Anthony Edet Ekpenyong

Evolution of reference signals for LTE-advanced systems

3GPP LTE Release 10 standards (also known as LTE-Advanced) adopted some of the state-of-the-art radio access technologies that include carrier aggregation, eight-layer downlink spatial multiplexing, and four-layer uplink spatial multiplexing. For facilitating these enhancements, reference signals have significantly evolved in LTE-Advanced. This article examines underlying design principles of the LTE-Advanced reference signals. Specifically, newly introduced dedicated demodulation reference signals and channel state information reference signals for downlink and improvements of demodulation reference signals and sounding reference signals in uplink are discussed.

Feedback Constraints for Adaptive Transmission

In this article, the feedback constraints that limit the potential of AMC and MIMO systems for FDD systems are presented. It has been shown that while adaptive transmission efficiently utilizes the available bandwidth and transmit power, feedback constraints could be a significant impediment. Channel prediction can be employed to compensate the impact of feedback delay but its effectiveness would depend on the accuracy of the channel model. It is also showed that feedback errors could cause outage regions where AMC is not feasible and could also impact the benefits of multiuser scheduling. Limited feedback is a major issue for MIMO-AMC systems because channel state information grows with the rank of the MIMO system, the number of transmission modes, and the number of users for broadcast systems. Since feedback transmission consumes resources that could otherwise be used for data transmission on the reverse channel, feedback design is extremely important especially for power-limited devices such as mobile stations. Finally, some open problems are outlined that are of interest to adaptive transmission systems

From homogeneous to heterogeneous networks: A 3GPP Long Term Evolution rel. 8/9 case study

Heterogeneous cellular networks (HetNets)—conventional macro base stations overlaid with low-power nodes—are being deployed for reasons of improved spectral efficiency, economy, and scalable network expansion. To optimally realize the benefits of HetNets, it is crucial to factor deployment aspects such as cell association, transmission power, spectrum allocation, and deployment density. This paper presents comprehensive dynamic system level simulations based on the 3GPP Long Term Evolution (LTE) standard modeling spatial, temporal, and frequency domain channel variations and ray-tracing based penetration losses in urban and indoor environments. The simulator takes into account the dynamic (subframe level) modulation and coding scheme and precoder selection, as well as finite rate channel state feedback and transmission aspects intrinsic to LTE. Considering a Rel. 8/9 LTE HetNet, two access schemes are analyzed: first, open access picocells, where results show that a single picocell can double the area spectral efficiency without any impairment of the macro-layer, and second, closed access femtocells, where results show that in typical scenarios, the user throughput is deteriorated on both the macro-layer and the femto-layer. Consequently, open access low-power nodes can be deployed as is, whilst closed access ones require additional interference avoidance mechanisms. These dynamic system level simulations are important for deriving accurate and practically relevant performance trade-offs, while commensurate with the time and frequency granularity of LTE.

Link Adaptation Control in LTE Uplink

Long Term Evolution (LTE) technology aims at addressing the increasing demand for mobile multimedia services in high user density areas. Channel aware scheduling across wide system bandwidth is one of the key techniques enabling this goal. To this end, the LTE scheduler heavily relies on an accurate and robust Link Adaptation (LA) function for predicting the best modulation and coding scheme (MCS) of a user. In this paper, we complement the popular block error rate (BLER) based LA with a scheme maximizing the throughput without BLER constraint and show that it indeed optimizes the spectral efficiency performance. We also describe a sub-optimal UL scheduler providing a good performance/complexity trade-off between a low-complexity basic scheduler and the optimal, but impractical complexity-wise.

Single and multi-cell scheduling in coordinated multi-point distributed antenna systems

Multi-cell coordination is a technique for interference mitigation in cellular systems. For joint-transmission (JT), the received signal power is boosted by diversity combining of signals from multiple transmission points. This however results in reduced frequency-reuse ratio and cell-splitting order, which may subsequently degrade the network-level throughput. In order to strike a balance between the diversity combining gain and frequency-reuse ratio, in this paper we propose a dynamic scheduling algorithm that adaptively schedules users in single-cell or multi-cell transmission mode to maximize the system sum throughput. Furthermore, basestation association for each user is meticulously addressed to avoid negatively impacting the frequency-reuse gain. The proposed algorithm is evaluated with system-level evaluation tools fully compliant with 3GPP LTE specification, thereby providing a realistic assessment on the gain of multi-cell coordination in terms of system capacity and coverage.