Chunlin Yan

Receiver Design for Downlink Non-Orthogonal Multiple Access (NOMA)

Non-orthogonal multiple access (NOMA) is one of the promising radio access techniques for further cellular enhancements toward 5G. Compared to orthogonal multiple access (OMA) such as orthogonal frequency-division multiple access (OFDMA), large performance gains were confirmed via system-level simulations. However, NOMA link-level simulations and the design of the receiver remain of great importance to validate NOMA performance gains. In this paper, we evaluate downlink NOMA link-level performance with multiple receiver designs and propose a novel NOMA transmitter and receiver design, where the signals of multi-users are jointly modulated at transmitter side and detected at receiver side. The predominant advantage of the proposed scheme is that at receiver side interference cancellation to the interference signal is not needed, thus low complexity is achieved. The performances of codeword-level SIC, symbol-level SIC and the proposed receiver are evaluated and compared with ideal SIC. Simulation results show that compared with ideal SIC, downlink NOMA link-level performance depends on actual receiver design and the difference in the power ratio split between the cell edge user and cell center user. In particular, it is shown that codeword-level SIC and the proposed receiver can both provide a good performance even when the power ratio difference between the cell center user and cell edge user is small and with real channel estimation.

A New THP Precoding Scheme with Effective Channel Optimization

Recently there is a trend to use Tomlinson-Harashima precoding (THP) to achieve better performance with full channel state information known at transmit side. In general, performance of THP precoding is governed by the the minimum absolute diagonal elements of the triangle matrix which is derived from QR decomposition to the channel matrix. In this paper we propose a new THP precoding scheme in which codebook based precoding is applied to optimize the effective channel after THP precoding with the aim of maximizing the minimum absolute diagonal elements of the triangle matrix. As a result, about 4 to 8 dB gains are obtained by employing codebooks of different sizes. The merits of the proposed scheme are low complexity, flexible tradeoff between complexity and performance by employing codebook with different sizes, and friendly to hardware implementation because of its fixed complexity. The theory analysis on the BER performance of THP precoding scheme is provided in this paper which is close to the simulation results. Numerical simulation results verify our conclusions.

A new THP precoding scheme with reduced data streams

Tomlinson-harashima precoding (THP) is one kinds schemes which is widely used to improve system performance or alleviate the complexity at receiver side with the assumption full channel state information known at transmitter. In THP precoding scheme bit error rate performance is always determined by the minimum square value of the diagonal elements of matrix R over the noise ratio where R is the triangle matrix of QR decomposition to the channel matrix. In this paper we propose a scheme in which number of data streams is reduced so as to obtain a larger minimum absolute diagonal elements of R , hence we obtain better bit error rate performance with even higher order modulation applied. In addition, the bit error rate formula of THP precoding is derived in this paper which is close to the simulation results. It can be applied for assessment of the THP precoding schemes with different data streams and modulation orders. Numerical simulation results verify the feasibility of the proposed scheme.

An AM-STC(Adaptive Modulation & Space-Time Coding) Scheme for Future Mimo Systems

Some flexible space-time codes such as thread algebraic space-time (TAST) can achieve the optimal multiplexing-diversity tradeoff. And TAST is very flexibility in adaptively adjusting its code rate. By exploiting this characteristic, we propose a novel link-adaptation scheme called AM-STC (adaptive modulation and space-time coding) scheme, which is an integration of adaptive modulation and flexible STC based on TAST code. By combining two schemes, we can get wider link adaptation against dynamic channel variation. Compared with conventional method where only adaptive modulation is used, it is clarified that our scheme has more flexibility and higher spectrum efficiency under the same BER environment

Novel hierarchical modulation scheme for multi-user transmission in relay systems

In relay systems, the quality of the received signal at a mobile user is quite different from that at a relay node making it a good application scenario for hierarchical modulation. In conventional multi-user transmission schemes for relay systems where hierarchical modulation is applied, high level power is always assigned to the same mobile user, at both the source and relay node. The performance of one mobile user is always superior to that of the other; therefore, the performance of mobile users is unbalanced. In this paper, we propose a novel multiuser transmission scheme that implements different hierarchical modulation signal in a relay node to achieve balanced bit error rate performance among the mobile users. The strategy we employ is that when high power is allocated to a mobile user at a base station, low power is allocated to the hierarchical modulation signal of the same mobile user at the relay node. Hence, the strengths of the signals at different mobile users, which are a combination of the signals from the base station and relay node, are comparable. Then mobile users can achieve excellent performance simultaneously, and a balanced performance can be obtained by using proposed scheme. Simulation results show effectiveness of proposed scheme.

Precoding for uplink multiuser MIMO with limited feedback

This paper investigates the optimization of precoders for uplink multi-user wireless communication systems, where the transmitters and receiver are equipped with multiple antennas. It is assumed that the centralized receiver has perfect knowledge of each uplink channel and designs the precoders for all users. The precoders are quantized to the vectors of a codebook, and the precoder vector index (PVI) is sent to each user through a limited channel. Since interference exists among users, the traditional single-user optimization-based precoding scheme exhibits poor performance. To improve the multi-user uplink sum-rate performance, a scheme incorporating an iterative multiple user PVI selection algorithm is proposed that iteratively projects one user channel into the null space of the precoded channels of other users, and obtains the PVI from the projected space. Simulation results show significant performance improvement compared to the conventional scheme. We also show that the proposed scheme achieves performance near to that for an exhaustive search, and outperforms the scheme that directly quantizes the precoders calculated from un-quantized joint optimization algorithms.

An Adaptive Transmission Scheme Robust to Variant Moving Speeds

Coherent space-time codes perform not so excellent in high moving speed case due to the inaccurate channel estimation. However, differential space-time code is appealing in this case because no channel estimation is needed. While in low and middle signal-to-noise ratio regions differential space-time codes are inferior to the coherent ones. To achieve an excellent performance in a various moving speed environments, we develop an adaptive transmission scheme in this paper where coherent space-time code is applied in low and middle moving speed cases, while differential space-time code is used in high moving speed case. Because the proposed scheme adopts the space-time codes wisely according to the moving speed, it outperforms the scheme which purely utilizes coherent space-time code or differential space-time code and thus is suitable for various moving speed environments. Moreover, two differential space-time codes based on differential Alamouti code are proposed in this paper with the merits of low decoding complexity, suitable for general number of transmit antennas and no code rate loss. The effectiveness of the proposed scheme and the proposed differential space-time codes is supported by numerical simulations.

A Flexible Tree Searching Scheme for MIMO Detection

Detection is one of the key technologies in multiple-input multiple-output system. In this paper we propose a new tree searching scheme which is flexible in the tradeoff between performance and complexity. Sorted QR decomposition based on minimum mean square error (MMSE) criteria is applied as a preprocessing to make the channel easier for tree searching and Fano bias, which is adjustable to achieve a good tradeoff between performance and complexity, is applied in the radius setting for different layer to more efficiently prune the leaves. Much smaller complexity is needed by the proposed scheme with negligible performance degradation compared with that of sphere decoding scheme, especially in the scenarios of larger number of transmitter antennas and high order modulation. Simulations accommodate our conclusions.