A Power-Efficient Implementation of In-Band Full-Duplex Communication System (ReflectFX)

Transcription

1 016 International Symposium on Signal, Image, Video and Communications (ISIVC) A Power-Efficient Implementation of In-Band Full-Duplex Communication System (ReflectFX) Seiran Khaledian, Farhad Farzami, Besma Smida and Danilo Erricolo Department of Electrical and Computer Engineering University of Illinois at Chicago skhale6@uic.edu ffarza@uic.edu smida@uic.edu derric1@uic.edu Abstract We propose a power-efficient implementation for ReflectFX, an in-band full-duplex wireless communication system. ReflectFX is based on backscatter modulation where the electromagnetic waves are modulated and reflected by the same antenna that receives them. With ReflectFX, the end-user receives self-interference free signals. At the end-user receiver, we use a bi-directional amplifier to amplify the reflected wave while providing sufficient power at the end-user demodulator. The bidirectional amplifier consists of two identical reflection amplifier using negative resistance and a 90, -3dB branch line coupler (BLC). A phase shifter circuit, which consists of four microstrip transmission lines, has been used to apply QPSK modulation to the reflected power. The designed ReflectFX end-user circuit, with power consumption of only 90μW, provides 17 db and 16.8 db reflection and transfer gains, respectively. Index Terms Full-duplex communication, backscatter modulation, negative resistance, low-power reflection amplifier, tunneldiode, bidirectional amplifier. I. INTRODUCTION The fact that half-duplex communication systems cannot transmit and receive signal at the same time and same radio frequency carrier has imposed constraints on the efficiency and flexibility of today s communication systems. Given the ever-increasing demand for high-speed data services, modern wireless networks will increasingly require more efficient strategies for usage of the available frequency spectrum. Full-duplex communication systems, on the contrary, allow nodes to transmit and receive signals at the same time and same frequency band and can potentially double the spectrum efficiency of wireless communication systems. Many previous works designed and implemented full-duplex communication systems during the last decade [1] [7], [7], [8], [8] [13]. To date, all of the proposed full-duplex systems are predicated upon the principle that the transmitter and the receiver must generate their own radio-carrier waves independently. In practice, this approach results in a high level of self-interference imposed on the receiver circuit, from the transmitter antenna. Consequently, many self-interference cancellation techniques have been proposed. They can be divided into three categories: 1) passive cancellation (antenna cancellation); ) analogue cancellation (RF cancellation); and 3) digital cancellation. Contrary to the previous full-duplex systems in which both nodes (base-station and end-user) suffer from selfinterference, a new framework to implement a full-duplex system by means of reflection power (ReflectFX) has been proposed in [14]. Within this framework the end-user receives a signal free of self-interference. ReflectFX is based on the reflection of electromagnetic wave between the antenna and its load interface. The reflected wave is then modulated by changing its amplitude and phase [15]. Reflection modulation has been used widely in radio frequency identification (RFID) [16] [18] but only in [14] for full-duplex communication system. This paper is an extension of our previous work in [14]. We propose here a power-efficient ReflectFX circuit implementation at the end-user. The ReflectFX circuit should satisfy the following two main criteria. First, the ReflectFX circuit needs to amplify the reflected signal to improve the coverage range because the end-user doesn t generate its own carrier wave. In addition, since the end-user load is not conjugate matched to its antenna impedance, the power available to the end-user demodulator is smaller than P match (i.e., the power available to demodulator when its impedance is conjugate matched with antenna). This leads to down-link (base-station to end-user) performance degradation. To guarantee no loss in the downlink performance, the ReflectFX circuit should at least provide P match at the demodulator circuit. In this paper, we propose an end-user ReflectFX circuit composed of three components: 1) a 90, -3dB branch line coupler (BLC) ; ) two identical reflection amplifiers ; and 3) a phase shifter circuit. The proposed ReflectFX circuit has been investigated analytically and simulated using Agilent ADS EM Circuit Cosimulation software. II. REFLECTFX COMMUNICATION SYSTEMS A. System model The ReflectFX communication system, illustrated in Fig.1, includes two nodes (base-station and end-user) that wish to exchange data over wireless channel. The end-user and basestation nodes transmit data at the same time over the same frequency band. Similar to conventional full-duplex systems, the base-station suffers from self-interference imposed on its receiver from the transmitter. At the end-user there is no /16/$ IEEE 4 ISIVC 016

