HSDPA: Big Brother Wireless

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Expected to be available to the end users by second half of 2005 high-speed downlink packet access (HSDPA) is the next step in the evolution in the WCDMA line of wireless access technologies. HSDPA is a packet-based data service in WCDMA downlink with data transmission up to 8-10 Mbps (and 20 Mbps for MIMO systems) over a 5MHz bandwidth in WCDMA downlink.

HSDPA implementations include Adaptive Modulation and Coding (AMC), Multiple-Input Multiple-Output (MIMO), Hybrid Automatic Request (HARQ), fast cell search, and advanced receiver design.

In the 3G scenario, HSDPA is a similar enhancement to WCDMA as EDGE is to GSM/GPRS networks. WCDMA in itself is capable of delivering data speeds up to 2 Mbps and HSDPA promises to hike data delivery by five fold to 10 Mbps in the 5 MHz channel. Theoretically, HSDPA can deliver data speeds between 10–14 Mbps. But it would be realistic to expect 2–3 Mbps downlink on average. In a shared environment, with an adequate coverage, 300 kbps to 1 Mbps downlink speed is being targeted.

HSDPA also shortens the round-trip time between the network and terminals, and reduces variance in downlink transmission delay. This makes HSDPA comparable to any wireless LANs and fixed-line broadband in terms of data throughput.

As the consumers become more mobile and get addicted to rich multimedia content over handsets, the demand for better network would rise. HSDPA would enable operators to deliver advanced mobile broadband services and applications. For an operator, who has already rolled 3G plans on WCDMA, upgrading to HSDPA would not be difficult. The HSDPA-ready WCDMA base stations require just a software upgrade and the network is ready to meet the demand for broadband access over wireless.

Key Mechanisms for HSDPA
Technical Features
Bandwidth 5 MHz
Downlink frame size 2 ms TTI
Channel feedback Channel quality reported at 500 Hz
Data user multiplexing TDM/CDM
Adaptive modulation and coding QPSK and 16 QAM mandatory
Hybrid ARQ Chase or incremental redundacy (IR)
Spreading factor SF=16 using UTRA OVSF channelization codes
Control channel approach Dedicated channel pointing to shared channel

Source: Nortel Networks

As a technology, HSDPA has all the characteristics to lower the cost per bit for data and can support services like interactive gaming, VoIP, and SIP-based multimedia. It also promises improved spectral efficiency delivering more consistent quality of service to a larger number of subscribers.

As it is being touted as an efficient technology that lowers the cost of the on-going network growth, HSDPA is all set to attract the attention of the operators who have already deployed WCDMA or are planning to go for a 3G rollout. Though GPRS and EDGE have already made their entry in India, the 3G plans none of the GSM operators is ready to come out in the open. It is also a fact that they need to do a lot in their present offerings before they migrate to WCDMA, which in turn would usher in HSDPA. And, this seems a couple of years away.  

Anurag Prasad

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