Extending polar modulation

How can designers achieve that goal while maintaining RF spectral performance? One strategy is to use polar modulation for WCDMA and other higher-bandwidth technologies. Widely used in GSM and EDGE systems, polar modulation eliminates the inherent conflict between power efficiency and amplifier linearity by allowing the input signal of the power amplifier to be constant envelope or to contain no amplitude variations.

In a polar modulation architecture, the I and Q rectangular baseband signals normally sent to the transceiver in a direct upconversion architecture are converted to a polar format having amplitude and phase components. This allows designers to apply the two modulation components differently and more efficiently. The phase signal is fed to the phase-locked loop (PLL) that's used as a phase/frequency modulator. The PLL-VCO output signal is then fed to a VGA or PA that operates near saturation/clipping level. Amplitude modulation is accomplished by controlling the gain of the VGA or PA. Since the amplitude of the phase-modulated signal produced by the PLL remains constant, it can be amplified using more efficient non-linear Class E or F amplifiers. This significantly reduces power consumption by the transmitter and ultimately contributes to longer battery life.

GSM systems use constant envelope modulation with Gaussian minimum shift keying (GMSK). Since the trajectory of the complex signal lies on a unit circle, the modulation can be described entirely by its phase component. EDGE systems triple the GSM data rate using differentially encoded 3π/8 8-phase shift keying (PSK) modulation. AM is added to the signal so that the transmit signal occupies the same 270 kHz bandwidth as GSM. These similarities simplify the extension of a GSM polar transceiver to EDGE.

WCDMA presents an entirely different set of challenges. This technology bundles multiple data channels and uses spread-spectrum hybrid PSK (HPSK) modulation to achieve higher data rates. The use of multiple channels creates a set of superimposed quaternary PSK (QPSK) patterns with different gains resulting from different spreading factors. A root raised cosine filter limits symbol smearing and restricts the transmit signal bandwidth to 3.84 MHz.

These distinctions place different demands on the design of the transmitter. GSM and EDGE systems require excellent phase linearity, low phase noise, and high efficiency. WCDMA systems demand high levels of accuracy over very wide bandwidths and wide amplitude ranges.

The polar architecture has been proven in GSM/EDGE solutions to deliver the lowest noise performance, which led to the removal of the transmit SAW filters. This concept can be extended to remove the transmit SAW filter in WCDMA without excessive current consumption that would be required with linear type architectures. These current savings are set to increase because the next-generation modulation schemes have increasing peak-to-average ratios. Since it supports all modulation formats, the polar architecture is also inherently suited to support true multimode power amplifiers. These factors will significantly reduce the overall size and complexity of next-generation solutions as shown in Figure 4.

New approach

To simplify the development of front ends in multimode phones and allow designers to reduce cost and PCB area, Sequoia Communications has developed an innovative architecture that takes advantage of polar modulation techniques to offer a single transmit path for all modes. The company's FullSpectra architecture provides the foundation for a family of monolithic, multimode RF transceivers. Its second-generation SEQ7400 supports tri-band WCDMA/HSDPA, quad-band EDGE, GPRS and GSM across seven frequency bands simultaneously, making this HEDGE transceiver applicable to most major networks worldwide. To help reduce component count and cost, the transceiver integrates all receive LNAs and WCDMA interstage filters. The device offers a standard analog interface and SCI or DigRF 2.5G control interfaces in a compact RF footprint. (Figure 5)

The benefits such a device offers in a multimode, multiband design are formidable. A single integrated device dramatically reduces engineering effort by eliminating the complexity and duplication of a stacked design. By integrating LNAs and eliminating receive interstage WCDMA SAW filters it drives down cost by reducing the design's bill of materials and minimizing PCB area. Using this new technology, designers can reduce RF board area by up to 70% and shrink RF component count by more than 40%.

Moreover, by supporting both quad-band EDGE and tri-band WCDMA interfaces in the same device, this new approach offers design teams unprecedented flexibility to develop platforms for use in different geographic regions and markets. And adding autonomous calibration and eliminating time-consuming manufacturing tasks, this new architecture promises to accelerate factory throughput and further improve handset-manufacturing costs. Finally, by reducing transmit and standby current requirements, this new approach allows designers to extend battery life in next-generation handset designs.

Conclusion

In today's highly competitive handset market, traditional stacked radio architectures is no longer a viable design option for multimode, multiband handsets. Their duplication of functions, higher BOM cost, and larger PCB area are a competitive liability. To meet customer demand, designers need a new, more efficient approach to the front-end design of multimode, multiband handsets.

Polar modulation offers the most promising transmit architecture option. Polar allows a single path to be used for all modulation schemes providing a size-optimized silicon approach, which can easily support next-generation multimode Pas. The inherent low-noise performance of the solution provides a battery-efficient approach to eliminating the WCDMA transmit SAW filters. Moreover, this efficiency advantage over other architectures will increase as the industry moves to high-order modulation schemes such as HSUPA and LTE.

ABOUT THE AUTHOR

Duncan Pilgrim is the director of product marketing at Sequoia Communications, San Diego, Calif. Prior to Sequoia, Pilgrim worked for RFMD, Mitel and GEC Plessey. He has 14 years of experience in cellular RF design and marketing.

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