6 Node Synchronisation

25.4023GPPRelease 17Synchronisation in UTRAN Stage 2TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

6.1 General

By Node Synchronisation it’s generally meant the achievement of a common timing reference among different nodes. In UTRAN although a common timing reference among all the nodes could be useful, it is not required. In fact different nodes’ counters (RFN and BFN), even if frequency-locked to the same network synchronisation reference, may be not phased aligned (see Figure 2).

Figure 2: Timing of UTRAN counters

However in order to minimise the transmission delay and the buffering time for the DL transmission on the air interface, it can be useful to estimate the timing differences between RNC and Node Bs, without the need to compensate for the phase differences between RNC’s and Node B’s counters.

On the other hand the achievement of a common timing reference among Node B’s may be used in TDD to support Cell Synchronisation.

For these reasons in UTRAN node synchronisation refers to the following two aspects:

– RNC-Node B Node Synchronisation;

– Inter Node B Node Synchronisation.

6.1.1 RNC-Node B Node Synchronisation

The Node Synchronisation between RNC and Node B can be used to find out the timing reference differences between the UTRAN nodes (RFN in RNC and BFN in Node B). The use is mainly for determining good DL and UL offset values for transport channel synchronisation between RNC and their Node B’s. Knowledge of timing relationships between these nodes is based on a measurement procedure called RNC-Node B Node Synchronisation Procedure. The procedure is defined in the user plane protocols for Iub (DCH, DSCH [TDD], HS-DSCH (Type1, enhanced CELL/URA_PCH operation), and FACH/PCH) and Iur (DCH, HS-DSCH).

When the procedure is used from SRNC over the DCH user plane, it allows finding out the actual round-trip-delay a certain service has (as the NODE SYNCHRONISATION control frames are transferred the same way as the DCH frames).

The procedure may also be carried out over a high priority transport bearer (beneficial when used between CRNC and Node Bs for the RNC-Node B Synchronisation purpose). Measurements of node offsets can be made at start or restart as well as during normal operation to supervise the stability of the nodes.

If an accurate Reference Timing Signal is used, the frequency deviation between nodes will be low, but could occur. If no accurate Reference Timing Signal is available, the local node reference oscillator must be relied upon. Then the RNC-Node B Node Synchronisation procedure can be used as a background process to find out the frequency deviation between nodes.

In the RNC-Node B Node Synchronisation procedure, the RNC sends a DL NODE SYNCHRONISATION control frame to Node B containing the parameter T1. Upon reception of a DL NODE SYNCHRONISATION control frame, the Node B shall respond with UL NODE SYNCHRONISATION Control Frame, indicating T2 and T3, as well as T1 which was indicated in the initiating DL Node Synchronisation control frame (see Figure 3).

Figure 3: RNC-Node B Node Synchronisation

In case of Node synchronization for HS-DSCH, the DRNC should transparently forward to the Node B a DL NODE SYNCHRONISATION control frame received from the SRNC, and should transparently forward to SRNC the UL NODE SYNCHRONISATION control frame received from the Node B.

In case of macro-diversity with recombining in the DRNC, the DL NODE SYNCHRONISATION control frame is duplicated in the DRNC on the different links, while the UL NODE SYNCHRONISATION control frames received from all the Node B’s are forwarded transparently to the SRNC (see Figure 4).

Figure 4: [FDD – RNC-Node B Node Synchronisation during soft handover
with selection/recombining in the DRNC]

6.1.2 Inter Node B Node Synchronisation

In the FDD mode Inter Node B Node Synchronisation could be reached via the RNC-Node B Node Synchronisation in order to determine inter Node B timing reference relations.

This could be used to determine Inter-cell relationships (considering T_cell) which can be used in the neighbour cell lists in order to speed up and simplify cell search done by UE at handover.

In TDD Inter Node B Node Synchronisation is used to achieve a common timing reference among Node B’s (see Figure 5), that allows to support Intercell Synchronisation.

Figure 5: Synchronisation of BFNs through TDD Inter Node B Synchronisation

In TDD Inter Node B Node Synchronisation may be achieved via a standardised synchronisation port (see subclause 6.1.2.1) that allows to synchronise the Node B to an external reference.

