## 2 Related Work  
The single-sideband (SSB) is sensitive in wireless communications due to the high ISI caused by the wireless environment. Thanks to the Hilbert transformation, it is feasible to generate the SSB signal from different types of modulation, e.g., PSK, QPSK, and QAM. However, for simplicity, the previous SSB-based researches are mainly applied in QPSK for increasing channel capacity. The result showed the successfully transmitted signal which compensates the ISI by using the turbo equalizer algorithm called widely MMSE estimation. The limitation of this approach is that the turbo equalizer is only used for QPSK then performance and feasibility are degraded in multipath fading channel because of the ISI increase [16].  
A notable research investigated the increased capacity by combining four single-sideband (4-SSB) QPSK [10]. This technology can double the amount of information by sending two QPSK symbols compared to the traditional SSB QPSK. Thus, undoubtedly, it increases the number of required SSB signals using additional Hilbert transform applied to make ISI. To solve this issue, the authors also applied the turbo equalizer to deal with QPSK modulation for the enhanced efficiency.  
The prior SSB-based researches are applicable in second generation (2G) and third-generation (3G) wireless technology. Besides, the research of QPSK 4-SSB using OFDM channel proves that the 4-SSB is applicable in fourth generation (4G) [11]. It also can be applied in the multipath fading channel by using the widely linear MMSE equalizer. The extension of this research is applying inter-canceller to improve the 4-SSB bit error rate efficiency. Currently, there is a need to apply the new idea of 4-SSB for the 5G wireless technology, but the existing researches mainly use turbo equalizers in QPSK.  
Typically, the research on 4-SSB M-QAM modulation using soft input soft output (SISO) equalizer over OFDM was proposed for increasing high data rate and capacity [12]. The researchers applied a new turbo equalizer algorithm for high order modulation. The evaluation results showed a consistent performance in AWGN and fading channel. However, we need to decrease the receiver complexity without losing orthogonality for the practical applications in 5G, e.g., MIMO system [17].  
To do this, the complexity was reduced by applying the shadow area constraints of QAM using multiple feedback successive interference cancellation with shadow area constraints (MF-SIC-SAC) [13]. Shadow equalizer is a type of equalizer which uses shadow area constraint (SAC) to evaluate the quality of decisions and avoid unnecessary multiple feedback procedures for reliable estimations. This algorithm feeds multiple candidates for symbol estimation. However, a reduced symbol candidate's decision is still required for realizing low energy and complexity in the equalizer.  
The guard interval discrete Fourier transform spread OFDM GI DFT-s-OFDM, and spectrally-preceded OFDM SP-OFDM are feasible candidates of OFDM technology to be applied in 5G [2,9]. The evaluation results of 4-SSB OFDM in uncoded environment showed good performance compared to traditional OFDM thanks to the high bandwidth efficiency from the feature of SSB-based modulation. However, decreasing bit error is required for high efficiency of bandwidth usage and efficient equalization algorithm. Also, we apply turbo code with interleaver parameters as defined by V5 GTF for the 5G radio specification [15] in 4-SSB. Our proposal then investigates and proposes the system design with high bandwidth efficiency and low complexity which implies the low energy consumption of the communication systems.