Janet A. Stern-Berkowitz

3D channel model extensions and characteristics study for future wireless systems

This work introduces a complete framework for three-dimensional (3D) channel extension based on two-dimensional (2D) channel models. In the 3D channel modeling, besides the azimuth angles, the multipath components of a receive-transmit pair are determined by elevation angles, correlations between elevation and azimuth, and the 3D configurations of a user equipments (UE) location and velocity. The 3D channel characteristics for an active antenna system (AAS) are studied in terms of aggregated array gain and channel covariance matrix. The 3D channel model is generated and examined through Monte Carlo simulations and a comparison of fixed vertical beamforming and adaptive vertical beamforming is presented. The comparison shows that adaptive vertical beamforming can potentially provide up to 8dB improvement in coupling loss. In addition, it is shown that for line-of-sight (LoS) the 3D channel covariance matrix can be decoupled into two independent covariance matrices with reduced sizes that effectively leads to reduced feedback overhead. For non-line-of-sight (NLoS), the correlation-based Kronecker model technique can be used to generate the 3D channel matrix with reduced complexity compared to the geometry-based technique.

A dynamic subframe set power control scheme for interference mitigation in reconfigurable TD-LTE systems

In 3GPP LTE, TDD spectrum allocation for uplink (UL) and downlink (DL) use is a function of a cells TDD UL/DL configuration which designates which time resources (i.e., subframes) are for UL and which are for DL. TDD dynamic UL/DL reconfiguration, under discussion in LTE Release 12, enables cells to quickly adapt the ratio of allocated UL and DL subframes to changing traffic patterns which can be different for neighboring cells. Having different UL/DL directions for the same subframe in adjacent cells can result in new destructive interference components, i.e., eNB-to-eNB and UE-to-UE, with levels that can significantly differ from one subframe to another. In this paper, UL power control mechanisms are considered to manage such interference, where different sets of UL subframes use different UL power control parameters. In particular, two schemes are studied where, in one, fixed subframe sets are chosen based on expected interference and, in the other, the eNB monitors interference levels in the subframes and dynamically reconfigures the subframe sets accordingly. The performance of each scheme is evaluated through system level simulations and it is shown that the UL power control with dynamic UL subframe set configuration outperforms the one with static UL subframe sets, with significant improvement achieved at cell edge.

Wi-Fi-LAA coexistence: Design and evaluation of Listen Before Talk for LAA

With licensed spectrum at a premium, recent interest has shifted to investigating the possibility of utilizing unlicensed bands as a means of providing additional bandwidth. License assisted access (LAA) and Long Term Evolution-Unlicensed (LTE-U) are radio access technologies (RATs) aimed at providing carrier-grade wireless service in the 5 GHz unlicensed spectrum. Since Wi-Fi devices are widespread in the unlicensed space, it is important to ensure that any new technology deployed coexists fairly with Wi-Fi and other preexisting technologies that already occupy this space. At the heart of the Wi-Fi-LAA coexistence problem is the difference in channel access mechanisms for these two technologies; Wi-Fi is a contention based channel access scheme, while Long Term Evolution (LTE) is schedule-based. As a result, introducing an LTE system as is into the Wi-Fi space would likely result in LAA gradually taking over the channel and starving Wi-Fi of access. In this paper we focus on schemes to address this very issue. In particular, we will focus on various design aspects of Listen Before Talk (LBT) schemes for LAA that aim to emulate the contention based scheme in Wi-Fi as a means of providing equal opportunity channel access for both of these technologies. We consider and evaluate the impact of various design aspects on fairness through several key performance metrics. Our simulation results demonstrate that LBT when applied to LTE can ensure good Wi-Fi-LAA coexistence.

Enhancing LTE-Advanced network performance using active antenna systems

This work exploits the benefits of adaptive downtilt and vertical sectorization schemes for Long Term Evolution Advanced (LTE-A) networks equipped with active antenna systems (AAS). We highlight how the additional control in the elevation domain (via AAS) enables use of adaptive downtilt and vertical sectorization techniques, thereby improving system spectrum efficiency. Our results, based on a full 3 dimensional (3D) channel, demonstrate that adaptive downtilt achieves up to 11% cell edge and 5% cell average spectrum efficiency gains when compared to a baseline system utilizing fixed downtilt, without the need for complex coordination among cells. In addition, vertical sectorization, especially high-order vertical sectorization utilizing multiple vertical beams, which increases spatial reuse of time and frequency resources, is shown to provide even higher performance gains.

Reevaluating cell wraparound techniques for 3D channel model based system-level simulations

Current wireless system-level simulation frameworks utilize cell wraparound in order to reduce boundary effects and provide an accurate evaluation of system performance. The current wraparound technique is purely distance based, where UEs always pick the closest cells, ignoring the radio propagation characteristics of the channel. While this wraparound methodology works well for a 2D channel model, it fails to account for the enhancements and spatial characteristics of the 3D channel model. In this paper we investigate the use of a radio distance based wraparound scheme for 3D channel models. This new wraparound methodology uses received downlink signal power to perform wraparound cell selection, allowing it to account for the radio propagation characteristics of the 3D channel model. We compare the performance of these two wraparound techniques with respect to coupling gain and other key metrics when considering a 3D channel model. Our results demonstrate the differences between the two schemes and highlight the improved accuracy offered by the radio distance based wraparound scheme.

Efficient adaptive vertical downtilt schemes in LTE-Advanced networks

This work introduces efficient schemes for adaptive vertical downtilt beamforming in Long Term Evolution Advanced (LTE-A) networks equipped with active antenna systems (AAS). A framework to determine cell-specific downtilt angles is proposed and solutions are provided with affordable complexities for practical applications. Instead of utilizing a single fixed downtilt, a cell-specific downtilt is obtained and adaptively adjusted across the system, which not only increases signal strength for desired users but also provides better control of inter-cell interference in the system. System level simulation shows that the proposed schemes achieve up to 10% cell edge and 5% cell average spectrum efficiency gain compared to the baseline system with fixed downtilt without involving complex coordination among cells.

Subscriber based smart antenna for wireless mobile networks: design, algorithm, and field test results

The use of smart antennas has recently been considered for wireless mobile networks such as third generation (3G) wireless systems. Until now, smart antenna designs and algorithms have almost exclusively been focused on base stations. In this paper, we present a novel smart antenna technology specially designed for mobile terminals to receive and transmit desired signals, called herein subscriber based smart antenna (SBSA). The SBSA can provide multiple directional beams as well as an omni beam. Its algorithm is based on a history of measurements from the individual beams, which can reflect a dominant angle of arrival at a mobile terminal The SBSA is low-cost and easy to implement. A series of field tests were conducted over a real cdma2000/spl reg/ network using cdma2000 phones equipped with InterDigitals Trident SBSA9 in order to validate algorithm assumptions and provide potential SBSA benefits. The results show that the SBSA can provide up to 3 dB link gain over traditional omni antennas, which theoretically translates to 50% coverage improvement as well as increased capacity and data rates. In addition, an SBSA equipped mobile requires up to about 2 dB less transmit power than a traditional omni antenna equipped mobile.