The optical version of the MOST physical layer has several well-known advantages. It has virtually no EMI problems. The SI-POF (step index plastic optical fiber) has low weight and the fiber and optoelectronics are low-cost. As the need for higher data rates grows, the feasability of the next generation of MOST on the currently attractive link components becomes an urgent topic. The article describes the physical layer for future Gigabit MOST.
Extending the speed of MOST into the gigabit range brings several challenges. The performance demands for every component are rising. On the transmitter side, it is necessary to modulate the LED quickly. In the fiber, the intrinsic bandwidth cannot accommodate the signal natively, and the high-frequency attenuation has to be dealt with. On the receiver side, a trade-off between bandwidth and sensitivity has to be addressed.
We will summarize the work on the EU-funded project POF-Plus, which has addressed several of the concerns about the components through new circuit techniques and the application of advanced signal processing. We will apply the experience and new results of POF-Plus to the power budget of MOST150 and extend it to 1.25Gbit/s. The remaining gap is small and a perspective on closing it will be outlined.
The target of the EU-Project POF-Plus was an “engineering solution of gigabit Ethernet over 50m of SI-POF”. The focus was thus particularly on developing and using practical components that could be mass-produced.
For the transmitter, new driving techniques for LEDs were investigated. A current peaking technique in a non-50 O environment was applied to the LED to quickly populate and deplete the junction region. The feasibility of the concept was proven in a discrete circuit on a PCB. A long-term test over 3,500 hours of continuous operation with the first version of the discrete driver with LED revealed a reduction in the optical modulation amplitude (OMA) of only 6.5% over time. However, the loss occurred almost entirely in the first 500 hours; after that point the OMA remained almost constant.
An improved version of the driver is in the making. The first prototypes display an improved performance over the first discrete version. The problem of bandwidth limitation has been solved in the transmitter. The gigabit driver is not capable of producing the same extinction ratio (ER), but has only a slightly smaller OMA even over temperature.
Receivers were also investigated inside POF-Plus. Measurements with different off-the-shelf components (PD and TIA) were done to find the best performance combination. In the last year of the project, this work was overtaken by the availability of a commercial prototype of an integrated PD/TIA solution with high bandwidth (by A3Pics). This unit delivered the best performance of all compared options. The higher bandwidth comes at the cost of more noise and a smaller photo diode, which in turn leads to a higher coupling loss at the receiver. (The loss was measured with the prototypes in molded fiber optic transceiver (FOT) packages.) These losses, however, can be compensated for in the electronics.
To compensate for the low pass characteristic of the fiber equalization of the frequency transfer curve is applied at the receiver. In the case of limited optical transmission amplitude, equalization at the receiver results in a smaller SNR penalty than at the transmitter. (This is in contrast to an electrical channel.)
Several architectures of different complexities have been investigated during the course of the POF-Plus project. We covered simpler solutions like self-adapting analogue peaking filters for a laser-based channel as well as sophisticated structures like a combination of feed-forward and distributed feedback equalizer (FFE/DFE) implementation for the RCLED-based channel. The best result was achieved with this FFE/DFE equalizer: Transmitting 1.25Gbit/s over the discrete LED driver, 50m of SI-POF and the commercial PD-TIA prototype. The equalizer testchip compensated the channel and a bit error rate of less then 10-10 was measured.
To read the complete article, which describes extending the equalizer approach and forward error correction to the received signal, and integration of results into power budget calculations, click here, courtesy of EE Times Europe Automotive.
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