It seems to me that a quick explanation of just what the heck a chip rate is would answer some of the questions that have come up about it. This is by NO MEANS a full explanation of what it is but I think that by explaining how it is used I could shed some light on the subject. Please excuse me if this has already been explained, and if it hasn't and my explanation causes more questions either you can send a private message or just respond to this post.
Chip Rate: a measure of the amount of spreading performed in a spread spectrum system.
Trigger: an event that causes something to happen, like your alarm clock goes off in the morning and you know it is time to get up.
Transmitter: Like a radio station that you recieve on your car radio.
In the past on this board people have referred to CDMA as a direct sequence spread spectrum wave form. This is the direct sequence part of that statement.
There are 2 ways to create a spread spectrum wave form, one is direct sequence spreading and the other is frequency hopping. In frequency hopping you use the transmitter (radio station) like a kids pogo stick. Every time a trigger (alarm clock) happens the radio bounces the pogo stick and lands on a new place. If you know where that new place is then you can continue the conversation easily if not then you lose the conversation. In direct sequence spreading the trick is to create lots of information out of only a little input. For instance if you make a 1 equal to 110001101010 the first time you see it and 1111100110010 the next time and both the sender and the reciever know what they each mean then you will have a secure communication.
Those are the basics of spreading, but here is the important part for the chip rate: The chip rate defines how much spreading you get for a single piece of information. What the heck do I mean? Remember that all of the information that we are dealing with here is digital (ones and zeroes) so it is easy to make a table that says "take the first one and make it equal to something". The problem is that if your something is shorter or longer than the senders something then you get quickly lost. This has gotten confusing so I will make it simple.
lets take the following code and translate it.
0001001000110100010101100111
If I said to you that the spreading on this code was 4 then you would take a 4 digit window and translate it with the following definitions. a 0001 is 1, a 0010 is 2, a 0011 is 3, a 0100 is 4, a 0101 is 5, a 0110 is 6, and a 0111 is 7.
the first 4 digits are 0001 and that = 1. The second four digits are 0010 and that = 2, do this through the entire code and you would get 1234567 as your translation.
What if I told you that the spreading is 3 and to use the following definitions. A 001 is 1, a 010 is 2, a 011 is 3, a 100 is 4, a 101 is 5, a 110 is 6, and a 111 is 7. Now we go back and try to translate using a spread of three (a 3 digit window).
Here is the code again: 0001001000110100010101100111
The first three digits are 000 and that is undefined, we are in trouble already. The second three digits are 100 and that is 4, the third group of three is 100 and that is 4. If you continue you get ?44321263?. In other words by changing the spread I changed the translation.
This is essentially what ERCY is doing. They seem to think that a 3,864,000 digit spread (window) is the same as the 3,684,000 digit spread (window) that cdmaOne has. If you looked above unless you have EXACTLY the same spread, and thus the same chip clock you have 2 people speaking 2 different languages. |
