EDN Access--11.21.96 PCS: not just another cell phone

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.

Refining silicon

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.

CDMA, PCS1900, and TDMA vie for PCS

Vendors designing for the PCS market have to sort through the alphabet soup of standards that is common in the communications industry. These standards involve two categories: "high tier," which supports macrocells and high-speed mobility, and "low tier," which supports low power and complexity. The two tiers roughly correspond to digital cellular and digital cordless, respectively. Two-way, high-tier standards operating around 1900 MHz include code division multiple access (CDMA), time division multiple access (TDMA), and PCS1900.

CDMA, the common name for the EIA/TIA Interim Standard (IS)-95, has found use in secure military communications applications for decades. Since the early 1990s, Qualcomm, which licenses CDMA technology, has been implementing CDMA for commercial use. To comply with the IS-95 standard, equipment manufacturers must obtain a royalty-bearing license giving them access to patents and documentation from Qualcomm. The IS-95 requires that equipment be dual-mode operation for Advanced Mobile Phone Service (AMPS) and CDMA. The standard stops short of specifying infrastructure interfaces.

CDMA created much excitement by early forecasts of 40-times greater capacity than AMPS on the same bandwidth. Although those claims were exaggerated, recent tests by Bell Atlantic Nynex (Trenton, NJ) show better than six times the capacity increase over AMPS. The trials led Bell Atlantic to expect at least a nine-times greater capacity than AMPS even with the same voice quality as that of a 13-kbps vocoder. CDMA licensees plan to use that capacity to cover the "CDMA footprint"—most of the United States—with PCS by 1998.

Each of CDMA's channels uses one of 4.4 trillion unique codes to transmit, or spread, a signal across the 1.25-MHz available spectrum. The intended receiver "knows" the code and, therefore, can distinguish the signal from the multitude of other signals and noise in the spectrum. A code, whose elements are called "chips," represents each data symbol. One of the challenges for CDMA technology implementation is to synchronize the chip times for a base station. Another challenge is the spreading process, which is more complex for reverse, mobile-to-base, links than for forward, base-to-mobile, links. Reverse links require that a signal transmitted by a low-power terminal be chip-synchronized and interference-tolerant when it arrives at the base station even though the signal travels different propagation channels (multipaths). Both the reverse and forward links also add a pseudorandom sequence to coding in case the terminals in adjacent cells have the same code sequence.

The reverse link also involves a "near-far" problem with transmitting power: A terminal near the base must adjust its transmitting power so that it does not swamp out chips from farther-away terminals. For CDMA to work, all terminals must have the same received power at the base station within a small tolerance. Fading, shadowing, and path loss in signals complicate this problem. The base station sends commands to the terminals for this power control interspersed in the speech frames at 800 bps, making each handset responsible for network integrity by adjusting its transmit power accordingly. Unfortunately, a terminal that incorrectly controls the power can drown correctly transmitting terminals. Also, a terminal that improperly spreads the signal can destroy the air interface for an entire base station.

Handoffs of mobiles moving from one base station to the next pose problems for all wireless protocols. CDMA uses "soft handoffs," in which mobile units maintain links from multiple bases. These soft handoffs help to increase capacity and reduce power consumption. CDMA terminals combine the signals from these stations as they would combine signals associated with multipath components and select the best signal at any time. The connection to a new station occurs before the old one breaks. The technology must revert to a "hard handoff"—a mobile unit that breaks the connection from one cell station and then re-establishes the connection with another—when a change in frequencies occurs as a mobile moves from cell to cell. A hard handoff reduces capacity and uses more power.

Qualcomm initially used 8-kbps coding, which suffered poor voice quality. The company is now using a variable-rate vocoder to improve voice quality and increase capacity. Variable rate adjusts to 13.3, 6.2, 2.7, or 1 kbps every 20 msec. By taking advantage of more bits, silent periods, and voice-energy fluctuations, variable rate delivers high-quality voice without decreasing system capacity.

PCS1900: a viable contender

The developers of the PCS1900 standard based it on Global System for Mobile communications (GSM) technology. GSM began commercial operation in Europe and Asia in 1991. Now, more than 200 service providers in approximately 100 countries offer the technology. In the past few years, operators and equipment manufacturers have been working to adapt GSM to the North American PCS market. The TIA-TR46 group has submitted the J-STD-007 standard to ANSI for approval as the standard for frequencies the FCC allocates at 1900 MHz.

PCS1900 is an "open" standard in more ways than one. It incorporates many aspects of communications engineering for a total system specification that spans the first three ISO model layers. Vendors do not have to license any technology to build compliant parts. "Open" also means that its developers left the standard open for creative implementations and different ways to build systems, wherever such creativity does not hinder interoperation between different vendors' products. Demonstrating this openness is the fact that PCS1900 uses a divide-and-conquer concept for specifying the switching centers, location registers, base-station controllers, base transceiver stations, smart cards, and handsets (Figure A ).

Another demonstration of PCS1900's built-in expandability is its growth since its inception. The original GSM 900 had 124 absolute-RF channel numbers (ARFCNs), each with eight time slots. The next extension added 50 channels. Digital Communications System (DCS) 1800, the European equivalent of PCS1900, now has 374 channels. In addition, half-rate speech coding doubles the time slots to 16 per channel. PCS1900 defines these RF channels to accommodate the peculiarities, such as fading and interference, of mobile channels. The standard identifies the physical channels for each time slot in all ARFCNs. Thus, DCS 1800 has a potential of 2992 physical channels per base transceiver station. Half-rate coding doubles this number of channels without greatly affecting voice quality.

