EDN Access--11.21.96 PCS: not just another cell phon
|Design FeaturesNovember 21, 1996|
PCS: not just another cell phone
Stephen Kempainen, Technical Editor
Thanks to the availability of low-cost digital chip sets, PCS is now becoming affordable and finding use in fax, e-mail, and palmtop computer functions. Capitalizing on that affordability, PCS providers and IC makers are hoping to lure consumers with imaginative and user-friendly features: large, easy-to-read displays; ergonomic touchscreens; and speech recognition.
Low-cost, high-functionality digital chip sets are becoming available for wireless base-station and terminal, or handset, applications, making personal communications service (PCS) affordable enough for mass-market deployment. As new capabilities emerge, PCS may even begin to rival wireline telephones as the predominant way to communicate. It may also find use as a way to facilitate e-mail and fax.
As evidence of the rising popularity of PCS, recent public auctions provide the spectrum for as many as six PCS and two Advanced Mobile Phone Service (AMPS) cellular-service providers in each area to compete for delivering the services that PCS customers want. Research-company Dataquest (San Jose, CA) predicts that, in one type of PCS, Global System for Mobile communications (GSM), mobile handset production will exceed 60 million by 1999.
In addition, three distinct and capable technologies—code division multiple access (CDMA), time-division multiple access (TDMA), and PCS1900, a GSM derivative—are striving to become the leading method for connecting handsets to base stations. All three technologies enable mass-market deployment because they increase the capacity per spectrum allocation for PCS (see box, "CDMA, PCS1900, and TDMA vie for PCS"). CDMA offers capacity and voice quality, and PCS1900 gains much from the maturity of the GSM technology and market in Europe and Asia. Meanwhile, TDMA is losing popularity because of perceived capacity and quality constraints. The important factors in this contest are not which acronym consumers like better but which technology offers them higher voice quality and reliability.
As PCS popularity and affordability grow, OEMs of handsets are vying to differentiate their products using not only color, size, and weight features, but also lower prices, lower power consumption, and improved user interfaces. For example, simply adding a dial tone to a mobile terminal can make it more user-friendly, and the ability to receive and respond to short messages adds value but requires improving the display and entry methods of the terminals. Large and easy-to-read displays are musts. Also, although no one wants a full keyboard on a PCS terminal, touchscreens have thus far proved cumbersome. However, easy-to-use touchscreen displays and speech recognition require the designers of ICs on which these systems are based to efficiently partition complex algorithms between hardware and software. Talk and standby times also depend on improved IC and algorithm efficiency.
Well aware of the requirements of PCS and the potential market it heralds, manufacturers of digital baseband ICs, such as Analog Devices, Lucent, and Texas Instruments, have begun to add features to their chip sets (Table 1). There are constraints, however. For example, chips for GSM terminals and base stations must get Full Type Approval (FTA) from the European Telecommunications Standards Institute (ETSI). Once these chips comply with the ETSI standard, it frees chip and handset designers to concentrate on the differentiating features that give products advantages in the market.For example, Analog Devices supplies chip sets for GSM and the PCS1900 standards. With software from The Technology Partnerships to complete the FTA for GSM Phase 2, the company's AD20msp410 chip set requires a radio subsystem, basic memory, a keyboard, and a display for a complete mobile station (Figure 1). The chip set comprises three CMOS components—an algorithm signal processor (ASP), a physical-layer processor (PLP), and a baseband converter (BBC) mixed-signal device—that perform the baseband signal-processing functions. The ROM-coded ADSP2178 ASP, a specialized DSP for GSM, contains all the on-chip memory to run the GSM algorithms. It handles channel equalization and full-rate speech transcoding. The ADPLP01 PLP performs all GSM layer 1 baseband functions. An embedded 16-bit Hitachi H8/300H microcontroller comes with the HIOS real-time operating system and a set of software-development tools. The layer 1 software comes with the chip set. The software for the GSM protocol stack, application layer, and terminal adapter—the differentiating factors—is available separately.
IC makers are also exploring other avenues to improve their offerings. One method of improvement is to increase their chips' power efficiency. The companies use the term "milliwatts per millions of instructions per second" (mW/MIPS) to measure efficiency. By improving algorithms that perform standard functions through an iterative process with wireless-system designers, chip designers can take the spec a step further to measuring milliwatts per function. Designers can optimize common functions, such as Viterbi decoding, integrated paging, and guidance functions in PCS equipment, for both power and memory efficiency to improve digital baseband-processing chips. In addition, although digital communication protocols include basic security, designers can further differentiate their products by innovative use of security features.