Another option to achieve the Inter Node B Node Synchronisation in a TDD system is the synchronisation of cells or Node Bs via the air interface ([3.84Mcps TDD – see subclause 6.1.2.2], [1.28Mcps TDD – see subclause 6.1.2.3]). This feature is not available for 7.68Mcps TDD.

6.1.2.1 TDD Node B Synchronisation Ports

This subclause defines the Node B input and an output synchronisation ports that can be used for Inter Node B Node Synchronisation. These synchronisation ports are optional.

The input synchronisation port (SYNC IN) allows the Node B to be synchronised to an external reference (e.g. GPS), while the output synchronisation port (SYNC OUT) allows the Node B to synchronise directly another Node B (see Figure 6).

Figure 6: Usage of Synchronisation Ports

This allows connecting Node B’s in a daisy chain configuration, so that a single external reference is enough and all remaining Node B’s can be synchronised (e.g. in case of indoor operation).

The Node B starts the synchronisation to the external reference when a valid input synchronisation signal is detected at the input synchronisation port.

If a valid synchronisation signal is detected, the Node B regenerates that signal at its output synchronisation port.

The electrical characteristics of the synchronisation ports shall conform to RS422 TIA/EIA 422 B [6] (output synchronisation port: subclause 4.1; input synchronisation port: subclause 4.2).

The synchronisation signal (illustrated in Figure 7a) is a 100 Hz signal having positive pulses of width between 5 μs and 1 ms, with the following exceptions:

– when (SFN mod 256 = 0) and not (SFN mod 4096 = 0), the pulse shall have a width between 2 ms and 3 ms.

This signal establishes the 10 ms frame interval, the 2.56 s multiframe interval, and the 4096 frames SFN period. The start of all frames in the cell of the node B is defined by the falling edge of the pulse. The required accuracy for the phase difference between the start of the 10ms frame interval is defined in TS 25.123 [15]. The time delay from the falling edge of the signal at the SYNC IN port to the start of the transmitted radio frame shall not exceed 500ns.

The start of the 256 frame period is defined by the falling edge of the pulse corresponding to the frames where SFN mod 256 =0 (i.e. of width between 2 ms and 3 ms, or between 4ms and 5 ms, respectively).

The start of the 4096 frame period is defined by the falling edge of the pulse corresponding to the frames where SFN mod 4096 = 0 (i.e. of width between 4 ms and 5 ms).

The synchronisation signal at the input port shall have frequency accuracy better than the one of the Node B.

The relative phase difference of the synchronisation signals at the input port of any Node B in the synchronised area shall not exceed 2.5 μs.

Figure 7: Synchronisation signal with 256 frames markers (Release 99)

Figure 7a: Synchronisation signal with 256 and 4096 frames markers (Release 4)

Synchronisation by a GPS receiver

The signal transmitted by a Global Positioning System (GPS) satellite indicates the GPS time that provides an absolute time reference. This makes the GPS receiver suitable for Inter Node B Node Synchronisation.

Inter Node B Node Synchronisation is achieved by relating the synchronisation signal (at the input synchronisation port) to the GPS signal. Since the period of this signal is 2.56 s, this implies that every 6400 frames the start of a 256 frame period coincides with an integer GPS second, i.e. a multiframe shall start when GPS time mod 64 = 0.

In general, at each start of a GPS second indicating the GPS time in seconds, the associated full SFN (the 12 bits value) can be derived as: SFN = (GPS time * 100) mod 4096. If the synchronisation port signal shall be derived from GPS, the special pulses for the 256 frames period and the 4096 frames period shall be present in the synch port signal when SFN mod 256 = 0 or SFN mod 4096 = 0, respectively, where the SFN in these equations is linked to the GPS time by the said equation.

Synchronisation by a Galileo receiver

The signal transmitted by a Galileo receiver indicates the Galileo time that provides an absolute time reference. This makes the Galileo receiver suitable for Inter Node B Node Synchronisation.

A Galileo receiver furthermore ensures integrity with a Time To Alert of 6s. The receiver shall only provide the date t, where t = t0 + Dt, Dt representing 15s (duration evaluated by the internal clock of the receiver) and t0 being the Galileo date evaluated 15s earlier, if it has not received and alert from the Galileo system between t0 and t. In the following paragraphs, t so calculated is called Galileo time.

