September 1, 1997

DECT ICs' bandwidth defeats noise, extends cordless-phone capabilities

DAVID MARSH, CONTRIBUTING TECHNICAL EDITOR

If mention of cordless telephones brings to mind clumsy, low-quality devices that fade out a couple of rooms away from their terminals, prepare for a design revolution. DECT technology banishes poor cordless performance, promising a raft of new applications and related telecomm services.

DECT's architecture provides the bandwidth that high-quality speech, data, and multimedia transmissions demand. DECT (digital enhanced cordless telecommunication) is the first cordless standard designed by the European Telecommunications Standards Institute (ETSI). Top on the list of ETSI's requirements for DECT were advanced but practical engineering, as well as application flexibility. Fulfilling these requirements, DECT is exceptionally flexible, suiting domestic cordless telephones, office PABXs, private wireless LANs, ISDN links, city and suburban mobile phones, and, in theory, small regional networks.

But DECT's high performance comes at a price. Terminals for DECT are complex devices, combining 1.9-GHz RF technology with enough DSP to power a respectable PC. First-generation DECT handsets comprise more than 300 components, but increasing integration slashes today's total component count to approximately 100. Because users are more interested in functionality than technicalities, basestation and handset costs must compare reasonably with wired alternatives, integrate seamlessly with the wired infrastructure, be compact and lightweight, and use battery power efficiently.

DECT's flexibility means that applications are diverging (see box, "Looking ahead," pg 138). For the next two to three years, the DECT industry expects that most growth will occur in domestic applications--for example, augmenting wired phones or replacing CT0 or other unlicenced standard cordless phones. Even if cordless data applications take off faster than the industry expects, hardware designs will remain much the same because DECT transparently carries speech and data--or both. DECT ICs and chip sets can dramatically reduce your development effort for implementing DECT voice/data handsets and base-stations.

Because DECT technology is complex and still relatively new, the silicon market remains wide open. To build a DECT handset or basestation, you need a baseband processor, IF/RF stages, an RF power amplifier, transmit/receive switches, and assorted filters and peripherals. Many manufacturers produce most or all of the major building blocks you need to implement a DECT handset or basestation design (Table 1). Although manufacturers such as Motorola make RF/IF parts that can be useful for DECT applications, established DECT silicon vendors include National Semiconductor, which now owns SiTel, Philips, and Siemens. Regardless of the silicon you choose, you still have to consider IF/RF filter requirements, external VCOs or VCO-support circuitry, and discrete component stages for level translation, power gain, and signal switching.

The minimum job that DECT IC chip sets fulfil is to transform speech/data between baseband and RF, interfacing with DECT's dynamic time- and frequency-sharing scheme. The DECT radio spectrum occupies 20 MHz from 1.88 GHz to 1.90 GHz, which divides into 10 equally spaced carriers with centre frequencies 1.728 MHz apart. For spectral efficiency, DECT uses Gaussian-filtered frequency-shift- keying (GFSK) modulation with a 0.5- bit time product. Each carrier has a total bit rate of 1152 kbps, and each carrier subdivides into 24 time slots using TDMA (time-division-multiple-access) multiplexing.

Speech transmissions employ a 32-kbps digital format using an ADPCM (adaptive-differential-pulse-code-modulation) technique that derives from the 64-kbps PCM format of the International Telecommunications Union. For normal speech telephony, each 10-msec DECT frame comprises 12 transmit and 12 receive frames to provide full-duplex communications (Figure 1).

Controller shapes design

The baseband controller determines the capabilities your design can support and, as such, has the most influence in your overall design. Baseband controllers suit differing mixes of lines or peripherals and furnish varying- capability DECT burst-mode control logic. The baseband controller provides a full-duplex interface between speech or data and the transceiver's IF/RF stages; integrates the codecs and the DECT burst-mode control logic; and can include peripherals, interfaces, and a mC. The DECT burst-mode control logic frames data for transmission, demultiplexes received data, and performs data encryption to secure transmissions from eavesdropping.

