In search of a Multiple Access Scheme (MAS) for 5G

5G – the 5^th Generation mobile technology might arrive at our door steps sooner than we think, supporting 3 typical usage scenarios: enhanced Mobile Broadband (eMBB), massive Machine Type Communication (mMTC) and Ultra-reliable Low Latency Communication (URLLC) [1]. Though architecture wise 5G is different from its predecessors, like in any other wireless communication technology, among many other things, the Multiple Access Scheme (MAS) becomes the key for 5G and selecting the suitable scheme will ultimately decide the future and success of 5G.

MAS (aka Multiple Access Method or Channel Access Method) is required when we have to share a physical media. This is not only applicable to wireless, but also to many wired networks including bus networks and ring networks. In wireless, MAS allows several User Equipment (UE) connected to the same multi-point transmission medium to transmit over it and to share the available radio capacity. MAS is based on a multiplexing method, allowing several data streams or signals to share the same communication channel or physical medium.

Multiplexing or muxing works by combining multiple analog or digital signals into one signal over a shared medium. Space Division Multiplexing (SDM), Frequency Division Multiplexing (FDM), Time Division Multiplexing (TDM) and Code Division Multiplexing (CDM) are some examples. Therefore the MAS using FDM becomes Frequency Division Multiple Access (FDMA), TDM becomes Time Division Multiple Access (TDMA) and CDM becomes Code Division Multiple Access (CDMA).  In FDMA, each user has a small part of the resource (spectrum) allocated all the time. In TDMA, each user has nearly all the spectrum allocated at a small duration of time. In CDMA, each user has all the spectrum all the time. While multiplexing is provided by the physical layer, multiple accesses also involves Media Access Control (MAC) layer.

So, in summary, Multiplexing is combining many signals on one media, while MAS is allowing many to access the media/resource at one time. Therefore, it can be stated that Multiplexing is a technique and multiple accesses is the way to use that technique.

Over the years, different generations of mobile technologies used different MASs to achieve different capabilities, more importantly the system capacity and spectral efficiency. 

Generation	MAS	Application
1G	FDMA	AMPS
2G	TDMA	GSM
3G	CDMA	UMTS
4G	OFDMA	LTE

Note: AMPS-Advanced Mobile Phone System, GSM-Global System for Mobile communications, UMTS-Universal Mobile Telecommunications System, LTE-Long Term Evolution

All the above MASs (except CDMA), which can be called as conventional MASs, are Orthogonal Multiple Access (OMA) technologies.  In OMA, different users are allocated to orthogonal resources in either time, frequency or code domain in order to mitigate Multiple Access Interference (MAI). As the expectations or the objectives of 5G (higher data rates – 100/1000 times 4G, low latency – 1ms Round Trip Time (RTT), massive connectivity, high density – 1M devices/km^2, enhanced indoor coverage etc.) are quite different from that of 4G and others, the Radio Access Technology (RAT), which is characterized by MAS, need to be flexible, reliable, energy efficient, spectral efficient and support diverse Quality of Service (QoS). OMA schemes are not sufficient to support these requirements, especially massive connectivity and diverse QoS. [2], [3], [4]

It’s worthwhile to look little deeper on the scenario of mMTC and why the current LTE or LTE- Advanced (LTE-A) will find it difficult to support mMTC. In LTE/LTE-A there are lot of interactive processes between the Node B (NB)/enhanced NB (eNB) and the UE before data is transmitted. This is ok for long time continuous sessions, as the signaling overhead averaged over time is less. However, in mMTC used in Internet of Things (IoT), the “thing” (including UE) transmits a small amount of data over s short period of time and there are millions of such “things”. Now if we use LTE/LTE-A for such a scenario, the signaling overhead suddenly becomes high and access efficiency becomes low.

Therefore, several Non-Orthogonal Multiple Access Schemes are proposed for 5G, especially to address the Machine-to-Machine (M2M) requirements. These include; Superposition Coding based Non-Orthogonal Multiple Access (SPC-NOMA), Multi –User Shared Access (MUSA), Sparse Code Multiple Access (SCMA), Pattern Division Multiple Access (PDMA), Resource Spread Multiple Access (RSMA), Non-orthogonal Coded Multiple Access (NCMA) and Interleaver-Grid Multiple Access (IGMA). The different Non-Orthogonal Multiple Access Schemes can be compared as below; [1], [5]

