The Next Wireless Wave is a Millimeter Wave
The past few years has witnessed the emergence of CMOS-based circuits operating at millimeter-wave frequencies. Integrated on a low cost organic packaging, this is the promise for high volume fabrication, lowering the cost and opening huge commercial impact opportunities. As standardization efforts catalyze the interest and investment of the industry, one can count on the spreading of millimeter-wave technology in the consumer electronic market place in the near future.
From: Vol. 50 l No. 8 | August 2007 | Pg. 22
In the past few years, the interest in the millimeter-wave spectrum at 30 to 300 GHz has drastically increased. The emergence of low cost high performance CMOS technology and low loss, low cost organic packaging material has opened a new perspective for system designers and service providers because it enables the development of millimeter-wave radio at the same cost structure of radios operating in the gigahertz range or less.
In combination with available ultra-wide bandwidths, this makes the millimeter-wave spectrum more attractive than ever before for supporting a new class of systems and applications ranging from ultra-high speed data transmission, video distribution, portable radar, sensing, detection and imaging of all kinds.
While at a lower frequency the signal can propagate easily for dozens of kilometers, penetrate through construction materials or benefit from advantageous reflection and refraction properties, one must consider carefully the characteristics (in particular strong attenuation and weak diffraction) of the millimeter-wave propagation, and exploit them advantageously. The free-space loss (FSL) (after converting to units of frequency and putting them in decibel form) between two isotropic antennas can be expressed as1
Fig. 1 Average atmospheric gaseous attenuation of millimeter-wave propagation at sea level.
FSL = 92.4 + 20 log F + 20 log D
where
F = frequency in gigahertz and
D = line-of-sight distance in kilometers
As an example, at 60 GHz the free-space loss is much more severe than at the frequencies usually used for cell phone and wireless applications. The link budget at 60 GHz is 21 dB less than the one at 5 GHz under equal conditions.2 In addition, other loss and fading factors increasingly affect the millimeter-wave transmission, such as gaseous (see Figure 1), rain, foliage, scattering and diffraction losses.
Fig. 2 Average storage capacity trends.
Beside the huge and unexploited bandwidth availability and the perspective of multi-gigabit to terabit networks, the potential of the millimeter-wave spectrum has many others attributes: enabling densely packed communication link networks, from very short range to medium range; leveraging frequency reuse to its paroxysm while increasing the security level of each link; integrating high efficiency radiating elements at the millimeter scale, leading to compact, adaptive and portable integrated systems; exploiting quasi-unlimited and unique electromagnetic signatures for detection, diagnostic or imaging.
Recently, the availability of standard CMOS technology enabling the design of MMIC circuits operating efficiently up to 100 GHz has revived the interest and investment in the 7 GHz of bandwidth unlicensed band in the 60 GHz spectrum. The specificity of the 60 GHz spectrum is the attenuation characteristics due to atmospheric oxygen absorption in the order of 10 to 15 dB/km over a bandwidth of about 8 GHz.
This attenuation precludes long-range communications, but provides an extra spatial isolation that is beneficial for frequency re-use in an indoor dense local network, reduces co-channel interference and provides extra safety for secure short-range point-to-point links. In addition to supporting multi-gigabit networks, this makes the 60 GHz spectrum a great opportunity for indoor ultra-high speed short-range wireless communications, targeting multimedia applications and others.
Fig. 3 Uncompressed video data rates.
Similarly, extremely fast growing opportunities for low cost commercial millimeter-wave systems are exploited at even higher frequencies, such as 77 GHz for automotive radar, 71 to 76 and 81 to 86 GHz for outdoor 10 Gbps networks, and 94 GHz for medical and security imaging. This just preludes terabits systems operating beyond 120 GHz and above.
The Multimedia Trend
The emergence of a multitude of “bandwidth hungry” multimedia applications has definitely had a leading role in the renewal of interest in the millimeter-wave spectrum. The conventional WLAN systems (802.11a, b and g) are limited to a data rate of, at best, 54 Mb/s. Alternative solutions such as UWB and MIMO systems will start becoming available to extend the speed up to 600 Mb/s, targeting 1 Gb/s and above in the near future. It is noteworthy that wireless networks tend to lag at least one generation behind wired LAN interconnect technology.3-4
Fig. 4 Uncompressed video data rates.
Two primary types of applications are driving the requirement for even higher data rates: ultra-fast file sharing and uncompressed high definition video streaming. Figure 2 illustrates the projected average storage capacity of PCs (desktop and laptop), reaching nearly 300 Gbytes in 2010, as well as the average storage capacity of embedded hard-drives and flash products. In the case of portable devices, especially in the case of smart cell phones, one can note a clear migration from micro-hard-drive toward high speed flash memory technology, exhibiting capacity up to 100 Gbytes and access speed exceeding the Gb/s in the horizon of 2010. It is obvious that today high speed wireless systems will lead to prohibitive synchronization time.