PCS1900 time-division-multiplexes each 200-kHz RF carrier into eight slots. Each terminal or mobile station periodically transmits on the "uplink" in every eighth slot and receives on the "downlink" in a corresponding slot at a different frequency. This intermittent activity of the mobile transceiver allows the mobile to measure the strength of signals from surrounding base stations. The mobile then reports this measurement to the base for calculating when a handoff should occur. All slots can accommodate user data, voice, control, or future services.

PCS1900's data-link layer uses eight categories of logical channels that can serve all functions, such as voice and control, with variable bandwidth. A frame/multiframe/superframe structure derives these channels, making PCS1900 a flexible protocol that can adapt to future needs. Its data-link layer also handles error control by using convolutional codes, CRC, and interleaving.

The PCS1900 voice coder models human speech relatively independently of language. The standard uses a regular-pulse-excitation-with-long-term-prediction (RPE-LTP) coding scheme. RPE-LTP delivers 260-bit blocks every 20 msec for a speech rate of 13 kbps. It divides the 260 bits into three priorities; 50 bits are in the highest priority, meaning that 50 bits have the most error protection and, therefore, are the most important and reliable for decoding. This coding scheme also uses voice detection to eliminate silence from coding. Half-rate coding provides a 6.5-kbps rate but requires more signal processing to get high quality. Full-rate enhanced coding is even more process-intensive but delivers excellent voice quality.

The full-rate, eight-slot/channel structure offers 22.8-kbps coding or 13 kbps with error correction. A half-rate version offers 11.4 kbps but increases the capacity to 16 slots/channel. The half-rate version increases demand on the DSP to maintain the voice quality and double the capacity. The timing base must be accurate because of the crowded spectrum. Each mobile station picks up signals from neighboring base stations and can synchronize to the base station in case a handoff is imminent. The system allows for timing errors, equalization in rapidly changing channels, and malfunction detection.

Like CDMA, PCS1900 also uses soft handoffs. The mobile station continuously scans adjacent channels to analyze RF quality and determine the best RF carrier available at the mobile station's location. When equal RF quality is available from two channels, the base station uses "load leveling," which evenly distributes the mobile stations among RF carriers, cell base stations, or areas.

The PCS1900 network layer defines functions for each portion of the complete system. It creates a messaging structure to handle many types of messages in a well-structured manner among the functional blocks (Figure B ). PCS1900 aims to achieve flexibility for the mobile switching centers to allow them to take advantage of the public switched-telephone-network infrastructure within which the technology must work.

TDMA: probably not the winner

The developers of the TDMA standard for North American PCS specified it in standard IS-136. This standard evolved from IS-54, which introduced digital service in the same frequency band as AMPS. Both IS-54 and -136 require dual-mode operation, in which the terminal operates in the mode available in the area. IS-54 provides three times the capacity of AMPS. IS-136, which has a different slot length from but in most ways resembles GSM, offers six times more capacity than AMPS by using half-rate coders. It also increases the overall signal rate by eliminating the IS-54 reliance on the AMPS control channel. In addition, IS-136 adds PCS capabilities, such as short messaging, broadcasting, low-power sleep-wake-up mode, and hierarchical cell structures.

Which technology will emerge as leader in the PCS race is unclear. The PCS providers have chosen both CDMA and PCS1900 more frequently than they've chosen TDMA. In addition, both PCS1900 and CDMA have capacity and quality advantages over TDMA. However, CDMA has not standardized the infrastructure above the base-station-controller level, but this standardization is now under way. PCS1900, on the other hand, has full network deployment on a few continents. The different speech-coding techniques of CDMA and PCS1900 may cause problems in voice quality: When you make a call from a CDMA terminal to a PCS1900 mobile station, the low-bit-rate speech transcodes to a different encoding format, and the speech quality may deteriorate.

The choice of which technology to use depends on analyzing which customer demographics a technology is serving, the intended applications, and which business model the service provider is pursuing, not on which technology customers perceive as the "best."

Table 1—Representative PCS digital baseband chips
Product	Standard	chips	No. of (V)	Power Availability	Features	Vendor
AKM	AK2388	PCS1900	One	3	Q1 1997	DSP, ARM7, 16-kbyte ROM, 12-kbyte RAM on chip;layers 1 and 2 software available
Analog Devices	AD20msp410 baseband-chip set	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
	DSP1627	TDMA, CDMA	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 licensees only	CDMA processor, 80C186EC µP, 8- and 13-kbps vocoder
Texas Instruments	ASIC backplane	PCS1900, TDMA, CDMA	One	3	PCS1900 and TDMA now	Integrated DSPC54x, ARM7, and software modules to provide single chip flexible enough tohandle all standards
VLSI	VP22020 VP22003	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 (408) 436-8580 www.akm.com	Analog Devices Inc Wilmington, MA (617) 937-1428 www.analog.com	GEC Plessey Scotts Valley, CA (408) 439-6049	GSM North America Alexandria, VA (703) 799-7383
Lucent Technologies Berkeley Heights, NJ (800) 372-2447	Motorola Phoenix, AZ (800) 521-6274 www.mot.com/sps	Philips Semiconductors Sunnyvale, CA (408) 991-2000	Qualcomm San Diego, CA (800) 266-2362
Siemens Cupertino, CA (408) 777-4500 www.sci.siemens.com	Texas Instruments Dallas, TX www.ti.com	VLSI Corp San Jose, CA (408) 922-5250 www.vlsi.com

Design Features

November 21, 1996

PCS: not just another cell phone

Stephen Kempainen, Technical Editor

Refining silicon

Terminal evolves to one ASIC

Table 1—Representative PCS digital baseband chips

For free information…

CDMA, PCS1900, and TDMA vie for PCS

PCS1900: a viable contender

TDMA: probably not the winner

Looking ahead