The Reverse Channel consists of the Access Channel and the Reverse Traffic Channel according to the state of the data connection. The Access Channel is used by the access terminal to initiate communication with the access network or to respond to an access terminal directed message. The Reverse Traffic Channel is used by the access terminal to transmit user-specific traffic or signaling information to the access network.
Along with the C.S0024 standard evolving from the release 0 to release A, the new Subtype 2 Physical Layer is introduced to support higher data rate transmission. This results in some modifications on both the forward channel and reverse channel structure. For details on the forward channels, see Forward Channel Overview . This section will discuss the key differences of reverse channel structure between subtype 2 physical layer and subtype 0 physical layer.
The following figure shows the reverse channel structure as defined by subtype 0 physical layer protocol.
The following table summarizes the walsh code, spread factor and duty cycle for each subtype 0 reverse channel. For more description on spreading factor (SF), see Spread Factor Description .
The subtype 2 reverse traffic channel has many changes in the definitions of the R-Pilot, R-RRI, R-ACK, and R-Data channels, and introduces two new reverse channels (R-DSC and R-Aux Pilot). Also, the R-Data channel uses a new timing structure (see Subtype 2 Reverse Link Frame Structure ).
The following figure shows the reverse channel structure as defined by subtype 2 physical layer protocol.
The following table summarizes the walsh code, spread factor and duty cycle (transmission duration) for each subtype 2 reverse channel. For more description on spreading factor (SF), see Spread Factor Description .
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In subtype 2 operation, the R-Data channel uses a new timing structure. The frame and slot timing is the same as subtype 0 (each 26.67 ms frame consists of 16, 1.667 ms slots), but the concept of the subframe is introduced. Each subframe consists of four contiguous slots (6.667ms). Also new is the concept of a subpacket. A subpacket is defined to be the smallest unit of a reverse traffic channel transmission that can be acknowledged at the physical layer by the access network. A subpacket is transmitted over four contiguous slots, thus a subpacket is transmitted in one subframe.
Each physical layer packet can be transmitted in up to a maximum of four sub-packets. When more than one sub-packet is transmitted, the transmitted sub-packets use a three sub-frame interlacing. That is, the transmitted sub-packets of a physical layer packet is separated by two intervening sub-frames, and sub-packets of other physical layer packets may be transmitted in the sub-frames between those transmitted sub-packets.
The following figure shows how the four sub-packets from a single physical layer packet (a 4096 bit physical layer packer was used) can be transmitted on the R-Data channel. Note that not all of the sub-packets will be sent if the AN acknowledges the packet as received before all sub-packets have been transmitted (early termination).
Spread Factor (SF) is a term which indicates which Walsh code set the Walsh code belongs to. For example, Walsh code 00000000 belongs to Walsh code set 8, and thus is said to have a spread factor of 8. The spread factor also indicates the length of the Walsh code (spread factor 8 indicates the Walsh code is 8 digits in length).
The diagram below shows an example of the Walsh code distribution of the subtype 0 reverse channels: