Abstract
Compared with the orthogonal frequency division multiplexing (OFDM) modulation, the orthogonal time frequency space(OTFS) modulation has a lower peak-to-average power ratio. It can effectively resist the time selective fading caused by the Doppler effect and has significant performance advantages over doubly dispersive channels. However, the conventional OTFS linear minimum mean square error (LMMSE) method has a high complexity and is not easy to process in real time. In order to solve this problem, we propose a low-complexity equalization algorithm with infinite norm constraints based on the optimal coordinate reduction. The equalization algorithm not only obtains the optimal solution through a certain number of iterations and avoids direct matrix inversion but also equalizes infinite norm constraints to improve the symbol detection performance gains. At the same time, the OTFS delay-Doppler channel matrix we utilize is sparse and the two-norm squares of each column vector equally reduces the complexity of optimal coordinate descent. Finally, the simulation in the underwater acoustic communication scenario we designed verify the effectiveness of the proposed equalization algorithm. The simulation results show that the performance of the proposed equalization algorithm is close to that of the LMMSE method, while its low complexity is ensured.
Highlights
Compared with the orthogonal frequency division multiplexing ( OFDM) modulation, the orthogonal time frequency space( OTFS) modulation has a lower peak⁃to⁃average power ratio. It can effectively resist the time selec⁃ tive fading caused by the Doppler effect and has significant performance advantages over doubly dispersive channels
The conventional OTFS linear minimum mean square error ( LMMSE) method has a high complexity and is not easy to process in real time
In order to solve this problem, we propose a low⁃complexity equalization algo⁃ rithm with infinite norm constraints based on the optimal coordinate reduction
Summary
本节对所提均衡方案进行性能评估,并与 OTFS 现有的算法比较。 首先考虑一个水声通信场景,假 设时变水声信道的最大时延为 τ max, 最大多普勒为 vmax,OTFS 调制中T和 Δf 分别决定最大多普勒 (1 / T) 和 时 延 (1 / Δf) , 即 vmax < 1 / T 和 τ max < 1 / Δf。 同时为满足一定的数据速率,OTFS 调制受系 统总带宽 B = MΔf 和时延 Tf = NT 条件的约束。 因此 可以通过选择 N,M,T(Δf = 1 / T) 参数来支持 OTFS 在时延 - 多普勒水声信道的有效通信。 考虑到时 变水声信道中,往往多径影响严重,最大时延 τ max 较 长,而多普勒影响一般在一定范围内,其最大多普勒 τ max 相对较小,因此可以选择较大的 T 来满足最大 时延,故 Δf = 1 / T 相对较小来支持最大多普勒。 另 外为实现低时延和保证一定数据速率,Tf 尽可能小, 带宽 B 尽可能大,因此应选择较小的 N 值和较大的 M 值,这里设置每帧发射 N = 16M = 64 个符号,采用 正交相移键控( QPSK) 调制,持续时间 T = 216 ms。 采用间隔 Ts = T / ( NM) = 0.25 ms,载波 fc = 6 kHz,声 速为 c = 1 500 m / s。 图 3 OTFS 的 OCD BOX,MP,MMSE 误码性能比较 [5] HADANI R, RAKIB S, MOLISCH A F, et al Orthogonal time frequency space( OTFS) modulation for millimeter⁃wave commu⁃ nications systems[ C] ∥2017 IEEE MTT⁃S International Microwave Symposium, 2017: 681⁃683
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