ACCESS PARAMETERS IN IDLE MODE
Initial PRACH (The first PRACH that UE transmit) is determined by the following formula.
Initial RACH Preamble Power = Primary CPICH TX Power – CPICH_RSCP + UL_Interference + Constant Value
CPICH_RSCP is the power directly measured by UE and all other parameters are calculated (obtained) from system information. The related parameter and system information is as follows.
System Information
|
Information Element
|
Example Value
|
SIB3
|
Maximum Allowed UL Tx Power
|
21 dBm
|
SIB5
|
Primary CPICH Power
|
-8 dBm
|
Power Ramp Step
|
3 dB
| |
Mmax
|
2
| |
NB01min
|
10
| |
NB02max
|
10
| |
SIB7
|
UL Interference
|
-92
|
If UE fail to get AICH for the PRACH, it increment the PRACH power by Power Ramp Step (SIB5) and transmit the PRACH again. UE repeat this process until it gets AICH or it retried the specific number specified in SIB5.
Following screenshot is an example of PRACH transmission and retransmission. This is from a test resultl of 34.121 8.4.2.1. In this test case, Network intentionally does not send AICH so that UE keep sending PRACH with specified power increment in the specified number of times.
AICH POWER OFFEST - Power level of the AICH relative to that of the primary CPICH.
Definition: Power level of the AICH relative to that of the primary CPICH.
IE Value Engineering Units
Allowed Range [-22 … +5] -22 … +5 dB
Recommended -7 to -5 -7 to -5 dB
Setting Tradeoff: If parameter is too small, the probability of acquisition indicator
misdetection and false alarm increase. Indicator misdetection leads to increased access time
and to increased uplink interference caused by the access attempt. A false alarm causes the
UE to send a RACH message packet (e.g., RRC connection setup) which is not expected and
not detected by the Node B, thus making the UE wait for a response. Higher layers have to
recover from this phenomenon resulting in increased access delay.
If parameter is too large, downlink capacity is unnecessarily consumed.
Dependencies/Constraints: None.
Traceability: TS25.331 [35] Sect. 10.3.6.3.
RRC Message Structure: SIB5/SIB6 FDD AICH Power offset
Power Ramp Step
Definition: UE transmit power increase between subsequent transmissions on the RACH
when no acquisition indicator is received.
Setting Tradeoff: If parameter is too small, the number of RACH preambles and the access
latency increase. If parameter is too large, the uplink interference increases.
Dependencies/Constraints: This parameter should be set to relatively small value if the
Preamble Retrans Max IE is set to a relatively large value, and vice versa, as their effect on
access latency and uplink interference is compounded.
Preamble Retrans Max
Definition: Maximum number of preamble transmissions in one preamble ramping cycle.
IE Value Engineering Units
Allowed Range [1…64] 1…64
Recommended 6 to 10 6 to 10
Setting Tradeoff: If parameter is too small, the probability of successful preamble ramping
cycle decreases. If parameter is too large, the uplink interference increases.
Mmax
Definition: Maximum number of preamble ramping cycles.
IE Value Engineering Units
Allowed Range [1…32] 1…32
Recommended 3 to 6 3 to 6
Setting Tradeoff: If parameter is too small, the RACH access may not take full advantage of
the time diversity provided by preamble ramping cycle repetition. If parameter is too large,
the uplink capacity consumption may unnecessarily increase.
