W-CDMA handsets face twice the bill-of-materials (BOM) costs of Edge handsets and close to three times that of GSM/GPRS devices. Clearly, designers of these multimode handsets must find new ways to reduce material costs.

Like most portable-system developers, handset manufacturers are seeking new ways to integrate functionality to achieve their cost and footprint goals. One of the more promising targets lies in reducing the complexity of W-CDMA front-end architectures. The evolution of the GSM handset architecture offers an excellent model. Early superheterodyne designs required both transmit and interstage SAW filters. Today, those functions have been rolled into the transceiver, and GSM designs also support the use of multiband power amplifiers.

A similar opportunity lies in the W-CDMA front-end architecture. As a direct spread-spectrum technology that disperses its transmissions across a wide, 5-MHz channel, W-CDMA's full-duplex transmission necessitates front-end electronics that attenuate the transmitter's wideband noise to prevent degradation of the receiver's sensitivity.

Common platforms can support GSM, EDGE and WCDMA. Same transmit and receive path swap out the duplexer.
Source: Sequoia Communications
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W-CDMA designers typically address this issue by using a duplexer along with bandpass filters in the receive and transmit paths. These designs also typically require external low-noise amplifiers (LNAs) and the associated board space to support them.

The complexity of this front-end architecture plays a key role in W-CDMA's relatively high cost. The external SAW filters and LNAs increase component count and BOM cost. And the board space needed to support those functions forces the designers of multimode handsets to pay an additional penalty in footprint and layout complexity.

Only recently have some designers managed to integrate the LNAs required for this function on-chip. But that strategy still requires the designer to route the signal on-chip to go through the LNA, then take the output of the LNA off-chip to a filter and back on-chip for baseband functions. Ideally, designers would like to eliminate the use of external filters to reduce component count and cost, and to simplify the board layout by keeping the signal on-chip.

Sequoia's multi-mode transceiver with integrated WCDMA RX filters. The receiver uses a direct conversion architecture.
Source: Sequoia Communications
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The only way they will be able to achieve these cost, footprint and configurability goals for their designs is by moving the frequency selectivity closer to the antenna. The elimination of external SAW filters in the receive path will allow designers to move to co-banded LNAs. And the removal of SAW filters in the transmit path will pave the way for the adoption of multimode, multiband power amplifiers.

Together, these advances will leave the duplexer as the sole determinant for frequency selectivity. By narrowing that function down to a single component, this architectural evolution will open the door to the development of common platforms capable of supporting GSM, Edge and W-CDMA through the same transmit-and-receive path by simply swapping out the duplexer.

Architectural innovation
One of the key distinctions between GSM/Edge and W-CDMA is the latter's full-duplex operation. This capability maximizes performance, allowing the receiver and the transmitter to operate simultaneously by connecting both paths to the antenna through a duplex filter. But this comes at the cost of additional system complexity.

Bridged-T network yields notch response using only passive components. Measured performance of Sequoia' implementation of this notch function.
Source: Sequoia Communications
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Sequoia Communications' new SEQ7400 RF transceiver design uses direct-conversion techniques combined with an innovative W-CDMA LNA and notch filter scheme to meet the requirements of W-CDMA, GSM and Edge modulation schemes. The direct-conversion architecture means it can adapt to the requirements of each modulation format by using the downconverter to translate the receive band to dc (zero-IF).

When the transmit power in a W-CDMA transceiver is at maximum output power, the duplex filter's limited isolation allows a signal of up to - 23 dBm to appear at the receiver's input. This transmit leakage signal represents the largest blocking signal seen by the receiver; if left unchecked, it can undermine receiver performance.

AM and FM time alignment is important in controlling spectral re-growth. Errors of greater than 2ns can cause spectral re-growth of the signal.
Source: Sequoia Communications
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In traditional W-CDMA receivers, designers have solved this issue with external LNAs and SAW filters. The SEQ7400 opts instead for a tunable notch filter. The filter's bridged-T network realizes a high-Q bandstop or notch response using only passive components. This notch function results in notch attenuation of greater than 20 dB over a 5-MHz bandwidth.

The integrated notch filter provides enough attenuation of the transmit signal to prevent the mixer receive path from saturating. This level of integration reduces the overall BOM cost for a triband W-CDMA handset by as much as $1.50 and saves 100 mm2 of board area.

Sequoia set of DSP algorithms maintain trajectory of complex signal. WCDMA transmit output spectrum is reduced to -163dBc/Hz at 45MHz.
Source: Sequoia Communications
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The polar modulator that Sequoia has developed for the transmit path was designed to be a multimode architecture capable of supporting cellular standards. The aim was to provide a cost-effective single RF path that could support both narrowband and wideband modulation schemes while being inherently linear enough to meet next-gen requirements. This polar modulator has been proven to meet the performance requirements with the release of Sequoia's SEQ7400 multimode transceiver. The next step in the transceiver evolution is the removal of the transmit SAW filters.

Receive channel noise
A SAW filter is placed between the transceiver and the power amplifier, and a duplexer is used between the output of the power amplifier and the input to the Rx LNAs to limit the noise that the transmit path leaks into the receive path. The presence of a Tx SAW filter complicates the implementation of multimode power amplifiers and adds cost and size to the solution.

For GSM/Edge solutions, the biggest challenge occurs at 10- and 20-MHz offsets, where careful design of the voltage-controlled oscillator — the main noise contributor — solves the problem. The wideband modulation associated with W-CDMA poses a greater problem. The challenging bands are the low ones — bands V, VI and VIII (824 to 849 MHz, 830 to 840 MHz and 880 to 915 MHz, respectively) — where the duplex frequency separation is only 45 MHz. This compares with the 80-MHz separation used for band II (1,850 to 1,910 MHz).

For a polar architecture, it is critical that the alignment of the amplitude and phase signals be accurate when they are recombined, since errors of greater than 2 nanoseconds can cause spectral regrowth of the signal. Sequoia has developed on-chip calibration routines that provide time-alignment accuracy of better than 0.5 ns.

The other cause of increased noise floors in a polar system is translation of the I/Q signals to polar format. This involves a nonlinear process that expands the bandwidth of the AM and FM signals, increasing the energy at the 45-MHz offset. It would seem reasonable to simply filter the AM and FM signals to reduce the signal bandwidths. Doing so, however, degrades the close-in spectrum and the error vector magnitude of the transmit signal.

The problem occurs because any practical filter implementation will change the trajectory of the complex signal, the main factor in causing the spectral regrowth. Sequoia solved the problem by developing a set of DSP algorithms that limit the peak frequency deviation and reduce the spectrum of the signals while maintaining the trajectory of the complex signal. These techniques reduced the W-CDMA transmit output spectrum to - 163 dBc/Hz at 45 MHz.

Duncan Pilgrim (dpilgrim@sequoiacommunications.com) is the director of product marketing at Sequoia Communications (San Diego). He holds an MBA from Wake Forest University and a BSEE from the University of Birmingham, England.