Non-orthogonal multiple access (NOMA), which combines multiple user signals and transmits the combined signal over one channel, can achieve high spectral efficiency for mobile communications. However, combining the multiple signals can lead to degradation of bit error rates (BERs) of NOMA under severe channel conditions. In order to improve the BER performance of NOMA, this paper proposes a new NOMA scheme based on orthogonal space-time block codes (OSTBCs). The proposed scheme transmits several multiplexed signals over their respective orthogonal time-frequency channels, and can gain diversity effects due to the orthogonality of OSTBC. Furthermore, the new scheme can detect the user signals using low-complexity linear detection in contrast with the conventional NOMA. The paper focuses on the Alamouti code, which can be considered the simplest OSTBC, and theoretically analyzes the performance of the linear detection. Computer simulations under the condition of the same bit rate per channel show that the Alamouti code based scheme using two channels is superior to the conventional NOMA using one channel in terms of BER performance. As shown by both the theoretical and simulation analyses, the linear detection for the proposed scheme can maintain the same BER performance as that of the maximum likelihood detection, when the two channels have the same frequency response and do not bring about any diversity effects, which can be regarded as the worst case.

High reliability is required, even in Internet of things (IoT) communications, which are sometimes used for crucial control such as automatic driving devices. Hence, both the uplink (UL) and downlink (DL) communication quality must be improved in the physical layer. In this study, we focus on the communication quality of broadcast DL, which is configured using orthogonal frequency-division multiplexing (OFDM) as a multiplexing scheme and turbo code as forward error correction (FEC). To reduce the frame-error rate (FER) in the DL, we consider two transmit-diversity (TD) techniques that use space-time block code (STBC) or cyclic-delay diversity (CDD). The purpose of this paper is to evaluate the TD performance and to enhance FER performance of CDD up to that of STBC. To achieve this goal, a channel estimation method is proposed to improve FER for CDD. For this purpose, we first evaluate the FER performance of STBC and CDD by performing computer simulations and conducting hardware tests using a fading emulator. Then, we conduct field experiments in the 2.5GHz band. From the results of these evaluations, we confirm that STBC and CDD improved FER compared with single antenna transmission. CDD with the proposed channel estimation method achieved almost the same performance as STBC by accurately estimating the channel frequency response (CFR) and appropriately adjusting the amount of cyclic shift (ACS). When moving a received device around Yokosuka Research Park, STBC and CDD, using spatial diversity with omni antennas for TD, improved the FER from 3.84×10-2 to 1.42×10-2 and 1.19×10-2, respectively.

In this paper, we show exact bit error rates (BERs) for orthogonal space-time block code (OSTBC) decoded-and-forward (DF) relaying networks over independent and non-identically distributed (INID) Rayleigh fading channels. We consider both non-adaptive DF (non-ADF) and adaptive DF (ADF) schemes for OSTBC relay networks with arbitrary multiple-input multiple-output (MIMO) relay antenna configurations. For each scheme, we derive the probability density functions (PDFs) of indirect link and combined links, respectively. Based on the derived PDFs, we express exact BERs and then, their accuracy is verified by the comparison with simulation results. It is confirmed that the transmit diversity gain of the relay node can be obtained when the relay is close to the source and then, the receive diversity gain of the relay node as well as ADF gain over non-ADF can be obtained when the relay is close to the destination.

A diversity strategy is efficient to reduce the fluctuation of communication quality caused by fading. In order to further maintain the communication quality and improve the communication capacity, this paper proposes a two-dimensional diversity approach by serially-concatenating spectral precoding and power normalized-differential space time block coding (PN-DSTBC). Spectral precoding is able to take benefit from a frequency diversity effect without loss in spectral efficiency. In addition, PN-DSTBC is robust against serious phase noise in an extremely high frequency (EHF) band by exploiting a spatial diversity effect. However, there is a problem that a naive concatenation degrades the performance due to the imbalance of equivalent noise variances over transmit frequencies. Thus, we examine an equalized PN-DSTBC decoder as a modified approach to uniform equivalent noise variances over frequencies. The performance evaluation using computer simulations shows that the proposed modified approach yields the performance improvement at any modulation schemes and at any number of transmit frequencies. Furthermore, in the case of 64QAM and two transmit frequencies, the performance gain of the modified approach is 4dB larger than that of PN-DSTBC only at uncoded BER=10-4.

