Woohyun Seo

Reliability Improvement Using Receive Mode Selection in the Device-to-Device Uplink Period Underlaying Cellular Networks

A new interference management scheme is proposed to improve the reliability of a device-to-device (D2D) communication in the uplink (UL) period without reducing the power of cellular user equipment (UE). To improve the reliability of the D2D receiver, two conventional receive techniques and one proposed method are introduced. One of the conventional methods is demodulating the desired signal first (MODE1), while the other is demodulating an interference first (MODE2), and the proposed method is exploiting a retransmission of the interference from the base station (BS) (MODE3). We derive their outage probabilities in closed forms and explain the mechanism of receive mode selection which selects the mode guaranteeing the minimum outage probability among three modes. Numerical results show that by applying the receive mode selection, the D2D receiver achieves a remarkable enhancement of outage probability in the middle interference regime from the usage of MODE3 compared to the conventional ways of using only MODE1 or MODE2.

Beam Subset Selection Strategy for Interference Reduction in Two-Tier Femtocell Networks

This paper examines an orthogonal random beamforming-based cross-tier interference reduction scheme for two-tier femtocell networks. In order to improve the immunity of both macrocell and femtocell users to cross-tier interference, we adopt a macrocell beam subset selection strategy. This beam subset selection strategy maximizes the throughput of the macrocell by optimizing the trade-off between the multiplexing gain and the multiuser interference. Simultaneously, the max-throughput scheduler suppresses the cross-tier interference with an adaptively reduced number of beams. Since the average cross-tier interference from the macrocell gradually decreases as the number of beams decreases, the beam subset selection strategy is capable of providing a spatial opportunity to the femtocell network. This spatial opportunity enables the femtocell network to take advantage of the selectivity arising from the correlation between the interference channel and the precoding matrix. Therefore, we also propose opportunistic channel selection and distributed power control strategies for the femtocell network. Both analytical and numerical results show that the proposed strategies collaboratively reduce the cross-tier interference in two-tier femtocell networks.

A Beamforming Codebook Restriction for Cross-Tier Interference Coordination in Two-Tier Femtocell Networks

This paper examines a novel multichannel multi-antenna-based cross-tier interference coordination strategy for two-tier femtocell networks. To reduce the cross-tier interference and improve the aggregate throughput of two-tier femtocell networks without exchanging feedback information by backhauling, we propose a beamforming codebook restriction strategy. The beamforming codebook restriction strategy enables the femtocell users to select the best channel that is robust to cross-tier interference before the transmission by reducing the size of the macrocell beamforming codebook. Although restricting the beamforming codebook increases the quantization error for macrocell users, the opportunistic channel selection strategy and the proportional fair scheduler compensate for the increased quantization error by exploiting the channel selection diversity gain and the multiuser diversity gain. Both analytical and numerical results demonstrate that the proposed strategies collaboratively improve the aggregate throughput of two-tier femtocell networks.

QoS-guaranteed transmission mode selection for efficient resource utilization in multi-hop cellular networks

This paper considers the problem of efficient usage of subcarriers in downlink orthogonal frequency-division multiple access (OFDMA) multi-hop cellular networks. Multi-hop transmission can either save or waste subcarriers depending on quality of service (QoS) requirements, interference from other cells, and the location of a given mobile station (MS). Preventing unnecessary usage of multi-hop transmission requires the use of a transmission mode selection (TMS) scheme, and we propose two types of TMS in this paper. The first of these is distance-based TMS (TMS-D), which determines the transmission mode based on the MSiquests location, and the second is subcarrier-based TMS (TMS-S), which selects whichever transmission mode uses fewer subcarriers. Numerical results on blocking probability demonstrate that TMS improves overall subcarrier usage efficiency, meaning that more MSs can be supported with low blocking probability within a cell. Furthermore, the performance of TMSS, even given its higher complexity, is shown to be superior to that of TMS-D.

