Forward link EVDO channels in Detail
A number of channels are transmitted in the forward direction to enable signalling, data and other capabilities to be handled. These channels include the Traffic channel, MAC channel, Control channel and Pilot. These are time division multiplexed.
Traffic Channel – This channels uses Quadrature Phase Shift Keying (QPSK) modulation for data rates up to 1.2288 Mbps. For higher data rates, higher order modulation techniques are used in the form of 8PSK with 3 bits per symbol or 16QAM with 4 bits per symbol. The levels of the I and Q symbols are chosen so that the average power becomes 1. The Incoming data to be used as the modulation comes from the from the turbo coder and is scrambled by mixing it with a Pseudo Random Number (PN) sequence. The initial state of the PN is derived from known parameters, and is unique for each user. Every packet starts at the same initial value of the PN sequence.
At the beginning of the transmission to each user, there is a preamble that contains the user ID for the data. Its repeat rate is determined by the data rate because lower data rates require higher repeat values. However even at its largest, the preamble will fill no more than half the first slot.
Control Channel – This channel carries the signalling and overhead messages.
Pilot – The differentiator between the cell and the sector is still the PN offset of the pilot channel and the pilot signal is only gated on for 192 chips per slot.
Medium Access Control (MAC) Channel – This channel carries a number of controls including the Reverse Power Control (RPC), the Data Rate Control (DRC) Lock, and the reverse activity (RA) channels.