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60 GHz operation to enable Gbps wireless communication

Summary:  

The increasing demands of society for technology driven appliances are pushing the trend to shift operation to higher frequencies, and the advancement in silicon technology makes this shift feasible.

Abstract

Operation at millimeter-wave frequency, where around 7GHz of unlicensed bandwidth is available in the 60GHz band, provides an opportunity to meet the higher data rate demands of wireless users. Advancements in silicon technology permit one to consider exploiting the 60GHz band for commercial applications (e.g., short range, wireless HDTV transmission) for the benefit of end users. This could enable, for example, wireless streaming of uncompressed high-quality video packets of a movie in few seconds due to data rates as high as multi gigabits per second.

No one can imagine carrying a laptop or any other portable device which is not connected to the internet, or even to a local network, from which you’re transmitting and receiving information. These can be ranging from simple text information to streaming video data that requires large data rates of few gigabits per second. Wireless communication at 60 GHz is claimed to be the most appropriate way to achieve the huge demands for the next generation consumer applications.

Definition

The unlicensed band centered at 60GHz lies in the extremely high frequency (EHF) band, which is the highest radio frequency band according to the International Telecommunication Union (ITU), running the range between 30 and 300GHz [1]. This frequency range is equivalent to wavelengths between 10mm to 1mm in free space. That is why it is also called the millimeter-wave (mm-wave) band. The Institute of Electrical and Electronics Engineers (IEEE) has another frequency band nomenclature that assigns 60GHz to the V band. The V band includes frequencies ranging from 40 to 75GHz [2].

Motivation

The increasing demands of society for technology driven appliances are pushing the trend to shift operation to higher frequencies, and the advancement in silicon technology makes this shift feasible. Data transmission is the current example of our choice. Communication rates of few gigabits per second are already available in consumer applications, which is a challenge to be supported by wireless channels. An example is shown in Figure 1. According to the current HDTV standard, up to 5.6 Gbps is required for TV display with a resolution of 1920×1080, 90 Hz frame rate and 30 bits per channel per pixel. Even higher data rates are expected to be available in the future. If such data rates are to be considered for wireless transmission, large bandwidths are required, which are available at higher frequencies.

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60 GHz band allocations

Several regulatory bodies in different countries started from 2000 to allocate unlicensed frequency bands around 60 GHz for the use of commercial products. As shown in Figure 2, typically 7 GHz (from 5 to 9 GHz) are available for wireless communication around 60 GHz. Following band allocations, several efforts to standardize communication over the available band were done by standardization organizations, resulting in standards such as IEEE802.15.3c [4], ECMA 387 [5], WirelessHD [6], WiGig [7] and IEEE802.11ad [8]. These standards split the 60 GHz band into four channels, with around 2 GHz each. Several digital modulations for different modes of operation are also defined for different applications, reaching data rates up to around 25 Gbps and transmission distances up to around 10 meters.

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Beamforming

Natural spatial isolation caused by propagation loss due to free space path loss (FSPL) and oxygen absorption  makes communication in this frequency band only viable over short ranges (till 10 meters). FSPL can be calculated in its simple form using the following equation:

FSPL (dB) = 20log (distance) + 20log (frequency) – 147.55

This shows around 88 dB loss for 60 GHz signal at 10 m. Figure 3 illustrates the oxygen absorption peak in the 60GHz region.

”]One advantage of the implicit attenuation and limited range of operation at 60GHz is the possibility of bandwidth reuse in two close offices. Directional propagation is used to enhance signal transmission and reception. In the transmitter, radiated power is concentrated towards the receiver instead of being wasted in unwanted directions. Similarly, gain is boosted in one direction and unwanted interferers can be spatially attenuated in the receiver. This suggests using multiple antennas at the transmitter, to direct and enhance signal transmission, and at the receiver, to improve the sensitivity and reduce interference. The size of an antenna is inversely proportional to the operating frequency. For example, 60GHz operation allows the use of 16-element antenna array that occupies the same area as a dipole antenna at 5GHz [10].

