Overcoming indoor coverage challenge in WiMAX

By BP Tiwari , www.beyond4g.org

Wireless community has stated numerous times about the importance of 700 MHz band which is considered “beachfront property”, implying that this band is very valuable.

But why is it valuable? The simple rationale most often cited is that the 700 MHz band has better propagation characteristics. Smaller cells at 2.6GHz so best suited for hot spots and 700MHz has a better indoor coverage, but more prune to interferences in urban environment.

WiMAX in particular has most of the deployments in 2.3, 2.5 and 3.5 GHz globally. If I refer reports from wimaxforum, it points 245 networks globally deployed in 3.5 GHz and nearly 100 networks in 2.3 and 2.5 GHz bands. While 2.5/3.5 GHz band has some advantages like it suits better for capacity driven deployment in areas which are highly dense in population, the other important consideration is availability of large chunks of spectrum in these new bands. While indoor coverage and mobility (especially in 3.5Ghz) are associated challenges in 2.5/3.5 GHz bands, WiMAX apparently has most of the deployments in these frequency bands globally.

For any operator who is planning a mobile WiMAX network, the major challenge in these bands is related to indoor coverage. Unlike radio planning in cellular world, wimax radio planning approach has seen little divergence from traditional means. Most of the new WiMAX networks are planned based on single wall coverage. Planning based on single wall or 1^st wall allows operators to decrease initial capex in a big way making the overall deployment economics much healthier. I am sure all of us who are involved in mobile wimax deployments will understand associated challenges of indoor penetration in higher frequencies.

So let’s assume an operator has planned his wimax network with 18db penetration loss which is good enough for most of the building material like glass, wood, brick and other types of commonly used building materials. However if there is a stone wall and the customer is in cell edge , we might face challenge as the penetration losses for stone for 90^th percentile is 20.7 db, which is much higher than what we have planned. There is a tradeoff, between quantities of coverage versus number of cell sites required to cover a particular geography. Every single db increases 10% more cell sites as per link budget. I see lot of wimax operators struggling with indoor coverage complexity and in most of the cases they return back customers deposits making the case non feasible for service.

The Russian wimax operator, Yota started their network with condensed capex and highest amount of achievable coverage by using very low building penetration losses. This has helped operator to improve cell radius and ultimately helped to reduce overall capex for initial deployments. Later the same operator introduced WiMAX-Wifi pocket router to improve their indoor coverage as their customer can place these devices near to window and work anywhere within 10-15 meters of the device [1] . This is a brilliant way to reduce initial cost of deployment and complement it with wifi to improve indoor coverage. In the second step, the Russian operator introduced Picocells to improve coverage to those areas where even WiMAX/Wi-Fi devices are not functioning.( since the initial network was planned for outdoor coverage)

Other operators in Asia are taking diverse strategies as their planning is based on moderate building penetration losses to strike a balance between capex required for coverage and indoor penetration. In almost all cases these operators are planning their wimax network for single wall coverage. I found this interesting device which is called WiMAX/WiMAX repeaters [2]. This repeater handles WiMAX signal for both indoor and outdoor, and UQ has developed the WiMAX/WiMAX repeaters together with cooperation from KYOCERA Corporation as both companies developed WiMAX/Wi-Fi repeaters released in September [3] . Release of this repeater is targeted by the end of fiscal year 2009 after license acquisition.

Now most of the repeaters normally amplify noise during their operation. This repeater is capable of performing MIMO and has advanced mechanism to handle especially noise. They have dual baseband processor in donor and acceptor side and this helps in reducing noise before repeating the desired signal. These types of devices could play very important roles in extending indoor coverage.

The last item that I would like to draw attention to, apart from the aspects of smart network planning, repeaters and in-building solutions is the use of higher order antenna systems to overcome the challenge of indoor penetration. If you are planning a wimax network in higher frequency, it might be a good idea to use 4TRX or even 8TRX system in bases station to reduce initial capex and exploit diversity gains in uplink. The individual cost of base stations with higher antenna systems might be higher initially, but the overall business case would be better as they will help immensely in reducing operational costs. Systems with higher antenna systems can also leverage Beam-forming technologies to provide additional downlink link budget gains to further improve capacity and coverage.

(The views expressed here are my own and does not reflects my company opinion)

Thanks