Improving linearity utilising adaptive predistortion

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BACKGROUNG TO THE RESEARCH

In today’s information-dependent world, there is an increase in demand for faster communication speeds and larger bandwidth. Current microwave frequency bands have become saturated and recent advancements in technology, especially in silicon-germanium (SiGe) technology, has resulted in a growing necessity to use new frequency spectrums such as the millimetre-wave (mm-wave) frequency band. In particular, the 60 GHz band is very useful for wireless communication. It offers a very wide frequency and higher transmission speed and it is unlicensed. The wireless transmitter needed for mm-wave communication comprises many building blocks, as shown in Figure 1.1, and of particular interest is the power amplifier (PA). The purpose of the PA is to amplify the input signal to an acceptable power level so that the signal can be transmitted from the transmitter to the receiver through the air interface.

PAs are the final subsystems prior to the antenna in the transmitter and directly affect the performance of the transmitter. Unlike wired line communications, wireless systems must share a common transmission medium. The available spectrum is therefore limited. The demand for greater spectral efficiency has resulted in orthogonal frequency division multiplexing (OFDM) being considered as the modulation scheme for communication systems in the mm-wave frequency band [2]. This modulation scheme results in amplitude and phase modulated signals with large peak-to-average ratios. These signals are very sensitive to disturbances that affect the amplitude and phase of the signal, such as non-linear amplification, which causes distortion in the output signal. This distortion causes the signal to expand to the other adjacent channels, resulting in interference, and deteriorates the performance of the communication system. For this reason it is important for a PA to operate linearly.

Linearity and efficiency are mutually exclusive. Designing linear PAs is possible, but most linear PAs are not efficient [3]. The PA efficiency is a very important factor especially in mobile communications. The design of linear and efficient mm-wave PA presents one of the most challenging design problems. Therefore, in order to achieve good linearity with sufficient efficiency, some kind of linearisation technique has to be implemented [4]. This research focuses on improving the linearity of PAs for mm-wave frequencies using adaptive predistortion (APD) as the linearisation technique. Predistortion’s chief attribute is its conceptual simplicity and it does not suffer from bandwidth limitations, which makes it suitable for wideband PAs.

JUSTIFICATION FOR THE RESEARCH

The huge bandwidth around 60 GHz is one of the largest unlicensed bandwidth allocations available. This band provides at least 3 GHz (59-62 GHz) overlap that is available worldwide, offering high data-rate communications. Even though this band suffers from severe attenuation of 10 dB/km due to oxygen absorption, this further justifies its use for short-range communication. The IEEE 802.15.3c and the WirelessHD task teams have defined standards for the 60 GHz band with 30 dBi gain for the antennas and 10 dBm output power for the PA [13]. In addition, OFDM has strict requirements for linearity. Therefore designing a PA at this frequency is challenging, as these PAs must deliver high linear output power and be efficient. Linearisation techniques can be used to improve the linearity of PAs and meet the demands of mm-wave communications.

Not much attention has been focused on linearisation techniques of PAs at 60 GHz. Therefore there is a need to investigate and characterise these linearisation techniques to determine their ability to improve the linearity of 60 GHz PAs. In addition there is an increasing demand for PAs to be reliable and cost-effective and to yield high performance to satisfy the mm-wave requirements. SiGe BiCMOS technology provides low cost and superior performance and can be used as the building blocks to realise these demands of mm-wave systems. The 3 GHz overlap unlicensed bandwidth allocation at 60 GHz has made this mm-wave spectrum lucrative for fast gigabit applications. This has resulted in numerous RF transceivers currently being developed to operate within the 60 GHz band. One of the most challenging functional blocks in the transceiver at 60 GHz is the PA [15]. Its function is to amplify the input signal and deliver high output linear power while being efficient, but its performance is severely affected by the scaled semiconductor technology and the operating frequency. It is for this reason that PA linearisation techniques should be investigated to improve the linearity and maintain efficiency, enhancing the performance of the PA. The first part of this chapter focuses on the fundamental concepts, parameters and figures of merit for PAs. The current semiconductor technologies as the building blocks of the PA are also discussed. The second part of this chapter analyses the various distortion components in the PAs. This important property contributes to the understanding of non- linearity in PAs. The last part investigates several linearisation techniques, evaluating their trade-offs and specifically concentrating on the predistortion linearisation technique for mm-wave PAs.

POWER AMPLIFIERS

The PA usually consumes the largest amount of static power of the transmitter. It is therefore desirable to operate the PA in the saturation region to achieve maximum power efficiency of the PA. The trade-off is that all real PAs have non-linear characteristics and this is most noticeable in the saturation region. This trade-off becomes even more stringent at 60 GHz, where desirable Si-based technology for low-cost production cannot provide sufficient output power in PAs while maintaining high linearity [16]. PAs are therefore designed and evaluated on several trade-offs, each trying to accomplish a conflicting requirement such as linearity versus efficiency or high output power versus minimum distortion. Consequently understanding the characteristics and operation of PAs is essential to ensure PAs are correctly designed to meet the requirements.

