Design and Implementation of CMOS Low-Noise Amplifier for 40-GHz Radio-over-Fiber (RoF)

System

Siti Maisurah M. H, Nazif Emran F. and S. A. Enche Ab Rahim

Advance Physical Technologies TM Research & Development Sdn. Bhd.

Cyberjaya, Selangor, Malaysia. maisurah@tmrnd.com.my

A. Marzuki1 and A. Nigam2 1School of Electrical and Electronics Engineering

University Sains Malaysia, Nibong Tebal, P. Pinang, Malaysia. 2Nattel Microsystem Private Limited

Goa, India.

Abstract— This paper presents the design of low-noise amplifier (LNA) implemented using 0.13-µm CMOS technology. The LNA was design to support the implementation of a remote antenna unit (RAU) constructed for a 40-GHz radio-over-fiber (RoF) system. The LNA designed for the RAU transmitter achieve a small-signal gain of 14.72-dB with a noise figure of 5.56-dB at 41-GHz. The LNA designed for the RAU receiver achieve a small-signal gain of 13.38-dB with a noise figure of 5.94-dB at 38.5-GHz. Both LNAs consumed 34.7-mW power from a 1.2-V supply voltage.

Keywords— low-noise amplifier; CMOS ; millimeter-wave; radio-over-fiber

I. INTRODUCTION Millimeter-wave technology has been an important

platform in developing high data rate wireless communications. With the current rising in bandwidth demand, current commercially available wireless transmission technologies will not be capable to support sufficient data transmission speed [1]. In addition, the traditional RF system suffers from high loss as the operation frequency of the system arises. Since fiber optic cables have very low losses and can be much lighter and smaller than corresponding RF cables, it has been seen as an ideal solution for future wireless communication systems [2].

Radio-Over-Fiber (RoF) is defined as a technology where light is modulated in radio frequency and transferred over optical fiber to facilitate wireless access. Exploiting the high bandwidth capability of fiber optic together with the flexibility of a wireless RF technology, RoF technology is seen as an ideal platform in providing seamless communication in both urban and rural area. The implementation of RoF system in providing broadband access is extremely important especially in rural areas where cable installation is not feasible.

A basic structure of a RoF system consists of a Central Station (CS) and a set of base stations called Remote Antenna Unit (RAU), as shown in Figure 1. CS transmits an electrical-to-optical signal or optically modulated RF signal to RAU via an optical fiber. The RF signal is recovered using an optical-to-

electrical signal converter also known as photo detector at RAU before being transmitted to the user at the Customer Premises Equipment (CPE) through wireless channel [3].

In this research work, the design of the RAU and CPE system can be implemented in two forms, which are in module-based or in an integrated circuit (IC). In module-based format, the RAU system is built based on off-the-shelf high precision modules for rapid system verification and testing. In IC chip format, the RAU system is miniaturized into CMOS-based integrated circuits. This approached is more practical in the sense that low-power and compact form of RAU system can be realized whereby it can be commercially viable. In this paper, the second method is chosen whereby the RAU system is designed in 0.13-µm CMOS technology.

This paper presents the design of low-noise amplifiers (LNA) implemented in RAU for the 40-GHz RoF system. Section II of the paper briefly describes on the proposed 40-GHz RoF system. The design of the LNA is discussed in Section III while the simulation results are presented in Section IV.

Fig. 1. Illustration of a Basic RoF System

978-1-4799-1337-4/13/$31.00 ©2013 IEEE

2013 IEEE INTERNATIONAL CONFERENCE ON CIRCUITS AND SYSTEMS

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II. PROPOSED 40-GHZ ROF SYSTEMS The RoF system proposed in this work uses 40-GHz as the

carrier frequency since it is targetted to operate in outdoor environment. Many of the previous demonstrated RoF systems uses 60-GHz as their carrier frequency since it is used for indoor or in-building transmission [4, 5]. 40-GHz technology offers lesser Friss propagation path loss compared to 60-GHz technology which has high path loss. The dry air absorption is also low compared to 60-GHz technology making it more attractive for outdoor communication.

