Multi-way relay networks: characterization, performance analysis and transmission scheme design

Multi-way relay networks (MWRNs) are a growing research area in the field of relay based wireless networks. Such networks provide a pathway for solving the ever in- creasing demand for higher data rate and spectral efficiency in a general multi-user scenario. MWRNs have potential applications in video conferencing, file sharing in a social network, as well as satellite networks and sensor networks. Recent research on MWRNs focuses on efficient transmission protocol design by harnessing different network coding schemes, higher dimensional structured codes and advanced relaying protocols. However, the existing research misses out the characterization and analysis of practical issues that influence the performance of MWRNs. Moreover, the existing transmission schemes suffer some significant limitations, that need to be solved for maximizing the benefits of MWRNs. In this thesis, we investigate the practical issues that critically influence the perfor- mance of a MWRN and propose solutions that can outperform existing schemes. To be specific, we characterize error propagation phenomenon for additive white Gaus- sian noise (AWGN) and fading channels with functional decode and forward (FDF) and amplify and forward (AF) relaying protocols, propose a new pairing scheme that out- performs the existing schemes for lattice coded FDF MWRNs in terms of the achievable rate and error performance and finally, analyze the impact of imperfect channel state information (CSI) and optimum power allocation on MWRNs. At first, we analyze the error performance of FDF and AF MWRNs with pair- wise transmission using binary phase shift keying (BPSK) modulation in AWGN and Rayleigh fading channels. We quantify the possible error events in an L-user FDF or AF MWRN and derive accurate asymptotic bounds on the probability for the general case that a user incorrectly decodes the messages of exactly k (k ∈ [1, L − 1]) other users. We show that at high signal-to-noise ratio (SNR), the higher order error events (k ≥ 3) are less probable in AF MWRN, but all error events are equally probable in a FDF MWRN. We derive the average BER of a user in a FDF or AF MWRN under high SNR conditions and provide simulation results to verify them. Next, we propose a novel user pairing scheme for lattice coded FDF MWRNs. Lattice codes can achieve the capacity of AWGN channels and are used in digital communica- tions as high-rate signal constellations. Our proposed pairing scheme selects a common user with the best average channel gain and thus, allows it to positively contribute to the overall system performance. Assuming lattice code based transmissions, we derive upper bounds on the average common rate and the average sum rate with the proposed pairing scheme. In addition, considering M-ary QAM with square constellation as a special case of lattice codes, we derive asymptotic average symbol error rate (SER) of the MWRN. We show that in terms of the achievable rates and error performance, the proposed pairing scheme outperforms the existing pairing schemes under a wide range of channel scenarios. Finally, we investigate lattice coded FDF and AF MWRNs with imperfect CSI. Con- sidering lattice codes of sufficiently large dimension, we obtain the bounds on the com- mon rate and sum rate. In addition, considering M-ary quadrature amplitude mod- ulation (QAM) with square constellations, we obtain expressions for the average SER in FDF MWRNs. For AF MWRNs, considering BPSK modulation as the simplest case of lattice codes, we obtain the average BER. Moreover, we obtain the optimum power allocation coefficients to maximize the sum rate in AF MWRN. For both FDF and AF relaying protocols, the average common rate and sum rate are decreasing functions of the estimation error. The analysis shows that the error performance of a FDF MWRN is an increasing function of both the channel estimation error and the number of users, whereas, for AF MWRN, the error performance is an increasing function of only the channel estimation error. Also, we show that to achieve the same sum rate in AF MWRN, optimum power allocation requires 7 − 9 dB less power compared to equal power allocation depending upon users’ channel conditions

Analysis and design of physical-layer network coding for relay networks

Physical-layer network coding (PNC) is a technique to make use of interference in wireless transmissions to boost the system throughput. In a PNC employed relay network, the relay node directly recovers and transmits a linear combination of its received messages in the physical layer. It has been shown that PNC can achieve near information-capacity rates. PNC is a new information exchange scheme introduced in wireless transmission. In practice, transmitters and receivers need to be designed and optimized, to achieve fast and reliable information exchange. Thus, we would like to ask: How to design the PNC schemes to achieve fast and reliable information exchange? In this thesis, we address this question from the following works: Firstly, we studied channel-uncoded PNC in two-way relay fading channels with QPSK modulation. The computation error probability for computing network coded messages at the relay is derived. We then optimized the network coding functions at the relay to improve the error rate performance. We then worked on channel coded PNC. The codes we studied include classical binary code, modern codes, and lattice codes. We analyzed the distance spectra of channel-coded PNC schemes with classical binary codes, to derive upper bounds for error rates of computing network coded messages at the relay. We designed and optimized irregular repeat-accumulate coded PNC. We modified the conventional extrinsic information transfer chart in the optimization process to suit the superimposed signal received at the relay. We analyzed and designed Eisenstein integer based lattice coded PNC in multi-way relay fading channels, to derive error rate performance bounds of computing network coded messages. Finally we extended our work to multi-way relay channels. We proposed a opportunistic transmission scheme for a pair-wise transmission PNC in a single-input single-output multi-way relay channel, to improve the sum-rate at the relay. The error performance of computing network coded messages at the relay is also improved. We optimized the uplink/downlink channel usage for multi-input multi-output multi-way relay channels with PNC to maximize the degrees of freedom capacity. We also showed that the system sum-rate can be further improved by a proposed iterative optimization algorithm

Error propagation in a multi-way relay channel

This paper studies the error propagation phenomenon for a multi-way relay channel based on binary phase shift keying (BPSK) modulation and network coding in the physical layer. Though physical layer network coding enhances the throughput of a multi-way relay system by reducing number of time slots, its error performance can be highly degraded since decision about a user's message depends on previous decisions. In this paper, the error probability of a multi-way relay system is examined through both analytical and numerical methods. We show that the system performance is significantly controlled by signal power and number of users accessing the single relay. Based on numerical analysis, the critical number of users to limit the error rate within an acceptable upper bound is found