Quasi Orthogonal Research Articles

Novel detection schemes for generalized quadrature spatial modulation

SummarySpatial Modulation (SM) is a technology that incorporates digital modulation and coding with multiple antennas to achieve high data rates along with improved energy efficiency. Quadrature spatial modulation (QSM), which extends the transmitted signals to in‐phase and quadrature domains, has recently been proposed to improve spectral efficiency (SE) for SM. In this paper, we have considered a combination of Generalized QSM (GQSM) and Quasi Orthogonal Space Time Block Codes (QOSTBC). We propose a new detection scheme for the GQSM and use three different prevailing detection schemes for QOSTBC. We present Bit Error Rate (BER) versus average SNR performance for the combined system using M‐QAM constellations with quasi‐static spatially uncorrelated Rayleigh fading channels and computational complexity in terms of numbers of addition/multiplication and search time. We have compared the considered system with prevailing GQSM system and shown that the prevailing system, as per active antenna permutation (AAP) in GQSM, offers number of transmitted bits ranging from 2 to 9 whereas the proposed system offers number of transmitted bits ranging from 6 to 16 in the same bandwidth. Thus, there is a visible gain in SE. Furthermore, we have compared the BER performance of the considered system with the performance of maximum likelihood detector (MLD) and spatial modulation (SM). In the considered system, the QOSTBC improves the diversity gain. Hence, we have shown that the considered Scheme 3 substantially outperforms the SM. Further, the considered system with Scheme 3 has nearly similar BER performance with the maximum likelihood detector (MLD), which has higher computational complexity.

On the performance of code word diversity based quasi orthogonal space time block codes in multiple-input-multiple-output systems

In the recent past, a lot of researches have been put into designing a Multiple-Input-Multiple-Output (MIMO) system to provide multimedia services with higher quality and at higher data rate. On par with these requirements, a novel Quasi Orthogonal Space Time Block Code (QOSTBC) scheme based on code word diversity is proposed, which is a multi-dimensional approach, in this paper. The term code word diversity is coined, since the information symbols were spread across many code words in addition to traditional time and spatial spreading, without increasing transmission power and bandwidth. The receiver with perfect channel state information estimates the transmitted symbols with less probability of error, as more number of samples is available to estimate given number of symbols due to the extra diversity due to code words. The simulation results show a significant improvement in the Bit Error Rate (BER) performance of the proposed scheme when compared with the conventional schemes.

Performance Evaluation of Correlation Properties of Quasi Orthogonal Prime Codes for Optical CDMA Communication

AbstractOptical code division multiple access (OCDMA) is one of the promising technologies to be implemented using all-optical networks. The performance of OCDMA system is largely dependent on the particular codes used for encoding. A proper choice of code family as well as code dimension is of prime importance due to various transmitter and receiver design considerations. This paper mainly aims at evaluation of desirable characteristics of various spreading codes based on the prime sequences. An extensive effort has been done in this paper to evaluate the correlation characteristics of various types of one-dimensional (1-D), 2-D and 3-D prime codes for OCDMA communication system. The detailed mathematical modelling has also been explained. It is observed that an addition in coding dimension leads to significant improvement in the code characteristics.

Quasi Orthogonal Space Time Block Code Over Nakagami-m Frequency Selective Fading Channels with Simple Decoding Complexity

In this paper, a Quasi Orthogonal Space Time Coding (QOSTBC) system over Nakagami-m frequency selective fading channels is introduced. The proposed scheme is carried out for the M-QAM modulation schemes assuming imperfect channel state information at the receiver. Inter symbol interference effect introduced due to the assumption of the frequency selective fading channels is eliminated by using the Orthogonal Frequency Division Multiplexing (OFDM) technology. Through the Bit Error Rate (BER) versus Signal to Noise Ratio (SNR) simulation results under different values of both Nakagami-m parameter and channel estimation error, the validity of the proposed technique is demonstrated. The results confirm that the BER in the case of frequency selective fading is overlapped with that of flat fading. This implies that the use of OFDM technology mitigates the effect of Inter-Symbol Interference (ISI) caused by frequency selectivity in the channel. Also, the results show that as the imperfect channel estimation error increases the performance degrades.