Inter Node B Node Synchronisation is achieved by relating the synchronisation signal (at the input synchronisation port) to the Galileo signal. Since the period of this signal is 2.56 s, this implies that every 6400 frames the start of a 256 frame period coincides with an integer Galileo second, i.e. a multiframe shall start when Galileo time mod 64 = 0.

In general, at each start of a Galileo second indicating the Galileo time in seconds, the associated full SFN (the 12 bits value) can be derived as: SFN = (Galileo time * 100) mod 4096. If the synchronisation port signal shall be derived from Galileo, the special pulses for the 256 frames period and the 4096 frames period shall be present in the synch port signal when SFN mod 256 = 0 or SFN mod 4096 = 0, respectively, where the SFN in these equations is linked to the Galileo time by the said equation.

Backward compatibility to Release 99

The Release 4 synchronisation port definition is backward compatible with the Release 99 synchronisation port in the following sense: It is possible to feed a Release 99 Node B with the Release 4 synchronisation port signal. This results from the fact that the Release 4 synchronisation port pulses defined for SFN mod 256 = 0 and those defined for SFN mod 4096 = 0 both meet the pulse width tolerance defined for SFN mod 256 = 0 in Release 99. So the Release 99 Node B will recognise these two classes of Release 4 pulses as valid Release 99 pulses for definition of the 256 frames multiframe start. The Release 99 Node B will, however, ignore the differences between the 256 frames period pulse and the 4096 frames period pulse: The result is the 256 frames multiframe synchronisation as specified for Release 99.

The opposite scenario, however, i.e. connecting a Release 99 synchronisation port signal (without the 4096 frames marker) to a Release 4 Node B, shall be excluded. This would cause confusion for the "synchronisation via radio interface" procedure. The TDD cells in Release 4 shall be either "reference" cells where the SFN is fully synchronised to an external reference, or they shall be "non-reference" without any external, local frame clock reference.

6.1.2.2 TDD Inter Node B Node Synchronisation procedure [3.84Mcps TDD]

The Node B synchronisation procedure is an optional procedure based on transmissions of cell synchronisation bursts in predetermined PRACH time slots according to an RNC schedule. Such soundings between neighbouring cells facilitate timing offset measurements by the cells. The measured timing offset values are reported to the RNC for processing. The RNC generates cell timing updates that are transmitted to the Node B and cells for implementation.

The synchronisation procedure has four phases to bring a network into a synchronised operation, the preliminary phase, the frequency acquisition phase, the initial phase and the steady-state phase. The procedure for late entrant cells is slightly different and is described separately.

For synchronisation via the air interface it has to be considered that as long as a cell is not synchronised the cell may interfere the neighbouring cells. This applies especially in case of late entrant cells where first the new cell has to be setup before the synchronisation procedure starts. By this Cell Setup procedure the SCH is already transmitting. The RNC shall therefore disable the downlink time slots on Cell Setup procedure by means of the Time Slot Status IE. When the cell synchronisation has been performed the downlink time slots shall be enabled by means of the Cell Reconfiguration procedure.

There should be at least one cell in each RNS which is synchronised by an external reference (e.g. GPS receiver). The RNC evaluates the absolute time thanks to these reference cells.

If the source is a Galileo receiver, the RNC shall only send corrections based on time t = t0 + Dt with Dt = 15s (duration evaluated by the RNC’s internal clock), and t0 the date evaluated by the Galileo measurement 15s earlier, if the Galileo receiver has not received alert message between t0 and t.

6.1.2.2.1 Preliminary Phase

1) The reference cells, synchronised by an external reference, shall initialise their SFN counter so that the frame with SFN=0 starts on January 6, 1980 at 00:00:00 GMT.

2) The RNC has to be informed at which of the cells the external reference clock is connected. Therefore, a ‘Reference Clock availability’ indicator is added within the RESOURCE STATUS INDICATION message that is sent from the Node B to the RNC when a Local Cell becomes existing at the Node B.

3) At Cell Setup a ‘Reference SFN offset’ may be given to the cells where the reference clock is connected in order to separate the synchronisation bursts from different RNC areas.

4) The RNC has to retrieve the reference time from the cells with the reference clock. For the reference time retrieval the DL Transport Channels Synchronisation procedure or the Node Synchronisation procedure on the PCH frame protocol (see TS 25.435 [4]) shall be used. The Node B shall consider the SFN derived from the synchronisation port and the Reference SFN offset given by the RNC.

