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The Wireless Edge: High Speed Downlink Packet Access

First, an important correction: In my last column on wireless data
uptake, I reported 500,000 EV-DO customers for Verizon--the number
that has circulated in various stories. Mobile Pipeline Editor Dave
Haskin questioned this number and obtained a clarification from
Verizon that the number refers to data subscribers for both 1xRTT
service and EV-DO. Most likely then, the majority of the customers are
using the earlier 1xRTT service. Now, on to HSDPA (High Speed Downlink
Packet Access).

Vodafone, one of the world's largest cellular operators, and Nokia
just completed testing of HSDPA, a new version of WCDMA (Wideband
CDMA), in Italy using Nokia infrastructure. The two companies reported
throughput rates of 1.5 Mbps. Various other companies have
demonstrated throughput speeds of over 1 Mbps for this technology, and
peak speed claims of over 14 Mbps are also common. But what speeds can
you realistically expect and what services will be available when?
Also, how does this technology line up with the competition, including
EV-DO, Flash OFDM and WiMAX?

First, let's look at what the technology is. HSDPA is an enhancement
to WCDMA, a technology that is also referred to as UMTS (Universal
Mobile Telecommunications System), the 3G path chosen by most GSM
operators around the world. Today, there are already some 75 UMTS
networks in operation around the world. In the United States, Cingular
(via its purchase of AT&T Wireless) has UMTS available in six cities.
However, the company is planning an aggressive deployment of HSDPA,
with 15 to 20 cities planned by the end of 2005 and most major
metropolitan areas by the end of 2006. The six cities with current
UMTS service will be upgraded to HSDPA as well. Globally, many UMTS
operators are planning on the HSDPA upgrade, and operators that have
not deployed UMTS yet are likely to go directly to HSDPA.

HSDPA is important for operators because current UMTS technology,
which based on 3GPP WCDMA Release 99 specifications, is extremely
efficient for voice service, but it is not optimized for data
services. The HSDPA upgrade, based on 3GPP Release 5 specifications,
keeps the same voice mechanisms but adds highly efficient data
capability. It does this through radio mechanisms such as higher order
modulation (16QAM in addition to QPSK), improved error correction,
dynamic adaptation of modulation and coding based on radio conditions,
and a tighter link between mobile and base station. It also adds a
packet scheduling approach that favors downlink transmissions to users
with the best radio conditions, thus allowing them to receive data at
higher throughput rates. Since conditions vary by users over time,
this results in what is called user diversity. All these approaches
working in combination result in at least a doubling of spectral
efficiency, and with later improvements on the roadmap, a further
doubling. Bottom line: The network can support many more users at
a higher speed.

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