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Allen Blighe offers his technical perspective on the evolution of the mobile network since the '90s


In the late 1990s, while working as an engineer in Dublin, Allen was involved in the early days of mobile network rollouts. At that time, 2G technology, driven by GSM (Global System for Mobile Communications), was the standard, with limited possibilities for mobile data. Looking back, Allen describes what was considered at the time high capacity in microwave radio. Fast forward 24 years to today and we’re seeing 10Gb/s EBand technology links being deployed, primarily driven by the insatiable demand for mobile coverage and data.


As we look ahead, Allen shares his insights into the future, particularly as mobile telephony advances beyond the realm of 5G.


Early Days


When I was a graduate engineer in 1999 working for a large Irish mobile network operator, I remember the upgrade of a key microwave link serving Dublin city passing through St Stephen’s Green shopping centre. This link was upgraded from a 32Mb/s single polar link to a cross polar link supporting 64Mb/s total. Back in those days, with only GSM in use at the time, the bandwidth requirements for a single 2G mast were less than 2Mb/s, and this upgrade seemed to be ludicrously over dimensioned for its purpose. Fast forward 24 years and we are deploying links of up to 10Gb/s using EBand technology. How did such radical growth occur?


Back in the late 90s mobile telephony was seen as a luxury and not essential, mobile data was unheard of, and home internet over fixed line was becoming popular. Mobile phone use grew massively from the late 90’s onwards, with SMS text messaging gaining popularity, giving an early indication of the possibilities of mobile data. The perception of the mobile phone changed steadily. No longer viewed as a luxury it became ubiquitous, replacing fixed line steadily in a huge cultural shift.


With that popularity any tolerance for poor coverage or slow network fault repair waned as people wanted to be contactable anywhere at any time. With emerging mobile data technologies such as GPRS and 3G, the shift to mobile data began in earnest in the 00’s. This began with primitive systems such as WAP, which developed in tandem with smart phone capability evolving into the high-speed mobile data experience that we take for granted today, i.e., high speed downloads and streaming to high definition, high-capacity devices. IoT already making its mark.


Microwave Radio


Microwave radio has always been a popular transmission medium for mobile networks due to its low cost and speed of delivery compared to fibre or other fixed line services. The growth of data per mast and per user has been massive and much of this hinging on the growth of microwave radio link capacity to meet these ever-increasing demands. Microwave radio presents a cheap and fast alternative to wired connections and allows rapid network rollout, unencumbered with the delays of digging up roads or land. Today, fibre rollout has improved, but microwave still provides a cheap and fast way to deliver the important first hop into the network for most mobile network sites, sometimes carrying this traffic over multiple radio links where fibre reach is not as extensive.


Traditional (“trad-band”) links in the frequency range 6 to 38GHz have evolved to meet the demand for increased capacity. This meant going from 3.5MHz channel sizes through 7, 14, 28 and 56MHz, and the use of cross polar technology to match a vertical and horizontal polarized channel into the same infrastructure, doubling the capacity. This is effectively having two radio links using the same dishes, radios and channels, avoiding interference by confining each link to transmit in either the vertical or horizontal plane, made possible by complex digital signal processing removing any residual interference. These days we are currently seeing the use of 112MHz channel sizes.


Over the years the modulation technology has improved vastly, going from static QPSK (quadrative phase shift keying) modulation to adaptive QAM (quadrature amplitude modulation), packing in more “bits per hertz” and adapting to provide as high a bandwidth as possible in different radio conditions e.g., rainy or foggy weather conditions.


As the technology improves, we are currently seeing the use of 4096QAM (aka 4k QAM), vastly improving the bit/hertz utilisation of available spectrum to deliver more and more capacity. Recently we have seen trad-band links with 112MHz channel size, with 4096QAM modulation in XPIC (cross polar) configuration delivering approximately 2.1Gb/s as a maximum capacity.