A Cyclotomic Lattice Based Closed Loop QOSTBC for Four Transmit Antennas

Drawback of the Space Time Block Code (STBC) is, for complex constellations, full rate and full diversity design exist only for two transmit antennas. In this paper, we propose a novel closed loop Quasi Orthogonal Space Time Block Code (QOSTBC) system based on cyclotomic lattices for four transmit antennas. It is a full rate and full diversity design where the information bits are mapped into four dimensional (4-D) lattice points. The bit error and symbol error performance of the system are evaluated by simulation.

An Outage Probability in Cooperative MIMO under (Alamouti, Orthogonal and Quasi Orthogonal STBC) Slow Fading Channel

Slow fading channel is one of the most important channels, which appears widely in a cellular mobile system. It has several problems such as bad effects of fading, which causes an attenuation to the signal. This paper presents a new scheme in cooperative communication system under slow fading channel to enhance and increase the quality of communication systems performance. This new scheme is called a cooperative multiple input–multiple output Antenna. The main idea of this scheme depends on transmitting multi copy of message via two paths. It’s performance has been compared with MIMO technique in term of outage probability. The results show that the negative effects of fading are mitigated and the outage probability and diversity gain are enhanced. Furthermore, the reliability in communication system under slow fading channel is improved.

준 독립적 특성 기반의 시간이동 LFM 신호를 이용한 거리-도플러 처리에 대한 연구

준 독립적 특성 기반의 시간이동 LFM 신호를 이용한 거리-도플러 처리에 대한 연구

On the convergence of Q-OR and Q-MR Krylov methods for solving nonsymmetric linear systems

This paper addresses the convergence behavior of Krylov methods for nonsymmetric linear systems which can be classified as quasi-orthogonal (Q-OR) or quasi-minimum residual (Q-MR) methods. It explores, more precisely, whether the influence of eigenvalues is the same when using non-orthonormal bases as it is for the FOM and GMRES methods. It presents parametrizations of the classes of matrices with a given spectrum and right-hand sides generating prescribed Q-OR/Q-MR (quasi) residual norms and discusses non-admissible residual norm sequences. It also gives closed-form expressions of the Q-OR/Q-MR (quasi) residual norms as functions of the eigenvalues and eigenvectors of the matrix of the linear system.

Quasi orthogonal Jacobi polynomials and best one-sided [formula omitted] approximation to step functions

We find the polynomials of the best one-sided approximation to a step function on [−1,1]. We prove that these polynomials are obtained by Hermite interpolation at the zeros of some quasi orthogonal Jacobi polynomials. We discuss the cases when there is uniqueness and, if there is not, we determine the extremal points of the convex set of the best approximants.

Comparative Study of Open-Loop Transmit Diversity Schemes with Four Antennas in DFT-Precoded OFDMA Using Turbo FDE and Iterative Channel Estimation

This paper compares the block error rate (BLER) performance of open-loop (OL) transmit diversity schemes with four antennas in the uplink for discrete Fourier transform (DFT)-precoded orthogonal frequency division multiple access (OFDMA). One candidate is a quasi-orthogonal (QO) - space-time block code (STBC) in which four-branch minimum mean-square error combining (MMSEC) is achieved at the cost of residual inter-code interference (ICI). Another candidate is a combination of STBC and selection diversity called time switched transmit diversity (TSTD) or frequency switched transmit diversity (FSTD). We assume a turbo frequency domain equalizer (FDE) associated with iterative decision-feedback channel estimation (DFCE) at a receiver to reduce the influences of the residual ICI and channel estimation (CE) error. From the extensive link-level simulation results, we show that a combination of STBC and TSTD (FSTD) is suitable as four-antenna OL transmit diversity scheme for DFT-precoded OFDMA using turbo FDE and iterative DFCE.

