Chuan Li

Spectrum aggregation based spectrum allocation for cognitive radio networks

In cognitive radio networks, the available spectrum holes are usually discontinuous. Hence, it is hard to exploit these spectrum holes because the bandwidth of an individual one may not be able to satisfy the wide bandwidth requirement imposed by secondary users (SUs). Spectrum aggregation (SA) enables SUs to integrate several spectrum holes into one channel with wide bandwidth which may support high bandwidth requirement. In this paper, we investigate the problem of SA based spectrum allocation. Specifically, we cast this problem into Multiple Knapsack Problems (MKP). Based on this scheme, we propose a spectrum aggregation algorithm to maximize the available bandwidth that SUs can access, as well as two spectrum allocation algorithms including an optimal algorithm and a suboptimal one. The proposed algorithms are evaluated in terms of total available bandwidth and spectrum utilization efficiency. Numerical results show that the proposed algorithms can significantly outperform the existing algorithms.

Robust uniform channel decomposition for MIMO communications

In point to point MIMO systems, uniform channel decomposition (UCD) has been proven to be optimal in BER performance and strictly capacity lossless when perfect channel state information (CSI) are assumed to be available at both the transmitter and receiver side. However, in practice, CSI is always contaminated by channel estimation error. In this paper, we proposed a novel Robust UCD scheme which is capable of improving the BER and capacity performance in the context of imperfect CSI compared with the conventional UCD scheme. We also derived the capacity lower bound of the MIMO channel using the Robust UCD scheme with channel estimation error.

An Improved Channel Estimation Algorithm for OFDM Systems with Virtual Carriers

In this paper, we propose an improved channel estimation algorithm for orthogonal frequency division multiplexing (OFDM) systems with virtual carriers. As conventional estimator can not estimate the channel transfer function (CTF) at virtual carriers, resulting in channel energy leakage after inverse discrete Fourier transform (IDFT), time-domain filtering method is not directly applicable. To circumvent this problem, we derive least squares (LS) method to estimate the CTF at virtual carriers by using the assumption of limitation of channel impulse response (CIR). Further, by exploiting the noise correlation between signal subspace and noise subspace, we use maximum a posterior (MAP) criterion to estimate the noise in signal subspace and then suppress the estimation error brought by it without the knowledge of channel statistical parameters. In addition to the training mode, the proposed method can also be extended and used in the tracking mode with decision-aided feedback. Simulation results show that the improved method is free of symbol error rate (SER) floor and its SER attains 2dB improvement in comparison with that of regular LS estimator.

Geometric-Mean-Decomposition-Based Optimal Transceiver for a Class of Two-Hop MIMO-Aided Amplify-and-Forward Relay Channels

A geometric mean decomposition (GMD)-based joint transceiver is investigated in the context of a multiple-input-multiple-output (MIMO)-aided relay-assisted two-hop amplify-and-forward relaying system. The canonical form of the optimal weighting matrix is derived, which is capable of achieving the maximum received signal-to-noise ratio (SNR) at the destination. Furthermore, the optimal power allocation is formulated as a geometric programming problem. The proposed optimal design of the linear weighting matrix achieves an approximately 5-dB SNR gain over the naive weighting matrix at the bit error rate (BER) of 10-3 as a benefit of efficiently exploiting the low-complexity linear signal processing capability of the relay.

Maximum-SNR Optimal Weighting Matrix for Multiple Relay-Antenna-Assisted Orthogonal Space Time Block Codes

An optimal linear weighting matrix is proposed for nonregenerative multiple relay-antenna-assisted orthogonal space time block codes, which results in the maximum received signal-to-noise ratio (SNR) at the destination. The proposed optimal design of the linear weighting matrix achieves an approximately-15-dB SNR gain over the naive weighting matrix at the bit error rate (BER) of 10-3 as a benefit of efficiently exploiting the diversity gain provided by the multiple relay antennas.

Achieving Maximum Degrees of Freedom of Two-Hop MIMO Alternate Half-Duplex Relaying System For Linear Transceivers: A Unified Transmission Framework for DF and AF Protocols

In this paper, a novel unified transmission framework in the context of both decode-and-forward (DF) and amplify-and-forward (AF) protocols was proposed for a two-hop multiple-input-multiple-output (MIMO) alternate half-duplex (HD) relaying system with M antennas for each node, which can achieve the maximum M degrees of freedom (DoFs) of the system, compared with the existing interference alignment (IA) schemes that can only get 3M/4 DoF. Furthermore, the proposed schemes require only two relays against three relays invoked in the existing IA schemes. Moreover, the proposed schemes are valid for the cases where the number of antennas M can be either even or odd, whereas the existing IA schemes can only apply for an even number of antennas M.

Interference Alignment for VFDM Based Uplink Transmission in Two-Tiered Networks

In this work, we study the problem of interference in a two-tiered uplink network which contains a macro-cell and multiple small-cells. We null the interference from small-cells to macro-cell by using Vandermonde-subspace frequency division multiplexing (VFDM). VFDM can exploit frequency selectivity and null space generated by the cyclic prefixes (CP) used by the macro-cell communication. We use an interference alignment (IA) scheme with an extension of a grouping method to align the interference between multiple small-cells into a lower dimensional-subspace, by using this grouping method we reduce the number of receive antennas required to null interference. Finally, we derive the achievable degrees of freedom (DoF).