Non-orthogonal multiple access based on physical-layer network coding

Ph.D.Non-orthogonal multiple access (NOMA) is a promising multipacket reception technique to increase spectral efficiency in 5G cellular networks. Physical-layer network coding (PNC) is a novel technique that turns wireless interference into network-coded information. This thesis investigates a new NOMA architecture, Network-Coded Multiple Access (NCMA), that combines the use of PNC with multiuser decoding (MUD) to boost performance. Previous work on NOMA that makes use of successive interference cancelation (SIC) to recover messages requires the signal powers of different users to be widely different. However, guaranteeing large signal power differences among users is not always possible.This thesis is a theoretical-plus-experimental investigation on NCMA as a practical 5G strategy for power-balanced NOMA.This thesis makes three major contributions toward the adoption of NCMA uplink and downlink in real practical NOMA systems.The first contribution is the use of NCMA as a practical and efficient scheme for power-balanced NOMA uplink. In particular, we build the first NCMA uplink system prototype operated on high-order modulations up to 16-QAM. When there is a disparity between the number of weak users and the number of strong users in a practical multiuser system, SIC does not work well if users with comparable received powers transmit together. Our experimental results on software-defined radio (SDR) showed that a simple two-user NCMA system prototype can achieve throughput significantly higher than that of SIC-based NOMA in such power-balanced scenarios. The previous NCMA prototype built in an earlier work was not efficient under the high SNR scenario because it could only support BPSK modulation. To further improve system throughput, in this thesis work, we built a new high-order modulated NCMA prototype,referred to as MIMO-NCMA. Direct generalization of the previous BPSK NCMA decoding algorithm to high-order modulations will lead to huge performance degradation, caused by the relative phase offset between received signals from different users. To circumvent the phase offset problem, we put forth (1) a MIMO-NCMA system with multiple receive antennas at the base station (BS), and (2) a low-complexity symbol-level PHY-layer decoder that can substantially alleviate the performance degradation. We implemented MIMO-NCMA on SDR and demonstrated the throughput gain achievable with high-order modulations. Overall, we provide a practically implementable framework for high-order modulated NCMA.The second contribution is the first rate-diversemultiuserNCMAuplink system prototype. While future 5G NOMA systems aim to support multiple users, all previous NCMA work was limited to two users. We first show that the performance of SIC-based NOMA further degrades under multiuser power-balanced scenarios. A three-user NCMA architecture in which all users use the same modulation,referred to as rate-homogeneousNCMA, can achieve substantial throughput improvement over SIC-based NOMA. Then, we put forth a new NCMA architecture, referred to as rate-diverse NCMA, in which different users may adopt different modulations commensurate with their relative SNRs. A challenge for rate-diverse NCMA is the design of a channel-coded PNC system. Ours are the first attempt to design channel-coded rate-diverse PNC. Experimental results on our SDR prototype show that the throughput of rate-diverse NCMA can outperform the state-of-the-art rate-homogeneous NCMA by 80%. Overall, rate-diverse NCMA is a practical solution for near power-balanced NOMA.The third contribution is the first NCMA downlink system prototype targeted for 5G unmanned aerial vehicle (UAV) communications. Mobile UAV BSs have received much attention