# Low-Cost Transceiver Architectures for 60 GHz Ultra  Wideband WLANs

Authors: S. O. Tatu; E. Moldovan; S. Affes
Journal: International Journal of Digital Multimedia Broadcasting
 (2009)

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## Abstract

Millimeter-wave multiport transceiver architectures dedicated to 60 GHz UWB short-range communications are proposed in this paper. Multi-port circuits based on90° hybrid couplers are intensively used for phased antenna array, millimeter-wave modulation and down-conversion, as a low-cost alternative to the conventional architecture. This allows complete integration of circuits including antennas, in planar technology, on the same substrate, improving the overall transceiver performances.


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## Body

## 1. Introduction

With the rapid growth
of wireless technologies, ubiquitous and always-on wireless systems in homes
and enterprises are expected to emerge in the near future. Facilitating these
ubiquitous wireless systems is one of the ultimate goals of the next
generations (4G and beyond) wireless technologies being discussed worldwide
today. This increasing interest for wireless connectivity has pushed the
regulatory agencies to provide new opportunities for unlicensed spectrum usage
with fewer restrictions on radio parameters. In order to provide more
flexibility in spectrum sharing, the FCC introduced (i) an underlay approach
with severe restrictions on transmitted power levels with a requirement to
operate over the occupied and the unoccupied spectrum across the 3.1–10.6 GHz
band and (ii) an opening of 7 GHz unlicensed spectrum at millimeter-wave
frequencies around 60 GHz, where oxygen absorption limits long-distance
interference. Both spectrums are suitable for ultra-wideband (UWB) short-range
wireless local area networks (WLANs) dedicated to high data rate applications,
which could be high-speed home or office wireless networking and entertainment,
such as high-definition television (HDTV).Conventional microwave UWB technology (3.1–10.6 GHz band) is
one of the most active focus areas in academia, industry, and regulatory
circles. Because of the power spectral density limitations, (−41 dBm/MHz) the
microwave UWB overlays existing wireless services (GPS, PCS, Bluetooth, and
IEEE 802.11 WLANs) without significant interferences.Compared to the conventional UWB technology, 60 GHz mm-wave
communications will operate in the currently unlicensed spectrum (57–64 GHz)
and will easily provide data rates up to several Gb/s [1–3]. In the case of
comparable bandwidths and data rates, an important advantage of using
millimeter-wave bands is the reduced ratio between the bandwidth and the
central frequency, leading the way to transceiver simplicity. However, the 60 GHz indoor channel presents a challenging environment for UWB wireless
communications. Therefore, future UWB WLANs will certainly be achieved using
smart antenna arrays to reduce the multipath effects together with the link
budget and the power consumption. The inherent power limitations and wireless
channel propagation characteristics dictate the short-range capability of 60 GHz communications. The strong signal attenuation allows an efficient reuse of
the spectrum. This helps to create small indoor cells for hot spot secure
wireless communications.


## 2. Fabrication Technologies

The successful integration of circuits and modules into the same
substrate is the key to a significant cost reduction. In particular, GaAs
technology has been developed to the point where 60 GHz monolithic microwave
integrated circuits (MMIC) are production ready [1, 3]. An alternative
technology based on silicon germanium (SiGe) promises to provide low-cost
millimeter-wave front-end MMICs, simultaneously maintaining the performances of
GaAs [1]. While complementary metal oxide semiconductor (CMOS) technology is
considered as an ideal solution in terms of cost and circuit integration, RF
CMOS for 60 GHz frequency requires more performance improvement [3].A very promising technology for small-scale integration is
represented by monolithic hybrid microwave integrated circuits (MHMIC) on very
thin ceramic substrate. It is to be noted that coplanar wire-bond
interconnections between chips and the flip chip assembly (direct electrical
connection of face-down, hence “flipped”) could be low loss at 60 GHz, whereas
multichip module technologies could well accommodate millimeter-wave
components along with IF and baseband circuits.A further improvement would be the monolithic integration of
antennas with MMIC or MHMIC chips in order to avoid significant interconnection
losses. We note that antennas are rarely integrated into the proposed
front-ends, decreasing the overall performances in terms of noise figure and
gain. The integration of circuits and antennas forming a single front-end module
offers several benefits, such as compactness, low-power consumption, and
multifunctionality.


