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Without precise timing, today's new communications signals can lose data in their translation, particularly in the areas of light-speed data and broadband systems. Due to the emerging standards in mobile telecommunications and digital broadcasting, the quality of the synchronization signal is more important than ever before. Telecommunications networks that are out of synchronization may suffer from static interference, bit errors and slow transmission rates.

The Global Positioning System (GPS), which comprises a network of 24 satellites, is the most accurate navigational system available today. Each satellite carries accurately controlled atomic clocks (cesium clocks) for transmitting timing signals worldwide. In addition to its position-determining capabilities, GPS technology enables many other timing and frequency applications. One of these applications involves the use of the cesium clock signal for timing synchronization and frequency stability control in wireline and wireless applications.

A new card-level GPS receiver module that provides high level synchronization for CDMA and Universal Mobile Telecommunications System (UMTS) mobile networks using the GPS signal and a high performance oven-controlled crystal oscillator (OCXO) has been developed. The module receives satellite signals at any place on earth. The GPS signal contains an inherent long-term stability of 1 pps as given by the atomic clock. This stability can be recovered only after proper treatment of the signal. The 4510 GPS module extracts and delivers the 1 pps stability as an output signal. However, the GPS signal is inherently noisy due to the selective availability coding on the signal. The 4510 receiver module consists of a GPS receiver circuit and an input interface for a 2.048 Mbps, 1.544 Mbps or 2.048 MHz synchronization signal together with a variable-voltage power supply. Figure 1 shows the 4510 GPS receiver module's block diagram The GPS module is fully equipped with a digital phase-locked loop and an internal OCXO that filters the received GPS signal for any noise, thus providing a G.811-complaint signal output. The unit also provides jitter and wander filtering and a holdover capability in accordance with recent recommendations made by the International Telecommunication Union (ITU). The auxiliary input is intended as a backup reference input in case the GPS signal is absent.

The GPS receiver module offers excellent maximum time interval error (MTIE) performance in harsh temperature environments as specified in the ETSI 300 019-2-3.2 specification, as shown in Figure 2. This performance makes it suitable for synchronizing second- and third-generation cellular mobile networks.

Third-generation networks, such as cdma2000 and UMTS, require base stations to synchronize the frames on the radio interface to a common reference. The 4510 receiver provides the signals necessary to accomplish this synchronization. Figures 3 and 4 show the unit's short-term stability (Allan deviation) and time deviation, respectively. In addition, the GPS receiver module's holdover mode keeps the base transceiver station operational, even when GPS satellite signals are momentarily lost due to interference problems.

Several versions of the 4510 unit are available. They differ in the level of stability and holdover performance of their synchronization function. Therefore, performance may be tailored to the requirements of the application. The range varies from cost-effective, low end types to ITU-T G.811 PRC card-level solutions.

The main features of the 4510 GPS receiver module are its integrated stability holdover functionality of less than lE-l0 per day (depending on the type of internal oscillator), its compliance with ITU-T G.811 with visual GPS signals and a UTC locked 1 pps output. In addition, the unit provides support for 1 + 1 redundant configurations, provides three different output signals (10 MHz, 2.048 MHz and 10 Mbits) and has an auxiliary 0.703-2/6/10 frequency input. Its holdover capability is compliant with ITU-T G.812 Type I - VI or ANSI Stratum 2, 3e, 3 SSU. The unit is a low cost, reliable and compact GPS receiver for original equipment manufacturers.
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Title Annotation:Global Positioning System
Publication:Microwave Journal
Article Type:Brief Article
Geographic Code:1USA
Date:Feb 1, 2000

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