How can one satisfy the high synchronization requirements for SCDMA in a normal cable network?
A. In order to get high throughput with SCDMA, the modems must all be time aligned within a fraction of the symbol rate. Otherwise, the “S” (synchronous) part of CDMA fails, and the data from one modem corrupts the data from other modems. The result is packet loss. The timing resolution is measured in nanoseconds. There are issues when you measure things in nanoseconds across a distance of 40 km (a short network) or up to 320 km (a long network): • minute changes in fiber path distance, caused by temperature (expansion and contraction of the glass itself) • expansion of the coaxial network (which is why every span has an expansion loop) • the fact that the speed of light also changes with temperature, in both fiber and coaxial line (Velocity of Propagation as a percentage of speed of light) Every 1 second, an SCDMA modem must be time-aligned, if the modem is more than 20 km from the headend, even if less than half of that network is overhead plant. This represents at least 60 to 80 per
