Toshiaki Koike-Akino

Optimized constellations for two-way wireless relaying with physical network coding

We investigate modulation schemes optimized for two-way wireless relaying systems, for which network coding is employed at the physical layer. We consider network coding based on denoise-and-forward (DNF) protocol, which consists of two stages: multiple access (MA) stage, where two terminals transmit simultaneously towards a relay, and broadcast (BC) stage, where the relay transmits towards the both terminals. We introduce a design principle of modulation and network coding, considering the superposed constellations during the MA stage. For the case of QPSK modulations at the MA stage, we show that QPSK constellations with an exclusive-or (XOR) network coding do not always offer the best transmission for the BC stage, and that there are several channel conditions in which unconventional 5-ary constellations lead to a better throughput performance. Through the use of sphere packing, we optimize the constellation for such an irregular network coding. We further discuss the design issue of the modulation in the case when the relay exploits diversity receptions such as multiple-antenna diversity and path diversity in frequency-selective fading. In addition, we apply our design strategy to a relaying system using higher-level modulations of 16QAM in the MA stage. Performance evaluations confirm that the proposed scheme can significantly improve end-to-end throughput for two-way relaying systems.

Denoising Strategy for Convolutionally-Coded Bidirectional Relaying

In this paper, we present a forwarding strategy for two-stage bidirectional relaying in which trellis-coded modulation (TCM) is employed. We reveal that adaptive network coding cannot resolve distance shortening occurred at specific channel conditions when a certain TCM is used. To overcome this issue, we introduce an improved amplify-and-forward (AF) scheme termed pseudo AF (PAF). The proposed strategy adaptively switches network coding and PAF according to the channel information. Computer simulations demonstrate that the proposed approach can improve throughput performance.

Denoising Maps and Constellations for Wireless Network Coding in Two-Way Relaying Systems

We investigate on the design of modulation schemes suited for two-way wireless relaying systems that apply network coding at the physical layer. We consider network coding based on denoise-and-forward (DNF), which consists of two stages: multiple access (MA) stage and broadcast (BC) stage. For the case of QPSK constellation in the MA stage, we introduce the modulation-related problems in DNF. We propose two approaches to solve those issues. One uses only QPSK constellations at the BC stage. The other allows the use of unconventional 5-ary modulations, optimized according to the channel condition. The performance evaluation shows that a significant improvement in end-to-end throughput can be achieved, in particular for Nakagami-Rice fading channels.

Coded Bidirectional Relaying in Wireless Networks

The communication strategies for coded bidirectional (two–way) relaying emerge as a result of successful amalgamation of the recent ideas from network coding and the broadcast/interfering nature of the wireless communication medium. In this chapter we consider the basic scenario in which two terminal nodes carry out a two–way communication, helped by a third, relay node. Insights are provided into several important ideas that arise in this simple communication scenario. The communication mechanisms can be divided into two groups, depending whether the relay node is or is not required to decode the messages from the terminals. For the class of techniques in which the relay denoises rather than decodes the messages from the terminals, we discuss the role of structured codes in achieving the highest aggregate rates in the system. The treatment of the topics is generally in an information–theoretic setting, but we also present the design of communication mechanisms when finite–length packets and practical modulation constellations are used.

Adaptive Modulation and Network Coding with Optimized Precoding in Two-Way Relaying

We propose a precoding strategy which controls amplitude and phase of receiving signals to improve throughput for two-stage bidirectional relaying. We consider the case when the nodes know channel state information (CSI) and can adopt adaptive modulation techniques. We introduce a novel scheme termed adaptive modulation and network coding (AMNC), which jointly optimizes modulations and network coding based on the CSI. For dynamic bit loading and power allocation, we propose a practical time-sharing method called the segmented precoding, in which a packet is split into several sub-packets, and a set of modulation and network coding is optimized in conjunction with amplitude and phase controls for each sub-packet. It is demonstrated that our proposed scheme can offer a significant improvement of achievable throughput for two-way relaying.

