4.1 MME ID

Each MME has three main identities. An MME code (MMEC) uniquely identifies the MME within all the pool areas. A group of MMEs is assigned an MME Group Identity (MMEGI) which works along with MMEC to make MME identifier (MMEI). A MMEI uniquely identifies the MME within a particular network.

1. UE Modes of Operation

PS mode 1 of operation: The UE registers only to EPS services, and UE's usage setting is "voice centric".
 

PS mode 2 of operation: The UE registers only to EPS services, and UE's usage setting is "data centric".

CS/PS mode 1 of operation: The UE is CS fallback capable and configured to use CS fallback, and non-EPS services are preferred. The UE registers to both EPS and non-EPS services.

CS/PS mode 2 of operation: The UE is CS fallback capable and configured to use CS fallback, and EPS services are preferred. The UE registers to both EPS and non-EPS services.

NOTE: UE can be either voice centric or data centric. This usage setting basically tells the network what’s more important for that UE (voice or data). For example a voice centric UE will disconnect from E-UTRAN and try GERAN/UTRAN if it is not able to obtain voice service in E-UTRAN while a data centric UE will not do that.

2. Paging in LTE

The UE looks for its identifier in the RRC paging message. Paging is used to:

  • Transmit paging information to a UE in RRC_IDLE.

  • Inform UEs in RRC_IDLE and RRC_CONNECTED about a system information change.

  • Inform UEs in RRC_IDLE and RRC_CONNECTED about an ETWS primary notification.

3. Radio Bearer in LTE

SRB (Signalling Radio Bearers)

It is used for carrying C-Plane traffic, i.e. RRC signalling messages. RRC messages can contain other messages, such as NAS messages. SRBs are used only for transmission of RRC or NAS messages. There are three SRBs, all must be established.

  • SRB0 is for RRC messages using the CCCH logical channel.

  • SRB1 is for RRC messages (which may include a piggybacked NAS message) as well as for NAS messages prior to the establishment of SRB2, all using DCCH logical channel.

  • SRB2 is for NAS messages, using DCCH logical channel. SRB2 has a lower-priority than SRB1 and is always configured by E-UTRAN after security activation.

 

All RRC messages are integrity protected and ciphered by PDCP. NAS independently applies integrity protection and ciphering to the NAS messages.

 

DRB (Data Radio Bearers)

It is used for carrying U-Plane traffic (user data). It carries IP packets.

4. LTE Numbering & Addressing

An LTE network area is divided into three different types of geographical areas

  • The MME pool areas - This is an area through which the mobile can move without a change of serving MME. Every MME pool area is controlled by one or more MMEs on the network.

  • The S-GW service areas - This is an area served by one or more serving gateways S-GW, through which the mobile can move without a change of serving gateway.

  • The Tracking areas - The MME pool areas and the S-GW service areas are both made from smaller, non-overlapping units known as tracking areas (TAs). They are similar to the location and routing areas from UMTS and GSM and will be used to track the locations of mobiles that are on standby mode.

 

 

 

 

 

 

 

 

LTE Basics

 

If we combine PLMN-ID with the MMEI then we arrive at a Globally Unique MME Identifier (GUMMEI), which identifies an MME anywhere in the world.

The M temporary mobile subscriber identity (M-TMSI) identifies a mobile to its serving MME. Adding the MME code in M-TMSI results in a S temporary mobile subscriber identity (S-TMSI), which identifies the mobile within an MME pool area. Finally adding the MME group identity and the PLMN identity with S-TMSI results in the Globally Unique Temporary Identity (GUTI).

 

4.2 Tracking Area ID

Each tracking area has two main identities. The tracking area code (TAC) identifies a tracking area within a particular network and if we combining this with the PLMN-ID then we arrive at a Globally Unique Tracking Area Identity (TAI).

 

4.3 Cell ID

Each cell in the network has three types of identity. The E-UTRAN cell identity (ECI) identifies a cell within a particular network, while the E-UTRAN cell global identifier (ECGI) identifies a cell anywhere in the world. 

 

5. Received Signal Received Power (RSRP)

It is the power of the LTE Reference Signals spread over the entire bandwidth and narrowband. It is expressed in decibels per milliwatt (dBm). A minimum of -20 dB SINR (of the S-Synch channel) is needed to detect RSRP/RSRQ. The reporting range of RSRP is defined from -140 dBm to -44 dBm with 1 dB resolution.

 

6. Received Signal Received Quality (RSRQ)

It is a measurement and it indicates the quality of the received reference signal. The RSRQ measurement provides additional information when RSRP is not sufficient to make a reliable handover or cell reselection decision. RSRQ is expressed in decibels (dB).

 

RSRQ = (N * RSRP) / RSSI

 

N – Number of used Resource Blocks

7. Received Signal Strength Indication (RSSI)

It is used almost in all the RATs to identify power received from the cell in idle as well as connected/dedicated modes. This helps UE always camped on to the best cell all the time. In case of drop in power measured using RSSI, either UE or network initiates the handover or cell re-selection is carried out. It is expressed in decibels per milliwatt (dBm).

8. Block Error Ratio (BLER)

It is defined as the ratio of the number of erroneous blocks received to the total number of blocks sent. An erroneous block is defined as a Transport Block of which the the cyclic redundancy check (CRC) is wrong.

9. Signal to Noise Ratio (SINR)

It is used in wireless communication and is defined as the ratio of signal power to noise power. SINR is expressed in decibels (dB).

10. Sounding Reference Signal (SRS)

Sound Reference Signal is an uplink reference signal which is used for measuring uplink channel quality over a section of the channel bandwidth. It is used for Frequency Selective Scheduling of PUSCH, not  PUCCH.

10.1 SRS-BandwidthConfig

  • It is broadcasted on SIB2.

  • It takes values from 0-7.

  • It is common to all UE's within the Cell

10.2 SRS-Bandwidth

  • It can be included in RRC Connection Reconfiguration Message.

  • It can take values from 0-3

  • It is always UE specific.