Wednesday, November 21, 2007

Tera-bits Per Second Over Fiber has reported that Tohoku University researchers in Japan have enabled Quadrature Amplitude Modulation (QAM) over fiber to move information at rates of hundreds of tera-bits per second. Here's a few quotes from the press release:

At the heart of the development is a technique already used in some digital TV tuners and wireless data connections called quadrature amplitude modulation (QAM). One glance at the Wikipedia explanation shows that it's no easy science, but the basics of QAM in this scenario require a stable wavelength for data transmission.

As the radio spectrum provides this, QAM-based methods work fine for some wireless protocols, however the nature of the optical spectrum means this has not been the case for fibre-optic cables ... until now.

The university team has solved the stability problem using a special laser that makes it feasible to pipe data down a glass fibre using the QAM method at blistering speeds. Although we shouldn't expect to be choosing from internet connections rated in Tbit/s anytime soon, the development could one day make us look back on ADSL as fondly as we now do our 56K modems.

Analog modems have used a form of QAM for years to move information from device to device across the Public Switched Telephone Network (PSTN) or voice network. QAM is also used by cable modems and ADSL modems to modulate (convert digital signals to analog) and demodulate (convert analog signals back to digital) communications signals.

Let's try to get a basic understanding of how QAM works - without any math! Computing devices (computers, PDA's, laptops, etc) use digital signals (1's and 0's) to process, store and manipulate information. Sending this information over long distances though typically involves a conversion or modulation of digital signals to analog signals on the sending device and a conversion or demodulation of analog signals to digital signals on the receiving device. QAM has been the method of choice for transmitting signals this way for years.

QAM combines amplitude modulation (think height of a sine wave) and phase shift (think of a sine wave moving along the x-axis relative to a zero degree reference) and allows multiple bits (combinations of binary 1's and 0's) to be transmitted for each cycle of a sine wave. I like to use the term multiple bits per cycle when I describe QAM.

QAM is categorized by the number of bits that can be transmitted in one sine wave cycle. To get a simple understanding let's take a look at 16-QAM. 16-QAM is considered rectangular QAM - the square root of 16 is 4 and this indicates that each cycle of a 16-QAM waveform can represent a 4 bit binary (1 and 0) pattern. Using the same method we can calculate 64-QAM represents an 8 bit binary (1 and 0) pattern because the square root of 64 is 8. 256-QAM can represent a 16 bit binary (1 and 0) pattern because the square root of 256 is 16, etc.

QAM signals are susceptible to instability and noise but it appears the Tohoku University researchers have figured out a way to stabilize optical signals and use QAM methods for tera-bit level data transmission. I have not been able to find any detail on the stabilization methods being used at this time.


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