A reader Adil M. asks , ” Why there is no Power Control for LTE in DL but does in UL ” ?
Let’s answer the question from the wireless communication perspective.
In wireless communications, one of the main challenge on the radio access side is to adapt the transmitting signal to overcome the variations
of the wireless channel. Wireless channel can have varying channel response under different conditions.
For a signal to be transmitted successfully from the transmitter to the receiver, it should adapt to the variations of the wireless channel. In
order to overcome the variations and channel response between the transmitter (BS) and the receiver (user). Different techniques can be
In order to protect the signal from the variations of the wireless channel, predefined procedures exist at MAC and physical layer (Layer 2,
and Layer 1) to prepare and massage the signal from getting corrupted from these variations. We are not going to list all of the ways and
protection mechanisms done at these layers to protect the signal. However , some specific ways to overcome these variations include.
Conventionally , power control has been used in previous generation technologies such as CDMA-based mobile communication systems such
as WCDMA, CDMA2000 to compensate for the instantaneous variations in the wireless channel conditions.
The transmission power is adjusted dynamically to overcome the variations of the wireless channel in instantaneous channel conditions.
The goal behind these adjustments is to maintain a constant Signal to Noise Ratio at the receiver end to successfully transmit data without
error probability becoming too high.
Transmit power control is increased when wireless link or radio link experiences bad RF coverage and vice versa.
If channel quality is great, transmit power control will be small and vice versa.
This mechanism results in almost constant data rate regardless of whatever channel variations take place on the wireless side.
For services such as voice transmission, this is great because by adjusting the power control ( as a type of link adaptation) we can maintain
the Signal to Noise ratio at a desired level as shown in figure below.
In case of mobile communications , for packet data traffic and services, we usually don’t need a constant data rate and throughput all the
time. Different services have different data and throughput requirements. And best effort services will work at varying data rates as well.
You may say, ” I would want as much high data rate as possible Azar for all my services ” . Fair enough if you need as much high data rate
as possible. For data services, variation in data service rate is not a big deal as long as the average data rate remains constant averaging over
short time interval. This means constant data rate is not required.
For data services, instead of power control . The link adaption is performed using other methods such as by dynamic rate control.
What happens is that in rate control the data rate is dynamically adjusted to compensate for different channel variations .
When channel conditions are favorable , data rate is increased and vice versa.
Effectively, rate control maintains the same Signal to Noise Ratio at a desired level by varying data rate not by adjusting transmission power
as shown in figure below.
Studies have been published already to proof that rate control is better than power control .
The advantage of rate control is that it means that power amplifier is always transmitting at full power and therefore it is used efficiently.
Whereas during power control base transmission. Power amplifier is not efficiently used as transmission power is less than the maximum
power available through the amplifier in most cases.
For practical purposes, data rate is adjusted by changing the modulation scheme and coding rate while power remains constant.
During excellent RF conditions, Signal to Noise Ratio is already high and the main limitation to increase data throughput is Bandwidth of the
wireless link ( radio link) . This is solved by applying modulation and coding scheme i.e., 16 QAM or 64 QAM with higher coding rate.
When it comes to Packet switched technologies such as LTE, we are mainly concerned with Data rate, unless if you are supporting Voice
Services or other supplementary services i.e., VoLTE etc. Let’s keep our focus on to Data Services alone for a moment. In order to provide data services, there can be two approaches to overcome
In Power Control: Transmitted power is varied in accordance with channel quality as explained above with diagram. Power is varied w.r.t. channel quality to provide a fixed data rate.
In Rate Control: Transmitted power remains fixed. However, in order to cope with variation in channel quality. Modulation and coding schemes are varied to compensate for channel variations. In this case data rate is variable, while transmitted power remains as explained in the picture earlier.
In LTE , Effectively Rate control lace in Downlink and Power Control takes place Uplink.
In case of 4G LTE Downlink , rather than varying power in the Downlink, full power is distributed uniformly over the whole bandwidth.
The same Power Spectral Density (PSD) is used on all DL channels. For example, PDSCH, PHICH, PDCCH etc.
PSD is the power of a signal divided by Bandwidth.
PSD = Power / Bandwidth.
In case of PSD, it is normalized to one resource block.
(Note: There are certain channels in DL, where power is varied accordingly.)
As compared to Downlink. In case of Uplink in LTE, Power control is used. As the battery of the phone(UE) is power limited compared to
base station power in the DL.
Uplink power control is used mainly for the following two reasons.
Hope that answers the question of Adil M. In case if you are still wondering how Power control is performed on the Uplink in LTE.
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