Optmizing Cyclic Prefix Length For DOCSIS 3.1 Upstream Transmission

The cable industry originated in United States with the first cable television being invented in 1948. Since the 1990s, many digital devices have been produced and many people have come to rely on the services provided by cable networks, which has lead to significant investment in cable network technologies. Interactive programs like GPS, on-line games and video conferencing, have been introduced in recent years. The customer demands of both uploading data (upstream) to and downloading data (downstream) from the Internet are increasing. There is an international telecommunication standard, DOCSIS, which sets rules for all cable systems. The latest version DOCSIS3.1 makes a leap forward in technology to offer larger bandwidth and faster data rates in both upstream and downstream transmissions. This thesis focuses on one key parameter, cyclic prefix length, of the upstream transmission system. The cable modem upstream uses Orthogonal Frequency Division Multiple Access(OFDMA) for data transmission, which allows a high volume of users to share the whole band at the same time. Each user occupies a set of sub-carriers for data transmission. The cable modem transmitter partitions the data into packets that fit into OFDMA frames and sends the data frames in time sequence to Cable Modem Termination System(CMTS). Different cable modems in the same frame may have different delays. In each frame, the cyclic prefix is fixed for all Cable Modems(CMs), and the constellation types can be varied for each CM in order to achieve the best possible data rate. Therefore, selecting an optimum Cyclic Prefix(CP) length for the system is an important issue. For all CMs in a frame, if the CP is long enough to cover the longest delay, the data transmission efficiency will be low. However, if the CP length is short, some CMs' performances will be degraded. To balance the trade-off between the frame length and the signal quality, the purpose of this research project is to generate a computer program which is able to choose the optimum CP length for a given cable modem upstream transmission system. The system must obey the rules set by DOCSIS 3.1. Several models were built to analyze and corroborate the distribution of interferences generated by a single sub-carrier when the CP length is insufficient to cover the channel delay. Finally, a series of mathematical equations, which can successfully estimate the distribution are developed. The developed Best CP Length Selection Program is described in detail. The signal processing procedures and computation methods are explained step by step. The useful data per minislot was chosen to be the criteria for the program. DOCSIS3.1 allows eleven CP lengths for the upstream transmission system. By analyzing these allowed CP lengths and computing the useful data per minislot for each, the best CP length can be determined. The program was tested for several hypothetical cable networks and generated logical results in all cases. The results show the Best CP Length Selection Program works properly and is reliable