Jianzhong Charlie Zhang

Modeling and Analyzing the Coexistence of Wi-Fi and LTE in Unlicensed Spectrum

We leverage stochastic geometry to characterize key performance metrics for neighboring Wi-Fi and LTE networks in unlicensed spectrum. Our analysis focuses on a single unlicensed frequency band, where the locations for the Wi-Fi access points and LTE eNodeBs are modeled as two independent homogeneous Poisson point processes. Three LTE coexistence mechanisms are investigated: 1) LTE with continuous transmission and no protocol modifications; 2) LTE with discontinuous transmission; and 3) LTE with listen-before-talk and random back-off. For each scenario, we derive the medium access probability, the signal-to-interference-plus-noise ratio coverage probability, the density of successful transmissions (DST), and the rate coverage probability for both Wi-Fi and LTE. Compared with the baseline scenario where one Wi-Fi network coexists with an additional Wi-Fi network, our results show that Wi-Fi performance is severely degraded when LTE transmits continuously. However, LTE is able to improve the DST and rate coverage probability of Wi-Fi while maintaining acceptable data rate performance when it adopts one or more of the following coexistence features: a shorter transmission duty cycle, lower channel access priority, or more sensitive clear channel assessment thresholds.

Cooperative communications for LTE-advanced—relay and CoMP†

SUMMARY n nThe Long Term Evolution-Advanced (LTE-A) system is currently under development to allow for significantly higher spectral efficiency and data throughput than the LTE systems. In a wireless system based on orthogonal frequency division multiplexing with frequency reuse factor one, the achievable cell spectral efficiency is often limited by the inter-cell interference or coverage shortage of base stations. In LTE-A, coordinated multi-point transmission/reception (a.k.a. multi-cell MIMO or base station cooperation) and relaying technologies are being introduced to clear these major performance hurdles. In this paper, cooperative communication technologies being discussed in LTE-A systems are presented, together with considerations on system design. Copyright

DoA estimation and capacity analysis for 2D active massive MIMO systems

Mobile data traffic is expected to have an exponential growth in the future. In order to meet the challenge as well as the form factor limitation on the base station, two-dimensional (2D) “massive MIMO” has been proposed as one of the enabling technologies for future wireless systems. In 2D “massive MIMO” systems, a base station will rely on the uplink sounding signals to figure out the downlink spatial channel information to perform MIMO precoding. Accordingly, direction-of-arrival (DoA) estimation of the underlying three-dimensional (3D) channel at the base station becomes essential for 2D “massive MIMO” systems to realize the predicted capacity gains. In this paper, we will analyze the performance of DoA estimation based on ESPRIT methods and study its impact on the capacity of 2D “massive MIMO” systems. To be specific, for ESPRIT-type algorithms, we will derive the closed-form expressions for the mean square errors of the elevation and azimuth angle estimations. These results will be used to obtain design intuitions for 2D antenna arrays at the base station as well as the capacity of the underlying 2D “massive MIMO” systems.

3D channel model in 3GPP

Multi-antenna techniques capable of exploiting the elevation dimension are anticipated to be an important air-interface enhancement targeted to handle the expected growth in mobile traffic. In order to enable the development and evaluation of such multi-antenna techniques, the 3rd Generation Partnership Project (3GPP) has recently developed a three-dimensional (3D) channel model. The existing two-dimensional (2D) channel models do not capture the elevation channel characteristics, making them insufficient for such studies. This article describes the main components of the newly developed 3D channel model and the motivations behind introducing them. One key factor is the ability to model channels for users located on different floors of a building (at different heights). This is achieved by capturing a user height dependency in modelling some channel characteristics including pathloss, lineof- sight (LOS) probability, etc. In general, this 3D channel model follows the framework of WINNERII/WINNER+ while also extending the applicability and the accuracy of the model by introducing some height dependent and distance dependent elevation related parameters.

Reduced complexity precoding and scheduling algorithms for full-dimension MIMO systems

Full-dimension multiple-input multiple-output (FD-MIMO) systems, in which base stations are equipped with a large number of antennas in a two-dimensional panel, has received considerable attention from academia researchers and industry practitioners. Compared with legacy cellular communication systems, FD-MIMO systems can achieve significantly higher spectral efficiency with high order multi-user MIMO (MU-MIMO) transmissions. However, as high-order MU-MIMO also incurs high precoding and scheduling complexity, it is critical to reduce complexity of these operations in order to realize throughput potential of FD-MIMO systems in practice. In this paper, we propose a reduced complexity algorithm to realize the high performance precoding technique, signal-to-leakage plus noise ratio (SLNR) precoding, and propose an efficient scheduling algorithm to enable high-order MU-MIMO transmissions in FD-MIMO systems. We further demonstrate the effectiveness of the proposed algorithms by using system level simulations.

