Non-orthogonal Multiple Access (NOMA) for Future Radio · PDF fileNon-orthogonal Multiple Access (NOMA) for Future Radio Access Kenichi Higuchi Tokyo University of Science February

Non-orthogonal Multiple Access (NOMA) for Future Radio Access

Kenichi HiguchiTokyo University of Science

1February 26, 2016 Kenichi Higuchi/TUS@Adachi_WS

Evolution of Radio Access (1) Non-orthogonal multiple access (NOMA) based on DS-

CDMA for 3G– Rake diversity in multipath fading channel appropriate for low-rate

data (thus large spreading factor)– Large capacity via statistical multiplexing effect for circuit-switching

voice users

Orthogonal multiple access (OMA) based on OFDM for 3.9G and 4G– High robustness against multipath interference with simple receiver

structure (e.g. by using frequency-domain channel equalization)– Channel-aware packet scheduling in both frequency and time domain multiuser diversity

– Good affinity to the MIMO channel transmission (again assuming relatively simple receiver structure)

2February 26, 2016 Kenichi Higuchi/TUS@Adachi_WS

Evolution of Radio Access (2)What about beyond 4G?

– In theory, NOMA with successive interference cancellation (SIC) achieves the multiuser capacity region.

• Especially important for improving the performance of cell-edge users with small system efficiency loss

– OFDM-like signaling with superposition coding

February 26, 2016 Kenichi Higuchi/TUS@Adachi_WS 3

We think that NOMA with more advanced transceiver can be a candidate multiple access scheme.

NOMA OMA NOMA with advanced transceiver?

3G 3.9G and 4G Beyond 4G

OMA vs. NOMAOMA

NOMA with SIC


User A’s signal, xA

User B’s signal, xB

Frequency

Narrow transmission bandwidth per user

No inter-user (intra-cell) interference

User A’s signal, xAUser B’s signal, xB

Frequency

Wide transmission bandwidth per user

SIC (successive interference cancellation) at the receiver

Decode of xAxA+xB+noise

xA

Decode of xB xBxB+noise

With inter-user (intra-cell) interference

No inter-user (intra-cell) interference

Study at Standardization Body: 3GPPNOMA, namely “downlink Multi-User Superposition

Transmission (MUST)” is now being discussed at the standardization body such as 3rd Generation Partnership Project (3GPP) as a further evolution of LTE, i.e., LTE Release 13.– 3GPP RP-150496, “New SI proposal: Study on downlink

multiuser superposition transmission for LTE,” Mar. 2015.

In the following, we will briefly explain the principle of downlink NOMA and our investigations, along with several issues for implementing NOMA in real systems.– K. Higuchi and A. Benjebbour, “Non-orthogonal multiple access (NOMA) with

successive interference cancellation for future radio access,” IEICE Trans. Commun. vol. E98-B, no. 3, pp. 403-414, Mar. 2015.


Downlink


Base station

Terminals

Principle of Downlink NOMA (1)Base station (BS) transmitter performs superposition coding

for non-orthogonal user multiplexing.– Each user’s information is independently channel coded

and modulated and then added with other users’ signals.


1

K

u uu

x p s

Tx power for user u Coded modulation symbol of user u

I

Q

I

Q

I

Q1 1p s

1p 2p

2 2p s x

Principle of Downlink NOMA (2)The user terminal conducts SIC.

– In the downlink, the decoding order of SIC should be in the order of increasing channel gain, |hk|2/Nk.

• Each user can remove interference from the user whose channel condition is worse than that user.


Received signal at user k

1

k k kK

k u u ku

y h x w

h p s w

Channel coefficient of user k

Noise + inter-cell interference(Power = Nk)

Decode of s1ky1 1kh p s

1s

Decode of s2 2s

Decode of s33 3kh p s

3s2 2kh p s

Principle of Downlink NOMA (2)The user terminal conducts SIC.

– User k can correctly decode user 1～k−1 signals and remove these signal components from the received signal.


2(NOMA)

2 2

1

log 1 b/s/Hzk kk K

i k ku k

p hR

p h N

Received signal at user k

1

k k kK

k u u ku

y h x w

h p s w

Channel coefficient of user k

Noise + inter-cell interference(Power = Nk)

Decode of s1ky1 1kh p s

1s


2s


3s

Capacity Region


NOMA with SICOMASymmetric

channel Capacity regions of two access schemes are identical.

Asymmetric channel

Capacity region of NOMA is wider than that of OMA.

R1 (b/s/Hz)

R2 (b/s/Hz)

3.5

3.5

ptotal|h1|2/N1 = 10 dBptotal|h2|2/N2 = 10 dB

0

ptotal|h1|2/N1 = 20 dBptotal|h2|2/N2 = 0 dB 0

0.20.40.60.8

1

0 1 2 3 4 5 6 7R1 (b/s/Hz)

R2 (b/s/Hz)

System Efficiency and User FairnessGeneralized system throughput based on α-proportional fair

– Parameter α controls the tradeoff between the system efficiency and user fairness.

To maximize the system throughput, the following resource allocation metric is to be maximized.


1

1

( )( )1

Kk

k

R tU t

User fairness

System efficiency

α0 ∞

1

1( , ; ) , ;( 1)

K

kk k

M W P t R W P tR t

Weighted sum of user throughput

As α is increased for better user fairness, weight for throughput of weak user increases.

α = 0α > 0

When NOMA is Effective?

NOMA is more attractive than OMA when the user channels are asymmetric and the system wants to take care of user fairness.


