RF Considerations for Wireless Systems Design Frank Jimenez Manager, Technical Support & Service 1
The Presentation Objective We will cover.. The available wireless spectrum 802.11 technology and the wireless time domain Fresnel zones and their impact Application wireless transport requirements The importance of the Site and RF Survey System design recommendations 2
Radio Channels in the Unlicensed Frequency Domain 3
Radio Channels in the 2.4 GHz Frequency Domain 4
Radio Channels in the 4.9 GHz Frequency Domain 4.94 GHz 4.99 GHz 5
The 802.11 Standard Originally Developed for indoor WLANs Is RF Half-Duplex technology, but bi-directional Bi-directional via Time Division Duplexing Is a CSMA/CA vs. CSMA/CD Technology Collision Avoidance achieved through transmit deferral 6
A Near Ideal Wireless Time Domain T = 0 T = 1 7
A Highly Congested Wireless Time Domain T = 0 T = 1 8
Fresnel zones Named after Augustin Fresnel (1788-1827), who contributed significantly to wave optic theory. (1) RF signal reflected off of First Fresnel zone boundary travels exactly ½ wavelength farther than direct path signal. Each subsequent Fresnel zone adds another ½ wavelength in distance to length of reflected path. (1) From Wikipedia, the free encyclopedia 9
Why are Fresnel zones important? As Fresnel zone numbers increase, separation between Fresnel zones decreases Fresnel zones are important with respect to reflected signals depending on their number If an RF path s reflection point is blocked from view of the antennas, then Fresnel zone relationships are not a factor 10
Fresnel zones and antenna height 11
Fresnel zones and antenna height 12
Fresnel zones and antenna height 13
Where do I start with my system design? Start by confirming the application to be supported, and it s wireless transport requirements. What protocols does it use? What is the transport bandwidth required? What is the allowable latency and jitter? Where are the traffic origins and destinations? 14
Where do I start with my system design? The Site and RF Survey Identify location of all application traffic interface points to the proposed system Determine all node and antenna locations Confirm line-of-sight (LOS) exists between nodes If LOS does not exist, find alternate wireless path(s) Perform RF spectrum survey of deployment environment Identify any special equipment needed (lifts, cranes, etc.) 15
What about non-line-of sight? Products in the marketplace are being advertised as nonline-of-sight (NLOS) technology. Is NLOS possible? NLOS Feasibility depends on the specific path, not all NLOS paths are possible NLOS systems are inherently signal distorted with variable bandwidth, latency and packet jitter performance NLOS systems may be a good fit for low data rate TCP applications, ie. Smart Grid and data acquisition systems NLOS systems are not recommended for video applications 16
Back to the system design.. Based on site survey, design a wireless topology that supports the application traffic routing, bandwidth, and latency requirements. Systems for streaming protocol applications should always be based on clear line-of-sight radio paths. NLOS signal distortion relegates these systems to acknowledgement protocol applications that do not require high bandwidth, low latency, and low packet jitter. 17
Antenna Selection Use narrowest beam width antenna available that will provide signal coverage to the intended nodes Usable beam width of an antenna is considerably broader than specified beam width Beam width is defined by the points on either side of the main signal beam where gain is reduced by 3 db Refer to an antenna s gain pattern to determine its effective beam width, so that you can properly utilize it in your system design 18
AS-050-N Sector Azimuth Gain Pattern 19
AS-050-N Sector Elevation Gain Pattern 20
And Finally ALWAYS specify a dual-radio nodes to avoid Bandwidth constriction Latency Packet jitter When reusing frequencies, isolate the links involved to avoid time domain congestion Accomplished by physical line-of-sight blockage, antenna cross-polarization, and directional antennas. 21
A high-performance design example 22
A robust linear loop mesh design example 23
In Summary A successful project outcome requires: Understanding the transport requirements of the application to be supported Understanding and managing the wireless frequency and time domains Performance of a proper site and RF survey Proper wireless topology design and implementation 24
Questions? 25