2 individual source for the transmitter since reflected power modulation is used, which leads to a self-interference free reception at the end-user. III. REFLECTFX END-USER CIRCUIT IMPLEMENTATION In this section, we propose a practical and power-efficient implementation of the ReflectFX circuit shown in Fig.. It includes a phase shifter and a bi-directional amplifier. The bi-directional amplifier consists of a BLC and two identical reflection amplifiers named RA1 and RA. Fig. 1. Full-duplex ReflectFX communication system (SI accounts for selfinterference). B. Preliminaires Based on ReflectFX framework, the received symbol at the end-user (i-th symbol) is: y m (i) = ξ B m (Z i )h B m (i)x B (i)+n m (i), (1) where ξ B m (Z i )=( λ 4πd ) G Bt G m p Bm (1 Γ(Z i ) ) is the average power attenuation from the base-station transmitter to the end-user receiver 1, λ is the wavelength, d is the distance between base-station antenna and end-user antenna, G Bt and G m are the base-station transmitter and the end-user antenna gains, p Bm accounts for polarization mismatch at the end-user antenna, h B m (i) models the frequency non-selective fading channel, x B (i) denotes the i-th symbol transmitted from the base-station, n m (i) is the AWGN noise at end-user and Γ(Z i )= Z i Z0, () Z i + Z 0 where Z 0 is the characteristic impedance of the antenna (usually 50Ω) and Z i is the receiver load. The backscattered signal x m (i) =x B (i t)(a st Γ(Z i )), becomes an analog network-coded combination of the messages sent by the base-station and the end-user (modulated by Z i ), A st is the structure-scattering factor, and t accounts for the propagation delay between the base-station and the enduser [14]. Note that both the received symbol y m (i) and the backscattered signal x m (i) varies with the receiver load Z i at the i-th symbol. Since the load is changing with the data, the receiving antenna cannot be conjugate-matched to the load and hence the received power may be diminished in comparison to conventional communication. In addition, ReflectFX circuits need to amplify the reflected signal to improve the coverage range because the end-user doesn t generate its own carrier wave. Therefore, we used negative resistance in the load to increase the received and backscatter power. 1 Note that in conventional communications, the load is matched to the antenna and hence Γ(Z 0 )=0. Fig.. ReflectFX circuit schematic in end-user node. A. Reflection amplifier A reflection amplifier (RA) is a one port circuit with negative input resistance Re(Z i ) < 0, so that when it is illuminated by an electromagnetic field, the reflection coefficient is bigger than one Γ(Z i ) > 1 and the reflected wave is amplified. Two types of reflection amplifier have been discussed in the literature. The first group used transistors to provide negative resistance [19] []. The second group used tunnel diodes as negative resistance circuit [3], [4]. In this work we used tunnel diodes due to their low DC power consumption. A tunnel diode is a p-n junction semiconductor using tunneling effect to exhibit negative resistance. When the applied voltage on a tunnel diode is increased, the current through it decreases. Tunnel diodes show negative input resistance in specific DC voltage bias range. In [3] a tunnel diode model of MBD5057- E8 has been used in a reflection amplifier circuit. We used the same tunnel diode model in this work. Fig. 3 shows the IV curve of the tunnel diode [3]. We designed a DC bias circuit to supply the tunnel diode and separate DC from AC signals. An impedance matching circuit is also required to achieve the desired gain in the desired frequency band while preventing possible oscillations. Oscillations happen when Im(Z i + Z 0 )=0and Re(Z i + Z 0 ) 0 [5]. Fig. 4 shows the schematic of tunnel diode based reflection amplifier performance in 5.8 GHz frequency band. The parameters value derived are listed in Table I. The simulation results for the reflection amplifier S 11 (reflection gain) and input impedance (real part and imaginary part) are shown in Fig. 5. For the reflection gain of 16.9 db at 5.8 GHz, a DC bias power of 45 μw is consumed. B. Bi-directional amplifier In this section, we describe the bidirectional amplifier configuration inspired by our previous work in [6] at 915 MHz. Because of the limited space, we will only describe the main features. Two identical one-port reflection amplifiers are 43

