Introduction

With a wide range of wireless standards and evolving global target carrier frequencies, infrastructure Original Equipment Manufacturers (OEMs) are faced with the challenge of rapidly addressing diverse market radio requirements while controlling development and inventory costs.

This challenge is further compounded by the need to deliver significant technical improvements within the radio equipment. Close to zero footprint, lower thermals, increased reliability and greater linearity to accommodate higher order modulation schemes are a few worth mentioning..

Figure 1: Challenges faces by Equipment Manufacturers and a possible solution.

Fig.1 demonstrates the challenge to OEMs and introduces a solution in Multi-Modal radio’s.. This term is used to describe a radio that has multiple personas. In essence, a platform configurable to address multiple target  frequencies through factory fit options, and capable of delivering multi-carrier multi-standard air interface support depending on the end market and deployment scenario.

While seemingly a utopian dream, various technologies have advanced to the point where Multi-Mode Wireless Radio’s are an emerging reality and will soon be used to address some of the emerging opportunities resulting from frequency liberalization.

Supply Chain Benefits of Multi Mode Radio

Figure 2: Benefits of Multi-Modal Radio

Multi-Modal Radio can offer benefits across all levels of the supply chain as illustrated in Fig.2. Having a flexible and programmable radio allows Network Operators and OEM’s the ability to provide support for new and evolving standards with minimal risk. It also enables Operators to  deliver coverage and services in response to usage scenarios from their customers. Clearly, there is likely to be most return on mobile high speed data close to business centres, transport hubs and urban areas. Other areas may benefit from lower cost voice coverage to compete directly with fixed line to the home.

The concept of Multi-Modal Radio is not new. Military applications have long been pursuing Software Defined Radio (SDR) as a truly reconfigurable platform that can support many different waveforms (or air interface standards).

Advances in Radio Frequency Design

Unfortunately, analogue circuitry has not tracked the cost reduction curve of Moore’s law in the same way  digital logic has been able to. However, there are still advances in the analogue domain that have enabled the digital domain to move closer to the antenna so delivering increased flexibility and lower cost points.

The data converter manufacturers are now able to provide support for Multi-Modal Radio through the use of direct conversion architectures. Such architectures allow for a much lower Bill of Materials (BOM) cost, by removing the need for radio Intermediate Frequency (IF) stages found in heterodyne transceivers. Fewer components result in a smaller footprint, lower testing costs and increased reliability through fewer points of failure.   Further more, there is a need for only one Local Oscillator (LO) providing the transmission frequency required by the Operator and  selectable frequency LO’s are now available, so enabling the same unit to service a number of frequencies and wireless standards.

Reducing Analogue costs with Digital Techniques

Moore’s law has held true for many years and while all CMOS devices have benefited from the journey into deep sub micron geometries, it is interesting to note that the number of pioneers on the bleeding edge is dwindling. Quite simply it is becoming harder and more expensive to develop solutions at geometries lower than 65nm and there are a decreasing number of applications where the business model makes sense to embrace the costs of addressing the engineering challenges.  Programmable Logic however, is uniquely positioned to defray the costs of continued innovation by targeting inherently flexible devices to multiple end markets. The transition from simple glue logic to powerful programmable SoC devices with the ability to deliver ASSP like functionality while retaining full flexibility, is at the heart of emerging muti-mode radio solutions.

Key elements of these devices are:-

·         Integrated SERialiser/DESerialiser (SERDES) to allow OBSAI and CPRI radio communication protocols to be implemented in FPGA.

·         An array of  Digital Signal Processing (DSP) blocks delivers low power, cost efficient implementation of multi carrier radio signal processing algorithms,

·         The availability of soft processor solutions with MMU[1] support and capable of running Linux offers another component of integration essential to continued innovation and cost reduction.

Power Amplifiers (PA’s) are well known to be the largest proportion of the cost of running the BaseStation, due in part to the requirement to be highly linear with the later signalling standards. Amplifier design is also progressing, but coupling this with advanced digital algorithms provides significant efficiency improvement with low equipment costs. Two well known methods of improving PA efficiency are Crest Factor Reduction (CFR) and Digital Pre-Distortion (DPD).

