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Leveraging WiMAX into LTE Success

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  • 1. Leveraging WiMAX into LTE SuccessBy 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 dominatethe 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 SubsystemsThe 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 downlink direction, including support for 64QAM modulation 2. MIMO Support: both WiMAX and LTE utilize multiple input multiple output (MIMO) radio technology to increase bandwidth and reduce signal-to-noise ratio 3. Overlap in channel and frequency ranges: both support 5MHz and 10MHz channel bandwidths and transmission in frequency bands in the 2GHz range
  • 2. 2However, 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 UL in LTE. SC-FDMA has a significantly lower peak-to-average power ratio (PAPR) and allows for a relatively high degree of commonality with the downlink OFDM scheme including reuse of the same radio parameters. 2. Support for FDD in addition to TDD: the latest WiMAX standards allow for FDD although the majority of existing equipment and approved interoperability profiles are only in TDD. 3. Support for 4x4 MIMO: representing another enhancement that aligns with the migration to WiMAX 1.5, current WiMAX solutions will need to add support for 4x4 MIMO to fully comply with LTE specifications. This, however, is an upgrade that may be put on the roadmap since most initial solutions and deployments will be 2x2 MIMO. 4. Increased flexibility in channel bandwidths and frequencies: LTE from the start is flexible enough to support channel bandwidths from 1.25MHz up to 20MHz and for transmission in frequencies from 700MHz up. 5. Smaller frame size: with an eye on support for voice services, the LTE frame size is 1ms versus a 5ms frame size in WiMAX. The smaller frame size leads to a challenging timing requirement and the need 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 a requirement for WiMAX systems today.Upper Layer ProtocolsDespite 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 providers’organizations. However, as one moves up the stack into the higher layer protocols and the application, theknowledge 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. WiMAX RAN LTE RAN MME Serving GW ASN GW S1-MME S1-U R6 R6 S1-U S1-MME X2 R8 BS BS eNodeB eNodeB R1 R1 LTE-Uu LTE-Uu Figure 1: WiMAX and LTE RAN Architectures Compared MC00261
  • 3. 3The 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. WiMAX BS LTE BS WiMAX BS Application eNodeB Application Connection Radio Measurement Dynamic Connection Radio eNB Measurement Dynamic RB RB RRM Mobility Adm ission Config & Resource RRM Mobility Admission C onfig & Resource Control Control Cont. Control Provision Allocation Cont. Control Provision Allocation Stack Stack RRC Manager 16 CNTL IPCS Manager X2AP S1AP R8 ASN CNTL GRE PDCP eGTP RLC MAC UDP SCTP IPSec MAC IPSec PHY IP PHY IP Key Layer 1 Layer 2 / Layer 3 Application Figure 2: WiMAX and LTE Base Station Software ComparedAs 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 WiMAX LTE Protocols LTE Key Differences Protocols BS to UE 16CNTL, MAC, RRC, PDCP, RLC, L2 more complex with segmentation / PHY MAC, PHY desegmentation at RLC layer MAC and MAC scheduler operate at 1ms frame size Security requires support for Snow3G at PDCP layer RRC and MAC must support FDD and TDD profiles Specific MAC channels for different service types including multicast / broadcast services Completely different encode / decode schemas and protocol information elements BS to BS R8, GRE, UDP, X2AP, SCTP, eGTP-u, Reliable transport over SCTP significantly IPSec, IP IPSec, IP more complex than UDP eGTP-u for tunneling the data from eNodeB to eNodeB in LTE versus GRE in WiMAX Minimal control plane latency (X2) to support higher speed mobility services RRM related messages carried over X2 versus relayed in the ASN GW in WiMAX Completely different encode / decode schemas and protocol information elements MC00261
  • 4. 4 Interface WiMAX LTE Protocols LTE Key Differences Protocols BS to Core ASN CNTL, GRE, S1AP, SCTP, eGTP-u, Separate control and data plane paths Network UDP, IPSec, IP IPSec, IP Reliable transport over SCTP significantly more complex than UDP eGTP-u in the data path versus GRE Completely different encode / decode schemas and protocol information elementsFrom 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., elementmanagement 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.ConclusionThe 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 equipmentvendor can rapidly get to market and compete for the next wave of LTE design wins.About Continuous ComputingContinuous 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 createhighly-optimized, field-proven wireless and packet processing network infrastructure. www.ccpu.comContinuous Computing is an active member of 3GPP, CP-TA, eNsemble Multi-Core Alliance, ETSI, FemtoForum, Intel Embedded Alliance, Multicore Packet Processing Forum, NI Alliance, PICMG and the SCOPEAlliance.Continuous Computing, the Continuous Computing logo and Trillium are trademarks or registered trademarksof Continuous Computing Corporation. Other names and brands may be claimed as the property of others. MC00261

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