Timing and synchronization are critical for proper operation of cellular infrastructure. Historically, in GSM and UMTS, two separate spectrum bands are needed, one for the transmission from the base station to the mobile device, and the other for transmission from the mobile device back to the base station. In GSM, UMTS and in LTE Frequency Division Duplex (LTE-FDD), frequency synchronization accuracy within 50 parts per billion (ppb) at the air interface is required. To meet this requirement, 16 ppb was specified as the requirment at the base station synchronization interface.
LTE-Time Division Duplex (LTE-TDD) and LTE-Advanced (LTE-A) use the same spectrum band for both upstream and downstream transmission, hence are more flexible in bandwidth allocation and more attractive for cellular operators. LTE-TDD and LTE- A services have the same frequency requirement as GSM, UMTS and LTE-FDD, however additional requirements for phase and time were added. The stringent phase and time synchronization requirements for LTE-TDD and LTE-A are varied from ±1500 nano sec to ±500 nano sec. The table below describes the frequency, phase and time synchronization requirements.
Addressing the frequency synchronization requirements was achieved by legacy SONET/SDH TDM-based backhaul network. However due to the migration to packet based backhaul infrastructure and the requirements for phase and time synchronization, this is no longer a valid solution. The synchronization shall be achieved using other technologies.
While GPS can deliver frequency, phase and time to the base stations, it introduces major problems. Many of the LTE-TDD and LTE-A small cells (metro cells, microcells and picocells) shall be located at street level, where GPS signal reception is poor. Furthermore, small cells base stations, may be located in-door in shopping malls, office buildings and sports arenas, where there is no GPS signal reception at all and where it is not feasible to connect to a remote GPS antenna. In addition GPS antennas are susceptible to performance degradation due to poor weather conditions or to weather damages (e.g. due to lightning). In order to address that, high quality, high cost holdover oscillator and costly support services will be required. On top of all of the above, GPS is also extremely vulnerable to jamming and spoofing.
As a result of all those major limitations, GPS is not economically or technically feasible at most base station locations. The solution is to form a reliable packet network based synchronization, that delivers frequency, phase and time. The IEEE 1588 Precision Time Protocol, also known as PTP, can accomplish that.
IEEE 1588-2008 is a two-way packet-based communications protocol designed to precisely synchronize clocks to sub-microsecond accuracy. The PTP standard incorporates hierarchical master-slave architecture for clock distribution. Under this architecture an ordinary clock is a device with a single network interface and is either a master or a slave. A boundary clock (BC) is a device with multiple network interfaces. One of them acts as the slave clock and the other acts as the master. A Transparent clock (TC) is a device that measures the time taken for PTP message to transit the device and provides this information to clocks receiving this PTP message.
The root timing source is called the grandmaster. The grandmaster is the prime master which transmits synchronization information to the other devices on the network. The synchronization information is timestamp carried by PTP UDP or Ethernet packets. The protocol defines events and general PTP messages. Event messages are messages with timestamp. An accurate timestamp is generated at both transmission and receipt. General messages do not require accurate timestamps. The PTP event messages are: Sync, Delay Req, PDelay Req and PDelay Resp. The PTP general messages are: Follow Up, Delay Resp, PDelay Resp Follow Up, Announce, Management and Signaling.
IPClock’s standard compliant PTP products have been widely deployed by equipment vendors in various markets and applications. The products leverage IPClock’s innovative clock recovery technology and provide high immunity to packet switched network impairments. The Timing accuracy in IPClock’s products is better than ±1µsec, and frequency accuracy is better than 16ppb on none-PTP aware, 10-switch GbE network under ITU-T G.8261 conditions. IPClock offers standalone products as well as IP core products. All IPClock products are adaptive to network impairments, enable zero touch approach, and eliminate the user’s need to configure servo in accordance to the network conditions. IPClock’s products enable significant cost reduction without compromising on clock synchronization accuracy, taking advantage of IPClock support of low cost oscillators. IPClock’s off-the-shelf products are designed to support future enhancements and field upgrades.