Speech codec for the European mobile radio system. - Peter Vary; Karl Hellwig; Rudolf Hofmann; Robert J. Sluyter; Claude R. Galand; Michele M. Rosso (ICASSP, 1988)

A regular-pulse excited linear predictive codec. - Peter Vary; Rudolf Hofmann; Karl Hellwig; Robert J. Sluyter (Speech Commun., 1988)

KISS-16V2: a one-chip ASIC DSP solution for GSM. - Dirk Weinsziehr; Harald Ebert; Gisela Mahlich; Joachim Preißner; Hans Sahm; Johannes Schuck; Harald Bauer; Karl Hellwig; Dietmar Lorenz (IEEE J. Solid State Circuits, 1992)

A VR-CELP codec implementation for CDMA mobile communications. - Luca Cellario; Daniele Sereno; Mario Giani; Peter Blöcher; Karl Hellwig (ICASSP (1), 1994)

The cellular text telephone modem - the solution for supporting text telephone functionality in GSM networks. - Matthias Dörbecker; Karl Hellwig; Fredrik Jansson; Tomas Frankkila (ICASSP, 2001)

RTP Payload Format for Global System for Mobile Communications Half Rate (GSM-HR). - Xiaodong Duan; Shuaiyu Wang; Magnus Westerlund; Karl Hellwig; Ingemar Johansson (RFC, 2010)

System aspects of the 3GPP evolution towards enhanced voice services. - Stefan Bruhn; Tomas Frankkila; Frédéric Gabin; Karl Hellwig; Maria Hultström (GlobalSIP, 2015)