A Reduced-State Ungerboeck Type MAP Receiver with Bidirectional Decision Feedback for M-ary Quasi Orthogonal Signaling

In this paper, we propose a generic receiver based on optimum maximum likelihood sequence estimation (MLSE) employing a generalized Ungerboeck metric with significantly reduced complexity for the general class of M-ary quasi orthogonal signaling (MOS) in severe multipath fading channels. First, by using the factor graph (FG) of the obtained generalized Ungerboeck metric for MOS and sum-product algorithm (SPA) framework, a highly efficient reduced state sequence estimation (RSSE) algorithm is proposed that operates on forward and backward directions and is directly applied to unwhitened channel matched filter (CMF) and code matched filter outputs at symbol rate. The proposed structure, generating the a posteriori probabilities (APP) by bidirectional RSSE recursions, are substantiated with symbol rate bidirectional decision feedback (BDF) based on surviving paths in order to eliminate the post- and pre-cursor inter-symbol interference (ISI) as well as multi code interference (MCI) due to the non-ideal properties of the signaling waveforms and multipath channel. It appears as the unification of Ungerboeck type reduced trellis equalization framework for MOS and achieves near-optimum performance especially for channels with large dispersion. Second, we identify a bias term through an error probability analysis which helps us improve the proposed receiver's performance. Furthermore, a tight approximation for the bit error rate (BER) of the proposed receiver is derived. These analyses lead to significant insight on the selection of system parameters and clearly demonstrate the high performance and efficiency of the proposed scheme by a proper choice of the signaling parameters.

A New Approach for OSTBC and QOSTBC

ABSTRACT A new approach for Quasi-Orthogonal space time block coding (QO-STBC) is proposed, with simple linear decoding via maximum likelihood detection. The conventional QO-STBC can achieve the full communication rate, but at the expense of the decoding complexity and the diversity gain due to the interference terms in the detection matrix. In this paper we propose a QO-STBC scheme that will eliminate the interference from the detection matrix. The proposed code improves the diversity gain compared with the conventional QO-STBC scheme; it also reduces the decoding complexity. A full rate full diversity order DiagonalizedHadamard Space Time code (DHSTBC) is presented Keywords Quasi-Orthogonal space time block coding (QO-STBC), full rate, full diversity order,DiagonalizedHadamard Space Time code (DHSTBC). 1. INTRODUCTION In traditional communication systems there is only one antenna at both the transmitter and the receiver. This antenna system is known as Single Input Single Output (SISO). SISO systems have a major drawback in terms of the capacity, as stated by the well-known Shannon-Nyquist criterion. In order to increase the capacity of SISO systems to meet the high bit rate transmission demanded by modern communications, the bandwidth and the power have to increase significantly. Fortunately, using the Multiple-Input Multiple-Output (MIMO) system could increase the capacity of the wireless system without the need to increase the transmission power or the bandwidth. Transmit diversity is a well-known technique to mitigate fading effects over a communication link. Alamouti [1] proposed a transmit diversity scheme that offers maximum diversity gain using two antennas at the transmitter. The pioneering work of Alamouti has been the basis to create Orthogonal space time block coding systems (OSTBCs) for more than two transmit antennas. First of all, Tarokh studied the error performance associated with unitary signal matrices [2]. Sometime later, Ganesan streamlined the derivations of many of the results associated with OSTBC and established an important link to the theory of the orthogonal and ‘amicable orthogonal’ designs [3]. It has been proved that a complex orthogonal design of STBCs that provides full diversity and full transmission rate is not possible for more than two transmit antennas [1]. To achieve full-rate transmission while maintaining much of the orthogonality benefits of OSTBC, Quasi Orthogonal Space Time BlockCode(QO-STBC) has been proposed [4-6]. A QO-STBC can achieve full rate but interference terms will appear from the neighboring signals during the signal detection which will increase the detection complexity and decrease the gain performance.

