# Uplink Transmit Power Control for Single-Carrier Grouped FDMA with Iterative Multiuser Detection
## 1 Introduction  
Single-carrier frequency-division multiple access (SC-FDMA), based on single-carrier modulation with frequency-domain equalization (SC-FDE) [1], is an uplink data transmission scheme for broadband wireless systems; it has a low peak-to-average-power ratio (PAPR) and has therefore been considered to be a replacement for orthogonal frequency-division multiple access (OFDMA), depending on the system parameters [2; 3; 4; 5; 6; 7]. SC-FDMA is also called \"linearly precoded OFDMA\" because it has an additional discrete Fourier transform (DFT) stage prior to the OFDMA structure. The low-PAPR characteristic of SC-FDMA benefits the uplink scheme of mobile user equipment by reducing the transmit power and the cost of manufacturing a power amplifier. In addition, the implementation and computational complexities of SC-FDMA are concentrated at the receiver end. Application of SC-FDMA to an uplink is advantageous in this regard, because the receiver is located at a base station (BS). Accordingly, the Third-Generation Partnership Project Long-Term Evolution (3 GPP-LTE) adopted SC-FDMA for uplink data transmission. Recently, SC-FDMA was also adopted in various major applications of 4 th-generation (4G) LTE advanced (LTE-A) and 5 th-generation (5G) new radio networks [8; 9] including heterogeneous networks [10; 11], multiple input and multiple output [12; 13; 14], machine-type communications [15; 16], and green communications [17]. The demand for wireless communication increased rapidly through the implementation of 4G and 5G. As the spectrum and power are limited resources, an improvement in spectral efficiency has always been a crucial challenge in wireless communication. To improve the spectral efficiency of SC-FDMA, a non-orthogonal multiple access scheme called single-carrier grouped FDMA (SC-GFDMA) was proposed, which is SC-FDMA with iterative multiuser detection (IMD) [5]. SC-GFDMA allocates a common set of subcarriers to several users simultaneously, rather than to only one user as in a typical SC-FDMA scheme. The sharing of a frequency band is a non-orthogonal feature that causes multiple access interference (MAI) [5; 6; 7; 18]. MAI can be efficiently eliminated through IMD; hence, SC-GFDMA can achieve higher spectral efficiency than that achieved by typical SC-FDMA, FDMA, OFDMA, and time-division multiple access systems [5]. Specifically, SC-GFDMA increases the spectral efficiency achieved in each time-frequency resource grid, which is called a subcarrier chunk in this paper, by stacking up users in each chunk (Figure 1); here, a chunk refers to a set of consecutive subcarriers and corresponds to the minimum unit of resource allocation in each frame, such as a physical resource block in LTE/LTE-A standards [8; 9]. In other words, SC-GFDMA increases the spectral efficiency achieved in each chunk without using additional time, frequency, and spatial resources. In terms of addressing the demand for higher-speed wireless communication with limited communication resources, SC-GFDMA can be a solution for future standards, as it enhances the performance of communication building blocks. In addition, SC-GFDMA inherits the low-PAPR characteristic and simple single-tap equalizer from SC-FDMA; the transmitter also has a similar structure to that of SC-FDMA, except for the user-specific interleaver. Thus, SC-GFDMA can be considered to be an alternative to SC-FDMA for uplink transmission in future wireless standards. The primary drawback of using SC-GFDMA (when compared with SC-FDMA) is the additional computational complexity imposed on the receiver side. In uplink transmission, this additional cost is on the BS side, and BS is generally more robust and can support more complex computations than user equipment can. Nevertheless, commercial network operators should consider a trade-off between the additional cost for higher computational complexity and the increase in spectral efficiency when implementing SC-GFDMA schemes.