10 Gbps Physical Layer for Single Twisted Pair: Page 2 of 5

September 10, 2015 //By Norbert Weber, Conrad Zerna, Fraunhofer IIS
10 Gbps Physical Layer for Single Twisted Pair
Applications like advanced driver assistance systems (ADAS) and passenger infotainment drive data rates in automotive vehicles. To realize the lightweight and fuel efficient cars of the future, it is mandatory to increase the data rate, to get more bits over the same channel in the same time. This article introduces an elegant method to increase the data bandwidth in a single twisted pair (STP) cabling.
Furthermore, standard complementary metal oxide semiconductor (CMOS) technology itself becomes more bandwidth limiting at the target speeds. The parasitic transistor capacitances and a limited maximum transit frequency, and the ESD (electro-static discharge) protection at the pads, have a major influence on the transfer function of the channel. Additionally, the targeted speed conflicts with transmit power requirements for the cable lengths common e.g. in automotive applications. It is therefore the specific goal to increase throughput by adding intelligence on the semiconductor side and keeping the transmission medium light and low-cost. The core requirements selected for the development of the new physical layer were chosen to be a low deterministic latency, low power consumption, and the use of a single twisted pair. The target bit error rate was fixed at 10-12. The implementation had to be feasible on a standard CMOS process.

System overview

To tackle the design problem at hand, it was necessary to undertake a paradigm shift and abandon classical NRZ (non-return-to-zero) binary coding with relatively slim signal processing. Firstly, we moved to a higher level modulation format and thereby limited the transmission bandwidth, which is now well below the notch of conventional twisted pair cables. The choice for the modulation format was also made through the calculation of the so-called channel capacity of the chosen cable. (The channel capacity gives a theoretical maximum of achievable data rates; practical implementations are well below that number.) This calculation showed that binary coding at available transmit powers could not carry a 10 GBps data rate. It also showed which modulation complexity would yield optimal data rate performance on the channel. Secondly, the signal processing was moved into the digital domain to acquire more complex and powerful signal processing algorithms.

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