Yusung Lee

Performance Analysis of MIMO System with Linear MMSE Receiver

This paper considers the uncoded multiple-input multiple-output (MIMO) system with linear minimum mean square error (MMSE) detection under ideal fast fading. The distribution of SINR at the output of the MMSE detection is derived for a small number of transmit and receive antennas. We present new approximation for the Gaussian Q-function driven by numerical simulation. Based on the SINR distribution and new approximation for Q-function, we analyze the performance of linear MMSE detection under ideal fast fading environment. By comparing the analytical results and Monte Carlo simulated results, we validate the analytical results.

An efficient CP compensation for SC-FDE with insufficient CP symbols

In this letter, we propose a method to mitigate interferences for single carrier frequency domain equalization (SC-FDE) with insufficient cyclic prefix (CP) symbols. The resultant channel matrices are not circulant due to some missing symbols. Unlike OFDM, however, the reconstruction of missing symbols using the estimated symbols is not effective. We propose that these missing symbols may be filled with the symbols in the following received block when the channel is static over the consecutive blocks. Since this method does not require estimation of symbols, the precision of detection can be enhanced. The improvement in the performance of this method is still limited since interferences are not completely removed. An iterative interference cancellation method is used to remove the residual interferences. The performance of proposed method is verified by simulations.

Preamble design for joint estimation of CFO and I/Q imbalance for direct conversion OFDM system

This paper proposes a preamble structure for joint estimation of carrier frequency offset (CFO) and I/Q imbalance. Because existing methods cannot guarantee correct solutions within all range of CFOs, we derive two constraints of phase rotations applied to repetition patterns. By using the preamble based on the presented constraints, CFO is optimally estimated, and I/Q imbalance estimation is possible in all ranges of CFO with low complexity. Numerical results verify the importance and necessity of these constraints for joint estimation.

Performance of MC-CDM Systems With MMSEC Over Rayleigh Fading Channels

In this paper, we investigate the performance of multicarrier code-division multiplexing (MC-CDM) systems with minimum mean square error combining (MMSEC) over a Rayleigh fading channel. An approximated expression on the symbol error rate (SER) for MMSEC is derived. The approximated expression provides an accurate approximation of the true SER, particularly for systems with higher modulation and a small spreading factor. Through the derived error performances, we also calculate the achievable diversity order. MC-CDM systems with MMSEC achieve the same asymptotical diversity order of 1, regardless of the spreading factor. Within practical signal-to-noise ratio (SNR) ranges, the effective diversity order is further increased as a larger spreading factor is used. The derived analytical expressions are verified by numerical and simulation results.

A RAKE Receiver With an ICI/ISI Equalizer for a CCK Modem

In this paper, we first derive the theoretical performance of a complementary code keying (CCK) code on an additive white Gaussian noise (AWGN) channel and over a multipath channel. To derive the error performance, we use the weight and cross-correlation distributions of the CCK code for optimal and suboptimal decoding, respectively, based on union bound. In addition, we propose a RAKE receiver for a CCK modem, which is suitable for a multipath environment with a large delay spread. The RAKE receiver principle is acceptable for modest multipath because it can coherently combine multipath components to provide signal-to-noise ratio (SNR) enhancement. However, as the delay spread is larger and the data rate of systems goes higher, intersymbol interference (ISI) generated due to multipath environments are increased. To handle the increasing ISI, the CCK modem needs an equalization technique to remove the ISI, together with RAKE processing. Thus, our proposed system is based on a channel matched filter (CMF) with a decision feedback equalizer (DFE). The CMF is applied for RAKE processing, whereas the DFE structure is used for ISI cancellation. In our system, ISI is calculated and removed by using a decoded CCK codeword.

Low-Complexity Detections for Downlink MIMO MC-CDMA Systems

In this paper, we investigate and propose the detection techniques for spatially layered multiple-input multiple-output (MIMO) multi-carrier code division multiple access (MC-CDMA) systems. First, we propose a noise-predictive linear detector. It has the same bit error rate (BER) performance as symbol-level detector and reduces the complexity significantly when the system load is almost full. We also propose a partial minimum mean square error (MMSE)-ordered successive interference cancellation (OSIC) based on multi-user detection, which first detects the most powerful interfering data symbols transmitted through the determined transmit antenna and then cancels their contribution from the received signal by the multiplexed data symbol vector. The nulling and cancelling processes between the user data symbols from the same transmit antenna are not performed. The proposed algorithms are verified by computer simulation

A RAKE receiver with ICI/ISI equalizer for CCK wireless LAN modem

We propose a new type of RAKE receiver for complementary code keying (CCK) codes, which is suitable for a multipath environment with large delay spread. As the delay spread becomes larger, inter-symbol interference (ISI) and inter-chip interference (ICI) generated due to multipath environments are increased. To handle the increasing ISI and ICI, the CCK wireless LAN modem needs ISI and ICI cancellation techniques, together with RAKE processing. So, our proposed system is based on a channel matched filter (CMF) with decision feedback equalizer (DFE) and includes a codeword DFE structure. In our system, ICI and ISI are calculated by using the detected CCK codeword. Also, our proposed receiver uses the error correcting capability of CCK codes, and it can remove ISI and ICI at the same time.

Interference Cancellation for Single Carrier Frequency Domain Equalizer without Cyclic Prefix

In this paper, we propose a method to mitigate the interferences for single carrier frequency domain equalizer (SC-FDE) without cyclic prefix (CP). SC-FDE systems suffer from the interferences when CP is appended insufficiently. Inter block interference (IBI) is introduced by the previous block, and inter symbols interference (ISI) is also generated by imperfection of the circulant channel matrix. The proposed method can effectively restore the channel matrix to be circulant, but the interference is also increased. Thus, some method, to make a compromise between compensating ISI and increasing the interference, is needed and is that the criterion is adopted to maximize the signal to interference plus noise ratio in this paper. However, the improvement of the performance is limited since the interferences are not removed. To overcome it, the interferences are cancelled using iterative MMSE receiver structures. The performance of the system outperforms that of conventional CP reconstruction method, and it is verified by computer simulations.

Joint Estimation of Carrier Frequency Offset and I/Q Imbalance for Direct Conversion OFDM System with Constrained Preambles

This paper addresses a joint estimation method for carrier frequency offset (CFO) and I/Q imbalance using constrained preamble. Because existing methods can not guarantee correct solutions within all range of CFO, we derive two constraints of phase rotation adjusted to repetition patterns. Using these constraints, proposed estimators can find the optimal estimate of CFO, and low complexity I/Q imbalance estimation is possible in all range of CFO with no exceptions. Numerical results show the importance of these constraints for joint estimation and, proposed scheme can achieve almost same bit error rate performance with perfect estimation scheme.

Effects of Subcarrier Interleaving on LDPC Coded MC-CDMA Systems

This paper describes the effect of subcarrier interleaving on low density parity check (LDPC) coded multi-carrier code division multiple access (MC-CDMA) system. Subcarrier interleaving in MC-CDMA system can achieve the randomization effect of burst error in frequency do- main, but it generates the distortion among the spreading codes due to the destruction of orthogonality. In channel-coded system ,h o wever, the achievable frequency diversity effect associated with the subcarrier interleaving in frequency domain also depends on the channel coding gain. So, there is a trade off between frequency diversity gain via sub- carrier interleaving or channel coding, and destruction of orthogonality. From the simulation results, frequency diversity gain by channel coding is more effective than subcarrier interleaving, especially for system with high order modulations and with low code rates.