Yong-Up Jang

A Two-Step Approach to Power Allocation for OFDM Signals Over Two-Way Amplify-and-Forward Relay

A two-way relay channel (TWRC) in which two terminals T 1 and T 2 exchange orthogonal frequency division multiplexing (OFDM) signals with the help of an amplify-and-forward (AF) relay T 3 is considered here, and an efficient technique for allocating powers to N parallel tones of OFDM is developed. A sum rate maximization problem is formulated by replacing the individual power constraints of the conventional sum rate maximization problem, which limit the power of each terminal, with the total power constraint limiting the sum of powers of all terminals. The maximization problem with the total power constraint yields a more efficient power allocation policy than the conventional problem with individual power constraints. It is shown that the closed-form solution of the maximization problem under the total power constraint can be obtained for a single-tone system (N=1). Based on this result, a two-step suboptimal approach is proposed in which the power is optimally assigned to each tone first, and then at each tone the assigned power is distributed to the three terminals. The proposed method is shown to assign 50% of the total power to relay T 3 irrespective of the channels. It is demonstrated that the proposed method is considerably simpler to implement than the conventional dual-decomposition method (DDM), yet the performance of the former is almost identical to that of the latter.

Performance Analysis of User Selection for Multiuser Two-Way Amplify-and-Forward Relay

This paper considers a multiuser two-way relay channel (MU-TWRC) in which a base-station and a selected mobile-station (MS) among K number of MSs exchange messages during two time slots with the help of a half-duplex amplify-and-forward relay. We analyze the performances of a MU-TWRC. The closed-form expressions of the lower bound of the average rates are derived. The asymptotic expressions of the average rates are also analyzed in the limit of large K. Analysis and simulation results indicate that the former matches well to the latter, and the performance of a MU-TWRC can increase as K increases or be bounded to some constant according to the channel conditions.

Capacity Evaluation of Various Multiuser MIMO Schemes in Downlink Cellular Environments

Presented in this paper is a study of the capacity evaluation of various multiuser MIMO schemes in cellular environments. The throughputs per user of the generalized zero-forcing with rank adaptation and vector perturbation schemes are compared with the capacity bound of the Gaussian MIMO broadcast channel, obtained by dirty paper coding under proportional fairness scheduling. The average cell throughputs of these schemes are also compared. From these comparisons, this study provides vital information for applying multiuser MIMO schemes in multicell environments

Frame Design and Throughput Evaluation for Practical Multiuser MIMO OFDMA Systems

This paper describes the design of a time-division duplexing frame with a variety of pilots for multiuser multiple-input-multiple-output orthogonal frequency-division multiple access (MU-MIMO OFDMA) systems, where the base station and users are equipped with four and two transmitting and receiving antennas, respectively. In addition, a simplified scheduling algorithm for the MU-MIMO OFDMA is proposed, and its computational complexity is analyzed. The proposed scheduling algorithm shows comparable sum achievable rates to the optimal MU-MIMO OFDMA scheduling that searches for user pairs in an exhaustive manner, whereas its complexity is fairly reduced. Furthermore, to verify the performance of MU-MIMO OFDMA systems that employ the proposed frame structure and scheduling algorithm, a system-level comparison of the average cell throughputs between the proposed MU-MIMO and the conventional MIMO OFDMA systems is numerically performed in a practical cellular environment. As a result, vital information on how we can apply MU-MIMO OFDMA schemes in cellular environments is provided.

Multiuser Scheduling Based on Reduced Feedback Information in Cooperative Communications

We introduce a multiuser (MU) scheduling method for amplify-and-forward (AF) relay systems, which opportunis- tically selects both the transmission mode, i.e., either one- or two-hop transmission, and the desired user. A closed-form expression for the average achievable rates is derived under two transmissions with MU scheduling, and its asymptotic solution is also analyzed in the limit of large number N of mobile stations (MSs). Based on the analysis, we perform our two-step scheduling algorithm: the transmission mode selection followed by the user selection that needs partial feedback for instantaneous signal-to- noise ratios (SNRs) to the base station (BS). We also analyze the average SNR condition where the MU diversity gain is fully exploited. In addition, it is examined how to further reduce a quantity of feedback under certain conditions. The proposed scheduling algorithm shows the comparable achievable rates to those of optimal one using full feedback information, while its required feedback information is reduced below half of the optimal one.

