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Peek at next-gen MOST PHY with 1Gbit/s

Posted: 18 Jun 2012     Print Version  Bookmark and Share

Keywords:MOST  Gigabit  equaliser 

The optical version of the MOST physical layer has a number of well-known advantages. It has virtually no EMI problems. The SI-POF 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.

The extension of MOST speed into the Gigabit range comes with several challenges. The performance demands for every component are rising. On the transmitter side, it is necessary to modulate the LED fast. In the fibre, 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 summarise the work on the EU-funded project POF-Plus [1], 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 will extend it to 1.25Gbit/s. The remaining gap is small and a perspective on closing it will be outlined.

EU-Project POF-Plus
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/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-50Ohm 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 [2]. A long-term test over 3500 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 occured 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 most performant combination. In the last year of the project, this work was overtaken by the availability of a commerical prototype of an integrated PD/TIA solution with high bandwidth (by A3Pics). It 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 moulded fibre optic transceiver (FOT) packages.) These losses, however, can be compensated for in the electronics, as will be discussed later.

Figure: Chip photograph of one of the equaliser prototypes.

To compensate for the low pass characteristic of the fibre equalisation of the frequency transfer curve is applied. It is done at the receiver. In the case of limited optical transmission amplitude, equalisation 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 [3] for a laser-based channel as well as sophisticated structures like a combination of feed-forward and distributed feedback equaliser (FFE/DFE) implementation [4] for the RCLED-based channel. The best result was achieved with this FFE/DFE equaliser: we transmitted 1.25Gbit/s over the discrete LED driver, 50m of SI-POF [5] and the commerical PD-TIA prototype. The equaliser testchip compensated the channel and a bit error rate of less then 10-10 was measured. A photo of one of the testchips can be seen in the figure.

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