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Leveraging WiMAX into LTE Success

By Todd Mersch, Director of Product Line ManagementIn business and technology, inflection points are a rare occurrence. Furthermore, the identification of inflectionpoints while they are still in progress is even rarer. The mobile broadband industry is at one of theseexceptional inflection points right now, but my chief concern is that parties on both sides of the competitivefence will get stubborn and miss the opportunity in front of them.

The inflection point I reference is the bifurcation of markets that thirst for the next generation of mobilebroadband serviced both by WiMAX and Long-Term Evolution (LTE) technology.

For those involved in either WiMAX or LTE, the recent past has more often than not been a focused debateabout which of these Orthogonal Frequency Division Multiplexing (OFDM)-based radio technologies is betterthan the other. I am not here to espouse the technical superiority of one or the other. The fact is, both WiMAXand LTE are finding their place in the market and both will enjoy deployments.

However, it is an undeniable fact that there is unprecedented global alignment from mobile operatorsthroughout the world behind the LTE standard. In fact, there are dozens of announced LTE migration plans. Assuch, WiMAX-only solution providers stand to miss out on this massive market opportunity if they do notevolve their WiMAX solutions to LTE and soon.

The good news is that for a WiMAX radio access network (RAN) equipment provider, the creation of an LTEsolution is -- pardon the pun -- an evolution of their WiMAX portfolio, not a complete do-over. However, time isof the essence since the very public plans of Verizon in the United States and NTT DoCoMo in Japan arespurring a flurry of activity by competing operators who do not want to be left behind. The opportunity is therefor WiMAX companies to leverage their real-world expertise in deploying OFDM networks, their proven OFDMintellectual property, and their existing economies of scale from on-going WiMAX design wins in order todifferentiate themselves from the traditional Tier 1 Network Equipment Providers (NEPs) who often dominate

the GSM landscape.

So what does it take to evolve a WiMAX base station to LTE? There are two key areas of focus. The first is theradio and physical layer (PHY) subsystem and the second is the upper-layer protocols and security aspects.The remainder of this article examines the necessary modifications and areas of re-use for both of thesesubsystems.

Radio and PHY Subsystems

The radio and PHY subsystems from WiMAX and LTE are, simply put, very similar. They share a commonunderlying technology, OFDM, and the migration of the existing WiMAX radio and PHY to an LTE version isnot as significant a transition as one might imagine. Key similarities between a WiMAX and LTE radio andphysical layer are as follows:

1. OFDMA Downlink: both WiMAX and LTE utilize the same transmission techniques in the downlinkdirection, including support for 64QAM modulation

2. MIMO Support: both WiMAX and LTE utilize multiple input multiple output (MIMO) radio technology toincrease bandwidth and reduce signal-to-noise ratio

3. Overlap in channel and frequency ranges: both support 5MHz and 10MHz channel bandwidths andtransmission in frequency bands in the 2GHz range

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However, despite these commonalities there are modifications required to transition from a WiMAX radio andPHY subsystem to an LTE one. Interestingly, in a lot of cases these enhancements align with the move fromMobile WiMAX 1.0 standards to the new Mobile WiMAX 1.5 capabilities, so a move to LTE now aligns withsupporting WiMAX 1.5 in the future as well. Enhancements required to migrate to LTE in the radio and PHYsubsystem are as follows:

1. Replacing OFDMA with SC-FDMA in the Uplink (UL): the 3rd

Generation Partnership Program(3GPP) has chosen to use Single Carrier Frequency Division Multiple Access (SC-FDMA) in the ULin LTE. SC-FDMA has a significantly lower peak-to-average power ratio (PAPR) and allows for arelatively high degree of commonality with the downlink OFDM scheme including reuse of the sameradio parameters.

2. Support for FDD in addition to TDD: the latest WiMAX standards allow for FDD although themajority of existing equipment and approved interoperability profiles are only in TDD.

3. Support for 4x4 MIMO: representinganother enhancement that aligns with the migration to WiMAX1.5, current WiMAX solutions will need to add support for 4x4 MIMO to fully comply with LTEspecifications. This, however, is an upgrade that may be put on the roadmap since most initialsolutions and deployments will be 2x2 MIMO.

4. Increased flexibility in channel bandwidths and frequencies: LTE from the start is flexible enoughto support channel bandwidths from 1.25MHz up to 20MHz and for transmission in frequencies from700MHz up.

5. Smaller frame size: with an eye on support for voice services, the LTE frame size is 1ms versus a5ms frame size in WiMAX. The smaller frame size leads to a challenging timing requirement and theneed for real-time performance up into the Medium Access Control (MAC) layer.

6. Increased mobility: the LTE standards require support for mobility up to 500kmph, which is not arequirement for WiMAX systems today.

Upper Layer Protocols

Despite the longer list of differences, the enhancement from WiMAX radio and PHY subsystems to LTEequivalents is an incremental upgrade. The changes required are generally extensions of existing capabilitiesin the WiMAX radio and PHY, and this is an area of deep expertise in WiMAX equipment providersorganizations. However, as one moves up the stack into the higher layer protocols and the application, the

knowledge gap becomes larger and the sheer time required to transition becomes greater.

Figure 1 places the WiMAX RAN architecture side-by-side with the LTE equivalent.