in the context of 5G highly mobile and flexible-configurable communication systems. Conventional NOMA downlink that makes use of superposition coding with SIC decoding does not work well in scenarios where the channel conditions of downlink users are not readily available at the transmitter side. This is the case, for example,in the UAV scenario where the UAV transmitter moves quickly,causing the channel conditions to vary in a very dynamic manner. We investigate NCMA as a solution for UAV NOMA downlink. In the absence of channel information,an NCMA transmitter allocates equal power to the superposed signals of different downlink users. A key challenge is how to achieve high NOMA throughput under such equal power allocation. Toward this end, NCMA downlink makes joint use of PNC and MUD together with a new superposition coding scheme, referred to as NCMA-based superposition coding. Although equal powers are allocated to the signals of different users, a relative phase offset between the signals is introduced to optimize PNC decoding and MUD. Our experimental results show that NCMA is robust against varying channel conditions. Moreover, the throughput of NCMA can outperform the state-of-the-art SIC-based superposition coding system and the TDMA system by 50% and 80%, respectively, demonstrating that NCMA is a practical solution to boost throughput in UAV NOMA.Overall, this thesis has solved a number of key issues in NCMA uplink and downlink, and has demonstrated its advantage to SIC-based NOMA in power-balanced settings. We believe NCMA will be a serious complementary technology to SIC for NOMA in future 5G systems.⾮正交多址接⼊（NOMA）是第五代移動通信（5G）中⼀項新興的多 址接⼊技術。通過同時同頻接收多個⽤⼾的數據，NOMA 可以提⾼ 5G 蜂窩網絡中的頻譜效率。物理層網絡編碼（PNC）是⼀種能將無線⼲擾轉 換成網絡編碼信息的新技術。本論⽂研究⼀種新型的⾮正交多址接⼊技術，網絡編碼多址接⼊（NCMA）。NCMA 結合了物理層網絡編碼以及傳 統的多⽤⼾解碼（MUD）⽅法來提⾼無線系統的吞吐量。前期關於 NOMA 的研究主要利⽤了不同⽤⼾之間功率⼤⼩的不同， 並採取連續⼲擾消除（SIC）的⽅式來實現。然⽽在實際 NOMA 系統 中，不同⽤⼾之間很⼤信號功率差異的條件並不能夠⼀定滿⾜。本論⽂ 是對 NCMA 作為功率均衡 NOMA 解決⽅案的理論與實驗研究。為了把 NCMA 技術應⽤於實際的 5G 系統中，本論⽂作出了以下三個主要貢獻：論⽂的第⼀個貢獻是提出 NCMA 作為功率均衡 NOMA 上⾏系統的 實⽤和有效的⽅案，並搭建了第⼀個⾼階調製下的 NCMA 上⾏系統原型 機。在實際的多⽤⼾系統中，當信道條件好的⽤⼾與信道條件差的⽤⼾ 之間存在數量差異時，在接收端就會出現不同⽤⼾接收功率幾乎⼀樣的 情況。這時傳統的 SIC 解碼⽅法不能得到好的解碼性能。我們通過基於 軟件無線電的實驗表明，在兩個⽤⼾接收功率相同的情況下，相⽐於基 於 SIC 的 NOMA ⽅式，NCMA 可以⼤幅度地提⾼系統的吞吐量。在早 期⼯作中構建的 NCMA 原型機只實現了低階的 BPSK 調製⽅式。為了 進⼀步提⾼系統的吞吐量（在中⾼信噪⽐的時候），我們搭建了新的⾼階 調製 NCMA 原型機，多天線網絡編碼多址接⼊（MIMO-NCMA）。我們發現在⾼階調製下，由於不同⽤⼾信號之間的相對相位偏移，直接將先前 的 BPSK 解碼算法推廣到⾼階調製將導致巨⼤的性能下降。為了解決相位偏移問題，我們提出了（1）在基站處具有多個接收天線的 NCMA 系 統，以及（2）基於碼元級別的低複雜度的物理層解碼器。我們在軟件無線電平台上實現了 MIMO-NCMA 的系統，並實驗證明了由⾼階調製⽽帶 來的吞吐量增益。我們為⾼階調製的 NCMA 系統提供了⼀個可實現的的 系統框架。論⽂的第⼆個貢獻是搭建了第⼀個不同碼率的多⽤⼾ NCMA 上⾏系 統。雖然 5G NOMA 旨在⽀持多個⽤⼾，但前期所有的 NCMA ⼯作僅限 於兩個⽤⼾，並採⽤相同的調製⽅式，稱之為碼率相同的 NCMA。我們 ⾸先表明，基於 SIC 的 NOMA 系統性能在多⽤⼾功率均衡的情況下會進 ⼀步降低，NCMA 可以帶來顯著的吞吐量提升。然後我們提出了⼀個新 的 NCMA 系統架構，允許不同⽤⼾選擇不同的調製⽅式與它們的信道條 件相匹配，稱為碼率不同的 NCMA。設計碼率不同的 NCMA 所⾯臨的挑 戰是設計帶有信道編碼的 PNC 解碼器。本論⽂⾸次設計了帶有信道編碼 的碼率不同的 PNC 解碼器。我們的軟件無線電實驗結果表明，碼率不同 的 NCMA 系統吞吐量可以超過現有的碼率相同的 NCMA 系統 80％。碼率多樣化的 NCMA 是近乎功率均衡的 NOMA 的實⽤解決⽅案。論⽂的第三個貢獻是提出了第⼀個針對 5G 無⼈機通信的 NCMA 下⾏系統。前期 NOMA 研究中的基站是靜⽌不動的，⽽移動無⼈機在 5G ⾼度移動和靈活可配置的通信系統中得到了很多關注。當下⾏⽤⼾的信 道條件在基站發射端不容易獲得的時候，利⽤疊加編碼与 SIC 解碼的常 規 NOMA 下⾏⽅案不能得到很好的性能。⽐如在無⼈機快速⾶⾏移動的 時候，無⼈機與⽤⼾之間的信道可能會以⾮常動態的⽅式變化。我們研究 NCMA 作為無⼈機 NOMA 下⾏的解決⽅案。在沒有信道信息的情況下， NCMA 發射端為不同的下⾏⽤⼾的疊加信號分配相等的功率。由此帶來的挑戰是如何在同等功率分配下實現⾼ NOMA 吞吐量。為此，NCMA 下 ⾏在接收端同時使⽤ PNC 解碼和多⽤⼾解碼，並在發射端設計了新的疊加編碼⽅案，稱之為基於 NCMA 的疊加編碼。儘管對不同⽤⼾的信號分 配了相同的功率，NCMA 在疊加信號中引⼊了信號之間的相對相位偏移， 從⽽優化了 PNC 解碼和多⽤⼾解碼性能。我們的實驗結果表明，NCMA 可以抵抗多變的信道條件。此外，NCMA 的吞吐量可以分別超過現有的 基於 SIC 的疊加編碼系統和 TDMA 系統 50％和 80％，證明 NCMA 是提升無⼈機 NOMA 下⾏吞吐量的實⽤解決⽅案。總的來說，本論⽂解決了 NCMA 上⾏鏈路和下⾏鏈路中的關鍵問題， 並且在功率均衡的實際場景中展⽰了 NCMA 對⽐基於 SIC 的 NOMA 的 優勢。我們相信 NCMA 將成為未來 5G NOMA 系統中⼀項重要的技術。Pan, Haoyuan.Thesis Ph.D. Chinese University of Hong Kong 2018.Includes bibliographical references (leaves 125-132).Abstracts also in Chinese.Title from PDF title page (viewed on 3 June, 2021)