Successful implementation of the proposed RoF system had been demonstrated in a live High Definition (HD) video transmission using 40-GHz downlink system [6]. The demonstrated downlink system refers to the data transmission in the direction from CS to RAU to CPE. The data is successfully distributed to RAU using a 20-km optical fiber and then transmitted wirelessly to CPE which is located 5-m away from the RAU. In this setup, both RAU and CPE were implemented using off-the-shelf modules. The demonstration setup is shown in Figure 2.

As a continuation of this work, it is the aim the project to miniaturize both RAU and CPE to make it portable and low in cost by fabricating the units in CMOS technology. An example implementation of such system has been demonstrated by Georgia Institute of Technology in 60-GHz applications [7]. The main motivation in using silicon at mm-wave is because of the higher level of integration offered at a high yield that leads into lower cost system [8].

The proposed architecture used to replace the RAU unit shown in Figure 2 is direct conversion. The frequency range allocated for the downlink transmission is from 40 to 42-GHz and for the uplink transmission is between 37.5 to 39.5-GHz. As shown in Figure 3, 500-MHz guard band frequency range is used to prevent interference during transmission.

Figure 4 shows the block diagram for the proposed RAU transmitter while Figure 5 shows the block diagram for the proposed RAU receiver. In both diagrams, the circuit blocks that will be integrated into a single-chip CMOS technology is highlighted. The chip will be mounted and wire-bonded onto a low-temperature co-fired ceramic (LTCC) board to enable it to connect to the other components in the transceiver, such as photo detector and antenna. From the link budget calculation done on the transmitter and receiver chain, it is estimated that 22-dBm of output power will be transmitted at the antenna for the RAU transmitter while around -70-dBm of input power will be received at the antenna for the RAU receiver.

Fig. 3. Frequency allocation for the proposed RoF system

Fig. 2. Demonstration setup for the proposed 40-GHz RoF downlink system [6]

10

DC

RF IN RF OUT

VGRF short

RF short

M1C1

TL1 TL2

VDD

TL3

C2

Fig. 4. Block diagram for the RAU transmitter

Fig. 5. Block diagram for the RAU receiver

III. DESIGN OF LNA The LNA used in both transmitter and receiver for RAU

has different specification, especially on the frequency and power requirement. The LNA for RAU transmitter needs to operate from 40 to 42-GHz with an output power around -20-dBm. On the other hand, the LNA for RAU receiver has lesser power requirement where it needs to operate from 37.5 to 39.5-GHz with an output power around -50-dBm.

The LNAs are designed using Silterra’s 0.13-um RF CMOS process. To ensure that the LNA will work well in mm-wave frequency, the layout of the active device has been optimize to minimize parasitic resistance and capacitance. Together with wiring effect, these extrinsic parasitic elements greatly affect the high frequency performance of the transistor [9]. The existing transistor provided by the foundry had been modified to become a double-gate transistor, as shown in Figure 6. Measurement results of the modified transistor shows that the transistor is able to work well in mm-wave frequency with highest maximum oscillation frequency, fmax achieved is around 122-GHz.

Fig. 6. Modified transistor used in the LNA

Five cascaded stages of common-source LNA are used to construct the LNA in both transmitter and receiver for RAU. Common-source topology is preferred than cascode topology due to its superior performance in noise-figure [10]. The schematic of the single-stage common-source circuit is shown in Figure 7. Both transmitter and receiver use the same topology with the differences lie only on the value of the passive components used. Transistor M1 is the double-gate transistor with finger width set to 2.0-µm and number of finger set to 10. TL1, TL2 and TL3 are transmission lines of type coplanar waveguide (CPW). CPW is preferred compared to microstrip line since it has higher obtainable inductive quality factor [11]. The CPW has been electromagnetic (EM) simulated previously to determine its characteristic impedance. Capacitors C1 and C2 are metal finger capacitor provided by the foundry. The capacitance has been simulated to ensure its resonance frequency is above 40-GHz.

