Xiaojia Lu

Comparison of Antenna Arrays in a 3-D Multiuser Multicell Network

A three-dimensional (3D) multi-cell multi-user system model along with spatial array processing are presented and studied in this paper. Most of the radio propagation in the literature has been studied and modeled in a two dimensional plane. The 3D propagation channel modeling has drawn attentions recently. This is due to the fact that, in some scenarios, the assumption of small elevation domain angular spread does not hold. With multiple antenna elements displaced in a 3D space, e.g., uniform linear array (ULA) or uniform planar array (UPA), the base station (BS) is capable of forming beams in vertical domain. This is particularly interesting for the interference limited scenario, since the 3D array processing provides another degree of freedom to transmit signal and eliminate the interference. Using the 3D multicell channel model, the impact of elevation domain angular spread and antenna analog beam pattern on the base station antenna design, digital beamforming and power control are studied.

Mobile transmitter digital predistortion: Feasibility analysis, algorithms and design exploration

This article addresses intermodulation challenges in carrier aggregation (CA) and multicluster type transmission scenarios in mobile transmitters. In such transmission schemes, emerging in 3GPP LTE-Advanced mobile cellular radio evolution, the spectrum of the signal entering the transmit power amplifier (PA) is of non-contiguous nature and thus severe intermodulation is created which may violate the spurious emission mask. To satisfy the stringent emission requirements and limits, devices may need to considerably back off their transmit power, compared to nominal value of ;23dBm, but this will reduce the uplink coverage. As an alternative, feasibility of digital predistortion (DPD) is explored in this article. A DPD solution is developed to control the most critical intermodulation components from terminal emission mask perspective with reduced complexity compared to conventional DPD solutions. The DPD is based on the Augmented Parallel Hammerstein (APH) architecture which can handle IQ imbalance and local oscillator leakage in addition to the PA nonlinearity while using simple digital linear estimation techniques. Furthermore, digital design exploration is carried out for the predistortion algorithm, implying that the needed computational resources are close to what is already available in most advanced mobile platforms and chipsets in the market.

Joint power control, receiver beamforming and adaptive multi base station coordination for uplink wireless communications

We concentrate on the sum power minimization problem for the uplink of wireless cellular systems in this paper. Given a certain quality-of-service (QoS) constraint per mobile station (MS), the optimal power control and receiver beamforming are jointly considered with adaptive multi base station set (MBS) selection for joint reception. An iterative optimization algorithm is proposed accordingly. Although the problem is non-convex in general, it can always be optimally solved via the proposed algorithm as long as it is feasible. By changing the size of MBS, the system performance in terms of the transmit power over all MSs is evaluated through system level simulations. The results show that the transmit power can be significantly reduced even with a small size MBS, which is of a great interests for uplink communications.

Low power implementation of digital predistortion filter on a heterogeneous application specific multiprocessor

Power-constrained mobile radio communication transmitters drive their transmit power amplifiers close to their saturation regions, which results in nonlinear intermodulation distortion that is especially harmful in multi-cluster and carrier aggregation transmission scenarios. Digital predistortion is a method for linearizing the transmitter and suppressing the most harmful spurious emissions at the transmitter power amplifier output. This paper describes a programmable implementation of a digital predistortion filter on a heterogeneous Transport Trigger Architecture (TTA) multiprocessor. The predistortion algorithm is based on a parallel Hammerstein polynomial model and the experimental results show that the proposed programmable architecture is capable of linearizing a 20 MHz LTE carrier in realtime with a power consumption that is suitable for mobile devices.

Adaptive Coordinated Reception for Multicell MIMO Uplink

We study the coordinated reception amongst multiple base stations (BS) in the uplink cellular system, where each mobile station (MS) is served by an associated multiple BS set (MBS). Under the individual signal-to-interference-plus-noise ratio (SINR) constraint per MS, power control and receive beamforming are jointly optimized with adaptive MBS selection to minimize the total transmit power. An iterative optimization algorithm is presented accordingly. The joint optimization problem is nonconvex in general, but it can be optimally solved by the proposed algorithm as long as it is feasible. To find the optimum, the proposed algorithm requires the exhaustive search over all BSs per MS. To reduce the complexity in the large-scale cellular network, we propose simplified schemes where a subset of BSs is preselected based on the large-scale fading factors. By limiting the search in the subset, the complexity is reduced significantly. Although the obtained power vector with the simplified algorithm is not optimal, a performance close to the macronetwork full coordination can be achieved by carefully choosing the sizes of the pre-selected BS subset and MBS. The significant advantage is proven by simulations.

