A digital polar transmitter for multi-band OFDM Ultra-WideBand

Linear power amplifiers used to implement the Ultra-Wideband standard must be backed off from optimum power efficiency to meet the standard specifications and the power efficiency suffers. The problem of low efficiency can be mitigated by polar modulation. Digital polar architectures have been employed on numerous wireless standards like GSM, EDGE, and WLAN, where the fractional bandwidths achieved are only about 1%, and the power levels achieved are often in the vicinity of 20 dBm. Can the architecture be employed on wireless standards with low-power and high fractional bandwidth requirements and yet achieve good power efficiency? To answer these question, this thesis studies the application of a digital polar transmitter architecture with parallel amplifier stages for UWB. The concept of the digital transmitter is motivated and inspired by three factors. First, unrelenting advances in the CMOS technology in deep-submicron process and the prevalence of low-cost Digital Signal processing have resulted in the realization of higher level of integration using digitally intensive approaches. Furthermore, the architecture is an evolution of polar modulation, which is known for high power efficiency in other wireless applications. Finally, the architecture is operated as a digital-to-analog converter which circumvents the use of converters in conventional transmitters. Modeling and simulation of the system architecture is performed on the Agilent Advanced Design System Ptolemy simulation platform. First, by studying the envelope signal, we found that envelope clipping results in a reduction in the peak-to-average power ratio which in turn improves the error vector magnitude performance (figure of merit for the study). In addition, we have demonstrated that a resolution of three bits suffices for the digital polar transmitter when envelope clipping is performed. Next, this thesis covers a theoretical derivation for the estimate of the error vector magnitude based on the resolution, quantization and phase noise errors. An analysis on the process variations - which result in gain and delay mismatches - for a digital transmitter architecture with four bits ensues. The above studies allow RF designers to estimate the number of bits required and the amount of distortion that can be tolerated in the system. Next, a study on the circuit implementation was conducted. A DPA that comprises 7 parallel RF amplifiers driven by a constant RF phase-modulated signal and 7 cascode transistors (individually connected in series with the bottom amplifiers) digitally controlled by a 3-bit digitized envelope signal to reconstruct the UWB signal at the output. Through the use of NFET models from the IBM 130-nm technology, our simulation reveals that our DPA is able to achieve an EVM of - 22 dB. The DPA simulations have been performed at 3.432 GHz centre frequency with a channel bandwidth of 528 MHz, which translates to a fractional bandwidth of 15.4%. Drain efficiencies of 13.2/19.5/21.0% have been obtained while delivering -1.9/2.5/5.5 dBm of output power and consuming 5/9/17 mW of power. In addition, we performed a yield analysis on the digital polar amplifier, based on unit-weighted and binary-weighted architecture, when gain variations are introduced in all the individual stages. The dynamic element matching method is also introduced for the unit-weighted digital polar transmitter. Monte Carlo simulations reveal that when the gain of the amplifiers are allowed to vary at a mean of 1 with a standard deviation of 0.2, the binary-weighted architecture obtained a yield of 79%, while the yields of the unit-weighted architectures are in the neighbourhood of 95%. Moreover, the dynamic element matching technique demonstrates an improvement in the yield by approximately 3%. Finally, a hardware implementation for this architecture based on software-defined arbitrary waveform generators is studied. In this section, we demonstrate that the error vector magnitude results obtained with a four-stage binary-weighted digital polar transmitter under ideal combining conditions fulfill the European Computer Manufacturers Association requirements. The proposed experimental setup, believed to be the first ever attempted, confirm the feasibility of a digital polar transmitter architecture for Ultra-Wideband. In addition, we propose a number of power combining techniques suitable for the hardware implementation. Spatial power combining, in particular, shows a high potential for the digital polar transmitter architecture. The above studies demonstrate the feasibility of the digital polar architecture with good power efficiency for a wideband wireless standard with low-power and high fractional bandwidth requirements

Ultra Wideband

Ultra wideband (UWB) has advanced and merged as a technology, and many more people are aware of the potential for this exciting technology. The current UWB field is changing rapidly with new techniques and ideas where several issues are involved in developing the systems. Among UWB system design, the UWB RF transceiver and UWB antenna are the key components. Recently, a considerable amount of researches has been devoted to the development of the UWB RF transceiver and antenna for its enabling high data transmission rates and low power consumption. Our book attempts to present current and emerging trends in-research and development of UWB systems as well as future expectations

Digital signal processing for fiber-optic communication systems

As the available bandwidth of optical fibers has been almost fully exploited, Digital Signal Processing (DSP) comes to rescue and is a critical technology underpinning the next generation advanced fiber-optic systems. Literally, it contributes two principal enforcements with respect to information communication. One is the implementation of spectrally-efficient modulation schemes, and the other is the guarantee of the recovery of information from the spectrally-efficient optical signals after channel transmission. The dissertation is dedicated to DSP techniques for the advanced fiber-optic systems. It consists of two main research topics. The first topic is about Fast-orthogonal frequency-division multiplexing (OFDM) — a variant OFDM scheme whose subcarrier spacing is half of that of conventional OFDM. The second one is about Fresnel transform with the derivation of an interesting discrete Fresnel transform (DFnT), and the proposal of orthogonal chirp-division multiplexing (OCDM), which is fundamentally underlain by the Fresnel transform. In the first part, equalization and signal recovery problems result from the halved subcarrier spacing in both double-sideband (DSB) and single-sideband (SSB) modulated Fast-OFDM systems are studied, respectively. By exploiting the relation between the multiplexing kernels of Fast-OFDM systems and Fourier transform, equalization algorithms are proposed for respective Fast-OFDM systems for information recovery. Detailed analysis is also provided. With the proposed algorithms, the DSB Fast-OFDM was experimentally implemented by intensity-modulation and direct detection in the conventional 1.55-μm and the emerging 2-μm fiber-optic systems, and the SSB Fast-OFDM was first implemented in coherent fiber-optic system with a spectral efficiency of 6 bit/s/Hz at 36 Gbps, for the first time. In the second part, Fresnel transform from optical Fresnel diffraction is studied. The discrete Fresnel transform (DFnT) is derived, as an interesting transformation that would be potentially useful for DSP. Its properties are proved. One of the attractive properties, the convolution-preservation property states that the DFnT of a circular convolution of two sequences is equal to the DFnT of either one convolving with the other. One application of DFnT is practically utilized in the proposal of OCDM. In the OCDM system, a large number of orthogonal chirped waveforms are multiplexed for high-speed communication, achieving the maximum spectral efficiency of chirp spread spectrum systems, in the same way as OFDM attains the maximum spectral efficiency of frequency-division multiplexing. Owing to the unique time-frequency properties of chirped waveforms, OCDM outperforms OFDM and single-carrier systems, and is more resilient against the noise effect, especially, when time-domain and frequency-domain distortions are severe. Experiments were carried out to validate the feasibility and advantages of the proposed OCDM systems