With long term evolution (LTE) technology gaining ground, mobile operators are facing challenges related to large amounts of data traffic, which is putting pressure on them to deliver more network capacity and enhance user experience. As a consequence, mobile operators are looking to upgrade or replace their backhaul equipment to ensure the faster transmission of voice and data through higher capacities. Backhaul solutions, therefore, play a key role in improving overall network performance and user experience. As network density increases with higher capacities, backhaul needs to be aligned with radio access capacities to avoid creating bottlenecks. This requires a continuous evolution in backhaul solutions to meet the growing demand for mobile broadband backhaul capacity. These solutions also need to be equipped with site-specific capacity requirements, which differ according to target data rates and population density.

tele.net takes a look at various backhaul technologies and the associated deployment trends likely to emerge in the future…

Copper backhaul

Digital subscriber lines (DSL) or copper-based cables have been the traditional choice for backhaul. However, while they can support transmission at speeds of 1.5 Mbps-2 Mbps, they cannot be scaled up easily for providing adequate bandwidth at a distance above a few hundred metres. This is a requirement for 3G and LTE usage. Bonding configurations are required for longer distances, which cost more with higher bandwidth requirements. For 3G backhaul, xDSL variants are used, under which bonding configurations can achieve bandwidths of hundreds of Mbps, thereby offering a viable offload solution. However, DSL bandwidth is inversely proportional to distance, which means a longer DSL connection distance between the cell site and the aggregation point could lead to a lower bandwidth connection. Thus, depending on the DSL variant in use, the reach of DSL backhaul for LTE is limited to around 500 metres.

Fibre optic

Since the early 1980s, legacy backhaul systems have delivered traffic between cell sites and mobile switching centres (MSCs) using copper-based T1 lines. However, the demand for backhaul bandwidth increased with the rapid growth of data traffic, which led to the widespread adoption of fibre-to- the-tower (FTTT) technology. Under this, optical fibre, which has a large bandwidth, was used as the physical medium in connecting cell sites to MSCs.

At present, several techniques are used for deploying fibre without any bandwidth constraints. For instance, the wavelength division multiplexing  technique combines multiple optical signals by carrying each signal on a different wavelength. An improvement over this is dense wavelength division multiplexing, which utilises close channel spacing to deliver even more throughput per fibre. The modern systems based on these new techniques can handle up to 160 optical signals, each with a bandwidth of 10 Gbps, achieving a capacity of 1.6 terabytes per second per fibre.

Thus, these systems can help reduce the need for additional fibre in current networks. As LTE gains further traction across the world, it will accelerate the demand for FTTT and require operators to upgrade their backhaul networks to fibre-based carrier Ethernet.

Microwave

At the moment, microwave technology is dominating the transmission technology market for mobile backhaul as it connects about 60 per cent of all base stations worldwide. It has achieved widespread deployment as it facilitates cost efficient and faster roll-outs of mobile broadband services. Over the past few years, due to the greater availability of spectrum, microwave can now provide bandwidth of over 1 Gbps per site, with a potential for going up to 10 Gbps. A recent report by Ericsson, titled “Microwave Towards 2020”, estimates that even as the total number of connections grows, microwave’s share of the market will account for around 50 per cent of all base stations. Even though its overall share in the backhaul market could decrease over the coming years, the volume of macro and small cell base stations will continue to increase, ensuring that the total number of microwave-connected base stations goes up as well. It will also play a key role in last mile access and complement the aggregation of networks. 

In the coming years, the primary challenge for backhaul networks will be to support the delivery of higher capacity and download speeds per user. Microwave networks are already capable of high bandwidth, and efforts are on to make them evolve so they can utilise available spectrum in a better manner. This will be achieved by improving link gain and through features like multilayer header compression,  adaptive multicarrier modulation, and radio link bonding. However, the availability of microwave spectrum in terms of more bandwidth and new spectrum will remain a crucial factor for these microwave backhaul innovations, and be essential for facilitating mobile broadband evolution.

Small cells

With LTE traffic increasing globally, small cell arrangements are being considered part of backhaul deployments. Small cells include low power microcells, picocells, and even femtocells, which are deployed on public and private infrastructure within the urban environment. So far, small cell deployments have mainly been concentrated in Europe (3G) and the US (LTE). However, they have not gained traction as their introduction in radio access networks as a complement to macrocell layers brings with it several backhaul challenges.

Typical outdoor small cell sites are 3-6 metres above street level on lamp posts or building facades. They require more cost-effective, scalable and easy-to-install backhaul solutions that support a uniform user experience across the entire radio access network. Traditional backhaul technologies like microwave, fibre and copper are being adapted to meet this emerging need. However, due to their positioning, small cells are typically without access to either a wired backhaul or without a clear non-line of sight (NLOS) to an existing macrocell or remote fibre backhaul point of presence.

Typically, frequencies below 6 GHz are required to ensure performance in locations where NLOS conditions exist. However, this kind of spectrum is limited and insufficient for small cell backhaul deployment. Thus, research is being carried out on higher frequency microwave systems in NLOS greater than 20 GHz and these have entered the commercial deployment stage. These systems can overcome obstructions over short hops between macro- and small cells through the use of diffraction and reflection techniques. This helps in achieving four to six times more capacity in backhaul compared to traditional NLOS solutions, which enables operators to deploy them at optimal locations.

At present, fibre optic backhaul for small cell deployment remains costly and logistically challenging to execute on a comprehensive scale. However, an October 2014 report by the GSM Association, titled “Wireless Backhaul Spectrum Policy Recommendations and Analysis” postulates that fibre optic backhaul for small cells will gain traction in cities with extensive fibre optic provisioning. These include Tokyo, Japan and Seoul, where small cells are currently serviced by fibre optic backhaul links in some locations. Meanwhile, traditional microwave equipment will remain the technology for small cell backhaul applications in the future.

Satellite backhaul technology

Satellite backhaul technology is used for rural areas without any wired broadband connectivity. Traditionally, these deployments do not provide next-generation

services and are limited to providing 2G coverage for voice and minimal mobile broadband. However, with the rise of small cell technology, operators have started exploring satellite backhaul as a viable alternative to traditional backhaul solutions.

Small cell networks are significantly less costly, and, when combined with low-cost satellite modems or routers, they could allow operators to expand coverage into rural areas more quickly and economically. Such networks can also allow them to operate smaller networks on board ships, in aircraft, or in remote mining areas.

At present, satellite backhaul solution providers are innovating on tackling higher data traffic protocols, using techniques like data compression, byte-level caching, predictive cache loading, data stream de-duplication, data compression, and protocol optimisation.

Going forward

In sum, small cells will be the mainstay of backhaul equipment in the near future due to their greater bandwidth and ability to be deployed at optimal locations. According to a recent study by ABI Research, the small cell backhaul equipment market will cross $4 billion in 2020. In terms of geography, the North American, European and Asia-Pacific regions will drive small cell installations between 2015 and 2020 as their LTE networks evolve. In addition, sub-6 GHz technology will be able to capture the largest share of the small cell backhaul market as additional spectrum is made available, while millimetre wave technology will be the fastest growing technology over the next few years.