Shrinking DSPs and embedded RISC chips enable combined DECT and GSM PC Cards
Bob Emmerson and David Greetham
The Digital European Cordless Telecommunications (DECT) standard is a pan-European standard for digital, cordless communications. Unlike the Global System for Mobile Communications (GSM) standard, it covers network-access technology rather than the specification of a complete network. DECT can provide access to virtually any type of network: voice (PBX), data (X.25), wireless (GSM), wired Public Switched Telephone Network (PSTN)/ISDN, or LANs.
As with GSM, DECT solutions were originally developed for voice transmissions: for example, a PBX with a number of cordless extensions. But DECT is starting to take off as a means for accessing data netw
orks from mobile computers. Wireless extensions are being added to Ethernet LANs, and small DECT-based systems for simultaneous voice and data made an appearance at Telecom '95 in Geneva in November.
The anytime/anywhere paradigm of mobile communications says that a mobile computer should give you the same communications channels on the road that are available in the office -- and without hassles. But in the wireless world of today's road warriors, notebooks have to be cabled to cellular telephones before GSM data services can be used. While this isn't a bad solution, it doesn't match the paradigm, particularly for data-based users. The paradigm also presents a problem when moving from a LAN to a WAN, because the mobile user must switch from a DECT device to a GSM PC Card.
Combining DECT and GSM
Most vendors see a combination of DECT and GSM on a single card as the optimal solution, although separate cards will be around for some time to come. Indeed, a dual-mode card with a
DECT air interface and a GSM fax modem would give notebook users both LAN and WAN access to corporate resources. A DECT- and GSM-enabled phone or notebook PC would be able to seamlessly switch from the home or office to rural environments, and it would make an ideal anywhere/anytime device as long as one number could be shared between different networks (e.g., PSTN, GSM, and DECT).
The key to this solution is developing communications chips that are smaller, less expensive, and, most important, consume less energy -- battery lifetime is a key issue. A DECT/GSM marriage requires a powerful combination of a digital signal processor (DSP) and a RISC processor to detect and process a multitude of different signals.This technology will first come to phones. Olivetti, however, introduced in mid-1995 the Net3, a DECT-based PC Card with a small stub antenna for use with wireless LANs (
see the photo
), and the company plans to introduce a combination DECT/GSM PC Card by next year.
DEC
T is currently being used in three main applications areas: business communications, including the provision of cordless PBXes and LANs and add-ons to regular wired PBXes; systems for small business and home use, where one or more handsets or terminals use a common base station; and public-access telepoint-type systems, where DECT is taking over from CT2 (for mobile-originated calls only). DECT can be used as a front end to GSM networks for high-density, metropolitan microcellular infrastructures, and it is also a suitable technology in developing countries for wireless local-loop applications.
DECT technology is also scalable. It has been assigned 20 MHz of spectrum in the 1.8-GHz band; this is divided into 10 carriers with a transfer rate of 32 Kbps. Time division multiple access (TDMA) technology is used to make a further time-domain division into 24 time slots, which provide 12 duplex (i.e., two-way) channels. A 32-Kbps transmission channel is therefore formed by the combination of a time slot and a
carrier frequency, and these 12 channels can be aggregated on the fly. Therefore, it's possible to establish high transmission rates up to 384 Kbps, which is the H.320 standard for desktop duplex video.
A look even further ahead reveals high-performance LAN, or HIPERLAN, another cordless technology that could provide 20 Mbps and thereby enable mobile access to multimedia services. Should this technology come to the office environment, the LAN/WAN mobility paradigm presents another problem: Second-generation systems carrying voice and data, such as GSM, will get faster and use packet switching, but they won't have the capacity needed for mobile multimedia.
For this, an ATM-based broadband ISDN solution is needed. This development is clearly way off; however, by the end of the decade, there will be more than 50 million powerful, portable devices, and multimedia communications will be the norm.
The North American equivalent of DECT is known as
spread-spectrum
. Wireless LAN solutions u
sing this technology are employed on both sides of the Atlantic, although the technology isn't used for voice in Europe. In large-size companies that have wired Ethernet or Token Ring LANs, wireless systems are used for flexible workgroup applications. DECT and spread-spectrum are the number-one candidates for this portion of the market, but DECT's many unique features make it better-suited for small, integrated voice/data solutions.
A typical GSM handset today comprises six chips: three RF ICs and three baseband processing ICs. The equivalent for DECT is similar: Three devices are used to amplify, modulate, and demodulate the RF signal, and the other three ICs perform baseband functions. According to Oliver Gunasekara of Advanced RISC Machines, by 1997 the semiconductor industry will be shipping a three-chip solution, which will be reduced to just two chips by the end of the decade -- one for RF and the other for baseband.
Smaller Die Sizes
Powerful 32-bit RISC processors c
an share their silicon substrate with DSPs and other circuitry. The ARM7 chip, from the U.K. semiconductor firm Advanced RISC Machines and one of the leading RISC architectures for deeply embedded applications, has a die size of just 3.88 square millimeters (compared to 169 mm(2) for a 486SL). The chip's performance/power consumption, a key parameter for battery life, is a staggering 580 MIPS per watt (versus 32 MPW for the 486SL).