Power consumption is of primary importance in PCS terminals. Mobile communication requires a set of flexible and tiered power-down modes. Most of Lucent Technologies' offerings include a mode such as sleep with slow internal clock, which increases standby time by powering only circuitry to accept wake-up calls from a base station. Besides lowering power consumption, Lucent continues to extend the Sceptre chip set for GSM and PCS1900. The set comprises three chips, including the DSP and GSM software, and has received FTA. Lucent is now offering samples of a specialized µC for its Sceptre family to complete the chip set. The communications protocol processor-cellular (CPP-C) supports higher PCS1900 protocols and the user interface and includes software-development tools.
Lucent offers several specialized DSPs based on the DSP1600 core. You can customize these DSPs for a technology by adding hardware accelerators, ROM, RAM, I/O, and peripheral features, depending on the application. For example, the DSP1618x24, which targets voice-plus-data-service GSM terminals, features an error-correction coprocessor (ECCP) for efficient Viterbi decoding, 24k words of ROM to hold extra software for data services, an external memory sequencer, and an 8-bit host interface for flexible µC adaptations.
Another low-power specialized DSP from Lucent implements the half-rate and enhanced full-rate voice coding for GSM and PCS1900. The DSP1628 adds maximum-likelihood sequencing estimation and convolutional decoding instructions to the ECCP and has 48k words of on-chip ROM and 7k words of on-chip RAM. The DSP1628 performs a 19-nsec, 52-MIPS cycle at 2.7V and uses only 0.55 mW/MIPS in operating mode.
To cover all of the PCS technologies, Lucent's DSP1627 supports the CDMA and TDMA standards. Lucent provides this programmable DSP so that designers can implement their own software on a high-volume production chip. The DSP1627 achieves 50 MIPS when operating at 2.7V and 70 MIPS at 5V and comes with 32k or 36k words of ROM and 6k words of on-chip RAM.
To facilitate mass-market deployment of base-station infrastructure equipment, Lucent's DSP1620 targets GSM, TDMA, and CDMA wireless applications. It operates at 120 MIPS from a 3V power supply. The DSP1620 incorporates 32k316 bytes of dual-port SRAM, which is useful in infrastructure equipment because designers can easily refine and upgrade the algorithms. Also, having this much processing power on one chip allows base-station designers to reduce the number of DSPs in the equipment.
Because the wireless market is evolving and standards makers frequently modify the protocols, some manufacturers are offering hardware and software modular blocks. They theorize that one flexible hardware platform can efficiently support all PCS. After many years of collaboration, communications-system and DSP designers have realized that some functions are common to many wireless protocols and that they can implement those functions in stand-alone modules. This work has also led to designing DSPs optimized for PCS equipment. The specialized communications DSPs, such as the Lucent DSP16xx and the TI C54x, use multiprocessing hardware-accelerator architectures, onboard RAM/ROM, and many millions of instructions per second to process the complex protocols that constitute PCS.Texas Instruments now calls its digital baseband offerings "hardware platforms" instead of chip sets. The company bases the hardware platform on an "ASIC backplane," which TI formulated by integrating the DSP C54x and the ARM 7 µP from Advanced RISC Machines (Los Gatos, CA) (Figure 2). With this base, you customize the ASIC backplane by adding all or some of the following modules: wireless-algorithm hardware accelerators; RAM, ROM, and flash; on-chip peripherals; ASIC gates; and analog modules. TI's hardware and software modules are available as a standard protocol matures. The TDMA modules, which support the EIA/TIA Interim Standard (IS)-54 standard, and GSM modules are stable and available. The company is developing the PCS1900 modules, and the CDMA modules will be available next year.
How, you may wonder, can you efficiently test an ASIC backplane integrating an ARM, a DSP, and everything but the kitchen sink? The answer comes from TI itself, which simplifies the chore by integrating the DSP, µC, and ASIC tools to provide the same development interface for all units. This approach provides an efficient testing framework for both the silicon and the system.
The workhorse of the ASIC backplane is the DSP C54x. TI tailored this family of DSPs for wireless-system terminals and infrastructure equipment. The C54x performs at 66 MIPS now, and TI expects that number to reach 100 MIPS next year. This processing power is enough to perform enhanced full-rate coding for PCS1900 or perform the processing-intensive infrastructure functions. Three power-down modes and single-cycle, parallel instructions decrease power dissipation.
