Multi-Gbit/s 60 GHz Transceiver Analysis Using FDM Architecture and Six-Port Circuit

Nazih Khaddaj Mallat ,Emilia Moldovan ,Serioja O.Tatu ,and Ke Wu

(1.Poly-Grames Research Center,école Polytechnique de Montréal,Montréal,Québec H3T 1J4,Canada;

2.Institut Nationalde la Recherche Scientifique,INRS-EMT,Montréal,Québec H5A 1K6,Canada)

Abstract:This paper presents an analysis and validation by advanced system simulation of compact and low-cost six-port transceivers for future wireless local area networks(WLANs)operating at millimeter-wave frequencies.To obtain realistic simulation results,a six-port model based on the measurement results of a fabricated V-band hybrid coupler,the core component,is used.A frequency-division multiplexing scheme is used by introducing four quadrature phase-shift keying(QPSK)channels in the wireless communication link.The data rate achieved is about 4 Gbit/s.The operating frequency is in the 60-64 GHz unlicensed band.Bit error rate(BER)results are presented,and a comparison is made between single-carrier and multicarrier architectures.The proposed wireless system can be considered an efficient candidate for millimeter-wave communication systems operating at quasi-optical data rates.

Keyw ords:millimeter-wave communications;Gbit/s data rates;passive components and circuits;six-port interferometer

1 Introduction

O ne of the goals of 4G wireless technologies is to simplify wireless systems in homes and enterprises.The coming 60 GHz WLANs are primarily aimed at applications with a short range and very high datarate,such as high-speed home,office,and high definition television(HDTV).

In 2001,the Federal Communications Commission(FCC)allocated a continuous block of 7 GHz of spectrum in the 57-64 GHz band for wireless communications[1](where oxygen absorption limits long-distance interference).Energy propagation in the 60 GHz band has many unique characteristics and brings advantages such as high security,immunity from interference,and frequency re-use[2],[3].There are many design challenges for millimeter-wave circuits,including the necessity for low cost,high power efficiency,and accurate computer aided design models.

Six-port circuits are proposed for low-cost high-performance millimeter-wave transceivers.The six-port is a passive circuit,first developed in the 1970s for accurate automated measuring of the complex reflection coefficient during microwave network analysis[4].It is a low-cost alternative to a network analyzer or beam direction-finding applications[5].Various millimeter-wave front-end architectures based on six-port devices have been proposed in recent years.These architectures use various fabrication technologies and modulation schemes[6]-[9].

Section 2 of this paper provides an analysis of a fabricated hybrid coupler that uses miniature hybrid microwave integrated circuit(MHMIC)technology and the six-port model.Section 3 provides an analysis of the proposed 60 GHz transceivers,and simulation results of single-carrier and multicarrier systems are presented[10].Conclusions are given in section 4.

2 MHMIC Hybrid Coupler and Six-Port Model

2.1 MHMIC Hybrid Coupler

The four-port 90°hybrid coupler is the core component of the six-port circuit and is designed and fabricated to operate in V-band.Using MHMIC technology,the six-port circuit is integrated on a 125μm alumina substrate with a relative permittivity of 9.9.

Fig.1 shows several microphotographs of the MHMIC 90°hybrid coupler.The RFshort-circuits(replacing via-holes)for coplanar transitions,and 50Ωloads are obtained using wideband open-circuited stubs.

The diameter of the coupler is around 700μm,and the 50Ωline width is nearly equal to the thickness of the alumina substrate.To characterize fabricated MHMIC circuits,on-wafer measurements are taken using a Cascade Microtech probe station(equipped with 150μm pico-probes)connected to an Agilent Technologies E8362B millimeter-wave power network analyzer(PNA)(Fig.2).Because of measurement limitations,60-64 GHz is considered for circuit characterization and system simulation.

▲Figure 1.Microphotographs of the MHMIC 90°hybrid coupler(with ports 1-4).

▲Figure 2.S-parameter measurementset-up.

Fig.3 shows the phase of transmission-scattering S-parameters(S12 and S13).A 90°phase difference is obtained over the 4 GHz band,and the phase imbalance is around 5°.Fig.4 shows the power coupling,matching,and isolation of the MHMIC-fabricated hybrid coupler.The power splits(S12 and S13)over the 4 GHz band are between-3 d Band-4 d B,very close to the theoreticalvalue of-3 d B.The return loss on input port 1(S11)versus frequency is higher than-20 d B,and the isolation between ports 2 and 3(S23)is higher than-15 d B(Fig.4).Because of circuit symmetry,measurements of equal isolations between ports 1 and 4,and ports 2 and 3 are obtained.Measurements of return loss at all ports(Sii)are also obtained.

