Part II: The Smoke and Mirrors behind Mobile Networks for 2G/3G/4G Technology
By Bradley Smith
Before we talk about the challenges and opportunities with 5G technology in a future article, I’d like to use this time to recap some of the technology that has been used to build and maintain the existing mobile network. My previous article introduced the basic building blocks of mobile networks, more like a "Dummies Guide to Mobile Networks in Australia".
I call this the “smoke and mirrors” behind mobile networks because there’s always lots of work required to maintain the network, once a site is built. That’s something that the public can’t really appreciate, because it’s meant to be a seamless operation that is maintained by the Carrier. It only really becomes an issue once things start to “play up” in the network – for instance, network congestion, mobile network interference, dropped calls, lack of coverage, etc.
There are various types of smoke and mirrors used by the Carriers to operate and expand the mobile network, but are primarily broken down into hardware and software. Hardware is primarily used to extend the footprint into areas where there is no mobile coverage footprint, whilst software is used to optimise the existing network and provide additional benefits for both the Carriers and the end users.
Repeaters
Back in the good old 2G days (ok, I’m showing my age here) there was a cheap way to expand the footprint in regional areas without having to cough up the big bucks. The installation of high gain repeaters (HGR’s) and low gain repeaters (LGR’s) were effective to cover areas such shopping centre floors, offices, and the like. They worked be repeating the cell coverage from the existing mobile network and adding 30 to 90 dB gain to the signal, then pumping it back in doors to where it was used by the end users. These were quite effective most of the time, however if they were installed incorrectly – or if there were change to the external landscape (eg. A building goes up next door), then this could have a detrimental effect on the LGR or HGR device, causing it to “sing” or simply not be able to repeat the signals effectively.
A repeater will “sing” when the input signal into the device is (unintentionally) amplified back into the mobile network, which is then input back into the repeater, to form an endless loop in the power being transmitted into the device and the network. This typically causes disruption to the mobile network in the form of dropped calls and interference, and can often burn out the repeaters due to the extra heat generated. As a result, their use was often limited, and were eventually phased out. Another unintentional side effect was that when the occupants of a premises moved, no-one knew who to contact with respect to getting the device removed from service. Also, we’ve come across several repeaters that should have been disconnected because a new site was built in the area, and the repeaters were not tracked and had been forgotten about.
An extension of the repeater concept was the use of medium gain repeaters (MGR’s) and frequency shifting repeaters (FSR’s) which came in around the early 2000’s and were compatible with the 3G networks. The FSR’s were able to overcome some of the aspects of “singing” that the LGR and HGR’s encountered by using a separate set of frequencies in the allocated spectrum. A piece of equipment on a tower at the “A” end would be used to translate and send the signal in a different part of the band which would then be picked up at the “B” end, translated back to the original spectrum and transmitted to the end user. This avoided the potential clash in frequencies over such a long distance, enabling the cell to be extended for a fairly low cost of the equipment deployment.
Home Zone, Cel-Fi and Wi-Fi Calling
Another piece of hardware that has been used to extend the footprint of the 3G networks is what Optus called the “Home Zone” technology, and was officially discontinued in July 2017. This was set up as either an add-on to an existing broadband or cable modem, or a swapout of the modem itself, which enabled a 3G signal to be broadcast within the home. This enabled the mobile handsets to accept incoming calls, texts, etc – and was particularly useful during the period where there were few other platforms to deliver text messages (eg. Facebook Messenger, Skype, iMessage, etc). There was also a commercial version of the Home Zone introduced, but these were not widely used.
Vodafone (and Telstra) had also introduced a signal amplifying device called Cel-Fi, for 3G/4G, that would sit near a window which was known to have had good signal strength and it would relay the signal to a far end device where typically no mobile network signal was present. This enabled the mobile signals to penetrate more effectively indoors, but were limited only to customers who asked for it, or who wanted to spend the $1,000 or so to acquire it. They sell on eBay today for well over $1,000.
About 12-18 months ago Optus (and then Telstra, and more recently Vodafone) introduced Wi-Fi Calling which enabled the eradication of the Home Zone technology, by enabling the handsets to use a VOIP-like channels on a Wi-Fi network to make and receive calls on their mobile phone. The Carrier (T/V/O) would issue an update to the handset settings which would need to be accepted by the user, and the new settings would be implemented often without the knowledge of the user.
Whilst this seems the next best thing since sliced bread, not everyone is dancing about it. Many people complain of difficulty hearing, intermittent speech and pure frustration when it’s been in use. (It can be disabled in the phone settings if desired.)
Mobile Network Features
The mobile network itself is undergoing rapid change. Before anyone ever knew what an “app” actually was, 2G data in the form of GPRS (2.5G) and EDGE (2.95G) was introduced as a layer that stole a couple of channels from the GSM network. This was one of my pet projects in Optus, upgrading 2G to increase the data speeds using EDGE. Now I’m slightly worried that this is one of my claims to fame.
Not everyone got the Memo about how this worked. In fact, there were a lot of confused customers who wandered into Optus World stores who wanted an explanation as to why their phone suddenly was displaying an “E” along the top of their screen. Was this an Error that needed to be fixed ? Fortunately after putting out a few bush fires we found that a brief educational session to the stores and reps gave them more understanding of the increase in speed that the customers were to experience as a result of the upgraded network. In GPRS the speed was 14.4k, but EDGE was able to provide around 40 – 50 kbps. Absolutely mind blowing speeds! Not. EDGE actually came in later on Optus than on Telstra and Vodafone, as the 3G networks were already being built and the expectation was that 3G was the way of the future. Oh well.
Later in my Optus employment I was PM’ing the introduction of the new 3G and 4G features into the mobile network. I must say that the Optus lab and processes that surround the introduction of the new technology are quite robust, which provides a rather smooth transition for the rollout into the live network.
Some of these features included the “stacking” of the 3G frequencies in a process called “dual cell” (DC) where two 5 MHz channels could be concatenated into a 10 MHz channel which would provide enhanced bandwidth (and therefore speed) to the end users.
Some features optimise the power that is transmitted by the base stations or mobile devices in order to lower the interference in the network. This is of significance as a 3G mobile network is typically limited by the amount of power introduced in the network – and if there is too much it will start to bring the edge of the cell in (and that is, quite dramatically) which would result in the impact of dropped calls and loss of data. So the importance of minimising power in mobile networks can’t be overstated – too much power in the spectrum reduces the effectiveness of the network, which results in lost capacity (can’t handle as many calls or data sessions) and can open up holes in the network near the borders of the cells.
Another set of features optimise the way calls or data sessions are handled on the network – whether they are sitting idly waiting for a data packet to knock on the door, or during the operation of a call. These sessions are scheduled for use in relation to other packets and sent optimally to avoid collisions.
Recap
There are many hardware and software features that operate behind the scenes to ensure that the mobile networks are operating at their full potential. Some of this technology has already come and gone – saying good bye to 2G, as well as Home Zone and most of the Repeaters.
The next article will look at the upcoming changes to infrastructure that the 5G networks will present and how our existing networks are going to be modified over the next 3-5 years in order to cater for these changes.
Without implementing the infrastructure for mobile technology we simply cannot expect “smart cities” to be a reality. And that is what we will focus on in the next article.