Design, Analysis and Simulation of Hybrid Integrated NRD Guide Based QPSK Modulator for LMDS

Applications At 28GHz

Santosh Kumar Bhagat, Amar Nath Yadav, Vivek Sharma, Nagendra P. Pathak Radio Frequency Integrated Circuits (RFIC) Group

Department of Electronics and Computer Engineering (E&CE) Indian Institute of Technology (IIT) - Roorkee

Uttarakhand, India skb.mait@gmail.com, amariitr@yahoo.co.in, vivs@ieee.org, nagppfec@iitr.ernet.in

Abstract—This paper reports design, analysis and simulation of QPSK modulator in Ka-band. The modulator is designed at 28GHz frequency in hybrid microwave integrated circuit (HMIC) technology. For feasible high frequency operation, the design is integrated with non-radiative dielectric (NRD) guide using NRD guide to Microstrip line transition. In simulation, the observed value of the maximum phase deviation from the quadrature phase shift is 50. Error vector magnitude (EVM) of the designed modulator is determined to be 11.9% for 400 Mbps digital data rate, modulated over 28 GHz RF carrier. The effect of amplitude and phase variations is analyzed through a constellation diagram, power spectrum and eye pattern.

Keywords-modulator; Ka band; switch; QPSK; NRD guide

I. INTRODUCTION Conventional radio frequency (RF) transmitters modulate a

baseband signal, containing desired information, and then typically filter them before up-conversion to RF. This RF signal is further amplified by a power amplifier (PA) and, thereafter, refiltered before being radiated through an antenna for wireless transmission. The RF signal, passing through all these components, gets infected with spurious side bands, reducing power level in its main band.

Direct carrier modulation technique mitigates such undesirable effects. As the name suggests, the RF carrier is directly modulated by the baseband signal. This not only reduces hardware complexity, but also, lowers cost for microwave and millimeter wave wireless applications. Direct carrier modulation based quadrature phase shift keying (QPSK) modulators use couplers with different semiconductor devices as switches [1]-[6]. Current work proposes a non-radiative dielectic (NRD) guide integrated millimeter/microwave QPSK modulator, utilizing Schottky diodes based switches.

The paper is organized as follows. Section II introduces architecture of the proposed QPSK modulator, explaining each of its constituent components. Design for operation in Ka-band along with verifying simulation results are provided in section III. Section IV discusses results achieved from the designed modulator.

II. ARCHITECTURE The proposed millimeter/microwave modulator consists of

two main parts. As shown in the snippet within Fig. 1, the first part consists of NRD guide to microstrip line transition, both at the input and the output of the modulator. NRD guide allows efficient signal transmission at high microwave and millimeter wave frequency ranges. Hence, RF carrier feeding to the modulator circuitry is done through this lossless guiding structure, with appropriate transitions [7].

The second part is the QPSK modulator itself, realized in hybrid microwave integrated circuit (HMIC) technology. The scheme, employed to achieve the required QPSK signal, combines RF outputs of two identical binary phase shift keying (BPSK) modulators, each of which operate on distinct (adjacent) data bits, simultaneously. The modulator's block diagram, as shown in Fig. 1, indicates its constituent modules. In particular, it is composed of a four-way power divider (PD), four parallel RF switches, two 00/1800 phase shifters and a broad-band quadrature combiner.

Figure 1. Proposed NRD guide based QPSK modulator

Authors are thankful to SERC, DST, Government of India for providingfinancial support for carrying out this research work through its grant no.SR/S3/EECE/0003/2010.

978-1-4673-5952-8/13/$31.00 ©2013 IEEE

Input to the four-way PD is provided through NRD guide transition. In turn, the PD equally distributes its input RF carrier power towards four RF switches. The baseband digital data controls the switching operation of these switches. An RF switch is ON for bit 1, while it gets OFF for bit 0. Hence, digital data controls the passage of input RF carrier through these switches.

Pair of adjacent switches, connected to a rat race coupler (RRC), exhibit operation of a BPSK modulator. Based on the controlling bit’s state, either of the two adjacent RF switches conduct the RF carrier, while the other blocks it. Now, depending on the port, at which the conducted RF carrier is received, the RRC either shifts the carrier’s phase by 0° or 180°, thereby, acting as a 00/1800 phase shifter. In sum, the two adjacent RF switches, along with connecting RRC, modulate the RF carrier with one bit data; thereby, achieving BPSK modulation. Finally, two such BPSK modulated RF signals, corresponding to two adjacent data bits, are combined using a branch line coupler (BLC). The BLC shifts the phase of one of the BPSK signal by 90°, hence, producing the required QPSK signal at its output. Following sub-sections give implementation details of each part of the proposed QPSK modulator, shown in Fig. 2.

