# PAPR Reduction in UFMC for 5G Cellular Systems
## 1 Introduction  
Nowadays, the wireless cellular networks are evolving rapidly to cope with the various advances in wireless technologies. In particular, Internet-of-Everything (IoE) applications and the promising fifth-generation (5G) services are the main drivers behind this revolution. To realize such type of networks, i.e., 5G and beyond, several innovative technologies at various network layers have been developed [1; 2; 3]. For instance, at the physical layer, new flexible and scalable modulation and multiple access technologies were proposed in order to provide high data rate systems (\(\sim\)20 Gb/s) [4], superior quality of service (QoS) end-users, and low latency (\(\sim\)1 ms), and energy consumption [1]. The currently used modulation scheme, in the fourth generation (4G) systems, is the orthogonal frequency division multiplexing (OFDM) [5; 6; 7]. OFDM can effectively combat the multi-path fading channel to provide reliable transmission and increase the system capacity. Therefore, OFDM uses a cyclic prefix (CP) and a guard interval between the OFDM symbols to avoid the inter-channel interference (ICI) and the inter-symbol interference (ISI), owing to the adjacent sub-carriers and symbols, respectively. Besides, OFDM suffers from large side lobes [8], complex design of power amplifier, owing to high peak-to-average power ratio (PAPR), synchronization errors in dataaggregation; therefore, not recommended for cognitive radio and IoE networks [9]. This stimulated researchers to develop and propose robust modulation formats that can satisfy the requirements of the new wireless networks [10; 11; 12; 13]. Recently, many alternatives to OFDM technology were proposed for the 5G air interface. These include filtered OFDM (F-OFDM) [14], filtered bank multi-carrier (FBMC) [3], universal filtered multi-carrier (UFMC) [1], and generalized frequency division multiplexing (GFDM) [15]. For example, in the FBMC modulation format, each sub-carrier is individually filtered using a filter bank to provide low out-of-band (OOB) emission, which results in lower ICI. On the other hand, FBMC shows more hardware complexity owing to the required filter length, which is impracticable in some applications, such as short-packet transmission [11]. In the UFMC system, the sub-carriers are packed and filtered as sub-bands. This reduces the filter length, making UFMC more suitable for dynamic spectrum allocation techniques in cognitive radio (CR) networks [16; 17]. Furthermore, UFMC provides lower side lobe radiation, low latency, and capability of supporting frequency segmentation and multi-service applications [15]. Additionally, it is suitable for short burst communications [11; 15], UFMC uses Dolph-Chebyshev filter for each sub-band [18], where in the pass band, the valuable signals are passed without any loss, whereas in the stop band the frequency response is decaying fast to avoid OOB emission [14; 19]. It is worth noticing that UFMC maintains the fundamental features of OFDM, making it relevant to multiple-input multiple-output (MIMO) applications. Additionally, in F-OFDM, the sub-bands are filtered after the CP addition for precise frequency localization. However, this method shows high hardware complexity, besides the filter tail is extending to the next OFDM symbol [14]. Nonetheless, UFMC suffers from high PAPR arising from the usage of multi-carrier transmission [11]. Several techniques for PAPR reduction were proposed in the literature [20; 21]. These techniques can be classified as either signal distortion [22; 23], encoding [24], or probabilistic methods [25; 26]. The first method uses clipping or filtering to limit the maximum level of the signal, hence reducing the PAPR. This introduces distortions in the transmitted signal, which reflects on the system bit error rate (BER) performance [21]. Additionally, the encoding schemes cause low data rate efficiency [27; 28].