4G RACH

Random access –

Random access (RA) is a basic procedure within cellular technologies for the terminal establishing uplink synchronization and starting the uplink transmission. The PRACH modulation is phase modulation as the sequences used are generated from Zadoff‐Chu sequences with phase differences between different symbols of the sequences. The random-access transmission is the only non‐synchronized transmission in the LTE uplink. Although the terminal synchronizes to the received downlink signal before transmitting on RACH, it cannot determine its distance from the base station. Thus, timing uncertainty caused by two‐way propagation delay remains on RACH transmissions. In LTE, the random access is used for several purposes, some of them are:

● Initial access or tracking area updates when establishing a radio link (RRC_idle to RRC_connected) ● To re‐establish a radio link after a radio link failure

● During the handover process to establish uplink synchronization to the target cell

● UL/DL data arrival during RRC_connected requiring random access procedure, when UL TA synchronization status is “non‐synchronized” or there are no PUCCH resources for SR available

● For positioning purposes when timing advance is needed for UE positioning

Random access procedure is handled by MAC and PHY Layer through PRACH (in UL) and PDCCH (in DL). RACH carries the preambles and occupies 6 resource blocks in a sub frame. RACH Process is as shown on Figure below: UE selects randomly a preamble from the list of preambles broadcasted in the BCCH then it calculates OLPC parameters (Initial Tx Power) and checks contention parameters (i.e. max. number of retries). After that, UE transmits the initial RACH and waits for a response before retry. Open loop power control ensures that each retry will be at a higher power level. Upon receipt of successful UL RACH preamble, eNB calculates power adjustment and timing advance parameters together with an UL capacity grant (so UE can send more info).

Random access settings have impact on mean call setup time. There is also slight capacity impact in case of heavy loaded network via interference mechanisms.