Page 62 - IJEEE-2023-Vol19-ISSUE-1

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