Advanced Vocoder Idle Slot Exploitation for TIA IS - 136 Standard

BellSouth has proposed a modification to the IS-136 digital cellular standard, which they refer to as Advanced Vocoder Idle Slot Exploitation (ADVISE), in which base stations can transmit auxiliary coded (redundant) bits on otherwise unused time slots to assist certain subscriber units (SUs). In this master's thesis, we investigate two aspects of ADVISE: 1) adaptive redundancy schemes by which the auxiliary bits are generated, and 2) autonomous detection mechanisms by which the SUs detect the presence of such auxiliary bits without external signaling. The adaptive redundancy schemes are designed as an overlay to the existing IS-136 forward error correction (FEC) design. We propose and test a scheme which starts with a base code of a given rate and derives lower rate codes by adding FEC overlay. When the channel is favorable, for example, only the coded bits from the base code are sent; as the channel condition deteriorates, the FEC overlay bits are sent along with the existing coded bits to increase coding gain. On the other hand, we propose an autonomous detection method which exploits the fact that some of these FEC overlay bits are mere repetition of the standard slot bits. The detection method makes use of a distance-based metric which, after simplification, performs hypothesis testing based on the Hamming distance between the two sets of received bits. If the Hamming distance is smaller than a threshold, the detection method declares ADVISE to be present and allows the channel decoder to use the auxiliary data. Otherwise, the method declares ADVISE to be absent, and the channel decoder uses only the data received in the standard slot. Thesis Supervisor: Dr. G. David Forney, Jr. Title: Adjunct Professor of Electrical Engineering, MIT Laboratory for Information and Decision Systems Industry Supervisor: Dr. A. Roger Hammons Title: Principal Engineer, Hughes Network Systems