LTE chips get ready to roll
The specification for 4G wireless, or long-term-evolution (LTE), is expected to be finished this month, and product rollouts are anticipated for 2010 or 2011. If you are wondering what the status of the chips that will enable LTE is, well, the pipelines seem planned and chip designers have their heads down to deal with the unique challenges of LTE systems.
Despite these challenges, there seems to be a definite creative enthusiasm about the possibilities and new ways of looking at networks, as broadband mobile wireless gets ready to roll. And, in a stroke of compatibility genius that seems rare in our industry, many of the chips will also be compatible with mobile WiMAX, thanks to some similarities in the RF signaling. Regardless of whether or not the two competing standards, LTE and WiMAX converge, we are likely to see chips doing double duty.
So what are the most important issues for LTE that system designers should know about?
"LTE and WiMAX differ in their uplink multiple access approach, with WiMAX using OFDMA for the uplink, while LTE adopts a single-carrier frequency division multiple access (SC-FDMA) approach. The crux of this decision by LTE designers is the technical challenges that broadband wireless presents to the device uplink's RF physical layer," notes Darcy Poulin, principal engineer RF systems & technical marketing, at SiGe Semiconductor.
Cecile Masse, senior RF systems engineer, RF group, analog devices (ADI), elaborates: "It is important to understand the nature of the modulated signal we are dealing with in LTE systems: its bandwidth, statistics, peak-to-average ratio, sensitivity to impairments like phase, amplitude distortion and the implications of using a scalable OFDM signal with sub-carriers modulated with different schemes or with partial usage of the available sub-carriers within a given channel bandwidth."
The deployment environment of LTE is also a concern, Masse noted. "Additionally, LTE is likely to be overlaid with existing services such as WCDMA and GSM. In these cases, system designers must take into consideration the implications of tolerance not only to the various LTE waveforms, but also to other in-band modulation types."
Rupert Baines, vice president of marketing at PicoChip, points out that important considerations for LTE go beyond the chip level. "It will be about small cells, not traditional big units. So small cells, like femtocells, are critical. And, at a genuine system level, driving intelligence towards the edge of the network will be essential. But because of all that, LTE represents an opportunity to sweep away a lot of the 'I wouldn't start from here' problems that have been inherent in the rather slow-moving, conservative world of network infrastructure."
As with all mobile applications, integration will be critical for LTE chips, and chip designers are looking for innovative ways to combine functionality. PicoChip takes a software-defined approach and reports that it has integrated the complete LTE physical layer.
The company recently announced plans for an LTE basestation reference design–the PC8618 picocell and PC8608 femtocell platforms–that are being designed in conjunction with PicoChip's development and research partners, MimoOn and Hong Kong's ASTRI.
Linear Technology has released the LT5557, a downconverting active mixer that covers the 400-MHz to 3.8-GHz bands
James Wong, a product marketing manager for Linear Technology, explains his team's approach to LTE integration, "We design performance to solve some of the most difficult problems facing LTE and basestation designers. For example, RF isolation in RF circuits is difficult to contain, and it's expensive to filter any undesirable RF artifacts. Our mixers (LT5557 and LT5579) have the RF transformers on-chip, facilitating exceptional isolation performance and eliminating external balun transformers."
Selection guide for LTE
For designers that need to select LTE chips, there are some key things to consider during the process. PicoChip's Baines cautions: "It's important to understand that the chips themselves are only a small part of any LTE solution. Just as important is the software to run on them. It is very important to differentiate between a chip you have to program from scratch, a chip with customizable library software, a chip with simple example code that is not actually suitable for a final implementation and a programmable chip with production-ready customizable code."
Brad Bannon, systems applications engineer, high speed converters group at Analog Devices (ADI), suggests that the most desirable specs to look for are high linearity (to meet spectrum quality and out-of-band emissions with the high peak-to-average ratio OFDM signal), low noise (for optimal radio link performance), gain flatness (for limited gain calibration when transmitting OFDMA signals up to maximum channel bandwidth), accurate power measurement (modulation agnostic, for optimum power transmission and high-efficiency design), and low power dissipation/low supply voltage.
To this list, Linear Technology's Wong adds spurious-free dynamic range (this limits the receiver's ability to resolve data integrity at low signal levels) and isolation/LO leakage. This last factor can cause out-of-band emissions exceeding the level required by local regulatory agencies, disqualifying a basestation from use in a given market. Baines points out that it is also important for the LTE device to be able to scale and adjust as the LTE specification changes.
While many chip and reference design products are in the design and roadmap stage, some products have already been released, particularly those that share a common platform with WiMAX.
ADI is in production with the AD9352 and AD9354 WiMAX transceivers, which can also be used in LTE designs. The company reports that it is researching LTE solutions now. PicoChip expects to have its reference design product available next year. For now, it is shipping the PC203 picoArray signal processor.
Linear Technology reports that a number of its products are currently in use in first-generation LTE basestation designs. These include the LT5557 downconverting active mixer and the LT5579 upconverting active mixer; the LT5575 I/Q demodulator; LT5570 and LT5581 RMS detector; wideband intermediate frequency (IF) amplifiers; wideband active filters; and high-speed analog to digital converters (ADCs).
No doubt, we will be hearing a lot about LTE as the standards evolve, are frozen and more products hit the market. The technical discussion is only just beginning, and upcoming technical conferences, such as next month's LTE: Towards Mobile Broadband conference in Dallas, TX, are likely to advance the conversation even more.
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