Keywords: Linearisation techniques, predistortion, power amplifiers, millimetre wave integrated  circuits, silicon germanium, heterojunction bipolar transistor, BiCMOS integrated circuits, intermodulation distortion.

TABLE OF CONTENTS :

• CHAPTER 1 INTRODUCTION
  • 1.1 BACKGROUNG TO THE RESEARCH
  • 1.2 RESEARCH PROBLEM AND HYPOTHESIS
  • 1.3 JUSTIFICATION FOR THE RESEARCH
  • 1.4 METHODOLOGY
  • 1.5 OUTLINE OF THE THESIS
  • 1.6 DELIMITATIONS OF THE SCOPE OF THE RESEARCH
  • 1.7 CONTRIBUTION TO THE FIELD
  • 1.8 PUBLICATION LEADING FROM THIS RESEARCH
  • 1.9 CONCLUSION
• CHAPTER 2 LITERATURE REVIEW
  • 2.1 INTRODUCTION
  • 2.2 POWER AMPLIFIERS
    • 2.2.1 PA class of operation
    • 2.2.2 PA topologies
    • 2.2.3 Output power
    • 2.2.4 Power matching
    • 2.2.5 Power efficiency
    • 2.2.6 Modulation schemes
    • 2.2.7 Non-linear phenomena in PA
    • 2.2.8 Harmonic distortion
    • 2.2.9 IM distortion
    • 2.2.10 Gain compression
  • 2.2.11 AM-AM and AM-PM distortion
  • 2.3 NON-LINEARITY COMPONENTS IN SIGE HBT
  • 2.4 SEMICONDUCTOR TECHNOLOGIES
    • 2.4.1 Active devices
    • 2.4.2 Passive devices
    • 2.4.3 Layout and parasitics
  • 2.5 PA MODELLING
    • 2.5.1 Quasi-memory-less non-linear model
    • 2.5.2 Memory effect non-linear model
  • 2.6 REDUCING DISTORTION IN PAS
    • 2.6.1 Predistortion linearisation operation
    • 2.6.2 Types of predistortion
  • 2.7 CONCLUSION
• CHAPTER 3 RESEARCH METHODOLOGY
  • 3.1 INTRODUCTION
  • 3.2 JUSTIFICATION FOR THE PARADIGM AND METHODOGOLY
  • 3.3 OUTLINE OF THE METHODOLOGY
  • 3.4 PA AND APD DESIGN METHODOLOGY
  • 3.5 SIMULATION SOFTWARE
  • 3.6 MANUFACTURING PROCESS
    • 3.6.1 SiGe HBTs
    • 3.6.2 MIM capacitors
    • 3.6.3 TLs
  • 3.7 MEASUREMENT EQUIPMENT
  • 3.8 MEASUREMENT SETUP
  • 3.9 CONCLUSION
• CHAPTER 4 MATHEMATICAL ANALYSIS
  • 4.1 INTRODUCTION
  • 4.2 MATHEMATICAL ANALYSIS
  • 4.3 CONCLUSION
• CHAPTER 5 PA AND APD DESIGN AND RESULTS
  • 5.1 INTRODUCTION
  • 5.2 PA AND VGA
    • 5.2.1 PA and VGA design
    • 5.2.2 Matching networks
    • 5.2.3 Biasing network
    • 5.2.4 Final PA schematic
  • 5.3 POWER DETECTOR
  • 5.4 ADC
    • 5.4.1 Comparator
  • 5.5 CONTROL LOGIC SUBSYSTEMS
    • 5.5.1 XOR gates
    • 5.5.2 Inverters
  • 5.6 DAC
  • 5.7 COMPLETE SYSTEM INTEGRATION
  • 5.8 SIMULATION RESULTS
    • 5.8.1 PA without predistortion
    • 5.8.2 PA with predistortion
    • 5.8.3 IMD3 simulations
  • 5.9 MEASUREMENT RESULTS
    • 5.9.1 DC biasing problem
    • 5.9.2 Future design improvements
  • 5.10 CONCLUSION
• CHAPTER 6 CONCLUSION
  • 6.1 INTRODUCTION
  • 6.2 CRITICAL HYPOTHESIS EVALUATION
  • 6.3 CHALLENGES AND LIMITATIONS
  • 6.4 SUGGESTED FUTURE WORK
  • APPENDIX A: CIRCUIT LAYOUTS

Philosophiae Doctor (Electronic Engineering)

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