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Michael Hamilton - Peraso
"Bandwidth is King” is the adage I heard about thirty years ago, and while nothing really unseats “Cash as King”, the value of frequency bandwidth certainly ranks as a contender. Not all bandwidth is created equal it seems; some spectrum slices of a few hundred MHz sell for billions of dollars, while vast expanses of millimeter wave (mmWave) territory are free to use. With 14 GHz of free spectrum in the 60GHz band available in North America and other regions, it would be reasonable to think that this territory would be heavily exploited, yet it is not.
Realistically, since the early days of LMDS, to WiMax, WiGig and recently the Terragraph program, mmWave has defied widespread adoption. Currently, the roll out of 5G FR2 Fixed Wireless Access has been delayed in favor of mid-band usage as carriers focus on low hanging fruit. While mmWave is a more complex technology to roll out, eventually carriers will embrace this spectrum as existing mid-band spectrum is fully utilized.
One of the reasons that the ramp up of mmWave adoption has been hindered is that the propagation characteristics of mmWave frequencies are unforgiving. Line of sight or a simple reflective path is almost always necessary, and penetration of building materials is generally poor. In fact, the absorption of energy in the 57 to 61 GHz range by O2 molecules is so dramatic, over even relatively modest ranges, that the band was deemed undesirable and granted the license free status in many regions globally. These realities limit applications which may require seamless mobile connectivity in uncontrolled environments and long-range operation.
Another reason is that mmWave is relatively new to the consumer world and a certain learning curve must be pursued to fit the technology to new applications. Nonetheless, progress is being made in semiconductor technology and systems engineering which is enabling applications such as Fixed Wireless Access (FWA) to demonstrate capabilities with a competitive edge over fiber and cable broadband services.
Over the last decade or so, driven largely by intrepid start-ups and a few advanced research groups, semiconductor technology has evolved and monolithic RFIC’s with powerful beamforming capabilities have been brought to the consumer and enterprise market. With the ratification of IEEE 802.11ad in late 2012, several companies introduced highly integrated beamforming 60GHz systems using CMOS and SiGe fabrication.
These products demonstrated that the technologies which were once only seen in aviation, satellite and military applications could be brought to the consumer market at a reasonable price. Furthermore, systems based on IEEE 802.11ad and the subsequent 802.11ay demonstrated some key differentiation from the traditional Wi-Fi systems operating in 2.4 and 5 GHz bands.
A concern for service providers is that operation in a license free band is subject to potential interference which can be detrimental to customers who demand reliable and constant data flow. The Wi-Fi 2.4 and 5 GHz bands have become very popular for all consumer applications and very crowded. No matter what is the “number on the box” for the latest Wi-Fi router, it is subject to the availability of clear spectrum in the neighborhood. Millimeter wave system greatly mitigate this problem by the nature of their tight-beam beamforming techniques which create true spatial separation between neighboring systems.
Millimeter wave radios use many RF beamforming elements, typically 16 to 64, in a compact array to create a tight beam, typically 5 to 15 degrees wide, which can be directed over a wide sector of space. This differs significantly from the approach used by traditional Wi-Fi which uses just a few antennas in a MIMO configuration which does not result in much beam narrowing, thus allowing significant interference between systems operating on overlapping channels.
The narrow beams created with mmWave beamforming also have an added benefit of making them more difficult to intercept by an off-axis listener, thus adding an additional layer of physical security. This can be especially important in applications where “low probability of intercept” is a key functional requirement.
Regulations carry a requirement for the use of a listen-before-talk (LBT) protocol to mitigate interference, but they do not carry strict requirements for RF requirements such as transmission channel masks, Tx power control, or adjacent channel rejection. Implementation differences may be significant and will impact network planning. The combination of beamforming and high antenna gain does provide special separation and isolation, but side-lobes and overlapping beams from neighboring networks may still have some impact. Some systems offer antenna options or even allow customized tailoring of the beam patterns and these features can reduce sidelobe effects.
