Tingfang Ji

Cell Association and Interference Coordination in Heterogeneous LTE-A Cellular Networks

Embedding pico/femto base-stations and relay nodes in a macro-cellular network is a promising method for achieving substantial gains in coverage and capacity compared to macro-only networks. These new types of base-stations can operate on the same wireless channel as the macro-cellular network, providing higher spatial reuse via cell splitting. However, these base-stations are deployed in an unplanned manner, can have very different transmit powers, and may not have traffic aggregation among many users. This could potentially result in much higher interference magnitude and variability. Hence, such deployments require the use of innovative cell association and inter-cell interference coordination techniques in order to realize the promised capacity and coverage gains. In this paper, we describe new paradigms for design and operation of such heterogeneous cellular networks. Specifically, we focus on cell splitting, range expansion, semi-static resource negotiation on third-party backhaul connections, and fast dynamic interference management for QoS via over-the-air signaling. Notably, our methodologies and algorithms are simple, lightweight, and incur extremely low overhead. Numerical studies show that they provide large gains over currently used methods for cellular networks.

LTE Femtocells: System Design and Performance Analysis

In this paper we consider a heterogeneous LTE network where femto cells are randomly deployed in a macro network. Femto cells are modeled as closed cells, namely only group member UEs can be associated with the femto cells. We demonstrate that inter-cell interference may prevent reliable operations for non-member UEs that are in proximity of a closed cell, which thus experience outage. We show how some of the novel features introduced in the Rel-10 specifications of the LTE standard can be leveraged by a suitable inter-cell interference coordination scheme (ICIC), which relies upon resource partitioning among different nodes to reduce the inter-cell interference problem. Additional significant improvements can be achieved when the proposed ICIC scheme is associated to a simple yet effective autonomous power control algorithm, described in detail in the paper, and further gains are demonstrated for UEs employing interference cancellation of broadcast interfering signals. We finally propose an enhanced ICIC method, based on a tighter coordination between macro and femto nodes, whose significant performance improvements advocate for suitable updates to the future LTE specifications.

Interference Management and Handoff Techniques in Ultra Mobile Broadband Communication Systems

Wireless communications of the next generation rely on large bandwidths and orthogonal frequency division multiple access (OFDMA) techniques to achieve high data rates. In a OFDMA-based wireless wide area network (WWAN), e.g., a cellular network, interference management and handoff are keys to the system performance, such as capacity, coverage, and stability. This paper introduces novel interference management schemes for use in a OFDMA cellular communication system that include dynamic fractional frequency reuse scheme and other cell interference based power control for interference control and avoidance and handoff schemes that employs disjoint links and beacons in a OFDMA cellular communication system.

Impact of Coordination Delay on Distributed Scheduling in LTE-A Femtocell Networks

We consider wireless femtocell networks where scheduling of transmissions is coordinated across multiple cells to mitigate interference and deliver fairness and quality of service (QoS). Coordination across multiple cells entails (i)~exchange of (serving and interfering) channel information and traffic priority (which is a function of packet delays, average rate, queue length), and (ii)~signalling of transmission powers over pilots so that neighboring cells can determine the expected SINR to pick the right modulation and coding scheme. Such an exchange entails delay in coordination resulting in scheduling decisions made on the basis of stale information received from other cells. In this paper, we study aspects of practical signalling and algorithm design to account for such coordination delay and study its impact. When the coordination across cells is done over a high latency backhaul, we design a hybrid signalling mechanism where minimal amount of additional over-the-air (OTA) signalling enhancement can lead to tremendous performance improvement. We demonstrate the benefits of our algorithms and mechanisms via comparison to a centralized scheduler and reuse one scheme using system simulations based on 3GPP evaluation methodology.

MIMO in wireless WAN — the UMB system

Theoretically, MIMO offers significant capacity benefits, but realizing these gains in deployed wireless systems remains a very challenging problem. MIMO has been used in wireless LAN systems with great success. However, due to the hugely varying spatial profiles and velocity patterns of the access terminals, integrating MIMO techniques in a wireless WAN system has been a difficult design problem. This paper introduces the design paradigms of ultra-mobile broadband, which is poised to become the successor of EVDO as the next generation of high speed wireless system. UMB allows the use of MIMO in various forms to suit the deployment scenario, and elicit as much gain as possible.