In this paper, the performance of orthogonal space-time block codes (OSTBC) for distributed multiple-input multiple-output (MIMO) systems employing adaptive M-QAM transmission is investigated over independent but not necessarily identically distributed (i.n.i.d.) generalized-K fading channels with arbitrary positive integer-valued k(inversely reflects the shadowing severity) and m (inversely reflects the fading severity). Before this, i.n.i.d. generalized-K fading channel has never been considered for distributed OSTBC-MIMO systems. Especially, the effects of the shape parameter k on the distributed OSTBC-MIMO system performance are unknown. Thus, we investigate mainly the significance of the shape parameter k on the distributed OSTBC-MIMO system performance, in terms of the average symbol error probability (SEP), outage probability, and spectral efficiency (SE). By establishing the system model, the approximated probability density function (PDF) of the equivalent signal to noise ratio (SNR) is derived and thereafter the approximated closed-form expressions of the above performance metrics are obtained successively. Finally, the derived expressions are validated via a set of Monte-Carlo simulations and the implications of the shape parameter k on the overall performance are highlighted.

This paper proposes a novel cooperative scheme combining distributed space-time block code (STBC) at physical layer, multiple access protocol at medium access control (MAC) layer and opportunistic routing without complicated routing algorithm for achieving high reliability for vehicle-to-vehicle (V2V) communications. The proposed scheme can reduce interference and collision, and achieve reducing redundant broadcast of safety-related messages for multi-hop vehicular communications on highway. In particular, we propose a novel algorithm of relay selection based-on position, speed and direction of movement to select intermediate vehicle stations (VS) with high contribution according to the transmission direction. Furthermore, in order to reduce interference and collision, we install a new timer to select a master relay vehicle station (MVS) which manages a packet transmission of whole network to trigger and synchronize transmitting timing of relay VSs (RVSs) in each hop. From the results of simulations, we can confirm that the proposed method can achieve reducing the redundant broadcast safety-related messages with keeping the packet loss probability by limiting the retransmission at each VS.

This paper presents the design of an ad hoc two-way two-hop relay network using physical-layer network coding (PNC) in which multiple antennas are used at all nodes. In the considered network, the Alamouti's space-time block code (STBC) is used for transmission while linear detection is used for signal recovery. In order to facilitate linear estimation, we develop an equivalent multiuser STBC model for the proposed network and design the sum-and-difference matrix which allows convenient combination of the transmitted symbols from the end nodes. In addition, a simple relay selection method based on minimum mean square error (MSE) is proposed for performance improvement. Simulation results show that the proposed network achieves diversity order 2 while requiring only polynomial complexity. Moreover, it is possible to achieve significant bit error rate (BER) performance improvement when the proposed relay selection algorithm is used.

Although Quasi Orthogonal Space Time Block Code (QOSTBC) is capable of achieving a full rate transmission for more than two transmit antennas, its generation entails a complex algorithm to create the transmission matrix for a very large number of antennas. This paper presents a simplified generation scheme of QOSTBC, which can be generalized to 2k antennas for any positive integer k. In addition, under a realistic massive MIMO with up to 16 transmit and receive antennas, the performance evaluation is conducted. It is verified that the proposed scheme provides significant performance with BPSK and QPSK modulation formats in Rayleigh fading channels.

In this letter, a cooperative scheme based on orthogonal frequency division multiplexing (OFDM) in vehicular communication system is proposed. In the conventional scheme, a destination exploits only one base station to communicate information. The proposed scheme can use an extra source from another base station through a relay, since the restriction of power in vehicle are less than cellular device. If a destination is distant from a base station, the performance is degraded. When a destination is distant from a base station, the proposed scheme employing space time block code (STBC) and cyclic delay diversity (CDD) has a higher bit error rate (BER) performance and throughput than the conventional scheme.

