Ever wondered why the majority of the mobile phones on the planet have an MCU and a DSP to provide the wireless modem function? Before getting into the answer, let's take a 30,000-ft view of a mobile phone communication engine. This engine is split into three layers that perform specific communication tasks. The lower the layer, the closer it is to the antenna; the higher the layer, the closer it is to applications and how the transmitted data is actually used by the consumer: voice, video, music, data files.
Layer 1 (physical layer) sits close to the RF (radio frequency) A/D and D/A converters—the analog front end—and requires heavy signal processing (multiply and accumulate—MAC functions) but does not consume significant memory size for software code, because it is made of few, repetitive, and computationally intensive, small kernels for data analysis and signal manipulation.
On the other side, layers 2 and 3 consume much less computation bandwidth, but result in a large program, as it contains code able to properly handle the communication protocol made of a complex state machine.
References: 3GPP TS 25.201 "Physical layer—general description".
When GSM was commercially launched in 1991, semiconductor companies had strong capabilities in mixed signal and SoC integration, while original equipment manufacturers (OEM) "controlled" the software layers—mainly L1, L2, and L3 to interface to higher level software, for example, the man-machine interface. The OEM could hence bring the needed differentiation to address network operators and end customers' needs.
During this time period, real-time operating systems (RTOSes) were only starting to be available and more importantly, C compilers were mostly available on MCUs and barely on DSPs. MCUs had better code density than DSPs, while DSP were much more efficient at running the physical layer and the voice coders. Naturally, OEMs focused on layer 1—with the semiconductor partners for the hardware/software integration—and layers 2 and 3 and implemented them on MCUs, while semiconductor companies handled mixed signal and SoC integration. When GSM technology was launched, no single player in the value chain had all the know-how necessary to create a complete single-chip solution for this application.
Is the Architecture About to Change?
Over time, architectures of embeddable processors have improved to a point that some DSP compilers now produce code with density competitive with MCUs. StarCore is a unique example of that new breed of processor cores. The StarCore architecture was defined according to a compiler-friendly design philosophy and features excellent control code density while exhibiting superior DSP performance.
The value chain has also tremendously evolved as the GSM technology matured. OEMs now face stiff competition from original design manufacturers (ODM), who are introducing unbranded cell phones into the value chain. The cellular modem function has now matured and stabilized; potential for differentiation has decreased. Requirements for a wireless modem black-box approach emerge from players who are focused on shorter term, very fast-to-market system solutions. Simpler and cheaper cellular modem solutions with low maintenance are key to getting quickly to the market.
Even if the technology has matured, it is still a challenge to integrate hardware and software. New companies, focused on intellectual property such as TTPCom, now provide full solutions ranging from software protocol stacks to complete reference designs.
This environment favors the simplification of the communication engine architecture to a point where the DSP can handle the complete modem function alone. The single-core DSP modem can efficiently handle layer 1, layer 2, and layer 3 of the protocol stack and no longer requires the use of an MCU.
What are the Benefits of this Solution?
First and foremost, a single-core DSP modem is a programmable solution that allows upgrades and fixes to be performed in the field. Other solutions that rely heavily on MCU alone and hardwired architectures cannot provide this level of flexibility because dedicated logic cannot be easily upgraded and any change requires a re-spin of the silicon.
The unification of layer 1, layer 2, and layer 3 on a single DSP gives code developers a much simpler platform for code development:
- Only one tool chain is used throughout the development cycle from code generation to final testing and validation.
- There is an array of communication schemes that can be implemented between layer 1 and layer 2 since this is all handled in software in a single device. Layer 2 and 3 development team can freely define interface with the layer 1 development team and even continue to refine it over the course of the project, if required.
- Revision control, maintenance, testing, and integration of the software are made easier with only one platform to maintain and support.
- This increase in flexibility further accelerates time-to-market for protocol stacks standard upgrades, feature set modifications, and last-minute requirement changes.
From a licensing perspective, semiconductor companies can deal with a single interface for the communication engine core at all levels: business models, support, maintenance, and product roadmap. This has the benefit of reducing the overall cost of ownership of the solution for both vendor and customer.
The evolution of air interface standards calls for more processing and more interoperability. Over a period of 13 years, we have seen GSM, GPRS, and EDGE, TDMA, CDMA 1x, and CDMA2000. WB-CDMA and its various implementations are now gaining solid ground while higher bandwidth standards like HSDPA are in final specification stages. For this reason, architecture scalability provides for a significant competitive advantage and protects semiconductor companies' investments. Binary code compatibility, another feature of StarCore LLC's family of products, guarantees for instance that a GSM/GPRS binary code can run on a higher performance core with no recompilation or reassembly. Developers can then add seamlessly the EDGE layer and accelerate their time to market. This key feature of the roadmap enables quicker integration of additional communication standards or services, such as WB-CDMA, 802.11, and GPS.
With the most recent advances in DSP technology and SoC implementations it is now possible to implement complete single-core modem solutions on a single DSP SoC including software and supporting DSP functions. Cores such as the StarCore SC1200 and SC1400 will enable better and simpler solutions while at the same time accelerating products to market at a lower cost.
About the Author
Philippe Bettler joined StarCore LLC as manager for Strategic Alliances in October 2002. Prior to joining StarCore LLC, he was business development manager for Infineon Technologies' DSP organization in San Jose, California. Philippe has 14 years of experience in the DSP arena with emphasis on software and systems. Philippe graduated from the University of Technology of Compiegne in France with a major in signal processing. He can be reached at
PR@starcore-dsp.com.
