When designing and deploying an outdoor Private Cellular Network there may arise a time where connectivity doesn’t quite reach as far as desired, or you need to extend the signal around a large attenuating object like a mountain. Within the US, CBRS FCC regulated power levels, controlled by SAS, limit the distance. Physics has a passive solution to the second, through reflections, to getting around some large objects that prevent coverage where it is needed. GXC has developed a product that allows you to actively solve both of these problems with GXC Cellular Mesh.
Last fall, I was privileged to be invited to discuss Private Cellular with B2B Tech Influencer, Evan Kirstel, and Hardik Jain, CTO and Co-Founder of GXC. During the livestream, Evan Kirstel asked me of my experience with GXC Cellular Mesh. I had only read about GXC’s niche gear but not experienced it firsthand. Because of that conversation, GXC sent me their gear to test out and play with the Cellular Mesh specifically.
Before we dig into Cellular Mesh, understanding limitations of Wifi Mesh will show the power of Cellular Mesh. Cellular Mesh can provide solutions for Enterprises like Wifi Mesh has done for the home.
As with all my other posts, I only received the gear to play with from GXC, beyond that this blog post is not sponsored. I have not received compensation for the following content.
Mesh Wifi in the Home
Wifi has become a common standard for wireless connectivity at home. From TVs to Laptops to IOT Sensors to Doorbell Cameras, Wifi is used for everything in the home for connectivity. Many home owners are discovering the limitations of a single Wifi radio that is placed on one side of a medium to large home.
Wifi dead spots abound in many homes, and a solution has arisen to reuse the existing spectrum for creating a Mesh Wifi network. There is no need to run cables in a home to provide the coverage when you use a Wifi Mesh network.
Most home Wifi vendors sell a Mesh AP that can be used to connect to another Mesh AP. Most Enterprise Wifi APs vendors tend to avoid Mesh or require Mesh to be enabled manually. The recommendations across the industry in Enterprise is APs are required to be hardwired for good reasons.
Wifi Mesh is defined in the standard with the 802.11s amendment. In Wifi frames, there are flags called “To DS” and “From DS” (DS stands for Distribution System) that helps you identify the direction of the signal. When both To DS and From DS are set to 1, like the PCAP below, the Frame is part of a Mesh network between to two APs.
When it comes to Wifi, all devices listen for the spectrum to be available before talking. There is no centralized control; everyone fights for airtime. When using a Mesh Wifi network, the RF spectrum is cut in half exponentially with each hop. So, only up to two hops are recommended. Each AP has to contend for airtime equally to everyone else including clients. The better Wifi Mesh systems utilize multiple radios and different channels for communicating with clients verses communicating to other APs. This works because there are a small number of clients in a home.
Mesh Wifi in Businesses
Many a new Wifi Engineer that doesn’t understand Wifi has attempted to bring the same practices used at home into businesses. Many small businesses are using Mesh Wifi and not realizing the technical costs that are affecting their network. I may work but not be optimal.
The concept is the exact same in a business as in a home. In Wifi there is no centralized control and all devices, clients and APs, have to fight for airtime. QoS provides a better chance for certain types of traffic to receive airtime, but every frame still has to contend for airtime. In a small business, this may work okay if there are a limited number of clients connected. As the number of clients increase, the effects are exasperated and performance starts to decline at an exponential rate.
Keith Parsons has a saying that if it doesn’t move, hardwire it. Because airtime is limited and the number of channels are limited, it’s best to hardwire a client if at all possible. The same concept applies to the Wifi APs. A Mesh AP, has to receive and rebroadcast a frame destined for a client which then cuts the available airtime in half before it reaches a client. When you approach hundreds of clients and hundreds of APs, you start wanting to preserve airtime across all channels as much as possible. In businesses it is best to dedicate Wifi airtime for Client to AP communication instead of AP to AP communication.
Wifi vs. Private Cellular
Wifi has succeeded and become the main form of wireless connectivity around the world for a number of reasons as talked about in the book Beyond Everywhere: How Wi-Fi Became the World’s Most Beloved Technology. Ultimately, Wifi is very inefficient when it comes to how it uses the spectrum, aka the PHY. A lot of airtime is wasted because of the lack of central control. Many wonder how Wifi even works with how much overhead airtime is “wasted in arbitration”. A single frame has only a very small payload surrounded by overhead headers and IFS times slots as Keith explains in this video:
“Co-Channel Interference, Wifi, 802.11, is 100 times more sensitive to other wifi than to non wifi. It’s built right into the protocol.”
Keith Parsons
Only one Wifi AP or client is allowed to talk at a time to prevent Co-Channel Interference. Keith Parsons calls this process “the game” involving the steps that Wifi devices follow to talk.
OFDMA, adapted from the cellular world and made available in Wifi 6, brings some ability for one device to talk to multiple clients on Wifi, but every Wifi device still has to fight for airtime with all other clients.
When it comes to Cellular technologies, the protocols are very different. Cellular is much more efficient with the usage of the spectrum because everything is centrally controlled by the Core and the Radios. OFDMA in Cellular cuts up the spectrum into smaller chunks that can be used to talk to multiple clients at the same. TDD and FDD schedule the usages of the spectrum (TDD via time slots and FDD via different frequencies) for who can talk instead of randomly playing “the game” as Wifi does.
Private Cellular Bands, such as CBRS, use Time Division Duplexing (TDD). All traffic is scheduled according to a spelled out pattern (shown above) separated by time. I talk extensively about these details in previous blog posts.
Most outdoor CBRS networks use Subframe Assignment 2 (SA2),line 2 above. While TDD is not truly Full Duplex, this can guarantee and prioritize traffic between the Mesh Node and AP over client traffic. This allows Cellular Mesh to be much more efficient with the spectrum than Wifi Mesh.
