A 9/18 KBD WIRELESS VIDEOPHONE SCHEME

A highly bandwidth efficient, fixed but arb i t ra r-ily programmable rate, perceptual ly weighted Discrete Cosine Transform ( D C T) based video communicator for qua r t e r common intermediate format (QCIF) videophone-sequences is presen-ted. Perceptua l ly weighted cost /gain controlled mot ion compensat ion and quad-class DCT-based compression i s appl ied without variable rate com-pression techniques and without adapt ive buf-fering i n o rde r to main ta in a fixed t ransmission rate, which can be ad jus t ed to any requi red value. I n th i s t rea t i se we opted for a source coded rate of 11.36 k b p s a n d the sensitivity-matched forward error correction (FEC) coded P rate became 20.32 kbps. A par t ia l forced up-date technique was invoked i n order to keep t r ansmi t t e r a n d receiver al igned amongst hostile channel conditions. W h e n using coherent pilot symbol assis ted 16-level quad ra tu re ampl i tude modulat ion (16-PSAQAM), a n overall signalling r a t e of 9 k B d was yielded. Over lower qual i ty channels 4 Q A M had to be invoked, which re-qu i r ed twice as m a n y t i m e slots to accommod-ate the resul t ing 1 8 k B d s t ream. Over the bes t Gaussian and worst Rayleigh channels signal-to-noise ratio ( S N R) values in the range of 7 t o 20 d B were needed for these modems i n or-d e r t o main ta in near-unimpaired image quality. I n a bandwidth of 200 kHz, similarly to the G S M speech channel, 1 6 and 8 videophone users can be suppor t ed, when using the 16QAM and 4 Q A M systems, respectively