5 BTS Requirements for Synchronization

3GPP45.010GSM/EDGE Radio subsystem synchronizationTS

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

5.0 General

The conditions under which the requirements of subclauses 5.4 and 5.6 must be met shall be 3 dB below the reference sensitivity level or input level for reference performance, whichever applicable, in 3GPP TS 45.005 and 3 dB less carrier to interference ratio than the reference interference ratios in 3GPP TS 45.005.

For EC-GSM-IoT, the conditions shall be met at the input level for reference performance of EC-RACH, and at the reference carrier to interference ratios of the EC-RACH, for the highest coverage class, as defined in 3GPP TS 45.005 for the supported TS option(s) of EC-RACH.

5.1 Frequency source

The BTS shall use a single frequency source of absolute accuracy better than 0.05 ppm for both RF frequency generation and clocking the timebase. The same source shall be used for all carriers of the BTS.

For the pico-BTS and Local Area multicarrier BTS classes the absolute accuracy requirement is relaxed to 0.1ppm.

NOTE: BTS frequency source stability is one factor relating to E-OTD LCS performance and the reader is referred to Annex C for the relationship between BTS frequency source stability and E-OTD LCS performance characteristics.

5.2 Timebase counters

It is optional whether the timebase counters of different BTS’s are synchronized together.

For COMPACT inter base station time synchronization is required such that timeslot number (TN) = i (i = 0 to 7) and frame number (FN) with FN mod 208 =0 shall occur at the same time in all cells. The timebase counters of different BTSs shall be synchronized together such that the timing difference between different BTSs shall be less than 1 symbol period, 48/13 s (which can be 1 or 3 bits depending upon modulation) measured at the BTS antenna.

If a cell defines a COMPACT cell in its neighbour list, time synchronization is required such that timeslot number (TN) = i (i = 0 to 7) and frame number (FN) with FN mod 208 =0 shall occur at the same time in both cells.

When extended DRX (eDRX) is supported in a routing area (RA) time synchronization is required such that any given timeslot number (TN) and frame number (FN) shall occur at the same time in all cells within the RA subject to an allowed tolerance. The timebase counters of different BTSs shall be synchronized together such that the timing difference between different BTSs (allowed tolerance) shall be less than 4 seconds measured at the BTS antenna.

5.3 Internal BTS carrier timing

The channels of different carriers transmitted by a BTS shall be synchronized together, i.e. controlled by the same set of counters. The timing difference between the different carriers shall be less than ¼ normal symbol periods, measured at the BTS antenna.

For pico-BTS and Local Area multicarrier BTS, the timing difference between different carriers shall be less than 2 symbol periods, measured at the BTS antenna.

5.4 Timing advance estimation

5.4.1 Initial timing advance estimation

When the BTS detects an access burst transmission on RACH, PRACH, or one or a sequence of access burst(s) on EC-RACH, it shall measure the delay of this signal relative to the expected signal from an MS at zero distance under static channel conditions. This delay, called the timing advance, shall be rounded to the nearest normal symbol period and included in a response from the BTS when applicable.

For the pico-BTS and Local Area multicarrier BTS, there is no requirement to measure this timing advance. However, either this measured value or a programmable value of timing advance shall be included in the response from the BTS when a timing advance value needs to be sent.

5.4.2 BTS Timing Advance Estimation for Positioning

A higher level of accuracy of the timing advance estimation by the BTS (reported to the SMLC, see 3GPP TS 43.059 [6]) is desired when the following positioning procedures are used:

– Multilateration Timing Advance procedure for assessing the timing advance in the serving cell and in non-serving cells, and

– Multilateration Observed Time Difference procedure for assessing the timing advance in the serving cell,

The actual algorithms and methods for estimation of the timing advance value are implementation dependent.

Moreover, a BSC that reports a timing advance value based on transmissions from an MS (e.g. the EC Multilateration Request message or EGPRS Multilateration Request message, see 3GPP TS 44.018 [8] and 3GPP TS 44.060 [9]) using the RLC Data Block or the Extended Access Burst method, shall establish a reported timing advance value = . It does so by first using the BTS estimated TA value = (based on the AB received on the RACH or on the EC-RACH (including blind physical layer transmissions for CC2, CC3 and CC4) identified when the MS starts the positioning procedure and using it as the assigned TA value = (i.e. the TA value sent to the MS using an assignment message on the AGCH/EC-AGCH is the value of the rounded off to the nearest symbol). It then adjusts the most recent estimated TA value as it receives subsequent updated timing advance estimation values from the BTS (i.e. the BTS provides an updated timing advance estimation for each subsequent burst it receives from the MS for the remainder of the positioning procedure wherein each burst sent by the MS uses the assigned TA).The final reported TA value ( is calculated by the BSC after it has received the last estimated TA value from the BTS based on the last transmission from the MS for the current positioning procedure and takes into account the MS Transmission Offset sent by the MS during the procedure (see 3GPP TS 44.018 [8] and 3GPP TS 44.060 [9] and 3GPP TS 43.059 [6]). This is shown in the formulas below.

(1)

where

(2)

(3)

where corresponds to the time as reported by the MS in the "MS Transmission Offset IE" (see 3GPP TS 49.031 [18]).

For the Extended Access burst method the number of timing advance estimations is N= 2 wherein both the RACH burst and the Extended Access burst are used for timing advance estimation. Similarly, for the RLC Data Block method the number of timing advance estimations correspond to N=5 (for the case of no HARQ retransmissions needed on the (EC-)PDTCH) wherein the AB received on RACH or EC-RACH and the 4 normal bursts used to receive the RLC data block are used for timing advance estimation.

Moreover, a BTS estimating the timing advance value for an MS using the RLC Data Block or the Extended Access Burst methods will be subject to an accuracy limitation inherent to its implementation and the radio conditions applicable when receiving transmissions from the MS performing the Multilateration Timing Advance procedure. This accuracy limitation is expressed as an assessment error of the reported TA value and shall be reported (to the SMLC via the BSS) as the BTS Reception Accuracy Level (see 3GPP TS 43.059 [6] and 3GPP TS 49.031 [18]) wherein the assessment error corresponds to the variance of the reported timing advance value of multiple timing advance estimations according to the equations below where N denotes the number of timing advance estimations and t_Ai,i=1…N denotes the estimated timing advance in estimation ‘i’.

The variance of the estimated timing advance value shall be evaluated using the formula:

(4)

s² is the unbiased sample variance

(5)

where and are calculated per the equations (1), (2) and (3) above.

For the Access Burst method the reported timing advance value corresponds to the estimated timing advance value derived from receiving the AB containing the EC Multilateration Request message or EGPRS Multilateration Request message.

When reporting the timing advance value to the SMLC the is rounded off to the nearest 1/64 of a normal symbol period (see 3GPP TS 49.031 [18]).

Similarly, for the SMLC to be able to accurately estimate the number of required BTSs to be used during the Multilateration Timing Advance procedure (in order to meet a targeted positioning accuracy) each individual BTS shall provide the BSC with BTS Reception Accuracy Capability information (see 3GPP TS 49.031 [18]) as follows:

– with a guaranteed timing advance assessment error, it shall always be capable of supporting at radio conditions down to the reference sensitivity level for RACH, if the BTS is capable of PEO operation, for all MTA radio access methods supported by the BTS. The BTS Reception Accuracy Capability shall be evaluated at the reference sensitivity level for PRACH/11 bits as specified in TS 45.005 [11].

– with a guaranteed timing advance assessment error, it shall always be capable of supporting at radio conditions down to the input signal level for reference performance for EC-RACH (CC1), if the BTS is capable of EC operation, for all MTA radio access methods supported by the BTS. The BTS Reception Accuracy Capability shall be evaluated at the input signal level for reference performance for EC-RACH (CC1) as specified in TS 45.005 [11].

The BSC, in turn, reports this guaranteed timing advance assessment error value as the applicable BTS Reception Accuracy Capability in the Assistance Information Response message sent to the SMLC in response to an Assistance Information Request message (see 3GPP TS 43.059 [6]).

The BTS shall comply with the indicated BTS reception Accuracy Capability in [90] % of the timing advance estimations.

5.5 Maximum timing advance value

The maximum timing advance value TA_max shall be 63. If the BTS measures a value larger than this, it shall set the timing advance to 63. In the case of GSM 400 the extended timing advance information element is supported and the maximum timing advance value TA_max shall be 219. If the BTS measures a value larger than this, it shall set the timing advance to 219. (3GPP TR 43.030 defines how the PLMN deals with MS’s where the delay exceeds timing advance value 63).

NOTE: The timing advance is always calculated in terms of number of symbols with normal symbol period irrespective of the actual symbol period used on the uplink.

5.6 Delay tracking

5.6.1 For circuit switched channels

For an MS in dedicated mode, the BTS shall thereafter continuously monitor the delay of the normal bursts sent by from the MS. If the delay changes by more than one symbol period, the timing advance shall be advanced or retarded 1 and the new value signalled to the MS.

Restricting the change in timing advance to 1 symbol period at a time gives the simplest implementation of the BTS. However the BTS may use a larger change than this but great care must then be used in the BTS design.

5.6.2 For packet switched channels

The BTS shall perform the continuous timing advance procedure for all MS working in packet transfer mode or in broadcast/multicast receive mode for which an PTCCH subchannel is assigned, except for an MS in dual transfer mode. Therefore the BTS shall monitor the delay of the access bursts sent by the MS on PTCCH and respond with timing advance values for all MS performing the procedure on that PDCH. These timing advance values shall be sent via a downlink signalling message on PTCCH. PTCCH shall not be assigned in case of an EC-GSM-IoT capable MS in EC operation.

The BTS shall update the timing advance values in the next downlink signalling message following the access burst.

The BTS may also monitor the delay of the normal bursts and access bursts sent by the MS on PDTCH and PACCH. Whenever an updating of TA is needed, the BTS may send the new TA value in a power control/timing advance message (see 3GPP TS 44.060).

