T2B.2 A Novel Wideband Digital Power Amplifier and Transmitter Architecture for Multimode Handsets1 Pierce Nagle, Member, IEEE, Radwan M. Husseini, Member, IEEE, Andrei Grebennikov, Member, IEEE, Walid K. M. Ahmed2, Senior Member, IEEE, and Finbarr McGrath, Member, IEEE. Abstract — This paper introduces a novel power amplifier architecture: “The wide-band Digital Power Amplifier (DPA)”. This device provides RF power amplification, phase/amplitude combining, and wideband amplitude modulation. The DPA is complemented with a Digital Modulator to realize a compelling Digital Transmitter (DTx) architecture. The basic functionality and advantages of the DPA and transmitter are presented. In addition, measured results for CDMA2000 are provided to demonstrate the performance capabilities of the DPA/DTx.
I. INTRODUCTION Emerging 2.5G, 3G and WLAN standards have adopted linear modulation schemes, forcing transmitters to maintain linearity throughout the lineup. These wide bandwidth transmitter products have both aggressive power efficiency and low cost requirements that are challenging the limits of today’s architectures. To date, the preferred choice for these transmitters has been the analog I/Q up-conversion architecture. Preserving the signal quality and linearity throughout the I/Q transmitter requires analog circuits that drain significant current, resulting in reduced power efficiency. Furthermore, such architectures fail to meet requirements for lower cost, smaller form factor, and multimode operation due to the plurality of band-specific components required such as driver amplifiers, off-chip SAW filters and linear power amplifiers. In recognizing the limitations of traditional I/Q architectures, it has become important to investigate the potential for polar-based transmitter architectures as an alternative [1]. This has motivated the work presented in this paper, which has resulted in the development of the polar-based Digital Power Amplifier (DPA) device and its associated Digital Transmitter (DTx) architecture, which are presented in this paper. This paper commences with an introduction to the DTx architecture. Throughout the paper, comparisons are drawn against state of the art I/Q-based, analog direct upconversion architectures [2]. We also present key innovations that have enabled the DPA and the DTx to not only overcome the shortcomings of traditional power amplifiers and I/Q architectures but also to deliver
compelling advantages in terms of cost, performance and multi-mode capabilities. Finally, the paper concludes with a summary of measured results of the DPA. II.
THE DIGITAL TRANSMITTER ARCHITECTURE
The Digital Transmitter takes an alternative approach to signal processing whereby the baseband signal is split into its digital amplitude and phase components. Figure 1 depicts a high level abstraction for this architecture, consisting of two ICs: the Digital Modulator (DM) IC and the Digital Power Amplifier (DPA) IC. This paper is focused on the DPA IC. Reference to the DM IC only serves to enhance the understanding of the operation of the DPA as part of the overall transmitter architecture.
DTX
Digital Modulator
Base - band
Amplitude
I
Digital Power Amplifier Q
Phase
Figure 1:The Novel M/A-COM Digital Transmitter Architecture.
The DM transforms digital I/Q baseband signals from the Cartesian domain to the polar domain. The phase and amplitude information signals are then applied separately to the DPA. Compared to IQ-based architectures, the DM replaces the combination of the analog modulator, the VGAs, and the SAW filters, while the DPA replaces all of the band-specific power amplifiers. The key advantages of the M/A-COM digital transmitter architecture can be presented as follows: 1. DTx does not utilize I/Q modulators: The amplitude and phase components of the signal are processed independently. The digitized amplitude is
1 All authors are with M/A-COM, Tyco Electronics; Pierce Nagle and Andrei Grebennikov are in Cork, Ireland and Radwan Husseini, Walid Ahmed, Finbarr McGrath are in Somerset, NJ, U.S.A. 2 Corresponding author. E-mail:
[email protected]
0-7803-8451-2/04/$20.00 ©2004 IEEE
171
2.
3.
4.