Beamforming is a signal processing technique used in sensor arrays for directional signal transmission or reception [11]. The term beamforming is derived from spatial filters that were designed to form pencil beams (Figure 4) [12]. As shown in Figure 5, an array of antennas with variable gain and phase shifting (or time delay elements) can form different antenna patterns, one of which is a beam with a specific radiation angle θ [10]. Time delays among different antenna paths need to be compensated by true time delay elements for coherent signal combination [13]. Assuming no channel bonding, signal bandwidth is around 2GHz, which is very small compared to the 60GHz carrier frequency. In narrowband systems, phase shift blocks can be used instead of true time delay elements as an approximation for the multi-path signal to add constructively [13].

 

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Beamforming implementation challenges

Phase shifting in a receiver can be implemented in four different ways: at RF after the LNA, at baseband after the mixer, in the LO path or using signal processing in the digital domain [15]. Signal combination in the digital domain uses the most hardware and is the most power hungry because the whole front-end is copied as many times as the number of antenna paths. Phase shifting at RF places lossy elements directly after the LNA which degrade the system gain, noise figure and bandwidth. System gain and noise figure are less sensitive to amplitude variations in the large LO signal. Thus, phase shifting at LO provides the lowest effect on signal quality. Phase shifting after the mixer causes insignificant deterioration of gain and noise figure (as compared to phase shifting at RF). Signal combination is performed at baseband in both LO and baseband phase shifting. In both cases, in order to avoid using multiple PLLs, LO signal should be distributed to different antenna paths. This includes other problems, such as cross-talk and low LO power levels.

System architecture

One antenna path of a direct conversion 60 GHz receiver is shown in the block diagram of Figure 6. After the receiving antenna, the receiver includes I/Q QVCO, LNA and I/Q mixers in the font-end. Phase shifting and signal-combination are performed at baseband together with LPFs, VGAs and ADCs. In a full system, the digital output signal is then processed in the DSP. The transmitter architecture uses similar system blocks, with the LNA and ADC are replaced by the PA and DAC, respectively.

Figure 6: 60GHz receiver architecture.

Enabling technology

In a mixed-signal chip that includes analog and digital circuits, CMOS technology is preferred over bipolar for large volume applications, where the digital part dominates. Moore’s law states that on-chip density of transistors doubles every two years. This doubling is due to the fabrication of transistors with smaller minimum length. Smaller size transistors enable operation at higher frequencies. That’s the reason for which operation at mm-wave became possible nowadays after it was just a dream years ago.

Scaling also has drawbacks. Smaller transistors usually require lower supply voltage due to the lower gate oxide. For example, the breakdown voltage is 1.8V for 0.18µm devices and 1.2V for 0.13µm ones [16]. This reduces the available voltage headroom, and thus, decreases voltage swings. The reduced supply voltage also limits the number of stacked transistors between supply and ground terminals. Smaller size transistors have more mismatch. This is because transistor mismatch is inversely proportional to the square root of its area according to the following equation:

[17].

where AVT is a technology constant (for example, AVT = 5 mVµm for NMOS device in 65nm technology).

Although devices are smaller in size, allowing for higher frequency operation, interconnects are not scalable as a consequence. Taking 60GHz as an example, wavelength in free space is 5mm. Assuming that the effective dielectric constant of a microstrip line is 4, the on-chip wavelength at 60GHz becomes 2.5mm. This means that a track length of more than 2500µm carrying a signal with frequency components of 60GHz will cause a considerable difference in signal characteristics. This suggests the use of electromagnetic wave simulators, such as Agilent ADS [18] or Ansoft HFSS [19], to model relatively long interconnects with the help of a parasitic extraction tools, such as Mentor Graphics Caliber [20] or Cadence Assura [21], for medium and short interconnects.

