5 BTS Requirements for Synchronization

05.103GPPRadio subsystem synchronizationTS

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 05.05 and 3 dB less carrier to interference ratio than the reference interference ratios in 3GPP TS 05.05.

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 class 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.

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 1/4 symbol periods, measured at the BTS antenna.

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

5.4 Initial Timing advance estimation

When the BTS detects an access burst transmission on RACH or PRACH, 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 symbol period and included in a response from the BTS when applicable.

For the pico-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.

5.5 Maximum timing advance value

The maximum timing advance value TAmax 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 TAmax shall be 219. If the BTS measures a value larger than this, it shall set the timing advance to 219. (3GPP TS 03.30 defines how the PLMN deals with MS’s where the delay exceeds timing advance value 63.)

5.6 Delay tracking

5.6.1 For circuit switched channels

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 update timing advance mechanism for all MS working in packet transfer mode for which an PTCCH subchannel is assigned, except for MS class A in dedicated 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.

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 04.60).

For MS class A in dedicated or 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 1/2 symbol periods for stationary MS. For MS moving at a speed up to 500 km/h the additional error shall be less then 1/4 symbol period.

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.

5.6.4 Pico-BTS delay tracking

The pico-BTS has 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

Optionally, the BTS may use a timeslot length of 157 symbol periods on timeslots with TN = 0 and 4, and 156 symbol periods on timeslots with TN = 1, 2, 3, 5, 6, 7, rather than 156,25 symbol periods on all timeslots.

5.8 Range of Timing advance

The timing advance shall be in the range 0 to TAmax (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 TAmax corresponds to maximum timing advance, i.e. the MS transmissions are 468,75 – TAmax symbol periods behind.