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 reverse channels, see Reverse Channel Overview . This section will discuss the key differences of forward channel structure between subtype 2 physical layer and subtype 0 physical layer.
The Subtype 2 forward pilot channel is the same as that used in Subtypes 0. It is an unmodulated, direct-sequence spread spectrum signal transmitted every half slot in bursts of 96 chips centered at the middle of the half slot. The Pilot Channel allows an access terminal to acquire the timing of the forward channel, provides a phase reference for coherent demodulation, and provides means for signal strength comparisons between sectors for determining when to handoff. The forward pilot channel is transmitted on the I channel at the full power of the cell/sector.
The Subtype 2 MAC channel contains several sub-channels: the Reverse Power Control (RPC) and Reverse Activity (RA) channels (similar to Subtype 0) and three new channels (H-ARQ, L-ARQ, and P-ARQ channels). The ARQ channels are time-division multiplexed with the DRC Lock and RPC channels in a 3 to 1 ratio where the ARQ channels are transmitted for 3 slots and the DRC Lock and R-PC channels are transmitted for 1 slot. All MAC sub-channels are spread with 128 bit Walsh codes (Subtype 0 uses 64 bit Walsh codes) and transmitted on either the I or Q channels depending on the MAC Index assigned to the access terminal. (See also MAC Code Channels Overview )
The RA channel transmits data indicating the activity level in the reverse link. If the AN detects that the overall reverse traffic channel activity is too high to support the currently used data rate, it sets the RA bit to a value of '1'. Along with other parameters, this value is used to reduce the reverse traffic channel data rate.
The combination of the R-PC, DRC Lock and ARQ channels are sent in parallel to all ATs with an open connection.
The preamble transmitted at the beginning of each packet either identifies the packet as a control channel packet (MAC index 2 or 3), or identifies the target access terminal for receiving the forward traffic channel or AT-directed control channel packet (the access network uses the MAC index to identify the target access terminal. The MAC index is assigned by the AN and sent to the AT as part of the procedure for establishing a traffic connection.). See MAC Index for details.
The traffic data and control data are time multiplexed together on the forward link. This multiplexed control channel carries the broadcast messages for the system. These messages include the Sync Message, Quick Configuration Message, the Sector Parameters Message, the Access Parameters Message, and the Broadcast Reverse Rate Limit Message. In addition, signaling messages addressed to individual access terminals are also carried on the multiplexed control channel.