5.6 Random access procedure
25.2243GPPPhysical layer procedures (TDD)TS
The physical random access procedure described below is invoked whenever a higher layer requests transmission of a message on the RACH. The physical random access procedure is controlled by primitives from RRC and MAC.
5.6.1 Definitions
FPACHi : : The ith FPACH number parameter i indicates the position of the FPACH, the first position is corresponding to the first instance defined in IE "PRACH system information list)" (see [15]).
Li : Length of RACH transport blocks associated to FPACHi in sub-frames
NRACHi : The number of PRACHs associated to the ith FPACH
nRACHi : The number of a PRACH associated to the ith FPACH ranging from 0 to NRACHi-1
M : Maximum number transmissions in the UpPCH
WT : Maximum number of sub-frames to wait for the network acknowledgement to a sent signature
SFN’ : The sub-frame number counting the sub-frames. At the beginning of the frame with the system frame number SFN=0 the sub-frame number is set to zero.
5.6.1A UpPCH sub-channel
In order to separate different ASCs, UpPCH has N sub-channels associated with it (numbered from 0 to N-1). N may be assigned the value 1,2,4, or 8 by higher layer signaling.
– Sub-channel i for UpPCH is defined as the UpPTS timeslot in the sub_frame where SFN’ mod N = i.
Where SFN’: the sub-frame number counting the sub-frames. At the beginning of the frame with the system frame number SFN=0 the sub-frame number is set to zero.
The following figure illustrates the eight possible sub channels for the case, N=8.
Figure 10A : Example of UpPCH subchannels
5.6.2 Preparation of random access
When the UE is in idle mode, it will keep the downlink synchronisation and read the system information. From the used SYNC-DL code in DwPCH, the UE will get the code set of 8 SYNC-UL codes (signatures) assigned to UpPCH for random access.
The description (codes, spreading factor, midambles, time slots) of the P-RACH, FPACH, S-CCPCH (carrying the FACH transport channel) channel; mapping relation of RACH and FPACH; ASC (available SYNC-UL sequences and available sub-channels) sets for each RACH are broadcast on the BCH.
Thus, when sending a SYNC-UL sequence, the UE knows which FPACH resource, P-RACH resources and S-CCPCH resources will be used for the access.
The physical random access procedure described in this sub-clause is initiated upon request from the MAC sub-layer (see [18] and [19]).
Before the physical random-access procedure can be initiated, Layer 1 shall receive the following information by a CPHY-TrCH-Config-REQ from the RRC layer:
– The uplink access position by higher layers.
– The association between which signatures and which FPACHs; which FPACHs and which PRACHs; including the parameter values for each listed physical channel.
– The length Li of a RACH message associated to FPACHi can be configured to be either 1 or 2 or 4 sub-frames corresponding to a length in time of either 5 ms or 10 ms or 20 ms.
NOTE 1: NRACHi PRACHs can be associated with to FPACHi. The maximum allowed
NRACHi is Li.
– The available UpPCH sub-channels for each Access Service Class (ASC);
– The set of Transport Format parameters for the PRACH message;
– The "M" maximum number transmissions in the UpPCH;
– The "WT" maximum number of sub-frames to wait for the network acknowledgement to a sent signature; (1..4) the maximum value supported by Layer 1 is 4 sub-frames.
– The initial signature power "Signature_Initial_Power";
– The power-ramping factor Power Ramp Step [Integer];
The above parameters may be updated from higher layers before each physical random access procedure is initiated.
At each initiation of the physical random access procedure, Layer 1 shall receive the following information from the higher layers (MAC):
– The Transport Format to be used for the specific PRACH message;
– The ASC for the specific Random Access procedure;
– The data to be transmitted (Transport Block Set).
– The type of random access
5.6.3 Random access procedure
The physical random-access procedure shall be performed as follows:
UE side:
1 Set the Signature Re-Transmission Counter to M.
2 Set the Signature transmission power to Signature_Initial_Power.
3 Based on the type of random access and the transport format indicated by MAC layer, an E-RUCCH or unique RACH used for the radio access is chosen, and then randomly select one UpPCH sub-channel and one signature respectively from the available ones for the given ASC. The random function shall be such that each of the allowed selections is chosen with equal probability.