A. NRD Guide Transition Lossless power transfer of high frequency carrier is

possible through NRD guide. In order to transfer this power to HMICs, efficiently, an NRD guide to microstrip line transition is required [7]. The structure consists of an NRD guide to suspended stripline transition, followed by a suspended line to microstrip line transition. The suspended stripline is placed in such a way that its field lines are parallel to that of the NRD guide. This couples the RF power from the NRD guide to the suspended stripline. Further, the field lines of a suspended stripline and a microstrip line are identical. Hence, placing them next to each other align their field lines and efficiently couples RF power from the suspended stripline to the microstrip line. Such NRD-guide transition is used to efficiently feed RF carrier power to the proposed QPSK modulator, at high microwave or millimeter wave frequencies. It is also used at its output, for lossless guidance of modulated RF signal.

Figure 2. Detailed layout of the porposed QPSK modulator

B. Four-Way Power Divider The first block in the proposed millimeter.microwave

QPSK modulator is a four-way power divider (PD). In general, an efficient multi-way power divider is required to distribute signal with equal amplitude and equal phase over a wide band. It is also required to maintain high isolation among its output ports. The four-way PD is realized through three modified Wilkinson power dividers (WPDs). In a conventional WPD, all branches are of length λg/4 and chip resistor of 100 Ω ensures isolation between its output ports. For easy integration of the chip resistor, branch length of each WPD is increased to 3λg/4 [8]. Two of these modified WPDs are connected at the two output ports of another (similar) WPD to realize four-way equi-phase, equi-power distribution among four isolated ports, as shown in the first part of Fig. 2.

C. RF Switches High frequency switches are used to either make or break

path for RF carrier in a circuit. These switches can be implemented using any semiconductor device. Current design incorporates Schottky diodes to realize shunt type, single pole single throw (SPST) RF switches. A shunt mounted diode switch offers high isolation between the input and the output. Further, T-type structure, with λ/4 resonator length, is used to improve isolation. Resonator’s open end is either grounded and kept open by the diode. This, in turn, switches OFF or ON the main RF path.

Four such SPST RF switches are connected to corresponding ports of the four-way PD. Digital data bits are used as bias voltages of diodes, used in these switches. Hence, digital bits control switching operation, which, in turn, allow them to control RF carrier flow.

D. Rat Race Coupler The passive RRC component is used act as a 00/1800 phase

shifter. An ideal RRC is a lossless, matched and reciprocal four-port network. Common four-port RRC of ring shaped transmission line (TL) sections is used in the current design. It consists of three λ/4 line sections and one line section of 3λ/4 physical length. For all port impedances of 50-Ω, the ring's characteristic impedance is calculated as 50√2-Ω. Terminating port 3 into a matched load of 50-Ω, an RRC’s output at port 2 exhibits a relative phase shift of either 0° or 180°, for respective RF input at port 1 or port 4.

E. Branch Line Coupler A 3-dB BLC is used because of 90⁰ phase difference

between its outputs at the coupled and the through ports. In order to achieve wide-band requirement at higher microwave or millimeter wave ranges, a two-section BLC is used [9], [10]. This, in turn, increases operational bandwidth of modulated output.

All the afore-mentioned blocks are combining as per the block diagram in Fig. 1, to implement a Ka-band QPSK modulator, as shown in Fig. 2.

III. DESIGN AND SIMULATION Design and simulation of each component block is done

before combining them together and realizing the required modulating operation in LMDS band. Table I shows lengths and widths of different micostrip line sections, with distinct characteristic impedances, that are used in QPSK modulator’s components design. The substrate material has 2.2 relative permittivity, εr , with 10-mil height, h, and 0.025 loss tangent tanδ. Agilent's LineCalc tool is used to determine physical dimensions.

TABLE I. LENGTH AND WIDTH OF QPSK MODULATOR COMPONENTS

Characteristic impedance (Ω)

Length (mm) Width (mm)

50.00 1.94 0.768 35.35 1.91 1.266 70.71 1.46 0.435 120.7 1.79 0.127

Fig. 3 depicts simulation result of the four-way modified Wilkinson power divider. The plot indicates equal insertion loss (IL) of around 10 dB at all output ports, while, no relative phase shift is observed in their signals, i.e., relative phase-shift between any two output ports is 0°. Moreover, isolation greater than 20 dB is seen throughout the 25-GHz to 30-GHz frequency range. Besides, around 30 dB input return loss at 28-GHz indicates sufficient RF power transfer to the circuit.

Microsemi’s Schottky diode, GC9931, is used in the design of each RF switch. Scattering (S) parameters simulation of one such SPST RF switch is shown in Fig. 4. The designed switch shows 15-dB isolation and 4.4-dB IL for its OFF and ON states, respectively.

As mentioned earlier, a 50-Ω chip resistance is used to terminate port 3 of the RRC. Fig. 5 depicts result for the designed RRC, which shows an IL of nearly 4-dB and RL near 18-dB. The phase difference between its two ports is around 180°.

