WiMax Deployment Models
There is likely to be a lot of variation in base-station equipment, tailored to the deployment scenario. Significant factors will be price, form factor, capacity, and performance. Two major types of base station will be:
- Cellular-style base stations: These will be for urban, suburban, high user density, business and residential deployments. They will use multiple RF carriers and sectorization, and support macro cells.
- Lower-cost base stations: These will be for rural, campus, indoor, and fill-in deployments. They will use a single RF carrier, and support micro and pico cells.
A key factor in design selection is base-station positioning, the ideal being good, tall broadcast points that are directly in the middle of the user population the operator wants to serve. More often, the operator has to use a mountain top, communications tower, or tall rooftop located somewhere that is noncentral to the target population. So operators will have a balance between big-stick cellular-type deployments that serve all directions, and have a lot of equipment costs associated with them, versus smaller standalone base station sectors.
Experience to date from Proxim suggests that, for fixed clients, full 360° coverage is not often required, even from a prime central location, 180° being the average. With WiMax Mobile this is likely to change, so operators need to have the flexibility to cope with this.
Vendors will naturally introduce enhancements to improve performance, such as smart antennas for non-line-of-site operation, MIMO antenna systems, and so on. There should be a vibrant subsystems market based on open base station architectures.
Smart antenna technology is a particularly interesting development because it can be used to make indoor CPE or indoor self-install modems much more viable, especially towards the edge of the cell, where signal strength is weakest. This is important because the cost of CPE and the cost of installing them are absolutely critical to the WiMax business case, and cutting out truck rolls is worth a lot. Figures from the WiMAX Forum suggest that smart antennas could, in some cases, effectively quadruple the non-line-of-sight and indoor self-install ranges obtainable at 3.5GHz compared to standard antennas -- from 0.5km to 2km for indoor self-install, for example.
It is widely expected, therefore, that multiple antenna schemes will be a key attribute of WiMax systems. Table 4 from last-mile wireless vendor SOMA Networks Inc. summarizes the options.
Table 4: Options for Multiple Antenna Schemes
|DL antenna processing scheme||Minimum number of antennas required in SS||Gain (over 1 antenna) with 2 Tx antennas at AP||Comments||Risk|
|Space time coding (STC)||1||3 dB, gain in throughput depends on operating SNR (x2 at low SNRs)||Works in any channel conditions at any SNR||Low risk, well understood technology|
|Beamforming (AAS)||1||6 dB, gain in throughput depends on operating SNR (�4 at low SNRs)||Works in any channel conditions at any SNR||Many implementation challenges|
|Spatial multiplexing (MIMO)||2||Throughput increases by factor of 2||Requires dense multipath and high SNRs||Many fixed wireless channels are AWGN, where MIMO has no gain|
|Source: Soma Networks, 2005|