4 Transmit the signature at UpPCH or other uplink access position indicated by higher layers using the selected UpPCH sub-channel at the signature transmission power. In the case that the Commanded Signature transmission Power exceeds the maximum allowed value, set the Signature transmission Power to the maximum allowed power.
5 After sending a signature, listen to the relevant FPACH for the next WT sub-frames to get the network acknowledgement. The UE will read the FPACHi associated to the transmitted UpPCH only in the sub-frames fulfilling the following relation:
(SFN’ mod Li)=nRACHi ; nRACHi=0,…, NRACHi-1, Here, FPACH to which UE should listen is decided according to the following formula:
FPACHi= N mod M,
Where, N denotes the signature number (0..7) and M denotes the maximum number of FPACHs that defined in the cell.
6 In case no valid answer is detected in the due time: Increase the Signature transmission power by P0 = Power Ramp Step [dB], decrease the Signature Re-transmission counter by one and if it is still greater than 0, then repeat from step 3; else report a random access failure to the MAC sub-layer.
7 In case a valid answer is detected in the due time
a) set the timing and power level values according to the indication received by the network in the FPACHi
b) send at the sub-frame coming 2 sub-frames after the one carrying the signature acknowledgement, the RACH message on the relevant PRACH. In case Li is bigger than one and the sub-frame number of the acknowledgement is odd the UE will wait one more sub-frame. The relevant PRACH is the nRACHi th PRACH associated to the FPACHi if the following equation ifs fulfilled:
(SFN’ mod Li)=nRACHi ;
Here SFN’ is the sub-frame number of the arrival of the acknowledgement.
Both on the UpPCH and on the PRACH, the transmit power level shall never exceed the indicated value signalled by the network.
Network side:
– The node B will transmit the FPACHi associated with the received UpPCH only in the sub-frames fulfilling the following relation:
(SFN’ mod L)=nRACHi ; nRACHi=0,…, NRACHi-1,
Here, FPACH number i is selected according to the following formula based on acknowledged signature:
FPACHi= N mod M,
Where, N denotes the signature number (0..7) and M denotes the maximum number of FPACH that defined in the cell.
– The Node B will not acknowledge UpPCHs transmitted more than WT sub-frames ago
At the reception of a valid signature:
– Measure the timing deviation with respect to the reference time Tref of the received first path in time from the UpPCH and acknowledge the detected signature sending the FPACH burst on the relevant FPACH.
For examples on the random access procedure refer to Annex CB.
5.6.3.1 The use and generation of the information fields transmitted in the FPACH
The Fast Physical Access CHannel (FPACH) is used by the Node B to carry, in a single burst, the acknowledgement of a detected signature with timing and power level adjustment indication to a user equipment.
The length and coding of the information fields is explained in TS25.221 sub-clause 5A.3.3.1 .
5.6.3.1.1 Signature reference number
The Signature Reference Number field contains the number of the acknowledged signature. The user equipment shall use this information to verify whether it is the recipient of the FPACH message.
5.6.3.1.2 Relative sub-frame number
The Relative Sub-Frame Number field indicates the current sub-frame number with respect to the sub-frame at which the acknowledged signature has been detected.
The user equipment shall use this information to verify whether it is the recipient of the FPACH message.
5.6.3.1.3 Received starting position of the UpPCH (UpPCHPOS)
The received starting position of the UpPCH (UpPCHPOS) field indirectly indicates to the user equipment the timing adjustment it has to implement for the following transmission to the network. The node B computes the proper value for this parameter according to the following rules: UpPCHPOS = UpPCHRxpath – UpPCHTS
where
UpPCHRxpath: time of the reception in the Node B of the SYNC-UL to be used in the uplink synchronization process
UpPCHTS: time instance 128 chips prior to the start of the UpPCH according to the Node B internal timing
This information shall be used by the UE to adjust its timing when accessing the network, as described in section [5.2 ‘Uplink Synchronisation’] .
