Synchronization for Future Mobile Networks

Patrick Diamond PhD, Director Systems Engineering, Semtech Corporation

Introduction

Let me begin with a very brief explanation of synchronization in mobile wireless networks. The radios used in these networks operate in very strict bands that need separation to avoid channel interference which reduces service quality. The synchronization discussed in this article deals with these requirements. The physical backhaul systems are also clocked however this clocking controls the ingress and egress of compressed voice packets in and out of the base station. This physical layer clocking is derived from the actual backhaul network itself.

The timing mechanisms and clock distribution for 2G, 2.5G, 3G mobile networks is old hat. In FDD (frequency division duplex) radio environments such as UMTS and WCDMA synchronization comes from the T1/E1 backhaul circuit. The base station builders apply complex electronic filters to these signals to purify them for use driving the radio. These are called “frequency only” radio systems. In TDD (time division duplex) radio environments GPS is used. These systems require both frequency and phase alignment control. Phase alignment cannot be derived from T1/E1 backhaul circuits therefore mandating the use of GPS.

The provision of synchronization for the future mobile networks will pose new challenges triggered by the following factors:

  • New mobile networks will offer new content-rich services, demanding faster backhaul links (e.g. IPTV, VoIP)
  • New mobile network technologies such as mobile WiMAX and LTE are bandwidth hungry. They are also capable of a much higher degree of spectral efficiency than today’s systems. This is key particularly for licensed spectrum usage.
  • The transport for traditional systems will be packet-based most likely using ethernet. 3GPP UMTS R5 already specifies an IP-based transport in the backhaul, as opposed to ATM specified for up to R4.
  • There will be an increased number of cell sites due to the increase in bandwidth delivered to the users, requiring more nodes and different timing distribution techniques. Some of these will be remote radio head systems.
  • Ethernet is the prime candidate as transport technology for future backhaul networks given the lower cost of the facility and switching equipment.
  • T1/E1 leased lines are expensive and compare poorly to Ethernet’s cost per megabit.

These points mandate that backhaul networks will be mainly based on packet-switched transport technologies in the future.

The convergence towards a packet-based transport network poses the challenge of providing a level of synchronization fulfilling the mobile networks requirements defined in the 3GPP, #GPP2 and IEEE 802.16e specifications.

The transmission of the clock information over a packet network eliminates the need for alternative mechanisms, such as GPS or expensive oscillators placed at the receiving nodes. This can provide significant cost savings in network equipment as well as installation and maintenance.

This synchronization solution transmits dedicated timing packets, which can follow paths followed by the data packets which reduces the cost of synchronization information delivery and simplifies implementation. The most popular of these techniques is IEEE 1588 V2, precision time protocol. The IEEE 1588 V2 protocol is designed to enable a properly designed implementers algorithm to deliver time as precisely as a GPS receiver. It is very important to understand that precise frequency and phase alignment can de derived from precise time. The fact that the same IEEE 1588 V2 implementation can supply FDD and TDD radio systems and CES transport systems with the synchronization signals they require.

Although a small amount of additional traffic is added to the network load, it does have the following advantages:

  • In the data path means “always on” operation
  • Multiple transmission paths reducing redundant clock system costs.
  • Usage of a single synchronization session for several data sessions
  • Support for legacy systems in mobile networks, where Circuit Emulation Service is employed to carry both TDM traffic (2G, ) and ATM traffic (3G)
  • Support for any generic packet-based transport (such as IP RAN)
  • Configurable packet rates for network conditions to maintain accuracy (e.g. packet rate <1pps to >1,00pps)
  • Far less susceptible to poor network conditions

For all these reasons IEEE 1588 V2 is the most suitable approach to provide synchronization for the backhaul section of future networks, where also legacy traffic could be transported over a PSN.

On the standardization front, the ITU has created a specification to measure this synchronization technique performance called G.8261 Specifically, the requirements in terms of noise, wander, MTIE, etc. are specified.

IEEE 1588 V2 has been extensively tested in these scenarios and has proven viable. It is very important for service providers and equipment vendors to understand IEEE 1588 V2 is a protocol only. The clock recovery algorithm that actually creates the synchronization signals is the domain of IEEE 1588 V2 solution supplier.