2.2 Six-Port Model

A six-port linear network is represented by a 6×6 characteristic matrix and is generally characterized by a standard two-port vector network analyzer(VNA).

The two-port measurements,required for every possible combination of two ports,are taken.The 4 ports not connected to the VNA are terminated with appropriate loads.This multiport circuit is composed of four 90°hybrid couplers and a 90°phase shifter(Fig.5).The input signals a5and a6are normalized waves,from the local oscillator(LO)and radio frequency signal,respectively.The output detected signals can be calculated based on the multiport block diagram and using the quadratic characteristic of the power detectors,as detailed in[6].

For each two-port measurement,a 2×2 sub matrix of the six-port characteristic matrix is determined.To avoid necessary circuits(fifteen in the case of a six-port),and to obtain realistic results with minimum fabrication cost,the six-port modelis implemented in 2010 Advanced Design System(ADS)software of Agilent Technologies.S-parameter measurements of fabricated 90°hybrid couplers interconnected by transmission lines are also used.

▲Figure 3.S-parameter phases of an MHMIChybrid coupler.

▲Figure 4.Magnitude S-parameter for transmission,return loss and isolation.

A matching of more than-15 d B and isolation of-20 d Bare obtained for the input ports(Fig.6).

▲Figure 5.Six-port circuit.

▲Figure 6.Simulation of S-parameter magnitude.

▲Figure 8.Percentage of I/Qerror over the 60-64 GHz band.

In a direct conversion scheme,the quadrature(I/Q)down-converted signals are obtained using a differential approach[6]:amplitude and phase,are observed.A differential approach is proposed in[6]and[7]that reduces these errors.Fig.8 shows the percentage of six-port down-converter quadrature error.

where V1,V2,V3,and V4are the six-port output detected signals,K is a constant,a is the amplitude of the LO signal,Δφ(t)=φ6(t)-φ5is the instantaneous phase difference,and α(t)is the instantaneous amplitude ratio between the RFand LO signals.

Harmonic balance simulations are performed for several discrete frequency points over 4 GHz.The RF and LO input powers are set to 0 d Bm,and the RFsignal phase is swept over 360°In practice,amplitude and phase imbalances are inherent because of design and fabrication constraints at 60 GHz.S-parameters in simulations can highlight such errors.

Fig.7 shows the six-port output signals in relation to the phase difference between RFand LO signals.

Theoretically,these signals must have equal amplitude and be shifted by 90°and its multipliers.Some imbalance-related errors,both in

▲Figure 7.Harmonic balance analysis of the six-port at 63 GHz.

Because of these inherent errors,I/Q signal phase difference is not exactly 90°and the shapes of demodulated constellations are distorted.However,for simple modulation schemes,that is,amplitude-shift keying(ASK),binary phase-shift keying(BPSK),and quadrature phase-shift keying(QPSK)recommended for low-cost transceivers,phase error of less than 5%is considered acceptable.

3.1 Single-Carrier Architecture

3 Proposed 60 GHz Transceiver

Recent studies have suggested that a V-band receiver based on six-port technology enables the design of compact and low-cost wireless millimeter-wave single-carrier communication receivers for future high-speed wireless communication systems[6]-[9].Millimeter-wave frequency conversion is performed using specific properties of the six-port circuit.This avoids the need for a costly conventional active mixer.A simplified six-port single carrier(SC)homodyne transceiver block diagram is shown in Fig.9.

The transmission path is simulated by an ADSloss link based on the Friis model.The free-space loss at 62 GHz is around 88 d Band is calculated using the Friis transmission equation

where PRis the ratio of power received by the receiving antenna,PTis the ratio of power input to the transmitting antenna;GTand GRare the antenna gains of the transmitting and receiving antennas,respectively;λis the wavelength(around 5 mm for 62 GHz),and R is the distance(10 m).In the transmitter part,the parameters are set as follows:LOpower=-25 d Bm,amplifier gain(A)=20 d B,and antenna transmitting gain(GT)=10 d Bi.These values are chosen in order to obtain transmitted signalpower of 10 d Bm(allowed by FCC for a V-band communications system).In the receiver,the antenna receiving gain is 10 d Bi,the low-noise amplifier(LNA)gain is 20 d B,and the six-port input signalpower is-38 d Bm.

▲Figure 9.Single carrier system block diagram.

