The mobile can start the random access transmission at the beginning of a number of well-defined time intervals, denoted access slots. There are 15 access slots per two frames which are spaced at 5120 chips apart. The structure of the random access transmission is shown in the following figure. The structure consists of one or more preambles followed by a message part that can be either 10 or 20 ms in duration. The minimum time between preambles and from the preamble to message part is 15360 chips. The PRACH preamble consists of a complex valued code that is obtained from a preamble scrambling code and a preamble signature.

PRACH Message Part

The following figure shows the principle of generating the PRACH message part. The PRACH message part consists of a control and a data part which are spread to the chip rate using spreading codes cc and cd respectively. The gains are adjusted by gain factors, βc and βd. The data and the control parts are summed as a complex pair and scrambled by the complex scrambling code Sr-msg,n.

Channel Codes: The channelization code for the control part is cc = C256,m where m = 16s+15, and s is the preamble signature. The channelization code for the data part is cd = CSF,m where SF is the spreading factor (32 to 256) and m = SF*s/16 . These are calculated automatically.

PRACH Message Structure

The structure of the PRACH message is shown in the following figure. The parameter k refers to the slot number that effectively determines the symbol rate.

The control part of the message consists of pilot bits(as shown in the following table) and two TFCI bits. If the message length is 20 ms, the TFCI bits are repeated for the second 10 ms frame. The TFCI bits are calculated from a 10-bit TFCI index.

Parameters

You can set the following parameters for the PRACH:

Scrambling Code: Specifies the code number used to set up the scrambling generator. Scrambling code is used along with the preamble signature to define the preamble bits. Valid range is 0 to 8191.

Number of Preambles: Specifies the number of preambles transmitted before the message part of the preamble. Valid range is 1 to 10.

Preamble Signature: Sets the preamble signature. Valid range is 0 to 15.

Preamble Power: Sets the relative power of the preamble. Valid range is 0 to –60 dB.

Symbol Rate: Specifies the symbol rate of the message. Valid values are 15 ksps, 30 ksps, 60 ksps, and 120 ksps.

Data Power: Sets the relative power of the data part of the message. Valid range is 0 to –60 dB.

Control Power: Sets the relative power of the control part of the message. Valid range is 0 to –60 dB.

Message Length: Sets the message length in frames. Valid values are 1 and 2. One frame is 10 ms.

TFCI Index: Sets the TFCI index. Valid range is 0 to 1023. The TFI index is used to determine the TFCI bits.

Data Source: Specifies the data source. Refer to Selecting the Data Source topic for more information on data source.