5) Now the RNC proceeds by updating the timing of all the remaining cells in the RNS, instructing them to adjust their clocks. Therefore, first the DL Transport Channels Synchronisation procedure on the PCH frame protocol shall be performed in order to determine the deviation from the reference SFN. The RNC then sends a CELL SYNCHRONISATION ADJUSTMENT REQUEST message to all the cells for SFN update, apart from the one(s) containing the reference clock. The cells shall adjust their SFN and frame timing accordingly.

6.1.2.2.1A Frequency Acquisition Phase

The frequency acquisition phase is used to bring cells of an RNS area to within frequency limits prior to initial synchronisation. No traffic is supported during this phase.

1) The cell(s) identified as reference cell, i.e. external reference clock is connected to, shall transmit continuously cell synchronisation bursts in every time slot where possible according to the information’s given in the Cell Synchronisation INITIATION Request message.

2) All other cells are considered as unlocked (i.e. not in frequency lock) shall listen for transmission from other cells and perform frequency locking to any transmission received. For setting the parameters within the Node B to listen for transmission from other cells, the Cell Synchronisation INITIATION Request message is used.

3) A cell shall signal completion of frequency acquisition to the RNC, as soon as it has locked its frequency to the received signal, fulfilling the Frequency Stability requirement set in TS 25.105 [17].

4) If the cell(s) have received transmission request on instructing the frequency acquisition and the cell(s) have performed frequency locking, the cell(s) shall begin transmitting the specified code for frequency locking of other cells.

5) When the RNC has received completion of frequency acquisition signals from all cells the frequency acquisition phase is completed.

6.1.2.2.1B Initial Phase

The procedure for initial synchronisation is used to bring cells of an RNS area into synchronisation at network start up. No traffic is supported during this phase.

1) For the synchronisation procedure it is useful to know which cells can “hear” each other. Therefore, all cells are instructed to transmit their cell synchronisation bursts in turn one after the other. The same cell synchronisation burst code and code offset is used by all cells.

2) Each cell shall listen for transmissions from other cells. Each cell shall report the timing and received SIR of successfully detected cell sync bursts to the RNC.

3) Upon reception of a CELL SYNCHRONISATION ADJUSTMENT message the cell shall adjust its timing accordingly. The timing adjustment shall be completed before the CELL SYNCHRONISATION ADJUSTMENT RESPONSE message is sent. It shall be implemented by adjusting the timing and/or tuning the clock frequency.

4) Steps 1 to 3 are repeated as often as necessary in order to reach the minimum synchronisation accuracy defined in TS 25.224 [16]. This serves the purpose to bring the network into tight synchronisation.
The SIR value within the cell sync burst reports is used by the RNC to define the schedule for the steady-state phase. I.e. to define when which cells transmit a cell synchronisation burst and when which cell synchronisation bursts shall be received. Cells which are sufficiently separated can be allowed to send the same cell synchronisation burst at the same time. Cells which are not sufficiently separated have to use different cell synchronisation codes and code offsets for distinctions.

6.1.2.2.2 Steady-State Phase

The steady-state phase allows cells to reach and/or maintain the required synchronisation accuracy. With the start of the steady-state phase traffic is supported in a cell. The steady-state phase starts with the Cell Synchronisation Reconfiguration procedure (see TS 25.433 [3]) which defines the synchronisation schedule. I.e. each cell gets the information when to transmit a cell synchronisation burst and when the individual cell synchronisation bursts from the neighbouring cells shall be measured.

For definition of the SFN when the cell shall transmit or receive cell synchronisation bursts, the SFN period is divided into cycles that have the same schedule. Within each cycle the Frame numbers for the cell synchronisation bursts are calculated by the number of repetitions per cycle and by an offset. Code and code offset are used to identify the individual cell synchronisation bursts.

1) The cell shall transmit a cell synchronisation burst and measure cell synchronisation bursts from neighbouring cells according to the information’s given in the Cell Synchronisation Reconfiguration Request message. Reception times for all relevant codes and code offsets shall be reported to the RNC with the Cell Synchronisation Report message.

