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Power Control in 3G systems

Introduction: Power control in general is all about controlling the transmitted power both in downlink and uplink direction for different reasons. Main aim of the power control is to transmit the signal with lowest possible power level, which maintains the required signal quality. Power of every transmitter is adjusted to the level required to meet the requested QoS. Determining the transmitter power level is a very sophisticated task due to dynamic variation of the radio channel. Whatever the radio environment is, the received power should be at an acceptable level. Target of power control is to adjust the power to desired level without any unnecessary increase in the UE transmit power.

Need for Power Control:

The main reasons of implementing power control are 1.Near-Far problem. 2.System level interference leads to reduced system capacity. 3.Limited power source of UE.

Following sections talks about the above problems in detail w.r.t 3G systems.

Power Control in 3G systems:

Power control is an essential feature of any CDMA based cellular system. Without utilizing an accurate power control mechanism, these systems do not operate.

Unlike FDMA and TDMA, which are bandwidth limited multiple access, WCDMA is interference limited multiple access. In FDMA and TDMA, power control is applied to reduce inter-cell interference within the cellular system that arises from frequency reuse while in WCDMA systems. the purpose of the power control is mainly to reduce the intra-cell interference. Due to the fact that in the WCDMA system the total bandwidth is shared simultaneously, other users can experienced as a noise like interference. In case power control mechanism is missing, common sharing of bandwidth creates a severe problem referred to as near-far effect (described below).

All signals should arrive at the base stations receiver with the same signal power. The mobile stations cannot transmit using fixed power levels, because the cells would be dominated by users closest to base station and faraway users could not get their signals heard in the base station. This phenomenon is called near-far affect. The main factors, that causes the near-far problem results from the path-loss variation of simultaneous users with different distances from the BS, fading variation, and other signal-power variation of the users caused by the radio wave propagation.

Basic approaches to power control:

 See main article on Power_Control for more information

In the uplink case, the optimal situation from the BS point of view is that the power representing one UEs signal is always equal when compared to the other UE signal regardless of their distance from the BS. In such cases SIR will be optimal and and BS receiver is able to decode the maximum number of transmissions. However, in reality, the radio channel is extremely unstable and therefore the transmission power of UE should be controlled very accurately by utilizing efficient mechanisms.

To achieve this, power control has been well investigated and many power control algorithms have been developed. These include distributed, centralized, synchronous, asynchronous, iterative and non-iterative. Most of the existing algorithms utilize either SIR or transmit power as a reference point in the power control decision making process.

The primary principle of Centralized Power Control (CPC)? schemes is that they keep the overall power control mechanism centralized. As a result they require a central controller which should have knowledge of all radio connections in the RAN. CPC is often referred to as optimum power control due to the upper performance bounds achieved by these algorithms. The CPC approaches bring about added complexity, latency and network vulnerability.

In contrary to CPC methods, Distributed Power Control (DPC)? methods do not utilize central controller. Instead they distribute the controlling mechanism within the RAN. Each base station tracks and updates the transmitted powers from the local mobile stations. Therefore the power control is distributed to all base stations and centralized controller is not required. The benefits of DPC are reduced computational complexity and small delay.

Power Control mechanism in UTRAN:

In WCDMA, power control is employed in both the uplink and the downlink.

The main target of the uplink power control is to mitigate the near-far problem by making the transmission power level received from all terminals as good as possible at the home cell for the same QoS. The mobile stations far away from the base station should transmit with considerably higher power than the mobiles close to the base station. Therefore uplink power control is for fine tuning of terminal transmission power, resulting in the mitigation of the intra-cell interference and near-far effect.

The situation is different in downlink direction. The downlink signals transmitted by one base station are orthogonal. Signals that are mutually orthogonal do not interfere with each other. However, it is impossible to achieve full orthogonality in typical usage environments. Signal reflections cause non orthogonal interference even if one base station is considered. Moreover signals sent from other base stations are, of course, non orthogonal and thus they increase the interference level taking into account that in a CDMA system the neighbor cells use the same downlink frequency carrier. This calls for downlink power control.

