Planning ahead for Wimax RF design

Andrew Nix and Eustace Tameh
16 Oct 2008

Planning a Wimax network usually takes place in two phases - a business plan that is used for costing and traffic/revenue estimates at the proposal phase, followed by a network plan that takes physical and geographical constraints into consideration when deciding where to site the actual basestations. The difficulties arise if this second plan differs substantially from the assumptions made in the proposal, and if the financial assumptions in the business plan then become unachievable. Just as seriously, if the second plan does not provide a sufficiently accurate and robust model of the network, then the topology may be sub-optimal, and both financial and quality of service objectives may not be met.

When choosing a planning tool or a consultancy to help plan a network, a deep understanding of Wimax standards and orthogonal frequency division multiplexing (OFDM) technology is vitally important, as well as having an accurate RF propagation engine that can accurately model complex multipath scenarios in large scale urban as well as rural environments. If the RF predictions are compromised at the start of the planning process, then the plans that result will be equally tenuous.

Legacy experience in 2.5G or even 3G network planning is not so relevant since, unlike TDMA and CDMA technologies, OFDM-based standards such as mobile Wimax and LTE rely on a variety of complex techniques - including MIMO and adaptive antenna systems - that depend on non-line-of-sight radiation for their performance. Achieving meaningful results demands an understanding of both the modulation and multiplexing techniques involved and the use and modes of deployment of the antennas.

The most critical question in the operator environment is how best to balance commercial and technical constraints in order to achieve the appropriate trade-off between cost and quality of service. For a successful deployment, it is necessary to understand from the outset what the cost of rolling out the network will be. This means getting to know both the OEMs and the customers, as well as assessing the characteristics of terrain - and this includes buildings and foliage. There are a number of different end-user applications for a Wimax network - including voice-based, video-on-demand and enterprise - and these will affect the usage patterns and capacity requirements and will thus also need to be taken into account when planning.

Planning methods

Looking in more detail at the various planning techniques in use by operators today, there are a number of pros and cons to each. For one, drive tests are a hugely expensive exercise, demanding a large amount of equipment and man-hours to map the signal coverage from a proposed basestation site. Nevertheless, some operators are distrustful of other techniques, and still prefer to use this empirical method for planning their networks.

Then there are 'business case' tools that deal mainly with subscriber numbers, capacity and revenue, without regard to the RF propagation characteristics of the area to be covered. These may be used to prepare a business plan at the licence application stage. However, the assumptions they make are rudimentary, and may well be incompatible with the technical constraints of the equipment when it comes to rolling out the real network.

As for vendor-specific tools, it is not widely known that even some apparently independent tool vendors embed the characteristics of specific OEMs' equipment in the basic assumptions of their planning process. The operator may then unwittingly be drawn into committing to one type of equipment at too early a stage in the design.

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