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When looking at the RAN you may not think of backhaul or fronthaul as a component, but it is a critical one. Think about it, without backhaul you have no connection to the core, and without fronthaul, you have no connection between the BBU and the radio.

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Let’s start with the backhaul. The backhaul is the connection between the BBU and the core, not a plug and play device just yet, although the small cell has that aspect of it. At the site, you need to have the components to make the connections happen.

First, let’s cover what the backhaul is. Today’s network will have an all-IP backhaul. What this means is that it will have an Ethernet connection to the router. The formats will all be IP-based for 4G and 5G connections. LTE is an all-IP format, as 5G will be. Remember that LTE is one of the building blocks of 5G.

In the days of CDMA and GSM, what we called 3G, they had traditional telco formats like T1 and DS3. These formats worked great at that time, and they were the foundation for what telco had to offer. However, they were over copper. They had limited bandwidth whereas today, with fiber, we can get more bandwidth. When building these systems, there is a need for more and more bandwidth. While DS3 could supply up to 155Mbps of bandwidth, it took more equipment to take it from IP to DS3 format and back again, so now Ethernet connections are the standard in most carrier backhaul systems.

What do you have in the backhaul and fronthaul components? You have the router at the RAN. Chances are the router will be Ethernet

in and Ethernet out. 3G systems used T1 and DS3 formats for the connection to the internet, but now all connections are pretty much IP in and out.

The standard connections could be copper, fiber, or microwave. Fiber is the most common for macro sites because they can deliver speeds greater than 100Mbps, in fact, as we go to 5G, the carriers will expect 1Gbps and up. Microwave is trying to catch up. You can find backhaul that can do 1Gbps links, but the hops are very short and LOS. You also should worry about latency, which is a real issue with fronthaul. We’ll get into that later.

The Wireless Deployment Handbook  Paperback and

The Wireless Deployment Handbook eBook that covers professional carrier end to end deployment of LTE small cells, CRAN, and DAS showing you the proper way to plan for deployment then execute.

Then, out of the router, you will have IP access which may go to a switch to distribute the data among the components in the BTS. While the primary purpose is to connect to the BBU for the backhaul, it also passes more information back to the core such as alarms and BTS status. There is also a control channel for the remote MME to manage the BTS. With the IP connection, there are so many things you can monitor and control Most OEMs already have most of this built into their alarming systems. They even look at temperature and open doors. Some carriers are running video back through the backhaul so that they can see what’s going on at the site when no one is supposed to be there. However, the data to the BBU is the top priority, and video is a convenience at best.

Fronthaul is a connection between the BBU and the radio. In the case of a macro site, you have a fiber run, generally in a hybrid cable, between the BBU and the radio head on or at the tower which could be a simple piece of fiber connecting the 2. The reason they call the cable a hybrid is that it will have 3 to 9 or more fiber strands running through it along with power for the radio heads. The power lines are copper lines for DC power up the tower. There can be AC power on these lines, but it would be low power, chances are DC or AC it will be 48V or less. Does it have to be big enough to carry the current to run 3, 6, or 9 radio heads on the tower? There is loss through the cable that, if the engineering is wrong then you could have problems. Radio heads need power to work.

Fronthaul at the tower is straightforward. However, in today’s world, we have small cells and remote radio heads that are part of CRAN, Concentrated RAN, systems, and we have radio heads that could be part of a cRAN, cloud RAN. The idea of these systems is that the controller, a BBU, will be at a remote location controlling several radio heads from that location. Generally, in CRAN they are called BBU hotels, making maintenance and control of multiple radio heads at remote locations a lot easier when the tech can go to the one location to make changes or upgrades.

So, fronthaul will have a router, and most of the time it is fiber. You could also have microwave. Copper is not too common because they want dedicated connections, fiber and microwave offer that. Copper does not.

The issue with fronthaul is that the latency must be very low, there are communication timing issues between the BBU and the Radio Head and the UE that are critical. You don’t want the packet to time out, so you have distance limitations with fiber and microwave. Fiber is clean and works very well. Some microwave systems have longer delays due to the conversion between the data and microwave which can be an issue when transmitting signals because if they time out, then it causes retransmissions which will cause problems in the network if there are too many.  Yes, there are delays through the microwave system usually from converting from IP to RF then from RF to IP on both sides. It takes processing power, and if there is a problem with the link, noise or interference, then the RF side will start data recovery and possibly retransmissions.

