Spectral resources are not inexhaustible, a fact that is becoming increasingly obvious as demand for them keeps growing. This is requiring operators to conduct proper planning for and use these resources efficiently.

Mobile broadband is ushering in a brand-new era of communications, in which wireless network traffic will grow explosively by up to 500 times in ten years. Traditional GSM voice subscribers are no longer satisfied with low-speed data access. This has led to some tough questions: What is the right way to leverage the spectral resources of the existing networks? How should the increasing demand for mobile data services be addressed? What should be done to protect the existing network equipment investment and ensure smooth evolution to LTE? Mobile operators worldwide are desperately looking for the answers, and the GL1800 Refarming solution has been proved to be one of the best.

1800MHz: Silver frequency band

The 1800MHz frequency band is now used mainly on GSM networks the world over, according to 3GPP in its definition of frequency bands for radio communications. It is also one of the bands defined for LTE FDD, others including DD800MHz and 2600MHz. In addition, Band 3 of 1800MHz has the richest spectral resources, as its paired FDD spectrum bandwidth reaches 75MHz. More than 50 operators in over 20 European countries each have more than 10MHz bandwidth in this band, according to statistics from existing networks. Mainstream Asia-Pacific operators in countries such as China, Australia, and Singapore also each have more than 10MHz bandwidth. This basically guarantees access to spectral resources for 1800MHz frequency refarming.

More importantly, voice traffic carried over GSM900 and GSM1800 is decreasing due to the increasing penetration of 3G services, subscriber migration, and 3G traffic being mostly carried over the 2,100MHz band. Frequencies in the 1800MHz band can therefore be gradually refarmed to be used for more advanced LTE networks.

The 1800MHz band features lower propagation and penetration losses than the mainstream 2600MHz band for LTE. It achieves wider coverage as it can multiply the coverage radius of a single LTE station, slashing the number of sites and reducing carbon emissions while ensuring high-quality coverage. That will greatly reduce TCO for operators and bring end users better mobile broadband QoE. The GL1800 Refarming solution results not only in significant savings on expenditure for new spectral resources but also faster LTE network deployment. In actual deployment, operators may also opt to reuse sites or even equipment based on the status of their existing network equipment, realizing smooth evolution to LTE from GSM. When it comes to purchasing new bands and planning new networks, a refarmed 1800MHz band can play a greater role among the existing spectral resources. It is a real silver frequency band.

Three technologies critical to refarming

To come up with an effective 1800MHz frequency refarming solution, three critical technologies need to be taken into account. The first is about how to reallocate frequencies and control interferences between neighboring GSM and LTE frequencies. The second is about the method of migrating GSM voice service subscribers to release part of the 1800MHz spectrum. And the third is how different networks should be coordinated.

For frequency reallocation, there are two mainstream methods commonly used in the industry, namely full refarming and partial refarming. Full refarming is suitable for mobile operators with well developed GSM and UMTS networks and rich spectral resources. The number of GSM/GPRS subscribers is decreasing as 3G services are growing and these subscribers are being migrated to 3G networks, leading to fewer loads on the GSM network at 1800MHz. Therefore, the 1800MHz spectrum can be fully refarmed to be used for LTE networks, while all the voice services are borne by GSM900.

Partial refarming is suitable for the operators with limited spectral resources who have no UMTS networks and have difficulty in subscriber migration or who have a large number of GSM subscribers that will remain stable in the short term. These operators need to consider how to retain the existing subscribers and provide competitive high-speed mobile data access. They may therefore phase in the band refarming to LTE by 5MHz or 10MHz spectrum bandwidth. Partial refarming is done by two methods – the sandwich method and the edge allocation method. The sandwich method releases the middle portion of the 1800MHz band of an operator to LTE by 5MHz or 10MHz spectrum bandwidth. Portions on both ends are still used by GSM. As for the edge allocation method, it allocates either end of the band possessed by the operator to LTE and keeps the other end for GSM. The sandwich method is recommended, given the GSM frequency reuse plan, interferences between frequencies and, in particular, interferences with other operators. Control over interferences between GSM and LTE can be done within the frequency band owned by an operator without needing to coordinate neighboring bands of other operators.

