RRC Connection Setup RRC Connection Setup RRC connection establishment involves the establishment of SRB 1. The E-UTRAN completes RRC connection establishment prior to completing the establishment of the S1 connection (i.e., prior to receiving the UE context information from the EPC). Consequently, AS security is not activated during the initial phase of the RRC connection. During this initial phase of the RRC connection, the E-UTRAN may configure the UE to perform measurement reporting. When the UE sends an RRC Connection Request to the eNB, this message is sent using the RLC-TM, and includes the UE identity, and the establishment [...]
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Improving RACH Success Improving RACH Success The dynamic range of a UE is defined by its minimum and maximum transmit power. On the other hand, the dynamic range of the random access procedure is defined by the product of the preamble step size (powerStep) and the max number of preambles (TransMax). For example, if powerRampStep = 2 dB, and preambleRetransMax = 10, the preamble dynamic range will be equal to 20 dB. This is far less than the UE’s dynamic range. Therefore, it is imperative the initial preamble transmit power, defined by the open-loop power control equation, is accurate. [...]
Timeline for PRACH Probes Timeline for PRACH Probes When the eNB sends a Back-off Indicator (BI) in the RAR sub-header, it is directing the UE to back off the next preamble transmission. Based on this BI value (x milliseconds), the UE will generate a random back-off value, say y, in the range of 0 to x milliseconds. The UE may transmit the next preamble, at the next valid subframe as defined by the eNB PRACH Configuration Index, after t+3+w+y. This back-off process randomizes retransmissions resulting in reduced chances of collision of preambles. To get updated with posts on new [...]
Preamble Transmission Power Preamble Transmission Power The random access is often the first transmission from the UE, and it is a short transmission (less than 3ms at most). Consequently, the network does not have an opportunity to power control the PRACH transmitted by the UE. Instead, the UE must estimate the minimum amount of power it needs to send the access request without causing excessive interference. The UE receives a number of key parameters for PRACH power control in SIB 2, including: 1.Preamble Initial Received Target Power: The power level the eNB would like to receive for a random [...]
PRACH Power Calculation PRACH Power Calculation After receiving the SIB Type 2 information, the UE determines the preamble transmit power. These parameters include the Preamble Initial Target Power that the cell wants to receive the preamble at, the maximum number of preambles the UE may send during the random access procedure, the power increase value in decibels between successive preamble transmissions and the cell Reference Signal transmit power per reference signal resource element. Typically, SIB2 is received at system acquisition time and cell reselection time. When the random access procedure is initiated, the UE executes the following sequence: 1.Measure [...]
Random Access Preamble Generation & Selection Random Access Preamble Generation & Selection This slide shows how the random access preamble is constructed. The eNB broadcasts root sequence index and cyclic shift, as well as number of non-dedicated preambles in a cell. There are 64 preambles available in a cell. A UE in the cell listens to the broadcast, generates 64 preambles, and randomly picks out one of the number of non-dedicated preambles (preamble 0 ~ # of non-dedicated preambles -1). The root sequence index and the cyclic shift are determined by network planning. The cyclic shift is related to [...]
Preamble Format Examples Preamble Format Examples There are four possible formats for the random access preamble, differing by the length of the fields within the preamble. Bigger preambles take longer to transmit, but are better suited for larger cells. The specific format to be used is set in SIB 2. SIB 2 also indicates when, and in which system frames and subframes, the preamble may be sent. The specific preamble sequence to be transmitted is selected randomly using the root index and cyclic shift information from SIB 2. There are a total of 64 different sequences available in each [...]
RACH Parameters in SIB2 RACH Parameters in SIB2 The parameters utilized by the UE Random Access Procedure are read from transmitted message blocks called System Information Blocks (SIBs). The UE reads these messages while it is in Idle Mode. In particular, SIB Type 2 provides a number of detailed parameters needed to generate the preambles and control transmissions on the RACH. These parameters include: The configuration of the Physical Random Access Channel (PRACH), such as the format to be used for the random access preamble and the sub-frames that can be used for access. Access control settings for managing [...]
Stage 2: Contention-Based Access Stage 2: Contention-Based Access The basic steps involved in contention-based access are: Step 2A: Once a UE has selected a cell, it listens to the system overhead information transmitted on the downlink. The System Information Blocks (SIBs) provide configuration parameters for the network; SIB2, in particular, includes key parameters for initial system access. Step 2B: The UE measures the Reference Signal strength and does a PRACH power calculation to determine the preamble transmit power. Step 2C: The UE selects a random preamble and sends it over the PRACH. The preamble indicates that a UE requires [...]
Initial Cell Selection Initial Cell Selection The cell selection criterion is defined in 3GPP standards 36.304 UE Procedures in Idle Mode. A cell is selected if the Srxlev value computed is > 0. Srxlev = Qrxlevmeas – (Qrxlevmin +Qrxlevminoffset) - Pcompensation Here is a definition of each term in the equation: Qrxlevmeas is the measured RSRP value of the cell the UE is evaluating. Qrxlevmin is the minimum Qrxlev value indicated by the cell in SIB1 message for cell selection. Qrxlevminoffset is sent by the cell in SIB1 as well. This value is used only in the case of [...]