2) Upon determination of an error in timing, the RNC adjusts the cell timing by means of the CELL SYNCHRONISATION ADJUSTMENT message. The timing adjustment shall be started at the beginning of the frame with the SFN given in the command. It shall be completed by the next cell synchronisation slot. Timing adjustments shall be implemented via gradual steps at the beginning of a frame. The whole adjustment shall be implemented with maximum stepsize of one sample per frame.

3) Step 1 and 2 continue indefinitely

6.1.2.2.3 Late-Entrant Cells

The scheme for introducing new cells into a synchronised RNS is as follows:

1) Late entrant cells (new cells being added without reference clock) or cells recovering from unavailability shall first be roughly synchronised. Therefore, first the DL Transport Channels Synchronisation procedure on the PCH frame protocol shall be performed in order to determine the deviation from the reference SFN. The RNC then sends a CELL SYNCHRONISATION ADJUSTMENT message to the late-entrant cells for SFN update.

2) Frequency acquisition of the late entrant cell is started by instructing the late entrant cell first to listen to the regular schedule of cell sync bursts of the surrounding cells. The transmission schedule of the surrounding cells shall be signalled to the late entrant cell within the CELL SYNCHRONISATION INITIATION REQUEST message. Frequency locking is reported using the CELL SYNCHRONISATION REPORT message.

3) In addition or instead of a regular schedule a single common cell synchronisation burst is transmitted in parallel by cells which are synchronised in the system and which are preferably the ones surrounding the late-entrant cell. The single cell synchronisation burst is initiated by means of the CELL SYNCHRONISATION INITIATION REQUEST message to the surrounding cells.

4) The late entrant cell shall correlate against the cell synchronisation burst according to the measurement information within the CELL SYNCHRONISATION INITIATION REQUEST message. The reception window shall be +/- 3 frames around the SFN frame given in the measurement information. The late entrant cell shall take the earliest reception as the timing of the system and adjusts its own timing and SFN number accordingly.

5) Thereafter, the late entrant cell shall start regular measurements after the reception of a CELL SYNCHRONISATION RECONFIGURATION REQUEST message and it shall report the timing of the measured cell synchronisation bursts to the RNC. In turn, the late entrant cell receives its own schedules for synchronisation transmissions and receptions and enters the steady-state phase.

6.1.2.3 TDD Inter Node B Node Synchronisation procedure [1.28Mcps TDD]

The Node B synchronization procedure for 1.28 Mcps TDD is an optional procedure based on the usage of the transmissions of the DwPCH to achieve Node B synchronisation over the air.

The main difference to the corresponding procedure for 3.84 Mcps TDD is the use of the DwPCH instead of the PRACH for synchronisation burst transmission and reception.

In addition, some extensions for the Steady State phase compared to the 3.84Mcps TDD solution have been specified:

– The ability to perform averaging of correlation results of several received SYNC_DL codes within a Synchronisation Cycle;

– The ability of the cell to perform self-adjustment of the timing based on measurements, and to report the accumulated adjustments to the RNC.

The synchronization procedure has three phases to bring a network into a synchronized operation, the preliminary phase, the initial phase and the steady-state phase. In addition there is a procedure for late entrant cells.

For synchronisation via the air interface it has to be considered that as long as a cell is not synchronised the cell may interfere the neighbouring cells. This applies especially in case of late entrant cells where first the new cell has to be setup before the synchronisation procedure starts. The RNC shall therefore disable the downlink time slots on Cell Setup procedure by means of the Time Slot Status IE. When the cell synchronisation has been performed the downlink time slots shall be enabled by means of the Cell Reconfiguration procedure.

There should be at least one cell in each RNS which is synchronised by an external reference (e.g. GPS receiver). The RNC evaluates the absolute time thanks to these "reference cells".

6.1.2.3.1 Preliminary Phase

1) The "reference cells", synchronised by an external reference, shall initialise their SFN counter so that the frame with SFN=0 starts on January 6, 1980 at 00:00:00 GMT.

2) The RNC has to be informed which of the cells are reference cells. Therefore, a "Reference Clock availability" indicator is added within the RESOURCE STATUS INDICATION message that is sent from the Node B to the RNC when a Local Cell becomes existing at the Node B.

3) At Cell Setup a "Reference SFN offset" may be given to the cells where the reference clock is connected in order to separate the synchronisation bursts from different RNC areas.