E-Band Technology


The last five years have also seen the widespread use of EBand technology in the 60-90GHz spectrum range in transmission networks, where very large channel sizes (up to 2000MHz) are used to deliver large capacity in the order of Gb/s. However, modulation capabilities of EBand systems lag behind trad-band, and we are seeing a maximum of 256QAM on current EBand links, delivering up to 10GB/s capacity.


The EBand spectrum range was previously under used, and so the possibilities for avoiding interference when deploying is greater. From a licencing point of view, governmental regulators tend to give “light touch” registrations for links instead of issuing specific licences for radio links as is typical for trad-band, making EBand very flexible for deployment. The very high capacity of these links again means high capacity available for RAN services and ultimately the end users.


As the spectrum is underutilised the microwave vendors have focused on using wide band channels with low modulation schemes as interference has not been a huge concern as it would be for trad- band links. However, roadmaps show that higher modulation schemes are imminent as EBand deployment matures and more bits/hertz are required.


For particularly busy links we are seeing operators delivering 112MHz XPIC (cross polar) trad-band links in parallel to high-capacity EBand links, aggregating the capacity to create microwave systems capable of more than 12GB/s per second. This provides more bandwidth between two sites, and additional resilience, as if one microwave link drops, the others are likely to stay up.


Future


As 5G proliferates and “5.5G” is in prospect, what trends can be seen in microwave to support the ever-increasing capacity demands? Vendors are forecasting use of larger 224MHz channels in trad-band links, bringing link engineering challenges in avoiding interference. They are also forecasting improved modulation up towards 16kQAM. This presents digital signal processing challenges for chip manufacturers. Qualcomm discuss their plans for chips to support 16kQAM here:



In the EBand space, vendors are working on increasing the maximum modulation towards 1024QAM (1k) which should bring a large growth capacity across all channel sizes. Also, other radio technologies in underutilised bands such as V-Band W-Band and D-Band are under development for mobile network backhaul, so as EBand use saturates and interference becomes more of an issue, fresh spectrum presents an opportunity for further growth. In addition, microwave vendors are working on ways to extend the reach of EBand using specialised antennas, allowing these links to go beyond their typical 1km reach towards 10km. This will compound the saturation of the band with longer links causing a higher probability of interference.


Ericsson, in a recently released video discussion on microwave, stated that they have achieved 100Gb/s over microwave in a test environment. I have heard other vendors discussing similar testbed results, so it is clear that the possible speeds over microwave will continue to grow, matching the demands of 5G and beyond.


Finally, 5G network requirements are also making latency (delay in sending and receiving data) more important, with strategies to split the network in front-haul, mid-haul and back-haul, placing network elements in the latency critical front haul segment in such a way as to minimise latency. The focus on microwave capacity may shift to a focus on latency as 5G use matures, and delay critical services are passing over these networks. Typical examples given to illustrate the importance of latency include the use of surgical robots for remote surgery, or a self-drive vehicle navigating via a 5G network. A lack of accuracy due to latency in such situations could mean life or death.


As mobile telephony evolves beyond 5G, microwave transmission continues to develop in tandem, with exponential growth in bandwidth/bits per hertz. Microwave systems are maxing out the possibilities of traditional frequency bands to the point of saturation with higher modulation and XPIC, proliferating the use of EBand spectrum with higher modulation and longer hops and expanding into newer W and D bands, all to keep up with mobile telephony demands. It continues to deliver a cheap and easily deployed alternative to fixed line transmission and has maintained its position as the cornerstone of mobile transmission and will do so for the foreseeable future.


About Allen Blighe, KTL Technical Lead


Allen works in Transmission on planning and operations for RAN and corporate data services since joining KTL in 2011, primarily in the microwave transmission space. With 24 years’ experience in the telecoms industry, he has worked in the design, rollout, and support of transmission networks from bespoke site-specific designs through to large scale upgrades of mobile networks.


To read more KTL technical insights blogs go to: https://www.ktl.ie/technical-insights


Keywords: Microwave Transmission, EBand Technology, EBand Spectrum, 5G, 5.5G

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