Wonjin Sung

Capacity analysis for distributed antenna systems using cooperative transmission schemes in fading channels

Wireless systems equipped with distributed antennas can provide enhanced data throughput as well as improved signal quality when appropriate cooperative transmission schemes among antennas are employed. In this paper, the performance of distributed antenna systems (DAS) under the influence of Rayleigh fading and log-normal shadowing effects is investigated for different cooperation strategies, and corresponding statistical distributions for both the signal-to-interference ratio (SIR) and the capacity are presented. Specifically, the ergodic capacity per remote antenna (RA) is determined when n RAs cooperatively transmit the desired signal to the target receiver, which is compared with the non-cooperative single-antenna transmission (SAT) case. Using the results, a preferable mode of operation for given geographic locations of the receiver is determined. We further propose to perform an adaptive operation between the cooperative and non-cooperative modes under the capacity maximization criteria, and quantify exact amounts of capacity gain over individual transmission modes.

Simple SNR estimation methods for QPSK modulated short bursts

An accurate signal-to-noise ratio (SNR) measurement is essential for cellular and satellite communication receivers for various purposes, including the demodulation algorithm adaptation to the time-varying channel conditions. In this paper, computationally simple SNR estimation methods, called the error vector magnitude (EVM) method and the minimum mean-square error estimation (MMSE) method are proposed. Analytic derivations as well as simulation results are presented to demonstrate the performance of the proposed schemes.

Robust frame synchronization for low signal-to-noise ratio channels using energy-corrected differential correlation

Recent standards for wireless transmission require reliable synchronization for channels with low signal-to-noise ratio (SNR) as well as with a large amount of frequency offset, which necessitates a robust correlator structure for the initial frame synchronization process. In this paper, a new correlation strategy especially targeted for low SNR regions is proposed and its performance is analyzed. By utilizing a modified energy correction term, the proposed method effectively reduces the variance of the decision variable to enhance the detection performance. Most importantly, the method is demonstrated to outperform all previously reported schemes by a significant margin, for SNRs below 5 dB regardless of the existence of the frequency offsets. A variation of the proposed method is also presented for further enhancement over the channels with small frequency errors. The particular application considered for the performance verification is the second generation digital video broadcasting system for satellites (DVB-S2).

Performance analysis of APSK modulation for DVB-S2 transmission over nonlinear channels

For increased bandwidth efficiency and receiver performance, standards for satellite broadcasting systems are evolving by utilizing efficient transmission techniques. The second-generation digital video broadcasting for satellites (DVB-S2) adopts the amplitude phase shift keying (APSK) modulation for enhanced performance over nonlinear channels. In this paper, we derive error rate bounds for APSK modulated symbols and generalize the bounds to the case of distorted constellation, which occurs when the maximum transmission amplitude is saturated by the soft-limiter type channel. The derived bound is shown to significantly improve the previously known result, to accurately predict both the symbol error rate and bit error rate in the entire signal-to-noise ratio (SNR) region of interest. Using the derived formula, the optimal input power level for the soft-limiter channel is determined, and the corresponding minimal error rates for 16- and 32-APSK are quantified. The result is also interpreted in terms of optimal input back-off (IBO) for nonlinear power amplifiers by evaluating the performance degradation as a function of IBO. Copyright

Group-Based Multibit Cooperative Spectrum Sensing for Cognitive Radio Networks

In cooperative spectrum sensing (CSS), a multibit combination rule shows better sensing performance than one-bit hard combination rules at the sacrifice of the reporting overhead. To overcome the tradeoff between the sensing performance and the reporting overhead, we propose a novel group-based multibit CSS scheme with a limited reporting overhead. The proposed scheme adopts contention-based reporting to restrain the reporting overhead while achieving multiuser diversity with an increased number of secondary users (SUs). Moreover, SUs report one-bit sensing results to a fusion center instead of sending multibit quantization information, and the rest of the information is embedded in the time slot. The simulation results demonstrate that, as the number of SUs increases, the proposed scheme improves the sensing performance and the average throughput of SUs, whereas the conventional one-bit or multibit combination schemes show a tradeoff between the sensing performance and the throughput of SUs.

