NAS Signaling Messages

The NAS signaling layer is used to exchange signaling messages between the UE and the CN.

1. Mobility Management:
LOCATION UPDATE, IMSI DETACH, AUTHENTICATION, TMSI REALLOCATION COMMAND, CM SERVICE, CM RE-ESTABLISHMENT

2. Call Control:
ALERTING, CALL CONFIRM, CALL PROCEEDING, CONNECT, CONNECT ACKNOWLEDGE, SETUP, RELEASE, STATUS

3. GPRS Mobility Management:
ATTACH, DETACH, ROUTING AREA UPDATE, SERVICE REQUEST, P-TMSI REALLOCATION, AUTHENTICATION AND CIPHERING

4. Session Management:
ACTIVATE PDP CONTEXT, DEACTIVATE PDP CONTEXT

EDCH Serving Cell

LTE UE State

Architecture of GSM, UMTS and LTE

SHO

Misunderstanding:

There are multiple RLs in both uplink and downlink during SHO.

In downlink, there are two/three RLs(radio links) during SHO. Please notice, radio link is just a logic concept which isn’t existed but just describes the connection between Node B and UE. But unlike downlink, there is only one RL in uplink. The reason is, in the downlink, scrambling code is used to distinguish the cells, whereas it’s used to identify UE in the uplink.

Therefore, Node B should send two/three copies of the same data via two/three different cells with different scrambling codes (2 way/3 way softer handover). At the same time, there is only one copy of data transmitting from UE to Node B. But all cells in active set will recieve it respectively and finally soft-combine it in RAKE.

The same mechanism is also applicable for E-DPDCH/E-DPCCH, as well as associated ul DPCCH and HS-DPCCH. As a result, all uplink channels have got diversity receiving gain.

ACK NACK DTX

ACK/NACK which is carried on uplink HS-DPCCH is sending from UE to inform Node B whether it has received/decoded certain transport block successfully.

The meanings of ACK/NACK/DTX are as obvious as their names express:

• ACK: Transport block is decoded correctly by UE;
• NACK: Transport block is decoded incorrectly;
• DTX: discontinuous transmission.

According to Chapter 6.2.6 ACK/NACK Transmit Power Reduction for HS-DPCCH in Mobile Communication Systems and Security by Man Young Rhee.

In Release 5, the UE always uses DTX in the ACK/NACK field of the HS-DPCCH except when an ACK or NACK is being transmitted in response to an HS-DSCH rtansmission. This means that if the UE fails to detect typical probability 0.01 of the HS-SCCH, the UE will use DTX in the corresponding ACK/NACK field. The Node B must avoid decoding this DTX as ACK if it is to avoid loss of the HS-DSCH TTI at the physical layer.

In summary:

1. If HS-SCCH is not decoded successfully(no matter which part is wrong, part 1 or part 2), UE will not send any ACK/NACK info on HS-DPCCH, which is DTX.
2. If HS-SCCH is successful but HS-PDSCH CRC is failed, NACK will be sent.
3. Only if both HS-SCCH and HS-PDSCH are decoded correctly, UE will send ACK.