Cooperative OFDM with Amplify-and-Forward Relaying with Timing Offset

This paper provides performance analysis of multiband OFDM UWB system that takes into consideration the effect of intersymbol interference and imperfect frequency and timing synchronization. The system performance is analyzed under UWB multipath fading channels, as specified in the IEEE 802.15.3a channel standard. Average signal-to-noise ratio is first derived for the system. Then an approximation technique is employed to obtain a closed-form average bit error probability that provides a profound understanding of the performance of the multiband OFDM UWB system. Simulation results validate the theoretical analysis.

By combing Spatial modulation (SM) with the Amplify-and-forward (AF) cooperative communication, a cooperative SM scheme with AF (AF-SM) protocol is presented, and the corresponding Bit error rate (BER) performance is investigated. Based on the performance analysis, the error probability of antenna index estimation ( P a ) and the error probability of symbol estimation ( P d ), which constitutes the overall average BER, are derived, respectively. As a result, tightly approximate closedform expressions of P a and P d are attained, respectively. Using these expressions, the closed-form overall average BER is achieved. During the analysis, the computational complexity of AF-SM detector is also provided in terms of the complex addition and multiplication numbers. Besides, the asymptotic BER at high SNR and the corresponding diversity gain are further derived, and the resultant diversity gain of N r + 1 is obtained for the system with N r receive antennas. Simulation results show that our theoretical analysis is valid, and provides good performance evaluation method for cooperative SM system. Moreover, the BER performance can be effectively improved as N r increases due to higher diversity gain.

Due to the great potential in extending propagation distance and enhancing the link reliability, amplify and forward (AF) cooperative communications have attracted much attention recently. However, the existing studies mainly focus on the energy consumption analysis and optimization, and also the channels used in communications are based on Rayleigh/Rician fading model, and/or it was usually supposed that the channel state information (CSI) is known or can be acquired by a training sequence. In this paper, blind signal detection for AF cooperative communication systems based on time division multiple access (TDMA) is studied, in which the channels are modeled as frequency selective fading and the CSI are unknown. A blind signal detection algorithm of partial amplify and forward (PAF) cooperative communication is proposed based on semidefinite relaxation (SDR). This method can recover the data without the training sequence and need not to implement blind equalization for the unknown channels. With the correlation between data of different links, the proposed method can obtain the optimal solution efficiently and achieve better performance of signal detection than the traditional algorithm. By simulations, the energy consumption efficiency as well as the overall system performance of the proposed strategy are verified.

The analysis and design of relay protocols is a hot issue in 3GPP Long Term Evolution--Advanced. In this paper, we discuss interference cancellation in a multiuser MIMO environment using Amplify-and-Forward (AF), Decode-and-Forward (DF) and De-Modulate-and-Forward (DMF) as relay protocols, and using Thomilson Harashima Precoding and Dirty Paper Coding as precoding techniques, with Zero-Forcing, Minimum Mean Square Error, Successive Interference Cancellation and Ordered Successive Interference Cancellation detection techniques. By using a combination of classical precoding schemes and detection techniques with weighted matrix, we propose a new interference cancellation technique that is capable of cancelling interference. The interference cancellation is managed by AF, DF and DMF relay node protocols and the interference free codeword is transmitted to the selected User Equipment. The proposed algorithm when used with DMF protocol shows best performance, compared to the conventional system or the no-relay system case, it gives best performance. The observation results shows that DMF protocol gives the best results for BER and Throughput performance in a high interference environment.

