74 lines
3.1 KiB
Plaintext
74 lines
3.1 KiB
Plaintext
[[interf_rep]]
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== Interference reporting
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According to 3GPP 48.058, section 6.1, the BTS shall periodically report the
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interference levels on *idle* channels using the "Radio resource indication"
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procedure. This is done by sending the `RF RESource INDication` message,
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which is specified in sections 8.6.1 and 9.3.21.
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// TODO: BSC -> MSC reporting (3GPP TS 48.008, section 3.1.3)
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=== Interference reporting parameters
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The interference band is calculated by the BTS based on the `Interference level
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Boundaries` and the `Averaging period`. These parameters are sent by the BSC
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over the A-bis/OML, and can be configured via the VTY interface.
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Below are the default values for them:
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----
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network
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bts 0
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interference-meas avg-period 6 <1>
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interference-meas level-bounds -115 <2> -109 -103 -97 -91 -85 <3>
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----
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<1> Averaging period (`Intave`) in SACCH multiframe periods (480ms).
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<2> Interference level boundary `0` (in dBm).
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<3> Interference level boundary `X5` (in dBm).
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The `Intave` parameter defines the averaging and reporting period. With the
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default value of 6 SACCH multiframe periods the BTS is instructed to report
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averaged interference levels approximately every 3 seconds.
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According to 3GPP TS 48.008, there exist five interference bands and six
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`Interference level Boundaries` (`0`, `X1`, ... `X5`). The BTS shall map the
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averaged interference levels (initially in dBm) into these 5 bands.
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----
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-115 dBm -109 dBm -103 dBm -97 dBm -91 dBm -85 dBm
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| <1> | <2> | <3> | <4> | <5> | <6>
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+----------+----------+----------+----------+----------+
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| band 1 | band 2 | band 3 | band 4 | band 5 |
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+----------+----------+----------+----------+----------+
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----
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<1> Interference level boundary `0` (outer).
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<2> Interference level boundary `X1`.
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<3> Interference level boundary `X2`.
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<4> Interference level boundary `X3`.
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<5> Interference level boundary `X4`.
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<6> Interference level boundary `X5` (outer).
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Unfortunately, it's not clearly defined by 3GPP how the BTS is supposed to map
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dBm values outside of the outer boundaries (`0` and `X5`) to band values. The
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ip.access nanoBTS, for example, would map values -120 dBm and -75 dBm to bands
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1 and 5, respectively. osmo-bts replicates this behavior.
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=== PDCH and dynamic timeslot handling
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The BTS may optionally report interference levels for PDCH timeslots. This
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may be useful for the BSC to determine whether dynamic PDCH timeslots might
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be better used for new circuit switched connections, or whether alternative
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PDCH resources should be allocated for interference reasons.
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NOTE: Currently osmo-bsc makes no use of PDCH interference reports, neither
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they get forwarded to the BSC co-located PCU over the PCUIF.
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For dynamic timeslots (`TCH/F_TCH/H_SDCCH/8_PDCH` and `TCH/F_PDCH`), the
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following expectations apply:
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* when in TCH/F mode: no interference reports, because the only sub-channel is active;
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* when in TCH/H mode: interference reports for *inactive* sub-channels only;
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* when in SDCCH mode: interference reports for *inactive* sub-channels only;
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* when in PDCH mode: optional interference reports;
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** measurements can be performed during IDLE TDMA frames.
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