Performance of Space Time Block Codes and Quasi Orthogonal Space Time Codes on Fading Channel

In this paper the performance of Space Time Codes (STC) & Quasi Orthogonal Space Time Block Codes (QOSTBC) has been analyzed. Use of multiple antennas is an emerging approach for recovering fading and reducing the bit error rate. Space diversity is introduced in space time codes. In Alamouti Space Time Code (STC) two transmit antennas are used that provide relatively better results than no coding approach. Alamouti uses a code matrix at transmitter and at receiver end it uses MLD and estimates the output signals. Space time block codes are with arbitrary number of antennas. It uses a huge code matrix at encoder and decodes using MLD. QOSTBC is one of the latest variants of the STBCs. The encoded symbols are mapped in the transmission matrix which is decoded at the receiver end using pair wise maximum likelihood decoding (MLD). Pair wise MLD is the specialty of the QOSTBC. The performance of STBC is far better than Alamouti and has no match with no coding. The performance comparison of QOSTBC, Alamouti, STBC and no coding is showed using simulations in MATLAB. Space time codes works with the price of redundancy bits and extra antenna price. Simulation results for QOSTBC also shows better performance with the price of slightly increased decoding complexity.

Performance Evaluation of Full Diversity QOSTBC MIMO Systems with Multiple Receive Antenna

Multipath fading is inherent in wireless communication systems. Diversity is the technique which takes advantage of multipath to mitigate the effect of fading and increase signal strength. Space Time Block codes (STBC) are used in MIMO systems to improve the performance by maximizing transmit and/or receive diversity. Among different schemes based on STBC, Quasi Orthogonal Space Time Block Code (QOSTBC) is able to achieve full rate transmission for more than two transmit antennas. Constellation Rotation QOSTBC (CR-QOSTBC) achieves full diversity and improves performance further along with full rate, to overcome the limitation of QOSTBC, which is unable to maintain orthogonality amongst the codes transmitted by different antennas. Higher diversity can be achieved by increasing uncorrelated paths between transmitter and receivers using higher number of receive antennas. This paper examines improvement in BER with reference to a number of receive antennas. Simulations were carried out under ideal as well as realistic environments, using least square technique with four antennas at transmitter side and variable receive antennas. Results of simulations presented in this paper indicate performance improvement of CR-QOSTBC over QOSTBC in flat fading channel environment. Simulation results also show performance degradation in BER when channel is estimated at the receiver.

The Enhanced Decoding Method for QO-SFBC System in Frequency Selective Fading Channel Environment

In this paper, we propose an enhanced decoding method to improve the performance of the quasi orthogonal (QO)-space frequency block coding (SFBC) in frequency selective fading channel environment. Generally, QO-SFBC obtains full diversity gain with more than two transmit antennas by using the constellation rotation in flat fading channel. However, QO-SFBC provides less diversity gain in frequency selective fading channel, because the discrepancy of channel frequency responses (CFRs) at adjacent subcarriers causes additive interferences. Although the conventional method using the simple averaging scheme for all CFRs at adjacent subcarriers can be effective for mitigating interferences, it is not quite robust to variation of CFRs, especially in high Doppler frequency environments. Therefore, to achieve better diversity gain in frequency selective fading channel environment, the selective averaged QO-SFBC decoding method based on mathematical analysis considering each CFR of elements in QO-SFBC encoding block is proposed. By using computer simulation, we show that the proposed method can provide better performance compared with the conventional methods and verify that the proposed method is attractive and suitable for implementation with stable operation when frequency selectivity is high.

Design of New Quasi-Orthogonal STBC with Minimum Decoding Complexity for Four Transmit Antennas

A new Space-Time Block Code (STBC) achieving full rate and full diversity for general QAM and four transmit antennas is proposed. This code also possesses a quasi-orthogonal (QO) property like the conventional Minimum Decoding Complexity QO-STBC (MDC-QO-STBC), leadingg to joint ML detection of only two real symbols. The proposed code is shown to exhibit the identical error performance with the existing MDC-QO-STBC. However, the proposed code has an advantage in transceiver implementation since this code can be modified so that the increase of PAPR occurs on only two transmit antennas, whereas MDC-QO-STBC incurs a PAPR increase on all transmit antennas.

