Difference between LTE and UMTS

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 dont 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.

Signaling 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 are 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 signaling 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 signaling unlike UMTS system which has CS

domain and PS domain. Because of a single domain in LTE the signaling 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 signaling 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 signaling 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 is any need to define activation time. Because of this

there is 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 is 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.

Signaling connection release: There is no signaling 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.