It used to be simple. Almost all cellular basestations were connected by separate backhaul connections that didn’t use any precious cellular spectrum, whether fibre, copper, point-to-multipoint wireless, point-to-point wireless or satellite. There were a few scenarios which used repeaters. More recently we’ve seen LTE relay and other technologies come into play for outdoor small cells. Here we look at several different levels of inband and out of band small cell backhaul.
Often called signal boosters, repeaters have been a popular solution with consumers for decades, providing what is considered instant relief to poor coverage problems indoors and in more remote areas. However they’ve also been a real problem to network operators and planners because if poorly installed they can sometimes cause havoc to others nearby. The technology has evolved and improved dramatically over many years, with very sophisticated and intelligent digital products now on the market. When professionally and independently installed, these can support multiple network operators, multiple bands and even multiple technologies (3G/4G). In most cases and especially outdoors, formal approval/permission from each network operator is required because these are transmitting on their licenced spectrum.
Mobile operators sometimes themselves have installed outdoor repeaters, extending the range of remote macrocells by lighting up hidden pockets and valleys otherwise masked by surrounding hills. Directional antenna can be used to avoid interference issues. These would enhance only their signal and not those of their competitors.
The difference between a repeater and a relay is that that while a repeater simply rebroadcasts the signal (in both directions), an LTE relay actively decodes, cleans the datastream by applying error correction and regenerates a higher quality signal. It's effectively a combination of a full smartphone (UE) and small cell (eNodeB). This means the system can take full advantage of advanced features such as 256QAM modulation rates and MIMO.
The 3GPP standard includes a comprehensive LTE Relay feature, with options for both FDD/TDD modes. The spectrum used can be the same as that of the end user (inband) or use a different frequency (out of band). Network capacity is shared from a parent or host basestation, called a Donor eNodeB, which need not necessarily be modified or even aware it is being used for this purpose. 3GPP Release 9 specification 36.806 includes two alternative architectures (A and B), and a total of four variants. As far as I am aware, nobody is implementing the more complex solutions which require a lot of changes to the donor eNodeB. All variants allow for multi-hop relays (e.g. a chain of cellsites distributed along a highway). The end-to-end architecture is shown below.
Tunnelled backhaul (Full Layer 3 Relay)
Tunnelled backhaul is my terminology for 3GPP's Architecture A Alternative 1 which packages up small cell backhaul into a regular 4G datastream, completely transparent to the donor eNodeB. Parallel Wireless can do this with their small cells combining both 3G and 4G. The nearby donor macrocell simply sees what looks like another end user device and pipes the datastream back to their HetNet gateway (operating as a Relay Gateway in the standards architecture above), which decomposes it back into the regular 3G and 4G interfaces for each relevant core network nodes. This avoids any special requirements to remotely configure or manage the existing donor macrocell sites.
A completely different solution which isn't a 3GPP standard comes from KUMU networks who we’ve been following for a few years now (see our previous detailed description of their technology). They tell me that after successful technical trials with Tier 1 operators they’ve recently started rolling out commercial trials of their full duplex technology, which sends and receives on the same frequency simultaneously using unique self-cancellation technology, effectively doubling spectrum use.
We’ve been tracking KUMU’s evolution at each of the Mobile World Congress events from 2014 through 2017. Mid 2016 saw their 2nd generation product, a module of 4 x 5 inches which incorporates several custom chips. They’ve recently announced a 3rd generation chipset which will integrate the analog circuitry into a single chip by mid 2018, allowing significant size, cost and power reduction.
Deploying their self-cancellation receiver module at a small cell site, it can share/reuse the same spectrum as the small cell, effectively doubling spectrum use. It still consumes capacity and resource blocks of the donor macro, but does so more efficiently than without. Again, there is no change required at the donor site.
Out of band wireless backhaul
Dedicated wireless backhaul products should generally be cheaper than LTE cellular solutions which are designed to cope with large numbers of fast moving end user devices with widely varying data traffic speeds, signal quality levels and services.
There are too many different backhaul technologies to discuss here (you can find our comprehensive list here), but in general the price of a dedicated backhaul link should be considerably lower than a standard macrocell LTE cellular radio.
Perhaps more importantly, taking the backhaul traffic out of band frees up precious cellular spectrum.
Where donor capacity makes financial sense
It would seem non-intuitive that using relatively expensive macrocell resources can be more cost effective than dedicated out-of-band alternatives. But it does make sense as a good first step where there is no incremental cost because it's using existing otherwise dormant installed capacity.
At the cell edge, a macrocell may need to use a significantly larger proportion of its resources to provide the same service compared with serving a nearby user. This is because modulation rates will be much lower, neighbouring cell interference may be higher and therefore more resource blocks (the fundamental capacity unit of each cell) are required to send the same volume of data.
Installing a suitably sited relay or remote small cell should result in a much higher efficiency LTE datalink, significantly increasing the efficiency of the existing macrocell and thus increasing its capacity.
However at some traffic level, the resource of the macrocell is exhausted and additional investment is required. There will be a choice between adding further macrocell basestation capacity (an additional sector or carrier frequency) or installing a dedicated out-of-band backhaul link (wireless or wireline). In most cases, this would justify a separate wireless backhaul product.
KUMU technology are aiming to move that threshold higher by doubling the available capacity of the LTE link. Parallel Wireless make more comprehensive use of the existing LTE capacity by supporting both 3G and LTE at the remote site. The actual transition cost point will vary depending on many factors including equipment price, installation and operational cost, available cellular spectrum etc. Definitely one for the accountants to figure out.