C1 and TL1 form the input matching network of the LNA while C2 and TL3 form the output matching network. TL2 is a source degeneration network used to improve the stability of the LNA. Instead of using a capacitor as the RF short at the bias voltage, VG and supply voltage, VDD, short circuited stub is used to give a more accurate result during the simulation.

IV. SIMULATION RESULTS Pre-layout and post-layout simulations have been carried

out for both the LNA using Cadence Spectre. EM simulation was carried out using Ansoft HFSS whereby the results were imported into touchstone format to enable it to be further simulated in Cadence. Figure 8 shows the layout of the single-stage common-source amplifier used for the RAU receiver designed in Cadence. The layout of the five cascaded stages LNA is not shown here since due to the unclear image of the transmission lines.

Fig. 7. Schematic of the single-stage common-source LNA

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Fig. 8. Layout of the single-stage common-source LNA designed for the RAU receiver

The s-parameter result of the post-layout simulation for the five cascaded stages of LNA for the RAU transmitter is shown in Figure 9. It can be seen that the LNA achieved a small-signal gain, S21 of 14.72-dB at 41-GHz. The S21 is higher than 13.26-dB across the frequency range from 40 to 42-GHz. At this frequency, the input and output return losses achieved are -36.89-dB and -18.26-dB respectively. The LNA achieved a noise figure of 5.56-dB at 41-GHz as shown in Figure 10. The LNA consumed a power of 34.7-mW from a 1.2-V DC supply.

Figure 11 shows the s-parameter results of the LNA for the RAU receiver. The LNA achieved a small-signal gain, S21 of 13.38-dB at 38.5-GHz. The S21 is higher than 9.74-dB across the frequency range from 37.5 to 39.5-GHz. The LNA achieved an input return loss of -33.36-dB and an output return loss of -19.5-dB at 38.5-GHz. As shown in Figure 12, a noise figure of 5.94-dB was achieved at this frequency. The LNA consumed a DC power of 34.7-mW form a 1.2-V DC supply.

Fig. 9. S-parameter results of the LNA for the RAU transmitter

Fig. 10. Noise figure of the LNA for the RAU transmitter

Fig. 11. S-parameter results of the LNA for the RAU receiver

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Fig. 12. Noise figure of the LNA for the RAU receiver

Table I summarizes the reported performance of mm-wave CMOS LNA working around 40-GHz. It can be seen that the proposed LNA presented in this work achieved a comparable performance with the rest of the LNAs. The figure of merit (FoM) used in the table is defined in Eq. (1):

[ ] [ ][ ] [ ]mWPowerdBNF

GHzFdBGainFoM⋅

⋅= (1)

TABLE I. PERFORMANCE COMPARISON OF 40-GHZ CMOS LNA

Ref. This work

(RAU receiver)

This work (RAU

transmitter) [12]* [13]*

Process 0.13-µm CMOS

0.13-µm CMOS

0.13-µm CMOS

0.18-um CMOS

Topology 5-stage

common-source

5-stage common-

source

3-stage common-

source

3-stage cascode

Gain (dB)

14.72 @ 41-GHz

13.38 @ 38.5-GHz

18 @ 41-GHz

15 @ 40-GHz

NF (dB)

5.56 @41-GHz

5.94 @ 38.5-GHz

6.3 @ 41-GHz

7.5 @40-GHz

Power (mW)

34.7 @ 1.2-V

34.7 @ 1.2-V

36 @ 1.5-V

36 @ 1.8-V

FoM 3.13 2.50 3.25 2.22

*Measured results

CONCLUSION Five-stage common-source LNA has been designed and

discussed in this paper. The LNAs were designed to support the building block of a RAU inside a 40-GHz RoF system. Post-layout simulation results show that the proposed LNAs achieved a gain of 14.72-dB and 13.38-dB for the RAU transmitter and receiver respectively. Both LNAs also exhibit good noise figure which is not more than 6-dB. The LNAs consume low power which is at 34.7-mW at 1.2-V DC supply.

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