Stage 4: Authentication and Security Stage 4: Authentication and Security Authentication ensures that the user is authorized to use particular network services. EPS AKA is the authentication and key agreement procedure used for the mutual authentication of the UE and the network. In LTE, encryption is possible for the NAS, RRC and user plane over the radio network. Different encryption algorithm options are provided for each of these protocol layers. Integrity protection is done on the NAS and the RRC signaling messages. Authentication requests and authentication response messages trigger counter pegs. Reason for failures are normally indicated in this [...]
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Stage 3: Network Attach Stage 3: Network Attach The UE initiates the attach procedure by transmitting an RRC Connection Setup Complete message (to the eNB) containing the Attach Request message intended for the MME. The Attach Request message contains the old GUTI or IMSI, the NAS key set identifier, the UE network capability and an ESM message container (which has the PDN Connectivity Request). The PDN Connectivity Request Message indicates whether the UE wants to perform the IP address allocation during the execution of the procedure and, when known, it indicates the UE’s Ipversion capability (IPv4, IPv4/IPv6, IPv6), which [...]
E-UTRA States E-UTRA States A UE is in the RRC Connected state when an RRC connection is established, otherwise, the UE is in RRC Idle state. The RRC connection is always initiated by the UE and, typically, the network initiates the release of the RRC connection. The RRC states can further be characterized as follows: RRC Idle: UE-controlled mobility, The UE monitors a paging channel to detect incoming calls and system information changes, The UE performs neighboring cell measurements and cell (re)selection, The UE acquires system information, and A UE-specific DRX may be configured by the upper layers. RRC [...]
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 [...]
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 [...]