GB2375692A

GB2375692A - Synchronising base stations in an UMTS terrestrial radio access telecommunications system

Info

Publication number: GB2375692A
Application number: GB0112166A
Authority: GB
Inventors: Anthony Peter Hulbert
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 2001-05-18
Filing date: 2001-05-18
Publication date: 2002-11-20
Also published as: GB0112166D0

Abstract

The invention relates to synchronising base stations in an UTRA (UMTS[Universal Mobile Telecommunications System] Terrestrial Radio Access) i.e. Node B synchronisation over the Air, system, by measuring timing differences between stations. The timing updates from a Resource Network Controller (RNC) are sent back to the base stations. Synchronisation communication is performed over a time period which includes part or all of the guard period. This period is bounded by the uplink and downlink pilot time slots (UwPTS, DwPTS). The communication operates in a 1.28 Mcps Time Division Duplex mode (TDD).

Description

IMPROVEMENTS IN OR RELATING TO MOBILE TELECOMMUNICATIONS SYSTEMS The present invention relates to improvements in or relating to mobile telecommunications systems, and is more particularly concerned with improving the synchronisation of base stations within such systems (Node B Synchronisation over the Air) UTRA (UMTS Terrestrial Radio Access where UMTS stands for Universal Mobile Telecommunications System) is a third generation digital cellular mobile radio system providing wideband data services in addition to conventional voice communications. It has several operating modes, one of which is known as Low Chip Rate TDD (Time Division Duplex) with a chip rate of 1.28 Mcps or 1.28 Mcps TDD. In UTRA the base stations are referred to as NodeBs and the mobile terminals are referred to as User Equipments (UE).

Reliable operation in the UTRA requires synchronisation between base stations within a compliant telecommunications system. To these ends, the synchronisation of base stations must be achieved at the levels of time slots, frames and multi-frames, where a multi-frame is a repeating cycle of a number of frames.

UK Patent Application No. 9919973.9 describes a method of providing synchronisation between a plurality of base stations in a telecommunications

system which comprises providing a random access channel in each cell. A mobile station receives a resource unit via the random access channel and a base station uses the random access channel in one cell to transmit a synchronisation signal to other base stations within the system. The base stations are synchronised together by means of transmissions between base stations. This involves measuring timing differences between the base stations in both directions to remove the effect of propagation delays. The timing differences are reported back to a central radio network controller (RNC) which computes the necessary timing updates and signals these back to the base stations. In such systems bases stations communicate with each other over the air, referred to in the art as Node B synchronisation, in order to provide measurement of time difference (by measured time of arrival). As a result these time differences are reported to a network controller and appropriate adjustments made.

The frame structure for 1.28 Mcps UTRA TDD mode known in the art is shown in Figure 1. Each radio frame of duration 10 ms is divided into two equal subframes of duration 5 ms. The subframes are further subdivided into 7 timeslots (Ts) designated Ts 0 to Ts 6 plus a Downlink Pilot Timeslot designated DwPTS and an Uplink Pilot Timeslot designated UpPTS. The DwPTS and UpPTS are assigned between timeslots Ts 0 and Ts 1. A guard period, GP, separates the DwPTS and UpPTS. Ts 0 supports a PrimaryCommon Control Physical Channel, P-CCPCH. Timeslots may be used for uplink (UE to NodeB) or downlink (NodeB to UE) transmission. Every transmission in a numbered timeslot, whether uplink or downlink consists of two equal length spread spectrum modulated data bursts separated by a fixed sequence of spread spectrum chips that is specific to a NodeB. The sequence is known as a midamble.

In a known prior art system concept, the DwPTS is used to establish downlink synchronisation in the UE (or after initial synchronisation also the midamble of

the P-CCPCH. The extra-long Guard Period"GP"is necessary to support large cells and the UpPTS (Uplink Pilot Timeslot) is used by the first stage of the Random Access, which is contention based. In the UpPTS there are 8 possible codes, designated SYNC-UL codes, where one is randomly chosen by the UE, which wants either to have access to the cell or to re-establish uplink synchronisation. This is the only uplink channel in 1.28 Mcps TDD, which is not synchronised to a high degree at the NodeB.

In the normal operation case (without any NodeB synchronisation method or without blanking of slots such as might be needed for providing location services), there are 2 points of switching between uplink and downlink per subframe, one is fixed and the second one can be chosen according the required asymmetry between uplink and downlink-see Figure 2. In Figure 2 the slots containing arrows pointing down the page represent downlink slots and the slots containing arrows pointing up the page represent uplink slots.

