Quasi-orthogonal Space-time Codes Research Articles

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.

Virtual MISO with joint device relaying and beamforming in 5G networks

In this paper, a novel scheme is proposed in which mobile users organize a virtual multiple-input single-output (MISO) structure with the aid of relaying based on device to device (D2D) communication in a 5G cellular network. The performance of distributed quasi-orthogonal space–time code (D-QOSTC) based virtual MISO for the cooperation among four communication nodes is investigated. Error propagation in the second hop of relaying in decode and forward (DF) relaying protocols, is one of the main challenges of virtual MISO systems and it can be mitigated by the joint use of selection relaying and beamforming techniques. We propose a novel beamforming technique for removing the end to end inter-symbol interference (ISI) of D-QOSTC scheme. By means of our proposed scheme, even though we have error propagation in the second hop of relaying, without any loss in bandwidth or rate, the full diversity order can be achieved. Moreover, the performance of bit error rate (BER) with closed form result is analyzed and compared to the simulation results. The results exhibit that the theoretical BER curves coincide with the simulation ones and the system achieves full diversity order. The simulation results also indicate a significant gain in power efficiency for the optimal selection relaying and fixed relaying compared to conventional schemes.

Space Time State Trellis Codes for MIMO Systems Using Reconfigurable Antennas

We present a novel class of space time trellis codes based on super quasi-orthogonal space time codes for a reconfigurable antenna system. We call them space time state trellis codes (STSTCs). We design for 4 trellis states and both 2 and 4 branch trellis structures for BPSK and QPSK constellations. The proposed codes take advantage of the additional degrees of diversity offered by reconfigurable antennas and changing propagation states. They achieve full rate, full diversity and high coding gain. Simulation results demonstrate their good performance. STSTCs achieve the diversity order expected of four transmit antennas while using only two reconfigurable transmit antennas. Our code out performs the existing super orthogonal space time trellis code, which fails to achieve full diversity in a reconfigurable antenna system. The proposed 4 trellis state STSTC is designed based on the following performance criteria: minimum coding gain distance of the minimum return path (CGD MRP), performance factor (PF), rank of the partial valid path (RPVP) on top of traditional minimum coding gain distance of parallel path (CGD PP). We show through analysis and simulations that minimum CGD PP is not a sufficient criteria and that additional performance gains are provided by considering the CGD MRP.

Signal Constellations of Quasi-Orthogonal Space–Time Codes for MIMO Systems

Orthogonal space---time codes (OSTCs) have recently captivated considerable attention due to their properties of full diversity and low implementation complexity. Quasi-orthogonal space---time codes (QOSTCs) provide full diversity and rate one, which enables simple pair wise decoding to be performed as well as better results to be obtained. In this paper, we derive effective OSTCs and QOSTCs for multi-input multi-output systems, which enable us to calculate coding gain of the OSTC. In addition, the fast maximum-likelihood (ML) decoding at the receiver can be performed separately. Since the QOSTC has a fast ML decoding, the symbol pairs can be decoded independently. With the constellation rotation of the symbol, rotated version of QOSTC is generated. OSTCs are designed to achieve the maximum diversity order for a given number of transmit and receive antennas subject to the constraint of having a simple decoding algorithm. This goal is achieved by properly choosing the signal constellations and comparing bit-error-rate (BER) to signal-to-noise ratio (SNR) for different quadrature pulse shift keying and quadrature-amplitude modulation techniques. The simulation results show that the QOSTC outperforms the OSTC at low SNR whereas it has higher complexity than the OSTC. From simulation result, we also compare the STC and space frequency code at BER versus SNR.

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.

PIC 그룹 복호화를 이용한 최적화된 Double-ABBA 유사 직교 시공간 부호

본 논문에서는 그룹(group)을 2개의 심볼(symbol)로 나누는 시스템을 제안하는데, 이 두 개의 더해진 심볼들은 다중화에 의해 분리된 후 역다중화 기술을 이용해 다시 합해진다. 제안된 시스템에서 간단한 PIC(Partial Interference Cancelation) 그룹 복호화 기술은 Double-ABBA(D-ABBA) 유사 직교(Quasi-Orthogonal) 시공간 부호(Space Time Code)를 위해 사용된다. 이 부호는 고차 MIMO(Multiple Input Multiple Output) 시공간 블록 부호화에서 복호화시에 복잡도를 줄여준다. 그리고 기존의 방법과 제안된 방법과의 성능을 비교한다.

A Linear Programming Receiver for Blind Detection of Full Rate Space-Time Block Codes

We present a new approach to the blind detection of full rate space-time block coded transmissions. Our method is based on linear programming and is globally convergent. We exploit the implicit structure of space-time block codes to cast the problem as linear programming to be solved efficiently. Unlike several known methods, the proposed technique is applicable to full-rate space-time block codes such as the popular Alamouti orthogonal code and the quasi-orthogonal space time code. Moreover, we utilize the code structure of these full rate codes to achieve simultaneous detection of all coded data streams without relying on successive schemes that are prone to error propagation. For orthogonal space-time codes, our algorithm allows near maximum likelihood detection of orthogonal codes without channel knowledge. We also can generalize the application of the proposed receiver to be used with nonorthogonal codes. Our proposed algorithm can provide performance competitive with existing techniques without imposing any conditions on the number of receive antennas.

