Dual Mode Index Modulation-Aided Orthogonal Chirp Division Multiplexing System in High-Dynamic Scenes
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摘要: 基于索引调制的正交线性调频分复用系统(OCDM-IM)要求部分子载波保持静默状态，这一方面削弱了正交线性调频分复用系统(OCDM)的时频扩展增益，导致OCDM-IM系统在高动态场景下受多普勒频移的影响仍然较为严重；另一方面，静默的子载波不携带传输信息，造成了吞吐量的损失。针对以上问题，该文提出一种新型的双模辅助索引调制的OCDM通信系统架构(DM-OCDM-IM)。该系统在OCDM系统的基础上，引入双模索引映射方案，拓展调制维度，既保留了OCDM系统在高动态场景下时频二维扩展抗干扰能力的核心优势，又实现了低阶星座调制下的高频谱效率。为了降低接收端复杂度，提出利用离散菲涅尔变换(DFnT)的特征分解来简化DM-OCDM-IM系统数字信号处理的接收算法。仿真结果表明，与现有的双模辅助索引调制的OFDM系统(DM-OFDM-IM)相比，所提的DM-OCDM-IM系统具有更强的抗多径衰落和抗多普勒频移的能力；与OCDM-IM系统相比，所提的DM-OCDM-IM系统提高了频谱效率的同时，仍然具有更强的抗衰落能力。Abstract:
Objective In high-dynamic environments, the Orthogonal Chirp Division Multiplexing (OCDM) system has attracted significant attention due to its inherent advantage of time-frequency two-dimensional expansion gain. The OCDM with Index Modulation (OCDM-IM) system extends the index domain of the traditional OCDM system, selectively activating subcarriers through index modulation. This reduces inter-carrier interference to some extent. However, the OCDM-IM system necessitates that certain subcarriers remain inactive, which, on one hand, diminishes the time-frequency expansion gain of the OCDM system and, on the other hand, leads to more pronounced Doppler interference in high-dynamic environments. Additionally, the inactive subcarriers do not contribute to data transmission, resulting in throughput loss. To overcome these challenges, this study proposes a novel communication system architecture, the Dual Mode Index Modulation-aided OCDM (DM-OCDM-IM). This architecture incorporates a dual-mode index mapping scheme and introduces new modulation dimensions within the OCDM system. The DM-OCDM-IM system preserves the interference immunity associated with the time-frequency two-dimensional expansion of the OCDM system while achieving higher spectral efficiency with low-order constellation modulation, offering enhanced communication performance in high-dynamic scenarios. Methods In this study, a DM-OCDM-IM communication system architecture is proposed, consisting of two main components: the dual mode index modulation module and the receiving algorithm. In the dual mode index modulation module, the DM-OCDM-IM system partitions the subcarriers in each subblock into two groups, each transmitting constant-amplitude and mutually distinguishable constellation symbols. This design expands the modulation dimensions and improves spectral efficiency. At the same time, low-order constellation modulation can be applied in a single dimension, thereby strengthening the system’s anti-jamming capability in high-dynamic environments. The constant-amplitude dual mode index mapping scheme also reduces performance fluctuations caused by channel gain variations and offers ease of hardware implementation. For signal reception, the system must contend with substantial Doppler frequency shifts and the computational complexity of demodulation in high-dynamic conditions. To address this, the DM-OCDM-IM employs a receiving algorithm based on feature decomposition of the Discrete Fresnel Transform (DFnT), which reduces complexity. The discrete time-domain transmit signal is reconstructed by applying the Discrete Fourier Transform (DFT) and feature decomposition to the received frequency-domain signal. Finally, the original transmitted bits are recovered through index demodulation and constellation demodulation of the reconstructed time-domain signal using a maximum-likelihood receiver. Results and Discussions The performance of the proposed DM-OCDM-IM system is simulated and compared with that of the existed Dual Mode Index Modulation-aided OFDM (DM-OFDM-IM) system and the OCDM-IM system under three channel conditions: AWGN, multipath, and Doppler frequency shift. The results show that, relative to the DM-OFDM-IM system, the proposed DM-OCDM-IM system exploits multipath diversity more effectively and exhibits stronger resistance to fading in all three channels ( Fig. 5 ,Fig. 6 ). When compared with the OCDM-IM system, the Bit Error Rate (BER) performance of the proposed DM-OCDM-IM system is significantly improved across all three channel conditions, particularly at high spectral efficiency (Fig.7(b) ,Fig.8(b) ). These results confirm that the introduction of the dual mode index modulation technique extends the modulation dimensions within the OCDM framework. Information is transmitted not only through index modulation but also through dual mode modulation, enabling higher spectral efficiency without increasing the modulation order. At the same time, the time-frequency expansion gain characteristic of OCDM is preserved, while receiver complexity is effectively controlled. These combined features make the proposed DM-OCDM-IM system well suited for communication in high-dynamic channel environments.Conclusions This paper establishes a novel DM-OCDM-IM system framework. First, by integrating a constant-amplitude dual mode index mapping scheme into the traditional OCDM system, the proposed design expands the modulation dimensions and allows the use of low-order constellation modulation in a single dimension. This improves spectral efficiency while enhancing system reliability in high-dynamic environments. Second, to reduce receiver-side complexity, a receiving algorithm based on feature decomposition of the DFnT is proposed, simplifying the digital signal processing of the DM-OCDM-IM system. Finally, the performance of the system is evaluated under AWGN, multipath, and Doppler frequency shift channels. The results demonstrate that, compared with the existed DM-OFDM-IM system, the proposed DM-OCDM-IM system exhibits stronger resistance to multipath fading and Doppler frequency shifts. In comparison with the OCDM-IM system, the proposed DM-OCDM-IM design preserves the time-frequency expansion gain of OCDM and provides stronger fading resistance at high spectral efficiency. Therefore, the proposed DM-OCDM-IM system offers superior adaptability in high-dynamic scenarios and has the potential to serve as a next-generation physical-layer waveform for mobile communications. -
表 1 DM-OCDM-IM(4,2,$ {M_{\text{A}}},{M_{\text{B}}} $)系统的索引调制查找表
索引
比特由星座映射器A调制的
子载波的索引子块符号向量 [0,0] [1,2] $ {[{\boldsymbol{S}}_{\text{A}}^{(1)},{\boldsymbol{S}}_{\text{A}}^{(2)},{\boldsymbol{S}}_{\text{B}}^{(1)},{\boldsymbol{S}}_{\text{B}}^{(2)}]^{\text{T}}} $ [0,1] [2,3] $ {[{\boldsymbol{S}}_{\text{B}}^{(1)},{\boldsymbol{S}}_{\text{A}}^{(1)},{\boldsymbol{S}}_{\text{A}}^{(2)},{\boldsymbol{S}}_{\text{B}}^{(2)}]^{\text{T}}} $ [1,0] [3,4] $ {[{\boldsymbol{S}}_{\text{B}}^{(1)},{\boldsymbol{S}}_{\text{B}}^{(2)},{\boldsymbol{S}}_{\text{A}}^{(1)},{\boldsymbol{S}}_{\text{A}}^{(2)}]^{\text{T}}} $ [1,1] [1,4] $ {[{\boldsymbol{S}}_{\text{A}}^{(1)},{\boldsymbol{S}}_{\text{B}}^{(1)},{\boldsymbol{S}}_{\text{B}}^{(2)},{\boldsymbol{S}}_{\text{A}}^{(2)}]^{\text{T}}} $ 
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