The jury is still out on how to meet the backhaul requirements of long-term evolution.
The roll-out of long-term evolution (LTE) mobile broadband technology is gaining momentum around the world, with more than 240 service providers commercially implementing it by the end of 2013, according to Global Mobile Suppliers Association (GSA). In South Africa, the incumbent operators have already rolled out the initial phases of their next-generation networks.
To ensure global uniformity, the 3rd Generation Partnership Project, or 3GPP - the LTE standards body - has detailed the requirements of the new radio and core networks. However, the organisation has not detailed the requirements for the packet backhaul network that links the two networks together. Therefore, the industry has been left to extrapolate requirements for the backhaul network based on the best-case radio interface capabilities.
This has led to exaggerated predictions and has been insufficient to plan backhaul networks. The factors of LTE channel capacity, radio propagation, cell site design and traffic aggregation need to be combined to realistically estimate the backhaul capacity requirements of LTE.
Base stations transmit and receive user traffic over the assigned LTE radio channel. Traffic volume is directly proportional to the LTE radio channel size. 3GPP defined channels of 5MHz, 10MHz, 20MHz and others in Release 8 of the LTE standard. The standard also defines radio interface spectral efficiency targets for download (5bits/Hz) and for upload (2.5bits/Hz) of this radio channel.
To estimate the available over-the-air LTE radio peak rate, providers need to look at how an LTE radio base station maximises the available radio channel. The main parameter is the received signal quality measured at the handset. The closer a handset is to the base station, the better the radio signal quality and the higher the radio peak rate.
All LTE peak rate figures assume the use of the entire LTE cell by a single user handset, and include a radio layer overhead. In reality, multiple handsets will share the available radio resources of the LTE cell, resulting in a lower peak rate, as well as lower average throughput per user.
Having estimated the LTE radio cell or sector peak data throughput, realistic cell site backhaul requirements can then be determined. Each LTE cell site typically combines three LTE radio sectors into one macro site.
The LTE standard is evolving, and many of its aspects are still theoretical.
Because LTE handsets are statistically distributed within these three radio sectors, and don't download at maximum peak rates all the time, backhaul capacity can be split and overbooked among individual sectors. The busier a macro cell is, the lower this overbooking factor needs to be. For low usage sites, the factor can be safely increased. Existing backhaul transmission networks for 3G, CDMA and WiMax have shown that factors between two to five work quite well.
Dense urban sites certainly have higher needs than rural sites. The size of the cell coverage area is also an important factor, as it determines the mean peak rates of the handsets served by the site.
Rural sites cover a much larger area, and the majority of handsets transmit at lower peak rates, as they are further away from the cell centre.
Of course, the LTE standard is evolving, and many of its aspects are still theoretical. For example, a major factor is the available antenna technology in use between the base station and the handset. Service uptake, cell site roll-out and antenna technology progress drives the requirements for backhaul capacity. This capacity is not rising exponentially, but is rather linear and likely to level off somewhere between 75Mbps-90Mbps.
Although multiple input, multiple output (MIMO) antenna technology will enhance the peak data rates of handsets, the radio signal conditions of a 'noisy' LTE environment that experiences interference are still forming a fundamental ceiling of how much data throughput improvement is achievable.
However, even for a dense urban area with large numbers of users, a three-sector macro cell using a 20MHz radio channel plus 4x4 MIMO technology is unlikely to require more than 150Mbps-200Mbps dedicated backhaul capacity.
Improving throughput, service quality
Adding LTE radio capacity will improve user data throughput and user experience. Given that the licensed LTE radio channel size cannot be easily increased, the alternative is to deploy more LTE cell sites covering smaller radio areas. Many smaller cells, such as picocells, can supplement three-sector macro cells.
Picocells host fewer concurrent user handsets, but typically provide better radio signal quality and throughput rates to each user. This will require separate, dedicated backhaul networks for high user demand environments like sports stadiums, business parks, conference centres or public areas.
The deployment of smaller cells within the coverage area of macro cells also adds radio interference and increases handovers. Advanced LTE radio network analysis and design adaptation is therefore required to minimise the side-effects of deploying many small radio cells to enhance LTE throughput.