## 3. MultiPort Millimeter-Wave Circuits and  Modules for UWB Applications

The transceivers for 60 GHz WLANs must be composed by
low-cost components having excellent electrical performances, designed
according to the UWB transceiver requirements.Millimeter-wave multiport circuits and modules represent a
low-cost unconventional approach.A complete multiport quadrature down-conversion theory,
validated by various simulations and measurements of a Ka-band direct
conversion receiver, has already been published [4]. Simulations have also been
performed for a V-band multiport heterodyne receiver [5]. The multiport
technology allows improved results in terms of conversion loss and requires a
reduced LO power compared to the conventional methods (as low as −20 dBm to
perform an efficient frequency conversion), as it has recently been
demonstrated in [6]. As known, a conventional diode mixer using antiparallel
diodes acting at LO-driven switches requires around +10 dBm LO power for the
same conversion loss. The excellent isolation between the multiport RF inputs
is another important advantage versus the conventional approach. Another recent
study [7] proves that the multiport mixer exhibits very good suppression of
harmonic and spurious products, due to the symmetry and specific multiport
properties.Figure1 shows the block diagram of the implementation of
multiport circuit for down-conversion purposes.Figure 1
The simplified block diagram of a multiport
circuit for down-conversion.The circuit uses four90° hybrid couplers and a 90° phase-shifter. The normalized output waves are represented on the same figure. As seen,
the output waves are linear combination of phase-shifted input signals. The
insertion loss is equal to 6 dB because each signal passes through two 3 dB
couplers.Figure2 shows the equivalence with conventional I/Q mixer
architecture. The millimeter-wave multiport in conjunction with a local
oscillator and two IF differential amplifiers (IFDA) will be used in order to
obtain two quadrature IF signals, avoiding the use of two costly
millimeter-wave mixers and reducing the power consumption [5]. It is to be noted that these IF circuits will realize
additional filtering functions.Figure 2
The equivalence between conventional I/Q
mixer—the multiport down-converter.The millimeter-wave modulator is an essential element for
the transmitter. Depending on specific modulation scheme, a multiport-based
modulator or a millimeter-wave switching network are proposed for low-cost UWB applications.The multiport modulator is composed by four90° hybrid
couplers, a 90° phase shifter and two pairs of monoports having a controllable
reflection coefficient, as seen in Figure 3. The phase and the amplitude of the
normalized output signal are related to the monoport return loss values.
Therefore, the direct modulation of a millimeter-wave signal can be easily obtained.
As an example, using short or open circuits, the return loss values are equal
to −1 and +1, respectively, and a direct QPSK multiport modulator is obtained [8].Figure 3
The simplified block diagram of a multiport
modulator.In addition, “Π” or “T” switch circuits using diodes (having appropriate
return and insertion losses at 60 GHz, and a bandwidth of several GHz) can be
designed and implemented for the transposition of the microwave UWB at 60 GHz.Intelligent antennas take advantage of both antenna technology
and propagation characteristics [9, 10]. The use of such antennas in millimeter-wave
front-ends has the potential to reduce multipath interference and to increase
the output signal to noise ratio.The advantages of the electronic beam scanning have been
extensively discussed in literature. The primary reason for using antenna
arrays is to produce a directive beam that can be electronically repositioned
(scanned). The antenna arrays consist of multiple stationary antenna elements
(which are fed coherently) and can use controlled phase shifters at each
element to scan a beam to given angles in space.In order to avoid the use of controlled phase shifters, for
low-cost millimeter-wave applications, the use of antenna arrays based on Butler
matrices is
proposed. This is a multiple beam array, each input port exciting an individual
beam in space. The Butler
matrix is a circuit implementation of the FFT, radiating orthogonal sets of
beams with uniform aperture illumination. The proposed choice is a4×4 Butler
matrix antenna
array with four orthogonal beams, spaced by 30°. If more discrete beams are
needed, the number of antennas and the complexity of the Butler  matrix increase rapidly.Figure4 shows the block diagram of a four element antenna
array. Its architecture is based on a 4×4 Butler
matrix having an original topology adapted to millimeter-wave frequencies, which
avoids any cross-line. The four patch antennas are connected to a multiport
circuit having four inputs and four outputs. This circuit is composed of four
90° hybrid couplers and two 45° phase shifters, implemented using λg/8
transmission lines. Due to their small dimensions, the patch antennas are
integrated on the same substrate.Figure 4
The simplified block diagram of a multiport
antenna array.For each multiport
output signal (b5–b8), an individual maximum is
obtained by shifting the angle of arrival in the 180° range. The side-lobes are
at least 8 dB below the main lobe and the angles of arrival corresponding to
maximum signals are around −45°, −15°, 15°,
and 45°. Therefore, the main lobe of the
antenna array can be shifted by 30° multiplies. A gain of around 10 dBi can be
obtained using this multiport antenna array [11].


