Page 62 - IJEEE-2023-Vol19-ISSUE-1
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58 | Msayer, Swadi, & AlSabbagh
5G and beyond support access to a very high data rate, Consider a downlink NOMA network system, shown in Fig.
massive communications with very low latency, high 1, with the source represented by the base station (;<), and
application mobility and support new applications with high three users: the near user (=5), the middle user (=6), and the
reliability and securing and preserving user data [10,11]. far user (=7). Assuming the near user has very good channel
conditions with the base station, and there is no direct
One of the new technologies used in 5G networks is Non- channel between them and the other two users due to a barrier
Orthogonal-Multiple-Access (NOMA) technology, through between them, assume that each node in the system has a
which it is possible to control traffic and enhance the single antenna (the base station and each user has one
spectrum where several users can share the same radio antenna). Also, consider the model with a fixed power
resource [12]. NOMA outperformed the traditional allocation has a power control circuit in the base station to
orthogonal multiple access (OMA) system that supported divide the transmit power among the three users and give the
previous generations from 1G to 4G. As we can see, signals a power weight.
Orthogonal Frequency Division Multiple Access (OFDMA)
has been used in 4G networks, which made a big leap in the Fig. 1: NOMA technique has two of three users out of
field of communications in the previous decade, but it does
not meet the increasing demand of users [13-15]. services.
The NOMA system relies on two basic techniques, it starts From Eq. (3), the achievable rate of the three users is:
by using the theory of the Superposition Coding (SC) process >8(59):; = log<( 1 + |>()|A**?+@) )
that takes place at the transmitter, where all the signals that .+>( -|.>*|.(?>>,+(((,-|*@.*|?)*?+3?@+@+@)@,-3,)A3*A)* ) (4)
are transmitted at the same sub band frequency are collected >8(69):; = log<( 1 (5)
and combined, each signal gives a specific weight of power >8(79):; = log<( 1 +
so that the least weight is allocated to the transmitted power (6)
to the closest (strongest) user, and the allocation begins to
increase to users who are farther from the base station Where:
[16,17].
6BC : The Rayleigh fading coefficient between the 78 and
The second technique is the process of Successive the 9C.
Interference Cancellation (SIC) at the receiving side, it can 6CD : The Rayleigh fading coefficient between the 9C and
be summarized as an iterative algorithm in which data is the 9D.
decoded in decreasing order of power levels [18,19]. 6CE : The Rayleigh fading coefficient between the 9C and
Decoding superposition signal that contains all signals were the 9E.
sending at the same band, then multiplying the farthest user’s
signal by the corresponding allocation weight and subtract it This model is employed to assess the data rates for the
from the decoding signal, then, the previous process is
repeated for the second stronger user until the desired user is
reached [20,21]. The paper is organized as follows: Section
2 is conducted to illustrate the model adopted in this paper to
analyze the NOMA downlink system, while Section 4
presents solutions to the considered problem in a cooperative
manner and shows the simulation results. Finally, the
achieved conclusions are summarized in Section 5.
II. SYSTEM MODEL three users with a barrier among them. Figure 2 shows the
Assume that the power allocation vector is ! =
[!!, !", !#, … … … , !$] . If the transmitted power per sub average achievable rates (bps/Hz), it’s clear that, because of
band is '%, then the user set power allocation should be [22].
no channel links among the middle and far users with the
base station, therefore, the achievable rate for them is zero,
the near user achieves data rate ranging from 13.14 bps/Hz
?$&'! '% !& = '% (1) to 23.11 bps/Hz at a transmitting power ranging from 0 to 30
and * = , = -
Where dBm. This justified with the outage probability for the three
?&$'! !& = *
users, it can be seen in Fig. 3, the second and third users are
out of service.
Then, the signal to interference noise ratio and the achievable III. COOPERATIVE NOMA DOWNLINK SYSTEM.
rate are [22]: ./!."0#1! Sometimes there are users located at the cell edge or
?$!'%&&./!."0#1$34" out of coverage, in that case their signal can be relayed by a
.((&)) *+ = (2) relay to improve the reliability for those users under poor
(3) channel conditions [22], Figure 4 illustrates the BS
/((&)) *+ = 012"(* + .((&)) *+) communicates with them via the near user by acting as a
Where decode-and-forward relay [23].
6& : Rayleigh fading coefficient between the 78 and the 9&.
:" : the variance of the AWGN.
'% : transmitted power