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Thread: Random Access Channel (RACH) Procedure

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    Default Random Access Channel (RACH) Procedure

    <h4>
    <a href="http://www.3g4g.co.uk/Tutorial/SK/">Senthil Kumar</a>
    </h4>
    <i>Last Updated: 28/07/2007</i>

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    <!-- Begin Main Code -->
    <br><br>
    RACH is a common transport channel in the uplink and
    is always mapped one-to-one onto physical channels (PRACHs). In one
    cell, several RACHs/PRACHs may be configured. If more than one PRACH is
    configured in a cell, the UE performs PRACH selection randomly.
    The parameters for RACH access procedure
    includes: access slots, preamble scrambling code,preamble
    signatures,spreading factor for data part, available signatures and
    subchannels for each Access Service Class(ASC) and
    power control information. The Physical channel information for PRACH
    is broadcasted in SIB5/6 and the fast changing cell parameters such as
    uplink interference levels used for open loop power control and dynamic
    persistence value are broadcasted in SIB7.
    RACH access procedure follows slotted-ALOHA approach with fast acquisition indication combined with power ramping in steps.

    Maximum of 16 different PRACHs can be offered in
    a cell, in FDD, the various PRACHs are distinguished either by
    employing different preamble scrambling codes or by using common
    scrambling code with
    different signatures and subchannels.With in a single PRACH, a
    partitioning of the resources between the maximum 8 ASC is possible,
    thereby providing a means of access prioritization between ASCs by
    allocating more resources to high priority classes than to low priority
    classes.ASC 0 is assigned highest priority and ASC 7 is assigned lowest
    priority.ASC 0 shall be used to make emergency calls which has got more
    priority.The available 15 access slots are split between 12 RACH
    subchannels.
    The RACH transmission consists of two parts, namely preamble transmission
    and message part transmission. The preamble part is 4096 chips, transmitted
    with spreading factor 256 and uses one of 16 access signatures and fits
    into one access slot.

    ASC is defined by an identifier i that defines a
    certain partition of the PRACH resources and is associated with
    persistence value P(i). The persistence value for P(0) is always set to
    one and is associated with ASC 0.The persistence values for others are
    calculated from signaling. These persistence values controls the RACH
    transmissions.
    INITIATING THE RACH PROCEDURE
    To start a RACH procedure,the UE selects a
    random number r, between 0 and 1 and if r&lt;= P(i), the physical layer
    PRACH procedure is initiated else it is deferred by 10 ms and then the
    procedure is started again.
    Once the UE PRACH procedure is initiated, then the real transmission
    takes place.
    PREAMBLE AND DATA PART TRANSMISSION
    As described above, the preamble part
    transmission starts first.The UE picks one access signature of those
    available for the given ASC and an initial preamble power level based
    on the received primary CPICH power level and transmits by picking
    randomly one slot out of the next set of access slots belonging to one
    of the PRACH subchannels associated with the relevant ASC.
    The UE then waits for the appropriate access
    indicator sent by the network on the downlink Acquisition Indicator
    Channel(AICH) access slot which is paired with the uplink access slot
    on which the preamble was sent.There are 3 possible scenarios possible.
    <ol><li>If the Acquisition Indication(AI) received is a positive acknowledgement, then UE sends the data after a predefined
    amount of with a power level which is calculated from the level used to send the last preamble.

    </li><li>IF the AI received is a negative acknowledgement, the UE stops
    with the transmission and hands back control to the MAC layer.After a
    backoff period, the UE will regain access according to the MAC
    procedure based on persistence probabilities.
    </li><li>If no acknowledgement is received, then it is considered that
    network did not receive the preamble.If the maximum number of preambles
    that can be sent during a physical layer PRACH procedure is not
    exceeded,the terminal sends another preamble by increasing the power in
    steps.The ability of the UE to increase its output power,in terms of
    steps to a specific value is called as open loop power control.
    RACH follows open loop power control
    </li></ol>

    References :
    1. Performance Analysis of RACH procedure in WCDMA by Jeyaratnarajah Niththiyanathan
    2. 3GPP TS 25.214, 3rd Generation Partnership
    Project: Physical layer procedures (FDD), V5.1.0<!-- End Main Code -->
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    Hi, How evaluate performance Randon Acces

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