recent progress in non-orthogonal multiple access · recent progress in non-orthogonal multiple...

Non-Orthogonal Multiple Access

Recent Progress in Non-orthogonal MultipleAccess

Zhiguo DingSchool of Computing and Communications Lancaster University

A collaboration with Princeton, SWJTU, FAU, USTC, BUPT, AUTH, CMCC, UTDallas, XJTU, QMUL, KCL, Southampton, Tsinghua, Peking and Bell Labs.

24 September 2017

Non-Orthogonal Multiple AccessOverview & Motivation

Outline

Overview and Motivation

A General FrameworkSingle-Carrier NOMAMulti-carrier NOMA

NOMA Assisted Wireless CachingMotivation and IntroductionNOMA Caching Strategies

Promising Research Challenges


Non-orthogonal Multiple Access (NOMA)What is multiple access (MA)?

Techniques to serve multiple users with limited bandwidth.An example for downlink multiple access

...data stream 1:

data stream 2:

data stream K:

BS: multiplexing the signals

...

k-th user

1-th user

K-th user

Receiver for the k-th user

Sig

nals se

nt b

y

the B

S

...


Non-orthogonal Multiple Access (NOMA)What kind of multiple access techniques have been used?

We have been using orthogonal multiple access (OMA).TDMA: Orthogonal (non-overlapping) time slots are allocatedto users.FDMA: Orthogonal (non-overlapping) frequency channels areallocated to users....


Non-orthogonal Multiple Access (NOMA) - (1/2)

Disadvantages of OMADilemma to realize a better trade-off between throughput anduser fairness, illustrated in the following example:

A user with a poor connection to the base station (BS) isserved by using OMA.Spectral efficiency is low since this user cannot utilize theallocated bandwidth efficiently.Since OMA is used, the bandwidth resources occupied by thisuser cannot be shared by the others.

Difficult to support massive connectivityRecall that the three key requirements for 5G are to supporthigh throughput, low latency and massive connectivity


Non-orthogonal Multiple Access (NOMA) - (2/2)

A promising solution is to break orthogonality → NOMAThe key idea of NOMA is to encourage spectrum sharingDetails for the advantages of NOMA are to be given in theremaining of this tutorial.

NOMA is gaining ground on the competition of multipleaccess techniques for the next generation wireless networks

Adopted by many 5G MA concepts, including power-domain(PD) NOMA, sparse code multiple access (SCMA), multi-usersharing access (MUSA), pattern division multiple access(PDMA), lattice partition multiple access (LPMA), etc.Used by 4G LTE-A, termed multi-user superpositiontransmission (MUST).Included in the forthcoming digital TV standard (ATSC 3.0).

Non-Orthogonal Multiple AccessA General Framework

Single-Carrier NOMA

Outline






Single-Carrier NOMA

Single-Carrier NOMA - Power Domain NOMA

All the users are served at the same time, frequency and code,but with different power levels.Users with better channel conditions get less power.Successive interference cancellation (SIC) is used.

[1]Y. Saito, A. Benjebbour, Y. Kishiyama, and T. Nakamura, “System level performance evaluation of downlinknon-orthogonal multiple access (NOMA)”, in PIMRC 2013.[2]Z. Ding, Z. Yang, P. Fan and H. V. Poor, “On the Performance of Non-Orthogonal Multiple Access in 5GSystems with Randomly Deployed Users”, IEEE SPL, 2014.


Single-Carrier NOMA

Single-Carrier NOMA - Cognitive Radio NOMA (1/2)Conventional power-domain NOMA allocates more power tothe user with poor channel conditions to ensures user fairness.But power domain NOMA cannot strictly guarantee the users’QoS targets.CR-NOAM can strictly guarantee the users’ QoS requirementsby using the fact that NOMA can be viewed as a special caseof CR networks.For example, consider the following two-user scenario:


Single-Carrier NOMA

Single-Carrier NOMA - Cognitive Radio NOMA (2/2)User 1 can be viewed as a primary user in a CR network:

If OMA is used, the orthogonal bandwidth allocated to user 1cannot be accessed by other users.Spectral efficiency is low since user 1 has a poor channel

The use of NOMA is equivalent to the application of thecognitive radio concept:

Specifically user 2, a user with better channel conditions, isadmitted to the channel occupied by user 1.Although user 1 causes extra interference at user 2 and hencereduces user 2’s rate, the overall system throughput will beincreased significantly since user 1 has a stronger connectionto the BS.

A new form of NOMA, CR-NOMA, can be developed by usingthe synergy between NOMA and CR.

