Reverse Channel Structure in CDMA
The Reverse CDMA Channel is composed of Access Channels and Reverse Traffic Channels. These channels share the same CDMA frequency assignment. Each Traffic Channel is identified by a distinct user long code sequence and each Access Channel is identified by a distinct Access Channel long code sequence. The following figure shows as example of the signals received by a base station on the Reverse CDMA Channel.
The reverse link employs the same 32768 length binary short PN sequences which are used for the forward link. However, unlike on the forward link, a fixed code phase offset is used. A long (242-1) PN sequence with a user-determined time offset is used to identify the subscriber (analogous to ESN in AMPS). The sequence is then modulo-2 added with a 42 bit wide mask.
The subscriber unit convolutionally encodes the data transmitted on the Reverse Traffic Channel and the Access Channel prior to interleaving. The transmitted digital information is convolutional encoded using a rate 1/3 code of constraint length 9 for the Access Channel and Rate Set 1 of the Reverse Traffic Channel. For Rate Set 2 of the Reverse Traffic channel, the convolutional code rate is 1/2.
The encoded information is then interleaved over a 20 ms interval. The interleaved
information is then grouped in code words which consist of 6 symbol groups each. These code words are used to select one of the 64 orthogonal Walsh Codes for transmission. On the reverse link, the Walsh Codes are used for information transmission. The reverse CDMA frequency channel can support up to 62 TCHs per Paging channel and 32 Access Channels per Paging Channel.