interference. Each user terminal (UT) has a single antenna while the relay is equipped with very large antenna arrays. To evaluate the impact of large number of relay antennas on the performance of the considered system, we derive a lower bound on the achievable rate. We also perform a large system analysis in the regimes of high signal-to-noise ratio (SNR), large number of relay antennas and large number of UTs. Numerical results are presented to verify the theoretical analysis.

In multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems, timing synchronization is important in order to find the correct start of OFDM frames and symbols at the receiver. In this paper, a joint transmit antenna selection and maximum ratio combining at the receiver (ST/MRC) are proposed to improve the timing synchronization performance of MIMO-OFDM systems. In this scheme, only the antenna with the highest channel power is selected at the transmitter. The timing metrics of all receive antennas are added together to realize MRC. In order to theoretically investigate the performance of the proposed scheme, the timing metric based on a time-domain repetitive synchronization sequence structure is adopted and the closed-form expression for the correct timing probability in the flat fading channel is derived. Then the proposed ST/MRC scheme for timing synchronization is applied in multipath fading channels. The advantage of the proposed scheme for timing synchronization is verified by simulations.

We consider a multipair two-way relay network where multiple communication pairs simultaneously exchange information with the help of a single relay. Each terminal has only a single antenna, while the relay is equipped with a very large antenna array. We further assume that channel state information is available at the relay node. We investigate the power efficiency of this network when very simple signal processing, i.e., maximum ratio combining (MRC) or zero-forcing (ZF), is used at the relay. When the number of relay antennas grows infinite, the transmit power of each terminal or relay (or both) can be made inversely proportional to the number of relay antennas while maintaining a given quality-of-service. We show that with very large antenna arrays, the two-way relaying scheme outperforms both the orthogonal scheme and the one-way relaying scheme.

Cooperative multi-relay networks are asynchronous both in time and frequency inherently due to their spatial distributed nature. This paper addresses channel estimation for amplify-and-forward relay networks using training sequences in the presence of both time and frequency offsets. A practical training scheme is provided to reduce the complexity at the relay nodes and to combat the effects of time and frequency offsets at the destination. By exploiting the training scheme, a piecewise channel estimator with respect to signal-to-noise ratio (SNR) is proposed to reduce the computational complexity of the minimum mean square error (MMSE) method. Simulation results show that our method approaches the MMSE estimator and outperforms the linear least-square (LS) estimator.