Another daily CDMA example
Everyone of us use the GPS (Global Positioning System) to know where we are or the way we must take to arrive somewhere as soon as possible.
Our GPS receivers calculate our position on Earth surface by means of tracking an electromagnetic signal transmitted by, at least, four satellites, to resolve our position: four satellites as we need to solve four equations with four unknown variables, three space dimensions plus the receiver clock.
How does our receiver know which satellite the signal tracked comes from?
In the US GPS system, this is done by means of a CDMA code. All the GPS satellites transmit their signal at the same frequency; in fact, there are more than one signal transmitted; there are five, so called from L1 to L5, but, in common receivers, just one is used (L1). The said L1 signal is transmitted at a frequency of 1575.42 MHz. Then, each satellite in the GPS system is ready to generate the signal modulated with a different CDMA code, that allows to the receivers on Earth to know what signal corresponds with what satellite; in the GPS system, this is known as the PRN code. Each satellite has its own PRN code. These codes are defined in the GPS ICD (Interface Control Document) and are saved inside each GPS receiver, so that it is able to distinguish between different satellites.
In fact, another bundle of different PRN codes exist, just known by the US Army, so that they could manage GPS data in a secret way, including some kind of satellite communications posibility.
In the other side, a russian parallel global positioning satellite system exists. It is called GLONASS. The principles of both systems are the same, but in the russian case, each satellite of the constellation transmits in a different frequency, all of them around 1602 MHz, with a bandwidth of 0.5625MHz per satellite.
In this case, no CDMA code is needed, as the satellites are distinguished by frequency, as it is explained at the beginning of Q1 lecture.
More general information:
http://en.wikipedia.org/wiki/Global_Positioning_System
http://en.wikipedia.org/wiki/GLONASS
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