Now I will talk about the power control issues in LTE.
Before we go into the LTE specific power control, let's try with our own thought process for this power control issue. In wired communication, the amount of energy (power) being sent from the transmitter reachs the reciever without much degradation. Just think about connecting two PC with a long ethernet cable (e.g, around 10 m) and connecting a device to your PC with a simple RS 232 cables (usually less than 2 m). If you measure the voltage at the two ends of the cables while communicating, you will not see much voltage drops between the source and the destination. (Of course there is a certain amount of voltage drops. but the drop is not so big that it cannot be recognized by the reciever). However, what if the transmitter and reciever is connected wirelessly ?
You can intuitively know that the energy drop will be tremandous. Anybody who tried to measure something wirelessly (e.g, test a mobile phone with antenna, not with cable connection with Network simulator or spectrum analyzer) would have experienced these energy drop every time they do the test.Then how do solve the problem ? This is the time that you assume you become a designer and have to come up with an idea to solve this issue.How do you handle this situation ?
The simplest way would be to use a very high gain amplifier on transmitter and blast huge powered signal to reciever.This method would be good when the reciever and the transmitter is in a reasonable distance. But what if the distance between reciever and transmitter is too close ? In this case, the strong signal from the transmitter may saturate the reciever.
Then how do you handle this situation ? You may try to tune down the transmitter amplifier power so that the receiver does not get saturated. If the distance between transmitter and reciever does not change and the channel condition (Humidity, precipitation, buildings) does not change, this kind of manual tuning would work. But can we do the same thing with mobile communication where the distance between reciever and transmitter changes very often and channel condition changes as well.
Now you have think up a solution to cope with this kind of varying distance and changing channel condition.
The way the people in this area came out is as follows :
i) Transmit send a signal to reciever
ii) Reciever measure the power of the signal from the transmitter
iii) if the measured power is too low, the reciever send a special command saying "increase the power". And if the measured power is too strong, it would send another command saying "decrease the power".
By this mechanism, the transmitter can change it's output power dynamically. This kind of power control mechanism is often called "Closed Loop Power Control" and the special command being used for power control is called TPC (Transmit Power Control) command. In short, Transmit send something and the reciever send a feedback to the transmitter and the reciever retunes itself by the feedback. This whole process forms a cyclic loop and this kind of control loop is called "Closed Loop" in control system theory.This kind of power control is used in almost all the mobile communication technology (e.g, CDMA, WCDMA, LTE and even in Bluetooth etc). This kind of power control process happens much more frequently than you may think. For example, in WCDMA case it would happen around 1500 times maximum within a second ideally. and in LTE case it can happen maximum 1000 times within a second.
If you are a person who is really interested in the power control mechanism and thought in very details about various communication environment, you may notice that there is a situation where we cannot use this kind of "command based power control" method. The 'command based power control'mechansm is based on a asumption that the transmitter and reciever has already established a call setup so thta they can exchange these command.
What if the transmitter and reciever is not in such a communication state ? For example, you just turned on your mobile phone and the mobile phone (transmitter in this case) has to send some signal to the base station (the reciever in this case).
How strong power the mobile phone has to transmit it's first signal ?This is very important.. if the mobile phone transmit the signal in too low power, the base station would not detect it.. and if it transmit it in too high power, it can interfere with the communication between other mobile phone and the base station. So it has to determine the proper transmit power level which would be strong enough to be properly decoded by the base station and weak enough not to interfere the communication between other mobile phone and the base station.
How do you handle this situation ? What kind of method the UE should use to determin the proper transmission power ?
It would not be easy to think out a solution intuitively...
Overall logic that is commonly used in mobile phone communication system is as follows :
i) Network (Base Station) is tranmitting a certain reference signal with a fixed power value
ii) Network transmit the information (e.g, Power) about the reference signal it is transmitting
iii) Network also transmit the maximum allowable power that UE can transmit.
iv) UE decode the reference signal comming from the base station and measured the power.
v) UE can figure out the path loss between the UE and base station by comparing the result of step iv) and step ii).
vi) Also from the information at step ii), UE knows how much power is allowed for it.
vii) From the result at step v) and step vi), UE can figure out how much power it can really transmit.
This kind of process is also called a power control process. But since this power determination process is not based on a feedback loop as in Closed Loop Power Control, it is called "Open Loop Power Control".
Power Control in LTE
For details of LTE specific power control, you have to refer to
"TS 36.213 - 5. Power Control" and if you want to have some hands-on experience of these, I would recommand you to try some test items on 36.521.
For PRACH Power, refer to "TS 36.213 - 6.1 Physical non-synchronized random access procedure".
Power Control in LTE can be summerized by the following equations. The main purpose for this section is to understand the every details of these equations.
Since it is not easy to embed the mathematical symbols in this blog, I would express these symbols in text form as shown below.
Let me continue later on these details. Stay tuned -:)