Sangjoon Park

Spatial resource utilization to maximize uplink spectral efficiency in full-duplex massive MIMO

In this paper, we consider a full-duplex massive MIMO system which includes a full-duplex BS with a large number of antennas and half-duplex uplink (UL) and downlink (DL) users with a single antenna. Due to the self-interference at the BS, the UL spectral efficiency is significantly decreased. In order to improve the UL spectral efficiency, we focus on the utilization schemes of the spatial resources which can be used to enhance the UL performance. If the BS uses the spatial resources as receive antennas, the BS can obtain an additional receive diversity gain for the UL desired signal. On the other hand, if the BS uses the spatial resources as transmit antennas, the BS can suppress a part of the self-interference by using the extended transmit beamformer including a part of the self-interference channel. Our analysis and numerical results show that there are some tradeoffs between achieving receive diversity gain for the UL desired signal and suppressing the self-interference. According to the system parameters, the utilization schemes of the spatial resources would be determined for maximizing the UL spectral efficiency.

Extended Detection for MIMO Systems with Partial Incremental Redundancy Based Hybrid ARQ

In this paper, an extended detection scheme is proposed for multiple-input multiple-output (MIMO) systems which employ partial incremental redundancy (IR) based hybrid automatic repeat request (HARQ). Based on the extended MIMO system model that interprets multiple retransmissions as a single transmission, the extended detection scheme estimates the entire transmitted symbols utilizing all information obtained up to the current retransmission. Compared with the conventional zero-forcing (ZF) detection scheme, the extended ZF detection scheme improves the post-processing signal-to-noise ratio (PSNR) of the newly transmitted symbols in every retransmission, as well as the repeatedly transmitted symbols. Simulation results verify that the extended detection scheme outperforms the conventional detection scheme. Simulation results also show that the extended minimum mean-square-error (MMSE) detection scheme can achieve a better error performance than the conventional maximum-likelihood (ML) detection scheme, and even the extended ZF detection scheme shows a comparable error performance to the conventional ML detection scheme.

Antenna ratio for sum-rate maximization in MU-MIMO with full-duplex large array BS

This paper analyzes the ratio of the transmit antennas and receive antennas in multi-user multiple-input multipleoutput with a full-duplex and large array base station (BS) and half-duplex users (MU-MIMO FLB-HU) systems. We consider the BS exploits zero-forcing beamformer and zero-forcing receiver. We derive the deterministic approximation of the downlink and uplink sum-rates considering inter-user interference and self-interference, respectively. Based on the analyzed results, we formulate an optimization problem in terms of the number of transmit and receive antennas to maximize the sum of downlink and uplink sum-rates subject to the number of total antennas at the BS. From the optimization problem, the optimal antenna ratio between the number of transmit and receive antennas can be obtained. We analyze that the optimal antenna ratio converges to the ratio between the number of downlink users (Kd) and uplink users (Ku) as the number of total antennas goes to infinity. Simulation results show that the optimal antenna ratio enhances the sum-rate performance compared to the same number of transmit antennas and receive antennas in the MU-MIMO FLBHU system. In particular, in the MU-MIMO FLB-HU system with Kd = 10 and Ku = 5, the optimal antenna ratio can achieve about 5~10bps/Hz performance gain compared to the same number of transmit antennas and receive antennas.

Clinical significance of soluble programmed cell death ligand-1 (sPD-L1) in hepatocellular carcinoma patients treated with radiotherapy

PURPOSE To investigate the clinical implications of the soluble programmed cell death-ligand 1 (sPD-L1) level in hepatocellular carcinoma (HCC) patients treated with radiotherapy (RT). MATERIALS/METHODS HCC patients treated with RT between June 2011 and March 2015 were prospectively recruited and sPD-L1 levels were measured using an enzyme-linked immunosorbent assay. Blood samples were obtained at the RT start, RT end, and 1-month follow-up. The associations of the sPD-L1 level with the clinical features and outcomes were analyzed. RESULTS Fifty-three patients with HCC were included. Thirty-four patients received conventional fractionated RT with hepatic arterial infusional chemotherapy, while 19 patients received stereotactic body radiotherapy (SBRT). The initial sPD-L1 level was significantly associated with stage, tumor size, portal vein tumor thrombosis, and venous invasion. The overall-survival was significantly poorer in patients with a higher level of initial sPD-L1 (≥1.315 pg/mL). A higher level of sPD-L1 at 1 month (≥12.9 pg/mL) was significantly related to early lung metastasis. The sPD-L1 level was significantly increased after RT and the change pattern of sPD-L1 was different between two RT schemes. CONCLUSIONS The level of sPD-L1 was associated with tumor aggressiveness and outcomes, suggesting its role as a possible predictive biomarker. The increases in sPD-L1 after RT suggests that combined treatment with RT and immune checkpoint inhibitors may be a promising therapeutic strategy in HCC.