Orthogonal-Like Space–Time-Coded CPM Systems With Fast Decoding for Three and Four Transmit Antennas

The Alamouti orthogonal space-time block code for two transmit antennas was designed primarily for QAM and PSK modulations, and we have previously generalized it for the continuous phase modulation (CPM), denoted as OST-CPM, by maintaining the orthogonality (for the fast ML decoding/demodulation) and the phase continuity of two signals from two transmit antennas. In this paper, we design orthogonal-like space-time coded CPM systems for three and four transmit antennas based on orthogonal and quasi-orthogonal space-time codes. Although the signals from transmit antennas in the proposed orthogonal-like space-time coded CPM systems are not orthogonal, the fast decoding/demodulation is maintained like the two transmit antenna case. Simulation results show that the performance of the proposed orthogonal-like space-time coded CPM systems for four transmit antennas is much better than that of the OST-CPM systems for two transmit antennas.

Complete Characterization of the Equivalent MIMO Channel for Quasi-Orthogonal Space–Time Codes

Recently, a quasi-orthogonal space-time block code (QSTBC) capable of achieving a significant fraction of the outage mutual information of a multiple-input-multiple-output (MIMO) wireless communication system for the case of nT = 4 transmit and nR = 1 receive antennas was proposed. We generalize these results to nT = 2n transmit and an arbitrary number of receive antennas. Furthermore, we completely characterize the structure of the equivalent channel for the general case and show that for all nT = 2n and nR the eigenvectors of the equivalent channel are fixed and independent from the channel realization. Furthermore, the eigenvalues of the equivalent channel are independent identically distributed random variables each following a noncentral chi-square distribution with 4nR degrees of freedom. Based on these important insights into the structure of the QSTBC, we derive tight lower and upper bounds for the outage probability achieved with QSTBC. Finally, by utilizing the special structure of the QSTBC, we propose a new transmit strategy, which decouples the signals transmitted from different antennas in order to detect the symbols separately with a linear ML-detector rather than joint detection, an up to now only known advantage of orthogonal space-time block codes (OSTBC).

Very-Low-Complexity Maximum-Likelihood Decoders for Four-Transmit-Antenna Quasi-Orthogonal Space-Time Code

This letter proposes two very-low-complexity maximum-likelihood (ML) detection algorithms based on QR decomposition for the quasi-orthogonal space-time code (QSTBC) with four transmit antennas [3]-[5], called VLCMLDec1 and VLCMLDec2 decoders. The first decoder, VLCMLDecl, can be used to detect transmitted symbols being extracted from finite-size constellations such as phase-shift keying (PSK) or quadrature amplitude modulation (QAM). The second decoder, VL-CMLDec2, is an enhanced version of the VLCMLDecl, developed mainly for QAM constellations. Simulation results show that both of the proposed decoders enable the QSTBC to achieve ML performance with significant reduction in computational load.

Optimal rotation angles for quasi-orthogonal space-time codes with PSK modulation

Full diversity of quasi-orthogonal space-time block codes with complex signals could be achieved by constellation rotation. In this letter, we derive the optimal rotation angle in the sense of maximizing coding gain for quasi-orthogonal codes with phase-shift keying (PSK) modulation using a geometry-based approach. We also prove that the rotated PSK signals with an even constellation size have higher coding gains (or diversity products) than those with an odd constellation size.

A Comparison of Orthogonal and Quasi-Orthogonal Space-Time Block Codes for Fast Fading Channels

This letter compares orthogonal space time codes and quasi-orthogonal codes when the wireless channels are fast fading. It is well known that a orthogonal space-time code is better than a quasi-orthogonal code in high signal-to-noise ratio (SNR) range and that a quasi-orthogonal code is better in low SNR range. In this letter, we show that a quasi-orthogonal space-time code is a better choice even in high SNR range when the channels are fast fading.

On the Construction of High-Rate Quasi-Orthogonal STBC for MIMO QR Demodulation

Orthogonal space-time block codes (STBCs) [1],[2] appear to be a very fascinating means of enhancing reception quality in quasi-static MIMO channels due to their full diversity, and especially their simple maximum-likelihood (ML) decoders. However, full-rate full-diversity orthogonal STBCs do not exist for more than two transmit antennas. Vertical layered space-time architecture (so-called the V-BLAST) [3] with a nulling- and cancelling-based detection algorithm, in contrast, has an ability of achieving high transmission rates at the cost of having very low diversity gain, an undesirable consequence caused by the interference and cancelling processes. The uncoded V-BLAST system is able to reach its ML performance with the aid of the sphere decoder algorithm [4] at the expense of higher detection complexity. Undoubtedly, the tradeoff between transmission rates, diversity, and complexity is inherent in designing space-time codes. This paper investigates a method to increase the nulling diversity gains for a general high-rate space-time code and introduces a new design strategy for high-rate space-time codes detected based on interference and cancelling processes, thanks to which high-rate quasi-orthogonal space-time codes for MIMO applications are proposed. We show that when nT transmit and nR = nT receive antennas are deployed, the first code offers a transmission rate of (nT - 1) with a minimum diversity order of 3, whereas the second one offers a transmission rate of (nT - 2) with a minimum diversity order of 5. Therefore, the proposed codes significantly outperform the V-BLAST as nR = nT. Simulation results and discussions on the performance of the proposed codes are provided.

Performance of quasi-orthogonal space-time code with antenna selection

The performance of a quasi-orthogonal space-time block code with antenna selection is considered. Transmit antenna selection as well as receive antenna selection is studied. A new selection rule is proposed that minimises the average bit error ratio and achieves full diversity even if only a simple zero-forcing receiver is used.

Improved quasi-orthogonal codes through constellation rotation

In this letter, we show that the performance of recently proposed quasi-orthogonal space-time codes can be improved by phase-shifting the constellations of the symbols constituting the code. The optimal rotation of the symbols increases the minimum distance of the corresponding space-time codewords, leading to substantially improved performance.