The move to 4G will also bring technical and spectrum challenges. Multimode radio will be key to LTE rollout, say David Hawke and Harpinder S Matharu, Xilinx
An important requirement for the rollout of 4G networks is the need for the new technology to co-exist for considerable time with the installed broadband wireless network (GSM, WCDMA, CDMA2000, and TD-SCDMA). Existing GSM and WCDMA wireless infrastructure is cost effective for basic services such as voice. However, the emergence of new multi-mode radio hardware, designed to be field upgradeable to support multiple air interface standards, is set to play a key role in the initial deployments of 4G networks.
Multi-mode radio technology allows operators to embed into their networks the flexibility needed for future transitions to new air interface technology with minimum hardware changes. In addition, multi-mode radio heads support remote connectivity in keeping with the growing trend toward distributed base stations (BTS) to reduce overall network cost and improve coverage. Industry consortiums and trade associations have taken on the challenge of defining standards-based remote connectivity interfaces for radio. Support for standards-based interfaces is critical to promoting a healthy ecosystem of low-cost, off-the-shelf hardware options for multi-mode radio development.
Programmable Hardware
The design of multi-mode radio heads is a daunting task that is well suited to the attributes of field programmable gate arrays (FPGAs). Today's FPGA technology is an ideal programmable platform for building cost-effective and flexible multi-mode radio systems on a chip. Indeed, FPGAs are already being used to implement the digital front end (DFE) of multi-mode radio, and are fast becoming pervasive across the industry.
First pioneered in 1984 by Xilinx, FPGAs by their very (programmable) nature deliver the flexibility, features, and field upgradeability required to build and maintain remote multi-mode radio hardware with lower costs and higher operational reliability. FPGAs, in their simplest form, comprise of a matrix of configurable digital logic blocks connected via programmable interconnect. However, the complexity and feature sets of these devices has evolved dramatically over time to current generations, which offer large amounts of on-chip memory, digital signal processing (DSP) blocks, embedded processors, multi-rate high-speed serial interfaces, and hundreds of input/ output (I/O) pins supporting varied standards. They use cutting-edge silicon fabrication technology to provide continuous innovation and improvements for even higher performance, lower cost, lower power, and better reliability.
Multi-Mode Radio
Multi-modal radio technology affords operators an opportunity to significantly reduce capital and operational expenditure by simplifying the supply chain and maintenance. Operating expenditure can be reduced further by using advanced digital algorithms such as Crest Factor Reduction (CFR) and Digital Pre-Distortion (DPD) on programmable hardware to achieve transmission efficiencies approaching 40 percent. By enabling dramatic improvements in power efficiency, these algorithms not only reduce the cost of operating equipment, but also significantly reduce the carbon footprint associated with running large networks.
The initial configuration of radio head supports a single GSM carrier. During a field upgrade, the operator could choose to upgrade to a new image supplied by the equipment manufacturer, as shown in configuration 2, thereby increasing the network capacity overnight. In configuration 3, during another upgrade, the same radio could be re-configured to additionally implement a single carrier of LTE.
In this scenario, network operators are able to continuously adapt and tune their networks to maximise revenue potential and attract new subscribers. The final result may be that the network, over time becomes an all LTE network, yielding the highest spectral efficiency of the available spectrum. This is not limited to GSM and LTE, but could similarly be applied to other standards.
Standards-based Interface
The Common Public Radio Interface (CPRI) and Open Base Station Architecture Initiative (OBSAI) are industry-wide initiatives led by wireless infrastructure vendors to standardise connectivity interfaces in base stations (BTS) to reduce cost and manage increasing system complexity. These standardisation efforts have helped to make multi-mode radio module-level expertise accessible, thereby stimulating greater ecosystem participation from small scale companies and startups. In turn, these new ventures apply the energy and skills of innovative teams across the globe to accelerating the wireless broadband evolution through faster technology development and adoption. Specifically, the delivery of nearly plug-and-play, off-the-shelf modules needed to create entire multi-mode radio systems will have a major role in lowering the CapEx and OpEx costs associated with base stations and backhaul networks.
With FPGAs, CPRI and OBSAI connectivity protocols are easily implemented through highly flexible, optimised, and low cost intellectual property (IP) cores. Currently, these soft IP solutions (programmable and changeable) support up to 3G line data rates, but have the ability to support configurable or automatically negotiated data rates ranging from high hundreds of megabytes all the way up to 6.144G line rates. Clock synchronisation and accurate cable delay measurement for installing multi-mode remote radio head topologies are ‘must-have' capabilities as part of these IP solutions, and are optimally implemented in FPGA devices to address the flexibility, lower cost, and lower power requirements of system vendors.
The modem or channel cards are connected to the remote radio heads using industry standard CPRI interface. The CPRI interface and switching function implements any channel card (also called modems) to any radio head with remote connectivity over an aggregated long range fiber. The DFE downlink path (transmit to user cell device) is comprised of digital up-conversion (DUC), crest factor reduction (CFR), and digital pre-distortion (DPD) functional blocks that are all implemented in the FPGA and connected to a digital-to-analog converter along with a feedback path from the power amplifier via analog-to-digital converter. The DFE uplink path (receive from user cell device) is comprised of filtering and digital down conversion (DDC) functions to an intermediate frequency (IF) at which all the base band physical layer processing is performed.