[3] Z. Ding, P. Fan and H. V. Poor, “Impact of User Pairing on 5G Non-Orthogonal Multiple Access DownlinkTransmissions”, IEEE TVT, 2016


Multi-carrier NOMA

Outline






Multi-carrier NOMA

Multi-carrier NOMA - Fundamental LimitsWhy to adopt hybrid NOMA

Asking all users to participate in NOMAcan cause significant complexityThis motivates the hybrid NOMAscheme

Users in a cell are divided into smallgroupsOMA is used to avoid inter-groupinterferenceNOMA is implemented among theusers within a single group

Who is to be grouped with whom?User pairing has an important impact on the performance ofNOMA.

[3] Z. Ding, P. Fan and H. V. Poor, “Impact of User Pairing on 5G Non-Orthogonal Multiple Access DownlinkTransmissions”, IEEE TVT, 2016


Multi-carrier NOMA

Multi-carrier NOMA - Resource AllocationResource allocation for hybrid NOMA is very difficult

maxpower ,bandwidth allocation

(Weighted) sum rate (1)

s.t. Power and bandwidth constraints

Power Allocation User Grouping

How to solve such a mixed integer non-convex optimizationproblem

Monotonic optimisation is applied to solve such a non-convexoptimisation problem to obtain an optimal solutionA low-complexity suboptimal solution based on SCO can beobtained with a performance gain close to the optimal.

[4] Y. Sun, D. W. K. Ng, Z. Ding, and R. Schober, “Optimal joint power and subcarrier allocation for full-duplexmulticarrier non-orthogonal multiple access systems,” IEEE TWC, 2017


Multi-carrier NOMA

Multi-carrier NOMA - 5G Variants

Various practical forms of multi-carrier NOMA have beenproposed for the 5G standard.

Multi-carrier NOMA achieves a favourable tradeoff betweensystem performance and complexity.

Both low density spreading (LDS) and sparse code multipleaccess (SCMA) are based on the idea that one user’sinformation is spread over multiple subcarriers.However, the number of subcarriers assigned to each user issmaller than the total number of subcarriers

This is referred to as the low spreading (sparse) feature ofthese two versions of NOMA.This feature ensures that the number of users utilizing thesame subcarrier is not too large, such that the systemcomplexity remains manageable.


Multi-carrier NOMA

SCMA: An Example of MC-NOMA (1/3)The key step of SCMA is how to map users to subcarriers.

SCMA Codebook

Mapping

Channel

Coding

Channel

Coding

Channel

Coding

Channel

Coding

SCMA Codebook

Mapping

SCMA Codebook

Mapping

SCMA Codebook

Mapping

Channel

Coding

SCMA Codebook

Mapping

Channel

Coding

SCMA Codebook

Mapping1 1 1 0 0 0

1 0 0 1 1 0

0 1 0 1 0 1

0 0 1 0 1 1

=

F

+

+

+

Codeword

1

Codeword

6

+

+

U1

U2

U3

U4

U5

U6

[5] H. Nikopour and H. Baligh, “Sparse code multiple access,” IEEE PIMRC, 2013.


Multi-carrier NOMA

SCMA: An Example of MC-NOMA (2/3)Consider an SCMA system with 6 users and 4 subcarriers.The key step to implement SCMA is to design the factorgraph matrix, which specifies which user’s encoded messagesare allocated to which subcarriers.A typical factor graph matrix for SCMA with 6 users and 4subcarriers is the following

subcarrier 1

subcarrier 2

subcarrier 3

subcarrier 4

U1 U2 U3 U4 U5 U6

where [F]i ,j = 1 means that the j-th user can use the i-thsubcarrier, and [F]i ,j = 0 means that this user cannot use thesubcarrier.


Multi-carrier NOMA

SCMA: An Example of MC-NOMA (3/3)

The sparse feature of SCMA is reflected by the fact that thereare only two non-zero entries in each column of F, i.e., eachuser employs only two subcarriers.Since one user can use multiple subcarriers, SCMA employsmulti-dimensional coding in order to ensure that the user’sinformation is effectively spread over the subcarriers.Because one user’s messages at different subcarriers are jointlyencoded, SCMA requires joint decoding at the receiver, wherethe message passing algorithm (MPA) is used to ensure lowcomplexity.

Joint decoding is an important feature of SCMA, whichdistinguishes it from power-domain NOMA, where SIC isemployed.


Multi-carrier NOMA

Pattern Division Multiple Access (1/2)PDMA is another type of multi-carrier NOMA, but the lowdensity spreading (sparse) feature is no longer strictly present

The number of subcarriers occupied by one user is notnecessarily much smaller than the total number of subcarriers.

Similar to the factor graph matrix for SCMA, the performanceof PDMA is largely determined by the design of the subcarrierallocation matrix, referred to as the PDMA pattern matrix.Consider a case with five users and three subcarriers, andPDMA pattern matrix

Q =

1 1 0 0 11 1 1 0 01 0 1 1 0

, (2)

where the entries of this matrix indicate how the subcarriersare allocated to the users.