Fig. 5 4G seamless connectivity including millimeter-wave systems.
Figure 3 illustrates the data throughput requirement for uncompressed video streaming. It appears again that the data throughput requirement is well in excess of 1 or 2 Gbps, following a progression from 5 to 10 Gb/s and above.
This demand has since pushed the development of technologies and systems operating at millimeter-wave frequencies, while maintaining a cost structure similar to the one of conventional WLAN systems. These throughput requirements of multimedia systems are dictated by interconnect and interface technologies such as PCI-express, High Definition Multimedia Interface (HDMI), Display Port (DP) or Unified Display Interface (UDI), as shown in Figure 4.
Two major standardization bodies, IEEE 802.15.3c and Ecma International TC32-TG20,5-6 are specifically considering these requirements, in the particular case of the 60 GHz spectrum, for applications ranging from very low cost peer-to-peer interface up to high performance Wireless Personal Area Networks (WPAN), including high definition uncompressed video streaming. Back-compatibility should also be considered to provide seamless connectivity across the technologies that will support the coming 4G communications infrastructure (see Figure 5).
Fig. 6 Module, CMOS MMIC, signal processing and high efficiency PHY-MAC technologies convergence toward low cost high performance millimeter-wave systems.
CMOS-FR4: A Low Cost Millimeter-wave Radio Platform
Since the mid-90s, many examples of MMIC chipsets have been reported for millimeter-wave radio applications using GaAs FET and InP PHEMT technologies.7 More recently, SiGe BiCMOS technology has also been demonstrated to be a viable alternative.8 Despite their commercial availability and their performance, however, these technologies struggle to enter the market because of their prohibitive cost and their limited capability to integrate advanced baseband processing.
The steadily increasing frequency range of CMOS process technologies has now made the design of low cost, highly integrated 24 and 60 GHz millimeter-wave radio possible in silicon.9-10 Proof of concept has been validated using CMOS 130 nm technology; however, CMOS 90 nm is the first technology node that enables high performance and power efficient implementation of 60 GHz transceivers suitable for high volume products.
Fig. 7 Millimeter-wave optimized transistor test structure, passive and active (S-parameters) modeling.
In addition, the optimum combination and co-design of CMOS technology with low cost FR4-based packaging technology is a requisite to ensure the minimal cost structure possible, the key for the successful deployment of ultra-high speed, high capacity, 60 GHz WPAN and video streaming applications.
Finally, innovative PHY, MAC, ADC and signal processing approaches are required to provide simultaneously ultra-high bandwidth, very high PHY-MAC efficiency at an affordable price and an acceptable power budget. As depicted in Figure 6, the convergence of module, CMOS MMIC, signal processing and high efficiency PHY-MAC technologies are the necessary key enablers of the coming generation of low cost, high performance millimeter-wave systems.
Fig. 8 V-band CMOS 90 nm chipset for multi-gigabit short-range multimedia applications.
Millimeter-wave CMOS Technology
The CMOS technology has advanced to a point that a complete chipset for millimeter-wave applications can be implemented using silicon. In a standard 90 nm CMOS technology it is now possible to achieve an Ft and Fmax beyond 150 GHz. Proper transistor geometry and layout, as well as complete and accurate modeling and optimized parasitic extraction methods up to the millimeter-wave frequency of interest are the entry point for such designs (see Figure 7).
The use of millimeter-wave low loss micro-strip line and micro-inductors for matching purposes are very characteristic of this new generation of millimeter-wave designs leading to more compact area and higher performance than its co-planar waveguide (CPW) counterpart. Power gain is in excess of 8 dB at 60 GHz and at a current density of 0.2 mA/mm enables reliable and low power circuit design. In addition, noise figures of 5.5 dB are achievable for similar biasing conditions, which make the optimization of low noise amplifiers easier. P1dB compression points of 4 to 7 dBm are reachable with fairly straightforward power amplifier designs. Fundamental frequency cross-coupled VCOs exhibiting phase noise better than –95 dBc/Hz at 1 MHz offset guaranties proper transmission and demodulation of multi-gigabit/s modulated signals. Figure 8 shows an example of a V-band CMOS 90 nm chipset developed for multi-gigabit short-range multimedia applications.
Fig. 9 A large panel area FR4-LCP multi-layer substrate, compact IWG filters and a wideband millimeter-wave feed-through transition.
Comparable figures of merit are also achievable at higher frequencies with the introduction of high volume production 65 and 45 nm CMOS technology, enabling now the design of low power E-band transceiver and targeting a high level of integration for systems such as 77 GHz automotive radar, 71 to 76 and 81 to 86 GHz 10 Gbps outdoor links, and 94 GHz imaging.
