Networking & Communications

Antenna Sharing, the WiMAX Buzz, and IEEE 802.xx

October 1, 2005 By: Wise Guy Sensors

Dear Wise Guy,

My setup includes two remote sensor nodes that I'd like to communicate with their respective base stations. The nodes are from different manufacturers and don't work together, but they do operate at the same frequency. Can I use one antenna and split the signal to hook up to each base station?

Wise Guy
Wise Guy

Signed, Split Personality

Dear Split,

Sharing a single antenna is indeed possible, but loaded with pitfalls. The most common snafu involves the transmitted signal coupling into the receiver input. Most wireless receivers have sensitivities in the –90 dBW range, while transmitters tend to operate with a radiated power of around 0 to –30 dBW. In a shared antenna configuration, some form of filter circuitry must block the transmitted signal from leaking back into the receiver-input stage. Such a filter needs to be directional so that it attenuates the signal being transmitted (the one that's leaking onto the receiver side), but not the signal being received.

It is indeed possible to share an antenna, but it's a whole lot easier to use separate antennas.

P.S. Here's a quick note on dBWs: 0 dBW = 1 Watt, –30 dBW = 1 milliwatt, –60 dBW = 1 microwatt, and –90 dBW = 1 nanowatt, a pretty small signal!

Dear Wise Guy,

What is WiMAX? And why are people so jazzed about it?

Signed, Why Max

Dear Why,

Worldwide Interoperability of Microwave Access (WiMAX), a technology based on the IEEE 802.16 specifications, enables the delivery of "last mile" wireless broadband access as an alternative to cable and DSL (see Figure 1). WiMAX is expected to provide connectivity for fixed, nomadic, portable, and eventually mobile wireless broadband applications—without direct LOS to a base station. WiMAX provides metropolitan area network connectivity at speeds of up to 75 Mbps and can transmit signals as far as 30 mi., though most WiMAX base-station installations will cover only 3–5 mi.

Figure 1.  A "typical" WiMAX application.
Figure 1. A "typical" WiMAX application.

Dear Wise Guy,

Will you please sort out this 802.15, 802.11, 802.16, 802.20 mumbo-jumbo?

Signed, Dots Enough

Dear Dots,

Figure 2 shows the committees of the Institute of Electrical and Electronic Engineers (IEEE) that have been working for years to identify key logical elements for delivery of data. Elements reside in the description and specification of the physical (PHY) channel as well as that channel's medium access control (MAC). Because it is frequently necessary to bridge the two delivery methods, such as when you plug your CAT5 Ethernet wire into your 802.11 WiFi router, the 802 specifications are meant to define how such interoperability should perform.

Figure 2.  An overview of the 802.xx world.
Figure 2. An overview of the 802.xx world.

You've taken a step into the wonderful and scary world of networking; consult or a local psychiatrist if you wish to proceed further!

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