4) The RNC has to retrieve the reference time from the cells with reference clock. For the reference time retrieval the DL Transport Channels Synchronisation procedure or the Node Synchronisation procedure on the PCH frame protocol (see TS 25.435 [4]) shall be used. The Node B shall consider the SFN derived from the synchronisation port and the Reference SFN offset given by the RNC.

5) Now the RNC proceeds by updating the timing of all the remaining cells in the RNS, instructing them to adjust their clocks. Therefore, first the DL Transport Channels Synchronisation procedure or the Node Synchronisation procedure on the PCH frame protocol shall be performed in order to determine the deviation from the reference SFN. The RNC then sends a CELL SYNCHRONISATION ADJUSTMENT REQUEST message to all the cells for SFN update, apart from the one(s) containing the reference clock. The cells shall adjust their SFN and frame timing accordingly.

6.1.2.3.2 Initial Phase

The procedure for initial synchronization is used to bring cells of an RNS area into synchronization at a network start up. No traffic is supported during this phase:

1) For the synchronisation procedure it is useful to know which cells can "hear" each other. Therefore, all cells are instructed to transmit their SYNC_DL Codes one-at-a-time.

2) Each cell shall listen to transmissions from other cells based on RNC schedule for initial synchronisation. The SYNC_DL sequence is transmitted continuously throughout each radio frame period. Each cell shall report the timing and received S/(N+I) of successfully detected SYNC_DL codes to the RNC.

4) Steps 1 to 3 are repeated as often as necessary in order to reach the minimum synchronisation accuracy defined in TS 25.224 [16]. This serves the purpose to bring the network into tight synchronisation.
The rapid updates allow the correction of the clock frequencies as well as the clock timings to be adjusted in a short timeframe. This rapidly brings the network into tight synchronization.
The S/(N+I) values are used to define the schedule for the steady-state phase. Cells which are sufficiently separated or use different frequency bands can be allowed to send the same SYNC_DL code at the same time. Cells which are not sufficiently separated have to use different SYNC_DL codes for distinctions.

6.1.2.3.3 Steady-State Phase

The steady-state phase allows the system to reach or maintain the required synchronization accuracy. There is a "basic method", and there are extensions which may be required under adverse circumstances, to achieve reliable measurements of SYNC_DL codes from neighbour cells, and to achieve immediate, fast timing corrections while reducing the Iub interface signalling load.

6.1.2.3.3.1 Basic method

With the start of the steady-state phase traffic is supported in a cell. The steady-state phase starts with the Cell Synchronisation Reconfiguration procedure (see TS 25.433 [3]) which defines the synchronisation schedule. I.e. each cell gets the information when to transmit a SYNC_DL code and when the individual SYNC_DL codes from the neighbouring cells shall be measured.

For definition of the "Synchronisation Frames", i.e. the SFNs when the cell shall transmit or receive SYNC_DL codes, the SFN period is divided into Synchronisation Cycles that include the same number of Synchronisation Frames. The interval from one Synchronisation Frame to the next is called a Repetition Period. Each Synchronisation Cycle has the same transmit and receive schedule.

To be specific, the SFNs which are used as Synchronisation Frames are calculated from the "Number of cycles per SFN period" and the "Number of Repetitions per Cycle" as follows (where Repetition Period may be a non-integer number):

Cycle length: 4096 / value of the IE ‘Number of cycles per SFN period’

Repetition period: Cycle length / value of IE ‘Number of repetitions per cycle period’

Synchronisation Frame SFN = floor ((k-1) * Cycle length + (i-1)* Repetition period)

k = {1, 2, 3, .. Number of cycle per SFN period} = cycle counter

i = {1, 2, 3, .. Number of repetitions within cycle period} = Repetition counter

This provides the set of Synchronisation Frames SFN within the SFN period or 4096 frames. Then the procedure works as follows:

1) Each of the cells transmits its own predetermined SYNC_DL sequence on the DwPCH and receives the specific SYNC_DL code of neighbouring cells according to the information given in the CELL SYNCHRONISATION RECONFIGURATION REQUEST message. All cells shall report the reception timing for each specific SYNC_DL code to the RNC with the CELL SYNCHRONISATION REPORT message.

3) Steps 1 and 2 continue indefinitely.

6.1.2.3.3.2 Extended method

The following extensions of the basic scheme are available: Averaging of measurements, and self-adjustment of the radio interface timing.