A calculator, from Sandesh Dhagle, shows how much more efficient cellular can be. In this example below, showing SA2, Special Subframe Pattern 7 (SSP7), and a 10MHz wide channel, 62.7% of Frames are Downlink and 22.9% are Uplink leaving 14.4% to overhead management and control frames. Much more efficient usage of the spectrum than the example Keith Parson shows in the above video.
This efficiency is the reason Cellular Mesh improves on Wifi Mesh for enterprises. Wifi absolutely has its place and benefits over cellular. But when it comes to Mesh, Cellular Mesh much better handles the spectrum efficiently.
Now that we have explored Wifi vs. Cellular efficiency, let’s dig into GXC’s Cellular Mesh technology. Cellular Mesh on a larger scale outdoors can be the solution for enterprise that Mesh Wifi is for home users.
Private Cellular Mesh with GXC
Unlike Wifi Mesh, the Mesh technology is not built into the cellular protocols. Everything GXC is doing had to be built from the ground up and is a proprietary solution. The connection between the Cellular AP and Mesh Node is proprietary patented technology. The connection from the AP and Mesh Node to clients just uses regular cellular technologies like 4G LTE and 5G. My iPad connects to both the Gateway AP and Mesh AP without issue as most clients that support CBRS will.
Above is the GXC Mesh Node and Access Point. Currently, it is multiple radios with multiple antennas interconnected with several cables. Hopefully, future models can reduce this down to a more compact unit. Each Node has a separate directional antenna. The lower square “pizza box” antenna is the Mesh Node or Antenna that connects to the Gateway basestation shown at the beginning of this post. The upper antenna is the AP sector antenna that rebroadcasts the signal in a new direction or to extend the signal further.
When I started to learn about Private Cellular, on of my sites wanted to connect a Cellular AP using CBRS as a backhaul. I quickly discovered one issue with Cellular Mesh is timing requirements. The remote radio has to be in sync with the Gateway and Cellular Core. On top of both the Gateway AP and Mesh AP are GPS modules to ensure timing is synced across all pieces.
The other item of note is that GXC’s Mesh is only available on their outdoor gear. Part of the issue is the requirement mentioned above about timing from GPS that isn’t often available indoors. The other issue is with RF reflections indoors, that cause too much interference when attempting Cellular Mesh. So, it is only available outdoors. Indoor small cells should be hardwired, just as Wifi APs are currently installed, when deploying an indoor network.
Bouncing Signal Around Corners
Next, let’s dig into how Cellular Mesh benefits a Private Network.
Notice the pizza box antenna above is pointing one direction and the Mesh AP antenna is pointing a different direction. This essentially allows you to extend and redirect the cellular signal around objects or in new directions. As I said at the beginning, physics has a partial solution.
One of my school districts, that used to be only connected using Microwave Radios until 2020, is situated in a valley that doesn’t have good line of sight to neighboring peaks.
The school is about 40 miles from a neighboring city where the closest fiber optics used to be located. During COVID shutdowns in the fall 2020, we finally connected this last school and community with fiber optics at a cost of millions of dollars. The path to connect the Fiber had to traverse a mountain pass through hard rocks and across federal property on a dam that provides power and water to millions.
Before the fiber connection, to reach the neighboring peak where a relay station is located, engineers used a common technique of reflecting microwaves off of billboard reflectors on a nearby mountain. This technique allows engineers to design a point-to-point network around objects. The signal is focused into a narrow beam but loses power with each bounce. The billboard reflectors are passive.
Physics provides this simple solution but often this can only be used to reach a narrow site. Like a laser beam can be focus and bounced with mirrors, radio signals can be bounced using a similar reflector technique. It doesn’t provide coverage to most of the surrounding area, as can be seen above, unlike a home Wifi Mesh setup. Regular clients are unable to connect as the link is dedicated to the backhaul.
GXC’s active Mesh nodes do not use a dedicated wireless backhaul, instead they smart select and register with the strongest GXC AP. This allows for additional client devices like iPads and smartphones to connect to such a donor AP when extra capacity is available.
Extending The Cellular Signal
Another one of my schools is located at the mouth of a canyon that leads to a National Forest. Less than half a mile from the school, carrier cellular signal goes from full bars to no signal because of the geography. About 2 miles up the canyon is a small town where cellular signal is non existent. Modeling this specific location in Google Planner looks like below with just a single tower at the school.
Notice how quickly the signal is blocked by the neighboring mountains. The carrier cell tower, a couple blocks from the school, is obviously using higher power signals but struggles with this location. The geography kills the cellular signal. Every summer, I go camping up this canyon and have to drive to this school to check in with family and work for both my Verizon and TMobile cell phones.
A single private cellular tower located on the school does not provide adequate coverage, just like the carrier tower.
Using a GXC Cellular Mesh changes connectivity for this little canyon. Placing the Mesh Node and Mesh AP on the mountain less than a mile from the school expands the coverage. Also the signal around the school is still useful for other devices instead of being dedicated as a backhaul as explained before.
Imagine a mining company or other outdoor business needing coverage in this area. GXC Cellular Mesh opens doors for higher throughputs than LORAWAN, HART, or Sigfox. It also is a much cheaper cost than running fiber optics.
Jeep Testing GXC Cellular Mesh
In my next blog post, I’ll dig into actual testing of GXC’s Cellular Mesh with my Jeep. I’ll explore some technical things that I discovered and how the system performs.
GXC’s Cellular Mesh can be a solution for businesses like how Wifi Mesh solves problems in a home. Cellular Mesh introduces a much more efficient usages of the spectrum compared to Wifi Mesh.
