AICT 2018 : The Fourteenth Advanced International Conference on Telecommunications Proposal of Power Saving Techniques for Wireless Terminals Using CAZAC-OFDM Scheme Takanobu Onoda, Ryota Ishioka and Masahiro Muraguchi Department of Electrical Engineering, Tokyo University of Science 6-3-1 Niijuku, Katsushika-ku, Tokyo, 125-0051, Japan E-mail: [email protected], [email protected], [email protected] Abstract— A major drawback of Orthogonal Frequency spectral efficiency, and additional power consumption of Division Multiplexing (OFDM) signals is extremely high DSPs. Peak-to-Average Power Ratio (PAPR). Signals with high On the other approach to overcome the problem, some PAPR lead to a lowering of the energy efficiency of the circuit topology for high-efficiency OFDM power amplifier Power Amplifiers (PAs) and the shortened operation time design have been proposed [2]. Here, a polar modulation PA, causes a serious problem in battery-powered wireless or an envelope tracking PA, is the most promising one. In the terminals. In this paper, we propose a new power saving polar modulation, OFDM signal is separated into phase technique for wireless terminals. It is the combination of modulation (PM) component and amplitude modulation a polar modulation technique and Constant Amplitude (AM) one. The PM component is input to the PA as the Zero Auto-Correlation (CAZAC) equalizing technique. quadrature signal with constant amplitude. On the other, the Proposed the polar modulation technique for the PA AM component is used as envelope tracking data, which employs a current control by changing the common-gate supplies to the PA as drain DC biasing from adaptive output stage bias in a cascade amplifier circuit. The CAZAC power supply using a DC-DC converter. In general, the equalization scheme makes the PAPR of M-array efficiency of PAs operating in saturation region is higher than Quadrature Amplitude Modulation (M-QAM) OFDM one in linear region. Therefore, the polar modulation PA signals into the PAPR of M-QAM single-carrier signals. improves energy efficiency under full-time operation in the By using CAZAC-OFDM signal, proposed polar saturation region. modulation PA exhibits overall efficiency of 40% at Error In this paper, we propose a new power saving technique Vector Magnitude (EVM) of -32dB. Furthermore, a for wireless terminals. It is the coupling technique between breakthrough technique which transcends barrier of 50% envelope tracking operation of PAs and CAZAC (Constant efficiency has been proposed. A prototype of Single Ended Amplitude Zero Auto-Correlation) equalizing. Here, we had Push-Pull amplifier (SEPP AMP) exhibits power gain of reported an original polar modulation PA using a cascade 15dB over 100MHz to 1GHz, the maximum efficiency of circuit topology [3]. One can control output power by 65% without polar modulation scheme. changing common-gate stage biasing in accordance with signal envelope, which improves energy efficiency of PA by Keywords-OFDM; CAZAC; polar modulation; SEPP; full-time operation in saturation region. Since the polar modulation is a simple method of separating into amplitude I. INTRODUCTION information and phase information, it can cope with all Orthogonal Frequency Division Multiplex (OFDM) modulation schemes. We also reported that one CAZAC system for high speed and high capacity communications is sequence in cooperation with IFFT signal-process converted recently attracting attention in wireless applications, e.g., the PAPR of the M-QAM OFDM signal into the PAPR of an 3GPP LTE, Wi-Fi and WiMAX. It is well known that one of M-QAM single-carrier signal. Here, this fact was our original the most serious drawbacks of the OFDM system is its high discovery [4]. peak-to-average power ratio (PAPR), which decreases the In this paper, this coupling technology has novelty and energy efficiency of power amplifier (PA) and increases effectiveness in combining the features of CAZAC-OFDM transmitter power consumption. In mobile communications, which is constant amplitude and the characteristics of this PA high PAPR signal negatively affects device battery life. which is power saving, where a more power-saving system To overcome the above problem, many techniques are can be created. The coupling technique reduces the PAPR of proposed: Partial Transmit Sequence (PTS), Selected 5 dB at the 16QAM-OFDM signal while the system imposed Mapping (SLM), etc. [1]. PTS and SLM techniques choose no penalties on the BER performances. By using CAZAC- respectively the phase factor and candidate data block to OFDM signal, proposed polar modulation PA exhibits overall minimize the PAPR of transmission signal, which improves efficiency of 40% or more at error vector magnitude (EVM) PAPR performance. However, those techniques need side- of -32dB which satisfies the requirement of IEEE 802.11 ac information in the receiver side, i.e., phase