Friday, 30 October 2015

Synchronization and Random access procedures (FDD)


1.1、Cell search
      During cell search, a UE detects P-SCH, S-SCH and DL reference signals to identify a cell and retrieve various cell information.
      There are two types of cell search:
         • Initial cell search: UE is first accessing the network (power up)
         • Non-initial cell search: UE is preparing for handover (LTE_ACTIVE state) or cell reselection (LTE_IDLE state)
1.2、Initial cell search
      A multi-step initial cell search procedure is shown below.
         • P-SCH detection at each raster frequency: P-SCH carries slot timing and allows frequency acquisition
         • S-SCH detection: S-SCH carries transmit antenna configuration for P-BCH
         • Cell-ID
         • Allows radio frame boundary detection
         • Allows CP length estimation
         • DL common reference signal sequence detection allows identification of the cell-ID

      It is possible to read the P-BCH (carrying any other system information needed to access the system).
1.2、Non-initial cell search
      A non-initial cell search for LTE consists of three possibilities:
         • Intra-frequency cell search (from LTE to LTE)
         • Inter-frequency cell search (from LTE to LTE)
         • Inter-RAT cell search (from other RAT to LTE)
1.3、Timing synchronization
      TC is necessary to keep timing alignment within CP limits. eNB estimates the necessary timing advance on received signals from the UE:
         • Packets from data transmission
         • UL synchronization signals (if not data is transmitted)
         • If no data is transmitted for a long period of time, measurements are voided
      eNB tells UE to advance or retard its transmission. Orders could be:
         • Incremental: Node B periodically sends +1/-1 for ΔT increment/decrement of transmission. ΔT is fixed.
         • Absolute: Node B sends  ΔTUE on a need basis. 
2、Random access procedures
      An L1 random access procedure comprises the successful transmission of RA preamble and RA response. Subsequent transmission happens on PUSCH/PDSCH and is not part of the procedure.
      A random access channel (RACH) is a set of 72 subcarriers (1.08 MHz) in a subframe or set of consecutive subframes.
Physical non-synchronized random access procedure
      The following steps are required for the L1 random access procedure:
1. Cell information received by the UE:
      a. Available random access channels (number frequency position, time period, and timing offset)
      b. Available preamble format for the cell
      c. Available ZC-ZCZ sequences
      d. Power ramping step size
      e. Maximum number of preamble retransmissions
2. Select available random-access channel
3. Randomly select a preamble so that all allowed selections are equiprobable
4. Initial preamble transmission power level is determined using an openloop power control procedure Ptransmit = Pinit
5. Transmission counter is set to the maximum number of preamble retransmissions
6. Preamble is transmitted
7. If an adequate preamble response is detected, a message "ACK on non-synchronized random access" is passed to the higher layers and the physical random access procedure is exited
8. If no adequate preamble response is received:
      a. If the transmission counter is not zero, another random access channel and preamble are randomly selected
      b. Transmission power is determined; preamble retransmission power level may be the same as or set larger than the initial preamble transmission power level in steps of Pramp dB with Pramp ≥ 0
      c. If  P transmit < P max :
            i. Preamble is retransmitted
            ii. Otherwise, a message "NACK on non-synchronized random access" is passed to the higher layers and the physical random-access procedure is exited

Budi Prasetyo

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