In 1993, Siemens introduced Gold, the first chip set to gain full GSM approval. Today, Siemens's new Goldplus GSM circuitry is implemented in 3-V technology for low power consumption, and it already has data-service capability. The 16-bit microcontroller has an address space of 2 MB and a system-interface block that comprises a series of GSM-specific interfaces and control functions.
The chip set's signal-processing circuitry contains two 16-bit DSP kernels for implementing the speech codec, channel codec, and other complex tasks -- all as DSP firmware. This means that
the same baseband chip can be used for 900-MHz GSM as well as for the 1800-MHz Digital Cellular System (DCS) standard, although the RF portion clearly must be different. DCS, which is derived from GSM, is basically designed for urban applications and employs a smaller cell size, offering higher densities of subscribers.
Philips Semiconductors introduced the first dedicated chip sets for DECT in 1992; the company now has an elegant two-chip solution. Its ABC chip (see the figure
"Philips's Two-Chip DECT Solution"
) performs all the baseband functions. The
A
in the chip's name indicates that it contains circuits for the adaptive differential pulse-code modulation (ADPCM) codec, the
B
refers to burst-mode control, and the
C
indicates that the microcontroller functions are fully integrated. The ABC chip uses Philips's own DSP core together with an 8051 microcontroller running at 14 MHz.
Different Frequencies
The ability to handle the
widely different frequencies that exist today is the most significant technical problem presented by a dual-standard solution. It implies the design of a sensitive, tunable front end that avoids interference from the adjacent digital circuitry.
However, according to industry experts, there is no technical reason why there shouldn't be two RF front ends to the same baseband IC. Thus, a dual-standard device could be made using just three chips. Although DECT and GSM have a number of different technical requirements, such as channel spacing, modulation, transmit-power range and control, speech coding and
bit rates
, channel rate and coding, and frame duration, embedded RISC and DSP systems are capable of handling them all.
Scientific Generics, a technical consultancy based in Cambridge, U.K., is currently engaged in the development of an advanced dual-mode GSM/DECT RF module that's based on today's technology and uses state-of-the-art chip sets. Behrooz Rashidzadeh, mobile commun
ications manager with the company, sees a clear trend toward further integration and anticipates that data-products manufacturers will concentrate on the development of dual-mode GSM/DECT data modems.
First-generation DSPs were used for the detection of a single-baseband signal, and programs were stored in a PROM, but static RAM (SRAM) memory is now used. This lets vendors continue to use the same basic chip set and merely change the instructions.
Third-generation DSPs, in combination with RISC processors, are able to detect and process different incoming signals. Since GSM is the most likely signal to be encountered, the DSP core is set to run this algorithm, but the microcontroller can periodically instruct the DSP to run the code for simultaneously detecting DCS 1800 or DECT. If either of these inputs is found, the processor changes over to manage this new signal (see the figure
"A Third-Generation DSP/RISC Processor Combination"
).
A RISC processor's strength is its a
bility to process a relatively limited number of instructions extremely quickly in order to carry out a complex task. An ultrasmall, low-power RISC processor, such as the ARM7, is therefore ideal for processing the instructions needed to turn a raw stream of digital data into speech or a fax.
A future processor, the ARM7T, will offer the same basic capabilities but will come with an additional instruction set to allow for 30 percent more program code in the same-size memory, This, in turn, can be used to implement dual-standard operation and provide a more advanced man/machine interface.
For its Spider communications microcontroller, GEC Plessy incorporated the 32-bit RISC engine of the ARM7 and added a serial I/O, power-control circuitry, a PC Card slave interface with 16-bit data, and a programmable memory interface. Further on-chip features include DMA and interrupt-control circuitry to provide a highly integrated communications controller.
GEC Plessy's Mantis, which is an enhanced versio
n of the Spider, has 16-bit-wide parallel I/O, a second universal asynchronous receiver/transmitter (UART), and a 32-bit-wide external data bus. Note that both microcontrollers use embedded cell technology, which means that all this functionality is contained in separate areas of silicon, but they are all on the same substrate and are connected by a common bus architecture.
A Range of Products
These developments are significant because they will enable PC Card vendors to create a range of products based on the same architectural platform. Moreover, the semiconductor industry is working its way toward an open architecture that will enable the fabrication of different vendors' circuitries on the same substrate. Therefore, in the future, it will be possible to put the circuitry needed for GSM/DECT/DCS alongside that of, say, the Spider. Today, the Spider, a 144-pin IC, connects to the six ICs needed for GSM on the PC Card's printed circuit board.