00.20.40.60.8

1

0 1 2 3 4 5 6 7R1 (b/s/Hz)

R2

(b/s

/Hz)

NOMA OMA0 1 2M R t R t

NOMA NOMA NOMA

0 1 21 1

1 1( 1) ( 1)

M R t R tR t R t

OMA OMA OMA0 1 2

1 1

1 1( 1) ( 1)

M R t R tR t R t

NOMA in Cellular Downlink In cellular downlink, the channel conditions vary significantly

among users due to the near-far effect.

When we want to improve the throughput of weak user …– In OMA, bandwidth allocation to strong user is severely

limited.

– NOMA with SIC• Wide transmission bandwidth for all users• Allocates large power to weak user

– Although the weak user does not use SIC, the impact of inter-user interference is small.

• Strong user uses SIC to cancel out the interference from weak user and small power allocation is enough due to strong channel.


Strong Weak Inter-cell interference

Example


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

10-1 100 101 102

Cum

ulat

ive

prob

abilit

y

User throughput (Mb/s)

OMANOMA

FTP traffic model 1 with λ = 0.8

Full-buffer traffic model with 10 users per cell

NOMA in MIMO Downlink

Key issues in our mind– Use of SIC instead of dirty paper coding (DPC)

• DPC is difficult to implement and sensitive to the error in channel state information feedback.

– Avoidance of increase in reference signaling overhead• In NOMA, the number of users multiplexed within a

same frequency is assumed to be beyond the number of BS antennas.

• If we assume the user-specific beamforming, the number of reference signals in NOMA may exceed the number of BS antennas.


NOMA with SIC appropriate for MIMO downlink

Proposed Method

BS transmitter: Intra-beam superposition coding– The number of beams is equal to that of BS antennas.– Within a beam, multiple-user signals are non-orthogonally

multiplexed based on superposition coding.• The number of reference signals is the same as in OMA

irrespective of the number of non-orthogonally multiplexed users.

UE receiver: Intra-beam SIC– Inter-beam interference is suppressed by the linear spatial

filtering.– Then, intra-beam interference due to superposition coding

is removed by SIC.• Effective use of SIC as in the SISO or SIMO downlink


Intra-beam superposition coding and SIC in NOMA with MIMO

Pictorial Example


BS

Equivalent normalized channel gain, g

Large Small

Beam 1

Beam 2

User 1

User 2

User 3

User 4

SIC of User 2 signal

User1 signal decoding


SIC of User 4 signal



Spatial filtering

Spatial filtering

Spatial filtering

Spatial filtering

fSignal to User1Signal to User2

fSignal to User3Signal to User4

Throughput Performance


0

10

20

30

40

50

10 20 30 40 50

Sum

thro

ughp

ut (M

b/s)

Number of users per cell

0

100

200

300

400

500

600

700

10 20 30 40 50

Cel

l-edg

e us

er th

roug

hput

(kb/

s)

Number of users per cell

1.5 times

Num. of BS antennas = 1Num. of BS antennas = 2

OMANOMA

OMANOMA

Num. of BS antennas = 1Num. of BS antennas = 2

Issues and recent investigations for implementing NOMA in real systems


Generation of Superimposed Signal (1) Two approaches under investigation at 3GPPSOMA (semi-orthogonal multiple access)

– Each user’s information is QAM modulated and summed among users with appropriate power allocations so that the resultant signal still forms higher-order QAM signal.

• E.g. QPSK + QPSK = 16QAM, 16QAM + QPSK = 64QAM


3GPP R1-151425, “Multiuser superposition schemes,” Apr. 2015.3GPP R1-151848, “Candidate schemes for superposition transmission,” Apr. 2015.

Near user modulation symbol (QPSK)

Far user modulation symbol (QPSK)

Superimposed symbol (16QAM)

Generation of Superimposed Signal (2)SOMA (semi-orthogonal multiple access) cont’d

– No need for defining new modulation symbols– Good performance even when SIC is not applied– Question: Do we need Gray mapping after superposition?

RA-CEMA (rate-adaptive constellation expansion multiple access)

– Superimposed signal forms 2mQAM similar to SOMA.– Mapping of coded bits of multiple users on m bits at

2mQAM modulation is adaptively controlled depending on the channel conditions of respective users.


A. G. Perotti and B. M. Popović, “Non-orthogonal multiple access for degraded broadcast channels: RA-CEMA,” in Proc. IEEE WCNC2015, New Orleans, USA, Mar. 2015.

Generation of Superimposed Signal (3)RA-CEMA (rate-adaptive constellation expansion multiple

access) cont’d– No need for defining new modulation symbols similar to

SOMA– By changing the bit mapping of multiple non-orthogonally

multiplexed users based on channel conditions, more detailed rate control is achieved.


I

Q

1111101100110111

1110101000100110

1100100000000100

1101100100010101

Far user = 1 bitNear user = 3 bits

OthersResource allocation

– In addition to the time/frequency block allocation in OMA, the power allocation needs to be appropriately conducted.

– Frequency block-distributed codeword mapping in LTE-Advanced should also be taken into account.

Control signaling– Transport format (modulation scheme, code rate, power,

etc.) of other users should be informed for SIC process.Reference signaling

– Additional format of reference signal appropriate for NOMA may be defined.

Hybrid ARQ in NOMA


ConclusionWe briefly explained the principle of downlink NOMA and our

investigations, along with several issues for implementing NOMA in real systems.


Thank you very much for your kind attention!

Documents

Non-orthogonal Multiple Access (NOMA) for Future Radio · PDF fileNon-orthogonal Multiple Access (NOMA) for Future Radio Access Kenichi Higuchi Tokyo University of Science February