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TopIntroduction
In 5G wireless communication systems, new radio access technologies are explored to satisfy the demanding requirements such as massive connectivity, low latency, and high data rate. Designing a suitable multiple access technique is an important aspect to improve system capacity (Jarray, et. al.,
2015; Al-Imari, et. al., 2014). Non-orthogonal multiple access (NOMA) is a promising multiple access technique for 5G cellular systems to improve spectral efficiency, provide low latency, high reliability, and massive connectivity (Yuan & Ding, 2016). NOMA techniques are divided into two categories, namely, power-domain and code-domain (Islam, et. al., 2016; Tao, et. al., 2015). This paper focuses on the power-domain NOMA in the uplink, where different users share the same time, frequency, and (possibly) code, but with different power levels (Benjebbour, et. al., 2015). To detect the desired signals while minimizing the interference, a SIC technique is employed at the base station (BS) receiver (Mollanoori & Ghaderi, 2014). The signals are assigned unique power levels so that the received signals arrive at the BS with an adequate power difference to allow the SIC receiver to decode them correctly (Dai, et. al., 2018). The SIC receiver sequentially detects the signals from the superimposed received signal at the BS. Once a user signal is successfully decoded, it is subtracted from the composite signal. Various modulations schemes (levels) can be used to modulate the transmitted signals such as BPSK, QPSK, and 16-QAM (Proakis & Salehi, 2008). Here the bit error rate (BER) is investigated for the NOMA receiver with and without channel estimation errors.
Recently, NOMA has been extensively studied in the last few years. For example, a switching between orthogonal multiple access (OMA) and NOMA scheme has been proposed by Janghel & Prakriya, (2018) to improve performance of the far user (FU) while ensuring that near user (NU) does not lose performance due to cooperative NOMA (Ding, et. al., 2015). In addition, a NOMA developed system has been proposed by Liu, et. al., (2017) and the system performance has been analyzed by proposing a power allocation scheme to guarantee the data rate by minimizing the outage probability.
In this paper, the BER performance of a two-user uplink NOMA system using successive interference cancellation (SIC), has been analyzed via simulation. Also, the BER for a given channel estimation error is investigated for BPSK, QPSK, and 16-QAM modulation schemes. The performance in the presence of different estimation errors is analyzed.
The main contributions of this paper can be summarized as follows:
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A simulation study of the BER performance of a two-user uplink NOMA using BPSK, QPSK and 16-QAM with a SIC receiver is introduced.
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For the above study, two scenarios are studied:
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Perfect channel estimation.
- •
Performance with a channel estimation error as a function of the modulation level.
The rest of this paper is comprised of four sections, and it is organized as follows. Describing the system model of a two-user uplink NOMA system with SIC reception is presented in the following section. The third section focuses on the simulation results of the BER performance for different modulation schemes with and without channel estimation errors. The conclusion of this research is contained in the final section.
TopSystem Model
This paper considers the uplink NOMA system which consists of two users and a BS receiver. The corresponding system diagram is illustrated in Figure 1. The first user transmits its signal, 
scaled by a power coefficient 
, to the BS over a channel with (scalar) coefficient 
. The second user sends its signal, 
with its power coefficient
, to the BS with channel coefficient 
. The received signal,
, at the BS is represented as:
,
(1) where
is the additive white Gaussian noise (AWGN) with zero mean and variance
. The transmitted signals
and
in (1) represent the transmitted signals
for each user with its power coefficient
.