# Nonlinear Distortion Cancellation using Predistorter in MIMO-GFDM Systems
## 1 Introduction  
The future generations of mobile networks, the fifth generation (5G), requires high data rates, flexibility, and also low latency. The 5G scenario includes increased system capacity, high quality of services (QoS), low power consumption, and low latency. Generalized frequency division multiplexing (GFDM) is one of the candidates of non-orthogonal waveforms that is currently being considered and evaluated by the 5 GNOW team [1]. GFDM is able to overcome orthogonal frequency division multiplexing (OFDM) weaknesses, such as a high peak to average power ratio (PAPR). This is because OFDM signals are the sum of exponential terms limited to rectangular windows, while GFDM is thesum of limited exponential terms with raised cosine (RC) filters. Given the assumption of having a normalized filter, the average power of the GFDM signal is certainly reduced by applying windowing, while remaining constant for OFDM. Therefore, PAPR is the same for the sum of exponential terms and OFDM signals, while it is statistically growing for GFDM [2]. Studies on PAPR in OFDM have been extensively carried out [3; 4; 5; 6]. GFDM also produces low out-of-band (OOB) radiation due to the use of raised cosine pulse shaping to produce more efficient bandwidth. The cellular system and base station on 5G requires a new and faster processor, baseband, and radio frequency (RF) device. High power amplifiers (HPAs) are an important element of any communication system to communicate the signals to a significant distance [7]. HPAs are used to convert low power RF signals into large power signals to drive the transmitter [8]. This is because the need for coverage and long-distance transmission in wireless communication systems currently requires HPAs [9]. HPAs have three operating areas, the cutoff region, the linear region, and the saturation region. In general, the HPA will be operated in the linear region approaching the saturation region in order to increase power efficiency. When the HPA input power is close to the saturation point, it increases power efficiency and extends the battery life of the mobile transmitter. However, the HPA will produce nonlinear distortion if it is operated in this area [10]. Nonlinear distortion due to HPA causes several effects, both for the users themselves and for other users. The effects on nonlinear distortion include amplitude distortion, phase distortion, and the spread of signal constellations [11]. The effects of nonlinear distortion also produce harmonic and intermodulation signals outside the frequency band, which results in a transmitted spectral signal, which in turn interferes with the adjacent channel [10]. Outof-band rises because the nonlinear effects also cause interference between adjacent subcarriers [12]. New waveforms for the physical layer (PHY) and diverse requirements for 5G cellular systems are being studied, where one of the candidates is non-orthogonal GFDM. This new wave still uses high M-ary modulation [13]. Just like other multicarrier systems, GFDM also uses HPAs on the transmitter side. The use of HPAs on massive multiple-input multiple-output (MIMO) systems for 5G technology is the downlink at the base station system [14]. Like single-input-single-output (SISO), MIMO transceivers are also sensitive to nonlinear distortion due to HPAs. However, the problem becomes more complicated with the combined problem of HPA nonlinear distortion at the MIMO-GFDM with high M-ary modulation. Recently, a study on HPA design for broadband and multiband has been proposed to support the base station transmitter on broadband communication systems [7]. Basically, research on RF design and implementation related to 5G systems with special emphasis on spectral contained waveform and small cell system scenarios has been conducted [15]. Furthermore, several studies on PAPR investigations, OOB radiation, and BER of the GFDM system if exposed to nonlinear distortion have been accomplished in recent years [16; 17; 18].