Synchronization in packet-based mobile

backhaul networks

By R P PORWAL

Mobile backhaul networks are evolving from circuit switched to packet switched technologies to meet exploding demand for bandwidth in both core

and access networks. A prerequisite in the evolution to packet-based backhaul in mobile backhaul networks is the ability to deliver carrier grade

synchronization over the packet network to remote wireless base stations and access platforms.

Synchronization in legacy mobile backhaul networksSDH/SONET network synchronization hierarchy

Network synchronization in legacy telecom systems is based on clock hierarchy, with the highest accuracy clock at the top, as illustrated below:

At the top of the hierarchy is the Primary Reference Clock (PRC) or Primary Reference Source (PRS) with clock accuracy of 1 part in 1011 [1]. PRC/PRS can be generated from a cesium (atomic) clock or from cesium clock-controlled radio signals, such as Global Positioning System.At the second level of hierarchy is Synchronization Supply Unit (SSU) [2] or Building Integrated Timing Supply (BITS). SSU/BITS includes holdover, a feature that allows it to generate a clock with higher accuracy than its intrinsic free running accuracy for a short period after it loses synchronization with PRC/PRS.The third level is the SDH Equipment Clock (SEC) [3] or SONET Minimum Clock (SMC). SEC/SMC also features holdover, but its holdover and free-run accuracy performance is lower than what is required for SSU/BITS.The second and lower levels of hierarchy will have clock accuracy equal to PRC/PRS, so long as their path to the PRC/PRS is not broken.While PRC/PRS and SSU/BITS are usually implemented as standalone products with timing-only functionality (no data transmission), SEC/SMC are almost exclusively implemented as a part of networking product, such as an SDH/SONET add-drop multiplexer.

Time and frequency synchronization in Mobile NetworksSynchronization is crucial for Mobile wireless networks because the radio used in these networks operate in very strict bands that need separation to avoid channel interference which reduces the call quality and network capacity.Poor synchronization has also negative impact for the handover between Base Stations. Mobile handsets derive the accurate frequency that they transmit and receive from the Base Stations. If the transmission frequencies are not very closely matched between adjacent cell sites, then “clicks” can occur when the call is being handed over (switches) between Base Stations. In the worst case, the call would drop because the mobile handset would not be able to immediately lock onto and acquire the new signal.It is clear that the need for synchronization has always been inherent in Radio Access Networks. Radio networks fall into two categories:Frequency Division Duplexing (FDD): Radio Access Networks in this category use two sets of frequencies for transmit/receive. These networks require frequency synchronization in order to accurately send and receive traffic. Traditional GSM and WCDMA (UMTS) FDD networks fall in this category and need only frequency synchronization.Time Division Duplexing (TDD): Radio Access Networks in this category use a single frequency for transmit/receive and a demarcation based on timeslots is identified for both transmission and reception. These networks require also precise time synchronization in order to accurately send and receive traffic. For example, CDMA2000, mobile WiMAX and CDMA2000 systems all demand microsecond-level time synchronization

When time synchronization is required, it also has strict requirements.For example the radio interface requirement for WCDMA (UMTS) TDD base stations is a frequency accuracy of ±50ppb; for the TDD mode there is the additional requirement for the phase alignment of neighbouring base stations to be within 2.5ms.In the case of CDMA2000 the time synchronization requirement is that for all base stations, the pilot time alignment error should be less than 3ms and shall be less than 10ms.

Synchronization in TDM based mobile backhaul networksConventionally GSM, WCDMA (UMTS) FDD base stations (CDMA2000 uses GPS receivers because of the need for time synchronization) that are connected via TDM networks can be synchronized via TDM timing (PDH or SDH). Following a master-slave architecture, the timing (frequency synchronization) is distributed over the TDM links from an accurate primary reference clock to slave clocks.With the replacement of TDM links with Packet Switched Networks (PSNs) such as Ethernet, IP or MPLS, this simple method of providing a frequency reference is lost, and frequency information must be made available in some other way.The solutions are discussed in the upcoming presentation here at the same place