Outage analysis and power allocation for space-time block coded two-way relaying systems

In this paper, we consider the two-way relay channel (TWRC) over which communication is performed using space-time block codes (STBC) with physical-layer network coding (PNC) protocol. The three-node two-way relaying (TWR) model consists of two end nodes (sources) simultaneously communicating with each other over a relay node where each node is equipped with two antennas and uses Alamouti's STBC to provide diversity. We consider non-reciprocal channels where fading channel coefficients are independent for opposite directions of the same link. We first derive closed form expressions for the outage probability of the STBC-TWR system and uncoded TWRC assuming equal transmit power at each node and decode-and-forward (DF) strategy at the relay. We show that the STBC-TWR system significantly improves the uncoded TWRC outage performance. Then, to further improve the performance, we propose an optimal power allocation in TWR scheme employing Alamouti's STBC and PNC protocol. The optimal power allocation is obtained by maximizing the sum-rate under the total power constraint. We show that the optimal power allocation considerably improve the outage performance as well as the sum-rate compared to the equal-power case.

In this paper, we study the power allocation of suggested cognitive relay network model to minimize the system outage probability subject to total and individual power constraints for cognitive relays and subject to interference constraints for primary user nodes. In suggested model, a source is assisted by cognitive relay nodes which allow coexisting with primary user nodes by imposing severe constraints on the transmission power so that they operate below the noise floor of primary nodes. A relay transmission scheme, namely, amplify-and-forward (AF) is considered. We present an optimal power allocation (OPA) scheme to minimize the system outage probability, based on complete channel state information among the source, destination and relay nodes and mean channel gains between primary user nodes and cognitive relays. The analytical power allocation strategies have been validated through numerical simulations. The results indicate that the AF with OPA has significantly better outage behavior and average throughput than AF scheme.

Physical-layer network coding (PNC) promises a significant gain In overall network throughput for multi-user cooperative communications. In a multi-unicast scenario, one drawback associated with PNC is an additional noise term, coined as network coding (NC) noise, which severely degrades the system data rate. In this paper, our contribution to address this challenging problem is source and relay power control with the objective of maximization of the minimum average achievable rate among all the source-destination pairs subject to a given total power constraint. We further develop a joint power allocation and relay selection scheme, which only rely on long-term channel statistics, to extend our results to general network topologies. We show that the joint optimization problem can be divided into two problems: optimal power allocation and optimal relay selection where the former is scalable and leads to a power assignment algorithm that exhibits the same optimization complexity for any number of sources in the network and the latter can be performed in a decentralized manner. By means of simulations, we validate our theoretical developments and verify the efficiency of our algorithm in improving the average achievable rate compared to a multi-unicast system with no power control or relay selection. We conclude that the proposed algorithm largely combats the adverse effects of NC noise while achieving near optimal fairness.

In this paper, we study the joint allocation of three types of resources, namely, power, subcarriers and relay nodes, in multi-relay assisted dual-hop cooperative OFDM systems. All the relays adopt the amplify-and-forward protocol and assist the transmission from the source to destination simultaneously but on orthogonal subcarriers. The objective is to maximize the system transmission rate subject to individual power constraints on each node or a total network power constraint. We formulate such a problem as a subcarrier-pair based resource allocation that seeks the joint optimization of subcarrier pairing, subcarrier-pair-to-relay assignment, and power allocation. Using a dual approach, we solve this problem efficiently in an asymptotically optimal manner. Specifically, for the optimization problem with individual power constraints, the computational complexity is polynomial in the number of subcarriers and relay nodes, whereas the complexity of the problem with a total power constraint is polynomial in the number of subcarriers.We further propose two suboptimal algorithms for the former to trade off performance for complexity. Simulation studies are conducted to evaluate the average transmission rate and outage probability of the proposed algorithms. The impact of relay location is also discussed.

We investigate a power allocation optimization problem under the scenario of multiple cooperative relays and one destination with multiple receiving antennas. We consider the relay protocol, decode and forward (DAF). An optimal power allocation scheme (OPA), through singular value decomposition (SVD) of the channel matrix, is developed among the relay nodes under a certain power constraint to achieve the maximum channel capacity. The expression of the optimal power allocation coefficient is derived using Lagrange multiplier. Simulation results show that the OPA can obtain higher capacity gain and lower outage probability comparing with the uniform power allocation (UPA) scheme. Moreover, we show that the OPA with DAF has better performance than it with AAF.