1) Averaging of measurements: For increasing the S/(N+I) values of measured SYNC_DL codes, it shall be possible for a cell to apply an averaging of SYNC_DL codes received from the same neighbouring cell, before deriving the receive timing from the correlation result (During the averaging period, the timing in the neighbouring cells transmitting the SYNC_DL codes should be “frozen” in order to avoid “blurring” of the averaged measurements). This optional averaging is supported by subdividing the Synchronisation Cycles into a number of "Subcycles" where in each Subcycle, a full set of SYNC_DL samples is received, and by averaging over the subcycles, such that at the end of a Synchronisation Cycle a full set of timing deviation measurements with improved S/(N+I) is available. The number of subcycles is configured by the CRNC.

This introduction of "Subcycles" implies a change in the equations how to calculate the Synchronisation Frames SFN: The Number of subcycles per cycle period IE is taken into account as follows:

Cycle length: 4096 / value of the IE ‘Number of cycles per SFN period’

Subcycle length: Cycle length / value of IE ‘Number of subcycles per cycle period’

Repetition period: Subcycle length / value of IE ‘Number of repetitions per subcycle period’

Synchronisation Frame SFN = floor ((k-1) * Cycle length + (I-1)* Repetition period)

k = {1, 2, 3, .. Number of cycle per SFN period} = cycle counter

j = {1, 2, 3, .. Number of subcycles per cycle} = subcycle counter

I = {1, 2, 3, .. Number of repetitions within cycle period} = Repetition counter

This provides the set of Synchronisation Frames SFN within the SFN period of 4096 frames.

NOTE 1: Subcycle length and Repetiton period can have non-integer values.

NOTE 2: If the number of subcycles per cycle is set to unity, the "subcycles" are identical to the "cycles", and no averaging occurs.

At the end of each Cycle, a full set of Time of Arrival measurements is available, with or without averaging. So these measurements can be further processed as in the basic method.

2) Self-adjustment of the radio interface timing: It should be possible for the RNC to allow the Node B to perform a timing correction based on its own measurements autonomously without requiring the RNC to calculate the amount of timing correction. This reduces the amount of Iub interface signalling while allowing for fast corrections of timing deviations. – So the RNC shall indicate the possibility of self-adjustment, by including a Propagation Delay Compensation IE into the CELL SYNCHRONISATION RECONFIGURATION message, in addition to the SYNC_DL code to measure. Whenever this optional IE is present, the Node B should use the respective SYNC_DL measurement (after potential averaging) to perform the self-adjustment at the end of a Synchronisation Cycle. – Whenever this IE is not present, no self-adjustment shall be performed. – In each measurement report where the Node B reports the measured Time of Arrival values, the Node B shall also include the accumulated phase adjustments since the last measurement report to the RNC for surveillance purposes.

6.1.2.3.4 Late-Entrant Cells

The scheme for introducing new cells into a synchronized RNS is as follows:

1) Late-entrant cells (new cells being added without reference clock) or cells recovering from unavailability shall first be roughly synchronised via Iub interface messages. Therefore, first the DL Transport Channels Synchronisation procedure or the Node Synchronisation procedure on the PCH frame protocol shall be performed in order to determine the deviation from the reference SFN. The RNC then sends a CELL SYNCHRONISATION ADJUSTMENT message to the late-entrant cells for SFN update.

2) The RNC should tell the late-entrant which SYNC_DL codes and carrier frequencies to listen for, corresponding to its neighbour cells signalled within the CELL SYNCHRONISATION RECONFIGURATION REQUEST message.

3) The late entrant then reports the timing of the SYNC_DL codes using the CELL SYNCHRONISATION REPORT message. The RNC knows the location of all cells and therefore should be able to compute a timing adjustment for the late-entrant that takes into account the expected propagation delays between the late-entrant and its neighbouring cells The RNC adjusts the cell and the cycle is repeated until the RNC is satisfied that the cell’s timing accuracy fulfils the requirements to be allowed to enter the Steady State phase.

6.1.2.4 Node B synchronisation for 3.84Mcps TDD MBSFN IMB

In 3.84Mcps TDD MBSFN IMB operation a timing reference of high accuracy is required at the Node Bs in the MBSFN. The required accuracy of Inter Node B Node Synchronisation may be achieved via an external reference, e.g. GPS or other means.