UMTS Network Synchronisation Charles W T Curry B.Eng. (Hons) – Chairman - Chronos Technology Ltd www.chronos.co.uk Introduction Mobile Operators are one of the most powerful customer groups with whom Network Operators have to develop commercial relationships. Business decisions taken in the early stages of UMTS/3G rollout will undoubtedly have a significant impact on the medium term viability of the business. The right partnerships need to be developed. Wireless operators are in the process of making a massive capital investment in UMTS licences and infrastructure. If the technology is to deliver increased speed over GSM, will the lessons learned from GSM be heeded, particularly regarding synchronisation? Now is an ideal opportunity to get it right. Additionally, services to be offered over UMTS include location services. How will these services be affected by poor synchronisation at the base stations? This paper explores the synchronisation requirements for UMTS networks and offers solutions for Core Network and Node B synchronisation, with suggestions for quality metrics. It also illustrates types of synchronisation problems experienced by the author when measuring real networks and explores the consequential effect on Node B behaviour. Mobile Operators’ Perspective From the “synchronisation-aware” Mobile operators’ perspective, the questions most likely to be to be asked should be: - Q. Can the Network Operator demonstrate why synchronisation is required for the successful operation of my UMTS/3G network and backhaul of my Node B traffic? Q. How can the service being offered to our end customers, be enhanced and protected over and above simple local traffic delivery? The quantity of backhaul bandwidth for UMTS/3G networks is likely to be relatively large given that 3G networks are predicted to have between 3 and 6 times more Node B’s than Base Stations in the 2G GSM world. If this is true then the business opportunity to suppliers of E1 access connectivity and significant core bandwidth umts synchronisation.doc v1.31 Page 1 of 30 08 Feb. 06 transmission capacity in providing the mobile operator with backhaul bandwidth may well be many hundreds of Millions of Euros. The 3G mobile operator must ensure that their chosen bandwidth supplier can meet the very exacting standards that are demanded for this solution. This local E1 Node B connectivity and backhaul bandwidth does after all contribute significantly to their business case and future success! A Network Operator who can demonstrate that they understand the need for synchronisation, can offer ways of optimising the local access infrastructure and can offer opportunities to enhance end customer offer, is the ideal partner for a 3G mobile operator. Network Operators’ Perspective A Network Operator that wishes to be the supplier of E1 Node B access connectivity and core transmission capacity to a 3G Mobile Operator, has to ask a few simple questions. Q. Am I able to guarantee the quality of synchronisation delivery through my local access network such that the jitter and wander characteristics meet the requirements of the UTRAN? Q. Can the local access architecture be simplified and made more cost effective? Q. What additional services may a prospective Mobile Operator require at remote Node B locations and can I offer a solution to meet those needs? Mobile Operators are looking to award significant business (often hundreds of Millions of Euro’s) with Network Operators that understand their remote site needs. A Network Operator who can demonstrate that they understand the local access architecture options and the need for synchronisation and can offer the “value-add” of guaranteed synchronisation will be in a strong position to win UMTS/3G backhaul network solutions. The UMTS Network and Synchronisation In order for Universal Mobile Telecommunications Services (UMTS or 3G) to work correctly, the absolute radio frequency (RF) stability of air-interface transceivers - Node B’s in the UMTS Terrestrial Radio Access Network (UTRAN) must be maintained to within defined standards. umts synchronisation.doc v1.31 Page 2 of 30 08 Feb. 06 Traditionally, the air interface has derived its RF stability from the incoming link of the telecom network used to deliver traffic with the base stations. Whilst the long term stability of these E1 links is often beyond question since they are directly traceable to a network Primary Reference Clock (PRC), the short and medium term variation in frequency (wander) may have a significantly detrimental effect on the air interface stability. Whilst some manufacturers are now designing low cost OEM Global Positioning (GPS) receivers into their Node B’s, risk of loss of GPS reception means that wireless network operators will still need to observe and make use of the E1 frequency stability information. If the