▁seem ▁reasonable ▁to ▁expect ▁the ▁transmit ▁power ▁to ▁be ▁constant . ▁However ▁it ▁is ▁not . ▁This ▁is ▁because ▁the ▁side b ands , ▁which ▁are ▁essentially ▁a ▁repet ition ▁of ▁the ▁data ▁in ▁the ▁main ▁car rier ' s ▁mod ulation , ▁are ▁filtered ▁off ▁with ▁a ▁sharp ▁filter ▁so ▁that ▁unnecessary ▁spectrum ▁is ▁not ▁used ▁up . ▁This ▁results ▁in ▁an ▁amplitude ▁mod ulation ▁and ▁is ▁why ▁T ET RA ▁requires ▁linear ▁ampl ifiers . ▁The ▁resulting ▁ratio ▁of ▁peak ▁to ▁mean ▁( R MS ) ▁power ▁is ▁ 3 . 6 5   d B . ▁If ▁non - linear ▁( or ▁not - linear ▁enough ) ▁ampl ifiers ▁are ▁used , ▁the ▁side b ands ▁re - app ear ▁and ▁cause ▁inter ference ▁on ▁adjacent ▁channels . ▁Comm only ▁used ▁techniques ▁for ▁ach ieving ▁the ▁necessary ▁linear ity ▁include ▁Cart esian ▁loops , ▁and ▁adapt ive ▁pred ist ort ion . ▁ ▁The ▁base ▁stations ▁normally ▁transmit ▁continu ously ▁and ▁( sim ult ane ously ) ▁receive ▁continu ously ▁from ▁various ▁mob iles ▁on ▁different ▁car rier ▁frequencies ; ▁hence ▁the ▁T ET RA ▁system ▁is ▁a ▁frequency - div ision ▁du plex ▁( F DD ) ▁system . ▁T ET RA ▁also ▁uses ▁F D MA / TD MA ▁( see ▁above ) ▁like ▁G SM . ▁The ▁mob iles ▁normally ▁only ▁transmit ▁on ▁ 1 ▁slot / 4 ▁and ▁receive ▁on ▁ 1 ▁slot / 4 ▁( inst ead ▁of ▁ 1 ▁slot / 8 ▁for ▁G