We describe a feasibility study for introducing a wideband speech (7 kHz) mode into the forthcoming GSM Adaptive Multi Rate (AMR) system. The AMR codec system is foreseen not only for the existing GSM network but also the future Third Generation mobile radio system UMTS. A wideband mode will offer speech services with an significantly improved perceptual quality in moderate channel error conditions. We discuss the requirements for the design of an AMR wideband (AMR-WB) speech codec and present an experimental source coding algorithm operating at a bit rate of 13.0 kbit/s. Subjective listening tests have shown that a clean speech quality equivalent to the 7 kHz ITU-T G.722 codec at 48 kbit/s can be achieved. Robust transmission in a GSM full rate channel is feasible with an appropriate error protection scheme.