Enhanced Data Rates for GSM Evolution (Edge) technology has reached maturity today in terms of worldwide coverage, stable infrastructure and the variety of handsets supporting Edge features. According to the 3G Americas trade group, more than 230 operators in over 120 countries around the world have Edge in their GSM networks in various stages of deployment. And commercial services are being offered by 160 operators in over 90 countries (see Reference 1 on page 58). Low-cost infrastructure upgrades and the availability of Edge features in the majority of mid- to high-end handsets provide a compelling price-performance argument for Edge deployment.

Key innovations that Edge has brought to the radio link include the introduction of higher-level modulation (8-PSK); multiple coding-modulation schemes MCS 1-9, which allow systems to adjust to operating conditions; and incremental redundancy, which provides link gains by combining different transmitted data. As a result, Edge offers increased data rates with a theoretical peak rate of 473.6 kbits per second and is effective in expanding data capacity with an average gain of over 3 times compared with GPRS (see Reference 1). Equally, if not more important, is the fact that Edge goes beyond improvements in radio performance. It supports the same quality-of-service architecture that is used by the Universal Mobile Telecommunications System (UMTS), which allows the evolution of services offered through future Third Generation Partnership Project (3GPP) releases.

While Edge provides significant improvements over existing GSM/GPRS networks, it can also coexist with other ra- dio-access technologies such as UMTS and 3GPP LTE. The next important factor contributing to Edge success is the accelerated introduction of wireless data services. Higher data rates offered by Edge coincided with user demand for wireless e-mail and content downloads such as music and video, extending to Internet Protocol Multimedia Subsystem and enterprise applications.

The Edge story is extending into the future, however. Standards bodies are already working on further enhancements that will increase Edge capabilities through the Geran (GSM/Edge Radio Access Network) initiative. Release 7 will introduce new features targeting an increase in peak data rates, spectral efficiency and capacity, reduction of latency, and simultaneous data and voice transmission. A number of new technologies have been considered, including higher-level modulation schemes like 16-QAM, reception via two radio channels and downlink diversity reception.

Evolved Edge emerges as an attractive technology for operators, as it offers efficiency comparable to 3G systems while relying on existing spectrum licenses.

Edge technology presents numerous challenges to wireless handset platforms. On one side, the complexity of new standards points to ever-increasing computational and memory requirements. On the other, commercial pressures demand that higher capability be offered at competitive power consumption levels and cost with respect to mature technologies such as GSM/GPRS. The answer to these challenges, which are far from unique, relies on the availability of fundamental technologies and the creativity of design teams. The optimization of the handset platform takes these factors into consideration:

• An advanced semiconductor manufacturing process,

• Multicore digital baseband architectures,

• Increased levels of integration ranging from core/memory to mixed-signal and RF on a single chip,

• Multichip packaging technologies,

• Software optimization techniques for reducing power consumption and

• A software-reconfigurable platform for flexible handset design.

Multicore architectures for wireless handsets have matured since the late 1990s, moving from voice-centric GSM to data-oriented GPRS platforms. To meet the increasing algorithmic complexity of the Edge data receiver design, however, chip set solutions have to introduce new capabilities for the computational signal-processing part.

Edge has introduced the clear dividing line for "sub-100 MHz" processors, which were not able to fulfill the requirements of the physical layer and required specialized hardware accelerators or a coprocessor.

Unlike GPRS, the complexity of Edge is that there is no de facto data receiver solution--algorithms that are adopted in chip set realizations range from the family of suboptimal trellis-search-based solutions (one can recall many acronyms such as RSSE, DDFSE, etc.) to more computationally demanding techniques. Depending on hardware-software partitioning, some or all computations can be offloaded to a hardware block--the trade-off typically goes toward preserving key parameter computations and prefiltering in software. In addition, critical performance figures of the data receiver are a function of synchronization capability as well as filtering techniques for interference suppression that are coupled with receiver design.

With the DSP processors intended for use in cellular baseband processors reaching speeds well above 200 MHz, the Edge data receiver can be implemented completely in software, with flexibility to adapt to changes in the standard, as well as changes required to implement and adjust for extensive field test cases and operator approvals (see Reference 2). This can be achieved by a low-power core design, which allows the overall power profile to be in line with or better than GPRS digital baseband chip sets while maintaining a high degree of flexibility.

