Non-Orthogonal Multiple Access (NOMA)
Contents
1. Introduction
2. General Overview
3. NOMA Systems
4. Advantages and Practical Considerations
5. Applications and Future Directions
6. Advantages and Practical Considerations
7. Conclusion
8. References
Introduction
2. General Overview
Multiple access schemes allow a large number of mobile terminals to share the allocated radio
resources such as frequency, time, and code in most efficient manner. Orthogonal multiple access,
in which radio resource allocated to a given terminal is not repeatedly allocated to another one, can
minimize interference between simultaneously serviced terminals.
NOMA is an approach to support multi-user with common resource in power domain while
orthogonal multiple access uses specific resource for a user in time domain or frequency domain[2-
4]. According to QoS of data for supported terminals, a base station decides whether to share an
allocated resource using NOMA scheme or not.
In NOMA scheme, each user in a set for a transmission has different power level and modulation
order by their channel status. Here, each user in a set with the same resource affects the other user
in the same set as interference. To enhance system performance, NOMA requires interference
cancellor in the receiver. Normally, a faraway terminal from the base station in a set serviced with
high transmission power is operated without interference cancellation, on the other hand the other
terminal near to the based station in the set serviced with low transmission power decodes data after
interference cancellation using the data to the faraway terminal. This sharing resource processing
can improve the system capacity up to about 30% compared to the OFDMA [4].
5.4.1.1.2.1. Downlink NOMA
NOMA is also known as a superposition coding scheme, hierarchical modulation, or layered
modulation which have been studied in the broadcasting communication system. It allows for
transmitting user-1 signal x
1
and user-2 signal x
2
over the same resource by superposition.
In other words, the output signal of FFT which sums two signals, each with its own individual power,
is given by the superposed signal x=
𝑝
1
𝑥
1
+
𝑝
2
𝑥
2
for NOMA, the received signals of user 1 and
user 2 are given as:
Y
1
= h
1
x+n
1
Y
2
= h
2
x+n
2
Where n
1
and n
2
are the receiver Gaussian noises and h
1
and h
2
are the channel coefficients.
Let us assume that user 1 has the better channel condition than user 2
|
ℎ
1
|
>
|
ℎ
2
|
Allocating more power to user 2,
𝑝
2
>
𝑝
1
. User-2 signal, corresponding to interference
in user 1, can be detected successfully and cancelled from Y
1
, which allows for detecting
User-1 signal. The successive interference cancellation leads to the following signal-to-noise
ratios (SNR) for user 1:
𝑆𝑁𝑅
1
=
𝑝
1
|
ℎ
1
|
2
𝑁
0.1
WHERE
𝑃
1
>
𝑃
2
and
𝑁
0.1
is the noise power density of n
1
. Meanwhile, SNR for user 2 is given
as
𝑆𝑁𝑅
2
=
𝑝
2
|
ℎ
2
|
2
𝑝
1
|
ℎ
1
|
2
+
𝑁
0.2
Where
𝑁
0.2
is the noise power density of n
2
, the capacities of user 1 and user 2 are
Represented respectively as
𝑅
1
=
𝑙𝑜𝑔
2
(1
+
𝑆𝑁𝑅
1
)
and
𝑅
2
=
𝑙𝑜𝑔
2
(1
+
𝑆𝑁𝑅
2
)
Assuming the received SNR of 20 dB and 0 dB for two users, respectively, Figure 5.4-2 shows
a sum rate of two users as varying the allocated power ratio. In this figure, it is clear that
NOMA provides more capacity than OFDMA.
Figure 5.4-2 Sum rate of two users (20dB and 0dB) with OFDMA vs. NOMA
At the transmitter, the data for a set of the same resource is merged with allocated power rate for
each user after the mapper, and the data of multi-user is transmitted as the form of N
HP
+N
LP
order
constellation where N
HP
is the constellation order of a high power allocated user and N
LP
is the
constellation order of a low power allocated user. At the receiver, the user with high power allocation
ratio decodes the received signal which is contained other signal as noise in a set and the user with
low power allocation ratio decodes own information after successive interference cancellation using
the data of user with high power allocation ratio.
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