This paper studies the performance of a half-duplex (HD) two-way relay (TWR) in power line communication (PLC) over a log-normal fading channel with impulsive noise. We consider the two common relaying protocols: amplify-and-forward (AF) and decode-and-forward (DF). For the DF relaying, we apply physical-layer network coding (PNC) and analog-network coding (ANC) to the PLC TWR. We derive analytic expressions for the average sum capacity and the outage probability of the system. The Monte Carlo simulations are provided throughout to validate our analysis. The analytical results show a tight approximation to the simulation results. We compare the one-way relay (OWR) to the TWR and show that the HD spectral efficiency loss incurred by the OWR can be sufficiently mitigated in PLC. However, the outage probability of the TWR is inferior to that of the OWR. To enhance the outage performance of the PLC TWR, we implement a hybrid PLC/wireless (HPW) system, where all nodes are equipped with the PLC and wireless capabilities. Data transmission occurs over the two parallel links. The diversity in the transmission allows the TWR to improve its outage performance in the AF and DF protocols. The impact of the impulsive noise, inherent to the PLC channels, is also highlighted in the simulation results. It is shown that the impulsive noise severely impairs system performance.

Optimal power allocation for two-way relaying over OFDM using physical-layer network coding

In this paper, the capacity of MIMO relay channel with decode and forward (DF) scheme is investigated, where each node within the system quipped with multiple antennas and located in an environment with distance-dependent path loss. In order to achieve the maximum capacity of the system, the optimal power allocation (OPA) among the nodes and the transmit antennas is analyzed under the assumption that relay node knows all channels instantaneous state information (CSI), and an implement algorithm of the OPA is derived. Numerical results demonstrate that adopting the proposed power allocation algorithm, the capacity of MIMO relay channel with DF relay scheme can be significantly improved.

Outage probability is one of the most important performance measures for cooperative communication systems. And the closed-form solution on exact outage probability is essential for many further studies, such as optimal power allocation and rate control. However, as far as we know, the issue remains open for multiuser linear network coded cooperation (LNCC), which is a technology integrating linear network coding into cooperative communication. Consequently, in this paper, we investigate the multiuser LNCC system with multiple cooperation time slots and many-to-one communication pattern. All the possible outage scenarios are fully considered and the closed-form solution of the system's exact outage probability is theoretically derived. In order to obtain the diversity order of the LNCC system, the asymptotic outage probability is also analysed. The theoretical analyses are verified by plenty of Monte Carlo simulations. In order to demonstrate the benefits introduced by LNC to the cooperative system, the outage performance comparison between the LNCC and the traditional Decode-and-Forward (DF) cooperation system is carried out. The results show that given the same number of users and cooperation time slots, the LNCC system's outage probability is greatly lower than that of the traditional DF cooperation system.

This paper investigates the outage performance of simultaneous wireless information and power transfer (SWIPT) in two-way relay channels, where two sources exchange their information via an energy harvesting (EH) relay with digital network coding (NC) employed. The relay harvests energy from the received radio-frequency (RF) signals transmitted from the two sources and then use it to forward the received signals. For such a system, we study the power splitting-based two-way relay (PS-TWR) transmission strategy and analytically derive an explicit expression of the system outage probability. In order to explore the system outage performance limit, we formulate an optimization problem to minimize the system outage probability by jointly optimizing the power splitting ratio and the time assignment ratio. Since the problem is non-convex and have no known solution method, we present a genetic algorithm (GA) to solve it. Numerical results demonstrate our theoretical analysis and show that the power splitting ratio and the time assignment ratio greatly affect the system outage performance. Moreover, it is also shown that for the SWIPT-enabled two-way relay system, the relay node should be deployed closer to the source with better channel quality and when it is positioned on the middle point between the two sources, the system gets the worst outage performance, which is very different from traditional non-SWIPT two-way relay systems.

