Energy and Bandwidth-Efficient Wireless Transmission

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 FCC’s 1999 Spectrum Policy Statement highlighted that “with increased demand of a finite supply of spectrum, the Commission’s spectrum management activities must focus on allowing spectrum markets to become more efficient”; and the Strategic Plan for Fiscal Year 2003–2008 (published in 2002) indicated its general spectrum management goal was to “encourage the highest and best use of spectrum” [1]. 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.

In a wide variety of communication systems, modulation as a fundamental technique plays a very important role in data transmission through air within a specified spectral bandwidth. A modulation signal, which is usually represented by a low-frequency baseband signal and is commonly referred to as an information-bearing signal, varies or modulates one of three parameters: amplitude, phase, and frequency of the radiofrequency (RF) carrier signal such that the baseband signal is carried by a varied parameter of the carrier signal through atmosphere propagation to the destination. Why does the information-bearing signal need to modulate a high-frequency carrier signal for this transmission? This is necessary because the size of the antenna used to radiate the signal to free space depends on the wavelength λ of the transmitted signal. The wavelength λ is equal to c/f, where c is the speed of light and equals 3 × 10⁸ m/s, and f is the frequency of the transmitted signal. For cellular communication systems, antennas are typically λ/4 in size [1]. If a baseband signal with a frequency of 15 kHz were to be transmitted through an antenna without modulating a carrier signal, the size of the antenna would be λ/4 = 5,000 m. However, it is only 8 cm if a carrier signal with a frequency of 900 MHz is modulated by such a baseband signal. For this reason, a high-frequency signal usually called a carrier signal is needed for all wireless communication systems to carry the modulation baseband signal. Thus, modulation is a necessary process in all wireless communication systems. Through this book, we mainly consider either the phase modulation scheme or a combination of an amplitude and phase modulation scheme such as M-QAM for its simple implementation and robust performance. In the phase modulation scheme, the information-bearing baseband signal is used to change the phase of a sinusoidal carrier signal whenever the polarity of the baseband signals changes. The phase change of the carrier signal, indicating either 1 or 0, is carried by the carrier signal.

An orthogonal frequency division multiplexing (OFDM) technique has been developed for wideband data transmission through multipath fading channels without the need for complex equalizers. The concept of OFDM dates back to the 1960s, when Chang [1] first proposed the synthesis of orthogonal signals for multichannel data transmission in 1968. Wideband transmission systems are more vulnerable to multipath fading because the fading notches have a higher chance of dropping into the transmission bandwidth. As its name implies, OFDM is a scheme of splitting a single data sequence at a high bit rate into many parallel sub-data streams at a low symbol rate to conventionally modulate orthogonal subcarriers in order to space these subcarriers close together in a certain bandwidth. OFDM has continuously developed into a very popular scheme for wideband digital communication systems, such as 802.11a/g/n/ac-based wireless local area networks (WLANs), digital television, audio broadcasting, and 4G mobile LTE communication standards.

In addition to the requirement of high spectral efficiency, power (or energy) efficiency—equivalent to battery life—is another important requirement for modulation techniques. In some applications, such as mobile handset devices, portable devices, and even satellite communication equipment, energy efficiency is crucial to achieve longer battery life or longer communication time. In these applications, to maintain minimum DC power consumption by power amplifiers, the power amplifier must operate in or close to the saturation region to maximize energy efficiency or minimize DC power consumption because the minimum DC current consumption occurs in a saturation region. However, a saturated amplifier introduces amplitude modulation to amplitude modulation (AM/AM) and amplitude modulation to phase modulation (AM/PM) conversions into the amplified signal, which is usually the amplitude- and phase-modulated signal. If such an input signal to a power amplifier that operates in or close to a saturated condition is a non-constant envelope modulation signal, its output will be affected by the AM/AM and AM/PM conversions. As a result, a nonlinearly amplified signal at the output of the power amplifier is affected by spectrum regrowth such that its output signal cannot meet the required spectrum mask or adjacent channel power ratio (ACPR) imposed by different standards and its error vector magnitude (EVM) is degraded as well. Thus, requirements of both energy efficiency and ACPR or spectrum efficiency impose constant or nearly constant envelope characteristics on the modulated signal to the power amplifier.

For both spectrally efficient transmission and robustly combating multipath fading transmission, orthogonal frequency division multiplexing (OFDM) with high-order modulation formats of M-ary QAM adopted by many wireless standards are widely used in modern wireless communication systems. These kinds of modulated signals naturally behave with non-constant envelope characteristics and have a very high peak-to-average power ratio (PAPR), which demands that radio frequency (RF) transmitter power amplifiers (PAs) operate with high back-off power from their P1dB point to perform linearly, resulting, however, in very low power or energy efficiency. If its back-off power is not high enough, or if it operates close to its P1dB point, a PA causes spectral regrowth, which leads to adjacent channel interference. Meanwhile, it also causes in-band signal distortion, which degrades the bit error rate (BER) performance. It is commonly known that achieving spectrum efficiency is controversial to having energy efficiency. To obtain high energy efficiency to reduce severe distortion caused by PA non-linearity, pre-distortion or linearization techniques have been widely used in applications. Generally, it is classified as digital baseband or RF analog pre-distortion, according to the operating domain.

In the previous chapters, we described the three major subsystems of modulation, demodulation, and power amplification in wireless communication systems. Starting with Chap. 6, we move to a top level, or the transceiver architecture that includes other functional blocks besides these three subsystems to achieve complete transmission and reception. In general, there are three types of common transmit and receive architectures available to the wireless radio frequency (RF) IC transceiver architect: superheterodyne, low (IF), and direct conversion, also known as zero-IF. Each of these architectures has its own advantages and disadvantages, and some of the potential issues related to the particular architecture can be minimized with either careful circuit design techniques or calibration methods. We describe and analyze what advantages and disadvantages these architectures have in practical applications and what challenges RF IC designers may face in their designs, and we discuss some of these design techniques and calibration methods in more detail in the subsequent chapters.

Similar to transmitter architectures, receiver architectures are mainly classified into three types: a heterodyne (or high intermediate frequency [IF]) receiver, a low-IF receiver, and a direct-down conversion (or zero-IF) receiver. In a low-IF receiver, the signal is mixed down to a low, but non-zero, IF that is usually set to one or two times the channel-spacing frequency and is compatible with the bandwidth of the desired signal. To further suppress the adjacent channel interferers, the low-IF may be set to half the channel-spacing frequency such that the image signal is the adjacent channel. Because of some advantages over the zero-IF receiver, such as being insensitive to DC offset and lessening the impact of the flicker noise, the low-IF receiver has been adopted by many IC design companies in the design of the radio frequency (RF) transceivers.

So far, we have described some main system design ideas and technologies used in wireless mobile and 802.11 RF transceivers. We have also discussed radio frequency (RF) analog impairments that can affect system performance, and introduced various calibration techniques that can minimize analog impairments. In this section, we introduce several practical transceiver products that recently have been or currently are available in wireless markets. The purpose of this section is to provide readers with an overview about how the actual IC products perform and how the described technologies’ advantages are applied to these products. The products chosen are among the most popular and advanced technologies used in wireless RF transceivers.