Friday, January 13, 2012

PROBLEM DESCRIPTION | Automatic and Optimized Cell-Mesh Planning in WiMAX

We look for the optimum conditions required by a fixed broadband wireless access system to cover remote rural users. We extend cellular automatic cell planning models to build an automatic cell planning tool. Our goal is to design a system to provide access to remote rural users under realistic conditions considering data networks. And also to find out how multihop topologies can improve over PMP. We describe these issues in the following.


We suppose a set of potential users, which are placed on real villages and country houses. Users are not necessarily uniformly distributed. We suppose that all the users have the same traffic requirements, they are fixed and have an external energy source. Every region corresponds to a real place in Colombia.
  • High population density, flat terrain: This scenario represents a city with uniformly distributed users, with shadowing caused by surrounding obstacles. One base station covers many users and usually operates saturated. There are also usually several base stations on the same site.
  • Medium population density, medium mountainous rural region: This scenario represents a typical rural region, where some of the users are uniformly distributed and some of them are placed on small towns or near roads or trails. We suppose that some of those users cannot be easily covered because of nearby obstacles.
  • Medium population density, mountainous rural region: We suppose a user distribution similar to the previous scenario. We suppose the existence of high mountains and rivers that cause deep canyons. There are several users with difficult coverage conditions, i.e., there are no privileged places that can cover a high percentage of the region.
  • Low population density, flat terrain: We suppose users widely separated from others. This is common in regions dedicated to agriculture, pasture lands, and forestry. In this case the main problem is caused by the long distance links. We also suppose some places with higher population concentration over the region average such as small villages.


Data models differ from voice systems in many ways. There are different QoS requirements, they are based on packet multiplexing and there are different transmission schemas that depend on link quality. QoS requirements for data networks include several criteria such as delay, delay variation, and guaranteed data rates. Base stations make use of statistical multiplexing to increase system capacity. An analysis of different transmission flows and the resources assignment problem. The base stations perform a process known as packet scheduling to assign transmission opportunities to packets. Some packets can have priority over others, to allow transmission of more urgent packets. Schedulers and multiplexing models for data traffic are difficult to use in the design process. Data networks like WiFi and WiMAX support AMC. As users have different spectrum efficiency values, they might require different number of slots on transmission frames to achieve the same data rate.


In PMP, a user connects to a single base station using a direct link. It chooses which base station to connect to from a set of available base stations, depending on link quality and available capacity. In multihop networks, information can go through several links until it reaches the base station. Packets transmitted through multiple hops have higher delay and require more capacity, i.e., multihop topologies extend coverage at the expense of more capacity consumption. Operation of multihop networks makes use of spatial reuse, controlled by a scheduler. Two different links on the same channel can transmit simultaneously if they do not interfere with each other. Our assumption is that there is only one active link among all links belonging to paths that end on the same base station, but links of users connected to different base stations can be active simultaneously even though they use the same channel.
In multihop topologies, users must decide not only which base station to connect to, but also the path that the packets should follow. The amount of resources required at every hop is not the same, as different links can have different modulation and coding schema. In our case, a certain node chooses the route that requires the lowest amount of resources. We limit our problem to routes up to two hops in relay topologies and up to five hops in mesh networks. A larger number of hops would be prohibitive in terms of delay and resources consumption.
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