Next-generation MOST PHY with 1Gbit/s keeping today’s optoelectronics and fiber

Next-generation MOST PHY with 1Gbit/s keeping today’s optoelectronics and fiber

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By eeNews Europe

Extending the speed of MOST into the Gigabit range brings several challenges with it. The performance demands for every component are rising. On the transmitter side, it is necessary to modulate the LED fast. In the fiber, the intrinsic bandwidth cannot accomodate 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 [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 molded fiber optic transceiver (FOT) packages.) These losses, however, can be compensated for in the electronics, as will be discussed later.

To compensate for the low pass characteristic of the fiber equalization of the frequency transfer curve is applied. It is done 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 [3] for a laser-based channel as well as sophisticated structures like a combination of feed-forward and distributed feedback equalizer (FFE/DFE) implementation [4] for the RCLED-based channel. The best result was achieved with this FFE/DFE equalizer: we transmitted 1.25Gbit/s over the discrete LED driver, 50m of SI-POF [5] and the commerical PD-TIA prototype. The equalizer 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 figure 1.

Figure 1: Chip photograph of one of the equalizer prototypes

We also extended the equalizer approach and investigated the application of forward error correction (FEC) to the received signal. The particular implementation of a Reed-Solomon block code is capable of turning a BER of 10-4 before into 10-12 after the FEC. The block code nature of the decoder is also capable of supressing propagated errors in the feedback structure of the DFE; no error bursts were measured. Project partner ISMB [6] did this part of the work and implemented a demonstrator on an FPGA platform. Due to the additional recoding of the Gigabit Ethernet data, which reduced the net data rate by 12%, the ISMB was able to achieve a 50m link with 7dB of optical margin [7].

So, in summary, the result of POF-Plus are two 50m demonstrations applying equalization and (in the second case) error correction and achieving error-free transmission with (in the second case) significant margin. When the improved components are used, the compensation of the fiber bandwidth alone results in only a small insertion loss of the equalizer (see below).

Integration of new results into Power Budget Calculations

The worst case in the power budget of MOST150 is described [8] as: an averaged transmitted power of -8.5dBm with an extinction ratio of 10dB, a coupling loss of 2.5dB at transmitter and receiver, an additional coupling loss for two inline connectors of 2dB each and a receiver sensitivity of -22dBm. This results, with an effective attenuation of 0.4dB/m, in a fiber of 11.2m length (see Table 1 below).

During the extension of the power budget to the optical physical layer of next generation MOST, we were making the following adjustments.

First of all, we left the parameters for LED wavelength range and spectral width unchanged. This results in the same effective POF attenuation. We did also not speculate about any improvements of the coupling in the inline connectors.

We have changed these parameters: We removed the TX coupling loss, because the driver prototypes were characterised in an FOT package in fiber coupled power. The coupling loss at the RX is higher because of the smaller photo diode and misalignment errors. The measured worst case loss was 5.2dB. The sensitivity or noise equivalent power (NEP) had to be adjusted as well.

In addition to the parameters of the MOST150 calculation we included an equalizer penalty, which is the effective insertion loss of the equalizer, and the gain of the FEC into the power budget calculation.

The result can be seen in table 1. Please note that we converted the transmitted and received power into OMA in order to calculate an SNR and compare the two power budgets. It can be seen that the proposed changes enable a POF length of 10m for a net data rate of 1.25Gbit/s. This is mainly facilitated through the higher fiber-coupled OMA and the application of forward error correction in the receiver.

We expect to see improvements in several components. The next generation of the driver will improve on the OMA over temperature. The coupling at the receiver will improve through better alignment of PD and lens or even some kind of concentrator. The TIA NEP could be lowered through an optimization for the bandwidth demand of the next-gen MOST.


We have reported experimental results and demonstrated in a power budget calculation that the next generation of MOST can keep its current cheap optoelectronics and fiber. The compensation for the lower performance of the optoelectronics was achieved through the application of signal processing.

The gap to achieve the same performance as MOST150 is small and we expect that it will be closed soon. This gained margin can then result in a higher fiber length or in reduced complexity of the signal processing. 

About the authors:

Norbert Weber, Ph.D has been head of the project group optical sensors and communications at the Fraunhofer Institute for Integrated Circuits, Germany since 1999. Currently his research interests include high-speed circuit design and optical communications, especially with polymer optical fibers.

Conrad Zerna is working in the project group optical sensors and communications at the Fraunhofer Institute for Integrated Circuits, Germany. He has developed several equalizers of different architectures for data transmission over POF for speeds up to 3.4Gbit/s.


[1] POF-Plus website (

[2] Bernd Offenbeck et al, “Results of Gigabit/s Transmission using RC-LEDs and New Driving Techniques”, ICPOF’09 PS104, September 2009

[3] Jan Sundermeyer et al, “Integrated Analogue Adaptive Equalizer for Gigabit Transmission over Standard Step Index Plastic Optical Fiber (SI-POF)”, Proceedings LEOS ’09. IEEE TuI2, October 2009

[4] C. Zerna, N. Weber, “Integrated PAM2 Decision Feedback Equalizer for Gigabit Ethernet over Standard SI-POF Using Red LED”, ECOC 2010, October 2010

[5] Standard IEC 60793-2-40 category A4.a2

[6] ISMB website (

[7] R. Gaudino et al, “First demonstration of real-time LED-based Gigabit Ethernet transmission on 50m of A4a.2 SI-POF with significant system margin“, in Proceedings of ECOC2010, post-deadline paper , October 2010

[8] “MOST – The Automotive Multimedia Network”, 2nd edition, Franzis 2011, Chapter 6.5

Table 1 – MOST next generation power budget calculation 






MOST next generation




Min TX power, average / dBm








Min ER /dB







Min TX OMA / uW






Fiber-coupled power at 85°C


Losses in the channel


Per piece or m / dB

Piece/length used

Total loss / dB

Piece/length used

Total loss / dB


Inline connector













Fiber length had to be reduced by 1.2m

Coupling endpoints






No TX loss, RX loss higher

Accumulated Loss









Min RX power average / dBm







Min RX OMA / uW







RX sensitivity / dBm




For BER=10^9



RX noise equivalent power / uW







Typical value

Equalizer insertion loss / dB






For 15m with NA=0.3 excitation (LED with molded lens)








Required RX SNR / dB




For BER=10^9


For BER=10^12

With application of FEC

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