New Minimum Decoding Complexity Quasi-Orthogonal Space-Time Block Code for 8 Transmit Antennas

In this paper, a new full-rate space-time block code (STBC) possessing a quasi-orthogonal (QO) property is proposed for QAM and 8 transmit antennas. This code is designed by serially concatenating a real constellation-rotating precoder with the Alamouti scheme. The QO property enables ML decoding to be done with joint detection of only four real symbols like the conventional minimum decoding complexity QO-STBC (MDC-QO-STBC). However, the proposed code is guaranteed to achieve full spatial diversity for general QAM unlike the MDC-QO-STBC which is specifically presented for only 4-QAM. By computer simulation results, we show that the proposed code exhibits the identical and slightly degraded error performance with the MDC-QO-STBC for 4-QAM and the Sharma's QO-STBC for 4 and 16-QAM, respectively. Finally, we present a new modified scheme of the original code so that there is no any discontinuity of transmission at each transmit antenna, without any loss of error performance.

Full-rate full-diversity space–time block codes for any odd number of transmit antennas

The authors propose a new class of space-time block codes (STBCs) achieving full-rate and full spatial diversity for general quadrature amplitude modulation (QAM) when using any odd number of transmit antennas under quasi-static Rayleigh fading channels. These codes are the extended works of the conventional Alamouti-ST constellation-rotating (CR) codes which are designed by serially concatenating CR precoders with the Alamouti scheme for an even number of transmit antennas. From the computer simulation results, it is observed that the best code in this class outperforms the existing ST-CR code and also exhibits error performance within only about 1-dB of the maximal ratio combining receiver. The codes possessing quasi-orthogonal (QO) characteristic are also included in this class, allowing simple maximum likelihood (ML) decoding with almost the same error performance as the best code in this class and the conventional QO-STBCs with full diveristy. These codes have identical or much lowered ML decoding complexity compared with the conventional QO-STBCs.

New Layered Space-Time Schemes Based on Group of Three Transmit Antenna

We present a new architecture of layered space-time codes as a combination of Bell Laboratory Layered Space-Time (BLAST) architecture and of special Space Time Trellis (STTC) Codes which are variations about the Super Orthogonal Space-Time Trellis Codes (SOSTTC) first introduced by Ionescu et al in [1] [2] and further improved by Jafarkhani et al. in [3]. The system we propose is named Super Quasi Orthogonal Horizontal Layered Space Time Trellis Code (SQOHLSTTC). It consists in a powerful Space-Time Block Code (STBC) based STTC originally derived for a three transmit antenna system, together with an original block based decoding algorithm. The decoding algorithm of SQOHLSTTC combines group interference suppression and group interference cancellation techniques. To implement the system, we propose a low complexity hard decision iterative decoding method. System performances confirm the great interest of the proposed transmission scheme.

Cyclic Shifted-and-Extended Codes Based on a Quasi-Orthogonal Sequence for a CDM Transmission Scheme

We have developed a code-division-multiplexing (CDM) transmission scheme for future road-vehicle communication systems, which uses cyclic shifted-and-extended (CSE) codes generated from a basic code with superior auto-correlation characteristics. This paper proposes to use a Quasi-Orthogonal (QO) sequence as the basic code. Its auto-correlation values are zero except at zero and middle shifts. When the CDM transmission is performed by the CSE codes based on the QO sequence, a desired correlation value is, at a receiver, interfered by the auto-correlation value at middle shift. Therefore, an elimination technique for the interfered correlation value is proposed and realizes zero cross-correlation characteristics within the cyclical shift interval. The new CDM transmission scheme based on the proposed scheme is evaluated by computer simulations in terms of the bit-error-rate performance.