Understanding polar modulation
Eliminating multimode issues for WCDMA and GSM/GPRS/EDGE handsets
BY EARL McCUNE
Tropian
Cupertino, CA
http://www.tropian.com
While the initial roll-out of UMTS in European countries is well underway, it is clear that operators must continue to rely on GSM/GPRS and EDGE technologies to provide service outside of UMTS-covered areas to gain critical 3G subscriber acceptance. For operators to maintain or even accelerate current adoption rates, their 3G networks must provide service superior to the 2G network performance which consumers are accustomed to and demand.
Handset manufacturers play an equally important role in this consumer equation by providing reliable, cost-effective handsets that are usable across multiple operating standards and market spaces. In addition to providing high quality and reliable voice service, today's networks and mobile devices must deliver video, email, Web access and other data services that weren't previously demanded of 2G networks. These additional capabilities add network and handset complexities which the end user is eager to adopt but cautious to pay for.
To realize the volume market opportunity for multimode 3G terminals, the solutions employed must be able to deliver on the performance expectations of customers; this is true both in terms of data rates and also power efficiency or battery life.
The solutions must be small and light enough that the industrial design and form factors of the handsets are not restricted – the key role that size and design of handsets play in their market success has been clearly demonstrated in the market. Finally to achieve the volume potential, these performance and size goals must be met with minimum penalty on system cost.
Traditional linear radio design is limited in its ability to achieve all these objectives together. Polar transmit architectures, however, offer significant advantages to addressing these needs.
Contrasting technologies
Most GSM/GPRS handset transceivers today use a tracking offset loop architecture. This transmitter architecture is optimal for constant envelope signals where the tracking phase-locked loop (PLL) acts as a combination of frequency translator, bandpass filter and hard limiter. The power amplifier (PA) performs burst ramping and power control. All signal modulation passes through the PLL, so its bandwidth must be wide enough to not distort the desired signal's angle modulation. An important feature of this design is that there is no inherent need for RF circuit linearity following the quadrature modulator.
In contrast, WCDMA and EDGE employ envelope-varying signals. They cannot directly use the tracking-loop and instead typically use linear transmitter designs. The linear WCDMA or EDGE PA usually operates at constant gain, so all output power control is provided by VGA stages earlier in the chain.
Some efforts have been made to modify the PA bias and offer a set of different operating modes for the PA to alleviate power efficiency issues at mid and low RF power output levels, but this comes with significant complexity and additional cost. The WCDMA transmitter must also control its output power over an 80 dB dynamic range without incurring significant distortion. Full-duplex operation in WCDMA further heightens the requirement to suppress the transmitter output noise in the receive band to well below the level of the receiver's input noise.
There are other differences between GSM and WCDMA transmitters. To support full-duplex operation, WCDMA normally employs a duplexer in place of the transmit/receive (T/R) switch used in GSM. WCDMA also uses an RF isolator that prevents the linear PA from distorting the output signal when the impedance match to the antenna is poor. Indeed, the only thing that these two transmitter designs actually have in common is an initial quadrature modulator stage and an antenna, illustrating the challenges involved in multimode design.
Fortunately polar modulation offers an alternative to standard GSM/GPRS, EDGE and WCDMA designs. One circuit is for the GSM/GPRS and EDGE half-duplex systems, the other for the WCDMA full-duplex system. All signals have constant-envelope phase components so linear RF circuitry is not required in either architecture.
Although the two circuits look nearly identical, the WCDMA implementation is more advanced than the GSM/GPRS/EDGE design and includes a duplexer and a larger power-control dynamic range. The additional configuration changes between the WCDMA and GSM modes are implemented digitally within the polar modulator and therefore require few modifications to the radio design from the manufacturer, thus reducing cost and time-to-market issues.
Uncommon design provides a common solution
There are major differences between the linear and polar WCDMA transmitter designs. In the linear design the power amplifier is operated at a fixed gain to maintain linearity. In the polar case, signal envelope variation and output power control are achieved by varying the gain of the polar output stage. This difference has an impact on transmitter design in two important aspects.
First, the need for RF circuit linearity preceding the polar-power stage is eliminated in the polar design. Second, the output noise, both in-band and out-of-band, is dramatically lower in the polar case because without the preceding linear gain, the amplification of input noise due to amplifier noise-figure does not occur. Instead, voltage-controlled oscillator phase noise dominates the noise floor from the polar-power stage. Phase noise is not white, and so is lower at receive-band offset frequencies than near the transmitter signal. The duplex filter can meet full-duplex operation noise performance with 40 dB or less rejection at the receive band, which eliminates the need for interstage filters.
To be economical in high-volume manufacturing, any radio design must exhibit very stable characteristics and large margins to the specifications. The latter refers to the situation in which individual manufactured units tend to perform better than the minimum design requirements. Measurements of the key performance parameters of at least one polar GSM radio show that this architecture can meet these criteria. These measurements included power spectral density, root-mean-square phase error and output power-control stability.
Polar transmitters are also ideal for EDGE because they operate with the output amplifier in compression and require no backoff from peak saturated power for any envelope-varying signal. So a 2-W PA for GSM, when used in a polar implementation, can provide 1 W of EDGE power. This is well above the current requirement of 500 mW for Class-E2 (low-band) operation.
A GSM/GPRS/EDGE polar transmitter also offers superior mode agility (see Fig. 1 ), defined as the ability to switch rapidly between the 0.3-GMSK modulation scheme used in GSM and the 8-PSK used in EDGE. Complete power-down and power-up ramping is performed between each time slot and the modulation configuration is changed in the digital signal processor following power-down and before the next ramp-up.
Fig. 1. A GSM/GPRS/EDGE polar transmitter offers superior mode agility.
Not only can the modulation type be set independently from slot to slot – as is required by the enhanced GPRS standard for EDGE – but the power level can be set on a slot-by-slot basis.
While operating in WCDMA mode, the polar transmitter offers stable output power control. Like the GSM transmitter, power control is still operated in an indirect closed-loop fashion. A power-control accuracy of within 0.5 dB has been demonstrated and spectral purity is maintained to 10 dB beyond the specified minimum output power.
The future is polar
Since conventional GSM/GPRS transmitter architectures are designed to deliver constant-envelope phase modulated RF signal, they therefore have little in common with recent power efficient designs for EDGE and WCDMA, which need to generate both phase and envelope-varying RF signals. As a result, combining these different transmitter requirements within one multimode handset can be an expensive proposition, often requiring separate signal paths. This results in dedicated modulators and Power Amplifier devices for each standard. By changing the transmitter's modulation system from the traditional linear quadrature (I/Q) architecture to polar architectures, modulators can be designed to deliver both constant-envelope and envelope-varying signals in a single device. Polar transmitter-based architectures separate the control for the signal phase and envelope modulation and by careful recombination of these signals can generate all of the above signals with a single design, and single PA. In addition, the fact that there is no requirement to maintain linearity within the PA means that circuits can now be designed with an emphasis on optimizing power efficiency – a separate topic, but clearly a further advantage for polar architectures.
Polar modulation technology has been implemented commercially today, creating a single, universal, multimode handset radio transmitter. Measurements have shown that one polar design readily supports the GSM/GPRS and EDGE signals, as well as the WCDMA signal. Other emerging technologies worldwide are increasingly using bandwidth-efficient, nonconstant envelope modulation schemes like OFDM. Polar modulation is well poised to enable development of multimode handsets for these multiple air interface standards.
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