Let’s look at the backhaul connection. You can have fiber, copper, microwave, or other connections.

Fiber connections:

The most desired connection to the core. Fiber allows a huge amount of bandwidth. Over 1Gbps of bandwidth is available with the right equipment. You have limitations, but it works well.

What options do you have for fiber?

• Dark Fiber – this is an unused dedicated fiber optic cable that to the customer’s purpose. In other words, you aren’t sharing it with anyone. A dedicated connection between the RAN and another site or the core or wherever you pay to have it sent. For dark fiber, the customer, you, will need to provide all the equipment to connect. You can get large amounts of bandwidth through dark fiber, 1Gbps, maybe more. Your limitation may be your gear. It is easy to scale dark fiber. If you run your dark fiber, it can be very expensive because you must get permits, right of ways, pay pole rents, maybe trench, and so on. It can get very expensive.
• Lit fiber – this is a shared fiber, and you connect to the carrier’s equipment. The carrier could be a telco or fiber carrier or anyone who offer service. It is usually cheaper, but it is not a dedicated connection. It will still connect between 2 points, but the bandwidth may be limited because you are sharing the fiber. You may have a problem scaling up and need to coordinate with the carrier to make changes.

When is fiber used/not used?

• Macro sites that require high-capacity could connect to the core or to another macro site to save on costs.
• Fronthaul for low latency and high-capacity to connect the BBU to the remote radio head in a CRAN option.
• Small cell sites when heavily loaded or no other option is available.
• CRAN Hotel BBUs to connect to high-capacity backhaul and to connect to remote radio heads for fronthaul creating a situation where you would have several fiber runs.
• In the case of C-RAN, Cloud RAN, it would be to connect the cell that is connected and controlled by a BBU in the cloud. New in 2016 and being tested in China and the USA.
• Fiber is not available everywhere. There are issues connecting to fiber in some areas.
• Fiber could be cost prohibitive to run to your specific site which has slowed the growth of small cell sites on remote poles. The cost of getting fiber to the pole may be more than the expense of the small cell and the installation of the small cell. That has been a problem that holds back the mass deployment of small cells.
• In some cases, you have only one fiber provider to choose from, and their costs may be probative.

The Wireless Deployment Handbook  Paperback and

The Wireless Deployment Handbook eBook that covers professional carrier end to end deployment of LTE small cells, CRAN, and DAS showing you the proper way to plan for deployment then execute.

Microwave Connections:

