A CDMA based bidirectional communication system for hybrid fiber-coax CATV networks

Hybrid fiber-coax CATV networks are considered a promising infrastructure for the implementation of future interactive services. Efficient utilization of the available CATV spectrum requires a communication system which is dedicated to the specific properties of these networks. This paper presents the physical layer of a bidirectional communication system for HFC CATV networks which is based on CDMA, a technique that has a certain robustness to the ingress noise found in the CATV bandwidth. Because the application of asynchronous CDMA results in a poor spectral efficiency, a transmission scheme based on synchronous CDMA is adopted. Using synchronous CDMA and QPSK modulation, a capacity of 64 channels of 64 kbit/s each can be achieved in a bandwidth of 6 MHz. It is found that small synchronization errors can be tolerated without significant performance degradation. Next, a description is given of the cable modems responsible for the communication between subscribers and a CATV headend. The design exploits the typical CATV network configuration to enable a cost-effective hardware realization. Sensitivity of system performance to linear distortion is investigated by computer simulation. Especially linearly varying group delay and uneven amplitude response are shown to be potential causes of performance degradation.