## 3 Experimental Setup and Results
### Results and Discussions  
In the demonstration, \(\sim\)7.5 Gbit/s data rate can be achieved using 2.5 GBaud/s 4-CAP modulation incorporating SM. The capability of the proposed active transmitter detection scheme in SM to provide SSD is shown in Figure 4a, where the channel gain difference ratio is \(\frac{\mathrm{h}_{2}}{\mathrm{h}_{1}}=0.5\). Here, the ideal active transmitter detection is defined as the perfect detection of the spatial bits. The measured ideal system bit-error-rate (BER) is thus described as:  
\[\mathrm{BER}_{\mathrm{ideal}}=\frac{\mathrm{Measured\ bit\ errors\ of\ CAP\ signals}}{\mathrm{The\ length\ of\ original\ bits}} \tag{5}\]  
where the original bits contain CAP signal bits and spatial bits. On the other hand, in our proposed transmitter detection scheme, the measured system BER is described as:  
\[\mathrm{BER}_{\mathrm{proposed}}=\frac{\mathrm{Measured\ bit\ errors\ of\ (CAP\ signals+\ spatial\ bits)}}{\mathrm{The\ length\ of\ original\ bits}}. \tag{6}\]  
Thus, compared with the ideal and the proposed transmitter detection scheme at the best-performed constellation rotation, i.e., 45\({}^{\circ}\) rotation [23], shown as the red and black curves, respectively, our proposed transmitter detection scheme only results in a slightly worse measured BER, implying a low-level detection error of spatial bits. Moreover, as 0\({}^{\circ}\) constellation rotation provides no SSD, compared with blue and red curves, our proposed transmitter detection scheme outperforms as SSD is provided, which in turn indicates the detection error of spatial bits is less than the BER improvement brought by SSD. It can be seen that about 0.61 dB signal-to-noise-ratio (SNR) improvement is achieved. To further explore the SSD performance at different constellation rotations using our proposed scheme, the 30\({}^{\circ}\) constellation rotation performance is also studied as in the green curve. The result further verifies the previous theoretical result of SSD performance at different rotation angles where ideal SM performance is assumed [23], i.e., the SSD is enhanced with  
Figure 3: Experimental setup. (**a**) Schematic diagram. (**b**) Photo. an increasing constellation rotation up to 45\({}^{\circ}\) under a specific channel gain difference ratio such as \(\frac{\text{h}_{2}}{\text{h}_{1}}=0.5\), thus showing the robustness of our proposed transmission detection scheme to offer SSD. The proposed active transmitter detection scheme in SM is also experimentally investigated under different channel gain distributions at the 45\({}^{\circ}\) constellation rotation, as shown in Figure 4b. An improved BER performance is observed with an increasing channel gain difference ratio from \(\frac{\text{h}_{2}}{\text{h}_{1}}=0.4\) to \(\frac{\text{h}_{2}}{\text{h}_{1}}=0.5\), which also follows the BER trend investigated in [23], i.e., a larger \(\frac{\text{h}_{2}}{\text{h}_{1}}\) enhances the SSD performance. However, a different BER result is observed when the channel gain difference ratio is further increased to \(\frac{\text{h}_{2}}{\text{h}_{1}}=0.66\). That is because, when \(\frac{\text{h}_{2}}{\text{h}_{1}}=0.4\), a large difference in received signal power between two channels is expected, which facilitates the spatial bits recovery. In fact, here, the error ratio of the spatial bits and CAP signal bits (short for error ratio) is in the range of [1:40 1:15], showing the dominant role of CAP signal errors. Thus, the BER performance is similar to the ideal SSD performance (without considering SM error) [23]. When \(\frac{\text{h}_{2}}{\text{h}_{1}}\) is increased to 0.66, a more negligible difference of two received signal power is observed, leading to a more demanding spatial bits recovery with the error ratio increase to the range of [1:2 1:1.2]. These increasing errors of spatial bits result in the overall performance of a slightly degraded BER in the scenario of \(\frac{\text{h}_{2}}{\text{h}_{1}}=0.66\).