A new forwarding strategy for wireless relay channels: Analog-encode-and-forward (AEF)

We propose two coding schemes combining decode-forward (DF) and noisy network coding (NNC) with different flavors. The first is a combined DF-NNC scheme for the oneway relay channel which includes both DF and NNC as special cases by performing rate splitting, partial block Markov encoding and NNC. The second combines two different DF strategies and layered NNC for the two-way relay channel. One DF strategy performs coherent block Markov encoding at the source at the cost of power splitting at the relay, the other performs independent source and relay encoding but with full relay power, and layered NNC allows a different compression rate for each destination. Analysis and simulation show that both proposed schemes supersede each individual scheme and take full advantage of both DF and NNC.

In this paper cooperation among users at the physical layer level is studied for the downlink. Distributed space time linear dispersion codes are proposed for the cooperative strategies amplify and forward (AF) and decode forward (DF), assuming no channel state knowledge at the transmitter. Both AF and DF strategies are evaluated in a cellular multiuser realistic scenario based on TDMA access, considering cellular reuse of the relay slot. Different configurations regarding the number of antennas at the relay and destination nodes are considered. The maximum achievable rate of cooperative schemes implemented by means of space time linear dispersion codes is compared with the theoretical capacity of the cooperative channel. Finally an analysis in terms of BER is given for different schemes.

In order to combine the advantages of amplify-forward (AF) and decode-forward (DF) in relay networks, several strategies have been developed making use of reliability information after decoding at the relay. The use of soft-output channel decoders enables forwarding reliability information for the decoded bits. This soft information can be forwarded in different ways, e.g. by transmission of the log likelihood ratios (LLRs) normalized to the power constraint. In this paper transmitting expectation values after decoding, the so-called decode-estimate-forward (DEF) scheme, will be shown to be the best choice in terms of the mean squared uncorrelated error at the receiver. Additionally, it will be shown that LLR combining is superior to maximum ratio combining at the receiver as the overall disturbance of the received signal is not Gaussian in the case of DEF. Furthermore, in the uncoded case LLR combing also improves the performance in terms of effective signal-to-noise ratio and bit error rate.

In this paper, we investigate symbol-level selective decode-forward (SDF) relaying for uncoordinated dense wireless networks. The main objective is to establish reliable communications for such wireless networks, which cannot have accurate link budget allocation due to signalling overhead limitation. To approach this goal, a novel half-duplex absolute difference based symbol-level SDF is developed to enjoy diversity gain at destination node with no signalling between relay and destination node. The proposed selection criteria is designed that no preset threshold is needed at relay, i.e., receive SNR threshold for uncoded systems or LLR threshold for coded systems. Therefore, it is more suitable for the considered uncoordinated dense wireless networks. It offers better performance, in terms of symbol error rate (SER), when compared with the conventional always decode-forward (DF) relaying and frame-level SDF. Computer simulations are carried out to evaluate the proposed approach with respect to the frame length and channel gain. Significant performance improvement is observed in terms of both SER and FER with various setup of frame length and channel gain.

The rotate-and-forward scheme was introduced in (Yang-Belfiore, 2010) to recover spatial diversity in multi-hop MIMO relay networks. It was shown that this scheme achieves the optimal diversity-multiplexing (DMT) trade-off in a two-hop relay network, with two antennas at the relay node. In this paper, it is shown that the scheme is DMT optimal for arbitrary number of antennas at the source, relay, and destination node.

In wireless networks with energy limited nodes, user cooperation is usually exploited to reduce the network energy consumption. In many practical scenarios, however, nodes' selfishness raises doubts on whether each node will be willing to spend its valuable energy in forwarding packets for other users. To analyze this problem, a non-cooperative game theoretic framework is adopted in our work. Using this framework, the critical role of altruistic nodes in encouraging cooperation is established, both for small and large scale networks. In a small network, where nodes utilize the Decode-Forward scheme to cooperate, we show that a relay node, with appropriate strategy and location, successfully turns the Nash Equilibrium from no- cooperation to full-cooperation. In the large scale network, we show that it is sufficient to have a vanishingly small fraction of the nodes to be altruistic, i.e., relay nodes, in order to ensure full cooperation from all the nodes in the network. This result hinges on using the appropriate forwarding policies by the altruistic nodes, as detailed in the sequel. Our work also establishes the sub-optimality of traditional relaying strategies, which ignore the game-theoretic aspect of the problem. An important aspect of our work is that only reward/punishment policies that can be realized on the physical layer are used, and hence, our results establish the achievability of full cooperation without requiring additional incentive mechanisms at the application layer.

