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[DV8]

Hi Guys,

There has been lots of talk about new wireless systems, but with the use of all sorts of other wireless devices like cell phones, PDA's Bluetooth devices, wireless phone etc. etc. I would think that there could be may issues with interference.

Can anyone shed some light on this for me?

Thanks

DV8

[emmsys]

Hi DV8,

You bring up a critical issue in all types of wireless technologies. Inteference is definitely a real problem and comes in different forms.  On the bright side, there are lots of ways to minimize interference, coexist with interference and in some cases, almost completely avoid interference. 

Some of the different types of interference include metal or other obstacles in the "wireless" path of a signal, water and devices talking in or around the same frequency as others. Note: some frequencies work better in certain environments. For example, a 132KHz transmitter will have no trouble near metal or water, while a 915MHz radio will. Putting a 2.4GHz ZigBee network on the same channel as a WiFi network that is currently transmitting a 10gig file will interfere with the ZigBee network.

Some ways to avoid or minimize interference includes placing the radio far from metal or water (again depending on frequency), setting your ZigBee network to a channel that does not overlap WiFi, sending smaller wireless messages (to reduce the chance of collisions) etc. This is definitely a huge topic so feel free to ask more detailed questions!

[DV8]

Thanks for the reply Emmsys. You seem to know your wireless stuff quite well.

You bring up some excellent tips, but I am particularly interested in knowing how sensitive these new devices are with electro magnetic fields generated by other wiring in the same proximity as the units?

Since the units are wireless they can be moved around quite easily, but one doesn't always know what lurks behind the walls and may misinterpret interference as a defective unit.

I wonder if these units would have a setting built in to them to be able to check the signal strength quality being received or sent out. Something similar to satellite systems so one can best optimize the placement of the unit to avoid interference problems.

DV8

[remlap]

You all bring up excellent questions and rebuttles with regards to the application of new wireless devices.

Could someone please explain the difference between 900MHZ products and 2.2 GHZ products aside from the frequency and operation and what some of the benefits are to each technologies?

[emmsys]

Hi DV8,

There are different types of sensitivities to look at. First, there are issues with signal reflection (aka multipath). A portion of a signal can bounce off obstacles before reaching its destination, resulting in transmission errors. The receiver might even get two of the same messages, one after the other. The error checking algorithms in wireless devices like WiFi would reject the message, resulting in retransmissions.

There is also the issue of signals close to the center frequency your device is transmitting on. These can cause interference if your receiver does not reject adjacent channels well. for example, in 802.15.4 an adjacent channel is +/- 5MHz so you would look for a transceiver than can reject interference in that range very well.

The next type would be alternate channel rejection. In this case, the signals are "further away" from your center frequency. In 802.15.4 that would be signals +/- 10MHz from the center frequency. Similar to above, these signals can still interfere with network performance.

In a nutshell, look for a receiver that can reject signals well. Transceiver performance varies so be sure to look at the specifications and pick which is best for you.

As for interference from wiring or other unknowns behind walls (like metal), it's recommended to do a site survey first. Wireless devices have methods of determining signal strength and link quality. In ZigBee, RSSI (received signal strength indicator) and Link Quality can be used. RSSI isn't an exact measurement of signal strength, but it gives a decent method of determining if there is interference between two wireless devices. The RSSI value increases as signal strength increases. Signal strength can be used to optimize placement of a device but given the nature of RF interference, the location picked might only be optimal for a short period. For example, a WiFi network could be installed later or all the cubicles in a room are moved around etc. A combination of antenna design and orientation, robust network technology (like mesh topology), channel selection etc. will all work towards a happier wireless network.

[emmsys]

Hi remlap,

The topic of 900MHz versus 2.4GHz is becoming much more popular now that 802.15.4b has been ratified. There are a number of differences to note:

900MHz is unlicensed in North America, Australia and Israel only while 2.4GHz is a worldwide unlicensed frequency band. If your products are for the North American market only then you can consider using 900MHz for it's longer range, less susceptibility to interference from obstacles (like walls) or other wireless devices since most phones nowadays are 2.4GHz, with WiFi, 802.15.4 and Bluetooth all running at 2.4GHz as well.

In 802.15.4 specifically, 900MHz throughput is significantly lower than 2.4GHz (40kbps versus 250kbps respectively). However, this gap has narrowed now that 802.15.4b was ratified because throughput at 900MHz was increased to 240kbps. If you will be piggybacking your ZigBee devices onto other ZigBee networks then you will likely go with 2.4GHz since most chip vendors provide ZigBee transceivers for that frequency band only. You will have the benefits of a tighter mesh network with the increased throughput.

Emmsys

[emmsys]

Hi Integrator,

There is definitely a lot of wireless activity in the villas you mentioned. However, most are running on different frequencies. The GSM pool cooler, the Ultrasonic IR, the band, the radios in the boats etc. are likely not running at 2.4GHz. What frequency is the wireless interface running at? Does this interface control the cameras, alarms etc.? The wireless Internet could cause interference with 2.4GHz systems if they are all running on the same channels. Ideally, the Hotspot will be setup on one of the 3 typical WiFi channels (1,6,11). This leaves a couple of 802.15.4 channels available without any direct interference.

There are definitely government rules restricting the use of RF frequencies. This is why, for example, in North America, 900MHz is unlicensed while in Europe it is 868MHz. Furthermore, there are restrictions for other aspects like power output, frequency hopping etc. so I strongly recommend studying the restrictions regarding wireless usage.

Emmsys

[emmsys]

Hi,

I'm not familiar with the standards bodies in Saudi Arabia but you can probably get info from sites like:

Industry Canada http://www.ic.gc.ca/

Federal Communications Commission (FCC) http://www.fcc.gov/

You need to find the standards body for the country you wish to deploy your solutions in. I will try to find more information on this for Dubai.

e

[Mesh]

Hi,

Great information in this thread! If I decide to use a certified platform (like a ZigBee off-the-shelf transceiver) then do I still have to worry about government standards? Should the certified platform take care of all those things for me?

Mesh

[emmsys]

Hi Mesh,

That depends. You need to consider power output regulations as well as ZigBee certification. If your application is based on a ZigBee public profile then you will need to ensure it meets the profile specification.

e