Note that data encryption is similar to GSM (Global System for Mobile communications), using a DAM (DECT-authentication-module) smart card. Because DECT's transmission- frame design imposes an intrinsic 10-msec transmit/receive delay, the baseband processor may integrate a dedicated echo-cancellation stage or perform this function with a DSP processor. VLSI's highly integrated baseband controller works well in a range of DECT applications--from handsets to repeaters (Figure 2). Note that early domestic baseband controllers support only three or six simultaneous conversations.

Apart from raw functional partitioning, look for baseband processor capabilities, such as programmable DECT time-slot allocation (for asymmetric data transmission), PCM interfaces (for peripherals such as subscriber-line ICs), and an IOM-2 interface. Siemens' 2-Mbps IOM-2 (ISDN-oriented modular extended) interface-bus standard connects telecommunication-network interface chips, such as transcoders, DSPs, and line interfaces, and is well- supported by vendors such as Philips and VLSI.

Anticipate that your design will need shielding and decoupling to prevent the output power stages from modulating sensitive VCOs and local power-supply regulation to minimise VCO phase noise. Most DECT ICs have to run two or three prima ry cells, so look for complementary power-management and voltage-regulation functions. Well-integrated power-management schemes selectively turn off all nonessential circuits between calls, cooperatively activating circuit blocks on demand.

After fitting your outline design spec, select ICs by how tightly coupled you want your system architecture to be. Chip sets such as Siemens' fourth-generation DECT ICs, are meant to run with baseband controllers that use a common RF interface. Handsets typically need only DECT's speech transmission and reception facilities and benefit from the highest degree of IC integration.

Designed for DECT handsets, Siemens' PMB4720 IC provides audio A/D and D/A converters, speech coding, a DECT burst-mode controller, and an enhanced 80C51-based mC block. User interfaces include a 163eight-digit LCD driver and a keypad port. The PMB4720 IC includes on-chip power management for the baseband and RF stages, including two linear voltage regulators and high/low supply-voltage threshold detectors. The mC core provides approximately four times normal 80C51 performance, coupled with 88-kbyte ROM and 3-kbyte RAM. The addressing scheme supports bank switching, and the chip has an interface to external EEPROM. You might be surprised by such memory emphasis, but anticipate needing approximately 48 kbytes of ROM even for a handset application.

Data-transmission focus

In high quantity, a baseband controller costs approximately $7 to $15. Because DECT is a high-volume business, cost savings count, and every vendor needs to provide baseband processors that address specific market needs. For this sort of money, VLSI's Vega VWS23101 IC contains all the analogue-to-DECT transmitting and receiving translation stages, mC, keypad, and LCD-control logic you need to connect analogue transducers and the user interface with DECT IF/RF transceivers (Figure 2).

To maximise application flexibility, Vega's DECT burst-mode logic controller supports slot-by-slot DECT frame control and mixed slot sizes per frame. The controller can transmit and receive in the same half-frame. The burst-mode controller architecture suits speech and data transmission and reception in handsets, single-line basesta tions, and WLL (wireless-local-loop) repeaters. Such an array of applications represents a wide target market compared with the more focused product family offerings from Philips and Siemens.

VLSI's Vega processor design also ignores conventional 8/16-bit wisdom, originating from a 32-bit ARM7 RISC core with a memory-efficient 16-bit instruction mode. VLSI's Patrick Edmond explains that his company chose a 32-bit processor over an 8- or 16-bit core, anticipating emerging DECT applications' needing the extra power.

Regarding core size, Edmond notes, "The ARM is about the same size as conventional 8-bit cores, so it's power for free." Edmond also says that VLSI aims for a wider product range because its core is flexible enough to allow it. "That is what manufacturers tell us they want," Edmond states. "If they invest in DECT know-how, they want to be able to use it for office systems as well as residential, or local-loop repeaters as well as fixed access units."

To assist application flexibility, Vega provides two package options that support as much as 128 kbytes of internal ROM or provide external data and address buses with a 1-Mbyte addressing range. You can develop applications with the external-memory configuration and migrate to ROM if code stability and production volumes allow it. The Vega's mC core has enough power to service the communications ports seamlessly and handle algorithms, such as echo cancellation in software, with VLSI providing example code you can use as is or adapt for different requirements.