Improving numerical forecast accuracy with ensemble Kalman filter and chaos theory: Case study on Ciliwung river model

A coded cooperative transmission scheme based on turbo encoding/decoding, in which only newly generated parity bits of the partner are sent if the user successfully decodes its partner's information in order to improve bandwidth efficiency is proposed. The proposed encoding structure introduces correlation between users' data over multiple frames, which offers effectively longer codes and facilitates high-performance iterative multi-user decoding at the destination. Additionally, the iterative decoding over multiple frames can provide time diversity besides spatial diversity inherent in user cooperation even for flat block fading channels. Simulation results show that the proposed scheme significantly outperforms direct transmission for the same transmitted power and bandwidth efficiency.

Reed-Solomon convolutional concatenated (RSCC) code has been widely applied in wireless and space communications. However, iterative soft-decision decoding of the concatenated code is yet to be developed. This paper proposes a novel iterative soft decoding algorithm for the concatenated coding scheme. The maximum a posteriori (MAP) algorithm is used to decode the inner convolutional code. Its soft output will be deinterleaved and then passed to the soft-in-soft-out (SISO) decoding algorithm for the outer Reed-Solomon (RS) code. The outer SISO decoder integrates the adaptive belief propagation (ABP) algorithm and the Koetter-Vardy (KV) list decoding algorithm, attempting to find out the transmitted message. If it is found, the deterministic probabilities of the corresponding RS coded bits will be fed back. Otherwise, the extrinsic probabilities that are yielded by the ABP algorithm will be given as the feedback. With the proposed soft information exchange decoding mechanism, error-correction potential of the concatenated code can be better exploited. Our simulation results show that significant performance improvement can be achieved over the existing decoding algorithms.

Recent research has shown that the cooperative diversity can efficiently combat the fading effects caused by the hostile wireless channels just as the multiple-input multiple-output (MIMO) antenna systems do. Besides, due to the inherent cost and size constraints by many small area wireless networks, e.g. wireless ad hoc networks and wireless sensor networks, in such environments, it would be almost impossible to deploy the antenna array in a mobile node. However, cooperative diversity can provide an economical and viable solution by sharing each mobile node’s single-antenna with others to form a virtual antenna array, while it still can maintain significant performance gains similar to MIMO systems. Meanwhile, the adaptive modulation is a proven technique which can significantly increase the spectral efficiency when perfect channel estimation can be achieved, and many wireless standards have adopted it as an implementation option. It also has been shown that combining these two techniques can greatly raise the system efficiency. Nevertheless, in the multiple nodes cooperative communication networks, the randomness of demodulation process at each relay node makes the task of correctly selecting the forwarding relay nodes very complicated. In view of this problem, a novel adaptive node selection algorithm is proposed for the destination node to effectively select the forwarding relay nodes to help the source node transfer the signals. In this paper, the decode-and-forward (DF) protocol is employed as the cooperative diversity signaling, and the constant-power variable-rate M-ary phase shift keying (M-PSK) is adopted as the adaptive modulation scheme. At the destination node, the maximal-ratio-combining (MRC) method is used to combine all of the signals from diversity branches with the main path transmitted directly from the source node. Because of the random phenomenon of the demodulation process, the outage requirement is used as the baseline in the first stage of the proposed node selection algorithm to choose the maximal transmission rate. Hence the maximal overall system throughput in terms of rate can be achieved. In the meantime, the first candidate forwarding sets of the relay nodes which collectively meet the required outage probability are also been chosen. Then in the second stage, we can further refine the number of the relay nodes among the first candidate forwarding sets by using the following criterion — maximum average rate per node. It will screen out the first candidate sets with the minimum number of the relay nodes. If only one candidate set is left at the end of the second stage, it will be the end of the proposed algorithm. However, if the result of the screening still has more than one set, then the final election among the second candidate sets would be the one which can give the minimum outage probability. In the simulation results, our proposed node selection algorithm can even further improve the spectral efficiency while the deleteriously fading effect induced by the wireless channels can also be effectively removed.

