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WCDMA PRACH Channel-Physical Random Access Channel


This tutorial section on WCDMA (UMTS) basics covers following sub topics:
UMTS Tutorial  UMTS channels  measurements  PHY layer  TDD vs FDD PHY  RRC States  Security  Cell search  Bands  UARFCN  Frame 


This page on WCDMA PRACH describes WCDMA Physical Random Access Channel(PRACH).It mentions links for LTE PRACH and GSM RACH channel basics.

Physical Random Access Channel (PRACH) is an uplink channel used by UE for connection request purpose. PRACH is used to carry the RACH transport channel data.

WCDMA PRACH fig1

The following steps are carried out during a random access burst:

1. The terminal decodes the BCH (PCCPCH) of the target cell to find out the cell specific spreading codes available for preamble and message parts, the signatures and access slots available in the cell, the spreading factor allowed for message part, the PCCPCH transmit power level.

2. The mobile randomly selects the signature and access slot to be used for the RACH burst.

3. The mobile estimates the downlink path loss and calculates the required uplink transmit power to be used for the random access burst.

4. A 1ms preamble is sent with selected signature.

5. The terminal decodes the AICH to see whether the base station has detected the preamble.

6. In case no AICH is detected, the terminal increases the preamble transmission power by a step given by the station, as a multiple of 1 dB and transmits in the next available access slot.

7.If AICH is received with the signature S of the PRACH, then the message part is sent.

WCDMA PRACH fig2

Structure of the random-access message part radio frame is mentioned in the figure2 above.

Spreading of PRACH message part is mentioned below in figure3

WCDMA PRACH fig3

PRACH RELATED LINKS

LTE PRACH channel  GSM RACH channel  UE RACH not reaching to the GSM network 

WCDMA UMTS RELATED LINKS

UMTS Network Architecture
UMTS Protocol Stack
UMTS Logical, Transport and physical channels
UMTS Spreading,scrambling and modulation
WCDMA MO CS call
WCDMA PS Call
CDMA vs GSM
WCDMA Compressed mode

RF and Wireless Terminologies


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