# Precoding for RIS-Assisted Multi-User MIMO-DQSM Transmission Systems
## 6 Results and Discussion
### Discussion  
RIS-based systems have strongly attracted attention since they improve the performance of the systems by optimizing the use of radiated energy. However, one drawback of the RIS-based system is the control of phases and amplitudes of the impinging signals in the RIS. Typically, an additional control unit in the RIS is required. However, this approach can be a challenge mainly when the RIS is located far from the transmitter or BS. Recently, RIS-assisted systems using spatial modulation have been considered as potential candidates for future 6G wireless communication systems [20; 21; 34]. However, previous works in this area are focused mainly on users with single Rx antenna (MISO) systems. The results obtained in this work show that the proposed system can effectively be used to pre-modify the amplitude and phases of the signals reflected by the RIS. This fact is important since in this case the signal processing required in the RIS is transferred to the BS where the hardware and software capabilities of the system make signal processing more adequate. Additionally, the utilized DQSM transmission scheme improves the overall performance and reduces the detection complexity of the system. Comparing the two proposed schemes, the RIS-MU-DQSM-I scheme has the lowest detection complexity. However, this scheme requires that \(N_{s}=N_{r}\) for perfect interference cancellation. This limitation is overcome by the proposed RIS-MU-DQSM-II scheme, which also outperforms by 2 dB and 6 dB in BER the RIS-MU-DQSM-I scheme for the same SE of 8 bpcu/user and 12 bpcu/user, respectively. It is worth noting that in comparison with the conventional MU-MIMO-SMux systems, the RIS degrades the BER performance of the system when a limited number of mirrors is used. However, when \(N_{s}\) is greater than \(N_{t}\), the RIS-based system takes advantage of diversity and significantly improves in BER.  
As expected, the proposed system clearly outperforms the previous AF-MU-DQSM scheme. This fact is mainly due to the total quantity of noise in the RIS-based systems compared with the relay-assisted scheme used as a reference. However, it is fair to mention that relay-based systems can offer an extended coverage range compared with RIS. Considering the correlated channel scenario, both systems using a relay or RIS result were significantly affected compared with the conventional one. This fact highlights the need for RIS-based systems with low spatial correlation. Although the proposed system transfers any signal processing over the impinging signals in the RIS to the BS, the RIS is still responsible to change the reflecting angles of the reflecting mirrors. For this reason, the RIS still requires a low-speed reverse channel to track the MSs in the network and share the CSI. However, the manner in which the RIS carries out this task is beyond the scope of this paper. The proposed system reduces the required signal processing in the RIS, which is only responsible for tracking the mobile user in the network. This characteristic makes the proposed system suitable for the implementation of RIS-assisted 5G/6G systems. However, in order to improve the system proposed here, it would be desirable to integrate other techniques that can further improve the system's performance. For example, considering orthogonal frequency division multiplexing (OFDM) systems, the proposed technique could be improved in its spectral efficiency and system capacity. On the other hand, incorporating techniques such as non-orthogonal multiple access (NOMA) could result in a system with the ability to overcome the challenges of the wireless environment like random fluctuations, shadowing, and mobility in an energy-efficient way [35].