During the simulations,the operating frequency was set at 62 GHz,and the transmitted QPSK-modulated signals were pseudo-randomly generated by ADSwith a symbol rate of 500 MS/s(communication data rate=1 Gbit/s).By using the limiters in the last stage of the receiver,output square waves are generated(Fig.10).For a bit sequence of 200 ns,the output demodulated(I)signals have the same bit sequence as those transmitted.The same conclusion is obtained for the(Q)signals.Fig.11 shows the bit error rate(BER)variation in relation to energy per bit over spectralnoise density(Eb/N0)for the same distance of 10 m.The six-port receiver architecture using the single carrier scheme has excellent BER performance,very close to the theoretical ideal.

▲Figure 11.BERfor 1 Gbit/s QPSKsignal.

3.2 Multicarrier Architecture

In[11],the same proposed six-port receiver using frequency division multiplexing(FDM)architecture is able to transmit 2 Gbit/s up to 10 m with BER of 10-9,as required for an uncoded HDTVwireless transmission in a home or office.By spacing the carriers,this data rate can be increased to 4 Gbit/s for a short-range communication of 10 m.The advantage of using FDM scheme is that no central timing synchronization is required for each subchannel.Each subchannelcan operate independently.A serial to parallel converter(S/P)with 2×4 parallel outputs,4 millimeter-wave LO,4 quadrature modulators,and a millimeter-wave combiner(C)are used to generate the 4 carrier FDM signal.An envelope simulation is then carried out(Fig.12).

In the simulation,the power of each LO used in the transmitter part is-25 d Bm,and their frequencies are 61,62,63,and 64 GHz.The received signal is amplified by an LNA(20 d B),split using a millimeter-wave power divider(D),and coherently demodulated using 4 six-port receivers(MPR1-R4)and 4 baseband(BB)circuits.Finally,a parallel to serial data converter(P/S)generates the output data stream.For each frequency,the same power balance as the single carrier system in Fig.9 is kept.The symbol rate per carrier(SRC)is 500 MHz,which provides an FDM signal at 4 Gbit/s.To cover the whole bandwidth of 4 GHz,and considering the ADSconvergence properties,the step simulation is fixed at 1/(7×SRC).

▲Figure 12.Multicarrier system block diagram.

▲Figure 13.QPSKFDMspectrum atthe transmitter.

▲Figure 14.BERof QPSKsignal(different subcarriers).

Fig.13 shows the QPSK multicarrier transmitted signal spectrum.Between the 61-64 GHz frequency channels,there are spectral regions called guard bands that act as buffer zones to prevent interference between frequency subchannels.By taking into consideration the SRC,simulation step,and envelope simulation frequency,the guard bands are minimized as much as possible.Fig.14 shows the BERvalues that correspond with each subcarrier.

The BERcurves are very close to each other,ranging from 10-10to 10-7for an Eb/N0of 13 d B.These variations are considered tolerable for good quality wireless communications,which usually requires a BERof less than or equalto 10-9.

Because of the uncorrelated subchannels and ADSconvergence limitation,we calculate the BERof the whole system using an analytical approach.The BERaverage of the millimeter-wave multicarrier system is the sum of the BERrelated to each subchannel[12],[13]and is obtained using

where Psysis the global error probability(or BER)of our system,N is the number of carriers used,and M is related to the modulation levels or number of bits per symbol(for QPSK,M=2).(BER)iis the BERof each subcarrier.

Fig.15 shows the average BERcurve of the system as well as the BER corresponding to the single-carrier communication system.By using multicarrier modulation techniques,and for BER=10-9,Eb/N0should be incremented by about 2 d B,which is not a critical disadvantage.In the meanwhile,a high data rate of 4 Gbit/s is attained.This has interesting implications for the new generation of mobile radio communication systems.

4 Conclusion

In this paper,a V-band FDM wireless link based on six-port receiver architecture is proposed.I/Q signal phase error of less than 5%is caused by fabrication errors in the MHMIC coupler.But this error would not degrade some digital modulation schemes,such as QPSK,because of their robustness.The millimeter-wave receiver,based on the six-port junction fabricated on ceramic substrate,is able to transmit at 1 Gbit/s using single-carrier modulation.However,using multicarrier modulation based on frequency division multiplexing(4 subcarriers),4 Gbit/s is reached.The proposed system allows the design of high-speed,high-performance,and low-cost wireless transceivers for future millimeter-wave systems.In the future,the entire six-port circuit will be fabricated and tested,and a full test will be done on the proposed FDM millimeter-wave architecture.

▲Figure 15.BERresults of QPSKmulticarrier system.

Acknowledgement

The authors gratefully acknowledge the financialsupport of the Fonds Québecois de Recherche sur la Nature et les Technologies(FQRNT)and the support of the Centre de Recherche en électronique Radiofréquence(CREER)of Montreal,funded by the FQRNT,for the MHMIC circuit fabrication.