Predictive transmit beamforming for MIMO-OFDM in time-varying channels with limited feedback

A limited feedback-based transmit beamforming technique for multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) is investigated in time-varying channels. The performance of the system is significantly degraded by outdated feedback information even when the channel varies slowly. To compensate for the impairment in time-varying channels, the optimal transmit beamforming vector for a future channel, which maximizes the expected effective channel gain, is derived by applying the autoregressive (AR) model to the channels. These are obtained at the receiver. Following this, schemes for the selection of beamforming vectors are proposed to reduce the feedback amount. These can effectively reduce the amount of feedback information by utilizing both the frequency and time correlation of transmit beamforming vectors. Simulation results show that the proposed techniques outperform existing schemes in terms of the bit error rate (BER) performance with the same amount of feedback.

Performance Analysis of Cognitive Radio Networks Based on Sensing and Secondary-to-Primary Interference

This correspondence considers a hybrid cognitive radio (H-CR) where the primary transmitter (PT) transmits message to the primary receiver (PR) whenever traffic occurs by allowing interference level I from the secondary transmitter (ST) to the PR, and the ST transmits messages to the secondary receiver (SR) by controlling the transmitting power based on spectrum sensing and interference level I. The ST opportunistically adopts maximum power when it detects that the PT is not transmitting and controls the transmitting power to restrict the interference level from the ST to the PR below I when alerted that the PT is transmitting. The closed-form expressions of the average rates for the H-CR are derived. Based on the analytical results, the detection probability α and interference level I are optimized to maximize the average rate of the ST by guaranteeing the average rate of the PT. Analysis and simulation results demonstrate that the former matches well to the latter and the average rate for the ST of the H-CR is better than that of two existing CRs where the ST transmits messages to the SR with maximum power only when the ST recognizes that the PT is not transmitting or with controlled power to limit an interference level from the ST to the PR below a constant irrespective of the PTs activity.

Opportunistic Power Control for Successive Interference Cancellation

This letter proposes an opportunistic power control for increasing the transmission rate of a secondary transmitter (ST) in frequency reused scenario. The environments considered herein have multiple primary transmitter/receiver pairs and one secondary transmitter/receiver pair using the same frequency band. In this scenario, we develop the opportunistic power control method for assisting successive interference cancellation (SIC) and optimal SIC order. Simulation results show that the proposed method improves the ST transmission rate significantly when compared to the method with no power control, and it exhibits a small throughput loss compared to the optimal technique while offering simpler implementation and using less transmission power.

Two-step Scheduling With Reduced Feedback Overhead in Multiuser Relay Systems

In this paper, we introduce a multiuser (MU) scheduling method for multiuser amplify-and-forward relay systems, which selects both the transmission mode, i.e., either one- or two-hop transmission, and the desired user via two steps. A closed-form expression for the average achievable rate of the proposed scheduling is derived under two transmission modes with MU scheduling, and its asymptotic solution is also analyzed in the limit of large number of mobile stations. Based on the analysis, we perform our two-step scheduling algorithm: the transmission mode selection followed by the user selection that needs partial feedback for instantaneous signal-to-noise ratios (SNRs) to the base station. We also analyze the average SNR condition such that the MU diversity gain is fully exploited. In addition, it is examined how to further reduce a quantity of feedback under certain conditions. The proposed algorithm shows the comparable achievable rate to that of the optimal one using full feedback information, while its required feedback overhead is reduced below half of the optimal one.