Figure 3. Magnitude and Phase of S parameters for four-way WPD

Figure 4. S-parameters for both ON and OFF states of RF switch

Branch line coupler is used in the QPSK modulator to provide 90° phase shift and to combine two BPSK signals. After S-parameter simulation, results of the BLC show that an IL of nearly 5-dB. Results are shown in Fig. 6. Furthermore, the RL and the isolation are observed to be greater than 15-dB. Besides it, the circuit produces the required 90° phase shift between its output ports.

Figure 5. Magnitude and Phase of S-parameters for RRC

Figure 6. Magnitude and Phase of S-parameters for BLC

The final step to design the QPSK modulator is to combine its various blocks as per Fig. 1. Final QPSK modulator layout achieved is shown in Fig. 2. By changing the supply, which corresponds to digital data, each switch can be operated as ON or OFF. S-parameter simulation result of the entire QPSK modulator layout is presented in Fig. 7. At 28-GHz phase differences are 90.609°, 91.572°, 92.974° and 84.845°. Maximum phase deviation from the ideal quarter phase is 5°.

IV. MODULATORS ANALYSIS QPSK modulator performance is better analyzed by Error

vector magnitude (EVM). EVM basically represents difference between amplitude and phase imbalance between input and output signals modulated by the modulator due to amplitude and phase error in modulator [11]. According to the IEEE 802.11aTM-1999 standard specifies EVM is required to be 15.8% less than that for different modulation types. The designed modulator demonstrates excellent EVM values at 28GHz and 400 Mbps data rate. The EVM value is 11.9%. For the other frequency EVM result is tabulated in table II. Other parameters which look in modulator design are constellation diagram, eye diagram, and spectrum which tells the considerable amount of information about modulator.

Figure 7. Magnitude and phase of QPSK

TABLE II. EVM VALUES AT DIFFERENT FREQUENCIES UNITS

FREQUENCY (GHZ) EVM (%)

27 16.9 28 11.9 29 16.3

Constellation diagram is used for visualization of modulated/demodulated signals. Constellation diagrams are often used to represent digital bits in terms of symbols, where each symbol is represented by a unique magnitude and phase in the analog aspect. Fig. 8 shows the constellation diagram at different frequencies around 28 GHz obtained from static measurements. Fig. 9 shows a constellation diagram at 28 GHz and 400 Mbps with increase in signal power at modulator

input. Spectrum of modulated signal tells about the main channel power and the bandwidth of the signal. Main channel power in this case is around 4dBm.

An eye diagram is viewed by overlapping the time domain signal traces for a certain numbers of symbol.. The open part of the plot represents the time that are can safely sample the signal with fidelity. In order to transmit the signal over the channel efficiently limit the spectral occupancy of a signal to improve bandwidth efficiency and remove channel interference. Therefore, the signal limited by filtering the signal with a different technique. Here, along with the modulator raised cosine filter is used. For raised cosine, signals the larger the roll off factor (α), the wider the opening.

Figure 8. Constellation diagram for different frequency

In this paper eye diagram is drawn by adding raised cosine filter in front of modulator with α = 0.3. Various eye diagram parameters are tabulated in table III at 400Mbps and 28GHz, small eye rise and eye fall allow eye to be open and less sensitive to timing errors and width of crossover represent the amount of jitter in the signal, and small is better.

Figure 9. Constellation diagram at 400Mbps and 28 Ghz

Figure 10. Spectrum 400 Mbps and 28 GHz

Figure 11. Eye Diagram at 400 Mbps and 28 GHz, α=0.3

TABLE III. EYE DIAGRAM PARAMETER

Eye Height Eye

Amplitude Eye Rise Eye Fall

0.028 0.534 3.035E-9 3.069E-9

QPSK modulator are design and analyzed earlier by many author also performance comparison is given in Table IV which shows modulator design with different semiconductor controlling device.

TABLE IV. COMPARISON OF MODULATOR WITH PREVIOUS WORK

Ref. [3] Ref. [2] Ref. [4] Ref. [5] This Work

Process GaAs pHEMT

GaAs MESFET HBT NEC

MESFET Schottky

Diode Freq. (GHz) 30-66 7-9 24-32 7.5-9.5 27-29

Phase Deviation (Degree)

0.13 < 0.4 ± 2 0.4 5

Insertion Loss (dB) ~5 < 9.5 13 10 22

V. CONCLUSION In this paper Ka band QPSK modulator has been designed

and simulated in Agilent ADS EM simulator at 28 GHz. To get modulated output from the modulator, the performance of phase shifters is almost accurate and they introduce minimum 5⁰ phase shift error into signal using Schottky diode. And EVM of 11.9% is obtained at 400Mbps which is under IEEE standard.

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