Jeng-Shin Sheu

An EM Algorithm-Based Channel Estimation for OFDM Amplify-And-Forward Relaying Systems

The issue of channel estimation is investigated for the orthogonal frequency-division multiplexing (OFDM) relaying system with amplify-and-forward (AF) operation mode. Channel estimation in the AF mode is unique in that both the source-to-relay and relay-to-destination channels are needed at the destination in order to perform optimum combining of the received signals from the direct (source-to-destination) and relay paths if a cooperative diversity transmission is employed. Until now, however, there have been only literatures to study the estimation of the composite channel of the relay path. In this work, an iterative expectation-maximization (EM) algorithm-based channel estimation is proposed for respective estimations of the source-to-relay and relay-to-destination links at the destination. Computer simulations show that the iterative channel estimator performs satisfactorily and converges rapidly.

A feasible method to implement optimum CIR-balanced power control in CDMA cellular mobile systems

Power control is essential in the design of direct-sequence code-division multiple-access (DS-CDMA) techniques. Optimum power-control (OPC) methods with carrier-to-interference ratio (CIR) balancing have been formulated as eigenvalue problems for frequency-division/time-division multiple-access (FDMA/TDMA) cellular systems. For the CDMA cellular system, its OPC was also formulated as an eigenvalue problem based on a large link-gain matrix. The OPCs with CIR-balancing were realized by solving eigenvalue problems of link-gain matrices. We reformulate the CIR-balanced OPC in CDMA cellular systems by benefiting from the power constraints as an eigenvalue problem based on a novel link-gain matrix. For a feasible implementation, a two-level hierarchical power-control structure is proposed to carry out the eigendecomposition which is required for the CIR-balanced OPC. Shortages of unbalanced CIR and global outage are two common issues in CIR-balanced power control. To tackle these two problems, a simple linear prediction method and an adaptive on-off strategy are proposed. Furthermore, because of the capacity limitation of wireless communications, a differential pulse code modulation scheme is presented to reduce the number of bits required for the transmission of command words in the two-level hierarchical power-control structure.

Joint Preprocessing Techniques for Downlink CoMP Transmission in Multipath Fading Channels

An elaborate coordination among multiple cells called coordinated multipoint (CoMP) transmission/reception is adopted by LTE-A to improve coverage and cell-edge throughput. This paper derives various joint precoding techniques among coordinated cell sites to address the serious co-channel interference (CCI) for downlink CoMP broadband transmission. They include joint time-division multiple access (J-TDMA), joint zero-forcing (J-ZF), joint minimum mean square error (J-MMSE), and joint null-space decomposition (J-NSD). The J-TDMA scheme is automatically free from CCI at the cost of sacrificing spectral efficiency. The J-ZF scheme enables complete suppression of interference, but is at the expense of demanding higher transmitter power on precoding process. The performance of J-MMSE is close to the J-TDMA since it makes a good tradeoff between interference suppression and transmitter power efficiency. In J-NSD, the CCI is completely removed but it comes with a penalty in that a users transmission is constrained to a smaller subspace. The precoding is done under the perfect channel state information and thus the simulation results in this paper provide an upper-bound reference for these precoding schemes.

An optimum downlink power control method for CDMA cellular mobile systems

Power Control is essential in the design of DS-CDMA techniques. The optimum power control (OPC) methods with carrier-to-interference ratio (CIR) balancing have been identified as eigenvalue problems. For the CDMA cellular system, Wu (see IEEE Trans. Veh. Technol., vol.48, p.571-5, 1999) formulated the CIR-balanced power control problem to an eigenvalue problem based on a newly proposed link-gain matrix. However, the size of the matrix is proportional to the number of active mobiles in a CDMA cellular system. Therefore the implementation will be infeasible due to the exceedingly large size. We present a novel link-gain matrix, whose size is only proportional to the number of cells of a whole CDMA system. Based on the proposed link-gain matrix, the CIR-balanced power control in a CDMA cellular system is formulated as an eigenvalue problem and the corresponding implementation algorithm is presented as well that is optimum in the sense that the interference probability is minimized. As a result of the small size of link-gain matrix, the proposed OPC algorithm has better convergence performance than Wus. The numerical results indicate that our proposed OPC method achieves the same performance as Wus regardless of the pretty small size of the proposed link-gain matrix.

