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,:A" MOTOROLA Technlcal Developments
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Volume 3 March 1983
DIGITAL SUBSCRIBER LOOP SYNCHRONIZATION TECHNIQUE
By Richard A. Comroe and David E. Borth
STATEMENT OF PROBLEM
The integrated services digital network (ISON) will permit new and diverse communication for the information society. The fundamental element of any ISON system is an end-to-end digital communica- tions capability. This end-to-end digital capability also presents the greatest technical challenge when attempts are made to utilize already existing subscriber loops designed for voice band audio. These metallic loops are commonly called digital subscriber loops (DSL) for this application.
A common DSL technique which alternately uses the line for transmission of data in each direction is called time compression multiplexing (TCM) or ping-pong. Implementations vary between systems in such details as modulation or line code, bit rate, and burst rate. Motorola, for example, has announced
a chip set which uses a unique line code called modified differential phase shift keying (MDPSK) to transmit 80 Kbls data bidirectionally at a 256 Kbls burst rate.1
The design of a reliable receiver for the reception of TCM transmissions over OSLs requires a significant design effort. Because of the nature of TCM transmission, most existing receiver tech- niques are not directy applicable. For example, because data is sent in bursts, bit and frame syn- chronization must be established prior to data demodulation. Most discontinuous transmission schemes make use of synchronization bits at the start of each burst to assist synchronization. However, in a TCM system, extra bits used for synchronization result in a reduction of bit rate and/or
line length and are thus to be avoided, if possible.
SOLUTION
The characteristics of the MDPSK line code in aTCM system can be exploited by a receiver to pro- vide reliable performance without the use of extra bits for synchronization. For example, independent of the transmitted data pattern, the MDPSK waveform has a zero crossing in the center of each bit dur- ing...