Cooperative communication combats the effects offadingwithoutthe need for multiple antennas.The cooperative networks use multiple distributed nodes that virtually behave like multiple antennas each having their own local oscillators. But the oscillators cause multiple Carrier Frequency Offsets (CFO), unknown channel gains and Multiple Timing Offsets (MTO). For a good performance of the system perfect synchronization is needed which is achieved by estimation and compensation of multiple offsets. Also, the coverage and capacity can be improved by employing intermediate nodes called relays within cooperative networks. These cooperative relay networks can perform in a better manner even when considering MCFOs, MTOs and unknown channel gains by having more computationally efficient decoders that can use Maximum Likelihood (ML) estimation subject to the Cramer-Rao Lower Bound (CRLB) on the variance of any impartial estimate. The simulation results show that frequency offset still exist after Amplify and Forward (AF) relaying. The performance of a conventional cooperative network seems to have improved with respect to Bit Error Rate of the system when an AF relay is hosted. The estimate of the frequency offset is chosen such that it gives the signal, a high probability of occurrence using ML estimator. In ML estimate, since the bias and variance is difficult to compute, a lower bound called the CRLB on the variance of any unbiased estimate is derived which is satisfied by the ML estimator.Keywords: AF, Cooperative Communication, CFO, ML Estimation and Compensation, OFDM

Cooperative communications can be used to improve communication reliability. However, the transmissions from different relaying nodes result in a great challenge in practice. The use of TDMA for relaying transmissions causes large transmission delay. In contrast, FDMA and CDMA associate with the issue of imperfect frequency and timing synchronization due to the simultaneous transmissions from the relaying nodes. In this work, we propose a space-time network coding (STNC) scheme utilizing transform-based coding to improve communication reliability while maintaining a stable network throughput and overcoming the issue of imperfect frequency and timing synchronization. Based on TDMA, a node in the network forms a linearly-coded signal from the overheard symbols and transmits it to the destination in its dedicated time slot. The pairwise error probability (PEP) is analyzed and design criteria of the STNC are derived to ensure achieving full diversity. Simulations are conducted to verify the performance of the proposed scheme and to reveal its advantage over a distributed space-time block coding scheme under timing synchronization errors.

This paper considers a virtual multiple-input single-output (VMISO) network, where spatially separated single-antenna radios cooperatively transmit the same data to a destination. The existing works for the VMISO system assume perfect synchronization between the nodes. However, in this paper, we investigate the effects of imperfect timing synchronization on the performance of this network. For that purpose, we develop a statistical model for the timing errors and derive the expressions for the bit error probability (BEP) using this model. The results indicate that the BEP performance is degraded due to imperfect timing synchronization and the BEP is dependent upon the number of radios per cluster, signal-to-noise ratio (SNR) and the distance disparities between the nodes.

This work explores amplify-and-forward (AF) two-way relay networks (TWRNs) working under flat-fading non-reciprocal channels conditions in the presence of both timing and frequency offsets. We showcase a novel pilot-based maximum likelihood estimator for the joint channel and carrier frequency offsets (CFOs), assuming the timing offsets are known. The proposed estimator has reasonable complexity, and we show that it meets the Cramer-Rao bound (CRB) for our model.

This paper presents an optimized turbo equalizer based on Laot's turbo equalization scheme by means of soft intersymbol interference (ISI) cancellation. Due to Berrou's turbo-principle, common turbo equalization schemes use the extrinsic information as a-priori input for the equalizer. In contrast to this, we propose the use of the full soft output channel decoder's information for the turbo interference canceller. The performance of this improved turbo equalizer by means of soft interference cancellation is evaluated in the environment of two channels, which introduce severe ISI. Simulation results show that the proposed turbo equalizer achieves a performance gain of up to 4 dB compared to Laot's original approach. The performance gain is achieved without any increase of computational complexity. Moreover, we apply a modified extrinsic information transfer (EXIT) chart to the proposed turbo equalizer and illustrate the convergence advantages in comparison to Laot's approach.

Traditional cooperative communications can improve communication reliability. However, the simultaneous transmissions from multiple relays are challenging in practice due to the issue of imperfect frequency and timing synchronization. In this work, we propose space-time network codes (STNCs) that overcome the issue of imperfect synchronization, reduce the large transmission delay, and still provide full spatial diversity in multipoint-to-point (M2P) and point-to-multipoint (P2M) transmissions. Relays form single linearly-coded signals from the overheard symbols and transmit them to the destination, where multiuser detection is utilized to detect the desired symbol. For a network of N client nodes, M2P and P2M STNCs result in a diversity order of N with only 2N time slots, a significant reduction from N2 time slots in traditional cooperative communications using TDMA.