Category	Power Domain Based	Code Domain Based					Interleaver Based
	SPC-NOMA	MUSA	SCMA	PDMA	RSMA	NCMA	IGMA
Scenario	DL: eMBB	UL: mMTC, URLLC DL: eMBB	UL: mMTC, URLLC DL: eMBB	UL: mMTC, URLLC DL: eMBB	UL: mMTC, URLLC	UL: eMBB mMTC, URLLC	UL: eMBB , mMTC, URLLC
Multiplexing Domain	Power	Code/ Power	Code/ Power	Code/ Power/ Spectral	Code/ Power	Code	Interleaver
Transmitting Overhead	Low/ Medium	High	Medium/ High	Medium/ High	Low	High	High

Note: UL-Uplink, DL-Downlink

When we design Non-Orthogonal Multiple Access Schemes, we need to consider following;

• Coverage
• Peak to Average Power Ratio (PAPR)
• Implementation Complexity
• Combination with Multiple –Input Multiple-Output (MIMO)
• Flexibility

Pictorially, TDMA, FDMA, CDMA, OFDMA and NOMA can be visualized as follows [6]. This shows how different users (denoted by different colors) gets the allocation of spectrum.

While there are multiple candidates for the 5G MAS in non-orthogonal domain, none of them seems to be a perfect choice at this point of time. As different schemes have both their merits and demerits, it’s likely that a combination of different MASs, including the conventional orthogonal schemes, will be used in 5G to achieve different objectives. However, as we get closer to 2020, where the finalized standards are expected to be released, more improvements on different schemes are possible with the expectation of all new schemes or approaches to combine different schemes. At the meantime, Research & Development Engineers and scientists will be quite busy discovering, improving and innovating the finest ingredients for the success of 5G.

References

[1] SUN Qi, WANG Sen, HAN Shuangfeng et al., “Unified Framework Towards Flexible Multiple Access Scheme for 5G” ZTE Communications, vol. 14, no. 4, pp. 26-33, October. 2016. doi: 10.3969/j.issn.1673-5188.2016.04.004

[2] WEI Zhiqiang, YUAN Jinhong, Derrick Wing Kwan Ng, et al., “A Survey of Downlink Non-Orthogonal Multiple Access for 5G Wireless Communication Networks”  ZTE Communications, vol. 14, no. 4, pp. 17- 23, October. 2016. doi: 10.3969/j.issn.1673-5188.2016.04.003

[3] Volker Jungnickel, Konstantinos Manolakis, Wolfgang Zirwas et al., “The Role of Small Cells, Coordinated Multipoint, and Massive MIMO in 5G” IEEE Communications Magazine, pp. 44-51, May. 2014.

[4] Peng Wang, Jun Xiao, Li Ping, “Comparison of Orthogonal and Non-Orthogonal Approaches to Future Wireless Cellular Systems”.

[5] YAN Chunlin, YUAN Zhifeng, LI Weimin et al., “Non-Orthogonal Multiple Access Schemes for 5G”  ZTE Communications, vol. 14, no. 4, pp. 11-16, October. 2016. doi: 10.3969/j.issn.1673-5188.2016.04.002

[6] Mahyar Shirvanimoghaddam, Sarah J. Johnson, “Multiple Access Technologies for Cellular M2M Communications”  ZTE Communications, vol. 14, no. 4, pp. 11-16, October. 2016. doi: 10.3969/j.issn.1673-5188.2016.04.006

Why should you select EoMPLS as the technology of choice for a green field CE deployment?

Introduction

Assume a Communication Service Provider (CSP) who does not have an aggregation network between the access and core. The CSP wants to use Carrier Ethernet (CE) technology to build the aggregation network. The CSP, most of the CSPs, intends to deliver/transport/aggregate the following services/traffic across the Carrier Ethernet Network (CEN) in an End-to-End (E2E) Internet protocol (IP) oriented architecture supporting the current and future demands;

o   Enterprise and corporate data services

§  Layer 3 Virtual Private Networks (VPN)

§  L2 VPN

§  L2 Point to Point (PP)

o   Consumer services

§  IP Television (IPTV) (multicast TV and unicast Video on Demand (VoD))

§  Broadband Internet

§  IP Voice

o   Wholesale services

§  Mobile backhauling

§  L2 VPN

§  L2 PP

IP/Multi-Protocol Label Switching (MPLS) has been a field proven technology and assume, like most of the CSPs, this CSP also has for the past several years implemented an IP/MPLS based core network and has built a team of experienced staff.

Options

To deploy a new CEN technology, the CSP has the following technology options available.
 