There are two basic types of power control: 1.Open Loop Power Control 2.Closed Loop Power Control

Open Loop Power Control:

The open loop power control technique requires that the transmitting entity measures the channel interference and adjusts its transmission power accordingly. In this process, the UE estimates the transmission signal strength by measuring the received power level of the pilot signal from the BS in the downlink, and adjusts its transmission power level in a way that is inversely proportional to the pilot signal power level. Consequently, the stronger the received pilot signal, the lower the UE transmitted power. This can be done quickly but the problem is that the interference estimation is done on the received signal, and the transmitted signal probably uses the different frequency (in case of FDD), which differs from the received frequency by the systems duplex offset. As uplink and downlink fast fading (on different frequency carriers) do not correlate, this method gives the power values only on average. If UTRA-TDD is employed, then both the uplink and downlink use the same frequency and thus their fading processed are strongly correlated. This means that open loop power control gives quite results with the TDD mode. Open Loop power control, is however used in WCDMA-FDD mode but only to provide initial power setting of the mobile station at the beginning of the connection.

Closed Loop Power Control:

In the closed-loop power control technique, the quality measurements are done on the other end of the connection in the base station and the results are then sent back to mobiles transmitter so that it can adjust its transmitted power. This method gives much better results that the open loop method but it cannot react to quick changes in the channel conditions. The UTRA-FDD mode uses fast closed-loop power control technique both in uplink and downlink. In this method, the received SIR is measured over a 667 microseconds period (one TS), and based on that value, a decision is made about whether to increase or decrease the transmission power in the other end of the connection. The transmit power control (TPC) bits are sent in every time slot within uplink and downlink. All power control signals contain either an increase or decrease command. In uplink, When BS receives the UE signal it compares the signal strength with the pre-defined threshold value at the BS. If the UE transmission power exceeds the threshold value, the BS sends a Transmission Power Command to the UE to decrease its signal power. If received signal is lower than the threshold target the BS sends a command to UE to increase its transmission power. In downlink, the roles of UE and BS are interchanged.

The fast-closed loop power control is also called the inner loop power control. Closed loop power control also includes another loop called outer loop.

The inner loop closed loop power control adjusts the transmitted power in order to keep the received SIR (SIRest) equal to given target (SIRtarget). If the used SIR value still gives a low quality bit stream, then SIRtarget must be increased. The SIR target is fixed according to the received BLER or BER. The setting of SIR target is done by the outer loop power control in order to match the required BLER. The required SIR depends on mobile speed and multipath profile. The inner loop and outer loop are visualized in figure below.

Algorithms used in case of inner closed loop power control:

1.The transmitted power is updated at each time slot. It is increased or decreased by a fixed value as described below: -if SIRest > SIRtarget then TPC command to transmit is 0, requesting a transmit power decrease. -if SIRest 2.Transmitted power is updated each five time slots which simulates smaller power update steps.

The power control step size in case of uplink is equal to 1 or 2 dB in WCDMA system. Values smaller than 1 dB can be emulated by taking larger PC update periods. The power update step size may be chosen according to the average mobile speed and other operating environment parameters.

Power Control during Soft Handover:

In soft handover? state, the transmission power of UE is adjusted by selecting most suitable TPC command from those received from different BS's (to which it has simultaneous radio links). In this case the UE receives more than one TPC command from different BS's independently. The received TPC commands may differ from each other which shall lead to the conflict situation for UE. The basic approach to resolve this problem would be to choose the TPC which may require the least power among the received TPC.

SSDT (Site Selection Diversity Technique)? is another power control solution in the case of soft handovers. The basic principle of SSDT is that the BS with the strongest signal is dynamically chosen as the only transmitting BS. Then the other BS's, to which the UE has simultaneously radio connections turn off their dedicatedd physical data channel transmissions. Therefore the transmit power is adjusted based on the power control command of the BS with strongest signal.

Power Control in compressed mode:

In compressed mode?, the transmission and reception of the BS and UE are ceased for a predefined period to make room for performing the inter frequency radio measurements. As a result there is also break in the transmission power adjusting mechanism. To resolve this problem, UE in uplink case is allowed to increase or decrease its transmit power with larger step sizes to reach the desirable SIR level as fast as possible.

Maintainer: ajay.khanna@hsc.com Views: ###

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