factor and specification [5]. candidate number information, in order to demodulate the Moreover, a breakthrough technique, which transcends received signal correctly, which result in degradation of barrier of 50% efficiency has been proposed. As well-known, a Class-B amplifier allows operating at a power efficiency of Copyright (c) IARIA, 2018. ISBN: 978-1-61208-650-7 115 AICT 2018 : The Fourteenth Advanced International Conference on Telecommunications 2 78.5%, while a Class-A amplifier operates at a power max |푥푛| efficiency of 50%. The Class-B amplifiers, however, are 0≤푛≤푁−1 (3) 푃퐴푃푅 = 2 , strongly difficult to achieve high frequency and high linearity E[|푥푛| ] operations at the same time. where E[∙] is expectation operator. PAPR represents Our proposal of new circuit topology is certain kind of amplitude fluctuation of each symbol. In order to improve the single ended push-pull amplifier, or SEPP AMP, using a accuracy of PAPR, the OFDM signal 푥푘 is converted to 퐿- complementary MOSFET technology. The circuit topology is times oversampled time domain signal [1]. suitable for monolithic circuit configuration and easily As shown from (2), the OFDM signal is composed of a applied commercially available process of CMOS foundries. plurality of subcarrier signals, which causes an increase in A prototype of the SEPP AMP exhibits power gain of 15dB amplitude fluctuation. A high PAPR signal increases the Input over 100MHz to 1GHz and the maximum efficiency of 65% Back Off (IBO) at the power amplifier in order to amplify the with no use of polar modulation scheme. transmit signal without distortion. In general, increasing in Up to now we have shown that the polar modulation IBO causes decreasing the efficiency of PA. system is effective with various modulation schemes [3][4]. B. CAZAC equalizing technique On the other hand, the contribution of this paper is to demonstrate the achievement of unprecedented power CAZAC sequence is constant amplitude and provides a efficiency by combining CAZAC-OFDM which achieved the good cross-correlation property. Therefore, CAZAC same PAPR as the single carrier in the world and power- sequence is used in wireless communication systems such as saving polar modulation system. channel estimation and time synchronization. The Zadoff- This paper is organized as follows. SectionⅡ presents the Chu sequence 푐푘 which is one of the CAZAC sequences is represented as proposed the operating principle of the coupling technique 푗휋푘2/푁2 between envelope tracking operation of PAs and CAZAC. In 푐푘 = 푒 , (4) 2 SectionⅢ, a performance of proposed system is presented. In where 푘 = 0, 1, … , 푁 − 1 denotes the sequence index. In Section Ⅳ, the operating principle of further improvement of this paper, CAZAC 푁 × 푁 matrix 푴 is represented as OFDM-PAs with SEPP and simulation results are given. 푐0 푐1 ⋯ 푐푁−1 Finally, SectionⅤ concludes this paper. 푐푁 푐푁+1 … 푐2푁−1 푴 = [ ⋮ ⋮ ⋱ ⋮ ]. (5) II. PROPOSED SYSTEM 푐(푁−1)푁 푐(푁−1)푁+1 … 푐푁2−1 In the proposed system, CAZAC-OFDM and polar In CAZAC-OFDM system, multiply the signal with M modulation are used together to improve energy efficiency. before the IFFT of the transmitter. frequency domain symbol ′ ′ ′ 푇 Below, we explain CAZAC-OFDM and polar modulation 푿′ = [푋0, 푋1, … , 푋푁−1] is represented as and propose a system with improved efficiency. A 푿′ = 푴푿. (6) description of OFDM system in section A, CAZAC Therefore, the CAZAC-OFDM time signal x′ is equalizing technique in section B, and polar modulation represented as technique in section C are shown. 푁−1 푗2휋푘푛 A. OFDM system 푥′ = ∑ 푋′ 푒 푁 푛 푘 (7) In OFDM system, the frequency domain symbol 푿 = 푘=0 푇 = 푁 ∙ 푐 ∙ 푋( ) . [푋0, 푋1, … , 푋푁−1] is modulated by 푁 size inverse Fast (푁/2−푛)mod 푁 푁/2−푛 mod 푁 Fourier Transform (IFFT). The discrete-time OFDM signal Receiver side can demodulate the original frequency with 푁 subcarriers is represented as domain symbol with using conjugate 푴퐻 [4]. 푁−1 Figure 1 shows PAPR performance of OFDM, CAZAC- 푗2휋푘푛/푁 푥푛 = ∑ 푋푘푒 , (1) OFDM and single carriers with using complementary 푘=0 cumulative distribution function (CCDF). Each signal has 64 subcarriers and oversampling factor 퐿 = 4. As shown from where and is discrete time index. On the other 푗 = √−1 푛 Figure 1, CAZAC equalization improves PAPR performance hand, receiver acquires frequency domain symbol by 풀 about 2.5 dB of PAPR when CCDF value is 10−3. In addition, applying FFT to received signal 풚. the PAPR of CAZAC-OFDM and M-QAM signal is same 푁−1 performance, which results from (7). −푗2휋푘푛/푁 푌푘 = ∑ 푦푛푒 C. Polar modulation technique 푛=0 (2) 푁−1 In polar modulation system, AM and PM components are −푗2휋푘푛/푁 = ∑(푥푛 + 푁표푖푠푒)푒 . input separately into PA as power voltage and quadrature 푛=0 modulation signal respectively. General polar modulation The PAPR of the OFDM signal (1) can be expressed as system supplies AM component into PA by using dc-dc Copyright (c) IARIA, 2018.