Performance of a fixed delay decoding scheme for tail biting convolutional codes

A fixed delay decoding scheme for tail biting convolutional codes is described and its performance is investigated. Unlike other proposed algorithms that have iterative structures, the scheme requires a fixed number of computations and is suitable for practical DSP implementations. An analytic bound on the error probability is derived to indicate the decoding complexity and performance trade-off, and to provide criteria to determine the decoding parameters. It is shown that for the rate 1/4 tail biting convolutional code used in IS-54/IS-136 digital cellular standard, the bit error rate of the decoding scheme has negligible degradation from the maximum likelihood performance while the decoding complexity is less than twice the complexity of a single Viterbi algorithm trial.

Interference Effects of Low-Power Devices on the UE Throughput of a CR-Based LTE System

Recently, the use of mobile devices has increased, and mobile traffic is growing rapidly. In order to deal with such massive traffic, cognitive radio (CR) is applied to efficiently use limited-spectrum resources. However, there can be multiple communication systems trying to access the white space (unused spectrum), and inevitable interference may occur to cause mutual performance degradation. Therefore, understanding the effects of interference in CR-based systems is crucial to meaningful operations of these systems. In this paper, we consider a long-term evolution (LTE) system using additional spectra by accessing the TV white space, where low-power devices (LPDs) are licensed primary users, in addition to TV broadcasting service providers. We model such a heterogeneous system to analyze the co-existence problem and evaluate the interference effects of LPDs on LTE user equipment (UE) throughput. We then present methods to mitigate the interference effects of LPDs by ‘de-selecting’ some of the UEs to effectively increase the overall sector throughput of the CR-based LTE system.

Performance improvement of channel estimation based on pilot structure variations for cellular OFDMA systems

An increasing number of systems are adopting orthogonal frequency division multiple access (OFDMA) as their high-speed wireless transmission method supporting multiple users, and impacts on performance of OFDMA systems under mobile cellular environments are currently being investigated. In this paper, channel state and quality estimation performance issues are addressed for varying types of pilot structures in the transmitted burst. It is demonstrated that significant performance variations can occur in estimation of the state and quality of the wireless link depending on the pilot pattern used in the preamble symbol. In particular, advantages of using disjoint patterns for downlink transmission from adjacent cells are quantitatively presented, by evaluating the bit error rate and the carrier-to-interference ratio (CIR) estimation performance.

Interference coordination of heterogeneous LTE systems using remote radio heads

In this paper, we present an operational strategy to mitigate co-channel interference (CCI) by using geographically distributed remote radio heads (RRHs). The inter-node CCI becomes a dominant performance degradation factor for heterogeneous network (HetNet) systems. Recently, there are emerging attempts in Third Generation Partnership Project to adopt advanced techniques to Long Term Evolution Advanced systems to mitigate CCI problems for HetNet systems, namely, the coordinated multipoint transmission (CoMP). However, the CoMP scheme cannot control the CCI generated from outside coordination boundaries. To resolve this problem, we propose a partial activation strategy by using RRHs deployed near cell edge which results in moving coverage boundary effects. Based on Monte Carlo system level simulations, performance of the conventional strategies and the presented strategy is evaluated. Simulation results show that the proposed scheme outperforms the enhanced inter-cell interference coordination and CoMP schemes especially for users located near cell edge areas.

Precoder and Capacity Expressions for Optimal Two-User MIMO Transmission with Per-Antenna Power Constraints

In this letter, multiuser multiple-input multiple-output (MIMO) precoding which achieves the Gaussian broadcast channel (GBC) capacity under the per-antenna power constraint (PAPC) is investigated. In particular, closed-form expressions for both the MIMO precoder and the corresponding sum capacity are presented for the two-user case. We show that the result can also be utilized to provide the two-user capacity expression under the total power constraint (TPC) of transmit antennas.