Design Considerations (especially outdoors)
Consider the data rate required at the edge of the cell
- Larger cells offer binary phase shift keying (BPSK) or quadrature phase shift keying (QPSK) data rates at the edge but require fewer hops to wired network.
- Smaller cells may offer quadrature amplitude modulation (QAM) (16QAM or even 64 QAM) at the edge hence increased performance BUT
- More APs are needed to cover the area
- More APs means more backhaul traffic leading to need for multiple backhauls to increase throughput and reduce hop count to wired closet.
- Approximate AP radius per speed offered and area of the cell
- AP with QPSK at the edge can have 0.619 m radius (in open space)
- 16-QAM rate AP offers 0.324-m radius per cell at the edge
- 64-QAM rate AP offers 0.226-m radius per cell at the edge
- Common distances
- MAP to MAP: 100 to 200 m
- Client to MAP: 80 to 150 m
- RAP to first MAP: 200 to 800 m ( If RAP is high, APs at the foot of the building are in the shadow area, and need to go through another MAP to reach the RAP)
- Maximum recommended number of hops to RAP is 4
- AP cell area can be approximated as
Area = Pi (3.14159) x Square of the radius
- For Mesh, neighbouring APs overlap forming a hexagon with aan AP on each side
Area = 2.598 x Square of the radius Number of APs per Km= 1 / (2.598 x Square of the radius)
- MAPs are commonly located on light poles and RAPs on towers or rooftops
- Issues with mounting on the pole
- The mounting hardware might not fit the pole.
- The antenna might end up too close to the pole.
- Power might not be available all the time.
- The pole might not handle the AP weight and wind load.
- All outdoor APs must be grounded
- Make sure that nothing is blocking the first Fresnel zone, even the mounting point itself.
- Poles placed near the antenna can also affect the antenna radiation pattern and create dead spots in the direction of the pole.
- The minimum distance between the antenna and the pole depends on:
- Pole thickness
- Antenna height
- the street angle, the lower pole antenna radiation may be too low to be in range with the next AP up the street.
- Frequency to be used (5 GHz APs need more clearance)
- the antenna should be placed far enough so that the pole does not block more than 5 degrees in the antenna field of view.
- Outdoor mesh APs should not be used to provide indoor coverage. Indoor APs should be used to relay signal from the mesh AP
- Surrounding environment will also affect the design
- Buildings in the path change the cell shape
- Light pole positions determine where MAPs can be placed
- APs and antennas in the same physical space interfere with each other even if they are not using the same channel soAPs should be separated to limit interference.
- Vertical separation is more efficient than horizontal separation.
- CCNP Wireless (642-732 CUWSS) Quick Reference Guide by Jerome Henry
- IPexpert’s CCNP CUWSS Wireless Voice on Demand (642-731)