Shulan Feng

A Fractional Soft Handover Scheme for 3GPP LTE-Advanced System

Mobility enhancement and carrier aggregation are important aspects to investigate as part of the LTE-Advanced study. Hard handover is the only handover scheme in LTE Rel-8. Generally, hard handover is fairly simple, but its outage probability is high and the handover procedure may be unreliable, especially for VoIP service. In this paper, we propose a fractional soft handover scheme based on the carrier aggregation. The main idea is to partially perform soft handover for VoIP, but non-VoIP service is only transmitted from source eNodeB or target eNodeB. The theoretical analysis and simulation results show that the proposed fractional soft handover scheme not only attains the lowest handover outage probability, but also improve the reliability of VoIP service.

Preamble Design for Non-Contiguous Spectrum Usage in Cognitive Radio Networks

Cognitive radios can significantly improve spectrum efficiency by using locally available spectrum. The efficiency, however, depends heavily on their transceiver design. In particular, being able to use non-contiguously aligned spectrum bands simultaneously is a critical requirement. Prior work in this area requires a control channel so that transmitter/receiver pairs can synchronize on their spectrum usage patterns. However, this approach can suffer from high cost and control congestion. In this paper, we propose an in-band solution for informing receivers the spectrum usage patterns. By judiciously designing packet preambles, we embed the spectrum usage patterns in each data packet. Using the legacy 802.11 preamble structure, we focus on choosing the appropriate preamble sequences to maintain reliable packet detection in the presence of noise and interference. We verify our design using simulation and show that it can lead to reliable packet transmissions comparable to those of contiguous spectrum usage. We also identify the impact of interference on our design and propose refinements to choose the preamble sequence using information on the interference.

Comb-Type Pilot Aided Channel Estimation in Non-Contiguous OFDM Systems for Cognitive Radio

The Non-Contiguous (NC) OFDM is a promising candidate transmission scheme in Cognitive Radio (CR) system. However, the NC available sub-carriers bring new challenges on system design. The pattern of known signals in frequency domain, such as preamble and pilots is vulnerable by NC subcarriers. Several recent researches have studied synchronization issues for NC-OFDM. In this paper, we study the comb-type pilot channel estimation and interpolation for NC-OFDM transmission. Firstly, three types of channel estimation strategies are revealed. And then, we proposed the Shift Pilot (SP) scheme by inserting some punched pilots on other available sub-carriers of data at transmitter. A preprocessor is introduced between channel estimation and interpolation at receiver to estimate channel values of punched pilots. The enhanced strategy achieves better performance with a reasonable complexity in NC-OFDM.

Inter-Cell Interference Management in LTE-A Small-Cell Networks

This paper is concerned with inter-cell interference in co-channel Long-Term Evolution-Advanced (LTE-A) small-cell networks (SCNs). A practical SCNs architecture with small-cell group muting (SCGM) is proposed to mitigate the interference between the hosting macro-cell and small-cells as well as interference between adjacent small-cells. Lower power consumption can also be achieved in the proposed architecture. Our system level simulation results show that the proposed solution outperforms todays default operational methods in terms of both macro-cell average and cell-edge throughput.

Joint Transmission for LTE-Advanced Systems with Non-Full Buffer Traffic

Joint Transmission (JT) as an example of downlink coordinated multi-point (CoMP) transmission can improve the overall system performance, particularly cell average and cell-edge throughput. Many studies are related to JT in homogeneous network and based on full buffer traffic which is not a practical scenario. We propose a JT method for LTE-Advanced systems with non-full buffer traffic in both homogeneous and heterogeneous networks. System-level simulations demonstrate that the proposed JT techniques clearly outperform single cell transmission scheme in terms of cell average and cell-edge performance.

