I took the plunge and went wireless the other day. Yes, I know! Nothing new…but this really is new! I’m talking about Broadband Internet Access that is entirely wireless! I realized long ago that once you go broadband there is no looking back. Jaded as I was, I just couldn’t get over the fact that I was spending upwards of $50.00 a month for this digital fix that I seemingly couldn’t do without. I struggled with this high-speed afliction as I continued to pay confiscatory monthly access rates.
NO MORE! I signed up for ClearWire, picked up my new wireless Modem and was online within 10 minutes of arriving at home. Clearwire deployed this latest Orthogonal Frequency Division Multiplexing Not Line of Sight (OFDM-NLOS) technology in Jacksonville but it was unavailable at my residence until recently. Well my wait is over, and it couldn’t come at a better time. For less than half of what I was previously paying to feed my habit, I could now access the Internet and communicate with the rest of the blogosphere untethered and cable free! Want to learn more about it? Get ready for some geek terms!
So just how do they do it? Radio waves with a new twist, or more precisely by utilizing old technology in new ways. The old technology in use is radio waves. Radio waves are electromagnetic carrier waves that travel at the speed of light along a path that resembles a sine wave. This sine wave undulates up and down vibrating at a particular frequency. The frequency of this wave is measured in units of cycles per second or Hertz, but, all carrier waves do not have the same properties. These properties are based on the frequency of the wave with low-frequency waves traveling further and being able to be bent, while high-frequency waves tend to travel in only in straight lines and for relatively short distances. Radio (low-frequency) waves can travel through solid objects like walls making them ideal for use in remote connections. Light (high-frequency) waves cannot bend so they are limited to line of sight applications.
This is all well and good, but what does it mean for our broadband signal? Well broadband is all about sending and receiving large amounts of data quickly! So we have two choices, we can send data fast but it has to be sent in a straight line or we can send data slow without the straightline limitation. Radio waves have historically been used to send data because they can be sent without the limitation of straightlines, however, they are relatively slow and can carry a limited amount of data. Here is where OFDM comes into action.
OFDM uses the old technology of radio waves to carry its data through obstacles without the requirement of line of sight (LOS). It does this through a technique called modulation or frequency division. The definition of modulation is to blend data into a carrier signal. So OFDM is blending data into a radio wave in order to get it out to its subscribers. But wait, aren’t radio waves a relatively slow delivery method? Yes, but that is not the whole story; When a carrier wave is modulated, it is no longer a single frequency but is spread out over a range of frequencies. The speed of the data spread across the modulated carrier wave is approximately the same as the bandwidth spread of the spectrum being used.
So how does it work?
Imagine taking an entire soccer team and putting each player in a separate car and asking them to take the turnpike to the end where they would play the game. How long would it take your entire team to get to the other end of the turnpike if the road was a single lane divided highway and if every toll booth had only a single lane open? Could you imagine speeding down the highway, slowing to a stop, then inching forward while each of 16 cars entered the toll booth in order, came to a stop, and payed its toll by dropping a quarter in the basket on the wall? Sure the highway travel was pretty darn fast, but the toll booth bottle neck is not allowing for a very fast overall transit if each car slows to a halt for multiple toll areas on the highway. Now mix in the rest of the cars also traveling on the highway. You’d have to get on the road pretty darn early to make sure that all 16 players arrived at the field in order prior to the start of the game.
Now imagine that same set of cars travelling down a 6 lane superhighway. At each toll area, the cars are able to split into one of 8 separate toll booths. Now imagine further that each toll booth can be passed at 30 miles an hour instead of 0 mph. Getting the picture? The cars wouldn’t have to travel exceptionally fast to arrive at the destination quickly as long as we can process more cars quicker at each toll area.
Here comes another term.
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The OFDM system also uses a technology called multiplexing to split the data in to multiple channels or carriers and transmit it across the spectrum. In other words, this technology simply increases the data rate by splitting the data into multiple slower speed channels to transmit and then re-combines the data into a single high speed stream when it is de-multiplexed inside the modem and delivered to the computer as a broad band stream.
A few more terms and we’ll have it perfectly straight. Using our analogy again we can think of frequency division as each tollbooth lane representing a different carrier frequency or simply multiple carriers. Orthogonal is a mathematical term used to describe the x, y, and z axes and functions that do not influence each other. To put it together, Orthogonal Frequency Division is where the spacing between carriers is equal to the speed (bit rate) of the message. Therefore, OFDM (Orthogonal Frequency Division Multiplexing) is a method of using many carrier waves instead of only one, and using each carrier wave for only part of the message, and each part of the message is spaced so as not to influence each other. “Orthogonal frequency division multiplexing is then the concept of typically establishing a communications link using a multitude of carriers each carrying an amount of information identical to the separation between the carriers.” (http://www.ofdm.com/library/whitepaper_wofdm_general.pdf)
So, this works in a perfect world but what happens if the cars pass each other or in effect arrive at the final destination out of order? Although, this wouldn’t harm our soccer team in the slightest you can probably imagine what that would do to your emails or the latest installment of a JibJab video. OFDM, in order to avoid the distortion caused by these out of order deliveries, sends the message bits slowly enough so that any delayed copies are late by only a small fraction of a bit time. In other words, the cars are cued up and sent to the turnpike a single car at a time with a little buffer of time between cars. To maintain the high delivery or bit rate, multiple carriers are used to send many low speed messages at the same time. These same messages are then combined at the modem to make up one high-speed message.
For more info check out the following links:
http://www.wave-report.com/tutorials/OFDM.htm
http://www.ert.rwth-aachen.de/Projekte/Theo/OFDM/www_ofdm.html
http://www.clearwire.com/service/howitworks.html
http://www.nextnetwireless.com/products_tech.asp