Part 2
So what do DSP chips actually do in a cell phone? They primarily handle baseband functions, which include control of the voice codecs, filtering, equalization, echo cancellation, error detector and correction, and encryption, as well as speech coding (compression) and decoding (decompression).
Voice recognition also will be implemented in DSPs. In many of the current 2G and 2.5G CDMA phones, DSP chips haven't proven to be fast enough to perform some of the synchronization and processing required by the spread-spectrum technique. Special ASIC chips have been created to deal with this problem. Some functions have been implemented with FPGAs too.
Looking Ahead
It isn't anticipated that the first full3G phones and systems will be available until some time in 2001. The earliest full implementation will probably take place in Japan. In the meantime, operators and cellular-equipment manufacturers will continue to augment, enhance, and otherwise improve the current 2.5G phones. In any case, current phones and systems will gradually evolve into full- fledged 3G designs over the coming years.
It could be as late as 2005 before full3G objectives are met. This gradual progress will be caused by the slower infrastructure growth due to the massive investment that's required. Most operator companies will delay the build-out.
TWO GENERATIONS OF CELLULAR/PCS PHONES
Study groups are already beginning to think in terms of the next or fourth generation. What is a 4G cell phone and what does it feature? Well, we can certainly expect such phones to operate at even higher microwave frequencies than current models. Plus, even higher data rates will be possible. What we may be looking at is a color video phone with voice recognition operating in the 2GI-1z+ range. Or, perhaps it will be just a headset with hearing-aid-size RF and processing circuitry.
Cellular/PCS Companies And Organizations
New Products For 3G Phones
A cell phone is basically nothing more than a very sophisticated two-way radio. The RF section is critical. But, it continues to become a smaller percentage of the entire phone as RF chip manufacturers place greater amounts of circuitry on-chip and as the processing and control functions become larger and more complex with multiple air interfaces and networking capabilities. Today, the RF chips have essentially become a commodity. Many manufacturers have similar competing lines. Virtually all of them use biCMOS circuitry, and many employ Site bipolar devices.
Recently, Motorola designers also licensed Atmel to make their RF chips to ensure a sufficient supply to this demanding market. Motorola has even developed a SIGe:C process that integrates the silicon and germanium with carbon to produce heterojunction bipolar transistors (HBTs) into their biCMOS RF chips. The SIGe:C process extends the transistor's fT to 50 GHz and the f^sub MAX^ to 90 GHz.
Newer chips typically contain multiple signal paths to accommodate the multiband operation required for 2.5G and 3G phones. A typical 3G phone permits operation in the 800- to 900-MHz band and either the 1800- or 1900MHz bands, or perhaps even both. Two separate signal paths can normally be used, one for 800 MHz and the other for 1800/ 1900 MHz, if external filters can be switched in to differentiate between the 1800- and 1900-GHz ranges.
Cell phones need power amplifiers (PAs) too. Most PAs are HEMT or HBT bipolar devices made with GaAs. Some LDMOS PAs, however, are used specifically for higher-power applications in base stations.
The trend is to use direct-conversion zero-IF (ZIF) receiver ICs. An example is the Othello chip set from Analog Devices Inc. It consists of the AD6523 direct-conversion receiver and the AD6524 frequency synthesizer chip. These are designed for use in a dual- band GSM (900-/ 1800-MHz) phone. Analog Devices claims that this chip set will reduce the overall cost and size of the circuitry from 30% to 50% over current 2G designs. The chip is implemented in 0.6-(mu)m biCMOS.
The AD6523 consists of a law-noise amplifier (tNA) that drives a variable-gain amplifier (VGA) and a received signal-strength indicator (RSSI) circuit. The output drives a pair of mixers which are driven by quadrature local-oscillator signals from the separate synthesizer chip. External low-pass filters reproduce the baseband signal that goes to ADCs. Contained in the transmit section are the baseband DACs and their filters. The processed baseband signals drive upconverters to produce the RF signal with modulation. Furthermore, a SAW filter feeds an RF VGA for feedback power control. Plus, an external PA boosts the signal to the final transmit power level.
