Distributed Space-Time Coding for Full-Duplex Asynchronous Cooperative Communications

Using OFDM and space-time coding to achieve asynchronous cooperative diversity has been proposed by Mei et. al. and Shin et al, where orthogonal space-time block codes, Alamouti code in particular, were used at relay nodes. This proposed space-time coding can not achieve the multipath diversity when the channels from nodes to nodes are frequency selective fading. In this paper, we also consider asynchronous cooperative communications where the channels are assumed to be frequency-selective fading. We design high rate distributed space-frequency codes to achieve both full asynchronous cooperative diversity and multipath diversities.

We consider an asynchronous cooperative communication system where two distributed transmitters are communicating with one destination. In this paper, a bounded delay-tolerant time interleave reversal Alamouti code (BDT-TIR AC) is proposed. BDT-TIR AC achieves full diversity, given that the path delay difference is within a tolerance limit. Furthermore, we design an efficient maximum-likelihood (ML) decoder. By employing a divide-and-conquer strategy, the original decoding problem is decoupled into several sub-problems in small size. In fact, these sub-problems contain either Alamouti code structure or only four symbols. By parallel decoding on the sub-problems, the proposed ML decoder exhibits low complexity advantage in large scale problems. Simulations of BDT-TIR AC confirm the full diversity gain. The bit error rate performance of BDT-TIR AC in the considered asynchronous scenarios is comparable with that of synchronized Alamouti code, and outperforms the latest delay-tolerant space-time code.

Space-time turbo codes have advantages of both space-time codes and turbo codes. Space-time turbo code proposed by Su and Geraniotis (2001) achieves full coding rate and full antenna diversity. This paper presents some improvements of the space-time turbo code using a two-antenna configuration. We propose an encoder in which a serial connection of an interleaver and a deinterleaver is removed and the constraint of the interleaver is reduced. We also propose a modified binary decoding which estimates more precise extrinsic information. Simulation results show the proposed encoder achieves full antenna diversity in spite of a simple configuration with reduced constraint of the interleaver. Furthermore, our proposed decoding provides a better performance than that of the Su's space-time turbo code.

To achieve full cooperative diversity in a relay network, most of the existing space-time coding schemes require the synchronization between terminals. A family of space-time trellis codes that achieve full cooperative diversity order without the assumption of synchronization has been recently proposed, where it is assumed that the delays occurring at the relays must be integer multiples of the symbol duration. In this paper, we consider a cooperative system in which the delays can be fractional symbol duration and, correspondingly, multiple sampling per symbol interval is used at the destination. We show that the previously proposed space-time trellis codes for the system with integer symbol delays can still achieve asynchronous full diversity in the new scenario. Furthermore, compared with the applications in the system with integer symbol delays, the decoding complexity of the optimal Viterbi algorithm for the codes does not increase in the system with fractional symbol delays.

In cooperative communication systems, due to their distributed nature, multiple carrier frequency offsets (CFOs) may occur and make the channel time-varying, thus most existing space-time codes (STC) to collect diversity gain for the co-located multi-input multi-output (MIMO) systems may not be applied directly. In this paper, we propose a family of distributed space-frequency codes (SFCs) for cooperative transmissions with multiple CFOs and flat fading channels. These codes can achieve full cooperative (spatial) diversity with only linear receivers, such as zero-forcing (ZF) and minimum mean square error (MMSE) receivers, instead of the computationally exhausted maximum likelihood (ML) detector. The codes in this family have different symbol rates, orthogonality and performances for different number of relay nodes, which can be adjusted by choosing different design parameters. Actually, frequency-reversal SFC (FR-SFC), and frequency-shift SFC (FS-SFC) proposed in our previous paper are special cases of the new codes. Furthermore, new SFCs outperform both FD-SFC and FS-SFC can be obtained, from the systematic design.

A new space-time coding (STC) scheme for multiple-input multiple-output synthetic aperture radar systems is proposed in this paper. In the new scheme, two successive signal periods are put into one transmit duration to minimize the time-variant channel effect. The even and odd components of the transmitted waveforms are modulated into distinct Doppler frequencies in the azimuth direction and can be separated by bandpass filter in the range-Doppler domain. The presented STC scheme can suppress the interchannel ambiguous energy caused by the time-variant channel responses, which typically occur in conventional STC schemes, and increase the signal-to-noise ratio of the echoes by a decoding matrix. To further suppress the interference, a waveform called time and frequency comb-like chirp (TFCC) is proposed. With the application of the complementary comb-like structure in the time and frequency domains, two TFCC waveforms are short-term shift-orthogonal and constant envelope. Moreover, these corresponding TFCC waveforms can share the same antenna and time gate without sacrificing any TFCC waveforms' peak level. The theoretical analysis and simulation results illustrate the feasibility of the proposed scheme.

