Random Access Channel (RACH) Procedure Senthil Kumar Last Updated: 28/07/2007
RACH is a common transport channel in the uplink and is always mapped one-to-one onto physical channels (PRACHs). In one cell, several RACHs/PRACHs may be configured. If more than one PRACH is configured in a cell, the UE performs PRACH selection randomly. The parameters for RACH access procedure includes: access slots, preamble scrambling code,preamble signatures,spreading factor for data part, available signatures and subchannels for each Access Service Class(ASC) and power control information. The Physical channel information for PRACH is broadcasted in SIB5/6 and the fast changing cell parameters such as uplink interference levels used for open loop power control and dynamic persistence value are broadcasted in SIB7. RACH access procedure follows slotted-ALOHA approach with fast acquisition indication combined with power ramping in steps. Maximum of 16 different PRACHs can be offered in a cell, in FDD, the various PRACHs are distinguished either by employing different preamble scrambling codes or by using common scrambling code with different signatures and subchannels.With in a single PRACH, a partitioning of the resources between the maximum 8 ASC is possible, thereby providing a means of access prioritization between ASCs by allocating more resources to high priority classes than to low priority classes.ASC 0 is assigned highest priority and ASC 7 is assigned lowest priority.ASC 0 shall be used to make emergency calls which has got more priority.The available 15 access slots are split between 12 RACH subchannels. The RACH transmission consists of two parts, namely preamble transmission and message part transmission. The preamble part is 4096 chips, transmitted with spreading factor 256 and uses one of 16 access signatures and fits into one access slot. ASC is defined by an identifier i that defines a certain partition of the PRACH resources and is associated with persistence value P(i). The persistence value for P(0) is always set to one and is associated with ASC 0.The persistence values for others are calculated from signaling. These persistence values controls the RACH transmissions. INITIATING THE RACH PROCEDURE To start a RACH procedure,the UE selects a random number r, between 0 and 1 and if r<= P(i), the physical layer PRACH procedure is initiated else it is deferred by 10 ms and then the procedure is started again. Once the UE PRACH procedure is initiated, then the real transmission takes place.
PREAMBLE AND DATA PART TRANSMISSION As described above, the preamble part transmission starts first.The UE picks one access signature of those available for the given ASC and an initial preamble power level based on the received primary CPICH power level and transmits by picking randomly one slot out of the next set of access slots belonging to one of the PRACH subchannels associated with the relevant ASC. The UE then waits for the appropriate access indicator sent by the network on the downlink Acquisition Indicator Channel(AICH) access slot which is paired with the uplink access slot on which the preamble was sent.There are 3 possible scenarios possible. 1. If the Acquisition Indication(AI) received is a positive acknowledgement, then UE sends the data after a predefined amount of with a power level which is calculated from the level used to send the last preamble. 2. IF the AI received is a negative acknowledgement, the UE stops with the transmission and hands back control to the MAC layer.After a backoff period, the UE will regain access according to the MAC procedure based on persistence probabilities. 3. If no acknowledgement is received, then it is considered that network did not receive the preamble.If the maximum number of preambles that can be sent during a physical layer PRACH procedure is not exceeded,the terminal sends another preamble by increasing the power in steps.The ability of the UE to increase its output power,in terms of steps to a specific value is called as open loop power control. RACH follows open loop power control References : 1. Performance Analysis of RACH procedure in WCDMA by Jeyaratnarajah Niththiyanathan 2. 3GPP TS 25.214, 3rd Generation Partnership Project: Physical layer procedures (FDD), V5.1.0
