Wireless Review, W-CDMA>
W-CDMA systems will operate in 5MHz, 10MHz and 20MHz frequency
carriers, which are orders of magnitude greater than today's CDMA
bandwidth. The corresponding chip rates for transmission over the air also
will be much higher. This means that the propagation characteristics will be
different from today's second-generation systems. Coupled with the fact
that the GSM carriers do not have operating experience in CDMA, this
focuses the initial objectives of a W-CDMA trial on understanding how
well W-CDMA works and on answering key financial deployment
questions.
The Microcell W-CDMA trial is being funded by the GSM Alliance, and
the results of the trial will provide alliance members with critical information
that will help them justify the economics and timing for potential W-CDMA
deployments.
Equipment Providers
Nortel Networks provided the W-CDMA trial equipment, both because of
its existing strong relationship with Microcell and the early availability of
its W-CDMA prototype equipment. The trial infrastructure equipment is
illustrated in Figure 1 and includes:
Figure 1. 3G W-CDMA network configuration at Microcell
• Two BTSs, where each base station has three sectors using a simple omni
antenna
• A GSM MSC providing interconnectivity to the public switched
telephone network (PSTN), and an ATM switch that provides IP over
ATM interconnectivity to Internet service providers (ISPs)
• Internetworking functions to interconnect to the PSTN and ISP networks
that are provided by an radio network controller and MSC simulator.
Early handset availability has tripped up carriers in implementing new
wireless technologies in the past. For W-CDMA, traditional issues include
the development of multimode GSM/W-CDMA handsets and multiband
handsets to deal with the different global frequencies. However, 3G
handsets present further complications because they no longer are mobile
phones but encompass high-speed packet data applications.
Matsushita (Panasonic), which recently formed a partnership with Nortel
Networks to develop 3G voice and data solutions, is providing three types
of handsets for the Microcell trial:
• An 8.8kb/s voice mobile phone
• A data PCMCIA type II card. This uses a Windows-based laptop for
dial-up connections and provides both 64kb/s circuit-switched and
packet-switched data connections.
• An experimental mobile. This provides up to 384kb/s packet-switched
connections, supporting voice and data, but not video. The mobile is
coupled with RF test equipment to generate and capture field data.
No Longer Today's CDMA
The Microcell trial system operates in the upper end of the 2GHz band,
using direct-spread CDMA over 5MHz band and transmitting at a chip rate
of 4.096Mc/s. Today's CDMA system operates in a 1.25MHz frequency
carrier at a chip rate of 1.2288Mc/s.
The trial uses asynchronous intercell operations. By contrast, CDMA2000
uses GPS to synchronize intercell communications. W-CDMA posed
different hand-off challenges, and early trials have been essential to
resolving these issues.
Another key differentiator is the rake receiver. CDMA uses rake receivers
to detect and combine multipath. Rake receivers give CDMA a significant
advantage over other wireless-access technologies by improving the
signal quality, especially in the fringes of a cell, and in enabling soft
handoff. Today's CDMA standards support rake receivers with four
correlators, which provides the ability to combine three distinct signal
paths.
The 3G trials around the world are varying the number of correlators in the
rake receiver to test their effectiveness under different propagation and
soft-hand-off conditions, such as in pedestrian and built-up urban areas.
Although the number of correlators in the Microcell W-CDMA trial has not
yet been disclosed, we can speculate that if this trial follows the Japanese
W-CDMA specifications, it supports six sector handoffs and hence has
seven correlators.
Timing Is Everything
Today, Microcell's GSM systems support data services up to 9.6kb/s.
GPRS and EDGE extend GSM system capabilities to support high-speed
packet data services. Therefore, they enable GSM operators to offer
services such as Internet access and video calls without changing core
infrastructure equipment.
The challenge for GSM carriers is whether to invest in GPRS and EDGE,
which may give time-to-market advantages, W-CDMA or both.
“W-CDMA provides large pipes, but the path is through GPRS/EDGE,”
said Bob Brown, GSM Alliance executive director. “GPRS and EDGE,
although not as fast, will give consumers the image of 3G and potentially
be sufficient to give the subscriber enough of a wow.”
Spectrum availability plays a huge role in determining deployment. If a
carrier has sufficient spectrum, then GPRS and EDGE will enable it to offer
high data-rate services. If the carrier has insufficient spectrum, then it
either has to acquire spectrum or clear out spectrum to enable the
deployment of a spectrally efficient W-CDMA system, much like the
800MHz analog carriers cleared spectrum for the deployment of today's
CDMA and TDMA digital systems.
In North America, spectrum availability is less of an issue because carriers
are not as far along in building out the digital networks and attracting
subscribers as their European counterparts. The larger issues are justifying
the cost of rolling out a new technology before fully exploiting the existing
GSM technology, the availability of EDGE vs. W-CDMA handsets and the
challenges of operating two distinct wireless systems.
Microcell plans to take a one-step-at-a-time approach to evolving its
network. Providing bandwidth is the first enabler.
“We need to balance the deployments,” said Tony Schultz, Microcell vice
president of network planning. “We are aggressively looking at GPRS in
the 2000 time frame. We are not as far along in our consideration of EDGE,
but we are seriously considering it perhaps in the 2001/2002 time frame,
with W-CDMA in 2003/2004.”
In addition to the radio-related challenges, Microcell is faced with evolving
the core network from a voice-based network to an architecture that
supports both voice and data.
Trial Results & Future Challenges
The Microcell trial will be completed by the end of this year. The results
will provide answers to fundamental deployment questions: Can the
existing GSM tower footprint be used for the deployment of W-CDMA, or
will additional tower sites be required? What is the in-building signal
penetration from an outdoor base station? The trial likely will continue next
year with new objectives. The extended trial may include:
• Services and applications. Since commencing the Microcell trial, Nortel
has begun two additional W-CDMA trials in Europe. The first is with
British Telecom in Suffolk, England, where the goals include understanding
the market and technology drivers for future mobile IP data and voice
services. The second is with France Telecom, which is conducting a trial
using a test system at the Nortel GSM headquarters in France. Nick
Sanvoisin, product manager at Nortel responsible for the UMTS trials, said
that beyond assessing the economic justification for deploying W-CDMA,
these W-CDMA trials demonstrate useful applications at the high data
rates.
• Expanded technology tests. The trial equipment may need to undergo
changes to support these new tests.
• Test environment for 3G vendor equipment. Other vendors already have
expressed interest in participating in the Microcell trial, and the trial is
expected to be expanded to provide a multivendor test environment.
GPRS and EDGE provide GSM operators with the opportunity to roll out
high-speed data services and to maintain the loyalty of their existing
subscribers.
Is there a danger that the United States will fall behind Europe? The answer
is unclear. Although some European operators have stated their intentions
to deploy 3G, most have indicated that they will implement GPRS.
Time-to-market and understanding the emerging wireless data opportunity
is viewed as more critical than the technology choice.
In an uncertain market, one thing that is certain is that the Microcell trial
will enable the North American GSM operators to be much closer to
answering the critical strategic question regarding when to implement
W-CDMA.
Salter (asalter98@aol.com) is an independent telecommunications consultant based in Moranga,
CA.
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