Just last week I wrote a blog that talked about how busy the WiFi spectrum is getting. It seems like every telecom business is using or has plans to use the spectrum in a big way. Since I wrote that blog I noticed the following article which outlines how wireless companies intend to deploy WiFi transmitters as part of their urban cell sites (and any other cell site that experiences congestion).
Just about every telecom business has some use for WiFi. Telcos and cable companies are using WiFi in their data routers to spread their data around homes and businesses. The wireless carriers are all planning on using WiFi to offload their tremendously busy licensed spectrum. Businesses use it to set up public hotspots. Settop manufacturers are going to use it to serve multiple TVs in your home. Devices that connect to your TV like Roku and Playstation use it. And it is becoming the default spectrum to use for the Internet of things and billions of devices are going to be made WiFi capable.
So this raises the issue of whether there is some point when there is just too many different people trying to use the same spectrum in the same area at the same time. People use the WiFi spectrum because it’s free. But like any radio spectrum, it has physical limitations. At some point we can simply overwhelm a given spectrum band in a given area and it will not work well for any of the applications trying to use it. It sure seems to me like we are headed towards that possibility with WiFi. I am sure that everybody remembers in the 90’s when cordless phones came out. The spectrum got so busy in some neighborhoods that the phones just wouldn’t work.
Without getting too technical, let me discuss some of the issues associated with radio interference. There are a whole lot of different ways that interference can affect a spectrum in a given location. Consider some of the following:
Adjacent Channel Interference (ACI). The WiFi spectrum is not one big swath of data but is divided up into discrete channels. Earlier versions of WiFi used one channel per transmission, but the latest standards allow for bonding channels together. Many of the problems that are experienced in the real world with WiFi is that many of the devices using it are not built to the same high standards that you find in licensed spectrum gear. And so there are numerous devices that bleed usage into adjacent channels. When such a device is transmitting, it then not only uses the assigned channel but pollutes the two channels on both sides.
Electromagnetic Interference (EMI). This is the interference that we all remember when listening to AM radio. This is interference that comes from some outside source that can range from microwave ovens, computer terminals, solar flares, doorbell transformers, and hundreds of other sources. WiFi is not immune from external interference and so part of the spectrum is eaten through native interference.
Co-channel Interference (CCI). This is the interference that comes when more than one user us trying to use the same channel at the same time. In the voice world this is known as crosstalk, which we have all experienced on cell phones from time to time. But in a data transmission this manifests as slower data speeds since each concurrent user loses part of their signal.
Common Mode Interference (CMI). This is interference that comes from using spectrum to conduct two-way transmissions. This is basically interference between transmitting and receiving WiFi signals at the same time.
As we put more and discrete WiFi paths in the same neighborhood the effects of each one of these types of interference get magnified. In any given area there is at least a little bit of all of these types of interference. That is inherent in the way that radio waves interact with each other on a physics basis. There are engineering techniques that can be used to minimize interference. For example, it’s typical to put the transmit and receive signals as far apart as you can get them. But if you out enough different signals into the same environment there comes a point where no techniques can overcome the sheer facts of physical interference. The spectrum can get overwhelmed and essentially becomes worthless until the demand on it reduces.
I know there are a lot of scientists and engineers who look at all of the planned used for WiFi and just shudder. Because in urban environments it is likely that the spectrum is going to get overwhelmed and none of the uses will work as they should.
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[…] urban environment where living units are crammed a lot closer together. I highlighted some of the forms of WiFi interference in another earlier blog. But there are always other situations where WiFi will not be a great […]