• Point to Point, (PTP) is where you have a dedicated microwave shot between to end points.
• Point to Multi Point, (PTMP) is where you have one control point connected to multiple endpoints.
• Latency varies, and it is hard to capture in a band. Why? Let’s review this list:
  • Distance – just like fiber, the farther the data travels, the higher the latency. In microwave, the longer the link, the higher the latency.
  • Equipment – specifically the radio equipment in this case. The longer it holds on to a packet the longer the latency. The longer it takes to process the conversion from RF back to IP, the longer the latency. The longer error correction takes to complete the longer the latency.
• Spectrum, microwave can be in many spectrums that serve many purposes. High-level explanations for the US market but they could apply to the world. These are the most common. Remember that the distance and dish size and engineering will affect throughput and latency.
  • 6GHz range – general for long-range shots. Point to Point LOS (Line of Site) microwave using larger dishes for longer shots. Licensed. Used early on, but the limitations in bandwidth and the large dish size have made them less attractive to modern sites. The dishes are over 6 feet and over. However, the FCC will allow 3-foot dishes in some situations. The limitations are the spectrum, licensing, and potential interference. The FCC did allow larger channels, but the current licenses in the US make it hard to get larger channels. Antenna size is an issue, but because the propagation of 6GHz is great, meaning it can travel far, it makes it hard to license without causing someone else problems. It was great with voice channels when they could travel great distances. Public safety in rural areas relies heavily on this because many of their sites are spread out.
  • 11GHz range – generally used for midrange shots. Point to point LOS microwave using mid-size dishes, around 4 foot or so, but the FCC will allow 2-foot dishes. Licensed. Used extensively I the past and is a good midrange solution. The FCC was going to allow smaller dishes, but this band usage is high and very dense in the USA. The throughput is just over 200Mbps if properly engineered.
  • 18GHz range – generally used for short to midrange shots. Point to point LOS microwave using 1 to 4-foot dishes. Licensed. These are an attractive solution with high bandwidth. Do the engineering because these links are heavily affected by weather, specifically, rain. Bandwidth through these links could be 100Mbps up to just over 300Mbps
  • 23GHzrange – generally for very short hops. Licensed. Point to point LOS microwave using smaller dishes, around 1 to 4 foot. High throughput, 100Mbps and up. Very prone to rain degradation. Very easy to license in the USA.
  • 24GHz range – generally used for short hops. Point to point LOS microwave using 1-foot dishes could go down to 8 inches. Not licensed, very easy to license. With a throughput of 100Mbps, some companies can get this band to over 700Mbps with proper engineering, but rain is a factor when it comes to engineering these links. Very limited on distance. Interference is usually low because of the propagation properties of this spectrum. This spectrum is good for short hops.
  • 2.4GHz and 5.8GHz range – the ISM band used for short hops, (although I have seen companies connect 15 to 20-mile links). Could be PTP or PTMP. Could be LOS or Near LOS or in some cases non-LOS. Not licensed. This sub 6GHz license free spectrum is a popular choice among non-carriers because the spectrum is free and the hardware is cost-effective using smaller dishes (or panels) which are easy to install and setup. No license makes it easy to deploy anywhere, and the low-cost equipment makes it affordable to deploy anywhere. A short hop solution but there are claims that are using the right size dishes that it can be a long-haul solution. The downside is that it’s prone to interference because anyone can put them up or any Wi-Fi hotspot may affect it. They are easy to deploy. Throughput varies on the engineering but generally, 10Mbps to 150Mbps. I have seen more throughput, but it takes the right design and engineering to get it.
  • E-band 71-76GHz and 81 – 86GHz range – generally for very short distances, prone to weather issues. Dishes are very small, under 2 foot. Point to point hops.  Licensed links, but light licensed, so getting the license is very easy in the US and Europe. These are a popular choice for short hops that could need up to 1Gbps of throughput. Very high throughput looks like a fiber connection.
  • 60GHz – generally for very short hops. Point to point, but there is talk of a multi-point product coming out. Dishes are 6 inches to 2 foot. Throughput is very high, over 1Gbps.

When is Microwave used/not used?

• Microwave is a cost-effective alternative to fiber, but can only be used in specific cases. Your paying for the hardware, so CapEx is higher. The OpEx is lower because the only reoccupying cost is license renewal and tower rent if you’re paying it, and maintenance.
• Microwave works for macro and small cells for backhaul or fronthaul.
• Microwave does have its drawbacks because it is a limited solution, although a very cost-effective one if you’re looking at OpEx.

So, when looking at fronthaul or backhaul you have:

• Router.
• A connection from point a to point b, fiber, microwave, or copper.
• Switch (if needed).

What is LTE UE backhaul?

It is backhaul that uses the carrier’s spectrum, just like the UE, User Equipment, your smartphone. If you have ever used your smartphone as a Wi-Fi hotspot, then you know the concept, using the carrier’s backhaul to create a new hotspot. Now imagine taking your usage and multiply by hundreds or thousands of megabits. The UE backhaul device in something that will use the carrier’s LTE spectrum for backhaul. This is something that is commonly used for internet access when there is no Wi-Fi available. The carriers all sell these units and many of today’s smartphones do something similar. However, they just use the standard signal. Using it for a tiny hotspot and for an eNodeB are 2 different things.

Let’s talk hotspot. Many vendors provide equipment that a user can add coverage quickly and easily. It is a quick Wi-Fi connection to the internet using the carrier’s LTE to connect to the internet. Everyone has Wi-Fi, and there are devices that create an instant hotspot. Verizon has the Mi-Fi, or you can use your smartphone as a hotspot. Every carrier has a wireless modem that you will provide a Wi-Fi hotspot. I think anyone who is reading this knows about the hotspots. I thought it would be a good example to get started.

