Ubiquity is a pinnacle that the cellular communication sector has hoped to reach for the past five years. To reach this goal, a series of networks must be built that allow consumers to use their phone anytime, anywhere.
The truth is ubiquity is far from becoming a reality. Across the world cellular carriers can't seem to agree on a single air interface for wireless operation. Just look at the coveted 3G market. While the ITU pushed for a single 3G spec, it settled for a single standard with five parts, making ubiquity again a secondary item.
But, despite battles on the standards front, the wireless community has pushed forward in its efforts to build mobile networks and phones that deliver worldwide coverage. To make this happen, they have focused their attention on developing multimode systems that can support CDMA, TDMA, GSM, GPRS, wideband CDMA (W-CDMA), and a host of other air interfaces in the same box.
So far, however, building RF front ends for multimode phones and base stations has been a difficult task. In particular, the transmit chain has been a headache for designers. Fortunately, a new technology, called polar modulation, is on the horizon and hoping to ease the multimode system development process.
Transmit challenges
Before diving into the specifics on polar modulation, let's first explore the RF challenges that today's mobile phone and base station developers face when building multimode designs.
Supporting one radio in a box is a tricky task. Supporting multiple radios can be a nightmare. To support multiple protocols, designers of wireless systems have built multimode systems using multiple super heterodyne radios. While this may be sufficient for integrating CDMA and AMPS, it falls short when demanding protocols like GPRS, EDGE, and W-CDMA are added to the box.
To solve this problem, many members of the design community have focused their attention on
the receive side of the transceiver chain. For example, over the past year a host of direct-conversion receiver solutions have been unleashed to increase integration in the receive path of a mobile phone or base station design.
Although still in their infancy, these highly integrated solutions could allow the designer to more easily integrate multiple receivers in a system. But, designers have not placed the same level of attention to the transmit side of the RF chain. Integrating multiple radios in a single box has a huge impact in the transmit chain. For example, integrating multiple RF front ends could force designers to add additional surface-acoustic-wave (SAW) filters, oscillators,
filters, and specialized mixers in their system architecture. This is not optimal for the price, power, and space-conscious cellular market.
A new solution
Polar modulation digitally controls the type of modulation used while processing the carrier amplitude and phase signals independently. A modulator works together with a nonlinear power amplifier operating in switched mode. Eliminating the requirement for linear operation enables the power amplifier efficiency to be maximized for each form of modulation.
Under the polar modulation scheme, multimode operation can be achieved by digitally switching between modulation standards on the fly. Phase information is used to control an on-channel voltage-controlled oscillator (VCO) driving the power amplifier. The amplitude data, also being digitally processed, modulates the power amplifier according to the standard required. Since phase and amplitude information are independently processed but digitally synchronized, various signals (both with and without constant envelopes) can be transmitted without regard to sacrificing performance parameters.
For standards employing non-constant envelopes such as EDGE or W-CDMA, a major problem in conditioning and transmitting the signal is the trade-off between linearity and efficiency. In order to minimize distortion of signal peaks, drive levels to amplifiers must be reduced to prevent signal clipping. Depending on the crest factor level, some form of linearization technique may also need to be employed to insure signal integrity. The immediate consequence of these efforts to preserve linearity is an overall reduction in efficiency.
In the polar modulation approach, the conventional functions of transmitter and power amplifier are integrated into a
two-chip solution (see Figure 1).
In this system, the phase and amplitude paths are digitally separated into two separate paths by the modulator.
The phase path signal drives an on-channel VCO. Simultaneously, the amplitude portion of the signal controls a transistor, which modulates the drain supply of the power amplifier.
In this architecture, the power amplifier operates as a switched-mode amplifier, remaining in compression while the drain voltage is varied. This allows the output power generated by the power amplifier to be directly proportional to the supply voltage on the drain. By modulating this drain supply, the carrier's amplitude information (for non-constant envelope signals) can be completely superimposed on the signal at the output of the power amplifier.
Additionally, changing the drain voltage in this manner provides an accurate means for power-level control. This removes the need for closed-loop power control used in many traditional transmitter designs. Problems such as diode power detection, dynamic range issues, and the circuit complexity associated with conventional power detection can be eliminated.
In addition to power control accuracy, an open-loop system provides a range of power level control. Using a W-CDMA signal on a handset reference design, power levels can be varied by over 80 dB. Maintaining the power amplifier in compression also maximizes the power amplifier's efficiency irregardless of the type of modulation used.
Variations
In order to make any new technology commercially viable, designers must consider the robustness with respect to temperature, supply voltage, load, and frequency. This is certainly true in the case of polarization.
Let's examine some of these issues while providing some theoretical context concerning the advantages of polar modulation in product manufacturability.
Measurements will be given showing the effects of variations to such parameters as output power, error vector magnitude (EVM), and power spectral density (PSD).
