WiMax Operator's Manual

CHAPTER 5 ■ STRATEGIES FOR SUCCESSFUL DEPLOYMENT OF PHYSICAL INFRASTRUCTURES 121
Mesh Networks and NLOS
Chapter 3 has already discussed the principles of mesh network operation fairly extensively.
This section focuses on the NLOS capabilities of meshes and how a mesh architecture relates
to the previously mentioned techniques.
Using a mesh topology does not exclude any of the methods for facilitating NLOS mentioned
earlier. A mesh network can use OFDM and/or adaptive array antennas, and in fact one
system manufactured by Radiant Networks (now out of business) did utilize a rather crude
mechanically steered antenna that was the functional equivalent of a smart array in certain
respects. Indeed, if one were to combine a smart antenna with a mesh, one would achieve a
powerful synergy, but so far no manufacturer has attempted this.
By itself a mesh offers a compelling solution to NLOS problems in that it allows the network
to route around obstacles—provided enough subscriber terminals are present to permit
circuitous routing paths. The drawback, of course, is that the multiple hops required to
describe such paths rob the network of capacity and can introduce latencies that compromise
time-sensitive applications.
As indicated earlier, the future of the mesh in public networks remains in doubt. The companies
advocating the architecture are, with one exception, all very small and may not be able
to survive in the overall broadband wireless marketplace where there have been too few
deployments to date to support a large and diversified population of equipment manufacturers.
And there are also issues involving the installation of the terminals that could compromise
the effectiveness of what is essentially a sound networking strategy.
In a mesh, as you have seen, the subscriber terminal and the base station are one and the
same. This means that the radio and antenna are likely to be self-installed and the antenna
placed indoors at low elevation. The result will be that the individual radio will be provided
with a number of possible transmission paths but none that are likely to be particularly good,
especially, if, as is the case with most current mesh systems, the antennas are omnidirectional.
Such antennas also give rise to a generally high level of background interference because the
signal is being propagated everywhere. But without adaptive array antennas, or physically
steered antenna such as Radiant makes, an omni is the only way to ensure that the signal can
reach multiple nodes in the network.
The choice of a mesh over a point-to-multipoint architecture is fundamental and will
affect every aspect of network performance and evolution afterward. It is a choice not to be
undertaken lightly. My view is that mesh products are less proven than point-to-multipoint but
may be indicated in certain circumstances. If, for instance, the network is being planned for a
dense urban core where tall buildings ring every conceivable base station location and prevent
the establishment of links with choice customer locations, then one may want to explore what
may be done with a mesh, in particular whether line-of-sight connections can be established
among initial nodes that would enable future prime customers to be easily reached. Unfortunately,
this exercise will have to be done manually, at least in part, because not many software
tools are designed for planning meshes. And, in any case, the fact that mesh equipment itself is
designed to be self-organizing rather than operator configurable limits the degree to which the
network can be planned.
As is the case with any other radio, the operator would want to examine the mesh radio in
terms of its other capabilities—can it support QoS, voice telephony, legacy protocols such as
Asynchronous Transfer Mode (ATM), and so on? NLOS is of little use if one cannot offer the services
and reliability the customer expects.