White Paper Leveraging WiMAX into LTE Success By: Todd Mersch, Director of Product Line Management Overview In business and technology, inflection points are a rare occurrence. Furthermore, the identification of inflection points while they are still in progress is even rarer. The mobile broadband industry is at one of these exceptional inflection points right now, but my chief concern is that parties on both sides of the competitive fence will get stubborn and miss the opportunity in front of them. CONTENTS Radio and PHY Subsystems pg. 2 Upper Layer Protocols pg. 3 Conclusion pg. 5 The inflection point I reference is the bifurcation of markets that thirst for the next generation of mobile broadband serviced both by WiMAX and Long-Term Evolution (LTE) technology.
2 For those involved in either WiMAX or LTE, the recent past has more often than not been a focused debate about which of these Orthogonal Frequency Division Multiplexing (OFDM)-based radio technologies is better than the other. I am not here to espouse the technical superiority of one or the other. The fact is, both WiMAX and 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 operators throughout the world behind the LTE standard. In fact, there are dozens of announced LTE migration plans. As such, WiMAX-only solution providers stand to miss out on this massive market opportunity if they do not evolve 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 LTE solution is pardon the pun an evolution of their WiMAX portfolio, not a complete do-over. However, time is of the essence since the very public plans of Verizon in the United States and NTT DoCoMo in Japan are spurring a flurry of activity by competing operators who do not want to be left behind. The opportunity is there for WiMAX companies to leverage their realworld expertise in deploying OFDM networks, their proven OFDM intellectual property, and their existing economies of scale from on-going WiMAX design wins in order to differentiate 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 the radio 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 these subsystems. Radio and PHY Subsystems The radio and PHY subsystems from WiMAX and LTE are, simply put, very similar. They share a common underlying technology, OFDM, and the migration of the existing WiMAX radio and PHY to an LTE version is not as significant a transition as one might imagine. Key similarities between a WiMAX and LTE radio and physical layer are as follows: OFDMA Downlink: both WiMAX and LTE utilize the same transmission techniques in the downlink direction, including support for 64QAM modulation MIMO Support: both WiMAX and LTE utilize multiple input multiple output (MIMO) radio technology to increase bandwidth and reduce signal-to-noise ratio Overlap in channel and frequency ranges: both support 5MHz and 10MHz channel bandwidths and transmission in frequency bands in the 2GHz range However, despite these commonalities there are modifications required to transition from a WiMAX radio and PHY subsystem to an LTE one. Interestingly, in a lot of cases these enhancements align with the move from Mobile WiMAX 1.0 standards to the new Mobile WiMAX 1.5 capabilities, so a move to LTE now aligns with supporting WiMAX 1.5 in the future as well. Enhancements required to migrate to LTE in the radio and PHY subsystem are as follows: 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.
3 Figure 1. WiMAX and LTE RAN Architectures Compared 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. 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. 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. 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. Increased mobility: the LTE standards require support for mobility up to 500kmph, which is not a requirement for WiMAX systems today. Upper Layer Protocols Despite the longer list of differences, the enhancement from WiMAX radio and PHY subsystems to LTE equivalents is an incremental upgrade. The changes required are generally extensions of existing capabilities in 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, the knowledge gap becomes larger and the sheer time required to transition becomes greater. Figure 1 places the WiMAX RAN architecture sideby-side with the LTE equivalent.
4 Figure 2. WiMAX and LTE Base Station Software Compared The similarities include a flat end-to-end all-ip access network, radio resource management resident in the base stations, and support for direct communication among base stations to coordinate handover and radio aspects. However, when one looks a little deeper, the differences begin to emerge. Figure 2 places the software architectures of WiMAX and LTE base stations side-by-side. As is highlighted in Figure 2, the software architectures begin to diverge significantly at the Layer 2 and Layer 3 protocols, both in the control and data planes. Critical differences are summarized in Table 1. Interface WiMAX Protocols 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 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 BS to BS R8, GRE, UDP, IPSec, IP X2AP, SCTP, egtp-u, IPSec, IP Reliable transport over SCTP significantly 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 BS to Core ASN CNTL, GRE, UDP, S1AP, SCTP, egtp-u, IPSec, IP Separate control and data plane paths Network IPSec, IP Reliable transport over SCTP significantly more complex than UDP egtp-u in the data path versus GRE Table 1.
5 From Table 1, it is clear that the gap widens as one looks further into the details of the upper layer functionality in an LTE enodeb versus a WiMAX BS. The best area of re-use for a WiMAX solution provider will come within the MAC, and more importantly the MAC scheduler. Since both WiMAX and LTE utilize OFDMA in the downlink, 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 provider to 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 not lost, however, as there exists a burgeoning ecosystem of solution providers that support the entire suite of LTE protocol 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 are starting and trials are broadly underway. It would be pennywise and pound foolish to waste energy trying to argue the superiority of one technology solution over the other when there is such a phenomenal opportunity for companies with key WiMAX assets to expand their addressable market to the over 4 billion GSM subscribers worldwide. WiMAX solution providers have the OFDM experience and intellectual property to compete and win large chunks of LTE infrastructure rollouts. By leveraging their existing radio and PHY systems 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. Corporate Headquarters 5435 NE Dawson Creek Drive Hillsboro, OR 97124 USA 503-615-1100 Fax 503-615-1121 Toll-Free: 800-950-0044 www.radisys.com info@radisys.com 2011 Radisys Corporation. Radisys, Trillium, Continuous Computing and Convedia are registered trademarks of Radisys Corporation. *All other trademarks are the properties of their respective owners. September 2011