Wireless connectivity is all around us and pervades everyday life to such an extent that it has become more of a necessity in terms of modern life, much like electricity or gas. The promise of the Internet of Things and the connecting of devices and gadgets is driving the need for a much simpler, yet more configurable, approach to providing wireless connectivity. In addition, the cellular wireless infrastructure is experiencing massive growth as more consumers, using smarter and faster mobile devices, demand an ever increasing amount of bandwidth.
Over the years, designing a digital wireless solution has required a lot of time, money and specialised resources. The work involved in mixing the analogue and digital domains has always been a challenge, with size being a big issue for RF designers to address. As chip sizes have shrunk, so pressure has grown on designers to deliver solutions that provide a consistent and reliable signal.
However, a number of new devices have been developed that are capable of smoothing the interaction between these domains. One is the LMS7002M wireless transceiver from Lime Microsystems, a device that can be field programmed in a similar manner to an FPGA.
The field programmable radio frequency (FPRF) transceiver design approach is identical to that of an FPGA. Architected for programming by the end system manufacturer, all the configuration elements – such as DSPs, D/A and A/D converters, filters, mixers, PLLs and amplifier stages – can be arranged as required. While the FPGA provides flexible programming in the logic domain of a system, the FPRF provides flexible programming in the RF or wireless domain.
TheLMS7002M is a 2x2 MIMO (multiple input, multiple output), fully programmable digital to analogue RF converter covering frequencies ranging from 50MHz to 3.8GHz. With a programmable RF bandwidth range of 0.1MHz to 108MHz, the device supports both TD and FD operation and is 3GPP compliant for all mobile standards and Wi-Fi.
The continual drive towards higher system capacity, operational agility and lower cost drives an update to the 3GPP wireless standards every 18 months. Many of the releases are evolutionary, but some will require massive upgrade or redeployment of the network.
Currently, operators are incorporating LTE Advanced (LTE-A) into their systems and are planning to adopt such near term features as IoT, M2M communication and common radio access network (CRAN) architectures. Advanced concepts such as RAN sharing, software defined networking (SDN) and network function virtualisation (NFV) using commercially available off the shelf equipment are also being discussed.
While there are a broad range of application areas for an FPRF device, those that combine both FPGA and FPRF devices are the most exciting, since this combination provides complete flexibility in a wireless system.
Near term applications of FPRF and FPGA functionality include software defined radios (SDR), self organising networks (SON), small cell base stations and machine to machine applications (M2M), while long term applications are RAN sharing, cognitive radio and other advanced radio architectures.
Figure 1 shows an example of a radio unit architecture that can be used to support all wireless standards and which offers the flexibility to cover frequencies ranging from 50MHz to 3.8GHz. In most applications, with proper selection of RF filters, these radio units can be reprogrammed after field deployment, providing complete flexibility to the operator.
A practical application of the radio architecture shown in fig 2 is inter band carrier aggregation, where a FPRF plus FPGA radio unit could be field reprogrammed to add 20MHz LTE to an existing 10MHz LTE carrier, thus tripling network capacity without deploying or changing any radio units.
This example also applies to SONs and cognitive radio applications, since the radio unit can sense and/or monitor interference existing and new radio signals and adjust (reprogram) its frequency and bandwidth to optimise network performance.
An example is the bladeRF SDR platform, which uses the LMS6002D and the Altera Cyclone IV to implement an open source SDR platform. This platform can be programmed from 300MHz to 3.8GHz with a modulation bandwidth of up to 28MHz.
M2M communications may require communication over multiple networks, such as Wi-Fi, 3G or 3GPP, and over multiple frequency bands. In this case, a flexible radio module providing multistandard carrier aggregation can provide the best overall solution. In this example, bandwidth and capacity can be shared on a 3G network, solving remote monitoring issues.
A natural use for FPRF in advanced 3GPP applications is RAN sharing. While this has been included in the 3GPP standard for some time, it has yet to be implemented. With every new feature or release of 3GPP, the cost to redeploy (for example, 3G to 4G) or to include enhancements (such as LTE-A) has to be shouldered by each operator. With revenue per user almost flat, RAN sharing becomes a practical way to reduce infrastructure costs.
The standard, known as TS 23.251, sets out two approaches to sharing an RAN: a multi operator core network (MOCN); and a gateway core network (GWCN). Regardless of the approach to RAN sharing, a flexible hardware approach using RF and logic provides the best solution because field programmability allows requirements that are unknown today to be addressed in the future.
As an example, consider a case where operator A upgrades a network including flexible RF and flexible logic to provide additional capacity in the radio unit. A second operator, (Operator B), could then lease RAN capacity from Operator A in order to reduce its capital expenditure, or capex. Operator A, meanwhile, can monetise the existing network, thus reducing its costs. A shared RAN network allows a third party to provide and lease RAN capacity to operators, reducing the capex required to enter a market, thus providing users with more choice and lower costs. Since channel and band plans not only vary internationally, by also between metropolitan areas, it is almost impossible to design a fixed hardware that can meet all requirements. A programmable hardware platform is the only practical solution.
In summary, the growth of wireless use and devices is forcing operators to develop better, more scalable and more cost effective base station solutions. An example of the kind of technology that could help address the growing operational costs for network operators is a wireless transceiver that has been designed to be field programmable for RF performance (FPRF).
The FPRF transceiver design approach matches closely that of an FPGAand, by combining both, operators have flexibility in the logic and RF domains. This flexible hardware approach enables a simpler methodology for carrier aggregation, SDRs, SONs and M2M applications, among others.
In the longer term, FPRF will be able to support RAN sharing, cognitive radio and other advanced radio architectures, reducing infrastructure cost for operators.
Author profile:
Mike Fitton is director of Altera's wireless business unit.