The main features characterizing the standard IEEE 1588 V2 are:

  • It employs a two-way methodology, where packets are sent back and forth from the clock master to the clock slaves. This feature overcomes high-amplitude, ultra-low frequency wander that defeats other methods such as adaptive clock recovery techniques.
  • It is virtually independent of the physical media and can flow over low-speed twisted-pair, high-speed optical fiber, wireless or even satellite links without requiring equipment design modifications
  • It is not limited to TDM circuit emulation like the in-band solutions, but it can support CES better than adaptive clocking by distributing a precise network clock to every IWF node in the system
  • It can be used for any pure packet-based network, hence providing synchronization for future backhaul networks to be deployed by mobile operators
  • It can distribute either time, frequency or both
  • It could be used by telecom operators to sell a synchronization service to customers (residential, wireless operators, etc.)
  • It is resilient because a failed network node can be routed around.
  • It is resilient because the synchronization can come from one or more clock master nodes.
  • The IEEE 1588 V2 packets are fully ethernet and IP standards compliant and backward compatible with all existing ethernet and IP routing and switching equipment.
  • There is no requirement for intermediate switches or routers to be IEEE 1588 V2 aware. They see these timing packets are normal packet data.

As pictured below in figure 1. IEEE 1588 V2 protocol calls for synchronization packets with time stamps to be sent from master clocks to all slave clocks and for individual slave clocks to send time stamped packets to the master. The clock master establishes a separate synchronization sessions with each of the slaves he serves, as represented in Figure 1. The master then returns these slave originated time stamped packets to the specific slave that sent. This process provides the vendors algorithm with the time stamps it needs to recreate the master time base. From this time base the synchronization signals used by the base station are generated. The IEEE 1588 V2 vendors algorithm filters any noise , packet queuing delay and propagation delay created by the intermediate nodes.

Figure 1: IEEE 1588 V2 separate synch session for each clock slave

Semtech have produced a highly integrated IEEE 1588 V2 based semiconductor product ToPSync™ suitable for Telecom applications. Typically each synchronization session produces around 5kbps of traffic (in each direction, uplink and downlink), which represents a small portion of the total capacity usually provisioned in a backhaul network.

Since April of 2005 Semtech has been in a live trial. The test bed of the live trial is shown in Figure 2. Two ToPSync™ boards, acting as IEEE 1588 V2 master and slave, are connected to a live metro Ethernet backbone operated by a major US carrier. The master is locked to an atomic clock. The synchronization packets travel through the metro Ethernet before reaching the slave board, which is connected to an Agilent Omniber 718 measuring the Time Interval Error.

Figure 2: Semtech IEEE 1588 V2 live trial test bed

Figure 3 shows the delay profile over 1 day period measured in the metro Ethernet network.

Figure 3: Delay Profile of the Metro Ethernet used in the trial

Figure 4 shows the results of the measured Maximum Time Interval Error. The top picture shows the MTIE against the G.823 synchronization interface mask for an E1 as used in a GSM base station. The measured MTIE is well below the mask, confirming the excellent capabilities of IEEE 1588 V2. The picture at the bottom of Figure 4 shows that the results also exceeded the PRC synchronization mask for this trial.

Figure 4: Measured MTIE from the trial against G.823 E1 sync interface and G.811 (PRC) masks

 

 

This article has provided a brief explanation of the synchronization techniques in traditional mobile wireless networks. The actual synchronization requirements for mobile networks require having a base station to base station aligned timing reference. This is essential to guarantee transport channel alignment. The reference minimizes frequency (FDD systems) and timeslot (TDD systems) interference, and provides the required QoS level for real-time services such as voice, two-way video, etc. For FDD systems, accuracy better than 50 ppb (parts per billion) is required, whereas TDD systems require a base station to base station time alignment better than 2.5μs. Ensuring the fulfillment of these requirements reduces the call drop rate and improves the quality of services by decreasing the packet loss. This is how it works today and will continue to work when new backhaul techniques are implemented.

The emerging wireless systems such as 3G R5+, Wi-MAX 802.16e, specify an IP-based backhaul transport aiming at cost reduction and increased efficiency. The convergence towards a packet-based backhaul transport is creating the problem of providing the required level of synchronization to the base stations without the support of synchronous platforms such as PDH or SDH.

The adoption of a packet switched network in the backhaul is also posing a challenge for the legacy traffic, still transported via TDM and ATM. For this traffic, circuit emulation techniques can be used to encapsulate bit stream into data packets, transport them via the packet-switched backhaul network and then revert back into bit stream at the receiving end.

The bottom line is that the adoption of a packet-based backhaul requires new techniques to transport the synchronization signal at the base stations.

The results of extensive testing of the Semtech ToPSync™ technology by Vodafone were presented at the November 2007 ITSF (International Telecoms Synchronization Forum) is London. The testing suite employed by Vodafone followed the ITU G.8261 specification. These results concluded the Semtech ToPSync™ technology performed superbly. Vodafone declared they will deploy the IEEE 1588 V2 technology for their next generation mobile wireless network synchronization solution. Other service providers are currently evaluating this technology as well.