A Novel Single-Carrier Frequency-Domain Equalization Technique without Prefix or Suffix for Underwater Acoustic Communications
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摘要: 单载波频域均衡(SC-FDE)是水声通信中实现高数据率传输的重要技术之一，但传输序列中周期性插入的循环前缀或保护间隔等前后缀降低了频带利用率和通信速率。该文在时间反转(TR)处理的基础上提出一种新型无前后缀的水声单载波频域均衡技术，即发射机传输无前后缀的单载波信号，接收机对各个数据块的后缀进行重构并利用频域均衡技术消除符号间干扰。重构数据块后缀作为接收机的核心部分，主要包括3个步骤：TR处理、干扰消除和后缀重构。首先，TR处理将来自接收阵元的多通道信号融合为单通道信号，并且其等效信道冲激响应(即q函数)的稳定主峰避免了后缀重构中的噪声放大。然后，通过干扰消除，去除来自前一数据块的块间干扰(IBI)和来自当前数据块的反因果干扰(ACI)。最后，根据q函数主峰及其右侧的因果部分计算信号的各个路径分量，实现数据块的后缀重构。实验结果验证了这一技术的有效性，并且性能优于现有无前后缀的单载波频域均衡技术。Abstract: Single-Carrier Frequency-Domain Equalization (SC-FDE) is one of the key techniques to achieve high data-rate transmissions in underwater acoustic communications. However, the periodically inserted prefixes or suffixes, in forms of cyclic-prefixes or guard intervals in transmission sequence, reduce the bandwidth efficiency and data rate. A SC-FDE technique without prefix or suffix is proposed based on the Time Reversal (TR) for underwater acoustic communications. At the transmitter side, the single-carrier signal without prefix or suffix is transmitted. At the receiver side, the suffix is reconstructed in each block before canceling the intersymbol interference with FDE. The suffix reconstruction, which is the core of receiver, contains three main procedures: the TR processing, interference cancelation, and suffix reconstruction. First, the TR processing converts receptions from an array of receiving elements into a single composite signal. The equivalent impulse response (namely the q-function) has a stable peak, which avoids the noise amplification. Next, the interference cancelation is conducted to remove the InterBlock Interference (IBI) from the previous block and the ACausal Interference (ACI) from the current block. Afterward, suffix reconstruction is achieved by calculating the multipath arrivals of each symbol, which are obtained from the causal part of the q-function. Experimental results validate the effectiveness of this technique and its superiority over the SC-FDE technique without prefix or suffix in the current literature.
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表 1 几种频域均衡接收机在一个数据块内的计算复杂度
步骤 FDE TR-FDE ACIC-FDE 复杂度 范例值 复杂度 范例值 复杂度 范例值 信道估计 $\varPsi $MNL ${\rm{7}}{\rm{.68}} \times {10^{\rm{5}}}$ $\varPsi $MNL ${\rm{7}}{\rm{.68}} \times {10^{\rm{5}}}$ $\displaystyle\sum\nolimits_{\psi {\rm{ = 1}}}^\varPsi {MN_{\rm{e}}^\psi L} $ ${\rm{8}}{\rm{.29}} \times {10^{\rm{5}}}$ TR处理 0 0 $({3 / 2})\varPsi MN{\log _2}N$ $7.78 \times {10^4}$ $({3 / 2})\displaystyle\sum\nolimits_{\psi {\rm{ = 1}}}^\varPsi {MN_{\rm{e}}^\psi } {\log _2}N_{\rm{e}}^\psi $ $8.{\rm{51}} \times {10^4}$ IBI消除 0 0 $({3 / 2})\varPsi {L_q}{\log _2}{L_q}$ ${\rm{2}}{\rm{.88}} \times {10^2}$ $({3 / 2})\varPsi {L_q}{\log _2}{L_q}$ ${\rm{2}}{\rm{.88}} \times {10^2}$ ACI消除 0 0 0 0 $({3 / 2})\displaystyle\sum\nolimits_{\psi {\rm{ = 1}}}^\varPsi {N_{\rm{e}}^\psi } {\log _2}N_{\rm{e}}^\psi $ $1.7{\rm{0}} \times {10^4}$ 前后缀重构 0 0 $\varPsi N{L_q}$ ${\rm{1}}{\rm{.92}} \times {10^4}$ $\displaystyle\sum\nolimits_{\psi {\rm{ = 1}}}^\varPsi {\left( {N_{\rm{e}}^\psi - {L_q}} \right)} {L_q}$ $2.{\rm{00}} \times {10^4}$ 均衡 2$\varPsi $MN $1.20 \times {10^4}$ 2$\varPsi $N $2.40 \times {10^3}$ $2\displaystyle\sum\nolimits_{\psi {\rm{ = 1}}}^\varPsi {\left( {N_{\rm{e}}^\psi - {L_q}} \right)} $ $2.5{\rm{0}} \times {10^3}$ FFT/IFFT $ \varPsi \left( {MN + {N / 2}} \right) \times {\log _2}N $ $5.70 \times {10^4}$ $({3 / 2})\varPsi N{\log _2}N$ $1.56 \times {10^4}$ $\begin{array}{r} ({3 / 2})\displaystyle\sum\nolimits_{\psi {\rm{ = 1} } }^\varPsi {\left( {N_{\rm{e} }^\psi - {L_q} } \right)} \\ \times {\log _2}\left( {N_{\rm{e} }^\psi - {L_q} } \right)\end{array}$ $1.6{\rm{3}} \times {10^4}$ 总计算量 $O\left( {\varPsi MNL} \right)$ ${\rm{8}}{\rm{.3}}7 \times {10^{\rm{5}}}$ $O\left( {\varPsi MNL} \right)$ ${\rm{8}}{\rm{.83}} \times {10^{\rm{5}}}$ $O\left( {\displaystyle\sum\nolimits_{\psi {\rm{ = 1}}}^\varPsi {MN_{\rm{e}}^\psi } L} \right)$ ${\rm{9}}{\rm{.71}} \times {10^{\rm{5}}}$ 
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表 2 信道模型参数与通信仿真参数
信道模型参数 数值 通信仿真参数 数值 水深/传输距离 20 m/1000 m 调制方式 QPSK 海底起伏方差 10–3 符号速率 16 ksym/s 载波频率/带宽 85 kHz/20 kHz 过采样率/滚降系数 3/0.25 发射深度/接收深度 13.4 m/9.5～17.0 m(5阵元，间隔约1.9 m) 信道估计 压缩采样匹配追踪(稀疏度15) 声扩展因子 1.0 数据块时长 25 ms(无重叠) 仿真时长 3.0 s 迭代次数 1, 2, 3
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表 3 水池通信实验参数
通信实验参数 数值 通信实验参数 数值 调制方式 QPSK 分数采样率 4 载波频率/带宽 85.5 kHz/8 kHz 数据块时长 40 ms 各功率下传输数据包数 3 信道估计 压缩采样匹配追踪(稀疏度30) 数据包时长 6.0 s 迭代次数 5 符号速率 6.4 ksym/s 接收机 TR-FD-DFE, ACIC-FD-DFE
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