Compressive Feedback Based on Sparse Approximation for Multiuser MIMO Systems

In multiuser multiple-input-multiple-output (MU-MIMO) systems, limited feedback schemes using a vector quantization codebook (VQC) for partial channel state information (CSI) can enable a base station (BS) to enhance the sum throughput at low feedback rates. However, the VQC has a limited throughput gain, although it requires a great deal of memory and complexity as the feedback rate increases. Therefore, we propose a high-precision feedback technique by using sparse approximation and compression called ¿compressive feedback (CF).¿ By using the sparse approximation with a unitary codebook and the compression via a ¿good¿ compression matrix, CF achieves greater sum throughput than VQC. Our analysis of the maximum achievable sum rate verifies the superiority of CF over VQC.

LLR-based symbol selective transmission with a near-optimal threshold to minimize BEP for demodulation-forward relay systems

Error propagation in the source-relay link limits the performance of demodulation-forward (MF) relay systems. In order to solve this problem and efficiently exploit the relay, we propose log likelihood ratio (LLR)-based symbol selective transmission with a near-optimum threshold to minimize the bit error probability (BEP). In the proposed scheme, the source information is forwarded by the relay only if it is likely to be detected correctly, as determined by a comparison of the LLR with the near-optimum threshold. To derive the near-optimum threshold, the BEP of the proposed scheme is analyzed in the case of binary phase shift keying (BPSK) signals. Simulation results confirm the accuracy of the analysis and prove that the proposed scheme efficiently mitigates error propagation.

A joint feedback reduction scheme using delta modulation for dynamic channel allocation in OFDMA systems

A novel feedback reduction scheme is proposed for dynamic channel allocation (DCA) in OFDMA systems. Applying this delta-modulation (DM)-based scheme to the channel gain of adjacent subcarriers reduces the feedback information to 1 bit per subcarrier. Since this reduced feedback information can lead to performance degradation in a bit error rate (BER), a modified amplitude craving greedy (ACG) method and improved DM are jointly adopted to minimize the loss. Simulation results verify that the proposed scheme can decrease feedback information by a factor of 1/M while retaining performance comparable to the full feedback case, which requires M bits per subcarrier

A new asymptotic analysis of throughput enhancement from selection diversity using a high SNR approach in multiuser systems

This letter presents our study of throughput enhancement achieved by multiuser diversity (MUDiv) in a multiuser system using a new asymptotic approach. The MUDiv gain is evaluated by deriving a new asymptotic formula with a closed form, which is flexible to the number of MUDiv orders. To do this, we simplify a Puiseux series instead of a extreme value theory used in the previous researches. The formula shows that the MUDiv gain is independent of the signal-to-noise ratio, and it is applied to analyze proportional fair scheduling. Finally, this analysis is verified using Monte-Carlo simulations of scheduling algorithms.

Performance Analysis of Wireless Dual-Hop Systems With Multirelay and Multiuser

This paper presents closed-form expressions for the end-to-end performance of wireless dual-hop systems with multiple amplify-and-forward relays and multiple users over Rayleigh flat fading channels. These results are based on the statistics of the harmonic mean of two random variables related to exponential distribution. Two generalized scheduling policies are considered in the systems for selection diversity from multirelay and multiuser: the centralized scheduling (CS) and the distributed scheduling (DS). Numerical results show that the performance of DS can be degraded when large number of relays are used relative to the number of users. Additionally, it is shown that DS is quite competitive with CS at small number of relays that can be considered to be desirable.

A New Full-band Feedback Scheme Using the Adaptive Grouping Method in OFDMA Systems

Opportunistic scheduling algorithms with link adaptation can increase system throughput, but have the drawback of requiring the channel quality information (CQI) to be transmitted from all the candidate users. In order to alleviate this problem, we propose a new feedback reduction scheme that preserves CQIs over all channels. The proposed scheme binds similar CQIs in groups to represent them more effectively, and uses a simple method for fast grouping. Simulation results show that the proposed scheme with this simple grouping achieves close to perfect feedback performance with respect to the original CQIs under reasonable feedback loads.