Briefing: High-speed optical transmission.
Part 1: The start-up
Ofidium is a 100 Gigabit start-up that refuses to follow the herd.
While the optical industry has chosen polarisation-multiplexing quadrature phase-shift keying (PM-QPSK) for 100 Gigabit-per-second (Gbps) transmission, the Australian start-up is developing a module based on orthogonal frequency division multiplexing (OFDM) modulation.
"For data rates higher than 100Gbps, it [OFDM] is the only way to go"
Jonathan Lacey, CEO
Orthogonality refers to how more than one signal, each carrying a data stream, can be sent over a fibre before being recovered at the receiver.
Polarisation multiplexing, as used by PM-QPSK and Ofidium’s OFDM, makes use of two independent signals, exploiting the fibre’s orthogonal polarisations. But OFDM also uses orthogonality in the form of multiple independent carriers.
PM-QPSK is referred to as a single-carrier scheme. Ofidium’s OFDM approach, in contrast, uses digital processing at the transmitter to generate a signal that has many hundreds of very closely-spaced, independent sub-carriers, each tolerant to optical impairments. At the receiver, digital processing transforms this comb-like signal back into a single data stream.
Ofidium claims OFDM line-side transmission delivers several advantages when compared with PM-QPSK.
OFDM lends itself to very high spectral efficiency, claims Jonathan Lacey, Ofidium's CEO: “For 100Gbps at 25GHz channel spacing – and for data rates higher than 100Gbps – it [OFDM] is the only way to go.” The spectrally efficiency gives OFDM higher tolerance to optical filtering and to polarisation-dependent loss encountered at 100Gbps. “It is very tolerant to optical filtering and to polarisation-dependent loss,” says Lacey.
OFDM also has implementation benefits.
Despite adopting an alternative modulation scheme to the rest of the industry, Ofidium benefits from the same optical components being developed for PM-QPSK as stipulated by the Optical Internetworking Forum (OIF) in its Framework Document.
And one costly aspect of a single-carrier PM-QPSK design is the fast analogue-to-digital converters (ADCs) used. Sampling rates of up to 64 Gsamples/s are required. According to Ofidium, its OFDM design uses a sampling rate 40% lower than single-carrier PM-QPSK. Ofidium is working with German mixed signal specialist Micram for its ADCs.
OFDM also uses the fast Fourier transform, a commonly available digital signal processor (DSP) design block. “The wireless world has been optimising DSPs for OFDM for decades,” says Lacey. “We borrow from the wireless guys.”
Lacey says the start-up never considered using its OFDM expertise to become an equipment maker. “If I look at the expertise of the company, that isn’t where we add value,” he says. In turn the level of investment needed for a system vendor start-up is 10x that of a module maker. “With Australian venture capital $10 to $100m is possible, not hundreds of millions.”
Lacey says its module design is already being tested by a leading equipment maker and that more details will be announced in coming months.
The start-up can also take encouragement from the views of Verizon, the first operator to deploy 100Gbps.
Verizon and AT&T have both been vocal in backing PM-QPSK as the 100Gbps modulation scheme, partly to encourage the industry to focus their R&D spending on developing one common technology.
Yet Glenn Wellbrock, director of backbone network design at Verizon Business, accepts that Ofidium is ‘most likely” OIF Framework Document compliant, and admits that the operator could use OFDM technology. However two requirements must first be met.
When a system vendor says it meets Verizon’s system link performance requirements, all the 300-pin optical module suppliers’ designs used by the system vendor meet the specification. Any OFDM-based module must also meet Verizon’s specifications to be considered.
The OFDM module also needs to be as cheap as - ideally cheaper than - the 300-pin PM-QPSK transponders.
If both these conditions are met then OFDM-based transponders “can be successful”, says Wellbrock.
References:
[1] “Modulation and multiplexing in optical communication systems,” by Peter Winzer, IEEE LEOS newsletter, February 2009
[2] Ofidium’s technology resources