Real-World Application:
Ubidyne, a company that specialises in digital radio wireless technology, has built an embedded radio active antenna technology platform. This commercial application of multi-mode remote radio technology was successfully implemented on a programmable hardware platform to maximise system flexibility and scalability while optimising performance and power. Ubidyne's multi-mode radio design is digital, small, and completely integrated to achieve greater efficiency and bandwidth utilisation for both current and next generation standards. It consumes less than 50% power as compared to legacy base station systems.
The company's first product, the Ubidyne uB900, is a full featured digital radio system that is completely embedded into the antenna, thereby eliminating the need for motors, mechanical tilt hardware, coaxial feeders and other bulky equipment. This multi-standard platform has an integrated multi-antenna (MIMO) transmission capability and reception-ready RF unit for radio systems supporting GSM, UMTS, HSPA, HSPA+, and LTE. Additionally, the system design is two to five times more reliable with built in redundancy and its unique "self-healing" capability. It is environment friendly in visual impact, cleaner in installation, and features a reduced carbon footprint.
Summary
While 4G technology holds promise for increasing operator revenue, it must co-exist with the current wireless infrastructure. This requirement creates significant challenges for system infrastructure vendors who must manage an increase in system complexity, while keeping costs low. Fortunately, programmable hardware platforms go a long way in providing the much needed flexibility to address multiple air interface standards and the varying spectrum allocations in different geographies. Commercial applications, such as Ubidyne's embedded radio active antenna technology platform demonstrate the highly innovative and unique value propositions of multi-mode radio for improving coverage, capacity, and power consumption of next-generation wireless base stations.
Consumer knowledge of mobile broadband services is on the rise, and service uptake is increasing. This is impacting on the way mobile is viewed by regulators, as well has having a technical imact.
Strand Consult recently produced numbers that show that In countries like Norway, Finland, Sweden and now also Denmark, the number of ADSL connections is now decreasing. In a country like Austria, over 35% of all broadband connections are now mobile and the number for Slovakia is 30%.
John Strand says, "If you examine the Nordic countries, 15% of all Danish broadband connections are mobile, 20% of Swedish and Finnish are mobile and 13% of Norwegian broadband connections are mobile. The market shares that mobile operators have gained by offering mobile broadband have appeared within the last 12 months – and it has been 12 months where we have seen a continual growth in sales.
"In 2008, each time one Dane chose fibre, five chose mobile broadband and we are currently seeing monthly sales of between 18,000 and 20.000 mobile broadband connections in Denmark – that has a population of 5.5 million. In the Nordic countries we are looking at 100.000 new connections a month in a region with 24 million people and sales of approximately 350.000 mobile phones a month.
Numbers like this have had an impact at a political level. The European Union has confirmed plans to invest €18 million in research into LTE.
Vivane Reding said, "Millions of new users will get ultra high-speed internet access on their portable devices, wherever they are. This will create tremendous opportunities and plenty of space for growing the digital economy," she added.
Dan Warren, Technical Director of the GSMA, would no doubt welcome that sentiment. But he cautions that there is work to be done, both by the industry and regulators, to bring LTE to market.
"On spectrum, there are two key areas of focus," he says. "The first is to ensure mobile broadband has access t the digital divivdend spectrum at 700MHz, and also upwards to 798 and 862 in other areas. The second is the extension bands at 2.5GHz, to give operatwors the ability to support large numbers of people in cities.
"But there is a key point beyod that. The alignment of sprectrum from one country to another, and the band plans withing that is important."
Warren says it is key for the industry to have certainty of which frequency bands it will be working in – to drive R&D efficiencies and economies of scale.
The industry already faces a challenge of supporting MIMO?in different frequencies. Warren warns that low frequency MIMO will place demands upon antenna design.
"Spectral diversity will bring antenna receiver diversity on the device side. MIMO at 700MHz carries significant radio challenges. Antennas on the device will need to be far enough apart to avoid cross antenna interference. At 2.6 or 3.5GHz they only need to be 1.5 inches apart, but at 700MHz they need to be a small number of iches apart.
"Now that's not a problem when antennas are embedded in laptops, but it creates issues for phone design.'
Spectral diveristy also promises to increase complexity on the network, which in turn brings increased O&M demands.
Spirent's Bill Burns said, "The networks of today are infinitely different from the networks of 20 years ago. In fact, they are very different from five years ago.
"The complexity of the next generation network design is not lost on the telecommunications industry. CTOs and CIOs are taking steps to assess the methods used to analyse performance of networks and its elements. Not only is there a need to ensure that network and services are meeting the QoS and QoE benchmarks, but evaluation methods also need to be cost effective and efficient."
The articles in the next few changes analyse some of those methods and approaches…