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UMTS (Random Access procedure) - 20

Random Access procedure [Under UMTS]
» Phy procedures (FDD) - 25.214 «
» Phy channels and their mapping on transport channels (FDD) - 25.211 «
» Spreading and Modulation (FDD) - 25.213 «

Let us come back to our initial flow of discussion started in UE power on article: UE power on - scanning/synchronisation/PLMN search - PLMN/cell selection - NAS request for RRC connection - RRC connection establishment.

RRC connection establishment start with RRC Connection Request message; this is to be sent over RACH/PRACH. At this point, UE presence is not known to network and so UE has to perform random access procedure over (collision prone) shared uplink channel (known as PRACH).

There are two physical channels which are of concern here: AICH (Acquisition Indicator CHannel) in DL and PRACH (Physical Random Access CHannel) in UL. Both channels are divided in slots, with AICH in sync with P-CCPCH and PRACH ahead of AICH by a known time interval (7,680 chips) as shown below.

PRACH access slots are organised in 12 sets, called RACH sub-channels. The organisation has been specified in 3GPP TS 25.214.

Network further allocates RACH sub-channels to 8 Access Service Classes (ASC). Simply speaking, ASC denote the priority of the service for RACH request is being made; 0 is highest priority and 7 lowest. In addition to RACH sub-channels for each ASC, UE extract information required random access like preamble scrambling code, set of available signatures etc.

Here is roughly how random access works (an example is shown in above diagram):

1) UE chooses one of the available signatures from system information (signature work like a reference when later network reply back we will see in later step).

2) It scrambles chosen signature with preamble scrambling code and sends it on one of the access slots. Access slot should belong to sub-channel of appropriate ASC. UE also has to estimate initial power with which it is to send preamble.

3) UE then wait on AICH. Network sends back received signature on AICH. This acts as an acknowledgement for UE.

4) Due to less power or due to collision with preamble sent by other UE, preamble might not be received by network. If UE does not receive ack over AICH, UE try re-transmission with 1 db higher power.

5) Once AICH ack is detected, UE transmits RACH message over PRACH. There is one-to-one mapping [3GPP TS 25.213] between preamble scrambling code and scrambling code that is to be used for RACH message. So UE knows which code to use for RACH message.

Also, as scrambling code used for RACH message is different than that used for preamble, even though other UEs send preamble at the same time as RACH message, node B can still retrieve RACH message.

In effect, what we have done above is: we used common code to get access and then once acknowledged, we used different but known code to send actual message.

Needless to mention, retries are made certain number of times (as indicated by network in system information) and then access is aborted.

References: UMTS by Sanchez and Thioune, and WCDMA for UMTS by Holma and Toskala.

Copyright © Samir Amberkar 2010§ §

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