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We always talk about backhaul when it comes to sites. Wireless, fiber, cable, and copper are all solutions. So do many of you really know about fronthaul? This is what we use when we connect a BBU to the radio head. Is it different that backhaul? That depends where you are in the wireless ecosystem.
For this article we are talking about remote radio head small cells, not the all in one unit. These are very common indoors and outdoors in larger deployment scenarios. This is also something that will be more common when CRAN, Centralized Radio Access Networks, become more common place. Eventually, the RAN will be controlled by the cloud, for a vRAN, virtualized RAN.
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So on Macro, think about when you run fiber up the tower from the BBU, this essentially is the fronthaul where it feeds from the BBU to the remote radio head, (RRH). The data in the BBU is sent out the radio head for transmission to the user equipment, (UE). So now imagine that the BBU is located miles away from the RRH. The RRH is located where coverage is needed. Most small cells are low power units. This is very similar to a small cell and would be deployed similar to a small cell. The BBU hotel is located in one area and the link from the BBU to the RRH. So the data will leave the BBU and go into a router or fiber box and then feed dark fiber or a radio link. Personally I am not a fan of this but they are very common in the market. I prefer a small cell that is standalone with all of the components in one unit just because it’s easy. However, there are many advantages to having one BBU controlling several RRHs.
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These systems do have their advantages because the control is all in one place and can do a better job of timing and synchronization and reducing interference. If you are designing a network, then it makes 
a lot of sense to have centralized control. Centralized control will be reducing self interference, which is huge and something that most field workers could care less about until it gets to the optimization and performance phase. Then it’s an issue. Interference has to be cleaned up to improve the performance. Remember that this is all about coverage and performance. In a Het Net system we would call this eICIC, enhanced Inter Cell Interference Coordination. This is one of the reasons the cells have neighbor lists, to avoid this situation. Learn more on eICIC here, http://arxiv.org/pdf/1302.3784.pdf
So, back to fronthaul In this case timing is critical, and by timing I mean latency between the BBU and the RRH. It needs to be a specific time, depending on OEM. The link must arrive in time to properly send the data and have the timing set up properly. If it is late, packets are lost, and then people complain or it has to be sent again causing congestion. This is critical in voice communications.
This is why many carriers like fiber for the front haul because it’s clean and fast. It is usually easy to predict the delays because they are predictable unless someone screwed up a fiber connection or put a sharp bend somewhere.
Wireless links are great because it is line of sight, normally, but the radios add delay, usually with the error correction. So the distances, from what I have seen is generally lower that fiber. This could all change tomorrow but as of this time they have limitations. There are several wireless link manufacturers like EBlink working to make the fronthaul wireless link better, http://e-blink.com/.
Now, if you are doing the installation, you just do what you’re told and make sure it’s a great installation. If you are doing the design, you need to be very aware of the latency and the link delays. They will add up. The fiber will have some delays, the router will have some delays, and the equipment will induce delays. This all adds up to either success or failure. Proper planning! If you are doing the optimization, the commissioning NOC should have a good understanding of the delays of the fronthaul. So if there is an issue it needs to be considered.
Did you ever work with a CRAN? If you have done work for Verizon Wireless on oDAS, (outdoor distributed antenna system), then you may have. You see the concept behind the oDAS when using a distributed RRH is basically that of CRAN. Get it? The hBBU is located in one place, centralized, and the RRHs are distributed around where the population is.
Why do you care? Because you want to make sure you have a successful installation, commissioning, and integration. Then the commissioning should go well and everyone gets paid for the work. If there are problems, you all need to put more time into it, and chances are good that time will eat into your profit. Just think about the bottom line, proper planning and quality work adds up to a quality system and profitable work.
Be smart, be safe, and pay attention to the plan! Look for oversights and point them out.
I am putting a small cell handbook together, it should be out soon. It will be geared towards deployment but a good reference overall. It will have most of what I post but also some extra notes is it. If your interested, feel free to sign up for my newsletter below.
