(第5空)CDMA for cellular systems can be described as follows. AS with FDMA, echo cell is allocated a frequency ( ) , which is split into two parts, half for reverse(mobile unit to base station) and half for ( )(base stat i on to mobile unit). For full -duplex ( ). A mobile unit uses both reverse and forward channels.Transmission is in the form of direct-sequence spread ( ),which uses a chipping code to i ncrease the data rate of the transmission ,resulting in an increated signal bandwidth.Multiple access is provided by assigning ( ) chipping codes to multiple users,so that the receiver can recover the transmission of an individual unit from multiple transmission.