# Research on Co-Channel Interference Cancellation for Underwater Acoustic MIMO Communications
## 3 Experimental Results
### Results and Analysis
The constellations obtained by IPNLMS_DADFE, RLS_DADFE, and CE_IC+DFE_IC receivers in decision-directed mode are provided in Figure 6. One can observe that the constellations are clearly separated obtained by the proposed CE_IC+DFE_IC for both transmitter 1 and transmitter 2; this demonstrates that the proposed receiver achieves a fantastic communication performance. The constellations from transmitter 2 are much more separated than those from transmitter 1. The reason for this could be that, since the depths of the transmitters are different, as shown in Figure 2, the gains in spatial diversity from transmitter 2 are higher than those from transmitter 1. To illustrate the flexibility of the proposed CE_IC+DFE_IC receiver, we compare the communication performance using different lengths of feedforward filters, feedback filters and IC filters. The lengths of different filters are the same for IPNLMS_DADFE, RLS_DADFE, and the proposed CE_IC+DFE_IC. The lengths of feedback filters and IC filters are half of the length of feedforward filters, while the length of the feedforward filters varied from 6.25 to 112.5 ms. The IPNLMS_DADFE, RLS_DADFE, and the proposed CE_IC+DFE_IC work in training mode. From Figure 7, one may observe that the output  
Figure 6: The constellation obtained by the proposed receiver. (**a**) The constellation obtained by IPNLMS_DADFE from transmitter 1. (**b**) The constellation obtained by RLS_DADFE from transmitter 1. (**c**) The constellation obtained by CE_IC+DFE_IC from transmitter 1. (**d**) The constellation obtained by IPNLMS_DADFE from transmitter 2. (**e**) The constellation obtained by RLS_DADFE from transmitter 2. (**f**) The constellation obtained by CE_IC+DFE_IC from transmitter 2. SNR obtained by the proposed CE_IC+DFE_IC receiver increases with increasing filter length; the result verifies the theoretical analysis in Ref. [30], concluding that the output SNR increases with increasing filter length. The SNR output obtained by RLS_DADFE decreases when the length of feedforward filter increases. In addition, the output SNR obtained by IPNLMS_LMS almost remains the same. This implies that the performance of IPNLMS_LMS does not rely on the filter length. The output SNR obtained by RLS_DADFE and IPNLMS_DADFE reaches its maximum when the feedforward filter length is 20 ms. One may find that when the length of the feedforward filter is less than 10 ms, it fails to decode; the reason for this is that, in Figure 3, there is no multipath located within 10 ms. Based on Figures 3 and 7, we conclude that, when the strong multipath is located within the window of the feedforward filter, the proposed method achieves a considerable performance. To further investigate the relationship between the lengths of filters and communication performance, the lengths of feedback filters and IC filters were fixed at 25 ms (80 symbols), while the length of feedforward filters was varied from 12.5-150 ms. The RLS_DADFE, IPNLMS_DADFE, and the proposed CE_IC+DFE_IC receivers also work in training mode. The results are shown in Figure 8. It can be observed that the output SNR obtained by our proposed CE_IC+DFE_IC receiver still slightly increases, while it decreases when obtained by DA-DFE. From Figures 7 and 8, it can be concluded that, when designing underwater acoustic MIMO communication systems, the filter length of the proposed CE_IC+DFE_IC receiver is more flexible. The filter length can be set to as long as possible to cover the long-time delay path and obtain a higher SNR output. Figure 7: Relationship between length of filters and the output SNR. The lengths of feedforward filters, feedback filters, and IC filters are varied. From Figures 7 and 8, we can observe that the gain in output SNR over Tx1 and Tx2 obtained by our proposed CE_IC+DFE_IC receiver is relatively small, while the gain obtained by DA-DFE is significant.