[6] S. Chen, et. al., “PDMA - a novel non-orthogonal multiple access for 5G radio networks”, IEEE TVT, 2016


Multi-carrier NOMA

Pattern Division Multiple Access (2/2)Consider a case with five users and three subcarriers, and PDMApattern matrix

U1

subcarrier 1

subcarrier 2

subcarrier 3

U2 U3 U4 U5

User 1 is able to transmit or receive on all subcarriers.User 5 uses the 6-th subcarrier only.Therefore, different from LDS and SCMA, some users mightbe able to use all the subcarriers.

SCMA requires that each user occupies the same number ofsubcarriers.This constraint is not required by PDMA and hence makesPDMA more flexible.

Non-Orthogonal Multiple AccessNOMA Assisted Wireless Caching

Motivation and Introduction

Outline








NOMA is shown to be compatible to many other importantcommunication techniques, such as mmWave, relaying,MIMO, etc.Little is known about the coexistence between NOMA andwireless caching

How to realize content pushing in a timely and robust manner?Content can be pushed to caching infrastructure duringoff-peak hours through wired connections.

How to cope with the non-ideal situation for wireless caching,when some users? requests have to be fetched from the BSdirectly?

When this situation happens, the spectral efficiency of wirelesscaching is reduced.An inevitable but frequently happening situation.

[7] Z. Ding, P. Fan, G. Karagiannidis, R. Schober and H. V. Poor, ”NOMA Assisted Wireless Caching: Strategiesand Performance Analysis”, TCOM, submitted.


NOMA Caching Strategies

Push-then-delivery Strategy

Push file i to CSt

Content pushing phase

……

Push multiple files to CSs


……

Each CS serves a single user

Content delivery phase

……

Each CS serves multiple users


……

OMA

NOMA

The NOMA principle is applied to the content pushing andcontent delivery phases, respectively.Both the cache hit probability and content delivery probabilityare improved.



Push-then-delivery Strategy - Content Pushing

CS2

BS

f1 f2 f3

CSm

f1 f2

CSt

f1

CS1

f1 f2 f3 f4

CSm+2

f1 f2

CSm+1

f1 f2

CSm-1

f1 f2 f3

OMA - BS pushes f1 to CSt NOMA - BS pushes 4 files to CSs

An illustration of the impactof NOMA on contentpushing, where 1 < m < t.

CSs denotes contentservers.CSm is closer to the BSthan CSt .In OMA, a single file ispushed to CSt .In NOMA, the BS pushesthree additional files byusing NOMA.A content server closer tothe BS is likely able todecode more pushed file.



Push-then-delivery Strategy - Content Delivery

BS

CSi

Ui,1

Ui,2

CSt

Ut,1

Ut,2

CSm

Um,1

Um,2

An illustration of the impactof NOMA on contentdelivery.

In OMA, each contentserver serves a single user.By using NOMA, anadditional user can beserved.



Push-and-delivery Strategy

OMA

NOMABS serves Um,k while

pushing new contentCSs serve users

BS pushes files to CSs

BS serves Um,k

directlyCSs serve users

BS pushes files to CSs

Content delivery phaseContent pushing phase



When this situation happens, the spectral efficiency of wirelesscaching is reduced, since the users’ requests cannot beaccommodated locally.The proposed push-and-delivery strategy treats this situationas an opportunity for the application of NOMA to improve thespectrum efficiency of wireless caching.



Push-and-delivery Strategy

BS

CSi

CSm

Um,k

CSt

Content delivery

Content pushing

The use of NOMA can ensure new content files can becontinuously pushed to content servers, in a more timelymanner, compared to OMA.This strategy can also been applied to D2D caching.

Non-Orthogonal Multiple AccessPromising Research Challenges

Promising Research ChallengesDifferent variants of NOMANew coding and modulation for NOMAHybrid multiple accessNOMA assisted cachingCoexistence between mmWave and NOMAMIMO and cooperative NOMAInterplay between NOMA and cognitive radioImperfect CSI and limited channel feedbackSecurity provisioning in NOMAEfficient resource allocation for NOMAImplementation issues of NOMAEmerging applications of NOMA

Non-Orthogonal Multiple AccessPromising Research Challenges

Thank you for your attention![8] Z. Ding, X. Lei, G. K. Karagiannidis, R. Schober, J. Yuan, and V.Bhargava, A Survey on Non-Orthogonal Multiple Access for 5GNetworks: Research Challenges and Future Trends, IEEE JSAC, 2017[9] Z. Ding, Y. Liu, J. Choi, Q. Sun, M. Elkashlan, C-L. I and H. V. Poor,“Application of Non-orthogonal Multiple Access in LTE and 5GNetworks”, IEEE Communication Magazine, 2017.[10] L. Song, Y. Li, Z. Ding, and H. V. Poor, “Resource management innonorthogonal multiple access networks for 5G and beyond”, IEEENetworks, 2017

Email: [email protected]

recent progress in non-orthogonal multiple access · recent progress in non-orthogonal multiple...

Documents