DSPs: Helping Basestation Software Defined Radio Evolve

Andy McCann, Analog Devices, Norwood, MA

With 3G becoming a reality, operators are eager to roll out feature-rich 3G networks that give customers a wider range of services, applications and capacity. However the challenges in deploying these networks are vast and ultimately impede its proliferation. The financial capital of creating the network is obvious, and one of the only competitive places for operators to apply price pressure is on infrastructure equipment OEMs. Secondly, the evolving dynamics of the market make it impossible to get a fix on what features and services customers will ultimately demand. To insure ‘future-proofing’, mobile operators desire a network that can add capacity, features and services instantly.

The need for flexibility, performance, and upgradeability are the necessary capabilities of 3G basestations, not just in the context of competitive equipment costs, but also in considering total cost-to-operate and cost-to-own figures. Recent developments in digital signal processing (DSP) are making 3G a reality, from pricing and performance, to flexibility and upgradeability.

SDR moves to the head of the class

Software-defined radio (SDR) has often seemed a distant objective for several decades because of high costs and a host of technical problems. These limitations have been especially evident in 3G W-CDMA basestations, where manufacturers have typically had to employ a complex mix of special-purpose electronic hardware and software to implement the core baseband section of the basestation. High costs, low flexibility and restricted operational performance have all stood in the way of a viable SDR basestation for 3G networks.

Developments in DSP are at long last enabling the commercialization of a software-based wideband receiver (or software radio), reducing the cost, size, complexity, and power consumption of a basestation (perhaps as much as fivefold). More importantly, can support a multilingual variety of air/modulation schemes and protocols simultaneously, switching between them whenever required. All the processing is done in software, so it is possible to load new protocols, upgrades and repaired code into the basestation as they are developed.

After years spent in the concept stage, the all-software basestation is now both technologically and economically feasible and is ready to significantly improve the economics of 3G for both operators and manufacturers.

Bumps in the Road for 3G

The economics of 3G are compelling: 3G networks deliver more capacity and more spectrum, but with costs that are lower than 2G (a voice channel of WCDMA costs about half the price of GSM). This means revenue-per-user is so much higher (at least 20 percent higher, according to DoCoMo). But the revenue economics and the pressure to deploy 3G still have not spurred a rapid rollout. One of the reasons is the availability of multiple interface standards poses the same interoperability dilemma for equipment manufactures and network operators as they faced with 2G, the second generation of mobile telephony. In addition, the bridge connecting currently deployed second generation systems to 3G systems is continually cluttered by legal, political, economic and cultural challenges. As a result, the flexibility of baseband processing solutions is becoming a central issue among infrastructure equipment manufacturers.

Clearly, without the emergence of a single, flexible, and scalable baseband processing platform which can be utilized for all of the competing 3G modes, infrastructure equipment OEMs are faced with escalating engineering costs, significant time-to-market delays and unacceptably high market risks.

Basestation requirements

A typical basestation design requires radio frequency (RF) and power amplifier expertise, as well as a high-speed baseband, executing specific DSP operations and complex control protocols. The processing budget for 3G is estimated to be about 500 times greater than for GSM. W-CDMA demands fast signal processing, which requires logic implementation. However, these applications also have complex control algorithms such as searching, multi-path tracking, and finger assignment, which are better-suited for implementation in software.

In conventional cellular basestations, each channel dedicates a receiver tuned exclusively to a specific band. In stark contrast, the flexibility of a processor-based digital stage means that the basestation can be 'reprogrammed' to work with new standards. While the concept of SDR based on a powerful processor has been well understood for at least 20 years, technical and cost challenges have made feasibility elusive until now.

The first challenge is speed – the basestation must operate across a wide frequency band. More challenging still is the need for dynamic range. In the conventional approach, each radio only deals with a narrow band – by filtering out interfering signals, the classic receiver concentrates on the desired signal, adjusting gain to optimize signal-to-noise performance and extracting a weak signal from noisy background. However, with a wideband receiver that is no longer possible; no signals can be filtered out, because they are all required.

As a result the receiver must have an extremely wide dynamic range, for enough sensitivity to accurately recover weak signals without being swamped by the louder ones.

Architecture issues

Some previous architectures used a hybrid approach, perhaps combining whatever performance could be mustered by a previous-generation DSP with the hardware power of FPGAs or ASICs. Such heterogeneous multi-device architectures are inefficient because they must support the worst-case loading in each area independently even though no one application would require such rigorous provisions. Heterogeneous hybrid architectures compound development and deployment headaches and the industry has ultimately desired a single, cost-effective processing element that could perform without the need for adjunct hardware support – a single homogeneous environment that can address the variety of tasks, move resources between them as and when required, and simplify aspects ranging from B.O.M. parts stocking to in-field technical support.

Flexibility and cost, both capital and long-term, are the key advantages of an SDR basestation. These are the key competitive differentiators for basestation manufacturers looking to produce products that are differentiated in terms of price and performance, and which can be adapted easily as 3G standards evolve. They are also important to 3G network operators who need low-cost solutions that offer the best possible performance and the flexibility to easily upgrade or modify the system. In the end, fully programmable SDR-based basestations are the only option for operators who want to survive in the 3G era.

Conclusion

It is clear that only the advent of a signal-processing engine that is simultaneously powerful enough to implement an SDR basestation and inexpensive enough to deploy widely has supported the chronic use of adjunct hardware in basestation design. But today, processors are changing that situation quite immediately. Operators will demand flexibility and cost-of-ownership scenarios that can only be met by SDR basestations, and mobile infrastructure OEMs simply have no choice but to respond accordingly.