High speed transmission of information suffers inevitably by intersymbol interference (ISI) induced by multipath propagation over frequency selective channels. ISI has been a major obstacle preventing the successful implementation of physical layer network coding (PLNC) systems in these environments. Another major obstacle is the high computational complexity of existing algorithms dealing with ISI, which is a determining factor in the design of relay nodes. In this paper, we propose a novel and computationally efficient decision feedback equalizer structure that is utilized at the end node and in conjunction with a simple amplify and forward (AF) technique employed at the relay. We derive the optimal equations for the decision feedback equalizer (DFE) coefficients and evaluate the end-to-end bit error rate performance in frequency selective channels, and provide both analytical and numerical simulation results that demonstrate the feasibility of the proposed approach under realistic channel conditions. The obtained results show that E2E performance is dominated by the channel with most severe frequency selectivity.

Multiple-input multiple-output (MIMO) and cooperative communications have been attracted great attention for the improvements of communication capacity, power consumption, and transmission coverage. The conventional fixed relaying protocols, amplify-and-forward (AF) and decode-and-forward (DF), have their own advantages and disadvantages, i.e. AF performs better than DF for low signal-to-noise ratio (SNR) region, while the reverse is true for high SNR region. Therefore, this paper proposes an SNR-adaptive forward (SAF) relaying scheme obtaining the advantages of both AF and DF. Furthermore, the proposed SAF does not need to switch between AF and DF when SNR changes. The main idea is to adaptively derive the soft information at the cooperative relay nodes based on the information of the received signal and the SNR. Besides, based on the theoretical analysis and the simulation results, it is affirmed that the proposed SAF achieves superior performance than both AF and DF for all SNRs. Moreover, the performance gain would be improved with the increasing number of parallel cooperative relay nodes.

Cooperative communication has been studied as a way to improve reliability of networks compared to conventional direct transmission scheme. We consider amplify-and-forward (AF) cooperative communication with multiple relays. In this system, there is a problem that which relay nodes should be selected and how much power should be allocated to the selected relay nodes. Most of conventional researches on this model have focused on only improving channel capacity or reliability, rather than saving power consumption in all over the network with maintaining such communication qualities. In this paper, we propose a new power allocation scheme for multiple relay nodes using game theory that focuses on transmission power efficiency. With computer simulation, we show that our proposed game theoretical approach has superiority to some conventional schemes from the viewpoint of transmission power efficiency.

Recently, the orthogonal frequency-division multiple access (OFDMA) based wireless communication system is usually used in interactive digital video broadcasting environments or the wireless metropolitan area network (WMAN). Therefore, the relative condition of the inter-user channel can be easily deteriorated. This paper proposes and evaluates the mutual amplify-and-forward (AF) cooperative communication between two users using modified space time block code (STBC) to provide the reliability for an OFDMA system with a single antenna in an inferior inter-user channel environment. As the results, an OFDMA system adopting the proposed cooperative communication is more robust to the inferior inter-user channel than the previous decode-and-forward (DF) cooperative communication.

Cooperative communication represents an effective way of introducing spatial diversity in wireless networks. Full performance gain of an ideal cooperative communication system requires the cooperating transmitters to form their signals such that the destination receiver is able to coherently combine the desired signals. This encompasses multiple implementation challenges. We introduce a more pragmatic approach to cooperative diversity based on modern error correction coding working with phase dithering. Modulation constrained information rate analysis for this approach is presented, and we show that the loss relative to the ideal cooperative diversity scheme is minimal. By incorporating further practical constraints, we show performance of block-oriented phase dithering within a quasi-static Rayleigh fading channel