Figure 1: WiMAX and LTE RAN Architectures Compared

WiMAX RAN

eNodeBeNodeB

Serving GW

S1-MME S1-U

MME

S1-U

X2

S1-MME

LTE-Uu LTE-Uu

ASN GW

R6 R6

R8

R1 R1

LTE RAN

BSBS

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The similarities include a flat end-to-end all-IP access network, radio resource management resident in thebase stations, and support for direct communication among base stations to coordinate handover and radioaspects. However, when one looks a little deeper, the differences begin to emerge. Figure 2 places thesoftware architectures of WiMAX and LTE base stations side-by-side.

Figure 2: WiMAX and LTE Base Station Software Compared

As is highlighted in Figure 2, the software architectures begin to diverge significantly at the Layer 2 and Layer3 protocols, both in the control and data planes. Critical differences are summarized in Table 2.

Interface WiMAXProtocols

LTE Protocols LTE Key Differences

BS to UE 16CNTL, MAC,PHY

RRC, PDCP, RLC,MAC, PHY

L2 more complex with segmentation /desegmentation at RLC layer

MAC and MAC scheduler operate at 1msframe size

Security requires support for Snow3G at

PDCP layer RRC and MAC must support FDD andTDD profiles

Specific MAC channels for differentservice types including multicast /broadcast services

Completely different encode / decodeschemas and protocol informationelements

BS to BS R8, GRE, UDP,IPSec, IP

X2AP, SCTP, eGTP-u,IPSec, IP

Reliable transport over SCTP significantlymore complex than UDP

eGTP-u for tunneling the data fromeNodeB to eNodeB in LTE versus GRE inWiMAX

Minimal control plane latency (X2) tosupport higher speed mobility services

RRM related messages carried over X2versus relayed in the ASN GW in WiMAX

Completely different encode / decodeschemas and protocol informationelements

PHY

MAC

StackManager

IP

X2AP

SCTP

RRC

RLC

S1APeGTP

Dynamic

Resource

Allocation

Radio

Admission

Control

Connection

Mobility

Cont.

RB

ControlRRM

eNB Measurement

Config &

Provision

eNodeB Application

PDCP

IPSec

PHY

MAC

StackManager

IP

X2AP

SCTP

RRC

RLC

S1APeGTP

Dynamic

Resource

Allocation

Radio

Admission

Control

Connection

Mobility

Cont.

RB

ControlRRM

eNB Measurement

Config &

Provision

eNodeB Application

PDCP

IPSec

PHY

MAC

StackManager

IP

R8

UDP

16 CNTL ASN CNTL GRE

Dynamic

ResourceAllocation

Radio

AdmissionControl

Connection

MobilityCont.

RBControl

RRMMeasurement

Config &Provision

WiMAX BS Application

IPSec

IPCS

PHY

MAC

StackManager

IP

R8

UDP

16 CNTL ASN CNTL GRE

Dynamic

ResourceAllocation

Radio

AdmissionControl

Connection

MobilityCont.

RBControl

RRMMeasurement

Config &Provision

WiMAX BS Application

IPSec

IPCS

WiMAX BS LTE BS

Layer 1 Layer 2 / Layer 3 Application

Key

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Interface WiMAXProtocols

LTE Protocols LTE Key Differences

BS to CoreNetwork

ASN CNTL, GRE,UDP, IPSec, IP

S1AP, SCTP, eGTP-u,IPSec, IP

Separate control and data plane paths Reliable transport over SCTP significantly

more complex than UDP eGTP-u in the data path versus GRE Completely different encode / decode

schemas and protocol informationelements

From Table 2, it is clear that the gap widens as one looks further into the details of the upper layer functionalityin an LTE eNodeB versus a WiMAX BS. The best area of re-use for a WiMAX solution provider will comewithin the MAC, and more importantly the MAC scheduler. Since both WiMAX and LTE utilize OFDMA in thedownlink, the MAC schedulers for downlink transmission may be leveraged from WiMAX to LTE.

Beyond the schedulers, however, LTE requires essentially a whole new set of protocols even into the transport(i.e. SCTP vs. UDP) and the time to develop these from scratch would force the WiMAX base station providerto miss the LTE window of opportunity to leverage their differentiating experience, tool sets (e.g., element

management systems, performance management systems, etc.), and existing economies of scale. All is notlost, however, as there exists a burgeoning ecosystem of solution providers that support the entire suite of LTEprotocol stacks, helping speed time-to-market for LTE network equipment.

Conclusion

The time for WiMAX solution providers is now, for the LTE market is expanding quickly. Deployments arestarting and trials are broadly underway. It would be pennywise and pound foolish to waste energy trying toargue the superiority of one technology solution over the other when there is such a phenomenal opportunityfor companies with key WiMAX assets to expand their addressable market to the over 4 billion GSMsubscribers worldwide. WiMAX solution providers have the OFDM experience and intellectual property tocompete and win large chunks of LTE infrastructure rollouts. By leveraging their existing radio and PHYsystems and working with the ecosystem of upper layer protocol software providers, a WiMAX equipment

vendor can rapidly get to market and compete for the next wave of LTE design wins.

About Continuous Computing

Continuous Computing

is the global source of integrated platform solutions that enable network equipmentproviders to overcome the mobile broadband capacity challenge quickly and cost effectively. Leveraging morethan 20 years of telecom innovation, the company empowers customers to increase return on investment byfocusing internal resources on differentiation for 3G, Long Term Evolution (LTE), Femtocell and Deep PacketInspection (DPI) applications. Expertise and responsiveness set the company apart: only ContinuousComputing combines best-in-class ATCA platforms with world-famous Trillium

protocol software to create

highly-optimized, field-proven wireless and packet processing network infrastructure. www.ccpu.com

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