5.6.3.1.4 Transmit power level command for the RACH message
This field indicates to the user equipment the power level to use for the RACH message transmission on the FPACH associated P-RACH.
The network may set this value based on the measured interference level (I) (in dBm) on the specific PRACH and on the desired signal to interference ratio (SIR) (in dB) on this channel as follows:
Transmit Power Level Command for the PRACH(PRXPRACH,des)
PRXPRACH,des is the desired receive power level on the PRACH.
The UE shall add to this value the estimated path-loss to compute the power level to transmit for the PRACH.
5.6.3A E-RUCCH procedure
Requests for the transmission of an E-RUCCH are controlled by higher layers [18].
The available eight SYNC_UL signatures on the primary carrier in a cell is devided into two subsets, one for the access of RACH information and the other for the access of E-RUCCH information. The available eight SYNC-UL signature on the secondary frequencies in cell are all for the access of E-RUCCH information.
When a Node B detects a SYNC_UL signature and acknowledges it on the related FPACH, it should do some recordings, including the FPACH channel number, the sub-frame on which the acknowledgement is sent and the SYNC_UL signature number. When a PRACH or E-RUCCH comes from a UE, the Node B should derive the related FPACH and the sub-frame on which the acknowledgement was sent for the UE and find the right record. The signature number in the record can help the Node B know the access type.
Random access procedure for enhanced uplink is basically same as random access procedure in subclause 5.6.3, only adding some new definitions.
LiE is the Length of E-RUCCH information transport blocks associated to FPACHi in sub-frames.
NRACHi is the number of PRACHs associated to the ith FPACH.
NE-RUCCHi is the number of E-RUCCHs associated to the ith FPACH and NE-RUCCHi equals to .
When SF of PRACH code equals to 16, LiE will be 2, otherwise LiE will be 1。
When SF of PRACH code equals to 4, SF of E-RUCCH will be 8, otherwise E-RUCCHs has the same SF with PRACH.
When nE-RUCCHi equals to nRACHi, E-RUCCH will share the same code resource with PRACH. And when SF of PRACH code equals to 4, the code resource assigned to PRACH including two codes (code i and code i+1) of SF 8, E-RUCCH can use the ith code of SF 8.
If FPACHi sent an acknowledgement for E-RUCCH information, the sub-frames on which an acknowledgement is sent on FPACHi is fulfilling the following relation:
(SFN’ mod LiE)=nE-RUCCHi ; nE-RUCCHi=0,…, NE-RUCCHi-1,
Where, SFN’ is the sub-frame number of the acknowledgement on FPACH
Accordingly, the code resource assigned to PRACH may be used by PRACH or E-RUCCH, we should make two prescript avoiding the collision between PRACH and E-RUCCH.
When Node B sent a FPACHi for in sub frame K，Node B could not send a FPACHi for before sub frame K+Li；
When Node B sent a FPACHi for in sub frame K，Node B could not send a FPACHi for before sub frame K+LiE。
The interval between the acknowledgement on FPACH and transmission of E-RUCCH is fixed for a UE. The UE will send at the sub-frame coming 2 sub-frames after the one carrying the signature acknowledgement. In case LiE is bigger than one and the sub-frame number of the acknowledgement is odd the UE will wait one more sub-frame.
The transmission power and the transmission timing are set according to subclause 5.1.1.7 and 5.2.7 respectively.
5.6.4 Random access collision
When a collision is very likely or in bad propagation environment, the Node B does not transmit the FPACH or cannot receive the SYNC-UL. In this case, the UE will not get any response from the Node B. Thus the UE will have to adjust its Tx time and Tx power level based on a new measurement and send a SYNC-UL again after a random delay.
Note that at each (re-)transmission, the SYNC-UL sequence and the UpPCH sub-channel will be randomly selected again by the UE.
Note: Due to the two-step approach a collision most likely happens on the UpPCH. The resources allocated to PRACH and E-RUCCH are virtually collision free. This two-step approach will guarantee that the RACH resources can be handled with conventional traffic on the same UL time slots.