## 4. Proposed Transceiver Architectures

UWB
technologies using short-pulse signals have been applied to radar systems since
the 1960s, and their communication applications have stimulated industry and
academia interest since 1990s. A network capacity of many hundreds of Mbps may
be required by some specific applications such as WLAN Bridge
for interconnecting Giga-Ethernet LANs, wireless virtual realty allowing free
body movements, wireless TV high-resolution recording camera, or wireless
Internet download of lengthy files. It is to be noted that, in addition to
communication applications, the UWB devices can be used for imaging,
measurements, sensors, and vehicle radars. In principle, there are two ways to
achieve such a significant network capacity: (i) by increasing the spectral efficiency and/or (ii) by using an
extended bandwidth. According to FCC definition, the transmission bandwidth of
UWB signals should be greater than 500 MHz or larger than 20% of the central
frequency. This open definition does not specify any air interface or modulation
for UWB. In the early stages, time-domain impulse radio (IR) has dominated UWB
technology and still plays a crucial role. However, driven by the
standardization activities, conventional modulation schemes, such as orthogonal
frequency division multiplexing (OFDM) or single carrier (SC), have also
appeared [3].As known, the
OFDM technique partitions a UWB channel to a group of nonselective narrowband
channels (using a simple modulation technique such as QPSK), which makes it
robust against large delay spreads, by preserving orthogonality in the
frequency domain. In order to meet at least the “500 MHz requirement” of the
UWB systems, two approaches can be used: (i) a high number of carriers (16, 32,
64, or 128) with a corresponding relatively low bit-rate/carrier, or (ii) a
small number of carriers (2, 4, or 8) with a corresponding higher
bit-rate/carrier. However, at 60 GHz, phase noise and carrier offset will
degrade the OFDM performances. Due to the complexity of the OFDM architecture,
in our opinion, only the second approach can be suitable for low-cost 60 GHz
UWB WLANs.The available
bandwidth, the efficient reuse of spectrum (due to strong signal attenuation at
60 GHz) makes flexibility, simplicity, and cost the most critical points. It
was demonstrated that transmissions using SC advanced modulation techniques and
directive antennas can achieve comparable performances with OFDM for a 60 GHz
indoor channel [3]. These modulations, such as M-ary quadrature amplitude
modulation (QAM) and M-ary phase shift keying (PSK), will increase the spectral
efficiency. Simulations of a 60 GHz high-speed multiport heterodyne receiver
have been recently published [5]. A bit-rate up to 400 Mbps has been achieved
using a 16 QAM modulation for an IF of 900 MHz. The proposed architecture
enables the design of compact and low-cost wireless millimeter-wave
communication receivers for future high-speed WLANs, according to the IEEE
802.15.3c standard. However, the millimeter-wave circuits’ bandwidth must be
increased to few GHz and the IF must be at least 2.45 GHz, to cope with Gb/s
bit-rates.In order to use such a modulation, a UWB
transmitter must be also designed. In the case of SC-AM, the use of the directive
antenna arrays based on multiports, of the heterodyne multiport I/Q
down-converter, and of the multiport modulator, is considered as very promising
for indoor low-cost UWB communications.Figure5 shows a
simplified block diagram of a millimeter-wave multiport direct conversion transceiver.
A multiple input multiple output (MIMO) architecture is proposed. Two-phased
array based on Butler
matrices are used. This solution appears optimal because few discrete beam
directions are generally sufficient in indoor WLANs. A 20 GHz microwave
oscillator and a frequency multiplier generate the 60 GHz signal. The DSP unit
modulates the carrier using a multiport direct modulator MP2. According to 60 GHz proposed standard, the required amplifier output power is +10 dBm. In
order to obtain the control of transmitted beams, the DSP will activate one of
millimeter-wave amplifiers to feed the transmitter multiport antenna array
MP3. The corresponding direct conversion receiver (DCRi) is composed
by a low-noise amplifier (LNA), a carrier recovery (CR) circuit, a multiport
down-converter (MP1, power detectors and differential amplifiers (DA)), and two
baseband amplifiers (BBA).Figure 5
The simplified block diagram of a multiport
direct conversion transceiver.The access to
the inphase (I) and quadrature (Q) signals will enable significant additional
capabilities, increasing the phase measurement accuracy and offering a
straightforward correspondence between the baseband phasor rotation frequency
and the Doppler shift if the same oscillator is used in the receiver part. The
carrier recovery circuit is used as reference signal and will compensate the
Doppler shift in a hardware approach.Figure6 shows
the proposed heterodyne architecture of a multiport transceiver. Due to the
increased gain of the receiver, omnidirectional antennas can be also used. An
additional millimeter-wave oscillator must be used in the receiver part due to
the nonzero IF. The Doppler effects and the inherent frequency shift between
millimeter-wave oscillators are compensated using a PLL circuit operating at
IF. The second down-conversion can use conventional means due to the microwave