Cooperative communication has been proposed to improve the disadvantages of the multiple-input multiple-output (MIMO) technique without using extra multiple antennas. In an orthogonal frequency division multiple access (OFDMA) system, a cooperative communication that each user shares their allocated sub-channels instead of the MIMO system has been proposed to improve the throughput. But the cooperative communication has a problem as the decreased throughput because it is necessary that users send and receive the information to each other to improve reliability. In this letter, the modified cooperative transmission scheme is proposed to improve reliability in the fading channel, and it can solve the problem for BER performance that is dependent on the errors in the first phase that exchanges the information between both users during the first time.

In this paper, we consider decouple-and-forward (DCF) relaying, where the relay encodes and amplifies decoupled data using orthogonal space-time block codes (OSTBCs), to achieve the maximum diversity gain of multiple-input multiple-output (MIMO) amplify-and-forward (AF) relaying. Since the channel status of all antennas is generally unknown and time-varying for cooperation in multi-antenna multiple-relay systems, we investigate an opportunistic relaying scheme for DCF relaying to harness distributed antennas and minimize the cooperation overheads by not using the global channel state information (CSI). In addition, for realistic wireless channels which have spatial fading correlation due to closely-spaced antenna configurations and poor scattering environments, we analyze the exact and lower bound on the symbol error probability (SEP) of the opportunistic DCF relaying over spatially correlated MIMO Rayleigh fading channels. Numerical results show that, even in the presence of spatial fading correlation, the proposed opportunistic relaying scheme is efficient and achieves additional performance gain with low overhead.

In this letter, a cooperative scheme is proposed for the broadcasting and cellular communication system. The proposed scheme improves bit error rate (BER) performance and throughput on the edge of a cellular base station (CBS) cooperating with another CBS in the same broadcasting coverage. The proposed scheme for the enhancement of BER performance employs two schemes by a channel quality information (CQI) between a broadcasting base station (BBS) and users. In a physical area, the edge of a CBS is concatenated with the edge of another CBS. When users are on the edge of a CBS, they transmit simultaneously the CQI to CBSs, and then a BBS and CBSs transmit signals by the proposed algorithm. The two schemes apply space-time cyclic delay diversity (CDD) and a combination of space-time block code (STBC) with vertical Bell Laboratories Layered Space-Time (V-BLAST) to a signal from a BBS and CBSs. The resulting performance indicates that the proposed scheme is effective for users on the edges of CBSs.

Space-time block code (STBC) with complex orthogonal designs achieves full diversity with a simple maximum-likelihood (ML) decoding, however, do not achieve a full transmission rate for more than two antennas. To attain a higher transmission rate, STBC with quasi-orthogonal designs were proposed, whereas there are interference terms caused by relaxing the orthogonality. It has an impact on decoding complexity because a receiver needs to decode two symbols at a time. Moreover, QO-STBC does not achieve full diversity. In this paper, we propose a scheme which makes possible to decode symbols one by one, and two schemes which gain full transmission diversity by upsetting the balance of the transmit power and rotating constellation.

The conventional hybrid STBC schemes can achieve less BER performance for STBC detection schemes than conventional STBC schemes since SM symbols interfere with STBC symbols. Therefore, this letter proposes the improved scheme for hybrid STBC systems. STBC and SM schemes are combined for the hybrid space-time block code system. Our approach effectively obtains both diversity gain and spectral efficiency gain. The proposed scheme offers improved BER performance since it uses iterative detection. Moreover, it increases the data rate effectively with a little performance loss.

The sub-channel is empty except each user's allocated sub-channel in an orthogonal frequency division multiple access (OFDMA) system. The scheme of cooperative communication using this empty sub-channel has been studied. But, because each user wastes the time slots in the cooperation phase, it is difficult to achieve the full rate. In this letter, a new cooperative communication scheme based on OFDMA is proposed to improve transmission rate in Rayleigh fading channel.

