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

1 Introduction
    One 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 125 μm alumina substrate with a relative permittivity
of 9.9.

    Fig. 1 shows several microphotographs of the MHMIC 90°hybrid coupler. The RF short-circuits (replacing via-holes) for coplanar transitions, and 50 Ω loads are obtained using wideband open-circuited stubs. The diameter of the coupler is around 700 μm, and the 50 Ω line 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 150 μm 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.

    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 dB and -4 dB, very close to the theoretical value of -3 dB. The return loss on input port 1 (S11) versus frequency is higher than -20 dB, and the isolation between ports 2 and 3 (S23) is higher than -15 dB (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 a5 and a6 are 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 model is 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.

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

    In a direct conversion scheme, the quadrature (I/Q) down-converted signals are obtained using a differential approach [6]:

    where V1, V2, V3, and V4 are the six-port output detected signals, K is a constant, a  is the amplitude of the LO signal, Δφ (t ) = φ 6(t )-φ 5 is the instantaneous phase difference, and α(t ) is the instantaneous amplitude ratio between the RF and 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 dBm, and the RF signal 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 RF and LO signals. Theoretically, these signals must have equal amplitude and be shifted by 90°and its multipliers. Some imbalance-related errors, both in 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.

    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 Proposed 60 GHz Transceiver

3.1 Single-Carrier Architecture
    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 ADS loss link based on the Friis model. The free-space loss at 62 GHz is around 88 dB and is calculated using the Friis transmission equation

    where PR is the ratio of power received by the receiving antenna, PT  is the ratio of power input to the transmitting antenna; GT  and GR  are the antenna gains of the transmitting and receiving antennas, respectively; λ is 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: LO power =- 25 dBm, amplifier gain (A) = 20 dB, and antenna transmitting gain (GT) = 10 dBi. These values are chosen in order to obtain transmitted signal power of 10 dBm (allowed by FCC for a V-band communications system). In the receiver, the antenna receiving gain is10 dBi, the low-noise amplifier (LNA) gain is 20 dB, and the six-port input signal power is -38 dBm.

    During the simulations, the operating frequency was set at 62 GHz, and the transmitted QPSK-modulated signals were pseudo-randomly generated by ADS with 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 spectral noise 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.

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 HDTV wireless 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 subchannel can 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 dBm, and their frequencies are 61, 62, 63, and 64 GHz. The received signal is amplified by an LNA (20 dB), split using a millimeter-wave power divider (D), and coherently demodulated using 4 six-port receivers (MP R1-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 ADS convergence properties, the step simulation is fixed at 1/(7×SRC).

    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 BER values that correspond with each subcarrier. The BER curves are very close to each other, ranging from 10-10 to 10-7 for an Eb /N0  of 13 dB. These variations are considered tolerable for good quality wireless communications, which usually requires a BER of less than or equal to 10-9.

    Because of the uncorrelated subchannels and ADS convergence limitation, we calculate the BER of the whole system using an analytical approach. The BER average of the millimeter-wave multicarrier system is the sum of the BER related to each subchannel [12], [13] and is obtained using

    where Psys is 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 )i  is the BER of each subcarrier.

    Fig. 15 shows the average BER curve 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 /N0 should be incremented by about 2 dB, 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.

    Acknowledgement
The authors gratefully acknowledge the financial support 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.

