On Two-Pair Two-Way Relay Channel with an Intermittently Available Relay

When multiple users share the same resource for physical layer cooperation such as relay terminals in their vicinities, this shared resource may not be always available for every user, and it is critical for transmitting terminals to know whether other users have access to that common resource in order to better utilize it. Failing to learn this critical piece of information may cause severe issues in the design of such cooperative systems. In this paper, we address this problem by investigating a two-pair two-way relay channel with an intermittently available relay. In the model, each pair of users need to exchange their messages within their own pair via the shared relay. The shared relay, however, is only intermittently available for the users to access. The accessing activities of different pairs of users are governed by independent Bernoulli random processes. Our main contribution is the characterization of the capacity region to within a bounded gap in a symmetric setting, for both delayed and instantaneous state information at transmitters. An interesting observation is that the bottleneck for information flow is the quality of state information (delayed or instantaneous) available at the relay, not those at the end users. To the best of our knowledge, our work is the first result regarding how the shared intermittent relay should cooperate with multiple pairs of users in such a two-way cooperative network.Comment: extended version of ISIT 2015 pape

Performance Enhancement of Coherent Optical OFDM System Using LMS Algorithm, Journal of Telecommunications and Information Technology, 2020, nr 4

Instability of the local oscillator causes phase noise – a phenomenon that is a disadvantage and is considered to be a major obstacle in the functioning of coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. An attempt has been made in this paper to reduce the effects of common phase errors generated by phase noise. In this paper, a least mean square (LMS) based algorithm is proposed for estimation of phase noise. Using this proposed algorithm, the major problem of phase ambiguity caused by cycle slip is avoided and the bit error rate is greatly improved. Further, there is no requirement for modifying the frame structure of OFDM using this algorithm. A CO-OFDM system with the 8-PSK technique is used to implement the algorithm concerned. Furthermore, the algorithm, using the 8-PSK modulation technique, is analyzed and compared with the existing QPSK technique and with other algorithms. The investigations reveal that 8-PSK outperforms existing LMS algorithms using other techniques and significantly reduces the bit error rate

Lattice Codes for Multiple-access Relay Channel: Decode-and-Forward and Compute-and-Forward

For the uplink relay-aided transmission in a cellular system, the spectral efficiency of mobile radio networks can be improved by introducing relays to assist the transmissions of mobile stations. Further performance improvement can be expected if each relay aids not just a single mobile station, but many simultaneously. In this dissertation, we study the multiple-access relay channel (MARC), in which multiple users transmit messages to a common destination with the assistance of a relay. There are two protocols to be considered: 1) dynamic decode-and-forward (DDF) protocol and 2) compute-and-forward (CMF)protocol. In a variety of MARC settings, DDF protocol is very useful due to its outstanding rate performance. However, the lack of good structured codebooks so far hinders practical applications of DDF for the MARC. For the DDF protocol, two classes of structured codes for theMARC are proposed: 1) one-to-one relay-mapper aided multiuser lattice coding (OMLC), and 2) modulo-sum relay-mapper aided multiuser lattice coding (MS-MLC). The former enjoys better rate performance, while the latter provides more flexibility to tradeoff between the complexity of the relay mapper and the rate performance. It is shown that, in order to approach the rate performance achievable by an unstructured codebook with maximum-likelihood decoding, it is crucial to use a new K-stage coset decoder for structured O-MLC instead of the one-stage decoder proposed in previous works. However, if O-MLC is decoded with the one-stage decoder only, it can still achieve the optimal DDF diversity-multiplexing gain tradeoff in the high signal-to-noise ratio regime. As for MSMLC, its rate performance can approach that of the O-MLC by increasing the complexity of the modulo-sum relay-mapper. Finally, for practical implementations of both O-MLC and MS-MLC, practical short length lattice codes with linear mappers are designed, which facilitate efficient lattice decoding. When the channel links from the users to the relay are weak, DF-based protocol may fail to decode all users at the relay. Aiming to solve this problem, we propose a new lattice coding based on the CMF protocol, where the relay only needs to decode an integerweighted-sum of users’ lattice codewords, re-maps it with a modulo-basedmapper and then forwards the corresponding codeword. Although the decoding at the relay is akin to the orthogonal CMF protocol, we relax the restriction imposed by previous works that the users have to be silent when the relay is transmitting to avoid interference. The key ingredient is the joint multiuser lattice decoding performed at the destination. This jointly decoding strategy not only complicates the corresponding code design but also the error analysis. Simulation results show that the proposed coding schemes outperform existing schemes in terms of outage probabilities and the achievable rates in a variety of channel settings.Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii Chapters: 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Motivation and Contributions . . . . . . . . . . . . . . . . . . . . . . . 2 1.2 Dissertation Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.3 Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2. Multiple Access Relay Channel and Preliminaries . . . . . . . . . . . . . . . . 11 2.1 System Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.2 Review of Lattices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.2.1 Lattice Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.2.2 Loeliger Lattice Ensemble . . . . . . . . . . . . . . . . . . . . . 17 2.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3. DDF-based Multiuser Lattice Coding for the Multiple-access Relay Channel . . 19 3.1 Relay-Mapper Aided Multiuser Lattice Coding Scheme . . . . . . . . . 20 3.1.1 Encoders: Nested Lattice Codebooks and Relay Mappers . . . . 20 3.1.2 Proposed K-stage Coset Decoders . . . . . . . . . . . . . . . . . 24 3.2 Performance Analysis of the Proposed Coding Schemes . . . . . . . . . 32 3.3 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 4. CMF-based Multiuser Lattice Coding for the Multiple-access Relay Channel . 50 4.1 System Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 4.2 Non-orthogonal Lattice Coding withModulo-sumComputation and Joint Decoding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 4.2.1 Encoding Process for the Users . . . . . . . . . . . . . . . . . . 54 4.2.2 Decoding and Mapping at the Relay . . . . . . . . . . . . . . . . 55 4.2.3 Two-stage Joint Coset Decoder at the Destination . . . . . . . . 56 4.3 Performance Analysis of the Proposed Coding Scheme . . . . . . . . . . 58 4.4 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Appendices: A. Proof of Lemma 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 B. Proof of the Rate Region of MS-MLC with the K-stage Decoder in Theorem 2 71 C. Proof of (A.1 b) for Lemma 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 D. Proof of (3.13) for Corollary 2 . . . . . . . . . . . . . . . . . . . . . . . . . . 78 E. Proof of Theorem 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8