Current and future applications

The large bandwidth allocated for the 60GHz frequency band could be used to transfer tens of gigabytes of data in few seconds. Short range indoor applications like broadband internet access and high speed point-to-point wireless communication could utilize this capability. The only wide-spread available product in the market that utilizes the 60 GHz frequency band is a one port kit to replace the HDMI cable in HDTV connections. Other products including 60 GHz Home Cinema projectors are coming into market. The coming effective target application is to embed 60 GHz operation in WiFi. 802.11 wireless routers will have additional great capabilities when using the large 60 GHz band together with the 2.4 and 5 GHz ones. 802.11ad wireless standard is expected to be finalized in 2012, with the expectations to be employed in commercial wireless routers.

Figure 7 gathers the main 60 GHz usage models, ranging from peer-to-peer high data rate communications to a complete home wireless high definition network between electronic devices.

 

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References

[1] International Telecommunication Union. [Online]. http://www.itu.int/
[2][3][4][5]
[6]

[7]

[8]

IEEE Std 521-2002, IEEE Standard Letter Designations for Radar-Frequency Bands, 2003.Su-Khiong (SK) Yong, Pengfei Xia and Alberto Valdes Garcia, 60 GHz Technology for Gbps WLAN and WPAN: From Theory to Practice, John Wiley & Sons, 2011.IEEE Std 802.15.3c-2009, Wireless MAC and PHY Specifications for High Rate WPANs, Oct. 2009.”Standard ECMA-387 – High Rate 60GHz PHY, MAC and HDMI PAL,” ECMA International, Dec. 2008.

“WirelessHD Specification Version 1.1 Overview”, WirelessHD, May 2010.

“Defining the Future of Multi-Gigabit Wireless Communications”, WiGig, July 2010.

Status of project IEEE802.11ad. [Online]. http://www.ieee802.org/11/Reports/tgad_update.htm

[9] Eino Kivisaari, 60 GHz MMW Applications, Helsinki University of Technology, Telecommunications and Multimedia Laboratory, Apr. 9, 2003. [Online]. http://www.tml.tkk.fi/Opinnot/T-109.551/2003/kalvot/60GHz.ppt
[10] Ali M. Niknejad and Hossein Hashemi, mm-Wave Silicon Technology: 60 GHz and Beyond, Springer, 2008.
[11] Wikipedia, Beamforming. [Online]. http://en.wikipedia.org/wiki/Beamforming
[12] B.D. Van Veen and K.M. Buckley, “Beamforming: A Versatile Approach to Spatial Filtering,” IEEE ASSP Magazine, vol.5, no.2, pp.4-24, Apr. 1988.
[13] Dixian Zhao, 60 GHz Beamforming Transmitter Design for Pulse Doppler Radar, M.Sc. thesis, Delft University of Technology, Feb. 2009.
[14] J. Bass, E. Rodriguez, J. Finnigan, C. McPheeters, Beamforming Basics, Connexions Web site, Jul. 27, 2005. [Online]. http://cnx.org/content/m12563/latest/
[15] Karen Scheir, CMOS Building Blocks for 60 GHz Phased-Array Receivers, Ph.D. thesis, Vrije Universiteit Brussel, Dec. 2009.
[16] John R. Long, RF Integrated Circuit Design, TU-Delft MSc. course lecture notes, 2009.
[17] M.J.M. Pelgrom, A.C.J. Duinmaijer and A.P.G. Welbers, “Matching Properties of MOS Transistors,” IEEE J. Solid-State Circuits, vol.24, no.5, pp.1433-1440, Oct. 1989.
[18] Advanced Design System (ADS), Agilent Technologies. [Online]. http://www.agilent.com/find/eesof-ads
[19] HFSS, Ansoft. [Online]. http://www.ansoft.com/products/hf/hfss/
[20] Software for IC Design and Circuit Design Verification, Mentor Graphics. [Online]. http://www.mentor.com/products/ic_nanometer_design/
[21] Assura Physical Verification. [Online]. http://www.cadence.com/products/mfg/apv/

 

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