On the Efficient Channel State Information Compression and Feedback for Downlink MIMO-OFDM Systems

This paper is concerned with the efficient compression and feedback of channel state information (CSI) in downlink multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems. Inspired by video coding, we propose a novel CSI compression and feedback scheme, referred to as hybrid transform coding (HTC). HTC consists of two coding types, i.e, selective time-domain coding (STDC) and differential time-domain coding (DTDC), which are adopted to exploit the correlation of CSI in both the frequency and time domains. We first develop closed-form expressions for the overhead-distortion performance of these two coding types in HTC. The parameters involved in HTC are then optimized based on the analytical results. Finally, the system-level performance of HTC is evaluated in both maximum eigenmode beamforming (MEB)-based single-user MIMO (SU-MIMO) and zero-forcing beamforming (ZFBF)-based multiuser MIMO (MU-MIMO) under Long Term Evolution Advanced (LTE-A) Release 10-based cellular networks. Simulation results show that HTC can significantly outperform the available alternative.

Adaptive Cell Association and Interference Management in LTE-A Small-Cell Networks

This paper is concerned with user association and inter-cell interference in co-channel Long-Term Evolution-Advanced (LTE-A) small-cell networks (SCNs) in which the transmit nodes of control plane and data plane are separated. We propose a novel algorithm to associate users to one of the small-cells, while maximizing the sum utility of average achievable rates within one macro-cell. Moreover, small-cell group muting (SCGM) is used to mitigate the inter-cell interference among adjacent small-cells. Numerical simulation results in an LTE-A simulator show that the adaptive cell association and SCGM schemes clearly outperform currently used methods in terms of macro-cell average and cell-edge throughput.

Cooperative Uplink Link Adaptation in 3GPP LTE Heterogeneous Networks

Heterogeneous Network (HetNet) where small cells served by Low Power Nodes (LPNs) are overlaid onto conventional macro cells is considered to meet the requirements of the increasing need for higher data rates. In this paper, we investigate the uplink interference situation in 3rd Generation Partnership Project (3GPP) Long-Term Evolution (LTE) HetNet and the corresponding impacts on the efficiency of UL link adaptation, with the focus on small cells. It is observed that the efficiency of UL link adaptation of LPN cells is seriously affected by a small number of dominant interferers. In addition, we study two Uplink Coordinated Multi-point (UL CoMP) schemes which exploit the fluctuation nature of UL interference strength to mitigate the influences. A simplified approach to perform cooperative link adaptation for HetNet is also addressed. System level simulations are carried out to evaluate these schemes, and promising performance gains are observed.

RSTD Performance for Small Bandwidth of OTDOA Positioning in 3GPP LTE

We studies the influence of multiple reference signals on different transmit ports for RSTD measurements in small bandwidth scenarios. Because of scalable measurement bandwidth in long term evolution (LTE), the performance of reference signal time difference (RSTD) suffers from small bandwidth. Conventional RSTD scheme is based on positioning reference signals (PRS) which use one transmit antenna and two receive antennas (1x2). In order to reduce the loss of accuracy caused by small bandwidth, three new schemes are proposed. Compared with the conventional scheme, the accuracy of three proposed schemes has been improved in ETU channel. We verify it with our simulation results. The detection probability of one proposal 2x2 scheme which splits PRS symbols on two transmit antennas is decreased by half the number of accumulating symbols on each transmit antenna. Other two schemes which accumulate cell-specific reference signals (CRS) and PRS non-coherently or coherently not only provide better RSTD accuracies but also improve detection probabilities.

A New Downlink Control Channel Scheme for LTE

To accommodate the realistic channel conditions better and thus improve the system throughput, 3GPP introduced the Physical Downlink Control Channel (PDCCH) in LTE. A downlink control information (DCI) message is generated and transmitted over the PDCCH. The User Equipment (UE) is required to perform blind decoding as it is not aware of the detailed control structure. However, blind decoding consumes a lot of UEs computational power, e.g. UE needs to do nearly 240 convolutional decodings on the dedicated search space (DSS) within about 0.15ms in the case of aggregation of 5 carriers. This is a big challenge for UE implementation in reality. In this paper, we propose a new downlink control channel scheme which can greatly reduce the PDCCH blind decoding complexity (from 240 to about 15 convolutional decodings) and power consumption. Another important benefit of this new scheme is that the DCI design can be much more flexible by the DCI index. Also, the new scheme can reduce the false alarm of PDCCH blind decoding and thus decrease the false uplink transmission and interference.