4 Synchronisation Issues

25.4023GPPRelease 17Synchronisation in UTRAN Stage 2TS

4.1 General

This clause identifies the different UTRAN synchronisation issues, i.e.:

– Network Synchronisation;

– Node Synchronization;

– Transport Channel Synchronisation;

– Radio Interface Synchronisation;

– Time Alignment Handling;

– Uplink Synchronisation.

The Nodes involved by the above mentioned synchronisation issues (with the exception of Network and Node Synchronisation) are shown by the Synchronisation Issues Model of Figure 1.

Figure 1: Synchronisation Issues Model

The UTRAN solutions for most of the identified items are described in clauses 6-10. Additional information on UTRAN synchronisation issues and the detailed specification of UTRAN solutions can be found in the following Technical Specifications:

– Summary of UTRAN Synchronisation Issues:

TS 25.401 [1] "UTRAN Overall Description", clause 9.

– Network Synchronisation:

TS 25.411 [7] "UTRAN Iu Interface Layer 1", subclause 4.2;

TS 25.104 [10] "UTRA (BS) FDD; Radio transmission and reception", subclause 6.3;

TS 25.105 [17] "UTRA (BS) TDD, Radio transmission and reception", subclause 6.3.

– RNC-Node B Node Synchronisation:

TS 25.427 [5] "Iub/Iur Interface User Plane Protocol for DCH Data Streams", subclause 5.5;

TS 25.435 [4] "UTRAN Iub Interface User Plane Protocols for COMMON TRANSPORT CHANNEL Data Streams", subclause 5.2.

– Transport Channel Synchronisation:

TS 25.427 [5] "Iub/Iur Interface User Plane Protocol for DCH Data Streams", subclauses 5.2 – 5.3;

TS 25.435 [4] "UTRAN Iub Interface User Plane Protocols for COMMON TRANSPORT CHANNEL Data Streams", subclauses 5.3 – 5.4.

– Time Alignment Handling:

TS 25.415 [27] "UTRAN Iu Interface User Plane Protocols", subclauses 6.5.4.

4.2 Network Synchronisation

Network Synchronisation relates to the distribution of synchronisation references to the UTRAN Nodes and the stability of the clocks in the UTRAN (and performance requirements on UTRAN internal interfaces).

The distribution of an accurate frequency reference to the network elements in the UTRAN is related to several aspects. One main issue is the possibility to provide a synchronisation reference with a frequency accuracy better than 0.05 ppm at the Node B in order to properly generate signals on the radio interface (see references TS 25.104 [10] and TS 25.105 [17]).

A general recommendation is to supply a traceable synchronisation reference according to reference ITU-T Rec. G.811 [18].

The clock to be implemented in UTRAN Nodes shall be chosen with characteristics that depend on the L1 adopted (see reference TS 25.421 [8] and TS 25.431 [9]) and on the Network Synchronisation strategy adopted. Already standardized clocks may be used (see references ITU-T Rec. G.812 [19], ITU-T Rec. G.813 [20], ETSI EN 300 462-4-1 [21], ETSI EN 300 462-5-1 [22] and ETSI EN 300 462-7-1 [23]).

For example in order to support STM-N interfaces at the RNC, the ITU‑T Recommendation G.813 (see reference ITU-T Rec. G.813 [20]) may be sufficient. The implementation in the UTRAN of a better performing clock (in terms of holdover) may be recommended for distribution of a 0.05 ppm during failures in the synchronisation network (EN 300 462-7-1, see reference ETSI EN 300 462-7-1 [23], EN 300 462-4-1, see reference ETSI EN 300 462-4-1 [21], or ITU‑T Recommendation G.812 type 1, type 2 or type 3, see reference ITU-T Rec. G.812 [19]).

4.3 Node Synchronisation

Node Synchronisation relates to the estimation and compensation of timing differences among UTRAN nodes. FDD and TDD modes have different requirements on the accuracy of the timing difference estimation and on the necessity to compensate for these differences.

Two types of Node Synchronisation can be identified, "RNC-Node B" and "Inter Node B" Node Synchronisation. Their usage differs and the requirements differ between the FDD and TDD modes.

"RNC-Node B" Node Synchronisation allows to get knowledge of the timing differences between RNC and its Node Bs.

"Inter Node B" Node Synchronisation may be used in the TDD mode to compensate the timing differences among Node Bs in order to achieve a common timing reference. The purpose of having a common timing reference is to allow Intercell Synchronisation, which is used, within neighbouring cells to minimise cross-interference.

Positioning / Localisation functions may also set requirements on Node Synchronisation.

4.4 Transport Channel Synchronisation

The Transport Channel Synchronisation mechanism defines synchronisation of the frame transport between RNC and Node B, considering radio interface timing.

DL TBS transmission is adjusted to fit receiver by adjusting the DL TBS timing in upper node.

4.5 Radio Interface Synchronisation

The Radio Interface Synchronisation relates to the timing of the radio frame transmission (either in downlink [FDD] or in both directions [TDD]). FDD and TDD have different mechanisms to determine the exact timing of the radio frame transmission and also different requirements on the accuracy of this timing.

In FDD Radio Interface Synchronisation is necessary to assure that the UE receives radio frames synchronously from different cells, in order to minimise UE buffers.

In TDD Radio Interface Synchronisation refers to the following two aspects:

– Intercell Synchronisation that is used to synchronise radio frames within neighbouring cells in order to minimise cells cross-interference, to allow frame wise hopping mechanisms among cells (e.g. Cell Parameter Cycling according to reference TS 25.223 [12]) and to make procedures involving more cells (e.g. handover) easier and more efficient;

– Timing advance that is used between UE and UTRAN in order to minimise UE-cell interference. In the 1.28 Mcps TDD option, timing advance is provided by uplink synchronisation.

4.6 Time Alignment Handling

The Time Alignment Handling procedure over Iu relates to the control of DL transmission timing in the CN nodes in order to minimise the buffer delay in SRNC. This procedure is controlled by SRNC.

4.7 Uplink Synchronisation

In 1.28Mpcs TDD Uplink Synchronisation is performed at Layer 1 for PRACH and uplink DPCH. This procedure includes the establishment of UL synchronisation and maintenance of the UL synchronisation.