# Architecture Exploration of a Backprojection Algorithm for Real-Time Video SAR
## 4 Experimental Results
### Performance Analysis of BP-VISAR Architectures with Sub-Aperture Processing  
For the BP-VISAR architectures with sub-aperture processing, the elapsed times for generating video SAR frames were measured for non-overlapped and overlapped modes. The average elapsed times are summarized in Table 7 and Figure 10.  
In Arch. S1, regardless of the overlap mode, the average elapsed time also increases with the BP-VISAR frame resolution, and most of the computational complexity is from the IF stage as in Arch. F1. Arch. S2 and S3 show significant improvements in overall  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multicolumn{6}{c}{(b)} \\ \hline  & **Resolution** & **I/O** & **RC** & **IF** & **SUM** & **Speedup** & **PSNR** \\ \hline \multirow{6}{*}{Arch. S1} & 256 \(\times\) 256 & & & & 1512.4 & 1 & n/a \\  & 512 \(\times\) 512 & & & & 5705.5 & 1 & n/a \\  & 1024 \(\times\) 1024 & &  
processing times over Arch. S1. As can be inferred in the profile data in Table 3, most of the acceleration is also from the parallelization of the IF stage. Once the IF stage is parallelized, it does not use most of the overall processing time as in Arch. S1.  
In non-overlapped mode, for one frame generation, the sub-aperture processing performs the IF stage \(M\) times with the range profile data for one processing data unit. In contrast, the full-aperture processing performs the IF stage once with the range profile data for \(M\) processing data units. Arch. F1 and S1 perform all the processing in the host system while Arch. F2 and S2 perform the IF stage in the GPU device and Arch. F3 and S3 perform the RC and the IF stages in the GPU device. Despite the architectural differences in full-and sub-aperture processing, such as the SRPB and SFB, the overall processing times of Arch. S1 to S3 are not significantly different from those of Arch. F1 to F3.  
In the overlapped mode, for one frame generation, the architectures of sub-aperture processing perform the IF stage once with the range profile data for one processing data unit. In contrast, the architectures of full-aperture processing do the same processing in the non-overlapped mode except for the reduced execution of the RC stage. Because the IF stage accounts for the majority of the total computation in the generation of high-resolution frames, the reduced number of executions in the IF stage can significantly impact the overall processing time. For the overlap ratio of 0.8 in Tables 6 (b) and 7 (b), the overall processing times of Arch. S1 to S3 are improved by ratios of 2.9-4.5 compared with those of Arch. F1 to F3. In Arch. S3, compared with Arch. S2, there is further acceleration in the RC stage by the parallelization in the GPU device regardless of the overlap mode. The I/O time is independent of the architectures but depends on the overlap mode.  
Figure 11 shows the generated BP-VISAR frames of different frame resolutions for the Gotcha dataset. The visual quality is enhanced with a higher frame resolution. Although the proposed architectures accelerate the processing significantly, there is no severe degradation in the PSNR of the generated BP-VISAR frames compared with the baseline architecture.  
Figure 10: BP-VISAR frame generation time with sub-aperture processing in overlapped mode (\(\alpha=0.8\)).