Seung Hoon Nam

Differential space time block codes using nonconstant modulus constellations

We propose differential space time block codes (STBC) using nonconstant modulus constellations, e.g., quadrature amplitude modulation (QAM), which cannot be utilized in the conventional differential STBC. Since QAM constellations have a larger minimum distance compared with the phase shift keying (PSK), the proposed method has the advantage of signal-to-noise ratio (SNR) gain compared with conventional differential STBC. The QAM signals are encoded in a manner similar to that of the conventional differential STBC. To decode nonconstant modulus signals, the received signals are normalized by the channel power estimated forgoing training symbols and then decoded with a conventional QAM decoder. Assuming the knowledge of the channel power at the receiver, the symbol error rate (SER) bound of the proposed method under independent Rayleigh fading assumption is derived, which shows better SER performance than the conventional differential STBC. When the transmission rate is more than 3 bits/channel use in time-varying channels, the simulation results demonstrate that the proposed method with the channel power estimation outperforms the conventional differential STBC. Specifically, the posed method using the channel power estimation obtains a 7.3 dB SNR gain at a transmission rate of 6 bits/channel use in slow fading channels. Although the performance gap between the proposed method and the conventional one decreases as the Doppler frequency increases, the proposed method still exhibits lower SER than the conventional one, provided the estimation interval L is chosen carefully.

Transmit power allocation for an extended V-BLAST system

Vertical Bell Laboratories Layered Space-Time (V-BLAST) is a promising system that realizes the enormous capacity of multiple-input multiple-output (MIMO) communications. We present an extension of V-BLAST, and propose an effective transmit power allocation scheme for the extended system. The proposed transmit power allocation scheme minimizes the bit error rate (BER) averaged over all detection stages, and requires small feedback overhead from the receiver to the transmitter. Simulation results show that the extended V-BLAST system with the proposed transmit power allocation scheme provides a significant reduction in the BER compared to the conventional V-BLAST system. When the minimum mean square error (MMSE) nulling is adopted, the extended V-BLAST system is found to achieve the BER performance comparable to that of the maximum likelihood (ML) detection for the conventional V-BLAST architecture.

Differential space time block codes using QAM constellations

The propose differential space time block codes (STBC) using quadrature amplitude modulation (QAM), which can not be utilized in the conventional differential STBC. Since QAM constellations have a larger minimum distance compared with phase shift keying (PSK), the proposed method has the advantage of SNR gain compared with conventional differential STBC. The QAM signals are encoded in a manner similar to that of conventional differential STBCs. To decode QAM signals, the signals received are normalized by the channel power estimated forgoing training symbols, and then decoded with a conventional QAM decoder. When the transmission rate is more than 3 bits/channel use in time varying channels, the simulation results demonstrate that the proposed method with the channel power estimation outperforms the conventional differential STBC.

Differential space time block codes using QAM for four transmit antennas

We develop differential space time block codes (STBC) using non-constant modulus constellations for four transmit antennas. The proposed method improves on the conventional differential STBC techniques because of the larger minimum distances of non-constant modulus constellations. The transmitted signals are modulated using pulse amplitude modulation (PAM). Encoding is similar to that of the conventional differential STBC, while the receiver is different. The signal at the receiver is divided by the estimated channel power and then decoded using a conventional QAM decoder which treats pairs of transmitted PAM symbols transmitted from two transmit antennas as one QAM symbol. For transmission rates greater than 2 bits/channel use, the proposed method outperforms the conventional differential STBC.

Differential Spatial Multiplexing for Two and Three Transmit Antennas

In this paper, we construct a differential spatial multiplexing method that can trade-off spatial diversity for increased transmission rate in differential multi-input multi-output (MIMO) systems. To achieve a desired multiplexing gain while providing simple encoding and decoding, Gram-Schmidt algorithm is used at the transmitter to construct the unitary transmission matrix. In addition, by applying optimum scaling factors to generate transmission matrix, the symbol error rates can be minimized. Because the transmission matrix itself is not the information we are interested in, the equivalent channel parameters for each transmitted information symbols are calculated and the linear signal model for the information symbols is derived. Since that model is exactly the same as that of the coherent spatial multiplexing, all kinds of coherent detection schemes can be directly applied. The advantage of the proposed scheme over the differential space-time block codes (STBC) and the effects of scaling factor are investigated by computer simulation. In high transmission rate regimes, the proposed differential spatial multiplexing method can outperform the differential STBC having the same transmit rate in the signal to noise ratios (SNRs) we are interested in.

A random beamforming technique in MIMO systems exploiting multiuser diversity

A random beamforming technique for MIMO systems that simultaneously obtains downlink multiuser diversity gain, spatial multiplexing gain, and array gain by feeding back only effective SNRs is described. In addition, power control using waterfilling is employed to improve the throughput of our method. In a slow fading channel, we prove that the throughput of the proposed method converges to that of eigen beamforming when many users are in a cell. The number of users required to achieve capacity bound increases with the number of antennas and SNR. However, the capacity bound is achieved even with a small number of users, e.g. 16 users is a cell, when the SNR is low, e.g., 0 dB, and the number of transmit and receive antenna is small, e.g., 2.

A Full-Diversity Full-Rate Space-Time Block Code Design for Three Transmit Antennas

A design of non-orthogonal 3 × 3 space-time block code (STBC) is proposed. The proposed design achieves full rate, full level diversity, and maximum coding gain by symbol rotation (SR) method. In addition, the proposed scheme has lower encoding complexity than the unitary constellation-rotation (CR) STBC, while two methods exhibit the same.

Design of punctured space-time trellis codes

We develop a method to puncture a space-time trellis code (STTC) that increases the code rate at the cost of increasing the frame error rate (FER). In order to minimize the FER, the puncturing pattern and constituent convolutional codes that preserve the diversity gain for the short error sequences are presented. The rate 4/3 and 8/7 punctured STTCs achieve two-level diversity gain when the constraint length is five and the frame size is 16. However, the punctured STTCs require 1.3 dB and 0.7 dB more power to attain the same FER as the STTC without puncturing, respectively.

High Rate Space Time Block Codes

High Rate Space-Time Block Codes (HR-STBCs) with greater than 1 symbol/transmission and simple decoding schemes are proposed. The HR-STBC demonstrates 3 dB E b /N 0 gain at BER = 10 -3 compared with the conventional STBC when three transmit antennas and two receive antennas are utilized.

Differential Space Time Block Codes Using Nonconstant Modulus Constellations for Four Transmit Antennas

We extend the differential space time block code (STBC) using nonconstant modulus constellations of two transmit antennas to four transmit antennas case. The proposed method obtains larger minimum Euclidean distances than those of conventional differential STBC with PSK constellations. We derive the symbol error rate (SER) performance of the proposed method and demonstrate the SER performance using computer simulations for both static and fast fading channels. For transmission rates greater than 2 bits/channel use and 3 bits/channel use, the proposed method outperforms the conventional differential STBC.