On display at Telecom '95 were
prototypes that enabled the following scenario: An ISDN line comes into a small office. This line connects to a fully featured DECT base station that enables up to six simultaneous wireless calls -- either voice or data. Pocket-size phones are used for voice, and PC Cards are used for data. Calls can be internal or external; external calls can be switched (via a small PBX or key system) or put on hold, and additional parties can be brought into a conversation. The ISDN line offers PC faxing and other data communications at the same time as voice communications. This scenario comprises a de facto combined voice-switch and wireless-LAN solution that's being regarded as a communications "killer app."
Two-chip DECT solutions are state of the art. The big breakthrough on the ISDN side of this development comes from VLSI Technology (Munich, Germany) and Hagenuk (Kiel, Germany), who have jointly developed the first single-chip solution for Euro-ISDN terminal equipment. Their new VLSI ISDN Processor (VIP) offer
s a programmable engine for ISDN communications, and it's being used as the ISDN interface and control component in Deutsche Telekom's new range of Europa phones.
The heart of this new chip is the ARM7 processor. The platform will be able to accommodate future ISDN services while maintaining the existing hardware configuration. The VIP also benefits from other on-chip features, such as a pulse code modulation (PCM) codec, a UART, a PCM/DSP interface, a keyboard scanner, a D-channel data-link controller, and a programmable memory interface.
VLSI has also introduced a complete software/hardware development system that runs on Unix workstations as well as on PCs. This will assist developers in porting their existing software and in developing new features for the VIP.
Although the chances for realization of the mobile communications paradigm are good, it still has a number of hurdles to overcome. According to Hakan Mitts of the Technical Research Centre of Finland (VTT), it will be hampered by
the fact that public networks and communications services are mostly based on circuit-switched technology, while private LAN networks use packet transmission.
Consequently, the integration of the two environments will be difficult. The technology that can change this situation is asynchronous transfer mode (ATM), the networking technology that will be used in public and private networks alike.
"Another key issue is the freeing up of precious radio resources as a result of deregulation and technology advances," adds Mitts. So far, radio-spectrum has been largely owned or controlled by public operators, but today more of the spectrum is being opened up for competition and private use. This means that radio-spectrum could be available for sharing among private and public environments. As a result, in the near future there will be less need to use different radio frequencies for private and public communications, and therefore less need for dual-mode equipment in the new frequency bands.
More Competition After Deregulation
Europe will become a more competitive telecommunications marketplace after deregulation occurs in 1998. At that time, multimedia notebooks will start to become commonplace business tools, and they will employ multistandard wireless communications enabling data communications among office, public, and private environments. They will also be able to handle voice.
Today, notebooks such as IBM's ThinkPad employ an advanced system-on-a-chip DSP (the MDSP2780), which has telephony interfaces that let PCs replicate and improve the functionality of phones. What will hinder the realization of the paradigm is the fact that a mobile PC becomes a de facto phone that needs telecommunications approval -- and that will remain a hassle for vendors.
WHERE TO FIND
Advanced RISC Machines
Cambridge, U.K.
Phone: +44 1223 400449
Fax: +44 1223 400410
GEC Plessy
Swindon, U.K.
Phone: +44 1793 518255
Fax: +44 1793 518198
Hagenuk
Kiel, Germany
Phone: +49 431 8818373
Fax: +49 431 8818374
Philips Semiconductors
Zurich, Switzerland
Phone: +41 1 4651389
Fax: +41 1 4621006
Siemens
Nuremberg, Germany
Fax: +49 911 9873321
VLSI Technology
Munich, Germany
Phone: +49 89 67206364
Fax: +49 89 67206101
illustration_link (9 Kbytes)
Wireless data is bursty, but the carriers can increasingly support high bit rates. Voice and data are continuous, but compressed voice
and video includes bursty data with a predictable pattern. This allows the data to be compressed in a variety of ways (e.g., empty slots can be filled with store-and-forward mail).
illustration_link (24 Kbytes)
Forming the core of Philips's two-chip DECT solution are the ABC chip that handles ADPCM codec, burst-mode control, and microcontroller functions. The system converts the 1.9-GHz antenna-input signal into two IF signals centered around zero (known as "zero IF chip"). This signal goes to the IF baseband receiver, which converts the analog signal generated by the front-end receiver into the data stream requir
ed by the burst-mode controller.
illustration_link (15 Kbytes)
An amplified RF signal is mixed down to an IF signal using a local oscillator (depending on carrier frequency) and then band-pass-filtered and undersampled. The sample values are processed within the DSP to produce partial results. The RISC processor, tightly coupled to the DSP by a high-speed bus, integrates these results to recover the digital data stream.
photo_l
ink (14 Kbytes)
Olivetti has developed a DECT-based PC Card with a small-stub antenna for use with wireless LANs. The Net3 clients use a PC Card that emulates an Ethernet or Token Ring card and detachable aerial that can be positioned apart from the notebook.
Bob Emmerson is a telecommunications journalist based in Eindhoven, the Netherlands. David Greetham is a consultant with Greetham Associates (Turnhout, Belgium). You can reach them on the Internet or BIX at
editors@bix.com
.
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