relevant standards are not met, the ability of the network to operate effectively will be adversely affected. Poor synchronisation in GSM networks is known to compromise cell handover particularly whilst calling from a moving platform; it can also be a cause of dropped calls whilst stationary. Mapping of E1 traffic though the Synchronous Digital Hierarchy (SDH) can cause pointer movements (VC12 pointers). This is known to cause base stations to temporarily cease functioning. The purpose of this paper is to analyse the synchronisation issues of the UTRAN. It will also propose synchronisation quality requirements in terms of metrics familiar to telecom network operators and define sync transport mechanisms which guarantee sync quality delivery from the core network to the access layer and Node B’s. Relevant Standards and Metrics The main standards for telecom network synchronisation are the European ETSI Standards series EN 300 462-1 to EN 300 462-7, International Telecom Union ITU Series G.811, G.812 and G.813 and the American T1X1 T.101. There is a lot of similarity between the language in the three standards’ bodies and in all, the metrics for telecom network synchronisation stability are now well established and very well defined in terms of Maximum Time Interval Error (MTIE) and Time Deviation (TDEV). References to synchronisation in the standards defining requirements for 3G networks can be found in many documents within the 3rd Generation Partnership Project (3GPP) family. However the technical specification capturing most of the requirements is TS 125 402 – “Synchronisation in UTRAN Stage 2”. References to other relevant 3G technical specifications can be found within TS 125 402. The classical specification for the RF accuracy of GSM base stations was .05 ppm or 5x10-8 and is found in ETSI TS 145 010 (TS 100 912). The equivalent UMTS specification can be found in TS 125 104 “UMTS: UTRA (BS) FDD; Radio umts synchronisation.doc v1.31 Page 3 of 30 08 Feb. 06 transmission and reception” and TS 125 105 “UMTS: UTRA (BS) TDD; Radio transmission and reception” but modifies the accuracy to ±5x10-8 over one timeslot. We can also find in TS 125 402 a specification for the relative phase difference between Node B’s of 2.5 µs. Let’s briefly review what the 3GPP specifications say about synchronisation: - ETSI TS 125 402: “UMTS: Synchronisation in UTRAN Stage 2” states that: - “A general recommendation is to supply a traceable synchronisation reference according to G.811. The clock to be implemented in UTRAN Nodes shall be chosen with characteristics that depend on the L1 adopted (see TS 125 421 and TS 125 431) and on the Network Synchronisation strategy adopted. Already standardised clocks may be used (see G.812, G.813, EN 300 462-4-1, EN 300 462-5-1 and EN 300 462-7-1). For example in order to support STM-N interfaces at the RNC, the ITU-T G.813 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, EN 300 462-4-1, or ITU-T G.812 type 1, type 2 or type 3).” ETSI TS 125 411 “UTRAN Iu Interface layer 1” sub clause 4.2 states “The jitter and wander performance requirements on the interface shall be in accordance with either G.823, G.824, G.825, whichever is applicable. The synchronisation reference extracted from the Iu may be used as UTRAN synchronisation reference. A general recommendation is to supply a traceable synchronisation reference according to G.811.” ETSI TS 125 104 “UTRA (BS) FDD & TS 125 105 “UTRA (BS) TDD: Radio transmission and reception”, sub clause 6.3 “The modulated carrier frequency of the BS shall be accurate to within ± 0.05 ppm observed over a period of one power control group (timeslot).” Wherever the synchronisation reference originates – network PRC or local GPS, this synchronisation information must be passed through the UTRAN in order to ensure that all clocks within the UTRAN, whether they be at RNC sites or Node B sites meet the requirements for correct operation. As already stated, Node B’s must have frequency accuracy of ±5 x 10-8 over one timeslot. This means that any frequency reference used by the UTRAN and ultimately delivered to the Node B must meet this accuracy at all times. From this perspective umts synchronisation.doc v1.31 Page 4 of 30 08 Feb. 06 TS 125 402 recommends G.811 (EN 300 462-6) traceable references for the whole UTRAN. Whilst the Core Switch sites and co-located Radio Network Controller (RNC) are most likely to interface to a transmission network at the STM-N rate and only theoretically needs to meet the SEC standard of G.813 or ETSI EN 300 462-5, two key issues must be considered. These are (i) are we exceeding the overall SDH network architecture with reference to SEC clock hop count, which is defined as not exceeding 20 in ETSI EN 300 462-2-1? and (ii) the need to maintain Node B frequency accuracy in the event of holdover at the RNC.