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Because 1 km radius cells have an area of 3.14 square km, while 10 km radius cells have an area of 314 square km. That is simple geometry
So if you decrease the radius by 10 times, you need to decrease the power by 100 times, in order to not increase the interference level?However, to go from 1km to 10km radius, you would normally need 100 times the power, due to the inverse square law.
Yes Area of a disk - Wikipedia, the free encyclopediaHey,
Thanks!
So you're actually saying that if you decrease the radius by 10 times, then you can increase the amount of cells by 100 times?
Yes.So if you decrease the radius by 10 times, you need to decrease the power by 100 times, in order to not increase the interference level?
Cell phones interact with the towers to increase the power when the cell tower is far away. They also compensate for the distance from the cell tower. Timing advance - Wikipedia, the free encyclopedia
2G systems
Most 2G cellular systems, with the notable exception of IS-95, are based on TDMA. GSM, D-AMPS, PDC, iDEN, and PHS are examples of TDMA cellular systems. GSM combines TDMA with Frequency Hopping and wideband transmission to reduce interference, this minimizes common types of interference.
In the GSM system, the synchronization of the mobile phones is achieved by sending timing advance commands from the base station which instructs the mobile phone to transmit earlier and by how much. This compensates for the propagation delay resulting from the light speed velocity of radio waves. The mobile phone is not allowed to transmit for its entire time slot, but there is a guard interval at the end of each time slot. As the transmission moves into the guard period, the mobile network adjusts the timing advance to synchronize the transmission.
Initial synchronization of a phone requires even more care. Before a mobile transmits there is no way to actually know the offset required. For this reason, an entire time slot has to be dedicated to mobiles attempting to contact the network (known as the RACH in GSM). The mobile attempts to broadcast at the beginning of the time slot, as received from the network. If the mobile is located next to the base station, there will be no time delay and this will succeed. If, however, the mobile phone is at just less than 35 km from the base station, the time delay will mean the mobile's broadcast arrives at the very end of the time slot. In that case, the mobile will be instructed to broadcast its messages starting nearly a whole time slot earlier than would be expected otherwise. Finally, if the mobile is beyond the 35 km cell range in GSM, then the RACH will arrive in a neighboring time slot and be ignored. It is this feature, rather than limitations of power, that limits the range of a GSM cell to 35 km when no special extension techniques are used. By changing the synchronization between the uplink and downlink at the base station, however, this limitation can be overcome.
(1) I'm not entirely sure on the details of TDMA in cellular systems. My class seemed to focus more on the CDMA spread-spectrum method where key codes are used instead and so each cellular phone can transmit whenever it wants. But I googled "TDMA cellular phone synchronization" and it led me to this wiki page
Time division multiple access - Wikipedia, the free encyclopedia
(2) and (3) THe distance isn't measured. THe base station monitors the power level from each hadnset and constantly tells it to adjust the power level. This is because the basestation is where all the handset signals meet up and are received and if any one is stronger than the others it drowns the others out. I know they do this with CDMA spread spectrum systems because this is a key requirement in the way they work since all handsets share the same frequencies simultaneously and any one signal that is too powerful when it arrives at the basestation will drown the others out. It may or may not be done with TDMA because TDMA is not as sensitive to this issue since you actually have a dedicated slot in time at a particular frequency channel.
You can surmise from (2) and (3) that cellular systems require the handset to not be moving too fast or else contact would be lost in the time interval between timing adjustments and power adjustements.
Having the basestation (BTS) to transmit spread spectrum signals simultaneusly indeed makes sure that each handset recieves each bunch of spread spectrum signals at the same time, therefore they're in phase and orthogonality is achieved.dknguyen said:The basestation has no difficulty transmitting out a bunch of spread spectrum signals simultaneously that are aligned so the codes work properly.
Even thought we discuss digital communication, the wirelessly received data is analog, isn't it?dknguyen said:What do you mean by ADCs for encoding? It makes slightly more sense if you said DAC, though I still wouldn't understand your question.
The near-far problem is a condition in which a strong signal captures a receiver making it impossible for the receiver to detect a weaker signal.[1]
The near-far problem is particularly difficult in CDMA systems where transmitters share transmission frequencies and transmission time.
In contrast, FDMA and TDMA systems are less vulnerable.
The issue of the dynamic range of one or more stages of a receiver limiting that receiver’s ability to detect a weak signal in the presence of strong signal has been around for a long time.
The near-far problem usually refers to a specific case of this in which ADC resolution limits the range of signals a receiver can detect in a direct sequence spread spectrum (DSSS) system such as CDMA.
The receiver’s AGC must reduce its gain to prevent ADC saturation.
This causes the weaker signal to fall into the noise of the ADC.
This is different from a condition of one signal interfering with another because if the ADC had sufficient resolution it would be possible to recover both signals.
Each handset needs to know only one code in order to decode the transmitted signal and detect the data signal, or to encode the data signal as I did right below.dknguyen said:So that a data sequence of 0110 is transmitted as 01101010 11100110 11100110 01101010. This is just a conceptual example and has a bunch of problems. The biggest is that one signal requires the knowledge of two codes
It's a great link, I'll read it with pleasure, thank youdknguyen said:Read this well:
**broken link removed**