Thursday 6 August 2009

Simplicity of LTE and its differences with UMTS RRC

LTE is gaining pace as the days pass by and everyday we hear some sort of milestone achieved by some company towards the ultimate aim of LTE product. RRC is one of the main components of Layer in the LTE protocol stack just as it was in UMTS. Certainly LTE RRC looks simpler in terms of the no of states and off course the length of the RRC document. Below are some of the important changes in LTE RRC and its difference with the UMTS system.

RRC State: In LTE there is only 2 RRC states i.e. RRC_IDLE and RRC_CONNECTED whereas in UMTS system RRC has a 5 state i.e. IDLE, CELL_FACH, CELL_DCH, CELL_PCH and URA_PCH. One of the reasons why we don’t have CELL_FACH and CELL_DCH state is because there is no concept of common and dedicated transport channel in LTE. In LTE the data transfer will be done through the defined shared transport channel. Therefore this will simplifies the RRC State machine handling and improves RRC performance. This will also simplify the RRM algorithm which decides RRC states.

Signalling Radio Bearers: In LTE there is only three SRB is defined i.e. SRB0, SRB1 and SRB2 whereas in UMTS system RRC has 4 SRBs i.e. SRB0, SRB1, SRB2 and SRB3 (optional).
SRB 0: In LTE SRB 0 use RLC TM entity over CCCH logical channel in DL whereas in UMTS system it SRB 0 is sent on RLC UM entity over CCCH logical channel in DL.

MAC entity: In LTE there is only one MAC entity which needs to configured whereas in UMTS system there is 4 different MAC entity based on different type of transport channel i.e MAC-d (DCH), MAC-c/sh (FACH, DSCH), MAC-hs (HS-DSCH) and MAC-e (E-DCH). In UMTS system the state machine which is handling MAC configuration is quite complex. During state transition from CELL_FACH to CELL_DCH or CELL_DCH to CELL_FACH lots of signalling was involved. In LTE, since there is only one MAC entity which is easier and simple to configure and thus have very simple State Machine.

Radio Bearer mapping: In LTE Radio bearer mapping would be much simpler than the UMTS system because of there is no common and transport channel defined in LTE.
In LTE there is no RRC connection mobility defined like cell update and URA update.

Domain Identity: In LTE, there is only one domain identity i.e. PS domain and which is implicit no need to specify anywhere in signalling unlike UMTS system which has CS domain and PS domain. Because of a single domain in LTE the signalling overhead and complexity in RRC design has been reduced.

System Broadcast Information: In LTE, MIB includes a limited number of most frequently transmitted parameters and SIB Type 1 containing the scheduling information that mainly indicates when the SI messages are transmitted where as in UMTS system, MIB includes the frequently transmitted parameters was well as scheduling information.

Channels: In LTE RRC there is no need to define the downlink transport channel configuration in the RRC Reconfiguration message as it uses only shared channel. This will reduce signalling message size effectively. All DL-SCH transport channel information is broadcasted in system information.

Power Consumption: The above point introduces another very critical feature of DRX calculation since all DL data is on the shared channel. E-NB can tell the UE when to decode/listen over the radio frame. This will optimize UE power consumption.

Paging Type: Since there is no CELL_FACH and CELL_DCH state in LTE there is only one type of paging required where as in UMTS system there is two type of paging defined.
Reconfiguration: In LTE there is only one reconfiguration message to reconfigure all logical, transport and physical channel where as in UMTS system there are number of reconfiguration message i.e. RB reconfiguration , TRCH configuration, PHY configuration. Thus there is less signalling message or overhead in LTE for the reconfiguration.

Reduced Latency: Since there is no RNC or NBAP protocol in LTE, this reduces the latency of the RRC connection establishment and RB management procedure.
Single UE identity: Since there is only one shared MAC entity, there is no need to define URNTI, ERNTI, HRNTI, SRNTI etc in LTE.

No Activation time: In LTE, there in no need to define activation time. Because of this there are lots of synchronizing complexity in 3G-RNC systems i.e. Synchronizing Radio link procedure based on activation time, synchronizing between the various MAC entity. This reduces significantly latency during establishment and reconfiguration of radio bearers.
RRC State: In LTE, there in no need to specify the RRC State in RRC message.

CQI Reporting: For network control mobility, there is one feature which become very important and critical i.e. CQI Reporting. As in LTE the CQI reporting should be fast and correct for taking decision for mobility.

Signalling connection release: There is no signalling connection release procedure in LTE, since there is only one domain i.e. PS domain. Also the UE context is shared between the MME and ENB and if UE is active in ENB then it should be active in MME also.


Long Term Evolution said...

I still doubt that we have a common bearer/shared one in LTE.
There are default and dedicated bearers available as it is in UMTS.
Please clarify on this.

Davide said...

1) Is it true that the operator cost per bit to carry a Voice over IP call in 3G PS is higher that in case of a 3G CS call?

2) Is it true that LTE radio solves the problem above, making the cost per bit of a VoIP call competitive with a classic CS-based call?

Simon Foo said...

1) Operator cost per bit is higher using voice over IP in 3G PS in particular using R99 PS than 3G CS call. The costs are resources like base station resource CE and transmission resources.
2) The key question is whether QoS can be guaranteed for VoIP riding on interactive/background PS service. There is some talk about IMS & VoLGA support for LTE.