On the basis of analyzing of some existing differential space-time coding (DSTC) schemes, we develop a differential space-time block coding scheme based on multiple levels quadrature amplitude modulation (MQAM) in this paper. By introducing MQAM, the developed scheme can avoid the performance loss of conventional DSTC schemes based on MPSK modulation due to the decrease of minimum distance between constellation symbols in high spectrum efficiency. Compared with the existing DSTC schemes, the developed scheme has lower computational complexity and simple decoder. Moreover, it can achieve higher code rate in the case of more than two transmitter antennas. Theoretic analysis shows that the scheme has higher coding gain than existing differential unitary space-time coding scheme and differential orthogonal space-time coding scheme, and corresponding simulation results also show that our scheme has lower bit error rate (BER) when compared with the above two schemes.

In this paper, Beam Pattern Scanning (BPS), a transmit diversity technique, is compared with two well known transmit diversity techniques, space-time block coding (STBC) and space-time trellis coding (STTC). In BPS (also called beam pattern oscillation), controlled time varying weight vectors are applied to the antenna array elements mounted at the base station (BS). This creates a small movement in the antenna array pattern directed toward the desired user. In rich scattering environments, this small beam pattern movement creates an artificial fast fading channel. The receiver is designed to exploit time diversity benefits of the fast fading channel. Via the application of simple combining techniques, BPS improves the probability-of-error performance and network capacity with minimal cost and complexity. In this work, to highlight the potential of the BPS, we compare BPS and Space-Time Coding (i.e., STBC and STTC) schemes. The comparisons are in terms of their complexity, system physical dimension, network capacity, probability-of-error performance, and spectrum efficiency. It is shown that BPS leads to higher network capacity and performance with a smaller antenna dimension and complexity with minimal loss in spectrum efficiency. This identifies BPS as a promising scheme for future wireless communications with smart antennas.

Spatial modulation (SM) as a new Multiple-Input Multiple-Output (MIMO) technique is based on transmitting part of the information by activating different emitting antennas. SM increases spectral efficiency and uses only one radio frequency chain. For full-duplex (FD) communication systems, self-interference (SI) is always a central problem. Therefore, combining FD and SM can drastically reduce the difficulty of Self-interference Cancellation (SIC) because of the single SI chain. A Full Duplex Spatial Modulation (FDSM) system is proposed and an active analog SIC is highlighted in this paper. Moreover, the impact of SIC accuracy on the system performance is studied. The results demonstrate that the accuracy requirement will increase as the INR (Self-interference-to-noise Ratio) increases. The FDSM system is less sensitive than the FD system in presence of high estimated error value. Furthermore, an SI detector is presented to resolve the influence of the number of detect symbols.

Multiple-input multiple-output (MIMO) techniques have become an essential part of broadband wireless communications systems. For example, the recently developed IEEE 802.16e specifications for broadband mobile wireless access include three MIMO profiles employing 2times2 space-time codes (STCs) and two of those, namely, Alamouti's STC for transmit diversity, and the 2times2 spatial multiplexing (SM) scheme, are mandatory on the downlink of Mobile WiMAX systems. The first of these has full diversity but it is only half rate, while the second is full rate but suffers from diversity loss. In this paper, we develop a full-diversity STC of size 2times2, which represents an interesting trade-off between these two schemes. More specifically, the presented code is of rate 3/4, it has full diversity, and it leads to simple maximum-likelihood (ML) detection whose complexity grows only linearly with the size of the signal constellation. This makes it a suitable candidate for future evolutions of broadband wireless systems toward higher data rates and performance.

Three desirable properties of a four-antenna spacetime block code are full rate, full diversity, and single-symbol decodability. Previously reported space-time codes that achieve all three properties do so at the expense of the peak-to-average power ratio (PAPR). A fourth desirable property of a space-time block code is that its PAPR be the same as that of the underlying quadrature-amplitude modulation alphabet. In this letter we introduce space-time codes for three and four transmit antennas that achieve all four properties; these codes use a diversity technique based on constellation stretching. Numerical results for quasistatic Rayleigh-fading channels show that, despite their low PAPR, the proposed codes are comparable in SNR performance to the best-performing single-symbol decodable space-time codes for three and four transmit antennas.