References
[1] P. Smulders, “Exploiting the 60 GHz band for wireless multimedia access: prospects and future directions,” IEEE Commun. Mag., vol. 40, no.1, pp. 140 -147, 2002.
[2] D. Cabric, M. Chen, D. Sobel, S. Wang, J. Yang, and R. Brodersen, “Novel radio architectures for UWB, 60 GHz, and cognitive wireless systems,” EURASIP J. Wireless Commun. Netw., vol. 2006, no. 2, p. 22, Apr. 2006.
[3] C. Park and T. Rappaport, “Short-range wireless communications for next-generation networks: UWB, 60 GHz millimeter-wave WPAN, and ZigBee,” IEEE Wireless Commun., vol. 14, no. 4, pp. 70 -78, 2007.
[4] G. F. Engen, “The six-port reflectometer: An alternative network analyzer,” IEEE Trans. Microw. Theory Tech., vol. 25, no. 2, pp. 1077-1079, Dec. 1977.
[5] T. Yacabe, F. Xiao, K. Iwamoto, F. Ghannouchi, K. Fujii, and H. Yabe, “Six-port based wave corellator with application to beam direction finding,” IEEE Trans. Instrum. Meas., vol. 50, no. 2, pp. 377-380, Apr. 2001.
[6] S.O. Tatu and E. Moldovan, “V-band multi-port heterodyne receiver for high-speed communication systems,” EURASIP J. Wireless Commun. Netw., vol. 2007, no.1, p. 45, Jan 2007.
[7] S.O. Tatu, E. Moldovan, Ke Wu, R.G. Bosisio, and T.A. Denidni, “Ka-band analog front-end for software-defined direct conversion receiver,” IEEE Trans. Microw. Theory Tech., vol. 53, no. 9, pp. 2768-2776, Sept. 2005.
[8] N. Khaddaj-Mallat, E. Moldovan, and S.O. Tatu, “Comparative demodulation results for six-port and conventional 60 GHz direct conversion receivers,” Progress In Electromagnetics Research, vol. 84, pp. 437-449, 2008.
[9] X. Xu, R. G Bosisio, and Ke Wu, “Analysis and iImplementation of six-port software-defined radio receiver platform,” IEEE Trans. Microw. Theory Techn., vol. 54, no. 7, pp. 2937-2943, Jul. 2006.
[10] N. Khaddaj Mallat, E. Moldovan, S.O. Tatu, and Ke Wu, “Ultra-high-speed six-port frequency-division-multiplexing V-band transceiver,” in IEEE Radio and Wireless Symp. (RWS 2011), Phoenix, AZ, 2011.
[11] S. O. Tatu, N. Khaddaj Mallat, and E. Moldovan, “Ultra wideband frequency division multiplexing millimeter-wave multi-port receiver analysis,” in 2009 European Microw. Week, IEEE Conf., Rome, pp 108-111.
[12] J. G. Proakis, Digital Communications, 3rd ed. New York: McGraw-Hill, 1995.
[13] Jun Lu, Tjeng Thiang Tjhung, Adachi, F., and Cheng Li Huang, “BER performance of OFDM-MDPSK system in frequency-selective Rician fading with diversity reception,” IEEE Trans. Veh. Technol., vol. 49, no. 4, pp. 1216-1225, Jul. 2000.

Biographies
Nazih Khaddaj Mallat (nazih@ieee.org) received his bachelor’s degree in electrical and computer engineering from the Lebanese University in 2000. He received his master’s degree from Ecole Nationale Supérieure des Télécommunications de Bretagne (ENSTB), France, in 2003. He received his Ph.D. degree in telecommunications from the University of Quebec Institut National de la Recherche Scientifique (INRS) in 2010. He is a postdoctoral fellow at Ecole Polytechnique de Montreal. Dr. Khaddaj Mallat’s main research interests are passive
microwave/millimeter-wave circuit design, and telecommunication systems. He is the IEEE Montreal section chair in 2011. He has served at many IEEE conferences: EPC2007, SMC2007, EPEC2009, CNSR2010, MWP2010, FBW2011, CCECE2012 and IMS2012.

Emilia Moldovan (moldovan@emt.inrs.ca) received her B.Sc. degree in electrical engineering from the Polytechnic University of Cluj-Napoca, Romania, in 1980. She received her M.Sc.A. and Ph.D. degrees in electrical engineering from the école Polytechnique of Montréal, Canada, in 2001 and 2006. From 1982 to 1997, she was a telecommunication engineer with the Quality of Service Department, National Company of Telecommunications, Rom-Telecom, Romania. She is currently a research associate at the Institut National de la Recherche Scientifique (INRS), Canada. Her research interests include passive microwave/millimeter-wave circuit design, and telecommunication and radar systems.

Serioja O. Tatu (tatu@emt.inrs.ca) received his B.Sc. degree in radio engineering from the Polytechnic University, Bucharest, 1989. He received his M.Sc.A. and Ph.D. degrees in electrical engineering from the école Polytechnique de Montréal in 2001 and 2004. From 1989 to 1993, he was an RF engineer and head of the Telecommunications Laboratory at the National Company of Telecommunications, Rom-Telecom, Romania. From 1993 to 1997, he was a technical manager at the Telecommunication Laboratory. He is currently associate professor at the Institut National de la Recherche Scientifique (INRS). His current research interests are the millimeter-wave circuit design, hardware and software radio receivers, and radar systems.

Ke Wu (ke.wu@ieee.org) is professor of electrical engineering and Tier-I Canada Research Chair in RF and millimeter-wave engineering at the école Polytechnique de Montréal. He was director of the Poly-Grames Research Center and founding director of the Center for Radiofrequency Electronics Research of Quebec (funded by FRQNT). He has authored or coauthored more than 700 refereed papers and a number of books and book chapters. He holds numerous patents. Professor Wu’s current research interests include substrate integrated circuits (SICs), antenna arrays, and development of low-cost RF and millimeter-wave transceivers and sensors for wireless systems and biomedical applications.

[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.

[Keywords] millimeter-wave communications; Gbit/s data rates; passive components and circuits; six-port interferometer