3D MIMO Outdoor to Indoor Macro/Micro-Cellular Channel Measurements and Modeling

3-dimensional Multiple-Input Multiple-Output (3D MIMO) systems have received great interest recently because of the spatial diversity advantage and capability for full-dimensional beamforming, making them promising candidates for practical realization of massive MIMO. For 3D MIMO system design, it is important to have full characterization of the 3D MIMO propagation channel, i.e., characterize both the elevation and azimuth characteristics of the wireless propagation channel, especially at the base station end. In this paper, we present first results from a measurement campaign for obtaining these characteristics. For those measurements we use a hybrid switched/virtual cylindrical array with 480 antenna elements at the base station (BS), and a switched array with 24 antenna elements at the user equipment (UE). We perform outdoor-to-indoor (O2I) channel measurements in an urban macro-cellular (UMa) and a micro-cellular (UMi) environments. We provide the elevation and azimuth angular spreads at the transmitter and receiver, and study their dependence on the UE height. With the increase in the UE height, the BS elevation spreads decreased from 1.24 deg to 1 deg and 1.15 deg to 0.78 deg respectively for UMa and UMi; the azimuth spreads remained approximately the same (7.5 deg). Based on the measurements done with the UE placed on different floors, we study the feasibility of separating users in the elevation domain. Users were separable in 44% and 54.17% scenarios respectively for the UMa and UMi environments.

3D channel models for elevation beamforming and FD-MIMO in LTE-A and 5G

3rd generation partnership project (3GPP) has recently published a technical report on 3-dimension (3D) channel models [1]. This paper summarizes deployment scenarios and evaluation methodologies of the newly developed 3D channel models, including antenna, pathloss and fast-fading modeling. In addition, recent development of the full-dimension multi-input-multi-output (FD-MIMO) study in 3GPP is also discussed.

Cluster-based analysis of 3D MIMO channel measurement in an urban environment

Massive MIMO (multiple - input - multiple - output) and full-dimensional MIMO systems in next-generation cellular communications systems as well as high-data-rate military systems have garnered considerable attention recently. For the assessment of their performance, knowledge of the 3D propagation channel characteristics, i.e., azimuth and elevation of the multipath components (MPCs) is essential. In this paper, we present first results of a 3D outdoor propagation channel measurement campaign performed in an urban macro-cellular environment. The measurements were performed with a 20 MHz wideband polarimetric MIMO channel sounder centered at 2.53 GHz. At each measurement location, parameters of all MPCs observed were extracted with RIMAX, an iterative maximum likelihood high-resolution algorithm. It was observed that MPCs naturally grouped into clusters. We then present a cluster-based analysis of the propagation channel providing some results of the intra and inter cluster parameters and their relevant statistics. Correlation between all extracted cluster parameters are also provided. Parameters such as elevation and azimuth spread (at the base-station) in this work have been used as input to recent international channel model standardization.

DoA estimation and capacity analysis for 3D massive-MIMO/FD-MIMO OFDM system

With the promise of meeting future capacity demands for mobile broadband communications, 3D massive-MIMO/Full Dimension MIMO (FD-MIMO) systems have gained much interest among the researchers in recent years. Apart from the huge spectral efficiency gain offered by the system, the reason for this great interest can also be attributed to significant reduction of latency, simplified multiple access layer, and robustness to interference. However, in order to completely extract the benefits of massive-MIMO systems, accurate channel state information is very critical. In this paper, a channel estimation method based on direction of arrival (DoA) estimation is presented for massive-MIMO OFDM systems. To be specific, the DoA is estimated using Estimation of Signal Parameter via Rotational Invariance Technique (ESPRIT) method, and the capacity of the system is analytically characterized. It has been shown that optimum precoding matrix for the massive MIMO OFDM system can be constructed from the DoA estimation at the base station.

Performance analysis of millimeter-wave cellular networks with two-stage beamforming initial access protocols

A two-stage beamforming (BF) approach is proposed for millimeter wave (mmWave) cellular networks, wherein each base station (BS) first obtains wide beams to its users during the initial access (IA), then refines to narrow beams during the beam refinement phase (BRP). The performance of the proposed approach is derived under three IA protocols. Compared to a single-stage approach where beam searching is solely finished during IA, the two-stage BF approach is shown to significantly increase the user-perceived throughput (UPT) for the exhaustive search IA protocol, and it provides a flexible framework to achieve good IA delay and UPT performance simultaneously.