The companion AD 6524 synthesizer chip is a fractional-N design using a single crystal oscillator. It generates four output signals which are usable in DCS 1800 and PCS 1900 phones. The Othello chip set is designed to support both GPRS and EDGE protocols for the coming high-data-rate services.
Analog Devices has further announced a new ZIF chip with Mitsubishi for 3G W-CDMA applications. In addition to the I/Q mixers, the chip has an overall gain of 95 dB with gain variable in 1-dB increments. It contains on-chip lowpass filters and the RSSI circuit as well.
Another receiver approach is Philips' near-zer\o-IF GSM transceiver chip. The UA3535HL doesn't translate the signal directly to baseband, but it generates an IF of 100 kHz, which is very low compared to the signal frequency. An IF at 100 kHz is far easier to filter than a higher frequency. An on-chip low-pass filter eliminates the higher IF An integrated channel bandpass filter provides the desired iF selectivity. The chip supports the 900-, 1800-, and 1900MHz frequency ranges.
The transmit section of the UAA3535HL is traditional with I/Q mixers to upconvert the baseband digital to a transmit IF A second circuit then provides the modulation and mixing to the transmit frequency. Designed to be controlled by external PLL circuitry, VCOs for the receiver and transmitter sections are on-chip. Three power- up input pins are featured on the chip too. They let various parts of the circuitry be powered down during idle times. The RXON, TXON, and SYN pins allow the receiver, transmitter, and synthesizer circuits to be turned on separately or in any combination. Additionally, the UAA3535HL is designed to work with Philips' OneC-GPRS chip to implement the GPRS protocol. EDGE-protocol capability is expected in the future.
Although ZIF is the trend on the receiver side of the RF chain, the trend on the transmitter side is toward PAs with greater linearity, especially in the CDMA chip sets. Because CDMA requires very wide bandwidths, the PAs must be far more linear than those used in TDMA and analog designs. While such amplifiers are far less power- efficient than those employed in other chip sets, the greater linearity is essential to reduce the intermodulation (IM) products to a level that's acceptable by the standards.
Furthermore, the CDMA chip sets require precise power control. The power of a cell-phone transmitter is controlled directly through a closed-loop process with the basestation. It ensures that the basestation receives sufficient power, but also a minimal power level that minimizes interference and reduces the noise floor in CDMA receivers. In the new CDMA 3G phones, power is controlled in 0.5- or 0.25-dB increments.
An example of an RF chip set optimized for CDMA is Conexant's CX74001 and CX74002 products. These chip sets support any of the existing or future W-CDMA standards. The CX74001 is a dual-band receive subsystem made in a biCMOS process. It features two signal paths, including an LNA, mixers, VGAs, and I/O demodulators, as well as two receive VCOs.
The CX74002 is a Site dual-band transmit subsystem. The UQ mixers upconvert the baseband signal to RF and provide the necessary modulation. Dual VHF and UHF PLLs are incorporated, eliminating the need for an external PLL. The final RF upconverter provides sufficient output to drive the external PAs. Other products in the Conexant series include the CX74004 Site dual-band LNA/downconverter and the CX74005 bipolar VGA/I/Q demodulator. All of these new chips were designed to minimize power consumption by up to 20% over existing competitive devices, thereby decreasing power drain and increasing talk/idle time. Plus, Conexant makes TDMA/analog chip sets. All chips support GRPS and EDGE protocols for data transmission.
Mike Civiello, director of marketing for the Wireless Transmitter Solutions Div. of Motorola, indicates that switches are an unexpected RF need in 2.5G and 3G phones. Most multiband/multimode designs require the use of RF switches. Transmit/receive (T/R) switches are common in 1500-/1900-MHz phones, but most new designs need multiple switches for band switching and other circuit-switching functions. These switches select the proper input/output pins on the transceiver chips and route the external filters. Motorola is developing a line of GaAs pHEMT bipolar switches for these applications.
The key to a successful 3G phone is processing power. All new 3G phones will have far more powerful DSP chips and may incorporate several DSP chips to accomplish this. The new 3G phones will have more computing power than the average PC of today. More and more functions are being pushed into DSP as a way to reduce the parts count and simplify the design.