The research efforts at the Georgia Electronic Design Center have been focused on the development of a millimeter-wave CMOS fully integrated single chip radio suitable for multi-Gb/s applications. A super-heterodyne architecture using high IF frequency has been chosen and optimized to support wideband modulated signals. In addition, low power mixed-signal circuit techniques and innovative high speed analog-to-digital conversion are used to enable the integration of very low power PHY operating at multi-gigabit and multi-giga samples/s.
FR4-LCP-Based Module and Antenna Technology
Liquid Crystal Polymer has emerged as a promising low cost alternative for millimeter-wave module implementation. It combines uniquely outstanding microwave performances at low cost and large area FR4 PWB processing capability. It appears as a platform of choice for the packaging of the future 60 GHz gigabit radio. 24 x 18 inch FR4-LCP multi-layer substrates are fabricated using high volume standard PWB production lines. An example of a large panel area FR4-LCP multi-layer substrate is shown in Figure 9.
Fig. 10 LCP planar antenna array example for broad beam short-range and narrow beam medium range applications.
Compact filter designs using planar and integrated waveguide (IWG) techniques have been validated and measured, exhibiting less than 2 dB minimum insertion for a relative bandwidth of 8 percent at 61.5 GHz, and a rejection greater than 20 dB at 6 GHz offset.6-11 A wideband millimeter-wave feed-through transition exhibiting less than 0.2 dB insertion loss has also been implemented.
One of the obvious attractiveness of the millimeter-wave is the small wavelength, allowing the integration of multiple radiating elements in an array configuration while occupying a minimum space (see Figure 10). Numerous antenna array solutions have been developed to address various application scenarios ranging from VSR (very short reach) omni-directional to point-to-point link.12-13
Such generic packaging platforms provide a path of choice toward the low cost integration of scalable SISO-MIMO radio systems (SM radio) using compact multi-sector phased-array architecture that overcomes simultaneously the fundamental limitations of millimeter-wave signal propagation and CMOS technology. The multi-sector architecture can either be integrated on a single large panel or in a compact 3D integrated millimeter-wave module, including an embedded filter and antenna phased array, as shown in Figure 11. Extended azimuth and elevation coverage, provided by conformal multi-sector configuration, and extended range (including non-LOS scenario) provided by high gain adaptive phased-array technology, are the breakthrough attributes of future commercial millimeter-wave systems.
Fig. 11 Compact 3D integrated millimeter-wave modules, including embedded filter and antenna phased arrays, to be integrated into a multi-sector phased-array architecture.
15 Gbps and HD-Video Millimeter-wave Test-bed
The GEDC has established an experimental millimeter-wave wireless test-bed, using 60 GHz as a demonstrator vehicle to study the channel characteristic of a real indoor environment. Researchers recently established a new world record for the highest data rate transmitted wirelessly at 60 GHz, achieving a peak data transfer rate of 15 gigabit/s at a distance of 1 meter, 10 Gigabit/s at a distance of 2 meters and 5 gigabit/s at a distance of 5 meters. In addition, high definition video streaming running at 1.485 Gb/s has been demonstrated through a one-inch thick wood table. Special efforts have been dedicated to the complete transceiver module implementation operating at a power budget well below the one hundred pico-joules range. Figure 12 shows the demodulated transmission of the multi-gigabit signal and the experimental set-up of the video transmission through a one-inch thick wood table.
Conclusion
Fig. 12 The demodulated transmission of a multi-gigabit signal and experimental set-up of the video transmission through a one- inch thick wood table.
The development of millimeter-wave radios at the same cost structure of radios operating in the microwave region opens a new field of innovation for system designers. The convergence of a FR4-based module, CMOS MMIC, signal processing and high efficiency PHY-MAC technologies becomes today’s reality, enabling the coming generation of low cost high performance millimeter-wave systems. The feasibility of ultra high speed wireless transmission beyond 10 Gbps has been demonstrated on a low power, low cost platform. A power budget well below the one hundred pico-joules/bit range has been achieved, already looking at the next level of innovation targeting 100 Gbps transmission and the femto-joule/bit power budget.
The spreading of millimeter-wave technology in the consumer electronic market place is on its way, leveraging bandwidth availability at various frequencies, ranges and levels of system complexity. Peer-to-peer ultra fast synchronization and adaptive WPAN, for data and video distribution, will drive the cost down and further eases the adoption of low cost CMOS-based millimeter-wave platforms for automotive radar, outdoor point-to-point/point-to-multi-point links, portable radar, security, sensing and imaging systems, including numerous medical applications.
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