Deep dive into commercial LTE networks

I was recently in Greece and took the opportunity to have a closer look at two commercial LTE networks in order to understand what capability and configuration operators are using in the field.

The networks in question were Cosmote and Vodafone. Cosmote has launched 4G since around November 2012, initially limited to PS devices (dongles, Mi-Fi etc) and later on added support for smartphones. Vodafone has had a very limited LTE offering since the end of 2012 also limited to PS devices, but has also since expanded its LTE network and added support for smartphones.

Spectrum:
Both operators are using re-farmed 1800MHz spectrum to support 4G services. Each operator seems to have re-farmed 20MHz of spectrum which is split into a 10MHz block and a overlapping 20MHz block. In busy/important areas the 20MHz block is used in other areas the 10MHz block. Obviously as the spectrum is overlapping the two blocks cannot be used in the same geographic area. Details are shown below.

Vendors:
Vendor details are not usually shared in the public domain although occasionally certain vendors/operators do make announcements about awarded contracts. In this particular case I could not find any announcements in the public domain but what I call "L3 signatures" indicate that Vodafone are using Huawei EUTRAN while Cosmote uses NSN.

Idle Mode Selection/re-selection:
Unlike 3G which uses both a quality (Qqualmin) and signal strength (Qrxlevmin) selection criterion, LTE only uses signal strength (at least in rel8). Vodafone have configured their Qrxlevmin to -128dBm RSRP while Cosmote uses -130dBm. Although the standard allows for up to -140dBm, both of these values can be considered quite low. The obvious benefit is that UEs stay in LTE for longer, the obvious question is what is performance like (especially in the uplink) at such low RSRP values?

Specifically to intra-frequency cell re-selection Vodafone UEs start searching when the RSRP is equal or less than -68dBm. They perform the reselection if the neighbour cell is 4dB stronger.

Cosmote UEs start searching when the RSRP is equal or less than -68dBm as well but perform the reselection if the neighbour cell is 2dB stronger.

From an inter-frequency (not applicable for these networks) and IRAT re-selection point of view, LTE uses priorities similar to HCS. The configured priorities are shown below. As can be expected 4G has the highest priority followed by 3G and finally 2G.

Vodafone UEs will start measuring lower priority RATs at -118dBm and reselect when the serving cell RSRP falls below -128dBm.
Cosmote UEs will start measuring lower priority RATs at -124dBm and reselect at the same threshold.

Intra-frequency mobility:
Intra-frequency mobility in LTE is governed by event A3. A comparison of the A3 configuration for each operator is shown below.

 a3-offset is IE x 0.5dB so Vodafone trigger a HO when the neighbour cell is a3-offset + hysteresis stronger, which equates to 3dB. Cosmote also trigger a HO at 3dB however they don't make use of the hysteresis.

Connected mode IRAT mobility:
Both operators use event A2 to trigger IRAT mobility actions. The mobility mechanism itself is through an RRC connection release with redirect (PS handover is not supported). Vodafone use a measurement based approach. Two A2 thresholds are defined. The first at -120dBm RSRP triggers measurements against 3G & 2G neighbour cells. Depending on what is detected by the UE the appropriate re-direct RAT is selected. The second A2 threshold at -126dBm, is used to trigger a blind redirect to 3G directly. This is used when the UE does not report anything back following the first A2 event.

Cosmote on the other hand only use the blind re-direct and the UE is always re-directed to 3G at -124dBm RSRP.

CSFB:
Both operators use "basic" CSFB (i.e no DMCR, no SIB tunneling) to 3G (2100MHz band).
Interestingly enough, Cosmote use a CSFB inter-working function as described here. Although this eliminates any TAC to LAC planning it does create an additional call setup delay as shown from the measurements below.

RRC connection management:
The RRC state machine in 4G is very simple as only two states are defined. Idle and Connected. To transition from RRC_CONNECTED to RRC_IDLE Vodafone use a 5s inactivity timer while Cosmote use a 30s inactivity timer. It can be expected that 5s lead to an increase in signalling while 30s will impact battery life (assuming connected mode DRX is shorter than idle mode DRX which in this case it is).

That is it, quite an interesting deep dive into commercial LTE deployments and it also establishes something of a baseline for other networks I look at.