For an MS in dual transfer mode the BTS shall follow the procedure described in subclause 5.6.1.

5.6.3 Delay assessment error

For circuit and packed switched channels, the delay shall be assessed in such a way that the assessment error (due to noise and interference) is less than ½ normal symbol periods for stationary MS. For MS moving at a speed up to 500 km/h the additional error shall be less than ¼ normal symbol period. For EC-GSM-IoT MS assigned CC2, CC3 or CC4 (see 3GPP TS 45.002) on the UL, the assessment error shall be less than ¾ normal symbol period for MS moving at a speed up to 50 km/h.

The control loop for the timing advance shall be implemented in such a way that it will cope with MSs moving at a speed up to 500 km/h, except for EC-GSM-IoT MS when it enters EC operation, where 50 km/h applies.

5.6.4 Pico-BTS and Local Area multicarrier BTS delay tracking

The pico-BTS and the Local Area multicarrier BTS have no requirement to track timing advance for any class of channels. However, it shall include either the measured timing advance as specified above or a programmable timing advance value in the response from the BTS when a timing advance value needs to be sent.

5.7 Timeslot length

5.7.0 Implementation options

Optionally, the BTS may use a timeslot length of 157 normal symbol periods on timeslots with TN = 0 and 4, and 156 normal symbol periods on timeslots with TN = 1, 2, 3, 5, 6, 7, rather than 156,25 normal symbol periods on all timeslots. This implementation option is illustrated in figure 5.7.4. When reduced symbol period is implemented, this option is further elaborated in subclause 5.7.2.

A BTS shall follow the implementation option of timeslot length with integer symbol periods for normal symbol periods, see subclause 5.7.2, on all transceivers in case EC-channels (EC-SCH, EC-BCCH, EC-CCCH, EC-PDTCH, or EC-PACCH) are mapped onto one or more transceiver resources.

Figure 5.7.1: void

5.7.1 Regular implementation with timeslot lengths of non-integral symbol periods

If the timeslot length for normal symbol period burst is 156.25 normal symbol periods for all bursts, then, a timeslot of length 187.5 reduced symbol periods shall be used for all bursts using reduced symbol period. This case is shown in Figure 5.7.2 and Table 5.7.1. In this case if there is a pair of different symbol period bursts on adjacent timeslots, then the guard period between the two bursts shall be 8.5 normal symbol periods which equals 10.2 reduced symbol periods.

Figure 5.7.2: Implementation using non integral number of symbol periods in both Normal Symbol Period burst and Reduced Symbol Period bursts.

Irrespective of the symbol duration used, the centre of the training sequence shall occur at the same point in time. This is illustrated in Figure 5.7.3 below. This means that the active part of a reduced symbol period burst shall start 12/13 μs (which is a quarter of a normal symbol period) later in time and ends 12/13 μs earlier.

Figure 5.7.3: Timing alignment between normal symbol period and reduced symbol period bursts

The duration of various components of the timeslot are illustrated in Table 5.7.1.

Table 5.7.1: Duration of various components of the time slot

	reduced symbol period Bursts		normal symbol period Bursts
	Symbols	Duration (μs)	Symbols	Duration (μs)
Tail (left)	4		3
Encrypted symbols (left)	69		58
Training sequence	31		26
Encrypted symbols (right)	69		58
Tail (right)	4		3
Guard period	10.5		8.25
Total	187.5		156.25

5.7.2 Implementation option for reduced symbol period bursts when integral symbol period option is used for normal symbol period bursts

In this implementation option, the length of timeslots for the burst with reduced symbol period shall be 188.4 reduced symbol periods for TN = 0, 4 and 187.2 reduced symbol periods for TN = 1, 2, 3, 5, 6, 7. This implementation is shown in Figure 5.7.4.

Figure 5.7.4: Implementation allowing integral number of symbol periods for normal symbol period bursts

The different burst lengths shall be obtained by changing the guard period lengths to values other than what is described in Table 5.7.1. The guard period lengths on adjacent timeslots shall be as described in Table 5.7.2.

Table 5.7.2: Guard period lengths between different timeslots

Burst Transition	Guard Period Between Timeslots (In terms of normal symbol periods)		Guard Period Between Timeslots (In terms of reduced symbol periods)
Burst Transition	TN0 and TN1 or TN4 and TN5	Any other timeslot pair	TN0 and TN1 or TN4 and TN5	Any other timeslot pair
normal symbol period to normal symbol period	9	8	10.8	9.6
normal symbol period to reduced symbol period	9.25	8.25	11.1	9.9
reduced symbol period to normal symbol period	9.25	8.25	11.1	9.9
reduced symbol period to reduced symbol period	9.5	8.5	11.4	10.2

5.8 Range of Timing advance

The timing advance shall be in the range 0 to TA_max (see subclause 5.5). The value 0 corresponds to no timing advance, i.e. the MS transmissions to the BTS are 468,75 symbol periods behind (see subclause 6.4). The value TA_max corresponds to maximum timing advance, i.e. the MS transmissions are 468,75 – TA_max symbol periods behind.