## 5 Experiments and Results
### Adaptive Beamforming of Oblique Backscatter Ionogram
Although the SNRs of Figure 16b,c are not as good as Figure 16a relative to the strongest signal, relaying on the adaptive beamforming techniques, the interference signal is greatly suppressed, while the desired signal is still maintained. Figures 17 and 18 demonstrate the performance of two adaptive beamforming algorithms when the desired signal is stronger and weaker, respectively. For a strong desired signal of 7 MHz, in Figure 17, as described in the above simulation results, the performance of SMI is poorer, and the leading edge of the oblique backscatter signal is not as steep as ESB and channel 1's. But when the desired signal is weaker, at 8.8 MHz, for example, SMI beamformer not only has a lower and more stable background noise level than channel 1, but also sharpens the echo leading edge which is much easier to be extracted. Meanwhile, ESB beamformer has a good processing effect for both 7 MHz and 8.8 MHz. But in Figure 18c, the processing effect of ESB for strong signal seems better. For channel 1, the signal at  
Figure 16: Normalized SNR of 6 MHz; (**a**) Channel 1; (**b**) SMI; (**c**) ESB. Figure 17: Normalized SNR of 7 MHz; (**a**) Channel 1; (**b**) SMI; (**c**) ESB. Figure 18: Normalized SNR of 7.8 MHz; (**a**) Channel 1; (**b**) SMI; (**c**) ESB. 937 km is only 2 dB stronger than that at 837 km, but with ESB algorithm, the difference is increased to be 15.4 dB, and while the strongest point in Figure 18b is still at 837 km. Figure 19 shows the oblique backscatter ionogram processed by the Constant False-Alarm Rate (CFAR) processor. Although the SNRs are less in ionograms with SMI and ESB, the contrast between the oblique backscatter signals and the background is improved obviously. Compared with single channel reception and conventional beamforming, the vertical echo signal after adaptive beamforming can be approximately ignored. The interference problem of 2-hop signals of the vertical echoes with linear array is also well solved [17]. And for the oblique backscatter ionogram of channel 1 in Figure 19a, due to the strong interference of some frequency points, after CFAR processing, the useful signals are also eliminated at the same time, which results in some discontinuous points along the leading-edge trace, such as at 5.8 MHz, 6 MHz and so on. A similar situation also appears in the oblique backscatter ionogram of conventional DBF as Figure 19b. For the ESB's ionogram, which is shown in Figure 19d, the signal can be clearly identified above 7 MHz, but below 7 MHz, the trace is not as clear as the ionogram processed by the SMI beamformer in Figure 19c. From 5 MHz to 9.2 MHz, the SMI ionogram has a continuous leading trace. Especially below 6 MHz, although the desired signals are rather weak, the SMI beamformer still makes a continuous and clear trace. This is also the main purpose of the various processing for the oblique backscatter ionograms. Then from a comprehensive point of view, although ESB has better performance for stronger signals, SMI is a more suitable adaptive beamforming processing method for the low power system and the miniaturized L-shaped antenna array described in this paper. Figure 19: The oblique backscatter ionogram after Constant False-Alarm Rate (CFAR) processing and removal of discrete noise; (**a**) Channel 1; (**b**) DBF; (**c**) SMI; (**d**) ESB.