Also based on the preceding two frequency refarming methods, space division may be used to effectively reduce mutual frequency interferences between GSM and LTE. Operators may push refarming from cities to suburbs, or vice versa, from the perspectives of network loads and subscriber needs. They may first refarm some frequencies for LTE to satisfy the demand for mobile data in urban areas, for example. As for non-urban areas, they do not need to refarm frequencies and may instead keep using their own full-bandwidth GSM bands since there is no strong demand for high-speed mobile data services in these areas.

Mutual interferences of the same band between LTE and GSM should be avoided, as they often take place when this same band is used for LTE in urban areas and still for GSM in non-unban areas. Geographically, a transitional zone may be set between a city and its suburbs, where this band is left unused, to spatially prevent interferences caused by the use of the same band for both technologies.

Migrating their GSM voice service subscribers is a tough choice for operators, who have to increase the data access capacity of the existing networks – they have to reduce the bandwidth of GSM and accommodate existing subscribers at the same time. There are now two critical technologies to deal with this issue: half-rate voice (HRV) and aggressive frequency reuse (AFR). HRV bears the same number of voice service subscribers with a half bandwidth. AFR saves on bandwidth by 25 to 50% depending on configurations ranging from S2/2/2 to S7/7/7. Operators may employ those technologies flexibly.

Coordination between GSM/UMTS and LTE networks is another critical issue in frequency refarming. It guarantees load balance between different types of networks and the inheritance of voice services, while providing network selection priorities based on services, loads, and subscriber characteristics. Meanwhile, mobility should be guaranteed between the two networks, including handover and interoperability between the CS and PS domains. 3GPP has already defined the coordination between heterogeneous networks and set down specifications, laying a foundation for coordination between networks using different technologies.

Three schemes for refarming solution deployment

If the preceding technologies are preconditions for deploying the GL1800 Refarming solution, then equipment performance and evolution capabilities are tied to the TCO, and the progress and feasibility of network deployment.

As the first of its kind, Huawei’s SingleRAN solution supports GSM, UMTS, and LTE as well as the five bands of DD800MHz, 900MHz, 1800MHz, 2100MHz, and 2600MHz, in the same cabinet. Its advantages become more outstanding in GL1800 deployment. This solution supports sharing of stations, transmission, maintenance, and RF. It helps operators evolve smoothly to LTE1800 from GSM1800, or realize coexistence of GSM and LTE in the 1800MHz band.

As can be seen from RF modules to antenna feeder deployment schemes, the core issue with deploying a new LTE1800 network on top of the existing GSM1800 network is the RF module-based antenna feeder solution. There are three deployment schemes by scenarios:

The first scheme is completely separating RF modules and antenna feeders. As GSM1800 RF modules do not support multi-mode capabilities and have limited transmission power, RF modules and antenna feeders are separated to ensure the quality of coverage. This scheme is characterized by high investment, the requirement for a site, and difficulties in deployment.

The second is separating RF modules but sharing antenna feeders. This scheme is suitable for scenarios where RF module’s multi-mode support capabilities or operating bandwidths are limited. Though requiring new RF modules, the scheme allows for the sharing of the antenna feeder system through combiners to save on the space where antenna feeders are mounted. The negative side of this scheme includes the addition of combiners and, typically, additional 3dB insertion loss, which affects the transmission power to some extent.

The third is sharing both RF modules and antenna feeders. This scheme fully reuses the existing antenna feeder system and RF modules with no need to add any hardware after they come out of the base band’s CRPI. Nonetheless, this scheme requires RF modules to have multi-mode capabilities and higher operating bandwidths, guarantee use in discrete frequencies at 1800MHz, and support future bandwidth increases. Meanwhile, they should accordingly have higher transmission power. In Huawei’s SingleRAN solution, the RF modules feature multi-mode capabilities, dual channels, full bandwidth, and high power, helping operators save on CAPEX and facilitating network deployment. The solution is the best choice for GL1800 refarming.

The 1800MHz band is one of the main frequency bands commonly used by mobile operators, and also an effective supplement to the GSM900 network. In the foreseeable future, with mobile data services leapfrogging, traditional GSM voice service subscribers will be gradually migrated to UMTS or LTE networks. Given current spectral resources, the GL1800 Refarming is one of the best solutions for keeping network competitiveness, reducing investment and rapidly deploying LTE networks.