5.
conditioned in baseband up to the reconstruction stage. The digitized phase signal is up-converted to RF using an efficient and programmable wideband phase modulator. DTx enables efficient processing of the phasemodulated RF carrier: In the polar domain, the phase-modulated RF carrier is constant-envelope. This characteristic allows efficient nonlinear processing to be applied throughout the phase path until the signal is recombined with the amplitude in the last stage of the DPA. This highly compressed signal characteristic is exploited in the design of the DM and driver stages of the DPA. DTx does not utilize SAW filters: As Figure 1 shows, no SAW filters are used in this architecture. The total wideband noise density is, in fact, attributed to the wideband noise contributions of the phase and amplitude paths. Hence, it is necessary to suppress the noise on both paths in order to achieve satisfactory wide-band noise levels. Noise suppression on the amplitude path has been implemented in digital baseband using a novel, simple and efficient M/A-COM proprietary signal processing technique that has resulted in over 35dB of noise suppression on the amplitude path, without altering the digital nature of the DPA. On the phase path, the DM has been designed to deliver a wideband phase noise of the order of –165dBc/Hz. This performance, in conjunction with capabilities of the driver stages of the DPA, has eliminated the need for SAW filters that are commonly used to suppress receive band noise in I/Q architectures. DTx is configurable for multiband/multimode operation: The DTx is tunable through an off-channel synthesizer to support different bands. There is no band-specific hardware and therefore it can be tasked for different frequency bands and allow for different modes of operation by simply re-configuring frequency, clocks, and filtering coefficients to support the desired standards. The potential for the Digital Transmitter architecture is captured in Figure 2. The DTx can serve as a multiband/multimode transmitter for products supporting dualband CDMA2000 as well as potentially other combinations such as GSM, EDGE. Furthermore, the DPA can operate efficiently and linearly in several bands simultaneously! DTx enables digital processing algorithms to optimize the efficiency of the DPA and mitigate AMAM and AM-PM distortions: Recombination of the digital amplitude with the phase-carrying RF carrier occurs at the last stage of the DPA, enabling the utilization of efficient signal processing algorithms to tightly control the power during the reconstruction of
6.
the signal and to improve the efficiency of the DPA. More details on this issue are covered in the digital optimization part of the next section. DTx does not require linear power amplifiers: DTx utilizes the novel DPA device, which will be discussed in more detail in the following section. D I P/ D U P/ S W I T C H P L E X
Multiband & Multimode Receiver Dualband CDMA2k Dualband CDMA2k+GSM WCDMA+GSM ……. BB
WiNova Digital Transmitter Digital Transmitter
Up_Cnv & PA DA PM AGC
WiNova TxPLL BB
Digital I
Digital Q
Figure 2: M/A-COM DTx for Multimode Products.
III.
THE DIGITAL POWER AMPLIFIER
A. DESCRIPTION The Digital Power Amplifier combines efficient multiband RF power amplification, efficient power control, wideband amplitude modulation, and phase/amplitude reconstruction in a single chip. This clearly represents a ground-breaking level of functional integration. The DPA consists of novel circuit approaches that convert digital amplitude to modulated RF waveforms at the desired RF transmit power. Within the DPA, the digitized envelope and phase modulated RF carrier are combined to produce a linear and efficient high power RF transmission. B. OPERATION Standard Power Amplifiers are generally biased into class-AB operation as a compromise between the opposing requirements of linearity and efficiency. The PA’s driver circuitry must also have very good linearity to minimize added distortion but this results in poor efficiency. The DPA overcomes such limitations and constraints. Through a novel circuit design technique, each bit of the digital envelope is transformed into a precise and instantaneous RF signal component in the DPA. The individual RF signal components are then properly combined to achieve the desired linear wideband amplitude modulation. Furthermore, the linearity of the DPA is not dependent on the linearity of the amplifying devices, as is the case with conventional power amplifiers. Rather, the amplifier’s linearity is only dependent on how
172
linear the combining of the individual RF signal components is. The Digital Power Amplifier is biased into non-linear, yet efficient, modes of RF operation (such as Class-B or Class-C) without compromising the linearity of the final transmission. This simultaneous retention of linearity and efficiency is a unique and fundamental benefit of the Digital Power Amplifier. Furthermore, since the Digital Transmitter processes a constant envelope RF carrier, the driver circuitry to the DPA can also be operated non-linearly, offering improved efficiency. Such efficiency enhancement is maintained over the entire power control range. Furthermore, the DPA design facilitates operation over a broad RF frequency range, enabling multi-band operation in one single chip, compared to multiple chips that are typically needed with conventional linear PA designs. C. CHARACTERISTICS When used in conjunction with the Digital Transmitter, the Digital Power Amplifier offers several advantages: 1. The DPA is capable of wideband amplitude modulation. Amplitude bandwidths associated with the major modulation schemes are readily accommodated (EDGE, CDMA, WCDMA) 2. Performing amplitude modulation at the last stage of the DPA gives reduced current drain over the transmit power control range. The final stage of the DPA is biased into Class-B or Class-C operation without compromising linearity and there is minimal quiescent current drain (