Compliance with recommended channel masks and utilization of receiver channel filtering will help adjacent channel networks to coexist without stepping on each other when beam isolation alone is not enough. Additionally, Automatic Transmitter Power Control (ATPC) can be used to ensure only enough power is transmitted under the current circumstances to maintain the link performance.
An advancement for FWA applications was the addition of spectrum from 66 to 71 GHz, which is not impacted by oxygen absorption. This allows 60 GHz networks to break away from the physical constraints which limited operation to several hundred meters, to now allowing ranges of a few kilometers with small, phased array beamforming antennas.
Regulatory changes have also been introduced which allow the use of parabolic reflectors to increase EIRP and system gain. Utilizing these antennas for CPE devices can further extend network coverage and enable backhaul links with dishes at each end that exceed 20km range.
The plots below illustrate the user available TCP/IP throughput vs.range and channel for various configurations of Peraso’s Perspectus modules. In some configurations the PR2141XM module is mated with a dish reflector to increase gain.
Perspectus Rate Vs Range.Point to Multi-point.Array Antennas.
Perspectus Rate Vs Range.Point to Multi-point.Dish at STA.
Peraso Perspectus Rate Vs.Range.Point to Point.Dish at AP and STAThe IEEE 802.11ad and IEEE 802.11ay standards allow for single carrier and OFDM modulation types. To date, all vendors have chosen to implement single carrier modulation in order to maximize RF performance as single carrier does not require as much RF linearity as multi-carrier modulation. With IEEE 802.11ad, user available data rates reach a little over 3Gbps and with IEEE 802.11ay the data rate can theoretically be greatly extended through utilization of channel bonding, MIMO, and higher-level modulation.
mmWave for License Free FWA
Fixed Wireless Access in any form provides an alternative to copper, cable, fiber, and satellite services and has often been used to provide service to underserved or remote communities. It has also been used in urban areas where installation of new hardwired infrastructure is too expensive or disruptive. In the past, wireless services have not done very well in direct competition where cable or fiber services are installed as the wireless systems have not been able to keep pace with the bandwidth offered by hardline installations. This situation has changed with the introduction of mmWave systems which offer competitive multi-gigabit service with very low data latencies.
Other than major carriers who leverage mobile infrastructure, FWA providers are generally smaller operators, often specializing in a geographic region. As such, they are very sensitive to the cost of hardware, installation, and operation. Operating in a license free band is an attractive incentive, but this may easily be negated if the cost of mmWave infrastructure equipment and CPE units is burdensome.
With a clear focus on the requirements of the target market, equipment providers have been able to leverage the latest generations of mmWave silicon and off-the-shelf modules to produce a range of attractive equipment. There are some interesting key features which define the equipment which is helping carve out a significant market.
While there are commonalities among the hardware systems offered, it is also important for each vendor to have the freedom to provide value-added features. These may come in the form of subscriber management tools, traffic management capabilities, optimized beam patterns and packaging solutions.
Modifications to the MAC firmware may be possible to optimize the air protocol for specific applications. For example, capabilities may be expanded to support dynamic network topologies such as a mesh. MAC modifications typically require intense software engineering effort with intimate knowledge of real-time MAC and PHY requirements.
Conclusion
Fixed Wireless operators are taking notice of new mmWave radio systems with the right combination of features, performance, and cost. Some operators are exclusively utilizing mmWave, while others take a heterogeneous approach using a mix of bands.
Peraso Perspectus 60 GHz mmWave Modules
To facilitate the development of new products, component vendors, such as Peraso Inc, have introduced integrated modules including the baseband, RF and antenna in a single, on cost effective PCB assembly. These allow system vendors to focus on value-added features, achieve a fast time-to-market, and provided terminals which can sell for a few hundred dollars.
With plenty of spectrum and low-cost hardware, and license free mmWave is the way to grow WISP networks in both coverage and bandwidth.
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