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Keeping in Time with the New World of Wireless
By Eliot Minor, OEM Technical Sales Manager Asia Pacific, Symmetricom Inc. (www.symmetricom.com)
Dec. 10, 2002
The new world of third generation (3G) wireless brings some very impressive changes in
our ability to communicate with portable communication devices. In today’s second
generation (2G) networks we are restricted to a voice only communication with often
marginal connection stability and sound quality. 3G networks and handsets will feature
full interactive color video allowing face-to-face conversations with CD quality sound.
Multimedia and games enabled by access to data services will make it easier to carry out
transactions and get the latest updates on constantly changing information from anywhere
around the world.
However, for this great revolution in information and communication to occur,
everything must be perfectly synchronized. In fact, to maintain proper Quality of Service
(QoS) on these new broadband networks, accuracies up to ten billionths of a second
(1/10,000,000,000) are required. Without this level of timing precision, our new world of
next generation communications may become an unacceptable jumble of distorted jumpy
images and unclear sound.
How Does Timing Work?
Time and frequency solutions work by ensuring the bits of information going out match
the bits of information coming in at the other end. The bits of information are carried
through various networks including IP access networks, SONET and SDH optical back-
haul networks, and the core PSTN networks. They are transmitted through multiplexers,
de-mulitplexers and transmitters at various transition points in the network. Each of these
transitions must accurately translate high-speed streams of data. Any timing
inconsistencies at these key transition points can cause data to be lost or corrupted.
Highly accurate GPS-based timing devices installed at key locations in the network such
as central offices, switching stations, and wireless base stations can usually overcome
these inconsistencies in information flow. Wireless base stations use this timing to
ensure accurate stability at their RF air-interface to enhance voice quality on handsets and
reduce the chance of dropped calls. In many cases a GPS receiver synchronization
module provides direct GPS timing for a stable air-interface as well as accurate phase
alignment between base stations for smooth cell hand-offs.
Why Accurate Timing?
One reason wireless service providers are looking for highly accurate sources of timing is
that their base stations are usually connected to core SDH and SONET backhaul
networks. Although these networks can act as a source of frequency synchronization, the
quality of the sync is often not in control of the wireless service provider. One reason is
that the synchronization signal is often cascaded from Mobile Switching Centers (MSCs)
to Base Station Controllers (BSCs) and eventually down to the wireless Base Station
(BTS). Along the way, various types of transients, wander and phase jumps can occur
resulting in a degradation of sync quality. These perturbations occur in varying levels at
different parts of every network. They are hard to predict as networks will change and be
reconfigured resulting in new types of impurities introduced into the sync and data
signals. One sure way to combat these perturbations is to deploy more sources of high
quality sync at the edge of the network where the BTSs are located.
Figure 1: Phase Hits and Wander occurring over SDH and PDH
Conformance to Standards
In order for 3G services to work properly, the absolute RF stability of air-interface
transceivers (base stations) in the Radio Access Network (RAN) must be maintained to
within defined standards. If the relevant standards are not met, the ability of the network
to operate effectively will be adversely impacted. Poor synchronization in networks is
known to compromise cell handover particularly while calling from a moving vehicle but
also while stationary. For networks where synchronization is needed for call hand-offs
between base stations, the accuracy requirement defined in the 3GPP2 standards is 1 x
10-10. This includes all networks based on CDMA IS-95 and CDMA2000 technology.
The emergence of 3G networks will also require accurate synchronization to insure
interoperability between 2G and 3G networks as well as between different 3G operator
networks. Regulations defined by regulatory bodies such as the FCC in the U.S. will
require such interoperability and conformance to standards such as E911 location
services.
Sync for 3G Location Services
The FCC has mandated that for 911 emergency calls the carrier must be able to locate a
handset to within 50m for 65% of the time and within 150m for 95% of the time.
Accuracy requirements are very stringent when trying to determine the location of a
handset in a wireless network. Triangulation calculations to determine handset location
normally involve network-based systems, sometimes combined with transmitters located
inside the handsets. For a robust location system, the time-base of all the elements of the
location system including the BTSs must be tightly controlled. This is only possible if
they are locked to a high precision synchronization source (such as GPS). In cases when
accurate sources of sync are not available directly at the base stations, location
measurement units (LMUs) can be added to the network to provide the timing accuracy
needed to meet FCC requirements (figure 2).
Figure 2. Configuration of LMUs for location measurement in a 3G network
Summary
The reason more wireless and telecommunications providers are adopting precision
timing solutions is simple. The better the sources of timing distributed throughout the
network, the easier it is for service providers to guarantee QoS in the network. With
accurate sources of timing and frequency, service providers are able to provide more
diverse and higher quality services. Service providers are learning that they can save
money as well, because having an accurate time and frequency source available helps
reduce certain maintenance costs in their systems and can be leveraged to increase
efficiency and overall capacity of their networks.
About the Author
Mr. Eliot Minor is currently OEM Technical Sales Manager for Symmetricom's
Asia Pacific Region. Mr. Minor is responsible for the sale of various
integrated GPS and Timing and Syncronization solutions into OEM customer
products. Customers include most of the major global network equipment
manufacturers in addition to smaller specialized wireless and network
equipment manufacturers. His focus is on the rollout and design of new 3rd
generation wireless systems, as well as helping customers meet the
continually increasing demands for accurate and precise synchronization and
timing solutions. Before joining Symmetricom, Mr. Minor spent about 10 years
working as a business development consultant for high-tech venture companies
in Korea and Japan, and is fluent in the Japanese, Korean and German
languages.
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