It is well known that the performance of the wireless communication systems can be enhanced by using multiple transmit and receive antennas, which is generally referred to as the MIMO technique, and has been incorporated into the IEEE 802.11n standard, i.e. one of the Wi-Fi systems. The MIMO technique has also been adopted by the 3GPP Long Term Evolution (LTE) and the 3GPP2 Ultra Mobile Broadband (UMB) standards. The space-time coding is a promising way to realize the gain in the wireless communications system using MIMO. The space time block code, and the orthogonal space time block code in particular have been proposed in research and implemented in practical wireless communication systems due to their low computational complexity in maximum likelihood decoding implementation. However, the code rates of the orthogonal space time block codes are usually lower than $3/4$ when the number of transmit antennas is greater than two, which limit their throughput. To increase the code rate and the throughput of the orthogonal space time block code for more than two transmit antennas, a family of new space-time block codes (STBC) for 3 and 4 transmit antennas in a MIMO wireless communication system have been proposed in this Ph.D. research. These new codes are orthogonal and achieve full diversity, full rate using Triple QPSK'' modulation, which is proposed in this research. We investigated the performance of these STBCs when the channel state information (CSI) is perfectly known at the receiver. The CSI is not known to the transmitter in all of the studies in this dissertation. An analytical bound of the probability of bit error is developed and the Monte Carlo simulation is used when investigating these new STBC codes. In practical wireless communication system, the CSI is generally not available at the receiver and it is usually estimated by the receiver through the pilot signals or training signals known to the receiver. No matter what channel estimation method is used, the estimate of CSI will not match the CSI exactly and is noisy. The performance of these new STBCs using theTriple QPSK'' modulation when the estimate of the CSI is employed to detect the symbol is investigated in this dissertation. An upper bound of the bit error probability is developed when the noisy CSI is used to detect the symbols. The performance of the new STBCs using the Triple QPSK'' modulation is also evaluated using Monte Carlo simulation. When the CSI is not available to either the receiver or the transmitter, the differential encoding and decoding algorithm is applied to these new STBCs, which use theTriple QPSK'' modulation. The results showed that these orthogonal STBCs achieve full diversity full rate using ""Triple-QPSK"" modulation with differential detection. This provides a means of achieving space diversity and multiplexing gain without the CSI. A new differential encoding and decoding algorithm is proposed and analyzed for STBCs with 3 transmit antennas, whose transmission matrix is not square matrix, as part of this Ph.D. research. The STBCs using this algorithm achieve full diversity the MIMO system can possibly provide.

Differential space-time coding (DSTC) has received much interest as it obviates the requirement of the channel state information at the receiver while maintaining the desired properties of space-time coding techniques. In this paper, by introducing star quadrature amplitude modulation (star QAM) method, two kinds of multiple amplitudes DSTC schemes are proposed. One is based on differential unitary space-time coding (DUSTC) scheme, and the other is based on differential orthogonal space-time coding (DOSTC) scheme. Corresponding bit-error-rate (BER) performance and coding-gain analysis are given, respectively. The proposed schemes can avoid the performance loss of conventional DSTC schemes based on phase-shift keying (PSK) modulation in high spectrum efficiency via multiple amplitudes modulation. Compared with conventional PSK-based DSTC schemes, the developed schemes have higher spectrum efficiency via carrying information not only on phases but also on amplitudes, and have higher coding gain. Moreover, the first scheme can implement low-complexity differential modulation and different code rates and be applied to any number of transmit antennas; while the second scheme has simple decoder and high code rate in the case of 3 and 4 antennas. The simulation results show that our schemes have lower BER when compared with conventional DUSTC and DOSTC schemes.

Antenna spacings, the angle spread and the Rice factor significantly affect the correlation of fading coefficients in a multi-antenna system. We study the effect of these parameters on the performance of different types of detection schemes in a BLAST architecture. We show that the detection method based on lattice decoding is more resistant to the correlated fading than other types of detection which are based on successive interference cancellation. We notice that highly correlated fades significantly degrade the performance of detection techniques based on successive interference cancellation whereas they slightly affect the lattice decoding algorithm. We also study the effects of correlated fades on some space-time block codes. We show that space-time block codes from orthogonal design are more resistant to correlation than other high rate linear space-time block codes.

We consider an asynchronous two-way relay network in which a few distributed relays assist in the communication between two single-antenna terminals through analog network coding. The asynchronous transmission between relays and terminals causes symbol misalignments and results in diversity loss in space–time codes (STCs). We propose a family of zero-padded time-reversal STC that can achieve full diversity with low-complexity maximum likelihood (ML) decoding, given a bound for the path delay difference. With ML decoding, the proposed code is decomposed into several independent parts, which greatly facilitate the decoding process. For two-relay scenarios, three code designs based on Alamouti code, quasi-orthogonal space–time block code, and multigroup decodable STC are provided for each relay with one, two, and four antennas, respectively. For three-relay scenarios, one code design based on quasi-orthogonal space-time block code is provided for each relay with one antenna. Proof of full diversity is established, and the decoding complexity order is analyzed for all four designs. Simulations confirm the full diversity gain of all designs. The bit-error-rate performance in asynchronous scenarios is almost similar to that in synchronous scenarios.