Development of a TDM/FDM Transmultiplexer

Recently, digital systems such as digital telephone switching systems and digital transmission paths are being introduced more and more into communications networks in which signals are handled with Time Division Multiplex (TDM). There still exists, on the other hand, a number of analog transmission paths in which signals are multiplexed with Frequency Division Multiplex (FDM). Therefore, it is important to develop a scheme to interconnect efficiently TDM and FDM systems. Transmultiplexer, which is an equipment directly converting PCM.TDM signals and SSB.FDM signals with each other, is expected to provide an efficient interconneetion between TDM and FDM systems. Authors have developed an experimental transmultiplexer which can provide mutual conversion between a 24-channel PCM primary group (1.544 Mbits/s) and two 12-channel FDM basic groups (60-108 kHz). The developed equipment is designed so as to achieve reduction of the computation rate and simplicity of the hardware by the following techniques: (1) The FFT (Fast Fourier Transform) technique is used to optimize the filtering and multiplexing processes. (2) The spectrum reversion of odd-numbered channel signals is achieved by inverting the sign bits of the PCM signals every other frame. (3) The bandpass sampling technique is applied to obtain SSB.FDM signals directly without frequency shifts at analog FDM stages. In order to evaluate the characteristics of the developed transmultiplexer, experiments have been conducted by connecting it with FDM terminal equipments. The experimental results showed that the transmultiplexer can offer better performance than the tandem connection of standard PCM multiplex equipments and standard FDM terminal equipments with reasonable hardware.