Of course, the ideal cell phone is a software radio, where virtually all of the processing functions are performed in software. The ideal software radio receiver consists of an LNA and SAW filter whose output goes directly to an ADC. All downconversion, demodulation, and other processing is carried out by the DSP The transmitter section is simply a DSP upconverter that feeds the external PA.
Needed: Sophisticated Power Management
The increased power demands of 3G phones make improved battery/ power management and control circuits more important than ever. While voice usage is demanding, longer continued use of a handset with email, Internet access, and other data activities places a major strain on the power system. Power-management circuits are necessary to ensure the long battery life, talk time, and idle time that today's 2G and 2.5G phone users have come to expect.
At the heart of the power system is the battery. Look for continued use of the lithium-ion (Li-ion) and nickel-metal hydride (NiMH) batteries currently used in 2G and 3G phones that will be found in new sizes and shapes. But new batteries based on other chemistries or technology may come along to help the power problem: Some possibilities are the zinc-air and lithium-ion polymer batteries.
For battery power management, there are four aspects to consider:
Battery charging: Most users want fast recharges. This is possible with the new Li-ion and NiMH batteries. Both types, though, require critical charging voltages and currents to ensure that the batteries aren't damaged during recharging.
Voltage regulation: Power to all circuits must be regulated, of course. But multiple voltage levels will be required, which means that multiple regulator chips or special chips will be needed to provide the desired voltages. High-side regulators and/or dc-do converters are a must. Many of the newer RF and support chips already contain low-dropout (LDO) regulators, making extra regulator chips unnecessary.
Shut-down circuits: Power-management chips will allow major parts of the phone circuitry to be shut down during idle times. This conserves battery life.
Power control: Many new RF and processor chips have built-in power control. An input line to the chip tells the chip to shut down. Such chips make power management easier to implement. And, they save costs and design time.
While power-management and control circuitry will be necessary for each phone, most designs can be implemented with the many new chips that exist from familiar vendors. Custom chips are made possible through a vendor that modifies or enhances an existing chip. Given the usual high volume of phone sales, even custom chips can be inexpensive.
Meeting The 3G Test Challenge
The success of a 3G design will be as good as and heavily dependent upon the available test and measurement equipment. As higher frequencies are used and more complex designs became the norm, the challenge to test equipment will increase. How does one test for compliance of a modern W-CDMA phone? What equipment does one have available that allows testing microwave outputs and data- transmission standards such as GPRS to make sure that all protocols and error rates are met?
Additionally, EMI/RFI testing is a major requirement to meet FCC regulations. Designers will have to search for newer and better test equipment to meet these needs. Signal generators, spectrum analyzers, protocol analyzers, field-strength meters, and other specialized pieces of equipment will be needed. While the cost of this equipment continues to escalate because of its complexity for dealing with multiple frequencies, modes, and protocols, that cost will be a necessary investment in order to achieve successful 3G designs.
Perhaps the most important new pieces of test equipment are those designed specifically for 3G phone and networking testing. Special test sets for GSM, TDMA, and CDMA protocols are becoming available. Protocol analyzers are particularly important to test high-speed packet data functions like GPRS and EDGE. Test-equipment leaders such as Tektronix and Agilent Technologies now have full lines of generic RF test equipment and many new 3G test products.
Typical of the available advanced test gear is Tektronix's K1297 Protocol Tester and K1205 Signaling Protocol Analyzer (see the figure). Both now have software support for the GPRS data- transmission system that's being introduced in 2.5G phones and will continue to be used in some 3G phones. This complex protocol with many interfaces is a "bear" to test. Steve Stanton, Tektronix's product manager for spectrum analyzers, states that "one of the greatest challenges for 3G engineers is testing for manufacturing and testing the multiple standards that will exist."
Other test-equipment manufacturers also are paying attention to the 3G market. One company, TAS, was founded on the principle of creating sophisticated test equipment to meet 3G communications needs. In addition, Credence Systems Corp., a manufacturer of ATE systems for chips, recently purchased Modulation Instruments Inc., a manufacturer of leading-edge cellular handset and basestation RF test systems.
Copyright Penton Media, Inc. Oct 2, 2000
(Copyright UMI Company 2000 All rights reserved.)
Publication date: 2000-10-02
© 2000, YellowBrix, Inc. |
