Design and Testing of a Digital Diplexer for DOCSIS 4.0 Networks

Abstract

The cable television industry has experienced massive growth since it began in the United States as a commercial business in 1950’s. As cable television subscribers continued to grow in numbers, the demand for higher transmission rates increased. This led to the advent of Data-Over-Cable Service Interface Specification (DOCSIS). DOCSIS was developed by CableLabs and contributing companies to supervise the manufacturing of new digital equipment and ensure the compatibility of products from different manufacturers. New versions of the DOCSIS standards were released consecutively over the years to fulfill the raising demand for larger bandwidths and higher transmission rates. The latest version of the DOCSIS standards at the time of this writing is DOCSIS 4.0, in which the upstream and downstream signals are separated in frequency to eliminate interference.

A diplexer is a three-port device that implements this frequency-domain multiplexing, allowing bidirectional data transmission. It multiplexes two ports (e.g. L and H) onto a third port (e.g. S). The frequency bands occupied by the signals on ports L and H are separated by a transition band, which means that the signals on ports L and H can coexist on port S without interfering with each other. Commonly, the signals on port L will occupy a lower frequency band and the signals on port H will occupy a higher frequency band. In that case, a lowpass filter connecting ports L and S and a highpass filter connecting ports H and S are implemented in the diplexer. Conventional diplex filters have fixed passband and stopband corner frequencies, which means that they must be replaced every time there is a change in the bandwidth allocation due to customer demand. Furthermore, a conventional diplex filter usually has a large transition band due to the challenges of building a shard filter using radio frequency (RF) components, which results in wasted frequency spectrum in the network. One possible solution to the problems above is replacing conventional diplex filters with diplex filters built using digital hardware, which offers an unique advantage that allows filter parameters to be adjusted freely and precisely.

This thesis aims to design a hardware efficient digital diplexer for use in hybrid fiber-coaxial (HFC) networks. A digital diplexer samples the incoming analog signals, then performs filtering digitally. The downstream and upstream sampling frequencies were optimized based on the DOCSIS 4.0 frequency division duplex spectrum options. The computation results showed that the ideal downstream sampling frequency is 3588 MHz, whereas the ideal upstream sampling frequency is 1616 MHz. Further, the frequency specifications of digital diplex filters were determined based on the frequency allocation defined by the DOCSIS standard. Multiple filter implementation structures were compared and contrasted to find a structure that supports high sampling frequencies at the lowest hardware cost. After careful consideration, block-based frequency domain filtering structure was selected and applied to the design.

Based on the filtering structure and parameters, a fixed point model of the digital diplexer was constructed in the Verilog hardware description language. A simulation was then conducted in ModelSim to verify the performance of the model in the FPGA development environment. Another fixed point model of the digital diplexer was built and tested in MATLAB. The testing results were evaluated and compared with the simulation results obtained in ModelSim, which aimed to verify the functionality of the designed diplexer. After that, more ModelSim and MATLAB simulations were conducted to verify that the designed diplexer achieves a signal quality that can support the highest modulation order specified in DOCSIS (4096-QAM) and allows for dynamic switching of the upstream/downstream transition point. In addition, several digital diplex filters with different sizes of transition band were designed and simulated in MATLAB. The results showed that digital diplex filters can achieve sharper transition bands and the 'wasted' bandwidth associated with higher split points can be reduced as compared to the conventional approach.