The capacity regions are investigated for two relay broadcast channels (RBCs), where relay links are incorporated into two-user broadcast channels to support user cooperation. In the first channel, the partially cooperative RBC, only one user in the system acts as a relay. An achievable rate region is derived based on the relay using the decode-and-forward scheme. An outer bound on the capacity region is derived and is shown to be tighter than the cut-set bound. For the special case where the partially cooperative RBC is degraded, the achievable rate region is shown to be the capacity region. Two Gaussian cases of the partially cooperative RBC are studied. For the system where the additive white Gaussian noise (AWGN) term at one receiver is a degraded version of the other, which we refer to as the D-AWGN partially cooperative RBC, the capacity region is established. For the system where the AWGN term at one receiver is independent of the other, which we refer to as the AWGN partially cooperative RBC, inner and outer bounds on the capacity region are derived and are shown to be close. Furthermore, it is shown that feedback does not increase the capacity region for the degraded partially cooperative RBC, but that it improves the capacity region for the nondegraded version. In particular, feedback improves the capacity region for the AWGN partially cooperative RBC. In the second channel model being studied in the paper, the fully cooperative RBC, both users can act as relay nodes. All the results for the partially cooperative RBC are correspondingly generalized to the fully cooperative RBC. In particular, capacity regions are established for the degraded and D-AWGN fully cooperative RBCs. The capacity region is also established for the fully cooperative RBC with feedback. It is further shown that the AWGN fully cooperative RBC has a larger achievable rate region than its partially cooperative counterpart. The results illustrate that relaying and user cooperation are powerful techniques for improving the capacity of broadcast channels

We investigate the effects of user cooperation on the secrecy of broadcast channels by considering a cooperative relay broadcast channel. We show that user cooperation can increase the achievable secrecy region. We propose an achievable scheme that combines Marton's coding scheme for broadcast channels and Cover and El Gamal's compress-and-forward scheme for relay channels. We derive outer bounds for the rate-equivocation region using auxiliary random variables for single-letterization. Finally, we consider a Gaussian channel and show that both users can have positive secrecy rates, which is not possible for scalar Gaussian broadcast channels without cooperation.

We investigate the effects of user cooperation on the secrecy of broadcast channels by considering a cooperative relay broadcast channel. We show that user cooperation can increase the achievable secrecy region. We propose an achievable scheme that combines Martonpsilas coding scheme for broadcast channels and Cover and El Gamalpsilas compress-and-forward scheme for relay channels. We derive outer bounds for the rate-equivocation region using auxiliary random variables for single-letterization. Finally, we consider a Gaussian channel and show that both users can have positive secrecy rates, which is not possible for scalar Gaussian broadcast channels without cooperation.

We propose a quasi-maximum-likelihood (quasi-ML) detection scheme with soft output and low decoding complexity for multiple antenna wireless channels modulated with phase-shift keying (PSK). These detectors, which we refer to as quasi-ML PSK decoders, deploy a nonlinear programming relaxation and are able to achieve the performance of exact ML detection in a wide variety of coded or uncoded and PSK modulated multiple-antenna channels. Yet, the decoding complexity or the soft quasi-ML PSK decoders is much less than of ML detectors using exhaustive search. We also demonstrate that the quasi-ML PSK decoders possess favorable complexity properties compared to the existing sphere decoders.

In this paper, we propose a practical 1-bit soft forwarding protocol for a network-coded two-way relay channel. Different from the conventional estimate-and-forward (EF) protocol, the proposed protocol forwards 1-bit soft information at the relay. We employ the joint trellis coded quantization/modulation (TCQ/M) to implement 1-bit transmission of the soft information. Also, the codebooks in the TCQ are designed to be adaptive to the source-to-relay channel conditions so that the system can achieve the full diversity gain over fading channels. Specifically, in the low source-to-relay channel SNR region, we apply the TCQ/M to the soft information based on the codebook generated by the LloydMax quantizer. In the high source-to-relay channel SNR region, where the soft information is equivalent to its hard decision, we design the codebook by repeating the soft information. It has been shown that the proposed protocol outperforms both the amplify-and-forward (AF) and the decode-and-forward (DF) protocols over fading channels.