Due to the advancement of FPGA’s, CFR and DPD algorithms can now be realised cost effectively. Xilinx has seen efficiency improved from 6% to 34% on certain amplifier types with UMTS using it’s CFR and DPD algorithms. This significantly reduces the OpEx costs of running network equipment, and contributes to a reduction in CO2 emissions due to energy savings.

Figure 3: Multi-Modal Radio platform. (Picture courtesy of AXIS Network Technology)

Fig.3 illustrates a Multi-Modal Radio, where all DSP processing is accomplished using FPGA technology. Signal processing techniques are deployed such that any mixture of air interface waveform can be realized using a single device and the board shown is capable of CDMA2000, WiMAX, TD-SCDMA, UMTS and 3GPP-LTE.

Mixing air interfaces

Given that all radio’s are inherently networked, the FPGA can be reprogrammed after deployment and this delivers interesting possibilities for operators to future proof their networks and adapt to emerging opportunities. For example, it may be desirable to configure a radio close to a business center with multiple carriers of GSM and a single carrier of 10MHz LTE to harness the emergence of LTE data cards with the business community. The same radio platform deployed close to a transport hub could be configured with multiple carriers of GSM and a 10MHz carrier of WiMAX to potentially leverage the  emerging mobile WiMAX community. Of course there are those who still question timeline and pace of adoption for WiMAX, but with the deployment of multi-modal radio’s, operators are deploying risk mitigation and the opportunity for greater network optimization.

Benefits of enhanced Supply Chain Efficiency

One of the key benefits of multi-modal radio platforms is supply chain efficiency.  A streamlined product portfolio delivers a number of benefits such as lower inventory cost and reduced engineering effort  to integrate, test and qualify a single multi-mode platform compared to  separate platforms for UMTS at 2.1GHz, WiMAX at 2.5GHz or WiMAX at 3.5GHz for example. All of the above could be supported from the same base PCB and digital circuitry, allowing significant Bill of Materials (BOM) reduction over having multiple platforms with differing base architectures.

Increasing reliability

Maximising a system’s reliability is key to reducing OpEx and increasing profitability. Radio electronics reliability is affected by a number of elements. One of those being thermal stress.

Radios are thermally challenging. RF electronics and power amplifiers tend to run hot, and it is this thermal activity which can adversely affect a unit’s reliability. Reducing component thermal stress in both the analogue domain (RF components and power amplifiers) and digital domain (digital radio processing) results in far superior equipment reliability.

Traditional radio subsystems have comprised of  hundreds of components, many of these in the analogue domain. However, in newer analogue architectures there is a significant reduction in component count leading to a statistically higher reliability figure. Comparable integration in the digital domain is available with FPGA technology. Multi carrier Digital Signal Processing, programmable OBSAI/CPRI connectivity and embedded processing for Operations and Maintenance (O&M) functions can now be supported in a single device  lowering power consumption, thermal heat dissipation and increasing reliability.

The reliability of Xilinx’ 90nm FPGA’s are reported as 2 FIT’s or Failures In Time. This means that statistically there would be only 2 failures per 1 billion device hours.

Consumer benefits

If Operators and OEMs were to leverage the inherent flexibility of FPGA’s to deliver multi modal radio’s  it would open up possibilities for new levels of network optimisation.

Firstly,  Networks could be optimised such that some cell sites could be tuned to deliver a wider OFDM bandwidth and fewer GSM or UMTS carriers in areas of high broadband data, enabling greater freedom to optimize the price plans for localized regions. 

Also it has been noted by many operators that usage patterns vary through the day with peak voice and data traffic occurring during different times of the day. Multi modal radio’s founded on FPGA technology supporting dynamic configurability such that they need not go off line during a reconfiguration, opens the possibility to optimizing the network on a cell by cell and as frequently as desired.

Delivering services that customers are willing to pay for, through  flexible, efficient and cost optimal radio hardware allows networks to be continually tuned and adapted to the ever-changing market needs. This strategy allows greater profitability and opportunity to increase ARPU depending on the various consumer personas targeted by the operator, whilst mitigating risks associated by deploying completely new networks.

Conclusion

Technology advances both in the analogue and digital domain are enabling Multi-Modal Radios today. Network Operators can take advantage of these emerging radio platforms to reduce OPEX and deliver  optimized network configuration that minimizes risk, boosts cell performance and maximizes the opportunity to boost revenue from emerging data centric services.