This paper compares the system performance of transmit diversity schemes for use in interference-limited cellular systems. We focus on evaluating the performance of Alamouti space time block codes (STBC) and cyclic delay diversity (CDD) codes, which are being actively considered for adoption within the 3 GPP-long-term-evolution (LTE) and 802.16e WiMAX standards. While the performance of transmit diversity schemes has been extensively evaluated in noise-limited scenarios, their performance in co-channel interference (CCI) limited cellular systems has received limited attention so far. As cellular systems adopt frequency reuse of one, co-channel interference from other cells becomes a major limitation to system performance. We present system simulation results comparing the performance of transmit diversity schemes using matched filter reception for a multi-carrier OFDM-based cellular system. In particular, the performance of STBC and CDD schemes is compared with respect to the evolving third generation 3 GPP-LTE standard. Our comparison highlights the spatial as well as the multi-user diversity aspects of these schemes and considers a variety of scenarios that will require their use. Our results show that CDD can significantly improve the system spectral efficiency and cell-edge performance over STBC, when used in combination with frequency-dependent scheduling. For open loop transmission without scheduling, the relative diversity gains of STBC and CDD at link layer depend on the interference scenario considered, while the system performance of these schemes is very similar.

This paper proposes an optimal power allocation method for two-way decode-and-forward (DF) relay networks when transmit power values at source nodes are the same. In this paper we consider the multiple access (MAC) capacity for DF relaying scheme. Using case studies, it analytically determines the optimal power values for the two source nodes and one relay node. The achievable sum rate is maximized under a sum power constraint for given squared magnitude of the channel coefficients. Finally, numerical results show that the achievable sum rate for proposed optimum power allocation is greater than or equal to that for equal power allocation.

In linear networks with physical layer network coding (PNC), the relay node performs the combination of the relaying data from both end users. Moreover, the adaptive power allocation (APA) scheme can optimize the transmission rate of PNC for relaying communication systems. However, when the relay nodes are deployed with different distance to both of end user nodes, it is called near-far effect (NFE). The NFE degrades the bit error rate (BER) performance with deteriorating the decoding performance of PNC. Therefore, in order to further optimize the transmission power allocation (PA) for both of end user nodes and relaying node we propose a total adaptive power allocation (TAPA) scheme in this paper. With the proposed TAPA scheme, the NFE of conventional PNC systems can be diminished and the BER performance is improved. Simulation results show that the proposed TAPA improves the BER performance especially for the severe NFE environments.

Orthogonal frequency division multiplexing (OFDM) is a multicarrier modulation scheme widely used in 4G. It is effective for reducing the effects of multipath delay in wireless communication. For OFDM system, total delay spread (Td) is less than symbol period (T); it can be combined with antenna arrays at transmitter and receiver (MIMO) to increase diversity gain called as MIMO-OFDM system. MIMO-OFDM improves system capacity on time-variant and frequency-selective channels. The quality of service provided by a system greatly depends upon the correct selection of modulation scheme. In this paper, analysis and simulation of OFDM-MIMO system with different digital modulation schemes like BPSK, 8-PSK, QPSK, and QAM is performed using Alamouti space–time block codes (STBC). STBC uses spatial and time dimensions for error control unlike traditional methods that uses only time dimension. A comparison study shows that for a given data rate and channel conditions, OFDM-MIMO using STBC with BPSK modulation is better compared with other digital modulation techniques in terms of error performance. OFDM-MIMO using STBC with BPSK modulation is further analyzed and simulated for various antenna configurations. It is observed that as numbers of antennas at transmitter and receiver side are increased, the system diversity increases and gives better performance in terms of BER.

In this paper we consider a multi relay Decode and Forward (DF) relay network assisted by Hybrid automatic repeat request (HARQ) in a Rayleigh block fading environment. The system uses an incremental relaying strategy in which the best relay is selected to cooperate with source. A Space-Time Block coding (STB) is used for cooperation between source and selected relay to transmit the data to the destination. Relays and destination use maximum ratio combining (MRC) technique to decode the sent message using previously received versions. We derive a closed-form expression of asymptotic outage probability at high Signal-to-Noise Ratio (SNR) regime. We also derive the average delay and throughput in a delay constrained application. The derivation method is based on the Laplace transform, which proves to be a very effective analytical tool when combined with the properties of the special functions. In addition, we investigate the optimum power allocation strategy to minimize the outage probability and consequently maximize the average throughput perceived by the end user. The simulation results show that the obtained analytical bound for the outage probability is very tight at high SNR regime. In addition the proposed power allocation scheme clearly outperforms the equal power allocation.