Gao - Energy and Bandwidth-Efficient Wireless Transmission

Springer International Publishing Switzerland 2017

Wei Gao 1

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One of the primary and long-standing goals of the Federal Communications Commission (FCC) has been to promote more efficient use of spectrum for the radiofrequency (RF) wireless radio transmission. The FCCs 1999 Spectrum Policy Statement highlighted that with increased demand of a finite supply of spectrum, the Commissions spectrum management activities must focus on allowing spectrum markets to become more efficient; and the Strategic Plan for Fiscal Year 20032008 (published in 2002) indicated its general spectrum management goal was to encourage the highest and best use of spectrum []. Even though there are other, different metrics, one of the accepted common spectrum efficiency metrics for wireless communications is in terms of bits / second / hertz . To approach this goal, many government organizations and commercial communications companies have adopted advanced modulation and amplification transmission techniques to utilize spectrum more efficiently and as much as possible.

Wireless technology is transforming our society and peoples lifestyles, with over six billion people communicating over cellular networks and over a billion people using Wi-Fi internet globally in 2014 []. Total mobile subscriptions are expected to grow to over nine billion by the end of 2019. Cellular systems of 3G and 4G provide wide coverage areas, full mobility and roaming, but traditionally offer relatively low bandwidth connectivity. On the other hand, Wireless Local Area Networks (WLANs) provide high data rates at low cost, but only within a limited area. Most smart cellular phones can automatically be connected to Wi-Fi networks from cellular data connections when they are located within hot-spot areas. Therefore, WLANs have come to be relied on as a suitable complementary technology to the existing cellular radio access networks. The addition of more and more new mobile and Wi-Fi subscribers could result in congestion in the limited radio spectrum. Hence, highly efficient use of spectrum becomes particularly crucial and important in wireless communication systems. One effective approach to high spectral efficiency is to use some advanced modulations that have been demonstrated in both academia and industry to reduce spectrum congestion.

In addition to spectrum efficiency, energy efficiency, also called power efficiency, is another high-priority desired performance factor in wireless communication systems. For a power amplifier in the transmission system, a main priority is energy efficiency, which refers to how effectively the amplifier input energy is converted to the desired output signal []: (1) resource allocations, (2) network planning and development, (3) energy harvesting and transfer, and (4) hardware solutions. Hardware solutions primarily focus on increasing the energy efficiency of power amplifiers (PAs) through either direct PA design architectures or modulation signal design techniques that aim at either constant envelope characteristics or pre-distortion (PD)-based linearization methods. In our discussion of hardware solutions, only modulation signal design techniques, however, will be covered and discussed in more detail.

There is no one modulation technique that possesses both the best spectrum efficiency and the best energy efficiency. Hence, a trade-off involving optimization between them should be made, depending on actual applications. In addition, in order to improve energy efficiency, polar transmitter [] have been developed around the last decade and some significant progress has been achieved in the recent years.

In classic polar transmitters, the input RF modulated signal to a power amplifier is first split into an envelope signal through an envelope detector and a phase-modulated signal via a limiter; then, the resulting low-frequency envelope signal is amplified by a low-frequency amplifier to control the supply voltage regulator of the final PA. The phase-modulated signal on the RF path to the input of the PA can be amplified by a highly efficient nonlinear PA. Finally, the amplified envelope signal and phase-modulated signal are combined inside the PA to restore the original RF-modulated signal as much as possible.

The envelope-tracking (ET) technique requires the supply voltage of the PA to be dynamically varied with the instantaneous value of the input RF signal envelope while the input to the PA is an original RF modulated signal. Compared with traditional fixed DC supply voltage, which has a significant amount of power loss as heat, in the envelope-tracking technique the RF signal envelope modulates the supply voltage of the PA to track the envelope of the input RF signal to reduce the amount of power dissipated as heat []. As a result, high energy efficiency is achieved because of reduced DC power consumption. Any nonlinear distortion caused by modulating the supply voltage of the PA with the envelope signal can be compensated for using the pre-distortion method on the envelope path.

Compared with pre-distortion (PD)-based linearization techniques, which have been developed for several decades and have been widely used in wireless communication systems, the requirements of the envelope amplifier or modulator for polar transmitters are very stringent because they are responsible for amplifying the envelope signal and then precisely combining it with the phase-modulated signal inside the PA to regenerate the originally modulated signal at the output of the PA. With regard to the ET technique, even though some improvements in power supply modulation have been made experimentally in the past years, it still has a long way to go before this technique can actually be applied due to the limitations of accuracy and bandwidth in practical implementations. In addition, the pre-distortion method is still needed to compensate for nonlinear distortion in either the polar transmitters or ET-based transmitters. Hence, the PD technique is a fundamental and necessary approach to the linearization of the PA.

This book mainly focuses on the most recent advances in both energy and bandwidth-efficient modulation and transmission techniques, especially as they are applied to cellular and WLAN transceivers. Due to the portable properties of these products, besides the requirements of energy and spectrum efficiency, low cost is also highly desirable as a third priority in these applications. Starting with some fundamental modulation properties, and then moving to energy and bandwidth-efficient overlapped raised-cosine pulse-shaping modulations and the most bandwidth-efficient modulation formats with intersymbol interference (ISI)-free Nyquist pulse shaping which are suitable to nonlinear and linear amplification channels, respectively, the discussion will cover the two most energy and bandwidth-efficient modulation types of techniques, or constant envelope modulations and nearly constant envelope modulations. In order to achieve highly efficient transmission strategies for non-constant envelope modulation signals, especially for those having larger peak-to-average power ratio (PAPR) values, a pre-distortion linearization technique of the power amplifier will be addressed in detail. In the linearization technique, a simple approximation to both memory effects and nonlinear behavior of a PA with the Volterra polynomial model will be first presented in the complex baseband domain, and then a pre-distorter as an approximate inverse of the PA will be introduced and analyzed by using the Volterra polynomial model as well. After that, modeling approximation analysis, performance simulation results, and pre-distortion implementations with both analog and digital realizations will be addressed. Finally, from a system-design point of view, RF transceivers for wireless communication systems will be discussed in detail, including most common design issues or challenges that RFIC designers may face and drawbacks or disadvantages that some certain architectures may have. Included in the discussion will be several commercial transceiver products that utilize highly efficient modulation schemes in cellular and WLAN applications so that reader can gain actual design strategies and ideas from these applications.