A precoding design for double space-time block coding (STBC) system is investigated in this paper, i.e., the joint processing of STBC and dirty paper coding (DPC) techniques. These techniques are used for avoiding dual spatial streams interference and improving the transmitter diversity. The DPC system is interference free on multi-user or multi-antenna. The STBC transceiver can provide the transmit diversity. Due to the benefits about offered by the STBC and DPC techniques, we propose a new scheme called STBC-DPC system. The transceiver design involves the following procedures. First, the ordering QR decomposition of channel matrix and the maximum likelihood (ML) one-dimensional searching algorithm are proposed to acquire reliable performance. Next, the channel on/off assignment using the water filling algorithm, i.e., maximum capacity criterion, is proposed to overcome the deep fading channel problem. Finally, the STBC-DPC system with the modulus operation to limit the transmit signal level, i.e., the Tomlinson-Harashima precoding (THP) scheme, is proposed to achieve low peak-to-average power ratio (PAPR) performance. Simulation results confirm that the proposed STBC-DPC/THP with water filling ML algorithm can provide the low PAPR and excellent bit error rate (BER) performances.

This paper proposes an alternate time-switched transmission technique for single carrier modulation system with frequency domain equalization. Additional maximal ratio combining diversity gain is obtained by adding a transmit antenna and a switch. Alternating transmit symbols result in zeros which make maximal ratio receive combining possible in the receiver. Simulation results show that it has better performance than the traditional algorithm at the expense of one additional antenna.

MIMO-OFDM combining OFDM and MIMO techniques achieves high spectral efficiency and is able to increase throughput. MIMO-OFDM systems can be classified into either “open loop” or “closed loop” depending on whether the CSI is fed back from the Rx to the Tx. As a closed loop scheme, SVD-MIMO is the optimal single user MIMO-OFDM transmission scheme while it requires knowledge of the CSI at both the Tx and Rx. In practical systems, Tx weight is fed back from the Rx to the Tx by limited bits and with feedback delay, which causes mismatch between the weight and the real channel especially if the channel exhibits time variation. Hence, the transmission performance of the SVD-MIMO scheme degrades. Therefore, the performance comparison between open loop and closed loop schemes against channel variation is very important for practical deployment of MIMO-OFDM systems. For that purpose, a unified performance calculation method for the open loop and the closed loop MIMO-OFDM schemes with finite and delayed feedback is developed in this paper. The method is effective for analysis of both STBC for the open loop and SVD-MIMO using codebook for the closed loop with per stream layer AMC. Also, to combat frequency selective fading in practical wireless channels, an interleaver is employed in this paper. In numerical analyses, it is found that simulation results agree well with the derived theoretical performance results. Secondly, from these results, the cross-over point of the throughput performance of two schemes in terms of UE velocity and SNR is found.

In this letter we design a new family of space-time block codes (STBC) for multi-input multi-output (MIMO) systems. The complex orthogonal STBC achieves full diversity and full transmission rate with fast maximum-likelihood decoding when only two transmit antennas are employed. By combining the Alamouti STBC and the multidimensional signal constellation rotation based on the cyclotomic number field, we construct cyclotomic orthogonal space-time block codes (COSTBCs) which can achieve full diversity and full rate for multiple transmit antennas. Theoretical analysis and simulation results demonstrate excellent performance of the proposed codes, while the decoding complexity is further reduced.

The end-to-end performance of dual-hop multiple-input multiple-output (MIMO) decouple-and-forward relaying with orthogonal space-time block code (OSTBC) transmission over Nakagami-m fading is analyzed. By considering the multiple antennas at all nodes, we derive exact closed-form and asymptotic expressions for the outage probability and symbol error rate, which enables us to evaluate the exact performance and reveals the diversity gains of the considered system. In addition, the closed-form approximation and asymptotic expressions for the ergodic capacity are also derived. We show that OSTBC transmission over relay systems yields a unit order of multiplexing gain despite the fact that full diversity order, which is equal to the minimum fading severity between the two hops, is achieved.