Analysis of Network Coded HARQ for Multiple Unicast Flows

In this paper, we consider network coded (NCed) Hybrid-ARQ (HARQ) for multiple unicast flows. The main contribution of the paper is the derivation of a throughput expression for NCed HARQ with arbitrary number of users in i.i.d. channels. We apply the result to Rayleigh fading channels for incremental redundancy (IR) and chase combining (CC) based NCed HARQ. We verify the analytical approach with simulations and observe substantial SNR (or energy) improvements over regular ARQ with network coding as well as classical (H)ARQ. The SNR gains in the high and low throughput regimes are mainly due to the network coding and HARQ aspects, respectively. For low SNRs, NCed HARQ with IR surpass the CC performance.

Capacity, MSE and Secrecy Analysis of Linear Block Precoding for Distributed Antenna Systems in Multi-User Frequency-Selective Fading Channels

Block transmission with cyclic prefix is a promising technique to realize high-speed data rates in frequency-selective fading channels. Many popular linear precoding schemes, including orthogonal frequency-division multiplexing (OFDM), single-carrier (SC) block transmission, and time-reversal (TR), can be interpreted as such a block transmission. This paper presents a unified performance analysis that shows how the optimal precoding strategy depends on the optimization criterion such as capacity, mean-square error, and secrecy. We analyze three variants of TR methods (based on maximum-ratio combining, equal-gain combining and selective combining) and two-types of pre-equalization methods (zero-forcing and minimum mean-square error). As one application of our framework, we derive optimal precoding (i.e., OFDM with optimal power and phase control) in the presence of interference limitation for distributed antenna systems; we find that without power/phase control, OFDM does not have any capacity advantage over SC transmissions. When comparing SC and TR, we verify that for single-antenna systems in the high SNR regimes, SC has a capacity advantage; however, TR performs better in the low SNR regime. For distributed multiple-antenna systems, TR always provides higher capacity, and the capacity of TR can approach that of optimal precoders with a large number of distributed antennas. Furthermore, we make an analysis of secrecy capacity which shows how high-rate messages can be transmitted towards an intended user without being decoded by the other users from the viewpoint of information-theoretic security. We demonstrate that TR precoding can be the best candidate among the non-optimal precoders for achieving high secrecy capacity, while the optimal precoder offers a significant gain over those non-optimal precoders.

Mode-evolution-based polarization rotator-splitter design via simple fabrication process

A mode-evolution-based polarization rotator-splitter built on InP substrate is proposed by combining a mode converter and an adiabatic asymmetric Y-coupler. The mode converter, consisting of a bi-level taper and a width taper, effectively converts the fundamental TM mode into the second order TE mode without changing the polarization of the fundamental TE mode. The following adiabatic asymmetric Y-coupler splits the fundamental and the second order TE modes and also converts the second order TE mode into the fundamental TE mode. A shallow etched structure is proposed for the width taper to enhance the polarization conversion efficiency. The device has a total length of 1350 µm, a polarization extinction ratio over 25 dB and an insertion loss below 0.5 dB both for TE and TM modes, over the wavelength range from 1528 to 1612 nm covering all C + L band. Because the device is designed based on mode evolution principle, it has a large fabrication tolerance. The insertion loss remains below 1 dB and the polarization extinction ratio remains over 17 dB with respect to a width variation of +/- 0.12 µm at the wavelength of 1570 nm, or +/- 0.08 µm over the entire C + L band.

Cycle Slip-Mitigating Turbo Demodulation in LDPC-Coded Coherent Optical Communications

We show that an iterative demodulation with soft-decision feedback information from FEC decoder can efficiently mitigate cycle slips. With 3% pilot insertion, the turbo QPSK demodulation achieves 1.05dB gain even in the presence of frequent cycle slips.

A 24-Dimensional Modulation Format Achieving 6 dB Asymptotic Power Efficiency

We propose modulation using the extended Golay code over the 24D hypercube, achieving 6 dB asymptotic power efficiency with 1 b/s/Hz/pol spectral efficiency. Noise tolerance is improved by 3 dB over DP-BPSK at a BER of 10−3.