8 Idle periods for IPDL location method
25.2143GPPPhysical layer procedures (FDD)Release 17TS
8.1 General
To support time difference measurements for location services, idle periods can be created in the downlink (hence the name IPDL) during which time transmission of all channels from a Node B is temporarily seized. During these idle periods the visibility of neighbour cells from the UE is improved.
The idle periods are arranged in a predetermined pseudo random fashion according to higher layer parameters. Idle periods differ from compressed mode in that they are shorter in duration, all channels are silent simultaneously, and no attempt is made to prevent data loss.
In general there are two modes for these idle periods:
– Continuous mode, and
– Burst mode.
In continuous mode the idle periods are active all the time. In burst mode the idle periods are arranged in bursts where each burst contains enough idle periods to allow a UE to make sufficient measurements for its location to be calculated. The bursts are separated by a period where no idle periods occur.
8.2 Parameters of IPDL
The following parameters are signalled to the UE via higher layers:
IP_Status: This is a logic value that indicates if the idle periods are arranged in continuous or burst mode.
IP_Spacing: The number of 10 ms radio frames between the start of a radio frame that contains an idle period and the next radio frame that contains an idle period. Note that there is at most one idle period in a radio frame.
IP_Length: The length of the idle periods, expressed in symbols of the CPICH.
IP_Offset: A cell specific offset that can be used to synchronise idle periods from different sectors within a Node B.
Seed: Seed for the pseudo random number generator.
Additionally in the case of burst mode operation the following parameters are also communicated to the UE.
Burst_Start: Specifies the start of the first burst of idle periods. 256Burst_Start is the SFN where the first burst of idle periods starts.
Burst_Length: The number of idle periods in a burst of idle periods.
Burst_Freq: Specifies the time between the start of a burst and the start of the next burst. 256Burst_Freq is the number of radio frames of the primary CPICH between the start of a burst and the start of the next burst.
8.3 Calculation of idle period position
In burst mode, burst #0 starts in the radio frame with SFN = 256Burst_Start. Burst #k starts in the radio frame with SFN = 256Burst_Start + k256Burst_Freq( k = 0,1,2, …). The sequence of bursts according to this formula continues up to and including the radio frame with SFN = 4095. At the start of the radio frame with SFN = 0, the burst sequence is terminated (no idle periods are generated) and at SFN = 256Burst_Start the burst sequence is restarted with burst #0 followed by burst #1 etc., as described above.
Continuous mode is equivalent to burst mode, with only one burst spanning the whole SFN cycle of 4096 radio frames, this burst starting in the radio frame with SFN = 0.
Assume that IP_Position(x) is the position of idle period number x within a burst, where x = 1, 2, …, and IP_Position(x) is measured in number of CPICH symbols from the start of the first radio frame of the burst.
The positions of the idle periods within each burst are then given by the following equation:
IP_Position(x) = (x IP_Spacing 150) + (rand(x modulo 64) modulo (150 – IP_Length)) + IP_Offset;
where rand(m) is a pseudo random generator defined as follows:
rand(0) = Seed;
rand(m) = (106rand(m – 1) + 1283) modulo 6075, m = 1, 2, 3, ….
Note that x is reset to x = 1 for the first idle period in every burst.
Figure 6 below illustrates the idle periods for the burst mode case.
Figure 6: Idle Period placement in the case of burst mode operation