Transport

ICellular networks (CDMA and UMTS) Synchronization of data streams is required in Wireless third-generation technologies like UMTS and cdma2000 to ensure reliable signal handoff between basestations. With the increasing demand  for high-bandwidth and real-time application, dropped connection are becoming intolerable. The solution is to establish effective management and distribution of a reliable reference clock throughout the entire network.Theoretically it was tempting to use the public-switched telephone network (PSTN) as the source for synchronization and distribute it via T1/E1 links to the rest of the network. Unfortunately, using the PSTN is not impracticable for many reasons: first the quality of the PSTN cannot be certain, excessive jitter and wander are add during transit. Practically, wireless networks generally operate using a reference clock backed by a holdover clock. All other network clocks must be traceable to the reference clock. The question is how to distributing a reliable reference clock throughout the entire network. GPS system can offer independent reference for any application, including wireless Time synchronization is not easy. It take time to propagate a reference clock to the users. Each line and nodes add error. After you set the user clock it will start to drift, so continuous recalibration is needed. Even when using one reference clock for entire network you can have a situation  when subsystem start to deviate at different rate until the connecting will start to drop. CDMAone and cdma2000 demand that the deviation will not exceed 1 part in 1,010 or 7.5 microseconds over a 24-hour period. Comparatively, UMTS wideband code-division multiple access (W-CDMA) and GSM networks require an accuracy of 5 parts in 108 or 4.3 milliseconds. To reach this level of synchronization subsystem must track a reliable reference clock and also have excellent holdover abilities in case the reference clock is not available. The reference clocks used to be sent over the network itself. It is an appropriate way most of the time, but it cannot guarantee adherence to the minimum requirements for high-speed data transfers across highly constrained wireless networks under all operating conditions. GPS is an excellent candidate for transporting a reference clock because each subsystem can have a direct connection to the same reference clock instead of an indirect connection over an unreliable network link. GPS is not all the time available. You might have a situation when not enough satellites or environmental conditions make the signal difficult to lock onto. In this case you need to have an holdover capability. Holdover starts when a clock holds the last frequency at which it was clocking when the reference clock signal failed to arrive. Two standard types of clocks used in basestations today that are up to the task of accurately holding a reference clock with such accuracy are highly precise ovenized quartz oscillators and rubidium atomic clocks. Quartz oscillators are the less-expensive option but require additional components. For example, quartz performance changes over temperature, so this has to be managed. Rubidium atomic clocks, on the other hand, provide reliability that's an order of magnitude greater than CDMA networks require. The primary difference between the two clocks is reflected during loss-of-power scenarios and how long it takes the clock to regain stability once the reference source has been lost. Quartz clocks can take four to 24 hours to stabilize frequency. As the quartz warms, its stability increases. Rubidium atomic clocks achieve 98 percent reliability within minutes. Rubidium units achieving lock within a 5-minute window with an absolute accuracy better than 1 part per billion. In contrast, quartz resonators depend on the bulk acoustic properties of a crystal and can take hours to achieve wireless levels of accuracy, even with a GPS reference.As in UMTS technology there is no need for an absolute time code you can used a local clock as a reference source as long as its long term stability is 50 parts per billion over a 10-year period. This level of stability is in the very high level of quartz technology and easily achievable by Rubidium atomic clock. Rubidium can run for longer than 10 years and exceed the UMTS requirements. Positioning systems in cellular networks There are a number of reasons for which it is useful to be able to pinpoint the positionof a mobile telephone: location sensitive billing, increased subscriber safety, intelligent transport systems (ITS), enhanced network performance and so on. The principal positioning techniques are: Propagation Time (PT), this involves measuring the time it takes for a signal to travel between a base station and a mobile station or vice versa. Time difference of arrival (TDOA), a mobile station can ¡°listen¡± to a series of base stations and measure the time difference between each pair of arrivals. If the base stations are transmitters, the transmitted signal must leave each base station at the same time or with a known offset; if the base stations are receivers there must be a known time relationship between the receiver clocks at these base stations. Angle of arrival (AOA), this involves measuring the angle of arrival of a signal from a base station at a mobile station; synchronization is not required. Carrier phase (CP), the phase of a carrier has the potential to provide position evaluations with an error less than the carrier wave length. The need is to maintain a continuous lock on the carrier signal. Failure to do so results in cycle slips and errors in position. In the GSM system a combining of the previous techniques is generally used; some studies have demonstrated that performances improve through the TDOA technique and in presence of BTS synchronism. In UMTS system best results are obtained through Time Aligned Idle Period Downlink (TA-PDL) technique, in which the mobile is required to make Time of Arrival measurements during the idle period of the serving base site and these periods are approximately time aligned in adjacent BTS. The utilization of rubidium clocks will be indispensable to offer location and positioning services in future mobile networks. Testing technology for wireless devices and network As wireless technologies have evolved over multiple generations, handsets and network infrastructure have become more complex and test requirements increasingly challenging. Atomic Rubidium Standard are use in the Handset Testing and Network Measurement devices.