Seong Rag Kim

A constrained MMSE receiver for DS/CDMA systems in fading channels

The convergence problem of minimum mean square-error (MMSE) receivers is discussed, and to overcome the problem, a constrained MMSE receiver is proposed. In addition, we propose the orthogonal decomposition-based least mean square algorithm to implement the constrained MMSE receiver adaptively. Through computer simulations, it is shown that the proposed receiver provides significant performance improvement in the bit-error rate over the conventional matched filter receiver and currently available MMSE receivers.

A call control scheme for soft handoff in CDMA cellular systems

a new direction-based handoff call control scheme (DBHCCS) for CDMA cellular systems is proposed. In the proposed scheme, a neighbor base station ignores unnecessary soft handoff calls requested by mobile stations which are not actually approaching the neighbor base station, while protecting real soft handoff calls by employing a queue. We proposed a new method of determining the priorities of queued handoff calls. In this method, a mobile station predicts the time when its handoff call is forcedly terminated, by using the changing rate of the received pilot strength from the serving BS and using the current time broadcast by the sync channel of the serving BS as a reference time. The service priority of queued handoff calls is determined based on the predicted forced termination time reported by mobile stations. The performance of the DBHCCS is compared with that of the first-in/first-out (FIFO) scheme and the measurement-based prioritization scheme (MBPS). The simulation results show that the probability of new call blocking for the DBHCCS is lower than that of the FIFO scheme or the MBPS queuing discipline. It is also shown that the probability of forced termination for the DBHCCS is less than or equal to that of the conventional CDMA cellular system with the MBPS queuing discipline.

Statistical Eigen-Beamforming With Selection Diversity for Spatially Correlated OFDM Downlink

Statistical eigen-beamforming is shown to be effective over a spatially correlated fading channel. In the paper, we investigate the use of the statistical eigen-beamforming with short-term selection for an orthogonal frequency-division multiplexing (OFDM) downlink. The contributions of the paper are as follows: (1) an efficient statistical eigen-beamforming method with limited feedback for OFDM has been proposed; and (2) exact and asymptotic expressions of the average bit error rate for (transmitter side) spatially correlated Rayleigh fading channels are derived. In particular, the asymptotic expression allows us to gain insight to statistical eigen-beamforming with selection diversity in terms of array gain and diversity order.

Eigenbeamforming with selection diversity for MIMO-OFDM downlink

Eigenbeamforming is shown to be effective over a spatially correlated fading channel. In conjunction with the short-term selection of eigenmodes, it can also provide good transmit diversity gain with a smaller amount of feedback. In this paper, we investigate the use of eigenbeamforming with short-term selection for orthogonal frequency-division multiplexing (OFDM) systems. We derive exact and asymptotic expressions of the average bit error rate (BER) for a spatially correlated Rayleigh fading channel. Especially. the asymptotic expression allows us to gain insight into eigenbeamforming.

Cholesky Based Efficient Algorithms for the MMSE-SIC Receiver

The minimum mean square error with successive interference cancellation (MMSE-SIC) receiver is known to achieve the capacity of multiple-input multiple-output (MIMO) fast fading channels in the presence of knowledge of the channel at the receiver. This paper presents efficient and numerically stable Cholesky decomposition based detection algorithms for MMSE-SIC, exploiting a property of ordering of MMSE-SIC. The proposed algorithms are shown to significantly reduce the computational complexity of existing efficient algorithms for SIC in MIMO flat fading channels.

Adaptive LMMSE receivers for CDMA systems over time-varying multipath fading channels

Adaptive linear minimum mean square error (LMMSE) receivers for frequency selective fading channels are developed by modifying the constrained LMMSE receiver of Kim, Jeong and Choi (see IEEE Trans. Commun., vol.48, p.1793-96, 2000). In particular, additional constraints regarding interpath interference are imposed on the constrained optimization proposed by Kim et al., and simple adaptation rules are derived by converting the constrained MMSE problem into an unconstrained optimization. It is shown that the instantaneous SINR of the proposed receiver is always higher than that of the RAKE receiver. Computer simulation results indicate that the proposed receivers can perform better than the existing RAKE and adaptive LMMSE receivers in multipath fading environments.

Incorporation of adaptive interference cancellation into parallel interference cancellation

Subtractive interference cancellation receivers estimate and subtract out the multiaccess interference (MAI) from the received signals. Thus, the receivers have potential to provide significant performance improvement over the conventional matched filter (MF) receiver. However, the interference from out-of-cells limits the performance improvement of subtractive interference cancellation receivers in multicell environments, since the base station contains the information of users only in its own cell. On the other hand, adaptive MMSE receivers can suppress all interference from out-of-cells as well as in-cell, nonetheless they show performance degradation in fading environments. This paper proposes an integrated scheme of a subtractive interference cancellation receiver and an adaptive MMSE receiver, and shows through computer simulation that the proposed scheme provides outstanding performance improvement over both receivers in multicell fading environments.

Multi-mode subtractive interference cancellation for asynchronous multi-path channels

Much work on multi-user detection has been done to increase the capacity and/or to enhance the performance of DS-CDMA systems. In this paper, we propose a subtractive interference cancellation (IC) called multi-mode interference cancellation (MIC). The proposed scheme overcomes the limitations of the successive (or serial) IC (SIC) and the multistage parallel IC (PIC) schemes, and can change the IC modes according to the processing speed of the hardware, the number of users, and the power spread of received signals. Especially, the structure is designed to work efficiently in the asynchronous multi-path channels. In addition, based on computer simulations over the asynchronous multi-path Rayleigh fading channels, we determine the system parameters and evaluate the performance of subtractive IC schemes.

Adaptive Hybrid Serial/Parallel Interference Cancellation in CDMA Systems

To increase the capacity of CDMA systems much work has been done on interference cancellation (IC) schemes, and recently a successive interference cancellation (SIC) for implementation perspective was introduced. In this paper, we propose an adaptive hybrid serial/parallel interference cancellation (AHSPIC) and present performance comparisons of IC schemes via computer simulation over a Rayleigh fading channel. We find that the AHSPIC has not only capacity improvement compared to the single-user detector and SIC, but also outperforms the parallel IC for the large number of users in which the IC schemes are supposed to play a major role.

A modified turbo principle for iterative detection and decoding

This paper presents a modified turbo principle (MTP) for iterative detection and decoding in multiple-antenna systems whose detector uses the max-log-MAP algorithm. The modified turbo principle intensionally introduces dependency between the messages of the max-log-MAP detector and the decoder. Our simulation results show that the proposed MTP significantly reduces the performance loss due to the suboptimal detection algorithm, max-log-MAP. We also substantiate our claims by analyzing the proposed MTP from an information theoretic perspective.