Space frequency block codes (SFBC) have been widely adopted with orthogonal frequency division multiplexing (OFDM) to achieve both spatial and frequency diversity. However, OFDM is very sensitive to carrier frequency offset (CFO), which would cause intercarrier interference (ICI) and degrade the system performance severely. In this paper, we propose a low-complexity partial parallel interference cancellation (PIC) based receiver with common CFO compensation, modified SFBC decoding, and iterative partial PIC for Alamouti coded SFBC-OFDM cooperative relay systems. For the proposed receiver, the received signal is first compensated by a common CFO value to mitigate the effect of different CFOs from the distributed relays, and then the result is processed by a modified SFBC decoder. The decoded data is subsequently fed back to reconstruct the ICI for use in the developed partial PIC procedure. As compared with a previous related work, the proposed partial PIC based receiver achieves better performance with lower computational complexity.

A number of peak-to-average power ratio (PAPR) reduction techniques have been proposed for orthogonal frequency division multiplexing (OFDM) systems; however, most of them require transmitting side information and thus reduce the bandwidth efficiency. The existing constellation extension (CE) schemes do not need to transmit side information. In this paper, we propose an improved CE scheme for PAPR reduction of OFDM signals with 16-QAM modulation. The proposed CE scheme has four extendable constellation points, the four corners in the 16-QAM constellation, and each extendable constellation point has two possible extended constellation points. The proposed CE scheme, when compared with previous related CE schemes, has fewer extendable constellation points and more corresponding extended constellation points, resulting in better BER performance with almost the same PAPR reduction. Computer simulations are given to demonstrate the effectiveness of the proposed scheme.

A number of peak-to-average power ratio (PAPR) reduction techniques have been proposed for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems; however, most of them involve very high computational complexity and are not applicable to MIMO-OFDM systems with space frequency block coding (SFBC). In this paper, we propose a low-complexity PAPR reduction scheme for SFBC MIMO-OFDM systems, where the input sequence is multiplied by a set of phase rotation vectors respectively and then each resulting sequence is decomposed into several sub-sequences based on the linear property of SFBC. After computing the inverse fast Fourier transform (IFFT) to convert each frequency-domain sub-sequence into a time-domain signal, we perform equivalent SFBC encoding operations in the time domain for generating candidate signal sets, where the one with the lowest maximum PAPR is selected for transmission. With the proposed scheme, we can generate a large number of candidate signal sets by computing only a few IFFTs. As compared to previous related schemes, the proposed one achieves similar PAPR reduction performance with much lower computational complexity.

Generally, tasks in a distributed system must reach agreement. This requires a set of processors to agree on a common value even though some components may be corrupt. There have been several significant studies on this agreement problem in regularized network environments such as the fully connected, broadcast and multicast networks. Recently, many large complex networks have emerged displaying a scale-free feature that influences the system to reach a common value in a novel way. This unanimity problem is called Byzantine agreement (BA). The BA problem is one of the most significant problems in designing a fault-tolerant distributed system. Unfortunately, existing BA protocols cannot cope with the new network environment, and the BA problem thus must be revisited. In this paper, a new BA protocol is proposed that adapts to the scale-free network (SFN) environment and derives its limit of allowable faulty components while maintaining the minimum number of message exchanges. The correctness and complexity of this protocol have been proved. It is observed that an SFN in conjunction with the proposed agreement protocol can tolerate the maximum number of faulty components.

Fault-tolerance is an important research topic in the study of distributed systems. To cope with the influence of faulty components, reaching a common agreement in the presence of faults before performing certain tasks is essential. However, the Byzantine Agreement (BA) problem is a fundamental problem in fault-tolerant distributed systems. In previous studies, protocols dealing with the BA problem focused on static networks; however, these do not perform well in dynamically changing mobile networks. The most well known mobile network is the Mobile Ad-hoc Network (MANET). To enhance fault-tolerance and MANET reliability, the BA problem in virtual subnets of MANET is revisited in this paper. The proposed protocol is called the Hybrid Agreement Protocol (HAP). It achieves agreement on a common value among all functional mobile processors in a minimal number of message exchange rounds, and can tolerate a maximal number of allowable faulty components in the virtual subnet of MANET.

A novel structure of training symbol is proposed for orthogonal frequency division multiplexing (OFDM) systems. With the proposed training symbol, which has repeated sample blocks and a sign pattern on each block, both timing offset and frequency offset estimations can be obtained. In particular, the frequency synchronization is accomplished using two successive stages to obtain both the fractional and integral parts of frequency offset with an estimation range of plusmnN/4 sub-carrier spacing. Simulation experiments demonstrate that the performance of the proposed scheme is, in most of the investigated cases, substantially superior to the traditional schemes, or has negligible differences. In addition, the proposed synchronization mechanism has a relatively low system complexity, making a good comprise between performance and complexity.