Access Channel
Application: In IS-95, up to 32 Access Channels are associated with the pilot offset of a particular base station. The mobile uses an Access Channel to transmit to a base station when it is not in the traffic mode. Overhead or control data and short messages are sent to the base station at a rate of 4800 bits/second. Mobile transmit power control and randomized timing are used to control or limit contention, which is the simultaneous reception of access messages at the base station. An Access Channel is identified by its unique long code mask, which in part depends on the pilot offset of the base station to which the mobile is transmitting.
Access Channel
Example: After the mobile initially acquires the pilot of a base station and reads the data on the Synchronization channel, it transmits to the base station on an Access Channel. This is the first indication that the base station has that the mobile is present and active. All communications with the base station occur using the Access Channel until the mobile is placed in the traffic mode.
Individual access messages are contained in short "bursts" of data called probes. Groups of randomly, but closely, spaced probes are called sequences. An access attempt consists of a few probe sequences. Closed-loop power control, incrementally increasing power, and randomized timing between access probes is used during an access sequence. The Access Channel uses rate 1/3 convolutional coding, times 2 symbol repetition, interleaving, 64-ary orthogonal modulation, long-code scrambling with an offset based on an access-channel mask for privacy, and quadrature PN spreading using offset or staggered QPSK. The initial portion of each access probe, called the probe preamble, uses a bit pattern which facilitates acquisition by the receiver at the base station.
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