Power Control in UMTS

Power control is necessary to keep the transmitted signal power level under control so as to minimize the interference and keep the quality of signal to a desired level. The main functions are:
1. Closed-loop power control
· Outer-loop power control
- Uplink outer-loop power control
- Downlink outer-loop power control
· Inner-loop power control
- Uplink inner-loop power control
- Downlink inner-loop power control
2. Open-loop power control
· Uplink open-loop power control
· Downlink open-loop power control

Closed-loop power control is the power control mechanism used in UMTS to solve near-far problem, minimize interference and to keep the signal quality to optimum level. Closed-loop power control is used in uplink (UL) as well as downlink (DL). However, the motive in both the cases are different. In uplink, signals from different UEs reach NodeB with different power strength, thus causing the stronger signal blocking the weaker one, resulting in near-far effect. In downlink, there is no near-far effect, but the UEs near the cell-edge or in high interference region may need extra power to overcome the increased other cell interference and weak signal due to Rayleigh fading.

Closed-loop power control can be divided into outer-loop and inner-loop power control. In case of uplink, the RNC manages the outer-loop and Node B manages the inner-loop and for downlink, UE manages the outer-loop and Node B manages the inner-loop.

Inner-loop power control (also called fast closed-loop power control), operates at 1500 times per sec (1.5 kHz) [From where did this value of 1.5 kHz come from? Answer: A UMTS 10 ms frame consists of 15 TPC commands. This results in a power control frequency of 1500 Hz (15/10ms)] and relies on the feedback information from the opposite end of the link (or channel) to maintain the signal to interference (noise) ratio to a target level set by the outer-loop power control. The transmission power is increased or decreased by a certain fixed step size depending on whether the received SIR is below or above the target SIR. Precise power control can lead to optimum use of bandwidth resulting in increase cell capacity.
The UL inner-loop power control lets the UE adjust its output power in accordance with one or more TPC commands received in the downlink direction. Remember the increase and decrease in power is limited by upper and lower bounds as defined in 3GPP TS 25.101. The upper bound, i.e. UE maximum output power, is set depending on the Power class of UE. This can also be set below the maximum capability of the UE through signaling when the link is established. The lower bound, i.e. UE minimum output power defined as the mean power in one timeslot (TS), and shall be less than -50 dB.
The DL inner-loop power control is used to control the transmission power of downlink channels at Node B as per the TPC commands received from UE. However, in some situations Node B may ignore the increase/decrease these TPC commands. For example, in case of congestion (high load scenario), the Node B can ignore the TPC commands from UE.

Outer-loop power control is used to set the target quality value for inner-loop power control, i.e it adjusts the target SIR in Node B which is used during inner-loop power control. Now the question is why do we need to adjust the target SIR? Outer-loop power control tries to keep the quality of a connection to desired value. Too high quality will waste the resources.

Open-loop power control is used to set the initial power of UE (in random access) and downlink channels. The TPC commands used in inner-loop power control are relative, so it needs a starting point and this is defined by open-loop power control. Also, this is useful in setting the power level in case of common shared channels, where it is difficult to send each UE the necessary TPC commands. In case of uplink, UE and broadcasted cell/system parameters are used to set initial access power on RACH. And in case of downlink, the measurement report of UE is used to set the initial power of downlink channels.
The open loop power control tolerance is ±9dB under normal conditions and ±12dB under extreme conditions.

[References: TS 25.214, TS 25.215, Section 7.2.4.8 of TS 25.401]

Why chips are called so?

Chips are code bits used for spreading the desired signal. Chips are called so as they do not carry any useful information and to distinguish them from data (bits).

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Why Open-loop power control mechanism does not solve "near-far problem"?

The Open-loop power control mechanism, used in CDMA based systems, requires the transmitiing entity (mobile station) to monitor the received signal strength and channel interference in the downlink and adjust its transmission power accordingly. Now uplink and downlink signals use different frequencies and there is large frequency separation of uplink and downlink bands in UMTS FDD mode (uplink frequency band is 1885–2025 MHz and downlink frequency band is 2110–2200 MHz). As such, uplink and downlink fast fading (on different frequency carriers) are not correlated. The downlink signal may suffer from different sets of diffractions and reflections that uplink signal may not encounter, thereby not giving a correct result. This is the reason that "open-loop power control" can not be used to solve "near-far problem". Usually this mechanism gives correct result only on average. Therefore open loop power control is used mainly to provide initial power setting for the initial access of system (RACH).

The open loop power control tolerance is ±9dB under normal conditions and ±12dB under extreme conditions.


Reference: 3GPP TS 25.101.

What is near-far problem?

Consider that there are 2 mobile stations (MS) transmitting at equal powers, but one is nearer to the base station (BS) compared to the other. The BS will receive more power from the nearer MS and this makes the farther MS difficult to understand. As we know, the signal of one MS is the noise for another MS and vice-versa. So the Signal-to-noise ratio (SNR) for the farther MS is much lower. If the nearer MS transmits a signal that is orders of magnitude higher than the farther MS then the SNR for farther MS may be below detectability threshold and it would seem that the farther MS is not at all transmitting. This situation is called "near-far problem" and is less pronounced in GSM than CDMA-based systems as the MS transmit at different frequencies and timeslots in case of GSM.

To overcome this problem, a power control mechanism is used so as closer MSs are commanded to use less power so that the SNR for all MSs at the BS is roughly the same.