As engineers well know, the performance of RF components varies greatly depending on the system architecture at hand. These variations can lead to reduced system performance in both a handset and base station.
Polar modulation shines at curbing these variations by digitally controlling most of the transmit signal conditioning from the mobile handset's baseband section to the power amplifier. Additionally, this modulation technique thwarts variation by operating the power amplifier in compression for all modulation standards.
A fundamental property of digital control is its invariance to changes in temperature, power
supply, and other outside effects. Controlled by a clock reference used throughout the transmit system, various events such as ramp control (for TDMA systems), signal processing, channel tuning, and amplitude/
phase synchronization can be precisely timed.
The control of these and their functions by the modulator
will not be altered with external changes in the environment. This results in significant performance margins as shown in Figure 2 for a base station design.
In this figure, temperature is varied from --10 C to +70 C, while the the carrier frequency is EDGE modulated and centered at 1842.8 MHz.
Additional robustness
Another feature providing design robustness is operating the power amplifier in switched-mode operation. Transistor parameters such as gm, Vbe, VT, fT, and are relatively insensitive to variation with the power amplifier operating in compression. This allows significant design flexibility by removing the need to build in significant design margins to allow the product to meet spec in a volume manufacturing environment.
For example, a conventional superheterodyne base station transmitter design may use six or seven amplifiers, various mixers, filters and attenuators, as well as a power amplifier. All elements here will change performance characteristics subject to changes in temperature, frequency, and supply. The combined effects of these variations result in significant design trade-offs to ensure linearity, signal-to-noise, and power ratings.
With polar modulation, most of the transmit functions associated with super heterodyne configurations are eliminated. What remains is digital control of the transmit process along with a power amplifier relatively invariant to outside changes.
Figure 3 shows EVM variation as a function of frequency and power level variations. A handset design was employed using EDGE modulation in the 800-MHz US cellular band. The EVM spec (rms) for this product is 5%.
The PSD for this same handset product was then measured over a supply variation of 2.9, 3.6, and
4.2 V. After running this test, it was shown that adjacent channel variations were limited to less than 2 dB, with a worst-case margin to spec of about 7 dB at 400-kHz offset.
In addition to removing output coupling losses through the use of open-loop power control, the handset solution also removes the isolator, saving about 0.7 dB of insertion loss. Given that every 0.1-dB improvement in output loss corresponds to approximately 1% efficiency improvement, the power added efficiency (PAE) of the system is improved by about 7%.
To ensure that these improvements would not jeopardize power amplifier performance vs. load
variations, a series of load pull measurements were conducted over a variety of conditions. At
a voltage standing wave ratio (VSWR) of 30:1 over a 360 phase range, adjacent channel power
continued to meet specification requirements at both 400- and 600-kHz offsets.
Further tests and measurements were completed over a wide range of load impedances at varying power supply levels and frequencies. These measurements confirm that performance specifications continue to be met with good margin without an output isolator.
Synchronization
Since the amplitude and phase signals are separated under the polar modulation technique, synchronization between these signals becomes a bigger concern. To solve this problem, each digitally controlled signal is precisely synchronized at the output terminals of the RF power transistor to maximize spectral purity and minimize EVM degradation.
As modulation rates increase, synchronization between the amplitude and phase paths becomes increasingly important. To illustrate, let's intentionally misalign the amplitude and phase modulation stages of an EDGE signal by adding delay to the amplitude path. These experiments demonstrated that the EVM (rms) meets spec until timing is skewed by 0.08 symbol periods for an EDGE signal. Figure 4 shows the effects of this same misalignment on PSD.
Comparing these two measurements shows that when amplitude and phase alignment is sufficient to meet PSD specifications, the EVM spec will be made with a wide degree of margin. For EDGE signals, a 20-ns difference in alignment still provides sufficient margin to spec.
In a practical implementation of the polar modulation design, delay variations do occur in both the transmitter IC as well
as in analog components such as the drain/collector modulator. The combined total of these variations are in the picosecond range.
Looking forward to UMTS signals having higher data rate capacities, delay control between the amplitude and phase paths becomes more critical.
Measurements in the lab, however, have shown that synchronization in a UMTS system is achievable. With a digital approach, precise control of these signals can be attained while minimizing adverse impacts from frequency, supply, and temperature extremes.
Other applications
The advantages of polar modulation technology can also be realized in a variety of other applications such as professional mobile radio (PMR), wireless PBX systems, wireless local loop (WLL) systems, wireless LAN (WLAN) equipment, and DSL products.
On the PMR side, polar modulation technology can be employed in TETRA systems (TETRA is
the European standard for PMR). Currently, designers are developing polar modulation in TETRA handsets. TETRA base station reference designs have also been considered.
Mark Heimbach is a program manager for Tropian. He holds an MSEE from the University of Wisconsin-Madison and a MBA from the University of Dallas. He has been employed at both Motorola and Nokia. He can be reached at mark.heimbach@tropian.com.
