Polar Loop Transmitter For Quad-Band GSM, GPRS, And EDGE Applications

Skyworks Polar Loop architecture leverages saturation-mode PA technology to maximize Power-Added Efficiency (PAE) in 8-Phase Shift Keying (8-PSK) transmit modulation. This approach has the potential to improve PA efficiency in Enhanced Data Rate for GSM Evolution (EDGE) mode by up to 10 percent when compared to conventional architectures.

Polar Loop architecture uses separate amplitude modulation (AM) and phase modulation (PM) feedback control of the output signal. With this approach, the problems with AM-to-PM as well as AM-to-AM of the nonlinear PA are essentially eliminated. In addition, this architecture allows for a large dynamic output power control range as required by the GSM specification.

Polar Loop transmitters use a standard In-Phase and Quadrature (I/Q) interface and do not require the extraction of AM and PM signals in the digital domain. The dual feedback loop ensures robust performance even under Voltage Standing-Wave Ratio (VSWR) variations without using an isolator. No external PA filtering is required to meet the transmitter noise in the receive band. The EDGE spectral mask is met with an rms error vector magnitude of less than 3 percent at 29 dBm at the antenna, corresponding to 2 dB above nominal maximum output power.

There is no mode change between Gaussian Minimum-Shift Keying (GMSK) and 8-PSK modulation, and the transmitter operates seamlessly in multislot Enhanced General Packet Radio Service (EGPRS). The polar modulation transmitters meet or exceed GSM-type approval requirements for both EDGE and GSM/GPRS in quad-band operation (850, 900, 1800, and 1900 MHz).

This White Paper compares the Skyworks Polar Loop architecture with other architectures for EDGE transmitters.

Introduction

The increasing demand for wireless data transmission has caused several changes to the original GSM technical standard in recent years. These changes include the added definition of multislot operation, where several timeslots with a Time Division Multiple Access (TDMA) frame can be assigned to a single user, the definition of additional Radio Frequency (RF) bands, and the inclusion of the EDGE modulation format.

EDGE modulation effectively triples the transmission rate using the same bandwidth as the original GMSK modulation. The penalty for this higher data rate is paid by the more stringent requirements to the received signal-to-noise ratio (SNR), which reduces the coverage area for EDGE reception compared to GMSK. However, there are also significantly tougher technical challenges for both the receiver and transmitter circuits.

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•White Paper: Polar Loop Transmitter For Quad-Band GSM, GPRS, And EDGE Applications