Sampling now, VLSI's VWS23112 Vega Vmax baseband controller targets WLL applications with the ability to support 64-kbps ISDN transport streams and perform LU7 Reed-Solomon error correction. Error correction is essential for data applications to reduce the nominal bit-error rate from speech-telephony quality (for instance, 1 in 10E-3) to data network quality, substantially less than 1 in 10E-9.

The Vega Vmax controller supports 64-kbps protected-mode data, which is a DECT mode that adds error-correction bits to the actual data, transporting 64-kbps throughput streams. The error-correction bits allow the system to correct errors (principally due to radio interference) transparently to the user. With a view on multiuser terminal applications, the Vega Vmax IC also includes a four-channel ADPCM transcoder.

IF/RF stages

Users shouldn't notice any difference in the radio system, but there are still choices that affect product costs--traditionally focusing on filter requirements. If you choose a highly integrated chip set, you'll be committed to using the companion ICs that partner your baseband controller. However much of a choice you have, you'll find that approaches to integrating DECT radio stages employ single- or double-conversion radio architectures. Both radio schemes build on the superheterodyne principle, in which tuned local oscillators combine with the communication signal to transform the signal up or down in frequency.

Incoming RF is bandlimited in RF filter 1 before low-noise amplification (Figure 3). By tuning Local Oscillator 1, incoming RF mixes down to a frequency within the IF stage passband, typically approximately 110 MHz for DECT. The mixing produces sum and difference frequency components that can combine to produce an output "image." Rejecting the image component is a classic filtering problem, and vendors take approaches from using multiple external filters to integral cancellation schemes that involve phase splitting and recombining the IF signal, as in Philips' UAA2067G transceiver IC.

In the figure, Local Oscillator 2 converts the second IF signal from typically 10 to 20 MHz to baseband. Splitting the frequency conversion into two stages divides gain between the IF stages, eases filter specification, reduces the likelihood of signal limiting, and improves stability.

The dominant DECT radio architecture is a single-conversion transceiver, in which one local oscillator mixes between RF and IF in one step. Building on the superheterodyne-receiver design (Figure 4), the incoming RF signal mixes with in-phase (I) and quadrature (Q) local oscillator signals. I-Q mixing produces a complex signal containing frequency and amplitude information, in which the Q component's leading the I signal means that the incoming frequency is instantaneously increasing.

In combination, the I and Q signals represent the constant envelope of an FM signal, but, individually, the I and Q channel amplitudes vary and must not limit. Transforming the signal frequency in one step stresses the IF filter, which is conventionally a SAW device rather than a cheaper ceramic or virtually free IC filter. Note that contemporary SAW filters cost less than $1 in volume.

Alternatively, a double-conversion radio transceiver eases IC-filter integration, combining similar I-Q mixing with two local-oscillator stages. By transforming the signal in two frequency steps, individual filter characteristics relax. Note that the modulator/demodulator that couples practical single-conversion radio architectures to baseband also provides secondary frequency translation, such as from a 110-MHz IF to a 1152-kbps receive data stream.

Single-chip RF?

As more single-chip transceivers appear from vendors such as National Semiconductor and Philips, you'll begin to notice that DECT IF/RF IC designs vary considerably. You'll still need separate functional blocks, especially for the power-amplifier stage, although the modest output power requirement of approximately 500 mW is a fraction of a GSM handset's output and, therefore, relatively easy. Some vendors, such as Temic, argue that the optimum integration route for the IF/RF stage demands multiple ICs, so that a sensitive node, such as the local oscillator, can have better decoupling than in a single-IC transceiver design.

Temic's new three-chip set represents the single-stage radio-conversion ap-proach with an IC pair that provides the IF/RF interface to a single-chip, two-channel transmitting and receiving front-end amplifier stage (Figure 5). The U2785B transmission stage contains the PLL and loop stabilisation that controls an off-chip local oscillator. The U2761B RF/IF stage houses the receive- signal circuits, a transmit/receive switch, and the transmitter power- amplifier predriver. The U7004B front-end IC uses a silicon-germanium (SiGe) process rather than GaAs or competing high-frequency, bipolar technologies, such as Maxim's 27-GHz silicon process.

