Optical DSPs promise tera-ops performance
By Patrick Mannion
EE Times
October 8, 2001 (11:59 a.m. ET)
MANHASSET, N.Y. — Startup Lenslet Labs has demonstrated an optically based digital signal processing engine (ODSPE) that has the potential to take DSPs from the current giga-operations-per-second (Gops) limit to tera operations per second (Tops) by 2005, the company announced at the inaugural Communications Design Conference in San Jose last week.
Targeted at wireless (third-generation basestations and fixed wireless access) and wireline (xDSL) communications, along with JPEG, MPEG, machine-vision and voice-recognition applications, the technology combines proprietary encode/decode and signal-conditioning techniques with off-the-shelf optical components. The result is a solution that has the flexibility and programmability of field-programmable gate arrays (FPGAs) and DSPs with the performance of ASICs — without the process limitations of all current silicon technologies.
"Moore's Law [as it applies to conventional silicon processes] is helping designers reach higher and higher integration levels and processor speeds," said Ron Levy, director of marketing and business development at Lenslet (Ramat-Gan, Israel), "but they still can't meet the capacity demands of both wireless and wireline communications systems." All the while, he said, the power consumption is rising on a parallel path with those higher integration levels. "Also, by 2010, the physics of those silicon processes will mean that Moore's Law will likely have run its course," he said. "ODSPE will take processing to a higher level and with a much faster growth curve [than Moore's Law]."
The company has already demonstrated an 8-Tops, 20-watt device. "Using conventional DSPs to get that performance," said Levy, "you would need 6,600 devices, thereby requiring 26,400 cm2 of board space and 3,300 W. FPGAs would need 40 devices and 200 W, while ASICs would need 21 devices and 133 W." As for pricing, Levy claims the ODSPE comes in at 1,000th that of current DSPs, again for the same performance level.
The very nature of the ODSPE's operation, combined with its high processing speed, will also allow designers to operate at a much higher level of abstraction to greatly accelerate the migration from basic research to product development, Levy said.
"Their technology is one of the most exciting developments I've seen in a while," said Will Strauss, president of Forward Concepts (Tempe, Ariz.). "If they can do what they say they can do, they could force everyone to reevaluate the potential of their own technologies going forward."
Lenslet is targeting a market that it believes is one of the fastest-growing in the industry. "The Semiconductor Industry Association predicts that the DSP market will start rising again in 2002 at a rate of 30 percent to reach $6 billion," said Levy. By 2004 the SIA sees it reaching $10 billion. This after a disastrous 2001 period in which the market is projected to drop 26 percent.
Lenslet's core competence is the processing of transforms such as fast Fourier transforms/inverse fast Fourier transforms (FFTs)/(IFFTs), discrete cosine transform (DCT), discrete Fourier transform (DFT), compression, vector-matrix multiplication, equalization and correlation, all of which are essential functions at the baseband level of most communications technologies — "and are the ones that demand the most processing power," said Levy.
Light's transform
However, while designs to date have thrown more DSP/FPGA/ASIC horsepower at the processing of those transforms, Lenslet is instead relying on the fact that as light propagates through optical elements it undergoes a type of transform. ("A simple lens can do an FFT," said Levy.) The input and output data is the light, and the optical elements that perform different mathematical operations on the light represent the linear transform.
The type of transform to be performed is controlled by altering the characteristics of an optical element within the device and also by patented algorithms — specific to each transform — that encode and decode the electrical signals in and out and perform the signal conditioning.
"One of the key differences here between ODSPE and conventional techniques," said Levy, "is that the transform is completed on the whole vector at one shot, and at the speed of light. Conventional processing techniques require the data to be broken down to bit-slice operations or DSP instructions — we operate on functions instead of instructions." That allows operation at a much higher level of abstraction and speeds time-to-market, Levy said. "The technology is also highly scalable, has low power consumption and uses inexpensive optical components," said Levy, namely vertical-cavity surface-emitting lasers (VCSELs), compound lenses, spatial light modulators (SLMs) and optical receivers.
Product pair
Lenslet has outlined two product implementations of its technology. The first is the EnLight256, a general-purpose, off-the-shelf, reconfigurable optical transform engine that supports standard transforms such as correlations, FFTs, DCTs and DSTs. Complementing that is the Customized-EnLight, an application-specific optical transform engine that is available as a component, part of a chip set or as a board-level solution.
To date, forays into optical processing have been limited to optical switches and routers, as well as some attempts at CPUs. "CPUs have a long way to go," said Levy, "and what we're doing is way beyond simple switching and transmission." The closest analogy, said Levy, would be the work done by AT&T on optical correlators. "But they gave up in the mid-80s," he said. "The algorithms were too complex."
Attempts to perform optical transforms specifically have been limited by what Levy classifies as the I/O barrier. "The problem was in the conversion of the electrical signal to multilevel optical signal [light modulation] and the reconversion to electrical after the transform was completed," said Levy.
"The speed of the typical liquid-crystal modulator [which is fed by a single light source and requires physical changes in the LC elements to produce 8-bit gray-scale output] tops out at 200 Hz — much too slow for DSP applications."
Lenslet's approach uses VCSELs that are driven by an encoded signal that produces a digitally modulated, continuous analog waveform. The VCSELs have a 1-GHz frame rate.
That optical output then passes through a series of compound lenses and a specially adapted SLM. The characteristics of the latter determine the nature of the transform to be performed. Those characteristics can be changed in microseconds, Levy said, and thus allow on-the-fly programmability for such applications as software-defined radios.
The light exiting the SLM is then put through another series of compound lenses and detected by a photodetector array operating at a 10-GHz frame rate. The technique uses free-space optical transmission with no proprietary channeling or lightwave transmission techniques.
While the ODSPE takes care of the transforms, other DSPs will be required for lower-level processing for both the encode/decode algorithms and other baseband functions of future designs. "These can all be low-cost, low-performance devices, as the core processing has already been done by the ODSPE," said Levy. "So, the cost of support chips is greatly reduced."
While the company is demonstrating product now, it expects prototypes to be available next year. "These will mostly [be] used for evaluation and feedback, but by 2004 we expect to be shipping product for designs entering the market in 2004," said Levy. "The important parts were the breakthroughs in the encode/decode algorithms. We're now working on modified algorithms for the various transforms to be performed."
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