operating frequency. To cope with data rates of 500 Mbps, the IF of the
heterodyne receiver is chosen at 900 MHz. If the data rate is increased to 1 Gb/s, the IF can be chosen at 2.45 GHz.Figure 6
The simplified block diagram of a multiport
heterodyne transceiver.Direct
Sequence (DS) UWB is often refererred to as impulse, baseband, or zero carrier
technique. It operates by sending Gaussian low-power-shaped pulses, coherently
received by the receiver. In view of the fact that the system operates using
pulses, the transmission spreads out over a wide bandwidth, typically many
hundreds of MHz or even several GHz. To enable data to be carried, DS UWB transmissions
can be modulated in multiple ways. For example, pulse position modulation (PPM)
encodes the information by modifying the time interval and, hence, the position
of the pulses; binary phase shift keying (BPSK) reverses the phase of the pulse
to signify the data to be transmitted.Therefore, in
order to use a larger bandwidth with reduced power consumption, a new method
based on the transposition of impulse radio ultra wideband (IR UWB) signals at
60 GHz-band can also be taken into account [12–14].As a low-cost 60 GHz IR UWB proposal, the transmitter part can be implemented using an
oscillator, a millimeter-wave switch and an amplifier. A pulse generator (1st PG) generates subnanoseconds pulses (e.g., pulse width around 350 ps, in
order to reach 3 GHz bandwidth). The 60 GHz carrier will be digitally pulse
position modulated (PPM) using a millimeter-wave switch. After amplification,
Gaussian pulses are emitted over several GHz bandwidth centered into the 60 GHz
band. In order to implement the receiver, either a mixer or a detector can be
used. If a mixer is used, a millimeter-wave oscillator is needed. The mixer can
be implemented using the low-cost, low-power consumption multiport
down-converter. The oscillator is not required when a topology with a detector
is chosen, as presented in Figure7. Therefore, the receiver is composed of three
main modules: a low-noise amplifier, a 60 GHz detector, and a correlator. A
pulse generator (2nd PG) is used to control the sample and hold
(S/H) circuit. The main advantage of this architecture is that no phase
information is needed, and thus, no sophisticated coherent stable sources or
carrier recovery circuits are involved. It is to be noted that the pulses can
also be modulated using the BPSK with minimal architectural changes in both
transmitter and receiver. In order to transmit data information, instead of
modifying the position of pulses as for PPM, the phase of subnanosecond pulses
will be reversed at the transmitter. The receiver must be able to observe these
180° phase changes; therefore, a multiport based phase-detector can be
successfully used. It is to be noted that in the block diagrams, the DSP
include required A/D and D/A converters.Figure 7
The simplified block diagram of an impulse-radio
transceiver.Transmission
on ranges up to 10 meters can be expected, due to the high free-space loss at the carrier frequency. The
Friis path loss equation shows that, for equal antenna gains, path loss
increases with the square of the carrier frequency. Therefore, 60 GHz
communications have an additional 22 dB of path loss when compared to an
equivalent 5 GHz system. However, antenna dimensions are inversely proportional
to carrier frequency. Therefore, more antennas can be placed within a fixed
area and the resulting antenna array will improve the overall antenna gain. The
directive antenna pattern of a beam forming antenna array improves the channel
multipath profile by limiting the spatial extent of the transmitting and
receiving antenna patterns to the dominant transmission path. This aspect opens
up new opportunities for wireless system design. The use of smart antennas will
also improve the link budget and will reduce the transmitter power.A consequence
of the confinement to smaller cells is that the channel dispersion is smaller
than the values encountered at lower frequencies because echo paths are shorter
on average. However, movements of the portable stations, as well as the
movement of objects in the environment, cause Doppler effects, relatively
severe at 60 GHz, because they are proportional to the carrier frequency. For
example, if persons move at a walking speed of 1.5 mps, the Doppler spread
result is 1200 Hz.Simulated BER
results are excellent. The BER values are less than 10-5 for energy per bit to
noise power spectral density (Eb/No) ratio of 10 dB [11]. These results prove
that simple multiport architectures can be suitable for low-cost
millimeter-wave transceivers for future 60 GHz WLANs.


## 5. Conclusion

Several millimeter-wave multiport transceiver architectures have
been presented in this paper. The millimeter-wave frequency conversion is
obtained using the proposed I/Q multiport down-converter, avoiding the use of
a costly active mixer or a high-power millimeter-wave LO (in the case of the
conventional diode mixers). A multiport direct modulator or a millimeter-wave switching network can be used to modulate
the millimeter-wave carrier. In addition, an antenna array based on a
multiport circuit provides four output signals corresponding to four optimal
directions of arrival.The proposed multiport architectures enable the design of
compact and low-cost wireless millimeter-wave transceivers for future UWB
wireless communication systems.



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*Source: 382695-2009-04-06.xml*