LTE uplink power control and base station antenna down tilt in a 3D channel model

This paper examines the impact of the uplink power control and base station (BS) antenna down tilt on the system level performance in a realistic multicell three dimensional channel model. The 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) adopts single carrier frequency division multiple access (SC-FDMA) as an air interface. The intra-cell interference is thus avoided but the inter-cell interference is still an issue. The system level performance depends not only on the transmitted power but also on the inter-cell interference level. Uplink power control scheme is used to limit the mobile station (MS) transmit power in order to reduce the interference. To evaluate the system performance, a realistic propagation environment is essential. We extend the IST WINNER II channel model (WIM II) in the elevation domain and includ the elevation beam pattern. We show mathematical expressions of link loss related to the BS-MS distance, down tilt angle, link loss, beam gain, and the effect of MS open-loop-controlled power. Based on the 3D channel model, the impact of BS antenna down tilt angle effect along with open loop power control scheme is studied.

A Novel Scheme for Sum Power Minimization in MIMO MAC Multicarrier Systems

The sum power minimization (SPmin) subject to per user rate constraint problem in the multiple access channel (MAC) of a multiple-input multiple-output (MIMO) multicarrier system is investigated. The SPmin can be transformed into a series convex problems and solved iteratively. The conventional solutions requires a large number of iterations. Furthermore, though in each iteration the optimality can be achieved by the efficient interior-point method, the slow convergence speed is still a burden that makes the calculation with large numbers of users prohibitively complex. We propose an efficient algorithm to find the optimal solution of the SPmin problem. The algorithm utilizes the convexity of the rate region to reduce the number of iterations. The convergence behavior of the algorithm is compared to the existing methods and shown to be much faster.

Closed loop transmission for LTE system downlink

This paper studies the downlink closed loop transmission in the 3G long term evolution (LTE) system. We propose a precoding matrix selection algorithm from a finite codebook. The precoding, antenna selection, adaptive modulation and coding (AMC) require the channel information fed back from receiver to optimize the system performance. We study the codebook design and precoding matrix selection in LTE system, particularly the single user (SU) in a fading channel scenario. As a comparison to precoding, the antenna subset selection is investigated. The link adaption based on the instantaneous signal-to-noise ratio is also studied. The proposed rank and link adaption algorithm select the precoding matrix and AMC that maximize the spectral efficiency under a certain frame error rate constraint.

Lattice Reduction Based Detection Algorithms in High Correlated MIMO-OFDM System

The lattice reduction (LR) aided detectors for multiple-input multiple-output (MIMO) systems have been shown to have a better performance than conventional linear detectors like zero forcing (ZF) or minimum mean square (MMSE) detectors. In this paper, we study the performance of the LR aided detectors for multiple input multiple output orthogonal frequency division multiplexing (MIMO-OFDM) systems in spatially correlated fading channels. Also, the computational complexity of the LR based linear detectors is studied with comparison to the conventional detectors. The simulation results show that the LR aided detectors are more robust to the spatial correlation than the conventional linear detectors and also have a low computational complexity close to the conventional linear detectors

Uplink power control, subcarrier allocation and beamforming in coordinated multicell systems

We propose a joint bit and power loading, beamforming, multiple base station set (MBS) selection and subcarrier allocation algorithm for a resource allocation problem in a uplink multicell coordinated multi-basestation reception system. The objective is to minimize the total transmit power with rate constraint per mobile station (MS). The resources that we are considering include power, subcarrier, beamforming and the MBS set. The algorithm iteratively loads bits to the best link and subcarrier for the current rate increment and updates the signal-to-interference-plus-noise ratio (SINR) targets of all MSs on that particular subcarrier. Given a fixed rate target per subcarriers, the algorithm gives the optimal solution, i.e, optimal power vector, beamforming vector and MBS set. The iteration continues until all MSs reach their rate targets. We also propose a simplified scheme of such an algorithm. The simulation results show that the proposed algorithms perform significantly better than the simple approach without considering subcarrier allocation, i.e., equal rate allocation over the frequency spectrum, especially in a strong interference limited scenario.