Abstract
Sparse code multiple access (SCMA) is a non-orthogonal multiple access (NOMA) uplink solution that overloads resource elements (RE's) with more than one user. Given the success of orthogonal frequency division multiplexing (OFDM) systems, SCMA will likely be deployed as a multiple access scheme over OFDM, called an SCMA-OFDM system. One of the major challenges with OFDM systems is the high peak-to-average power ratio (PAPR) problem, which is typically studied through the PAPR statistics for a system with a large number of independently modulated sub-carriers (SCs). In the context of SCMA systems, the PAPR problem has been studied before through the SCMA codebook design for certain narrowband scenarios, applicable more for low-rate users. However, we show that for high-rate users in wideband systems, it is more meaningful to study the PAPR statistics. In this paper, we highlight some novel aspects to the PAPR statistics for SCMA-OFDM systems that is different from the vast body of existing PAPR literature in the context of traditional OFDM systems. The main difference lies in the fact that the SCs are not independently modulated in SCMA-OFDM systems. Instead, the SCMA codebook uses multi-dimensional constellations, leading to a statistical dependency between the data carrying SCs. Further, the SCMA codebook dictates that an UL user can only transmit on a subset of the available SCs. We highlight the joint effect of the two major factors that influence the PAPR statistics - the phase bias in the multi-dimensional constellation design along with the resource allocation strategy. The choice of modulation scheme and SC allocation strategy are static configuration options, thus allowing for PAPR reduction opportunities in SCMA-OFDM systems through the setting of static configuration parameters. Compared to the class of PAPR reduction techniques in the OFDM literature that rely on multiple signalling and probabilistic techniques, these gains come with no computational overhead. In this paper, we also examine these PAPR reduction techniques and their applicability to SCMA-OFDM systems.
Highlights
Non-orthogonal multiple access (NOMA) solutions are being actively studied to address the massive connectivity requirements for 5G and beyond 5G (B5G) communication systems [1]
WORK In this paper, we showed that optimizing the Sparse code multiple access (SCMA) codebook design to just have a low constellation peak-to-average power ratio (PAPR), for example through low-projection codebooks, is only applicable for lowrate users
We showed that the PAPR statistics of SCMA-orthogonal frequency division multiplexing (OFDM) systems are influenced by the joint impact of the phase bias in the multi-dimensional modulation scheme and the placement of the SCs that carry the SCMA codewords
Summary
Using SCMA as a multiple access scheme over OFDM means that the individual SCs are not independently modulated; motivating the need to characterize the PAPR analysis for SCMA-OFDM systems. By considering such a cross-layer systematization perspective, we motivate the fact that significant PAPR reduction can be achieved through the setting of static configuration parameters Such gains could come without any additional overhead to the system that is typically introduced by most multiple signalling based PAPR reduction techniques that work with the assumption of independently modulated SCs, i.e., no a-priori knowledge of any statistical dependencies in the transmitted signal [6]. With SCMA-OFDM systems, we can exploit the statistics known in advance through the novel aspects we present in this paper to reduce or even eliminate the complexity and sidelink information overhead typically incurred by these PAPR reduction techniques.
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