Telecom Articles by Samir Amberkar: wp-lte-allocations-explained-p6

1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
1.10
1.11
1.12
1.13
1.14
1.15
1.16
1.17
1.18
1.19
1.20

2.1

3.1

3.2

3.3

3.4

3.5

3GPP Modem
Simulator

Test Your
Knowledge

LTE Allocations Explained

Signaling path

User data follows path of

EPS bearer ⇔ DRB ⇔ DTCH ⇔ UL/DL-SCH ⇔ PUSCH/PDSCH

PDCP ↔ RLC ↔ MAC ↔ PHY

and (dedicated L3) Signaling messages follow path of

SRB1/2 ⇔ DCCH ⇔ UL/DL-SCH ⇔ PUSCH/PDSCH

ESM/EMM ↔ RRC ↔ PDCP ↔ RLC ↔ MAC ↔ PHY

Not only layer 2 entities (and PDCP) are same for above paths but also both signaling and user information is treated the same way in both paths. PDCP makes some distinction between SRB and DRB - IP header compression (which of course not applicable to signaling messages), integrity protection only for signaling messages.

Though User data and dedicated L3 signaling occupy major chunk of allocations, allocations are also needed for other purposes like random access response. To understand bit more about intent of allocations, let us look briefly at various steps involved in radio access.

Radio access steps

1. Choose best cell of most correct PLMN

Overall reference: [36.304:4.2]

(1a)

PLMN selection

PLMN and RAT (GSM, UTRA, E-UTRA) selection is done by NAS/MM based on PLMN data available in SIM/USIM and previous history as per procedure detailed in [23.122:4.4.3, 5/fig 2a,2b]. While NAS doing PLMN selection procedure at power-on, AS would have already started 1) scanning RF channels (1a & 1c below) and 2) indicating "strongest cells" with their PLMN identities to NAS as and when they are found. If selected PLMN is among these, NAS may choose to stop PLMN search and instruct AS to "camp on cell" of selected PLMN. In the other case, NAS would inform selected PLMN to AS and wait for AS to come back once cell from selected PLMN is found.

(1b)

Synchronisation or Cell search

On power ON, PHY start scanning RF channels (all or based on previous history) in E-UTRA bands. Scanning involves looking for primary and secondary sychronisation signals (For FDD, PSS will be last symbol of 0^th & 10 ^th slots of center - around DC subcarrier - 62 subcarriers, SSS will be at a symbol before PSS). In addition to time and frequency sychronisation, result of cell search would be determination of PHY Cell ID of the cell. Ref: [36.213:4.1], [36.211:6.11].


Copyright © Samir Amberkar	Page 6 of 17