What is a cell extender? There is a practice where many carriers will use a cell extender that will have a UE relay backhaul to extend the signal. This is also like a smartphone hotspot or a Mi-Fi unit because it was just to help a few customers but extends the carrier’s signal instead of Wi-Fi. This is a type of repeater to extend the macro’s signal, a cell extender. This is a way for the carrier to extend the coverage just a little bit farther. It’s a way to provide coverage someplace quickly and easily. These were common in 2G, 3G, and now LTE. It is a simple and quick way to install a repeater to extend carrier coverage down an ally. In the old days of DAS, this is what they did. They would take the signal where it was strong or use an antenna and amplifier to increase the strength to get it into a dead spot. People paid a lot of money for these systems.

It’s not a simple cell extender, and let me tell you why. Now you are talking about putting the small cell in an area where there is a loading issue. This goes beyond coverage. The data and spectrum usage could go through the roof! If you set it up like a cell extender with backhaul to the macro site, then guess what! You will see an overloaded macro sector! The macro not only has to deal with all its users but all the small cell or Mini macro users too. This sucks up all the spectrum and bandwidth for that sector.  What can be done? Read on!

I am bringing this up because now there is talk about using the UE backhaul for small cells, mini-macros, and macro cell sites. It’s making a more powerful cell extender. It sounds like a great idea on the surface. This is a cheap, quick and easy backhaul. However, what is the drawback? It’s not as easy as you think, the carrier needs to set up the donor site properly. I mentioned it earlier, and it is not something you just throw out there to feed a cell site. It draws a ton of data.  It sounds like a great idea on the surface. It looks like a cheap, quick and easy backhaul.

The donor site needs to break the bottleneck. You need to dedicate spectrum in the macro eNodeB that will be feeding the UE backhaul. This will alleviate the spectrum usage for the regular users on the macro sector. We don’t want them to get knocked off if the small cell US backhaul overloads the macro. This will make it so that the users on the macro don’t get shut knocked off if the small cell pulls the entire spectrum of its users. This will allow the small cell UE backhaul to have a dedicated pipe. It needs to have dedicated spectrum for this purpose. Then the small cell will know how much backhaul spectrum it has to available. By the way, not an easy change, changes in the eNodeB and possibly the core need to be considered as well as neighboring sites. This “dedicated backhaul spectrum” needs to be set aside for this sector and others too. It takes some planning and changes.

You could still have the data bottleneck at the macro’s backhaul. That’s another issue that needs planning.

So now you dedicated part of the band to the UE backhaul, which seems OK. Remember that the carrier paid a lot of money for that spectrum and now they are choosing to use it for backhaul. So the pipe is limited based on coverage and availability. It is a quick and easy to add UE backhaul, but is this the best use of the spectrum? Will you lose something in this backhaul? Yes, you have delay issues, timing issues, and neighbor issues. All of this is a problem when building a site for any type of real loading. Go to the links below to learn more.

However, what’s the real issue? Is it all the problems I mentioned above? They are all technical issues that good engineers will resolve. This appears to be a cheap and quick solution. But that’s not the real issue, is it? The carriers paid a crap ton of money for spectrum. Is backhaul a smart way to use this resource? Is that billion-dollar investment there to save some CapEx for the company? I thought it was for the customers! Backhaul could have been something in the unlicensed band for a lot less money. It could be a fiber link for more money. Is this an easy out or will it cause problems down the road because the spectrum is only going to get more and more valuable? Do investors want to see that spectrum used this way?  I don’t see the auctions being a cheap alternative to providing backhaul.

So just because it looks cheap and easy doesn’t mean it’s a good move strategically. Don’t get me wrong; the UE relays, the repeaters serve an important purpose for coverage and filling holes, I am just saying be strategic and think it through. For more information hit the links below to learn about these solutions.

If delays were lower, this would be a great technology for fronthaul, now that would be something!

Resources:

• https://www.nttdocomo.co.jp/english/binary/pdf/corporate/technology/rd/technical_journal/bn/vol12_2/vol12_2_029en.pdf
• http://lteworld.org/blog/introduction-relay-nodes-lte-advanced
• http://wireless.skku.edu/english/UserFiles/File/1569472705.pdf
• http://www.ericsson.com/res/docs/2013/lte-in-band-relay-prototype-and-field-measurement.pdf
• http://www.interdigital.com/research_papers/2012_01_13_system_architecture_for_a_cellular_network_with_ue_relays_for_capacity_and_coverage_enhancement
• http://www.airspan.com/products/airvelocity-2/ 
• www.cradlepoint.com

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