Data is playing an increasingly important role in cellular handsets, requiring new designs to receive and transmit data as well as voice. The transition from circuit−switched to packet-based radios, demonstrated by the success of GPRS, has enabled further enhancements to data-driven applications. The availability of Enhanced Data rates for GSM Evolution (EDGE), a packet-based system, doubles the data-rate capability of GPRS for many handset functions, such as e-mail access, video cameras, and MP3 players. Fortunately, the POLARIS™ 2 TOTAL RADIO™ polar modulator from RF Micro Devices (Greensboro, NC) helps conserve power consumption and cut heat in compact EDGE handset designs even with these additional functions.

EDGE uses 3π/8-shifted 8PSK modulation, frequently referred to as eight-state phase-shift-keying (8PSK) modulation. Supporting 8PSK over normal GMSK adds reasonable but significant complexity to the development of cellular radio transceivers, power amplifiers, and the overall handset. The primary consideration is that 8PSK is a nonconstant envelope signal, which includes an amplitude signal component. As a result, good EDGE performance requires accurate phase and amplitude control. In contrast, GMSK uses only phase modulation.

Reduced battery life and heat dissipation are two of the most significant parameter for GPRS- and EDGE-based handsets, especially when using multislot transmit operation (two or more bursts are received or transmitted within a single frame). Problems arise from poor transmitter efficiency, especially for the power amplifier. In voice-only handsets, the duty cycle of the power amplifier is only 12.5 percent of the overall transmit/receive frame, which is one burst (time slot) out of eight. With Multi-Slot Class MSC12 multislot operation, this jumps to a 50-percent duty cycle and quickly increases the dynamic power consumption and the heat of the power amplifier. Many handset manufacturers are targeting MSC12 for near-term handsets, but a significant barrier to their production has been the problem of the heat dissipation.

Polar modulation is a relatively new approach to improving the efficiency of the transmit system. Polar modulation dates back several decades but has not been used commercially in handset developments until recently. The two primary forms of polar modulation are small-signal and large-signal polar modulation. Some systems are based o the use of one or more feedback loops or paths, while others employ at least a partially open loop. Each approach has trade-offs.

All current polar approaches for EDGE separately process or create the amplitude and phase signals, but ultimately recombine these signals prior to transmission. Large-signal polar modulation recombines the phase and amplitude signals at the power amplifier. In the case of the POLARIS™ 2 TOTAL RADIO™ solution, amplitude modulation is provided by varying the collector voltage of the amplifier, a method historically known as plate modulation. The POLARIS 2 transmit system is implemented using an all-digital design which provides excellent repeatability from device to device and enables pure digital interfaces with the baseband circuitry (Fig. 1). Some competing large-signal polar modulators use an analog approach, which is subject to process variations, which can lead to performance variations in large volumes.

Small-signal polar modulation (Fig. 2) strips off the AM signal at the output of the I/Q modulator using an amplitude-modulation (AM) detector. This signal is then fed into the voltage control input of a variable gain amplifier (VGA). The VGA recreates the modulation by varying the signal level to the input of a linear power amplifier. In this case, the phase and amplitude are recombined at the VGA. Some of these same techniques can be used with large-signal polar modulation. The primary difference is the VGA is where the recombination is done for small-signal polar modulation, and the power amplifier is where the recombination is performed for large-signal polar modulation.

The phase signal can be generated in several ways. Many systems create the phase signal at baseband and use a standard in-phase/quadrature (I/Q) modulator to provide frequency upconversion either to an intermediate frequency (IF) or to RF. The translational loop architecture is the most common approach for Gaussian minimum-shift-keying (GMSK) transmit systems, where the modulation is initiated at baseband and the radio performs the upconversion to RF. Adding amplitude capability to the standard translational loop is one method to used to implement polar modulation. In the POLARIS 2 solution, a fractional-N synthesizer generates the phase modulation.

The polar loop system represents a variation to the polar theme. The system can employ either large-signal or small-signal polar modulation and can have one or two feedback signals to represent the amplitude signal, the phase signal, or both. In the case of amplitude feedback, a power detector is often used at the output of the power amplifier. This further lowers the efficiency of the transmit path by a few percent. The feedback paths provide value by monitoring the output for changes in phase or amplitude driven by changes in VSWR. The feedback enables a real time method to compensate for these changes, but not without some penalty. In general systems with feedback have increased complexity, higher power consumption and lower efficiency, but can do well with VSWR changes. The POLARIS 2 solution uses a large-signal polar modulation system in an open-loop configuration between the transceiver and the power amplifier. This requires no post power-amplifier detector as in traditional polar loop systems, and is a benefit of the POLARIS 2 implementation.

A standard I/Q modulator can be used as an alternative to polar modulation (Fig. 3). Phase and amplitude information are sent as baseband signals to the I/Q modulator which upconverts both signal components and feeds them to the base of the power amplifier. This approach requires good carrier suppression to be effective. The POLARIS 2 solution avoids this requirement with no upconversion and no carrier signal.

The I/Q modulator approach requires surface-acoustic-wave (SAW) filters for each band in the transmit path, adding cost and size to the overall solution. With tri- or quadband handset designs, the number of SAWs (and the cost of this approach) increases. The filters are needed to reject noise several megahertz from the channel frequency in order to meet the ETSI receive-band noise requirement. (Transmit-band SAW filters are not required with the POLARIS 2 system as the noise in the receive band is low enough without them.) Additionally, the I/Q modulator line-up requires use of a linear power amplifier where the efficiency is less than a saturated power amplifier (Fig. 7).