UK Patent Application No. GB 0007144.9 describes a synchronisation scheme for high chip rate (3.84 Mcps) UTRA TDD mode. In this a NodeB synchronisation burst was transmitted in a random access channel timeslot which was blanked at infrequent regular intervals for this purpose. This is not practical for 1.28 Mcps TDD since tight synchronisation must be maintained in this mode. Moreover, removing a whole timeslot would present an unacceptable overhead on the system which has only 7 timeslots per subframe as opposed to 3.84 Mcps TDD which has 15 timeslots per frame. Therefore it is proposed not to use"traffic timeslots"for the purpose of NodeB-Synchronisation. This leaves the DwPTS or the UpPTS.

The UTRA 1.28 Mcps TDD mode operates a combined TDD/TDMA structure.

The synchronisation of base stations must be achieved at the levels of time slots, frames and multiframes where a multiframe is a repeating cycle of a number of frames. The above techniques"stealing"uplink and downlink time slots (i. e. diverting them from normal usage) for the synchronisation burst are not really practicable in their efficiency. It is an object of the invention to provide synchronisation communications (synchronisation bursts) efficiently, and is particularly appropriate for this system and mode. This present invention addresses the synchronisation of base stations using radio transmissions in the same band of frequencies as that provided for communications with mobile stations. It also addresses only the active transmissions from the Node B's in order to achieve a synchronised system. The scheduling of the transmissions and processing of the measurements in the RNC can use the same methods as that described in prior art such as UK patent application GB 0007144. 9 In a telecommunication system, a method of synchronising a plurality of base stations comprising measuring timing differences between the base stations, said timing differences being reported back to a central radio network controller (RNC) which computes the necessary timing updates and signals these back to the base stations characterised in that synchronisation communication is performed over a time period which substantially includes the guard period, the guard period being the one flanked by uplink and down link pilot time slots, UwPTS and DwPTS, which have otherwise specific roles within the standard protocol in normal use Therefore the"stolen"time i. e. the period which is used for the synchronisation burst, is over at least the whole of the guard period.

The invention will now be described by reference by way of examples only and with reference to Figure 3.

Figure 3 shown the standard time slot comprising a Downlink Pilot Time Slot (DwPTS), followed by a Guard Period, followed by the Uplink Pilot Time Slot (UwPTS).

In an example according to the invention, the period chosen for a Node B synchronisation, i. e. the Tnbs burst, both the DwPTS, the GP and the UpPTS are temporarily removed from normal operation and replaced by a single downlink transmission. This is shown by arrow A under the figure 3. By combining the three fields the length of the transmitted sequence is increased. This provides improved processing gain and therefore supports larger cell ranges and increases the reliability of the signal. The transmitting NodeB would transmit the extra long sequence, on a regular basis, as scheduled by the Radio Network Controller. The neighbouring NodeBs measure this specific sequence. The extended transmitted sequence consists of more chip elements, allowing for a code or code set with improved correlation properties. This could allow the monitoring of at least 2 NodeBs within one step (using cyclically generated sequences). Because the transmission takes place in the period of the DwPTS/GP/UpPTS there is no other contemporaneous transmission so the signal can be transmitted at maximum NodeB power if desirable/necessary.

In various examples which fall in the scope of the invention Tnbs may extend greater or be shorter than the period described in the example above.

In another example Tnbs extends generally from the beginning of the DwPTS (and over the whole of it) and extends to the end of the UpPTS, as shown by arrow B below the figure.

In a further example, as shown by arrow C, the Tnbs extend substantially over just the guard period. In some circumstances it may be a few cps shorter than the guard period.

Essentially the invention extends effectively the downlink period into what would have been the uplink period where the base station would be receiving (listening) for mobiles. This is a slight penalty. However in some instances the base station may also be able to listen to normal UpPTS for mobiles at the same time, particularly if using suitably spread coding or orthogonal techniques. The advantage however in transmitting for longer time at the same period as in the invention means that the energy transmitted is increased which improves range as well as enhancing discrimination against other signals.

Claims

Claims 1. In a telecommunication system, a method of synchronising a plurality of base stations comprising measuring timing differences between the base stations, said timing differences being reported back to a central radio network controller (RNC) which computes the necessary timing updates and signals these back to the base stations characterised in that synchronisation communication is performed over a time period which substantially includes the guard period, the guard period being the one flanked by uplink and down link pilot time slots, UwPTS and DwPTS, which have otherwise specific roles within the standard protocol in normal use
2. A method as claimed in claim 1 herein the time period runs from between the start of the start of the synchronisation downlink slot and end of the synchronisation uplink slot.
3. A method as claim in claim 1 wherein the time period runs from the end of the guard period of the DwPTS and the beginning of the guard period of the UwPTS.
4. A method as claimed in any preceding claim wherein said telecommunication system is UMTS terrestrial Radio Access.
5. A method as claimed in claim 4 wherein said communication system operates in 1.28 Mcps TDD mode.

<Desc/Clms Page number 8>
6. A telecommunication system adapted for use as in any of claims 1 to 5.