The availability of data services is being eyed by mobile operators as a key way to increase user revenue. The problem is today’s mobile networks are all about voice services, with only a small proportion of users - roughly ten per cent in developed areas, less in developing markets - using any data capability at all other than text messages to mobile phones.
It seems the only way to make more profit from voice services is to increase efficiency – call charges are increasingly competitive and usage is growing at the expense of wired networks. Any enhancements aimed at increasing data usage need to ensure there is no compromise in voice service quality or availability.
Apart from purely technical decisions, operators must also determine how to deliver services that will encourage users to upgrade to devices supporting both new and legacy technologies seamlessly.
The evolution of mobile networks to support data services really began with GPRS. However, transfer speeds were too slow to support a comfortable user experience and take-up was disappointing.
Today phones support multiple technologies – typically GSM, E-GPRS and W-CDMA - as well as connectivity via Bluetooth and possibly wireless LAN. The first HSPA devices are also coming to market, raising potential download speeds to around 1Mbit/s.
The next step
3G LTE (long term evolution), as proposed in 3GPP Release 8, aims to increase cell data capacity by at least five times over the current implementations of HSPA. It will support more users per cell, as well as higher speeds to individual users, and is intended to match DSL speeds currently available to the home.
A simplified protocol structure and re-definition of the functional split between network elements and basestations is intended to raise efficiency while making all VoIP networks possible.
Another key feature of LTE is its potential to deliver sufficient bandwidth to support video services to individual users. Channel bandwidths from 1.25 to 20MHz are supported in the standard, with ultimate performance requiring a 20MHz spectrum allocation, and providing a headline data rate of 100Mbit/s. Support for the higher speed channel and increased transmission reliability – both of which are needed to reach LTE’s low-latency target – comes from the OFDM-based (orthogonal frequency division multiple access) air interface and MIMO (multiple input multiple output) diversity.
Challenges ahead
The first LTE-based networks are expected to roll out in 2009/2010. In contrast to other cellular technologies, conformance tests for LTE are expected to be available more than two years ahead of any service introduction. This will ensure user devices are available in volume when the network services are finally launched.
At the same time though, this timescale challenges everyone in the food chain including component suppliers, users, network equipment developers, and of course, test equipment companies which must support the standards with measurement capabilities throughout the product lifecycle - from early R&D through to volume manufacturing.
As with HSPA, user-equipment chipsets are being designed to have as long a life as possible, so manufacturers can recover massive investment costs over a longer period.
This means the headline rate supported by a chipset will be much greater than the data rate available to the device in a network. However, vendors will want to confirm correct operation up to the maximum rate that the chipset is designed to support.
Consequently, test instrumentation needs to be capable of supporting higher rates and channel bandwidths early in the design cycle. With HSDPA, for example, this means that test equipment needs to support at least 7.2Mbit/s today, although networks are set up to deliver a maximum of 1Mbit/s to an individual handset.
Another measurement challenge lies in the changing block diagram of both network and user equipment.
New generations of basestation radios rely more heavily on ‘software radio’ architectures, which allow operators greater flexibility to upgrade their capabilities. Now, standards such as DigRF are moving handset design along similar lines.
Such systems use greater integration of digital and RF into single devices to eliminate or hide traditional test interfaces. They also make use of software correction in the digital domain to achieve the desired RF performance. With the addition of MIMO, data from independent transmit and receive channels is combined to maximise correct reception. These changes require new measurement processes and equipment to allow the correlation of digital and RF data.
Renaud Duverne is European wireless R&D marketing development manager at Agilent Technologies.
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