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Random Access


The Random Access Channel (RACH) is an uplink transport channel. The RACH is always received from the entire cell. The RACH is characterized by a collision risk and by being transmitted using open loop power control.


RACH access slot Structure
RACH access slot numbers and their spacing


RACH preamble is of length 4096 chips and consists of 256 repetitions of a signature of length 16 chips. There are a maximum of 16 available signatures. All 16 preamble signature codes available in every cells.

The 10 ms RACH message part radio frame is split into 15 slots, each of length Tslot = 2560 chips. Each slot consists of two parts, a data part to which the RACH transport channel is mapped and a control part that carries Layer 1 control information. The data and control parts are transmitted in parallel. A 10 ms message part consists of one message part radio frame, while a 20 ms message part consists of two consecutive 10 ms message part radio frames. The data part consists of 10*2k bits, where k=0,1,2,3. This corresponds to a spreading factor of 256, 128, 64, and 32 respectively for the message data part.


Random-access message part radio frame
Structure of the random-access message part radio frame


The Acquisition Indicator Channel (AICH) is a fixed rate (SF=256) physical channel used to carry Acquisition Indicators (AI). Acquisition Indicator AIs corresponds to signature s on the PRACH.

AICH Structure
Structure of Acquisition Indicator Channel



The Access Preamble Acquisition Indicator channel (AP-AICH) is a fixed rate (SF=256) physical channel used to carry AP acquisition indicators (API) of CPCH. AP acquisition indicator APIs corresponds to AP signature s transmitted by UE.

The Collision Detection Channel Assignment Indicator channel (CD/CA-ICH) is a fixed rate (SF=256) physical channel used to carry CD Indicator (CDI) only if the CA is not active, or CD Indicator/CA Indicator (CDI/CAI) at the same time if the CA is active. The structure of CD/CA-ICH is shown in figure 25. CD/CA-ICH and AP-AICH may use the same or different channelisation codes. The CD/CA-ICH has a part of duration of 4096chips where the CDI/CAI is transmitted, followed by a part of duration 1024chips with no transmission that is not formally part of the CD/CA-ICH. The part of the slot with no transmission is reserved for possible use by CSICH or possible future use by other physical channels.

Uplink Common Packet channel (CPCH) is an extension to the RACH channel for packet-based user data.

PCPCH Access Example:


CPCH Structure
PCPCH (similar to RACH) and AICH transmission as seen by the UE


DPCCH
PCPCH
AP-AICH
CD/CA-ICH
AP
CD/CA
Dedicated Physical Control Channel
Physical Common Packet Channel
Access Preamble Acquisition Indicator Channel
Collision Detection/Channel Assignment Indicator Channel
Access Preamble
Collision Detection/Channel Assignment


Indicators are means of fast low-level signalling entities which are transmitted without using information blocks sent over transport channels. The meaning of indicators is specific to the type of indicator. The indicators defined in the current version of the specifications are:
  • Acquisition Indicator (AI)
  • Access Preamble Indicator (API)
  • Channel Assignment Indicator (CAI)
  • Collision Detection Indicator (CDI)
  • Page Indicator (PI)
  • Status Indicator (SI)

    Indicators may be either boolean (two-valued) or three-valued. Their mapping to indicator channels is channel specific. Indicators are transmitted on those physical channels that are indicator channels (ICH).


    Further reading: 3GPP TS 25.211


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