Feature	Provider Bridging (PB) / QinQ / Institute of Electrical and Electronic Engineers (IEEE) 802.1ad	Provider Backbone Bridging (PBB) / MAC in MAC / IEEE 802.1ah	PBB-Traffic Engineering (PBB-TE) / IEEE 802.1Qay	MPLS-Transport Profile (MPLS-TP)	IP/MPLS	Ethernet over Synchronous Digital Hierarchy (EoSDH)
Control Plane			Centralized Server	Centralized Server	MPLS	Centralized Server
Provisioning	Command Line Interface (CLI),  Element Management System (EMS)	CLI, EMS	EMS/Network Management System (NMS) only	EMS/NMS only	CLI, EMS	EMS/NMS only
Multipoint support	Yes	Yes	Yes	Yes (with VPLS)	Yes	No
Media Access Control (MAC) learning	CSP needs to learn all the MACs	reduced MAC learning	Not automatic	Yes (with VPLS)	Yes (Virtual Private Local Area Network (VPLS)	N/A
Protection	Spanning Tree Protocol (xSTP)	xSTP	Ethernet Ring Protection Scheme (ERPS)	Not mature Note 1	50 millisecond (ms), ring/mesh	50ms, ring
Operation			SDH like	SDH like		SDH like
Addressing scheme	Ethernet	Ethernet	Ethernet	different	Ethernet	N/A
TE	Poor	Poor	Good	Good	Good	Poor

Note 1: RFC 6372 (MPLS-TP Survivability Framework) released on September 2011

Feature	PB / QinQ / IEEE 802.1ad	PBB / MAC in MAC / IEEE 802.1ah	PBB-TE / IEEE 802.1Qay	MPLS-TP	EoMPLS	EoSDH
Maturity	High	Low	Low	Low	High	High
Interoperability	High	Low	Low	Low	High	High
Scalability	Low	High	High	High	High	Low
Separation of customer networks	High	High	High	High	High	High
Vendor stickiness	Low	High	High	High	Low	Low

Standard Defining Organizations have already published several standards for the IP/MPLS based CENs considering the current and future IP oriented service requirements. Following are some of the recent standards;

o   Broadband forum’s TR-221 (Technical Specifications for MPLS in Mobile Backhaul Networks)

o   Broadband forum’s,  WT-224 (MPLS in Carrier Ethernet Networks)

IP/MPLS in the CEN is required to support IP VPN and IP multicast features to support 4^th Generation (4G) mobile services such as Long Term Evolution (LTE) (all IP architecture).

To deliver the above mentioned multiple services on a single CEN with required service features, IP/MPLS is the most suited and matured technology. It is also needed to inter-op with the CSPs existing IP/MPLS core, especially for E2E seamless services.

It’s also noted that most of the access network uplinks are Ethernet or becoming Ethernet, while SDH/Plesiochronous Digital Hierarchy (PDH) networks are becoming outdated and obsolete. Hence investing on SDH is pointless.

Recommendations

Among many others, following are the most important recommendation for the CSP.

• The CEN shall be Transmission Agnostic.
• Aggregation of topology - to reduce the numbers of physical interfaces required at higher levels of the transport / switching hierarchy.
• Consolidation of network and transport protocols - to reduce the complexity of logical interfaces required at higher levels of the transport / switching hierarchy.
• To have different traffic types physically and logically aggregated, so that they can be transported by the Core Network.

• Development of CEN shall be closely mapped to service development strategy.

• The CEN shall be EoMPLS based.
• When selecting an CEN site, following shall be considered;
• number of access nodes in proposed topology
• total bandwidth demand and future bandwidth forecast that has to be aggregated and carried
• cost of alternate aggregation options and possible service impacts in a failure or other adverse condition
• All the network elements of different switching capacity and network shall have high availability features.
• The service delivery architecture within CEN shall be layer 2 based except for multicasting which shall be IP multicasting. However layer 2 multicasting features shall be available for customer multicasting services.
• The services and equipment shall be certified with Metro Ethernet Forum (MEF).
• Virtual Routing and Forwarding instance (VRF) shall not be brought to CEN level, unless it’s required for 4G Radio Access Network (RAN) backhauling (LTE) in the future as specified in Broadband forum TR-221.

CEN Requirements

Any CEN architecture needs to support key requirements of availability, stability, Quality of Service (QoS), performance, multicast support, Time Division Multiplexing (TDM) support, management and security. In the context of an EoMPLS based CEN,  that translates to what is explained below.

Availability (Resilience)

Since the CEN interfaces with the access layer, the resiliency is a key factor to avoid service outage due to the node failure or link failures. This is achieved by adopting multi-homing topology for the interconnection between CEN and the service edge network as well as the CEN and access layer where feasible. However, in order to avoid the complexity of the CEN, not more than two connections towards service edge network is recommended. Network side connections used for multi-homing requirements shall be terminated on physically separated line modules.

Equipment/node level high availability shall also be employed to ensure service and network availability due to failure of critical hardware and software modules of the CE node. The following redundancy mechanisms shall be available in all the network elements of CEN.

