Young Jin Sang

Heterogeneous Cellular Networks with Flexible Cell Association: A Comprehensive Downlink SINR Analysis

In this paper we develop a tractable framework for SINR analysis in downlink heterogeneous cellular networks (HCNs) with flexible cell association policies. The HCN is modeled as a multi-tier cellular network where each tiers base stations (BSs) are randomly located and have a particular transmit power, path loss exponent, spatial density, and bias towards admitting mobile users. For example, as compared to macrocells, picocells would usually have lower transmit power, higher path loss exponent (lower antennas), higher spatial density (many picocells per macrocell), and a positive bias so that macrocell users are actively encouraged to use the more lightly loaded picocells. In the present paper we implicitly assume all base stations have full queues; future work should relax this. For this model, we derive the outage probability of a typical user in the whole network or a certain tier, which is equivalently the downlink SINR cumulative distribution function. The results are accurate for all SINRs, and their expressions admit quite simple closed-forms in some plausible special cases. We also derive the average ergodic rate of the typical user, and the minimum average user throughput - the smallest value among the average user throughputs supported by one cell in each tier. We observe that neither the number of BSs or tiers changes the outage probability or average ergodic rate in an interference-limited full-loaded HCN with unbiased cell association (no biasing), and observe how biasing alters the various metrics.

Outage Probability for Heterogeneous Cellular Networks with Biased Cell Association

In this paper we develop a tractable framework for SINR analysis in downlink heterogeneous cellular networks (HCNs) with flexible cell association. The HCN is modeled as a multi-tier cellular network where each tiers base stations (BSs) are randomly located and have a unique transmit power, path loss exponent, spatial density, and bias towards admitting users. We implicitly assume every BS has full queues. From this model, we derive the outage probability of a typical user in the network, which can be viewed as a spatial average of SINR over all users in the network. We observe that deploying more or less BSs does not change the outage probability in interference-limited HCN with unbiased cell association, and observe how biasing affects the metric.

A Self-Organized Femtocell for IEEE 802.16e System

Femtocell is a small base station with low power which can be installed by users to get better data rate in the indoor environment. The power control scheme of the femtocell BS is the most important issue in the femtocell implementation but there still exist deadzones if macrocell users are located very close to the femtocell. In this paper, we have proposed a new frame structure for the IEEE 802.16e based femtocell. Self-initialized segements selection and preamble allocation scheme are also proposed. Using the proposed preamble structure and self-initialization scheme, macrocell users can detect a macrocell BS even though they are located very close to the femtocell. The simulation results show that macrocell users can survive and communicate with the macrocell BS even though they are in the femtocell coverage. Additionally, it is shown that the proposed system provides the highest throughput of the macrocell and the femtocell inside the femtocell coverage.

On the Uplink SIR Distributions in Heterogeneous Cellular Networks

In this paper, the uplink (UL) signal-to-interference ratio (SIR) distributions in a two-tier heterogeneous cellular network (HCN) are analyzed by considering the power control effect and approximating the UL interferer locations as a non-uniform Poisson point process (PPP). Approximated closed-form SIR distributions are provided according to the tier, association rule, power control strategy for each tier, and resource partition (common or dedicated) and are shown to be quite close to the true distributions in a wide range of network parameters.

Load Distribution in Heterogeneous Cellular Networks

This letter provides the distributions on the association region of a small-cell BS (sBS) and that of a macro-cell BS (mBS) in a two-tier heterogeneous cellular network (HCN) as functions of the power and density ratios between tiers. Computer simulation results confirm that the proposed distributions are quite accurate for a wide range of network parameters. By using the association region distributions, the load distribution of a BS in each tier is derived. From the results, it is shown that the load is more dispersive in tier s than in tier m, which can be used for devising improved association, resource allocation and load balancing schemes in an HCN.

Performance Analysis of Proportional Fair Scheduling with Partial Feedback Information for Multiuser Multicarrier Systems

We consider multiuser multicarrier systems, where each non-symmetrically distributed user reports only partial indices of subcarriers whose normalized SNRs exceed the prede- termined threshold so that only partial feedback information is available at the base station. To accommodate both efficiency and fairness, the base station is assumed to perform the normalized SNR based proportional fair scheduling (PFS). Using the simple and quite well matched approximation of the ordered channel capacity, the approximated ergodic capacity of the full SNR feedback scheme is derived as well as the exact expression of the ergodic capacity of the partial index feedback scheme. Their asymptotic behaviors as the number of users and average SNR increase are also derived. An important finding through the per- formance analysis and numerical examples in this paper is that, also in the non-symmetrical multiuser distributed multicarrier system performing the PFS with limited feedback, the ergodic capacity of the optimal asymptotic growth rate can be achieved, keeping the average feedback overhead bounded regardless of the number of users.

The modified proportional fair scheduling algorithms for real-time applications in multiuser multicarrier systems

In this paper, we propose two modified proportional fair scheduling algorithms for multiuser multicarrier systems. Compared to the existing proportional fair scheduling algorithms, our proposed algorithms are devised to support the delay constraint for the real-time applications as well as to guarantee a certain level of fairness. The simulation results show that the proposed scheduling algorithms not only almost achieve proportional fairness with low complexity but also support real-time service users.

Performance analysis of a decode-and-forward based hybrid-ARQ protocol

Cooperative hybrid-ARQ protocols have been widely studied because the efficiency of time resources of them is higher than that of cooperative protocols. In this paper, a cooperative hybrid-ARQ protocol based on incremental redundancy hybrid-ARQ technique is proposed and analyzed with respect to the relay location. Based on the analysis, the optimum initial rate is searched and the maximum throughput is obtained with respect to the relay location.

Multiuser adaptive transmission technique for time-varying frequency-selective fading channels

This paper proposes an efficient multiuser adaptive modulation and coding (AMC) scheme that considers inevitable feedback delay by employing short-term and long-term channel state information (CSI) in time-varying frequency-selective fading channels. By taking the statistic of the true signal-to-noise ratio (SNR) at a given predicted SNR value into account, the required transmit power to meet the target packet-error-rate (PER) can be obtained and used for user selection, power allocation, and modulation and coding set (MCS) selection. In addition, a simple and useful approximation method of obtaining the required transmit power is proposed. The performance of the proposed scheme is shown to be much better than that of conventional schemes without considering the feedback delay or the prediction error. The proposed scheme can also reduce the feedback resource while maintaining the system throughput by allocating different feedback resources to different users according to their prediction error variances.

Partial feedback schemes for MIMO-OFDMA systems using random beamforming: analysis andoptimization

In this paper, partial feedback schemes for a multiple input multiple output orthogonal frequency division multiple access system using a random beamforming are analyzed and optimized. For partial feedback schemes, the partial channel quality indicator feedback schemes and the partial channel quality rank indicator feedback schemes are considered. For these schemes, we first derive the effective downlink spectral efficiencies by considering the required uplink resource for feedback together over block fading channels. Then, by using the analysis, the amount of feedback overhead per user is optimized to maximize the effective downlink spectral efficiency according to the system and channel parameters. From the analysis and numerical examples, we show that the partial channel quality rank indicator feedback scheme provides better performance than the partial channel quality indicator feedback scheme unless the channel rapidly varies owing to its feedback efficiency, and the proposed adaptive control of the feedback overhead can improve the performance of practical systems, such as the 3rd generation partnership project 3GPP long term evolution, in practical scenarios. Copyright