LTE Random Access Procedure: Traps and PitfallsBy Andreas Roessler, Rohde & Schwarz
Thatâ€™s the reason why I link the abbreviation LTE more to the term LONG TERM EMPLOYMENT. The technology itself is high complex, utilizing advanced procedures on top of what we know from existing standards such as WCDMA or HSPA. This blog picks interesting aspects of LTE and takes a closer look while providing some explanations around it. A basic understanding of the concepts used for LTE and standardized by the 3rd Generation Partnership Project (3GPP) is assumed while reading.
It all starts with a base station called eNodeB (enhanced Node B, eNB) providing a LTE cell. This eNB transmits primary and secondary synchronization signals on the downlink to allow a mobile terminal (user equipment, UE) to find the timing and derive the radio frame start. Furthermore, the local-oscillator frequency of the UE can be locked to the eNB carrier frequency and the cell identity (cell ID) is derived. Apart from identifying the cell in which a UE is present, the cell ID is also required to process the cell-specific reference signals transmitted by the eNB. Subsequently, the broadcast channel (PBCH) and further channels can be decoded which contain relevant information to the cell infrastructure.
Initial Access ProcedureOnce the UE successfully finished a cell search, it will attempt a random access to register on the network which â€“ for the contention-based procedure â€“ consists of four steps.
Step (1) Random Access Preamble (RAP) on Physical Random Access Channel (PRACH).
In a subframe allowed for PRACH transmission the UE transmits a certain preamble (for a given configuration there are 64 possible preambles, refer to  for details). Upon reception of that the eNB detects which preamble from the given set the UE chose and also estimates the transmission timing to later eliminate the delays due to the propagation.
Step (2) Random Access Response (RAR)
As an answer to the preamble, the eNB is required to send a RAR to the UE within a certain time window. With the RAR the eNB replies where in time and frequency it detected the preamble, instructs the UE to align its timing so its uplink will be synchronized, and assigns a temporary ID. An uplink grant will lead to the next step. Further details can be found in .
Step (3) Layer 2/Layer 3 Message (MSG3)
Using the assigned resources the UE sends information to the eNB such as the temporary ID assigned in the previous step, its unique 48 bit UE identity or, if available, its Cell Radio Network Temporary Identifier (C-RNTI), a connection request, tracking area update, etc.
Step (4) Contention Resolution
If the eNB successfully received the MSG3 it replies with the unique ID to address a UE. This allows other UEs that may have attempted a random access at exactly the same time to detect a collision and start another attempt. The UE that is addressed with this message employs Hybrid Automatic Repeat reQuest (HARQ) to confirm a successful registration and transition to the state LTE_ACTIVE by sending an acknowledgement (ACK).
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Figure 1. PRACH measurement with R&SÂ®CMW500 Wideband Radio Communication Tester.
Before a random access procedure starts, several parameters need to be specified that get broadcasted in the System Information Block Type 2 (SIB2). The preamble initial received target power, a delta offset, and the power ramping step multiplied by the preamble transmission counter define the actual power with which the UE transmits its preamble on PRACH. Depending on the noise, path-loss, and fading, the dynamic range of the power can be quite large and may result in a varying signal quality depending on the UEâ€™s digital scaling, quantization, analogue power amplifier, and RF circuit and components.
The PRACH configuration index together with a PRACH frequency offset, a RACH root sequence, a cyclic shift, and a flag indicating a un-/restricted set define where in a time-frequency resource a preamble can be transmitted and how it should be calculated. A simple error is that the UE transmits a preamble in the wrong subframe, or on a wrong frequency offset. This can be verified with a spectrum analyzer using frame trigger.
A more convenient option is the Rohde&Schwarz CMW500 Wideband Radio Communication Tester with the LTE RF Measurement option â€“ PRACH (option CMW-KM500). Not only the power, subframe number, and frequency offset can be verified, but also an accurate timing offset can be measured. Furthermore, the chosen sequence number for the configured set is detected, an EVM, magnitude and phase errors vs. subcarrier are displayed, and power dynamics are shown. The constellation diagram shows the nature of the preamble, which is a CAZAC sequence. CAZAC stands for constant amplitude zero auto-correlation. Statistical counters allow qualitative results over many measurement trials that can be automated (see Figure 1).
Progressing through the random access procedure, higher layer functionality is required. Tests if the UE obeys the rules regarding the windows in which it can retransmit a preamble or message, the number of retransmissions, the offsets in the power ramping steps, the timing-adjustment, and of course the correct handling of IDs such as the C-RNTI, can all be verified with the CMW500 and the corresponding protocol test case packages.
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Figure 2. R&SÂ®CMW-KF506 Physical Layer test case package for LTE FDD highlighting test cases targeting LTE random access procedure verification.
If stack functionality is not yet available or developed by another team the physical layer test case packages that come with the CMW500 can be used for verifying preamble transmission and random access procedure. Figure 2 shows the test case package in the project explorer, a software tool that is used to manage, run or modify test campaigns.
The random access procedure is a fundamental functionality within LTE not only used for initial access, also for restoring uplink synchronization after being in discontinuous transmission mode (DTX). Its verification is an important step in the design process and Rohde&Schwarz offers with the R&SÂ®CMW500 Wideband Radio Communication Tester the right instrument and software tools.
Donâ€™t miss the next blog on â€œMaking sense of Sounding Reference Signals (SRS) in LTEâ€.
 3GPP TS 36.211 V8.9.0 Physical channels and modulation (Release 8), 2009-12.
 3GPP TS 36.321 V8.9.0 Medium Access Control (MAC) protocol specification (Release 8), 2010-06.