Hardware Component of CE Node	High Availability  mechanism
Route processor	1:1
Switching fabric	1:1
Power supply	1+1 Note 2
Power feed	1+1 Note 2
Cooling system	1+1 Note 2
Any other control plane module	1:1
Any other switching plane component	1:1

Note 2:   single component shall be able to take the full load of the CE node

The network shall use International Telecommunication Union-Standardization (ITU-T) G.8032 version 1 & 2 (ERPS) wherever possible to achieve sub 50ms protection and recovery for Ethernet in ring topology in case of a node or network failure.

Following software level high availability features shall be implemented;

• Non Stop Routing (NSR) for
• Label Distribution Protocol (LDP)
• Resource reservation Protocol (RSVP) TE
• Border Gateway Protocol (BGP)
• Open Shortest Path First (OSPF)
• Protocol Independent Multicast-Sparse Mode  (PIM-SM) and PIM-Source Specific Multicast (SSM)
• In Service Software Upgrade (ISSU)

Bidirectional Forwarding Detection (BFD) requirement shall be analyzed for following in the future stage

• LDP
• RSVP
• BGP
• OSPF
• PIM-SM and PIM-SSM

Non Stop Forwarding (NSF) requirement shall be analyzed in future stage.

Stability

Stability of the CEN and its Network Elements (NE) are very important. This should ensure consistent performance of the NE. Mean Time Between Failures (MTBF) and Mean Time To Repair (MTTR) values shall meet 99.999% node availability requirements.

QoS

CE QoS model is essential to provide dynamic quality of service feature without overbooking the bandwidth for applications. It should be able provide better service to selected traffic, depending on the individual requirements of different types of service and also to meet requirements in customer Service Level Agreements (SLA).

The CEN shall be able to handle Layer 2 (802.1Q), Layer 3 (Differential Services Code Point (DSCP)) and MPLS (Experiment (EXP)) QoS/Class of Service (CoS).  The CEN/NEss need to support classifying, marking, remarking, scheduling, shaping and policing for all the above QoS/CoS models at all egress and ingress ports whether access side or network side. Within the CEN, the nodes shall be able to support at least 4 hardware queues for traffic per port. The control and management traffic within the node shall be handled separately from the user traffic. Hierarchical QoS shall be analyzed in future stages.

Performance

Scalability of the CEN determine by providing sufficient bandwidth to be able to guarantee a committed level of performance for the full service portfolio of end users. CEN design to achieve the certain QoS requirements/Key Performance Indicators (KPIs) defined with the set of services/products. The CEN must be able to handle unpredictable surges in traffic, and appropriate load. The network utilization has to be maintained within 70% to facilitate the introduction of services and for the unpredictable surges in traffic.

Multicasting

To support IPTV and other multicast applications, the CEN shall support IP multicast protocols.  Layer 3 based (PIM) multicast technology is preferred over Layer 2 technology for scalability and flexibility reasons. Layer 2 multicasting features shall be available for customer multicasting services.

Supporting TDM services

TDM services shall be supported in the form of Circuit Emulation Services (CES) using Synchronous Ethernet (EtherSync/SyncE) or IEEE 1588v2 for frequency and time of day synchronization. At least E1 and STM-1 CES shall be supported. Enabling CES shall be done if the no. of TDM services few compared to the Ethernet services. If not, separate TDM equipment shall be used.

Management

All the CE elements should be able to address the management domain requirements. The standard functional entities such as: Fault management, Configuration management (Fulfillment support), Security management, Performance management and Inventory management on all Network Nodes will be required.

For service management, the network and the nodes shall support following Ethernet Operation Administration and Maintenance (OAM) standards;

• IEEE 802.1ag (Connectivity Fault Management (CFM))
• IEEE 802.3ah (Ethernet in the First Mile (EFM))

Security

The CEN addresses the security which provides confidentiality, integrity and availability of specific services. The following areas have been identified and will be equipped with necessary security mechanisms,

·         Node security

·         Access security

·         Interconnection security – User to Network Interface (UNI) and Internal-Network to Network Interface (I-NNI)

·         Protocol security – UNI and I-NNI

Standardization

MEF, the Broadband Forum, Internet Engineering Task Force (IETF), IEEE and ITU-T are the main Standard Defining Organization (SDO) with regard to the CEN.

The EoMPLS is a field proven and matured technology in implementing CENs. Though the standards are available, CSP needs to standardize this architecture and protocols. All the future network developments and deployments in the future need to align to these.

It is recommended that CSP get involved with these SDOs, especially the MEF and the Broadband forum. It is also recommended that CSP get the MEF certification for its services (E-Line, E-LAN, E-Tree and E-Access) and use MEF compliant equipment in the CEN (MEF 9- Ethernet Services at the UNI, MEF 14- Traffic Management Phase 1). Click here to see the 5 CE attributes defined by MEF.