Terminal evolves to one ASIC
The traditional terminal architecture is evolving as fast as the ICs that comprise it. The baseband portion of the terminal used to comprise three devices: the DSP, the µP, and the ASIC. The programmable DSP handled speech coding, modulation, security and error control, detection, equalization, and some RF-interface functions. The µP handled the user interface, protocol stacks, and system software, and the ASIC performed high-throughput functions, such as filtering, synchronization, and channel coding. It was also the catchall for assorted glue logic.
Now, however, for size, weight, reliability, and power saving, more manufacturers are throwing the entire baseband function into one ASIC. For example, Qualcomm offers a mobile-station modem (MSM-2), which integrates the CDMA processor and 80C186EC µP and includes software-controlled power management and support for AMPS. It performs both 8- and 13-kbps, "pure-voice" encoding, which Qualcomm developed, and is also available in a stand-alone voice encoder, the variable-rate Q4413 vocoder/echo canceller.
Qualcomm offers the MSM-2 and a baseband-analog processor (BBA-2) only to the company's CDMA licensees. These two chips provide the core functionality for CDMA terminals that support EIA/TIA IS-95. Separate royalty-bearing license agreements for the terminal, the infrastructure hardware, and the software to complete the CDMA equipment allow you to buy chips from Qualcomm or its licensed chip vendors. The license also provides some documentation on implementing CDMA as the IS-95 standard specifies.
Another company that now offers chip sets for GSM terminals, VLSI is working with OEMs on designs for PCS1900. These new products will be available by next year. The company's GSM chips use the VP22003 kernel processor, which integrates an ARM processor, an adaptive Viterbi equalizer, a channel coder, and peripheral and I/O sets. The device contains battery management, charging control, and power management, which disables inactive portions of the device. All development tools are available for the GSM kernel and the ARM processor. VLSI is also a licensee of Qualcomm's CDMA technology and is working on products with other licensees.
With the demanding growth about to happen in PCS worldwide, more companies will focus on ease of use and voice quality. The players will have to constantly improve and develop to stay in the race. The pressure is on the IC vendors to supply winning designs.
|Table 1—Representative PCS digital baseband chips|
|DSP, ARM7, 16-kbyte ROM, |
12-kbyte RAM on chip;layers 1 and 2 software available
|Analog Devices||AD20msp410 |
|PCS1900||Three||3||Now||FTA hardware and software, embedded 16-bit µC, processing layer 1 software provided, layers 2 and 3 available|
|Lucent Technologies||DSP1628x24||PCS1900||One||3||Now||FTA hardware and software, 24k-word ROM, 4k-word RAM, supports data services, layer 1 software|
|DSP1628x48||PCS1900||One||3||Now||48k-word ROM, 7k-word RAM, supports enhanced full- and half-rate vocoding, 52 MIPS while consuming 0.55 mW/MIPSat 2.7V|
|One||32k- or 36k-word ROM, 6k-word RAM, some software available for TDMA, 50 MIPS while consuming2.1 mW/MIPS at 2.7V|
|CPP-C||PCS1900||One||3||Samples now||Communications protocol processor-cellular for higher level protocol and user interface|
|Qualcomm||Q5357 MSM-2||CDMA||One||CDMA |
|CDMA processor, 80C186EC µP, 8- and 13-kbps vocoder|
|Texas Instruments||ASIC |
|One||3||PCS1900 and |
|Integrated DSPC54x, ARM7, and software modules to provide single chip flexible enough tohandle all standards|
|PCS1900||Two||3||Now Viterbi, and channel coder||Kernel includes embedded ARM|
|For free information…|
|When you contact any of the following manufacturers directly, please let them know you read about their products in EDN. Note: All Web sites start with http:// unless otherwise stated.|
|AKM Semiconductor Inc|
San Jose, CA
|Analog Devices Inc|
Scotts Valley, CA
|GSM North America|
Berkeley Heights, NJ
San Diego, CA
San Jose, CA
|CDMA, PCS1900, and TDMA vie for PCS|
Rappaport, Theodore S, Cellular Radio and Personal Communications, Volume 2: Advanced Selected Readings, IEEE Publishing, 1996.
Kakaes, Apostolos K, "GSM and DCS 1900: Evolution to PCS," ICC '96 Tutorial and Workshops, June 1996, Dallas, TX.
|You can reach Technical Editor Stephen Kempainen at (415) 643-1760, fax (415) 643-9513, email@example.com.|
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