Characteristics and modelling of inter-cell interference for orthogonal frequency-division multiple access systems in multipath Rayleigh fading channels

In a multi-cellular system, inter-cell interference is one of the major impairments that limit system performance. Accurate knowledge of inter-cell interference is a key step towards determination of link performance and system capacity efficiently. For orthogonal frequency-division multiple access (OFDMA) systems, the inter-cell interference has been studied in the literature mainly through computer simulation, which is very time-consuming, particularly in a heavily loaded operation. Through an analytical approach, this study investigates the characteristics and modelling of inter-cell interference for OFDMA systems in multipath Rayleigh fading channels. Analytical results indicate that the inter-cell interference exactly has a Gaussian distribution, provided that it is modulated by phase-shift keying. Otherwise, the inter-cell interference is multivariate Gaussian-mixture distributed. In addition, the cross-correlation of interference among subcarriers is insignificant, according to the analytical result. Simulation results also indicate that the cross-correlation can be safely ignored while modelling the inter-cell interference. In summary, the authors accurately characterise the inter-cell interference and analytical results provide an efficient way to generate inter-cell interference for OFDMA multi-cellular systems, which is very time-consuming through real computer simulations.

On channel estimation of orthogonal frequency-division multiplexing amplify-and-forward cooperative relaying systems

This study investigates channel estimation for amplify-and-forward (AF) relaying systems. For a two-hop relaying system, channel estimation of a relay path (source-relay-destination) with AF relaying is unique because both individual source-to-relay (S-R) and relay-to-destination (R-D) links are necessary at the destination to perform the optimum combination of signals received from the direct path (source-to-destination) and relay path if employing a cooperative diversity transmission. Until now, most literature about channel estimation on relaying systems has focused on the composite channel of the relay path, and not its individual links. Current methods to separate the estimation of both individual links require the relay to either deliver a quantised version of the S-R link estimate through an independent feed-forward channel, or to perform discrete Fourier transform/inverse discrete Fourier transform operations before amplifying the received signals. In this study, the respective channel estimations are done at the destination. First, the destination utilises a relay amble (R-amble) to obtain channel estimation of the R-D link. Next, exploiting the R-D link estimate, a maximum-likelihood estimator is proposed for the S-R link in a decision-directed (DD) way. Simulation results show that the proposed scheme out performs the current relay-assisted methods in most circumstances. In addition, numerical results reveal that the mean-square error can be significantly reduced in a DD way for symbol error rates of interest.

A Novel Convergence Accelerator for the LMS Adaptive Filter

Due to its ease of implementation, the least mean square (LMS) algorithm is one of the most well-known algorithms for mobile communication systems. However, the main limitation of this approach is its relatively slow convergence rate. This paper proposes a booster using the Markov chain concept to speed up the convergence rate of LMS algorithms. The nature of Markov chains makes it possible to exploit past information in the updating process. According to the central limit theorem, the transition matrix has a smaller variance than that of the weight itself. As a result, the weight transition matrix converges faster than the weight itself. Therefore, the proposed Markov-chain based booster is able to track variations in signal characteristics and simultaneously accelerate the rate of convergence for LMS algorithms. Simulation results show that the Markov-chain based booster allows an LMS algorithm to effectively increase the convergence rate and further approach the Wiener solution. This approach also markedly reduces the mean square error while improving the convergence rate.

On the adaptability of OFDM-CDMA forward link with time-frequency spreading in multi-path fading channels

OFDM-CDMA (orthogonal frequency division multiplexing, code division multiple access) with joint time and frequency spreading has a great potential to achieve very high spectrum efficiency by exploiting both the temporal and spectral characteristics of the multi-path fading channels. A full realization of this potential however is contingent on the systems ability to adapt to the changing operating environments. In this paper, we explore this adaptability for the forward link of such a system by adapting the receiver detector, spreading pattern, spreading format and spreading code to the channel and traffic conditions. Numerical results show that significant performance improvement can be achieved as compared to the non-adaptive counterparts.

Cell sectorization and power management for inter-cell interference coordination in OFDMA-based network MIMO systems

Network multiple-input multiple-output (MIMO) aims at mitigating the inter-cell interference and improve the performance of cell-edge users (CEUs) through multi-cell cooperative transmissions. This paper investigates the cell sectorization and power management on top of fractional frequency reuse (FFR) based network MIMO systems. It is shown that both the signal strength and interference quantity greatly depend on the layout of cell sectorization. Specifically, a commonly used one is shown to be inappropriate. It is found that there exists a method of sectorization that causes less interference to cell-center users (CCUs) and meantime offers better signal quality to CEU during network MIMO transmissions. Also, results show that the proposed method of managing power can improve the signal quality of other cells CEUs and not degrade the performance of intra-cell users at the same time.

Performance of optimum transmitter power control in multimedia CDMA cellular mobile systems

Power control is very essential to the design of a CDMA cellular system. There have been several studies done on the estimation of the performance bounds of the transmitter power control scheme. However, these researches were focused on the single-type service. This paper investigates the optimum multimedia power control (OMPC) for multimedia CDMA cellular systems. The stability of the OMPC is formulated as an eigenvalue problem. It is also shown that both uplink and downlink systems have the same system performance. Simulation results indicate that the OMPC can satisfy the effective requirements of QoS among various service types.