Power Control in 3G

Presented By:Presented By: Manish SrivastavaManish Srivastava

Contents

Introduction to 3G Need for Power Control Near-Far Effect Interference in CDMA

Cellular Systems Types of Power Control Conclusion

Some Key Terms…

•Coherence Bandwidth: Measurement of the range of frequencies over which the channel can be considered “flat” or in other words, the approximate maximum bandwidth or frequency over which two frequencies of the signal are likely to experience comparable or correlated amplitude fading.•Multipath Fading: Multipath propagation is the phenomenon that results in radio signals reaching the receiving antenna by two or more paths. The effects include construction and destructive interference and phase shifting of the signal. Destructive interference causes fading. It causes jitter and ghosting effect in television transmissions and deceives the radar receiver. •Shadowing: Signal fading caused due to obstacles.•MS: Mobile Station•BS: Base Station

Introduction to 3G

3G, short for 3rd Generation, is a term used to represent the 3rd generation of mobile telecommunications technology. This is a set of standards used for mobile devices and mobile telecommunication services and networks that comply with the International Mobile Telecommunications-2000 (IMT-2000) specifications by the International Telecommunication Union.

3G finds application in wireless voice telephony, mobile Internet access, Fixed Wireless Internet access, video calls and mobile TV.

Low signal reception is due to improper power control. (Near Far Effect)

Efficient power control is very important for CDMA network performance.

It is needed to minimize the interference in the system.

Reducing the interference results in direct increase in system capacity.

Need of Power Control

In the uplink direction, all signals should arrive at the base station’s receiver with the same signal power. The mobile station cannot transmit using fixed power levels because the cells would be dominated by users closest to the base station and faraway users cannot get their signals heard in the base station. The phenomenon is called the near-far effect.

In order to solve this problem we require uplink power control.

Near – Far Effect

The downlink signals transmitted by one base station are orthogonal (Signals which 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 only one base station is considered. Moreover, signals sent from other base stations are of course non-orthogonal, thus they increase the interference level.

The signals should be transmitted with the lowest possible power level, so that it maintains the required signal quality.

fig. Near–Far Effect in the Uplink Direction (MS: Mobile Station)

Without Power Control:

Tx level MS a = Tx level MS b = Tx level MS c

Rx level MS a < Rx level MS b < Rx level MS c

With Power Control:

Tx level MS a > Tx level MS b > Tx level MS c

Rx level MS a = Rx level MS b = Rx level MS c

Interference Problem

•In CDMA cellular systems, reducing the interference results in direct increase in system capacity.

•Interference can be reduced by: Sectorisation, voice activity monitoring, beam forming techniques, diversity techniques (SSTD), power control.

•Power control is needed in both 3G and near-far problem.

TYPES OF POWER CONTROL

Power Control

Inner Loop(Open & Closed)

Outer Loop

Power control compensates for: distance, shadowing and multipath fading.

Distance and Shadowing affects: On both FW(BS to MS) and Rev.(MS to BS) Links.

Mobile measure signal on the FW link and adjusts its power accordingly.

INNER LOOP POWER CONTROL SYSTEM

Multipath fading: Frequency separation between FW and Rev.

links >> coherence BW of the channel.

Hence, both links fade independently. Base station has to tell the mobile how to adjust its power.

The rate of change in the channel is function of: mobile speed, number of fading resolvable paths, carrier frequency, etc.

OPEN LOOP POWER CONTROL SYSTEM

• 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 MS 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 MS transmitted power.

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.

FAST CLOSED-LOOP POWER CONTROL TECHNIQUE

•In this method, the received SIR is measured over a 667 microseconds (one Time period), 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.

TWO SPECIAL CASES FOR FAST CLOSED LOOP POWER CONTROL

Soft Handover: How to react to multiple power control commands from several sources. At the mobile, a “power down” command has higher priority over “power up” command.

Compressed Mode: Large step size is used after a compressed frame to allow the power level to converge more quickly to the correct value after the break.

Fig. Uplink Closed Loop Power Control

Fig. Downlink Closed Loop Power Control

Thank You!!!