Eui-Rim Jeong

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.

Data-aided approach to I/Q mismatch and DC offset compensation in communication receivers

A digital signal processing technique for compensating both the I/Q mismatch and the DC offset in communication receivers is derived with an emphasis on direct-conversion architectures. The I/Q mismatch and DC offset are estimated in a least-squares sense using a training sequence. Also, a group of training sequences that minimizes the mean square error of the estimate is determined. The advantages of the proposed technique are demonstrated through computer simulation.

Adaptive Predistortion With Direct Learning Based on Piecewise Linear Approximation of Amplifier Nonlinearity

We propose an efficient Wiener model for a power amplifier (PA) and develop a direct learning predistorter (PD) based on the model. The Wiener model is formed by a linear filter and a memoryless nonlinearity in which AM/AM and AM/PM characteristics are approximated as piecewise linear and piecewise constant functions, respectively. A two-step identification scheme, wherein the linear portion is estimated first and the nonlinear portion is then identified, is developed. The PD is modeled by a polynomial and its coefficients are directly updated using a recursive least squares (RLS) algorithm. To avoid implementing the inverse of the PAs linear portion, the cost function for the RLS algorithm is defined as the sum of differences between the output of the PAs linear portion and the inverse of the PAs nonlinear portion. The proposed direct learning scheme, which is referred to as the piecewise RLS (PWRLS) algorithm, is simpler to implement, yet exhibits comparable performance, as compared with existing direct learning schemes.

Adaptive Compensation for Power Amplifier Nonlinearity in the Presence of Quadrature Modulation/Demodulation Errors

This correspondence proposes techniques that jointly compensate for amplifier nonlinearity and quadrature modulation/demodulation (QM/QDM) errors. The proposed methods are derived based on the polynomial predistortion (PD) employing the indirect learning technique and do not require any additional feedback loop for QM/QDM-error compensation. Compared with the existing joint compensation technique, the proposed methods need some additional parameters to be estimated but exhibit faster convergence and better performance. The advantage of the proposed technique is demonstrated through computer simulation.

Least squares frequency estimation in frequency-selective channels and its application to transmissions with antenna diversity

A new data-aided frequency estimator for frequency-selective fading channels is introduced. The proposed estimator is developed based on a least squares (LS) error criterion and can estimate frequency offsets without the need for channel information. Statistical analysis indicates that the resulting estimate is unbiased and tends to approach the Cramer-Rao lower bound (CRLB). Simulation shows that the proposed LS method is preferable to existing techniques in mobile communications. The application of the LS estimator to systems with transmitter antenna diversity is also considered. In particular, it is demonstrated that the LS method can be successfully applied to third-generation wireless communication systems.

Data-aided frequency estimation for PSK signaling in frequency-selective fading

A new data-aided frequency estimator is introduced for phase-shift keying signals transmitted over frequency-selective fading channels. This estimator is developed based on a maximum likelihood criterion. It assumes the use of a special class of pilots, called near-i.i.d. (independent identically distributed) sequences, with impulsive fourth-order moments. With the help of such pilots, the proposed method can estimate frequency offsets without the need for channel information. The pilots of GSM and IS-136 mobile communication systems have been observed as being near-i.i.d., and statistical analysis indicates that the proposed estimate is almost unbiased if the pilot is near-i.i.d. The advantage of the proposed estimator over conventional methods is demonstrated via computer simulation.

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.

A Computationally Efficient Criterion for Antenna Shuffling in DSTTD Systems

The application of double space-time transmit diversity (DSTTD) scheme to multicarrier systems, such as orthogonal frequency division multiplexing (OFDM) systems, requires calculating the determinations of the antenna permutation matrices for all subcarriers, resulting in a heavy computation load. In this paper, we show that the signal-to-noise ratio (SNR)-based antenna shuffling criterion for DSTTD systems can be reduced to a simple criterion that evaluates determinants of 2 times 2 submatrices of the 4 times 4 equivalent channel matrix. The new criterion can lighten the computational load by about 95%. Furthermore, it is shown that the minimum mean square error (MMSE)-based criterion for antenna permutation can also be reduced to the same criterion.

Digital predistortion based on envelope feedback

A new digital predistortion (DPD) technique based on envelope feedback is proposed for linearization of power amplifiers (PAs). Unlike conventional DPD techniques that need frequency down converters (FDCs) in the feedback path to recover the complex envelope of the PA output, the proposed technique does not need an FDC. Instead it employs two envelope detectors, estimating the envelope of the PA output and that of the difference signal between the PA input and the PA output. It is shown that the complex envelope of the PA output can be estimated from the two envelope feedbacks if the sign of the phase distortion of the AM-PM characteristic remains the same for all PA input magnitudes. Simulation results show that the performance of the proposed DPD is comparable to the conventional DPD employing an FDC.

Beamforming and PAPR Reduction for MISO-OFDM Systems

A peak-to-average power ratio (PAPR) reduction technique for multi-input single-output (MISO) orthogonal frequency division multiplexing (OFDM) systems with beamforming is proposed. This technique consists of adaptive antenna selection and phase rotation. By using the proposed method, a large reduction in PAPR can be obtained at the expense of a small signal-to-noise ratio (SNR) loss. According to the simulation results, the PAPR is reduced by 10% in terms of complementary cumulative distribution function (CCDF) with negligible SNR loss. If a 1 dB SNR loss is allowed, a 33% reduction in PAPR can be achieved.