3 parameter value parameter value parameter value l 1 (mm) 0.7 l (mm) 0.76 l 3 (mm).8 l 4 (mm) 7.1 l 5 (mm) 1.7 C 1 (pf) 400 C (pf) 6.8 C 3 (pf) 0.3 L 1 (nh) 1 TABLE I REFLECTION AMPLIFIER PARAMETER VALUES Fig. 3. IV characteristic curve of a tunnel diode (model: MBD5057-E8). where G = S 11 (RA) is gain of the reflection amplifier. The simulation results for the bi-directional amplifier of Fig. 6 for the demodulator impedance of 35 Ω is shown at Fig. 7, where S 11 is the reflection gain from port 1 and S 41 is transfer gain from port 1 to the receiver port 4. Fig. 4. Reflection amplifier schematic. TD accounts for tunnel diode model MBD5057-E8. l 1 l 5 are microstrip transmission lines with width of w = 1.81 mm and length indicated in table I. V DC =0.08 V and substrate with permitivity of ε r =3.66 (FR4) and thickness of 0.17 mm and tan δ =0.01 is used. Fig. 6. Bi-directional amplifier schematic. Z 0 =50Ω. Fig. 5. The reflection amplifier S 11, and input impedance simulation results. integrated with a four ports BLC to realize a two port bidirectional amplifier, as shown in Fig. 6. In this configuration, port 1 is the RF input and port 4 is connected to the demodulator circuit Z L, port and 3 are through ports connected to the reflection amplifiers (RA1 and RA). Looking at Fig. 6, the amplitude of the scattered wave (shown with negative sign) are related to the amplitude of incident wave (shown with positive sign) as follows: V = 1 V 1 +, V 3 = j V 1 + V + = GV, V 3 + = GV3,V+ 4 = Γ L V4, V4 = j V V 3 + = j GV 1 +, V1 = j GV 4 + = j Γ(L) GV 1 +, (3) Fig. 7. Bi-directional amplifier reflection gain S 11 and transfer gain S 1 gain. C. QPSK modulator circuit To apply QPSK modulation to the reflected signal, a phase shifter circuit of Fig. 8 is added to the bi-directional amplifier. The phase shifter consists of four micro-strip transmission lines with 90 phase delay to each other and two SP4T switches to select between them. Switches are controlled by their voltage control pins V c1 and V c to select between phases. 44

4 In order to realize a practical configuration of the circuit meander transmission lines has been used to achieve desired phase delay between switches. (a) S 11 =16.5 db 0 Fig. 8. QPSK phase shifter consists of micro-strip transmission line with characteristic impedance of 50Ω and relative phase delay of 90. D. Implementation Fig. 9 shows the layout of the proposed ReflectFX circuit at the end-user. This layout has been simulated with ADS co-simulator. The power (db) and the phase of the backscatter signals are shown in Fig. 10. It can be seen that for all the constellation points the amplitude is the same. The 90 phase shift with respect to each other provides the QPSK constellation points. (b) S 11 =16.55 db 7 Fig. 9. The proposed layout for QPSK backscatter modulation. (c) S 11 =16.45 db 16 IV. CONCLUSION A power-efficient implementation of ReflectFX end-user is proposed. Based on the ReflectFX framework, the end-user use a single antenna as TX/RX and use reflection modulation instead of individual source of transmission power. The proposed circuit consists of two identical reflection amplifiers integrated with a BLC to form a bi-directional amplifier, and a QPSK modulator which is simply a phase shifter. The reflection amplifier is designed using tunnel diode to provide negative resistance. Tunnel diodes exhibit input negative resistance at small DC voltage/current bias. Consequently, we provided a reflection amplification of 17 db with only (d) S 11 =16.5 db 107 Fig. 10. The modulated reflected wave power and phase (a) T 3 =7.7 mm, (b) T =3.9 mm, (c) T 4 =15.6 mm, and (d) T 1 =11.6 mm. 45

5 45μ W power consumption. For the bi-directional amplifier, a reflection gain of 17 db and a transfer gain of 16.8 db are achieved at the cost of 90μ W power consumption. The designed bi-directional amplifier is then integrated with phase shifter to apply QPSK modulation to the reflected power. Theoretical investigation, design procedure and simulation results for each components of the proposed ReflectFX circuit are provided in the paper. V. ACKNOWLEDGMENT This work was partially funded by the National Science Foundation CAREER award # REFERENCES [1] M. Duarte and A. Sabharwal, Full-duplex wireless communications using off-the-shelf radios: Feasibility and first results, in Proc. Asilomar Conf. Signals, Systems and Computers, 010, pp [] D. W. Bliss, P. A. Parker, and A. R. Margetts, Simultaneous transmission and reception for improved wireless network performance, in Proc. IEEE Workshop on Statistical Signal Processing, 007, pp [3] M. Duarte, C. Dick, and A. 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