Performance of Symbol-Level Combining and Bit-Level Combining in MIMO Multiple ARQ Systems

The performance of symbol-level combining (SLC) and bit-level combining (BLC) in multiple-input multiple-output (MIMO) multiple automatic repeat request (ARQ) systems is investigated. Considering the zero-forcing (ZF) detection under the assumption of perfect packet elimination for SLC, the two performance characteristics of Chase combining (CC) with SLC and BLC in MIMO multiple ARQ (MMARQ) systems are analyzed. First, the performance of a packet with the highest hybrid ARQ (HARQ) round in CC-SLC-ZF is not affected by the HARQ rounds of the other packets simultaneously sent. Second, the performance gain of CC-SLC-ZF over CC-BLC-ZF for a packet can be improved when the packets with higher HARQ rounds are simultaneously sent. The latter indicates that CC-SLC can provide an improved error performance for the packets transmitted only once when there is at least one retransmitted packet simultaneously sent. Therefore, even though incremental redundancy (IR) provides a significantly larger coding gain than CC, CC-SLC can provide a better throughput than IR-BLC as the average block error rate of a retransmitted packet approaches zero. Simulation results verify that the analyses remain valid regardless of the detection scheme and the throughput of CC-SLC at the high SNR region can be better than IR-BLC in MMARQ systems.

Plasma Cell-Free DNA as a Predictive Marker after Radiotherapy for Hepatocellular Carcinoma

Purpose Cell-free DNA (cfDNA) is gaining attention as a novel biomarker for oncologic outcomes. We investigated the clinical significance of cfDNA in hepatocellular carcinoma (HCC) patients treated with radiotherapy (RT). Materials and Methods Fifty-five patients with HCC who received RT were recruited from two prospective study cohorts: one cohort of 34 patients who underwent conventionally fractionated RT and a second of 21 patients treated with stereotactic body radiation therapy. cfDNA was extracted and quantified. Results In total, 30% of the patients had multiple tumors, 77% had tumors >2 cm, and 32% had portal vein tumor thrombus. Optimal cut-off values for cfDNA levels (33.65 ng/mL and 37.25 ng/mL, before and after RT) were used to divide patients into low-DNA (LDNA) and high-DNA (HDNA) groups. The pre-RT HDNA group tended to have more advanced disease and larger tumors (p=0.049 and p=0.017, respectively). Tumor response, intrahepatic failure-free rates, and local control (LC) rates were significantly better in the post-RT LDNA group (p=0.017, p=0.035, and p=0.006, respectively). Conclusion Quantitative analysis of cfDNA was feasible in our cohorts. Post-RT cfDNA levels were negatively correlated with treatment outcomes, indicating the potential for the use of post-RT cfDNA levels as an early predictor of treatment responses and LC after RT for HCC patients.

Subframe shift for full-duplex base station in TD-LTE systems

This paper proposes a subframe shift method to provide the full-duplex operation for the full-duplex base station (BS) in the current time-division long term evolution (TD-LTE) systems. For the full-duplex operation using the proposed subframe shift, we also propose a frame structure for the simultaneous transmission and reception of full-duplex BS operating in the current TD-LTE system and a signaling method for the downlink channel quality informations using the proposed subframe shift method based on the interference from the co-scheduled uplink channel. The proposed method shows a gain of approximately 50% in average system throughput and provides the access to the full-duplex BS for the half-duplex users as well as the full-duplex users in the current TD-LTE systems.

Impacts of Frequency Selectivity on the Error Performance of Time-Domain Equalizers in MIMO Systems

In this paper, we investigate the impacts of frequency selectivity on the error performance of time-domain equalizers in multiple-input multiple-output (MIMO) systems. Two types of time-domain equalizers are considered: linear equalizers and decision-feedback equalizers. Under the assumption of the perfect channel estimation at the receiver, the error performance of both linear and nonlinear equalizers is evaluated according to the frequency selectivity, i.e., the number of channel taps, by numerical simulations. The simulation results show that both linear and nonlinear equalizers achieve the error performance improvements as the number of channel taps increases. Therefore, in MIMO systems under frequency-selective fading channels, the diversity gain dominates the inter-symbol interference effects in terms of error performance and the increased frequency selectivity can provide the performance gain at the price of higher computational complexity.

Error Performance of Successive Interference Cancellation Schemes in MIMO Systems

In this paper, the error performance of various successive interference cancellation (IC) schemes is analyzed in spatially multiplexed multiple-input multiple-output (MIMO) systems. First, we classify the successive IC scheme by their ordering strategy. In addition, considering the channel coding is usually applied for the conventional wireless communication systems, we apply the channel coding to the spatially multiplexed MIMO systems and evaluate the block error rate (BLER) as the error performance of the systems. Using numerical simulations, the error performance of IC schemes is shown according to various system parameters, e.g., the number of transmit and receive antennas, the utilized linear filter, and the MIMO fading channel characteristics. In this way, this paper analyses and compares the error performances of the successive IC in various perspectives.

Multi-stage turbo equalization for MIMO systems with hybrid ARQ

A multi-stage turbo equalization scheme based on the bit-level combining (BLC) is proposed for multiple-input multiple-output (MIMO) systems with hybrid automatic repeat request (HARQ). In the proposed multi-stage turbo equalization scheme, the minimum mean-square-error equalizer at each iteration calculates the extrinsic log-likelihood ratios for the transmitted bits in a subpacket and the subpackets are sequentially replaced at each iteration according to the HARQ rounds of received subpackets. Therefore, a number of iterations are executed for different subpackets received at several HARQ rounds, and the transmitted bits received at the previous HARQ rounds as well as the current HARQ round can be estimated from the combined information up to the current HARQ round. In addition, the proposed multi-stage turbo equalization scheme has the same computational complexity as the conventional bit-level combining based turbo equalization scheme. Simulation results show that the proposed multi-stage turbo equalization scheme outperforms the conventional BLC based turbo equalization scheme for MIMO systems with HARQ.