Re: GSM-900

But since we transmit in x freq, then the receiver must tune to x feq

> in order to receive the signal right? Why transmit at x freq and > receive at y freq? > Or I have misunderstood. Kindly enlighthen.

Many (but not all) two-way radio transmission systems that operate in full duplex (which means there are two full-time transmission paths, one in each direction) use different frequencies for each of the two paths.

One path, known as the downlink, uses frequency X to transmit from the base station to the mobile, and the other path, known as the uplink, uses frequency Y to transmit from the mobile to the base station at the same time. Under this scenario, the handset transmits on Y to a base station receiver tuned to Y, while the base station transmits on X to the handset, which is tuned to receive X. Operating in this manner is known as frequency division duplex, or FDD.

Using paired frequencies that are sufficiently far apart allows the receiver at each end to be able to operate without getting overloaded and desensitized by the transmitter at the same end. If the receiver were tuned to the same frequency being used to transmit at the same time, it would pick up its own transmitter's strong signal and wouldn't be able to pick up the much weaker signal coming from the other end.

Some communications networks use a single frequency for both transmissions, but alternate the use of that frequency in time so that neither end is actually trying to receive when it is transmitting, known as time division duplex or TDD. One way of operating in this manner is to use "simplex" transmissions, such as on ham radio bands or old-fashioned taxi dispatching systems, where you say "over" when you are finished and then the other party keys its transmitter on to respond. Another way to accomplish it is to alternate between transmitting and receiving at a constant, high rate, with both units carefully synchronized.

Keeping the units synchronized at a high enough rate for high-quality speech is complex, and even more so when the distance between the two units can vary considerably, since for every mile of distance, there is a delay between transmission and reception of 1/18,600 second. Thus, if the system is designed for a maximum transmission distance of

20 miles and a minimum of 0 miles, there must be at least 1/9300 second of dead air at the beginning and end of each time slice to keep the two transmissions from overlapping, wasting at least 1/2325 second for each pair of time slices (1 in each direction). If the time slices themselves are short, as they must be for conversational speech that isn't going to tolerate significant delay due to the time compression and decompression involved, a significant amount of transmission time is wasted.

And that's for just a single two-way voice transmission. GSM networks combine many conversations into a single paired radio channel, which is itself time-sliced, utilizing time division multiple access (TDMA); but GSM separates the TDMA uplink and downlink transmissions by frequency, using FDD. So GSM is an FDD/TDMA system. In applications where an appreciable time delay is acceptable, TDD/TDMA can be used, in which transmission time on a single frequency is sliced up between up- and downlinks, each of which is further time-sliced into multiple communications channels.

Michael D. Sullivan Bethesda, MD (USA) (Replace "example.invalid" with "com" in my address.)

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Michael D. Sullivan
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