Partitioning the transmitter buffer well away from the VCO-control IC helps reduce power-output and signal- impedance changes modulating the VCO--for example, when a user touches the antenna. In transmit mode, the 10.368-MHz data stream mixes with the local-oscillator signal, passing through a frequency-doubler stage that increases interstage isolation. A driver amplifier further decouples the VCO from the output power stages.

In receive mode, an I-Q phase splitter drives the mixer to reduce nominally 1.9-GHz RF to 110.592-MHz IF. A SAW filter bandlimits the signal to approximately 1.1 MHz and introduces approximately 3-dB insertion loss, for which the following IF amplifier/limiter compensates. At this point in the receive path, the amplifier/limiter extracts the RSSI (received signal- strength indicator) with 80- to 90-dB dynamic range. The RSSI tells the baseband-controller logic to locate busy or vacant channels and interface with DECT's cooperative frequency-division-multiplexing arrangement. Before the 1152-kbps DECT receive-data signal output, an FM detector-demodulator reconstructs and lowpass-filters the data signal that you connect directly to a baseband controller.

Mind the GAP

Do not neglect software- and prototype-development issues when considering which silicon to use. If you choose a highly integrated chip set, you'll find that some 90% of implementation effort goes into unexpected ancillaries, such as microphone and ear-piece acoustic optimisation and, more predictably, software development. Vendors often quote GAP (generic-access-profile) compliance, which means that the hardware supports the physical-layer requirements to implement GAP.

GAP is a software issue, effectively the lowest common denominator of network-access requirements DECT equipment needs to meet. GAP's principal objective is to ensure that compliant equipment coexists and communicates transparently, regardless of the vendor. GAP concentrates on speech telephony; higher level access profiles add supplementary services, such as ISDN and DECT-to-GSM interworking.

Siemens' hardware-prototype environment optionally includes PRO-DECT development software. Philips offers the dedicated OM5878 DECT protocol software that implements a proprietary communications protocol over DECT's common air interface to provide a 9600-bps V24 link. VLSI provides example routines for medium-access-control (MAC) layer communication and echo cancellation. But, as Rob Shepherd, telecommunications systems engineer at Cambridge Consultants, notes, the software that silicon vendors supply is designed as an enabler for their ICs.

Understanding that DECT applications will pursue different roads in the next few years, Cambridge Consultants offers a modular DECT software suite based on Unix STREAMS technology with a standard port to National Semiconductor's baseband controller. The modular-software concept guarantees GAP compliance and provides a clean mechanism for higher level profiles to introduce future services, such as multiple 64-kbps ISDN connections and area network control.

Want to know more?

Disappointingly, you won't find a lot of DECT information on silicon vendors' Internet Web sites. You can download overviews and data sheets in Acrobat format from VLSI and National Semiconductor. Philips publishes useful paper application notes for its ICs, including interesting coverage of the the image-rejection scheme in the UAA207x IC family (see AN96106). With DECT, the Internet does what it is classically supposed to do--provide an open communication channel that connects research establishments, commercial operators, and other interested parties. Using the Lycos search engine, for example, a simple "DECT" text enquiry produces 1350 references, with perhaps 5% of real interest.

For more information on DECT, see Walter Tuttlebee's excellent edition, Cordless Telecommunications Worldwide, a 500-pg reference you can preview at ourworld.compuserve.com/homepages/CORDLESS/. For frequent updates of the DECT forum's recent activities, preview the offerings at www.dect.ch/. If you're an IEE member, you can borrow from the IEE's telecommunications library (www.iee.org.uk). Or, read the proceedings from the DECT '97 Conference in London in January, published by IBC at www.ibc-uk.com/conf/CR186/. And, finally, for a more personal experience, attend the Sixth Annual DECT Congress, held at the London Marriott on 22 to 24 Sept 1997; for more information, phone IIR in the United Kingdom at +44 171 915 5055.

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