Cellular service providers are deploying an advanced version of the high-speed, mobile 3G data services already offered on many of their networks in an attempt to accelerate the adoption of broadband-enabled wireless computing. By doing so, they’re hoping to fight off a growing challenge from alternative Wi-Max networks, which deliver data (and increasingly voice calls) over long distances – potentially threatening the core business model of these companies.
Already, 18 cellular networks worldwide have launched or upgraded a version of the High-Speed Downlink Packet Access (HSDPA) network in 14 countries, with another 29 nations expected to be upgraded by the end of this year, according to the Global Mobile Suppliers Association. The networks promise respectable speeds of 1.4-3.6 Mbps – the limit of today’s mobile phones and PC cards – although the HSDPA technology can actually support up to 14 Mbps.
Philippe Keryer, president for mobile radio activities for Paris-based network equipment vendor Alcatel, says that speed is critical because mobile providers don’t want to leave customers disappointed (as many were with 3G networks). “People are inevitably going to compare this to their DSL line,” says Keryer, so operators “need to have service that can compare.”
While it’s taken the cellular companies some time to commit to HSDPA networks – equipment makers have been eager to sell them on it since 2004 – the changeover, once initiated, is a quick process. That’s because HSDPA is primarily a software upgrade on the network end.
Bill Krenik, manager of wireless advanced architectures for Texas Instruments, which is a major wireless semiconductor supplier, says the fundamental difference between HSDPA and current 3G technology is that the equipment operating the antennas that communicate with the mobile devices adapts on the fly. Rather than broadcasting one form of signal, they tailor their signals to each user based on the quality of the link. “The base station is going to assess the quality of the channel and use the coding and signal that’s most suitable,” says Krenik.
When the connection is good, for example, an HSDPA base station will transmit a more efficient signal, called 16-QAM, where each “symbol” transmitted and received corresponds to 16 bits of information – four times more per symbol than a 3G transmission. Over a strong connection, the base station will also transmit less error-correcting information, the coding that enables the phone to check whether it got a clean feed; a tighter code means faster transmission of the data packets the user wants.
And when a phone does detect a problem and asks the base station to resend garbled or lost data, HSDPA responds much faster than 3G. In 3G networks, such “retransmission requests” initiate a long, slow hunt for packets at various layers in the network. In HSDPA, the outgoing data is buffered in memory added to the base stations, which can thus respond immediately to the retransmission requests. Krenik says this feature, by reducing network traffic, can double data throughput on a cellular network.