Microwaves hold future for rural comms

In recent years, the Asia-Pacific region has bucked the global trend of declining telecom growth. The uptake of mobile services and popularity in many territories of, for example SMS in the Philippines has seen significant growth in both subscribers and network infrastructure.

However, the increased demands for services and extending coverage to rural and remote areas has created challenges for mobile networks looking for solutions to manage their remote mobile backhaul.

Microwave radio transport has for many years provided a flexible, quick-to-deploy and cost-efficient means of transporting traffic between RBSs (radio base stations). More than 50% of the world's GSM RBSs are connected using microwave radio links.

Engineering the mobile backhaul for these networks in today's market can however be a challenge where the ever-increasing demand for coverage needs to be satisfied while the availability of spectrum resources becomes ever more limited.

To meet licence coverage obligations or to expand their subscriber bases, many mobile operators are now looking to roll out coverage to remote and often hard-to-reach areas.

In the Asia-Pacific region the challenges can be escalated where communities are situated on offshore islands, over large flood plains, among dense foliage and surrounding hills.

They may also be, by character, low-revenue or low-subscriber-density markets which places pressure on the operators to reduce both the infrastructure implementation and the operational costs.

For example, French Polynesia's national communications provider, OPT, recently extended its GSM coverage through the islands of the Marquesas archipelago. Mobile phone coverage is vital for the islands' economic development and tourism and is becoming increasingly important for maritime safety.

The very nature of the Marquesas where subscriber densities are relatively low and the geography heavily foliated and mountainous accurately reflects the challenges facing mobile operators extending coverage to these locations.

In rural areas, the distance between mobile phone RBSs is typically up to 20-35 km and up to 70 km or more with extended cells to remote communities. These distances rule out low cost, millimetre wave products that are not suitable due to high path attenuation and rain fade.

The traditional backhaul solution has been to engineer longer distance links in the 7-15 GHz bands, although they frequently have far more capacity than is actually needed for a remote/rural RBS site. Together with the capital cost of this radio equipment, these bands do require much larger, more expensive solid dish antenna and more robust mechanical mounting, tower strength and stiffness.

For transmission over longer distances dual antenna space-diversity configurations are also often essential to deliver the required level of availability, further increasing equipment and infrastructure costs.

The capital and operational costs for these over-dimensioned radio systems can be a significant determinant in the economies and viability of rural mobile phone coverage.

New alternatives

Network planners often overlook the distance potential or reach of the low microwave and upper UHF frequency bands.

There is a perception that the low frequencies are 'gone' or that mobile services have taken all available spectrum. While large chunks of the 400 MHz, 800 MHz, 900 MHz and 1500-2200 MHz spectrum are predominately assigned to mobile services, there are numerous spectrum opportunities in these ranges for narrowband, high-efficiency links with adequate capacity for rural BTS linking.

Recent advances in RF design, electronic integration and the implementation of high performance QAM (quadrature amplitude modulation) techniques have resulted in a new breed of integrated low-capacity, point-to-point digital radio systems developed to provide efficient, reliable and cost-effective transmission solutions.

High-performance 64 QAM modulation has been made practical using techniques of feed-forward, decision-feedback adaptive equalisers and forward error correction, enabling economic systems to be designed for low-cost cellular and other mobile phone radio linking.

These equalisation techniques help minimise potential transmission degradation due to multi-path and other interference sources. Selectable modulation, high system gain and the choice of operation in the UHF or low microwave frequency bands offer working ranges of typically 70 km.

Cost-efficiencies

Grid-type dish antennas are usually employed for the low-frequency microwave bands at 1.4 GHz and 2 GHz. At 400 MHz and 800/900 MHz, low-cost Yagi types can be used, but grids are preferred for their better front-to-back ratio performance. Most administrations require the use of relatively high-performance antenna for fixed links in these bands to optimise frequency reuse.

Grid antennas are cheaper than the solid types required at 7 GHz and above and the use of grids reduces tower wind loads. Antennas operating at the low-frequency bands have wider beamwidths than at higher microwave bands, reducing the requirements for tower stiffness - a further cost saving.

Feeder costs are reduced as coaxial cable can be used instead of more expensive elliptical waveguides. As all the RF equipment may be mounted indoors, further savings are possible - both in capital cost and in ongoing maintenance - when compared with split-mount implementations common above 7 GHz.

Bandwidth efficiency

Spectrum in the lower microwave bands is valuable and needs to be used judiciously. At 32 QAM, a full E1 bearer can be accommodated in 500 kHz of spectrum while still meeting the ETSI 1.4 GHz mask.

However, not every BTS needs a full 2 Mb/s E1 connection and a lower capacity fractional E1 connection will often suffice.

As long as the BTS equipment is presented with the standard framed E1 connection, small sites might be served with just five to 10 timeslots at 64 Kb/s. This type of fractional E1 link can be accommodated in a bandwidth of 75 and 150 kHz.

The integrated cross-connect feature of these new radio products enables grooming of circuits to make best use of bearer capacity.

To connect rural RBS sites, mobile phone operators need 'easy to engineer, easy to deploy' connections that help keep capital expenditure as low as possible. Fibre, millimetre wave and unlicensed linking products are unsuitable for these long-haul applications.

High-performance digital radio systems that use low microwave and UHF bands are now available that can transport E1 and fractional E1 payloads economically and with new levels of bandwidth efficiency.

As rural RBS sites do not often require a full E1 connection, fractional links may be employed to conserve spectrum with the integrated cross-connect feature of the new radios used to provide grooming.

Such links now meet mobile phone operators' needs to deliver high-quality long-distance connections to rural and remote locations more cost-effectively than traditional methods.