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Will WiMAX, LTE coexist in multimode?

Posted: 18 Jun 2008 ?? ?Print Version ?Bookmark and Share

Keywords:WiMAX? LTE? 4G standard? multimode technology?

After two years of verbal skirmishing and specsmanship, there are signs that the WiMAX and Long Term Evolution (LTE) camps may be seeking a negotiated settlement.

For many players, there are compelling reasons for peace. Saving money tops the list. A head-to-head battle over the next few years would require an outlay of multiple billions of dollars in equipment deployment. It would also be confusing for end users, and might even determine a winner and loser in a very high-stakes game.

Until recently, much has been made of the differences between the two 4G wireless-communications candidates, usually by comparing performance characteristics and ignoring architectural similarities.

But from a technology perspective, how different are the WiMAX standard and the Third-Generation Partnership Project's Long Term Evolution specifications? More important, what multimode technologies are beginning to surface that make a standards-oriented battle for market supremacy pointless in the long run?

'80% similar'
Earlier this month, Sean Maloney, executive VP at WiMAX champion Intel Corp., hinted that the two standards should be harmonized because they are "about 80 percent similar." Maloney added that the chip giant is looking into ways to integrate the technologies. It is technically possible to create a chipset that could be used for both, he said.

Maloney's comments might be interpreted as a response to speculation at February's Mobile World Congress that WiMAX could find a place within the LTE standard. Vodafone Group CEO Arun Sarin tossed out that suggestion during his keynote in Barcelona, Spain. Intel, of course, considers WiMAX the more mature standard.

While the feelers may not qualify as a love fest, they come at a time when emerging semiconductor technologies promise to make LTE-WiMAX multimode operation a reality in the not-too-distant future. In that context, spending billions to deploy standard-specific networks becomes unattractive.

"The differences are more political than anything else," said Nadine Manjaro, senior analyst for wireless infrastructure at ABI Research. Although Manjaro predicted the standards would merge, she also said LTE will not be a formal standard until 2009 or 2010. Thus, she said, it would be 2015 before any merger takes place.

Peas in the pod
The single most important similarity between LTE and WiMAX is OFDM signaling. Both technologies also employ Viterbi and turbo accelerators for FEC.

From a chip designer's perspective, that makes the extensive reuse of gates highly likely if one had to support both schemes in the same chip or chip et. From a software-defined radio (SDR) perspective, the opportunity is even more enticing. Flexibility, gate reuse and programmability seem to be the answers to the WiMAX-LTE multimode challengeand that might spell SDR.

LTE and WiMAX may be two peas in an OFDM pod, but they are not twins. Here are three significant differences:

  1. 1. Both use OFDMA in the downlink. But WiMAX optimizes for maximum channel usage by processing all the information in a wide channel. LTE, on the other hand, organizes the available spectrum into smaller chunks.

    WiMAX pays a price for high channel utilization, however, because processing that much information might require a 1,000-point fast Fourier transform. LTE can get by with a 16-point FFT. This translates into higher power consumption, because it's difficult to design fixed-function WiMAX hardware that is also efficient in LTE designs. An architecture that exploits the principles of SDR, however, could reconfigure its FFT function for better power efficiency.

  2. LTE uses single-carrier frequency division multiple access (SC-FDMA) for uplink signaling, while WiMAX sticks with OFDM access (OFDMA). A major problem with OFDM-based systems is their high peak-to-average power ratios. The average power spec cited in marketing presentations does not show the whole picture. Unfortunately, the system's power amplifier has to be designed to handle peak powerand the PA is the single-largest power consumer in a handset.

    LTE opted for the SC-FDMA specifically to boost PA efficiency. "If you can improve the efficiency from 5 percent up to 50 percent simply by changing modulation schemes, then you save a lot of battery time," said Anders Nilsson, principal system architect at multimode specialist Coresonic AB. WiMAX's OFDMA has a peak-average ratio of about 10dB, while LTE's SC-FDMA's peak-average ratio is about 5dB.

    The difference also affects the baseband chip, Nilsson added, because of the need to support two modulation schemes in the uplink. Programmable solutions are flexible enough to reuse gates and keep power low in LTE mode.

  3. Although both the IEEE 802.13e standard and the evolving LTE standard support frequency division duplexing (FDD) and time division duplexing (TDD), WiMAX implementations are predominantly TDD. LTE seems to be heading in the FDD direction because it is true full-duplex operation: Adjacent channels are used for uplink and downlink. LTE can therefore quote a better spec for downlink data rates, albeit at a cost of placing very severe latency requirements for forward error correction. The bottom line is that the WiMAX radio is much simpler.

    These differences make designing a chip or chipset to support both standards more difficult, but they also have network infrastructure consequences that might be more easily resolved by harmonization instead of competition. Certainly, from the handset designer's perspective, there is no clear winner.

    The battery life and power efficiency of the chip or chipset are critical to market success, said Fannie Mlinarsky, an independent consultant specializing in wireless testing and design. Power is a big issue for WiMAX and LTE because megabit-per-second capability means running the DSP hard and making the chips more power hungry.

Multimode via SDR
While it is true that SDR has a well-deserved reputation for being expensive and overhyped, it is just as true that telecom chip designers are already adopting SDR techniques. They need to, simply to accommodate changes to ever-evolving standards.

The classic definition of SDR is having arrays of general-purpose processors running virtually all functions in software. This approach can be expensive and may not be able to hit the price/performance targets of high-data-rate technologies such as WiMAX and LTE, said Mlinarsky.

New approaches that use innovative hardware architectures, on the other hand, seem just about ready for prime time. One early entry is from Wavesat, which has a long history of designing OFDMA chips. The company has inked agreements with Compal Communications, a mobile-products ODM, to develop mobile WiMAX products, and with Willcom, a Japanese telecom company, to develop XG-PHS broadband wireless products using Wavesat's Odyssey 8500 chipset.

The chipset is, in reality, a 4G platform that can implement any OFDM-based technology, said Vijay Dube, senior VP of marketing at Wavesat. Odyssey 8500 is based on eight DSP cores.

Coresonic AB also has a multimode platform based on a new architecture: single instruction stream, multiple task. SIMT can achieve the performance of a very long instruction word architecture, but with lower control overhead and much lower program and memory usage, said CEO Rich Clucas.

Multimode baseband solutions for LTE and WiMAX are challenging, said Mlinarsky, but designing the front-end chip is truly daunting for several reasons, not the least of which is the wide spectrum covered by the two standardsabout 4GHz. LTE would likely support the 900-1,900MHz bands. WiMAX has had to scramble to find available spectrum and, depending on region, may operate from 2.3GHz to 3.5GHz.

BitWave Semiconductor's programmable RF transceiver promises a way through the multimode thicket. Prototypes of BitWave's Softransceiver RFIC are already in the hands of selected ODMs, said chief marketing officer Russell Cyr. Handsets and femtocells that incorporate the technology should launch next year. BitWave's technology digitally tunes passive circuit elements to make the analog functions such as LNAs, filters and mixers programmable.

With these new technologies in play, a little harmonization will go a long way. LTE is still very much in its development stage. Nor is WiMAX standing still, said Will Strauss, president of Forward Concepts. "The 802.16m task group is working to complete improvements that will make it look a lot like cellular, with such things as hand-offs," Strauss said.

- Jack Shandle
WirelessNet DesignLine

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