Kazuki Takeda

A Field Trial of Unlicensed LTE (U-LTE) in 5.8 GHz Band

To alleviate network congestion and improve the spectrum efficiency, it is natural to consider operating Long Term Evolution (LTE) on unlicensed spectrum, which is called Unlicensed LTE (U-LTE). In order to get practical experience with U-LTE, NTT DOCOMO and Huawei built up a U-LTE trial system operating over bandwidth of 20MHz in 5.8 GHz unlicensed bands and a field trial was conducted in 2014. The U-LTE system operated in real time and was field tested in typical indoor environments. In this paper, we report the field trial results for both U- LTE and Wi-Fi systems. Our objective is twofold: firstly, coverage and capacity performance in single cell are presented to confirm the potential effectiveness of using U-LTE in the operator network. Secondly, the intra-Radio-Access-Technology (RAT) coexistence performance conducted in different interference scenarios disclosed a challenging problem, i.e., the straightforward introduction of LTE into the unlicensed spectrum is not recommended and interference avoidance technology is necessary in U-LTE.

Uplink Power Control for Dual Connectivity

Small cell enhancement is a key technology to meet future network capacity demands. Dual connectivity has been introduced in LTE Release 12 to enhance the LTE operation in heterogeneous deployments and improve the overall system performance. However, in the scenarios of dual connectivity, due to distributed scheduling in two eNodeBs (eNBs) connected with non-ideal backhaul, the total required transmit power of a user equipment(UE) will easily exceed its maximum capability and power scaling is needed. In this paper, several semi-static power splitting schemes will be proposed based on power spectral density (PSD), path loss and average power consumption ratio to solve this problem. Simulation results demonstrate that our schemes can greatly decrease the probability of power scaling and reduce block error rate (BLER), thus achieving significant gains in uplink throughput.

A field trial of LTE in unlicensed bands with SDL (Supplemental Downlink) transmission

We built up a Long Term Evolution (LTE) trial system operating in 5.8 GHz unlicensed bands and conducted a real time field trial in typical indoor environments. The user throughput of LTE with the 20MHz carrier in the unlicensed bands, which is used as a Supplemental Downlink (SDL) (always downlink), is evaluated and compared with that of Wireless Local Area Network (WLAN) with the same bandwidth. Our test results show that LTE achieves better coverage and larger capacity in the 5.8 GHz unlicensed spectrum than WLAN alone. In addition, our test results on the intra-Radio-Access-Technology (RAT) coexistence performance conducted in different interference scenarios clarifies that an interference avoidance technology is necessary in LTE in unlicensed bands especially for dense deployment such as inter-operator coexistence case.

Performance of downlink control information signals using decision-feedback channel estimation for EPDCCH

In the Release (Rel.) 11 Long Term Evolution (LTE)-Advanced radio interface, the Enhanced Physical Downlink Control Channel (EPDCCH) is specified, which transmits downlink control information (DCI) signals. The EPDCCH is frequency-domain-multiplexed with data symbols and is used to transmit more DCI signals than the Physical Downlink Control Channel (PDCCH) for Rel. 8 LTE due to the application of multiuser multiple-input multiple-output (MIMO) and coordinated multi-point (CoMP) transmission and reception. This paper presents the effect of decision-feedback channel estimation (DFCE) using soft-symbol estimation of the DCI signals for the EPDCCH. We propose a DFCE method that improves the channel estimation accuracy by using soft-symbol estimation of the control signals based on the a posteriori log-likelihood ratio (LLR) at the soft output Viterbi algorithm (SOVA) for convolutional coding in addition to the demodulation reference signals (DM-RSs). Computer simulation results show that the DFCE is effective in decreasing the transmission power of the DM-RSs to approximately half compared to that for only conventional DM-RSs to achieve the same target block error rate from a low to high Doppler frequency region in a frequency-selective Rayleigh fading channel.