A software Edge implementation has the advantage of a handset hardware design that remains stable while performance is optimized in the type of approval and interoperability test process that is used on multiple vendors' infrastructure. The cost-effective and power-efficient solution meets the stringent wireless handset requirements. In addition, a software-based, future-proof platform--something that's increasingly important with standard evolution--allows for the addition of performance-enhancing techniques ranging from proprietary algorithms for enhancing Edge to in- corporating such other advanced processing techniques as single-antenna interference cancellation or elements of Evolved Edge.

Looking further into the functions of the handset, there is an increased requirement for high-level applications, ranging from audio to video and imaging-based services. In this domain, the capability is based on the resources of the MCU core: Typically, solutions based on the ARM7 core present a highly optimized communications platform suitable for data cards and entry-level handsets, but require significant hardware support for multimedia functions or dedicated DSP core or hardware acceleration for audio/video.

The next level of multimedia capability is introduced with the combination of the more powerful ARM9 core and an enhanced DSP core.

For example, as depicted in Figure 1, a typical multicore solution has a DSP sub-system, which consists of the DSP core, L1 code and data memories (configurable as cache or SRAM), unified L2 memory and a set of DSP peripherals. The DSP subsystem handles the channel equalization, data receiver and voice coding/decoding functions. If there are "spare" Mips available after those tasks, the DSP subsystem can also perform some multimedia functions, usually at lower power than the MCU, due to the better instruction set available for such operations. The MCU subsystem consists of the microcontroller core and cache. The ARM cores available from Advanced RISC Machines are almost universally used for the MCU block. Multimedia interfaces for display and image-capture devices can be supported by the dedicated bus subsystem. It may include a Parallel Peripheral Interface controller, supporting a multibit camera sensor or video input interface as well as a dedicated external bus interface for parallel liquid-crystal displays, which eliminates noise and loading on the main external memories interface. The data-movement needs of the multimedia interface devices could be supported by means of the multichannel direct memory access controller, which supports necessary video formats.

A typical partitioning of an Edge handset is depicted in Figure 2. The elements of the chip set include the RF transmit/receive portion, analog baseband with mixed-signal and power management blocks, and digital baseband. A complete phone design includes the chip set, memory modules, applications modules (camera, display, etc.) and peripherals such as Bluetooth or a secure digital or MultiMediaCard.

The depicted solution is based on a proven and stable platform approach with partitioning that is cost-effective in today's process and package technology. At the same time, this platform is extremely competitive in terms of power, both active and standby, which has been achieved by integration of power management as well as architectural choices on digital baseband.

This architectural approach allows for seamless migration to an increased level of integration, using either a system-in-package or system-on-chip, that will inevitably come with the move to the next semiconductor process node (i.e., 65 nanometers).

In addition to providing the Edge solution, the platform's scalability makes it possible to extend the architecture to multimode operation. For example, the proposed platform can be easily extended to the TD-SCDMA standard with all the attributes of an advanced digital baseband platform, including the capability to handle new features and absorb a number of hardware features in an efficient way. It can also achieve the necessary speed for a broad class of use requirements; provide scalable power consumption; efficiently handle control code; and support the optimizing the compiler for production code quality. In addition, it directly benefits from the years of investment in performance/cost improvement and power reduction in GSM, GPRS and Edge. This includes the use of dynamic voltage scaling to match power consumption to processor performance, and the use of such advanced RF and mixed-signal techniques as direct conversion receivers and sigma-delta data converters.

The TD-SCDMA solution using the Edge solution on the described platform has been a performance leader in field trials conducted in recent years. During this time, the platform's scalability has proven to be a real asset for those handset developers that would like to position their companies for the future. n

References

1. "Mobile Broadband: Edge, HSPA & LTE," 3G Americas white paper, September 2006.

2. "DSP for Handsets: The Blackfin processor" in Software defined radio--baseband technology for 3G handsets and basestations, Tuttlebee, Walter (ed.), ISBN 0-470-86770-1, John Wiley & Sons.

Zoran Zvonar is the manager of the Systems Engineering Group at Analog Devices Inc., focusing on the design of algorithms and architectures for wireless communications. He is a co-editor of the Radio Communications Series in the IEEE Communications Magazine and Software Radio Technologies: Selected Readings, IEEE Press/John Wiley.

 See related image